[clutchslip]

Even if you have raced 20+ years, it seems that you do not understand that bottoming out is a dynamic event, not a static event, and that is why you do not lose control even if you bottom out.......

Well, I guess I have had enough, after reading all this stuff.

I went to pass on the inside into turn five at Laguna Seca. Hard braking bottomed the forks and I lost control. The bike would not slow down enough to make the turn, because it was bouncing on the front tire rather than absorbing the bumps. I cut off the guy I was passing (almost took him out) and went into the MotoGP trap outside of five. I didn't go down only because of experience on and off road. This is what bottoming out can do to control. (Probably a mid 1.30s lap).

I have two other examples of losing the front from bottoming. The point is that you never want to bottom your front end, unless you like risking your life for no apparent reason.

------

I need to improve the brake dive on my street DDC. It is becoming a liability. I run the compression at negative settings to deal with street bumps and dips. Yes, I have the wizbang separate sensors. This is why I was reading this thread. I want suggestions on whether to adjust pre-load, raise compression, or change oil, to help with the brake dive. I will not ride with the front bottoming out. THAT IS DANGEROUS.

 

[speedfinn]

Well, I guess I have had enough, after reading all this stuff.

I went to pass on the inside into turn five at Laguna Seca. Hard braking bottomed the forks and I lost control. The bike would not slow down enough to make the turn, because it was bouncing on the front tire rather than absorbing the bumps. I cut off the guy I was passing (almost took him out) and went into the MotoGP trap outside of five. I didn't go down only because of experience on and off road. This is what bottoming out can do to control. (Probably a mid 1.30s lap).

I have two other examples of losing the front from bottoming. The point is that you never want to bottom your front end, unless you like risking your life for no apparent reason.

------

I need to improve the brake dive on my street DDC. It is becoming a liability. I run the compression at negative settings to deal with street bumps and dips. Yes, I have the wizbang separate sensors. This is why I was reading this thread. I want suggestions on whether to adjust pre-load, raise compression, or change oil, to help with the brake dive. I will not ride with the front bottoming out. THAT IS DANGEROUS.

..........

First you should understand what is "bottom out", and how long and why and where the front end bottoms out.
If the front end bottoms out, and front end/ front wheel of the bike stays on a straight (or constant) line, bottom out is no problem.
If the front end bottoms out too much/ too long, front end/ front wheel starts to act like a snake, and goes from side to side.
In both cases front end uses 100% of its total suspension travel.

Yes, front suspension should always use 100% of its suspension travel, that 100% includes also bottoming out. In ideal situation that bottoming out happens every time when you hit your brake mark and start braking. And depending on how much your bikes front end can carry load (weight shifting + braking force), it gives you a limit how much your maximum brake force can be when you start braking.
If you are using too much braking force compaired to the front end load handling capacity, front end will bottom out too much/ too long and front wheel starts to act like a snake, and goes from side to side.

 

[speedfinn]

Lets beat the dead horse...!

Sorry for hurting your eyes. I think we just need to agree to disagree, or at minimum take it outside of this thread.

Yes.

Fair I didn't get into hard bottom of damper rod against bottom of cartridge vs hydraulic bottom vs a coil bound spring.

You are thinking only about the actual mechanical suspension bottom out limit.

You are not thinking about what happens when and how suspension bottoms out, and how that affects bike handling (or does not affect bike handling) and how long bottom out lasts, and what is the lean angle and longitudinal (braking) force and lateral (turning/leaning) force, and does it compress tyre just a little bit or totally or something between, is there bumps (sharp short ones or big long one), etc.
Thats the point when I am saying that there are multiple different kind of bottom outs and zip tie can not tell you s**t about them.

You should understand that bottoming out is very dynamic and complex situation. And when suspension uses full travel and bottoms out in controlled manner and at bottom out limit compressing and extending forces are in some what balanced, tyre is not overloaded, and you get the maximum out of the bike and suspension and tyre.
Zip tie can not tell you s**t about that dynamic and complex situation.

Maybe the most important info from zip tie is, that if you have bottomed out, and you have not crashed, everything works just as designed, and you should not make any suspension changes.

And the only real indicator what you should remember is; does your front end squirm at max brake? Because you can bottom out and not squirm and bottom out and squirm. Thats the real indicator when you should consider stiffer springs (not preload, not compression damping).

Rebound is primarily control of the sprung mass

No. Rebound is primarily control of the UNsprung mass.

and compression controlling the unsprung mass,

Primarily no.
In a simplified optimal situation you just control the forces going in to spring (compression) and coming out from the spring (rebound). And only mass that moves up and down is UNsprung mass (forks/swing arm, wheel, brake disk, brake caliber), and sprung mass (bike and rider) continues on a steady horisontal line.

but I dont see how this matters to this thread. Just because I am slow doesn't mean I dont understand how suspension works,

As you said, you are "No means an expert, only in the intermediate (yellow) group (got there 2 years ago) at my own trackdays but figure toss in a few comments."
I bring this up only because all the legendary bs has the most strongest endorsement in mid pace groups.
10-15 years ago I was also In that same mental group with the same misbeliefs.

but it does seem I have a different view of how it works vs your view. Fair enough, I didnt mean to get into a discussion of all the suspension parameters. And I'll stick to my suspension guy and his resume of winning here in Canada.

As I said, maybe the source does not have time or arse to walk you through the basics.
Maybe you should next time ask your winning suspension guy if he agrees with me. You might be surprised.

 

[J_D_W]

Lets beat the dead horse...!

Against my better judgment I might as well try to understand you logic while I catch up on some WSBK. Besides, this thread seems to be mostly dead.

You are thinking only about the actual mechanical suspension bottom out limit.

Interesting comment, maybe then you are talking about something else. When I (and most I would assume) talk about bottoming your suspension we are talking about mechanical suspension bottom. I suppose we can decide if we mean where it starts the bump rubber on the shock and hydraulic lock on the fork, but generally I mean where no more stroke is available to you, none left. I am not aware of another bottom out case. What is your definition of 'bottoming out' your suspension?

You are not thinking about what happens when and how suspension bottoms out, and how that affects bike handling (or does not affect bike handling) and how long bottom out lasts, and what is the lean angle and longitudinal (braking) force and lateral (turning/leaning) force, and does it compress tyre just a little bit or totally or something between, is there bumps (sharp short ones or big long one), etc.
Thats the point when I am saying that there are multiple different kind of bottom outs and zip tie can not tell you s**t about them.

Please outline the multiple bottom outs then. And I agree the zip tie is not a data logger. So since you are so anti-zip tie I assume you have a AiM or 2D system on your bike with suspension potentiometers. Might be good to see a graph of a lap with the following traces GPS Speed, Brake G, Lean Angle, Throttle %, Front suspension travel, Rear suspension travel. I would be interested in seeing the graph and how long and often your suspension is at 'bottom' (ie full suspension travel used) in a lap. Would be good to know the track as well as I might be able to see a video of a lap online to put the corners etc into context.

You should understand that bottoming out is very dynamic and complex situation. And when suspension uses full travel and bottoms out in controlled manner and at bottom out limit compressing and extending forces are in some what balanced, tyre is not overloaded, and you get the maximum out of the bike and suspension and tire.
Zip tie can not tell you s**t about that dynamic and complex situation.

A physical bottom condition is not complex, the fork cannot compress any further, it is basically a rod. But I will agree it can be dynamic, especially at the tire. I dont understand how you can control compression at true bottom as bottom (full travel) by definition means the fork can no longer compress so the only thing that can compress is the tire.

Maybe the most important info from zip tie is, that if you have bottomed out, and you have not crashed, everything works just as designed, and you should not make any suspension changes.

I think the most important thing it tells you is how much of your suspension stroke you are using somewhere in the lap.

And the only real indicator what you should remember is; does your front end squirm at max brake? Because you can bottom out and not squirm and bottom out and squirm. Thats the real indicator when you should consider stiffer springs (not preload, not compression damping).

No. Rebound is primarily control of the UNsprung mass.

Primarily no.
In a simplified optimal situation you just control the forces going in to spring (compression) and coming out from the spring (rebound). And only mass that moves up and down is UNsprung mass (forks/swing arm, wheel, brake disk, brake caliber), and sprung mass (bike and rider) continues on a steady horisontal line.

Sorry, I dont think physics work like that. If the wheels simply followed the road under a steady state motorcycle then you could not have the pitch moment of interia around the CoG. Remember physics works such that if a wheel leaves the ground or travels over a hallow that the suspension will exert an equal and opposite force to both sprung and unsprung parts of the bike. The ratio of the sprung to unsprung is important here, for heavier bike we can get closer to the desires state of a high ratio of sprung to unsprung. Hitting a bump (compression) puts a force into the tire, into the wheel and therefore into the suspension. The unsprung to sprung ratio comes into play as the heavy (relatively speaking) bike does not lower towards the wheel on compression, so the item in movement and being controlled is the unsprung wheel (and brakes and 2/3 of the spring mass etc etc ... the unsprung mass). On rebound the wheel is still in contact with the ground (if you have a nicely behaving wheel, again where the unsrpung to sprung ratio helps) and the rebound is now controlling the 'push away' of the bike (sprung mass) on the spring and transmitted force from the bump. I believe chapter 6 of Tony Foale's chassis design book and chapter 2 of John Bradley's Volume 3 could confirm this thinking unless I have completely misunderstood them. In fact you got me thinking if I had my sprung and unsprung mixed up so I looked in John's book and on page 64 "In general, the rebound system is primarily focused on controlling the sprung mass while the compression system is primarily focused on controlling the unsprung mass" I agree it is not intuitive as you think 'I add compression damping to stop brake dive' which would make you think you are controlling the sprung mass but the brake dive is that pitch moment around the CoG and just because the bike is pitching it does not mean you are controlling the sprung (bike) mass. As a rider on the bike itself really all the sensation of damping seems to be on the sprung mass since that is the riders perspective.

As you said, you are "No means an expert, only in the intermediate (yellow) group (got there 2 years ago) at my own trackdays but figure toss in a few comments."
I bring this up only because all the legendary bs has the most strongest endorsement in mid pace groups.
10-15 years ago I was also In that same mental group with the same misbeliefs.

Something I find interesting in your posts is that you do not seems to state your definitions very well. And I can't seem to find details on your level etc, maybe my search foo is not up to standards. Given you pick on the mid-pace groups I can only assume you are in the fast group or a racer trying or at a pace within seconds of the lap records at your tracks? Given your advanced 'use all the suspension all the time' technique and how beneficial it is.
I run mid 2:20's (last weekend an example session was 2:28, 2:25, 2:25, 2:26, 2:24 full laps) at Calabogie (Ontario Canada), lap record (set by Canada Superbike rider) is 2:00.4 (dont quote on the .4, can't remember though I did witness it), fast trackday group range in 2:25 to 2:07 range if I ignore racers in the group.
I define bottom out of suspension as the point in the suspension stroke such that you can no longer compress the fork. Further I mark this limit when I do not have fluid or spring in the fork and collapse the fork. Bottom for me means I have no other compression available. I have confirmed that even with fluid and air gap (180mm from memory, log out in trailer) I can collapse my forks to the hard bottom limit. For reference I have NIX 30 front setup and TTX GP rear shock.

So you got out of the mid-pack pace and learning more complex suspension understanding for 10-15 years ... fair enough, first set of forks I serviced was 27 years ago (damper rod). But taking 14 years off from track riding didn't help when it comes to lap times.

I guess what I am getting at is, why should we consider your information more correct compared to mine? Please provide the graphs from your data logger, definition of your suspension bottom and maybe the physics that work in your situation. For me, I am going to continue to keep my suspension from hard bottom. And I know I will never be close to track records and therefore dont need your advanced 'bottom out the forks' technique.

As I said, maybe the source does not have time or arse to walk you through the basics.
Maybe you should next time ask your winning suspension guy if he agrees with me. You might be surprised.

So far I dont see the details to explain your thinking so maybe you can cover the basics as you understand motorcycle dynamics and I can learn something?
Since I am close to using all my fork travel at the moment I have left a message with the person who helps with my suspension and will see what he thinks. Maybe once you define your version of bottoming out (since there seems to be multiple versions) I'll question him a bit more when I see him next.

Jeff

 

[speedfinn]

Sorry, I dont think physics work like that. If the wheels simply followed the road under a steady state motorcycle then you could not have the pitch moment of interia around the CoG. Remember physics works such that if a wheel leaves the ground or travels over a hallow that the suspension will exert an equal and opposite force to both sprung and unsprung parts of the bike. The ratio of the sprung to unsprung is important here, for heavier bike we can get closer to the desires state of a high ratio of sprung to unsprung. Hitting a bump (compression) puts a force into the tire, into the wheel and therefore into the suspension. The unsprung to sprung ratio comes into play as the heavy (relatively speaking) bike does not lower towards the wheel on compression, so the item in movement and being controlled is the unsprung wheel (and brakes and 2/3 of the spring mass etc etc ... the unsprung mass). On rebound the wheel is still in contact with the ground (if you have a nicely behaving wheel, again where the unsrpung to sprung ratio helps) and the rebound is now controlling the 'push away' of the bike (sprung mass) on the spring and transmitted force from the bump. I believe chapter 6 of Tony Foale's chassis design book and chapter 2 of John Bradley's Volume 3 could confirm this thinking unless I have completely misunderstood them. In fact you got me thinking if I had my sprung and unsprung mixed up so I looked in John's book and on page 64 "In general, the rebound system is primarily focused on controlling the sprung mass while the compression system is primarily focused on controlling the unsprung mass" I agree it is not intuitive as you think 'I add compression damping to stop brake dive' which would make you think you are controlling the sprung mass but the brake dive is that pitch moment around the CoG and just because the bike is pitching it does not mean you are controlling the sprung (bike) mass. As a rider on the bike itself really all the sensation of damping seems to be on the sprung mass since that is the riders perspective.

Before we get to the other questions, do we agree or not, on the following fundaments:
The most important thing is grip.
Grip can be achieved only if tyre is on the surface.
Optimal grip can be achieved only if load on the tyre is constant.
Optimal laptime can be achieved only with optimal grip.
Tyre is part of the wheel and wheel is part of the unsprung mass.
By controlling the unsprung mass you controll tyre, grip and laptime.

 

[Piper]

The one time when its really ok to have the suspension bottom out on track is when braking in a straight line, and for example resulting in the rear getting levered into the air etc.
If you brake like this, then it is unlikely that a zip-tie will tell you anything since it will always just indicate that the suspension bottomed out on the brakes.

However, when coming off the brakes into a corner, you want the suspension to be able to come back into its operating range, and stay there through the corner.
i.e. you do not want the suspension to bottom out mid corner, or top out on the throttle off the turns if possible.

=> is this what you guys are getting at?

 

[J_D_W]

Before we get to the other questions, do we agree or not, on the following fundaments:
The most important thing is grip.
Grip can be achieved only if tyre is on the surface.
Optimal grip can be achieved only if load on the tyre is constant.
Optimal laptime can be achieved only with optimal grip.
Tyre is part of the wheel and wheel is part of the unsprung mass.
By controlling the unsprung mass you controll tyre, grip and laptime.

• I will agree that you want the tire on the track as much as possible with the best grip you can get from your tires.
• I will agree that the tire and wheel is part of unsprung mass.
• I dont agree that controlling just the unsprung mass you control tire grip and laptime, but if you base your physics on the bike being a constant and the wheels follow the track I can see how you can state this. I am just not sure how physics work in that case but willing to listen
• I dont agree that you can have constant tire load throughout a lap. By "constant load" in your statement I take that to mean that the weight per area (PSI or Pa) is the same constantly for all cases of the contact patch. That is riding down the straight there is x Pa on the tire but in a corner the same tire still has x Pa on the tire.
  I suppose if you could have a constant load on the tires it would simplify the dynamics but I think we know load on the tire depends on many things on the track and unless you can hit exactly the same line every time there is no way to be that constant between laps let alone sessions. For example, we know the contact patch size is different between corner types (high speed vs low speed, with camber vs against the track camber etc). I would also put forth a hypothesis that the point load (psi if you will) over the different contact patch sizes would be different and therefore not constant on the tire in different cornering cases. But this is where you can explain your understanding of motorcycle dynamics. Willing to 'listen' based on your assumptions.

=> is this what you guys are getting at?

Honestly I am not sure what speedfinn is getting at so this is why I would like him to define his version of bottoming out in a bit more detail. At this point I am intrigued enough in his version to continue this enough to see if I get the details or the suspension "basics" of his understanding.
I personally do not think hitting true bottom (lack of additional compression in the suspension stroke) is a good thing. I will accept that in an 'oh crap' case it may happen but I would not design or setup my bike to be in this state on purpose regularly, which seems to be what speedfinn proposing.

Jeff

 

[speedfinn]

The one time when its really ok to have the suspension bottom out on track is when braking in a straight line, and for example resulting in the rear getting levered into the air etc.
If you brake like this, then it is unlikely that a zip-tie will tell you anything since it will always just indicate that the suspension bottomed out on the brakes.

However, when coming off the brakes into a corner, you want the suspension to be able to come back into its operating range, and stay there through the corner.
i.e. you do not want the suspension to bottom out mid corner, or top out on the throttle off the turns if possible.

=> is this what you guys are getting at?

Yes, all the above is also my point(s) on this.

 

[speedfinn]

• I dont agree that you can have constant tire load throughout a lap. By "constant load" in your statement I take that to mean that the weight per area (PSI or Pa) is the same constantly for all cases of the contact patch. That is riding down the straight there is x Pa on the tire but in a corner the same tire still has x Pa on the tire.
  I suppose if you could have a constant load on the tires it would simplify the dynamics but I think we know load on the tire depends on many things on the track and unless you can hit exactly the same line every time there is no way to be that constant between laps let alone sessions. For example, we know the contact patch size is different between corner types (high speed vs low speed, with camber vs against the track camber etc). I would also put forth a hypothesis that the point load (psi if you will) over the different contact patch sizes would be different and therefore not constant on the tire in different cornering cases. But this is where you can explain your understanding of motorcycle dynamics. Willing to 'listen' based on your assumptions.

Ok, if we change semantics to be more precise:
Most optimal grip can be achieved only if load on the tyre is as constant as possible.
Agree, yes no?

For example, we know the contact patch size is different between corner types (high speed vs low speed, with camber vs against the track camber etc). I would also put forth a hypothesis that the point load (psi if you will) over the different contact patch sizes would be different and therefore not constant on the tire in different cornering cases. But this is where you can explain your understanding of motorcycle dynamics. Willing to 'listen' based on your assumptions.

No. Contact patch size is not different between corner types (high speed vs low speed), because lateral (centrifugal) force and bike+rider mass in high speed and low speed corners are the same.

Track camber yes, then you have different lateral (centrifugal) force, and different contact patch size, because the force/ load on the tyre is different.

 

[J_D_W]

As I said, maybe the source does not have time or arse to walk you through the basics.
Maybe you should next time ask your winning suspension guy if he agrees with me. You might be surprised.

from the suspension person: "We love to see the oring about 3-4 mm off the bottom after your fastest laps. <snip> for us mortals without dataloggers, we just look at the oring and if we can keep it off bottom, then great."

This does not surprise me as an answer but if you take this as agreeing with you then I still dont understand your point on bottoming.

Zip tie does not tell you where and why and how long front end bottomed out, so do not think of it as a Data Recorder.
Any any reasoned suspension Tech will tell you that.

My suspension tuner also stated "It CAN bottom in a straight line, but you really need data to determine where it is using the most amount of travel". I never said you can't, though I believe for most (all?) people on this forum ( especially me ) they should not bottom their forks. Most people on this forum are not at a level where it matters enough or maybe control their bike repeatedly enough to ensure that the bottom out case is 'safe' each time. (level of safe probably different per rider). And I am sure you are going to reply with "see, told you, he agrees" but to agree I need to understand your POV. I know for myself squirming under brakes is not something I want (or can deal with, I just dont work that way when entering a corner).

Zip-tie in front fork should be banned, because it causes much more problems than bottoming out...

I don't understand why knowing how much travel you have used in a lap is a negative. I suppose what the rider does this this information could be detrimental to the performance of the bike, but using only 10% of your travel is also not good and can be shown with a zip-tie.


So I went back through your comments in this thread to try to put together what I think you are trying to say without reading the anti-Moss, anti-zip tie type etc comments.

Are you trying to simply state? Optimal lap times are obtained if during any threshold braking phase (full brake pressure you can apply without flipping over frontwards) you should be using as much fork travel as possible. This can (and should in your opinion) include bottoming the fork. Generally threshold breaking at the track is upright and not during the trail braking phase. And is not every corner as you dont threshold break into every corner.

If this is the case, I will not disagree with the general case (use as much fork travel as you can). I dont see how zip-tie is against this and causes more problems. I dont see what I have stated that goes against this other than I dont think (the average track rider) should outright bottom your fork in this case. Very few of us are Toprak Razgatlioglu and therefore, speaking for myself and many I know from track, having ~5mm or so of suspension left during heavy breaking is not going to negatively effect a lap enough compared to some safety margin that it may add.
And yes, I am generalizing about the capabilities of posters here, not saying they are 'slow' but am assuming those who want to learn and question things like zip-tie location are probably not at the pointy end of pro races with the ability to constantly brake the same for the same corner.

Even yourself agree that a fork with no movement (stiffened up) can have traction issues so that would mean a bottomed fork at the wrong time would be detrimental to traction.

Or they would have to stiffen up front end so much, that they would loose too much front end traction, and again they would loose at least a couple of seconds per lap.

I know I am not capable of hitting the same point on track every lap and apply the same brake pressure lap in and lap out for the same corner. Your approach of stating people should bottom their forks each lap might work for you but you might be loosing people simply because they know they have limits. And as my suspension tuner stated "us mortals without dataloggers, we just look at the oring and if we can keep it off bottom". That said, I would still like to see your data log graphs showing when and for how long you bottom your forks.
It seems a 'rule' that states 'you should use as much of the fork travel as possible with 1-5mm left' would be safer in the long run for common track riders vs a 'you should bottom your forks whenever you brake hard' (not sure how to define it exactly) rule. Moss 80/20 rule might be good enough for street and slow group track, 3-5mm off bottom using zip-tie might be good enough for fast trackday goers. But many of your comments speedfinn come across as though there are no rider skill ranges at play and that its black and white you should either have a data logger or just 'send it' and bottom your forks.

Jeff

 

[speedfinn]

from the suspension person: "We love to see the oring about 3-4 mm off the bottom after your fastest laps. <snip> for us mortals without dataloggers, we just look at the oring and if we can keep it off bottom, then great."
My suspension tuner also stated "It CAN bottom in a straight line, but you really need data to determine where it is using the most amount of travel". I never said you can't, though I believe for most (all?) people on this forum ( especially me ) they should not bottom their forks. Most people on this forum are not at a level where it matters enough or maybe control their bike repeatedly enough to ensure that the bottom out case is 'safe' each time. (level of safe probably different per rider). And I am sure you are going to reply with "see, told you, he agrees" but to agree I need to understand your POV.
It seems a 'rule' that states 'you should use as much of the fork travel as possible with 1-5mm left' would be safer in the long run for common track riders vs a 'you should bottom your forks whenever you brake hard' (not sure how to define it exactly) rule. Moss 80/20 rule might be good enough for street and slow group track, 3-5mm off bottom using zip-tie might be good enough for fast trackday goers. But many of your comments speedfinn come across as though there are no rider skill ranges at play and that its black and white you should either have a data logger or just 'send it' and bottom your forks.

Typical front fork has 80 mm compression travel.
Lets assume that it is a track bike, so no hydraulic bottom out stoppers, so all 80 mm comp travel is usable.
And oil level in front forks is also typical, so air spring is typical.
Lets say bike is S1000RR and rider pace is mid pack.
When he brakes hard, not even max, he easily uses most of the comp travel, lets say 70-75 mm.
Bike uses 70-75 mm because now you have most of the bike and rider weight on the front wheel (weight transfer) and also you have some of the braking force which is compressing front forks.
Then there is a small typical small 20-30 mm tall bump (~25 mm would be one inch) on the track surface, and front fork bottoms out for 0.2 sec, because of very typical track conditions.
And now your O-ring indicates bottoming out.
Do you now understand why it sounds totally ridiculous when your winning suspension person says:"We love to see the oring about 3-4 mm off the bottom" ?
3-4mm vs typical track surface with small bumbs. For real...?

 

[Alex_UK]

Zip ties are unreliable. You need the data showing the travel in the braking zones. One hard bump can make a zip tie lie.

 

[J_D_W]

Typical front fork has 80 mm compression travel.
Lets assume that it is a track bike, so no hydraulic bottom out stoppers, so all 80 mm comp travel is usable.
And oil level in front forks is also typical, so air spring is typical.
Lets say bike is S1000RR and rider pace is mid pack.
When he brakes hard, not even max, he easily uses most of the comp travel, lets say 70-75 mm.
Bike uses 70-75 mm because now you have most of the bike and rider weight on the front wheel (weight transfer) and also you have some of the braking force which is compressing front forks.
Then there is a small typical small 20-30 mm tall bump (~25 mm would be one inch) on the track surface, and front fork bottoms out for 0.2 sec, because of very typical track conditions.
And now your O-ring indicates bottoming out.
Do you now understand why it sounds totally ridiculous when your winning suspension person says:"We love to see the oring about 3-4 mm off the bottom" ?
3-4mm vs typical track surface with small bumbs. For real...?

No, I dont understand that its ridiculous. If the o-ring is still 3-4mm from bottom mark then it means none of your 20-30mm bumps I came across had a large enough force (vertical acceleration) to get through the tire flex and put enough load into the fork to compress the fork further against compression damping and spring force. But you are equally correct that if the force of the bump was enough to cause the fork to compress I would have the o-ring at the bottom. If the o-ring was at bottom I would add maybe some compression damping and next time around maybe it is now 2mm from the bottom due to the same forces (I hit the same bumps). Or maybe its 5mm from bottom because I missed the most square of your 25mm bumps. A 25mm bump does not match to 25mm fork movement (I assume we can agree on that?). A 25mm sharp square edge bump has significantly more vertical acceleration vs a 25mm bump that ramps up over a distance (a hunk of fairing that is angled ?) and that ramped bump would not impart the same vertical force. We are not dealing with a solid wheel, so the tire composition contributes a lot to how much force is transmitted to the suspension.

The 3-4mm from bottom o-ring shows that in your last session you did not generate a force (via braking, turning, bumps etc) enough to compress your forks more than the 76mm (80mm - 4mm example). Maybe next session in turn 5 there is a turtle (animal) crossing the track and I hit it, maybe while also applying additional braking and the combined force is an additional 4mm of fork compression, fair enough, o-ring at bottom due to the unusual circumstances, I would not make a change to the suspension to compensate for the bottoming case. I am not arguing that you dont actually know where on the lap(s) you used the travel or if its more than once or just once, maybe it wasn't the turtle you ran over and it was that you went faster through the corner with heavy G loading. Understood, wish I had the data logging to tell me.

Zip ties are unreliable. You need the data showing the travel in the braking zones. One hard bump can make a zip tie lie.

They don't lie ... they don't tell you the complete picture, agreed. But they still tell you what they are meant to do, show you the max travel you used. I agree data logging is better, I would like suspension data logging on my track bike, anyone willing to send me the pots let me know and I will provide address. Want to send me a full data logging system, even better!
If you have the same bike as above (track bike, no hydraulic bottom out, 80mm travel, sags set in range) and your zip-tie after a fast lap (for the rider) is 40mm from bottom, I would say it also tells you something. You can brake harder? Your spring is not correct though your sag # is? You have too much compression damping configured? etc

I accept it, I am thick and dont see things the same way as y'all. I am OK with agreeing to disagree. Maybe someone else reading all this will come to some conclusion that is helpful for them, or not.

Jeff

 

[Alex_UK]

I ended up going down a rabbit hole of never ending stiffening up the front because I was looking at the zip tie. Turns out half the time it was just slipping...
The other way to check it, that I like, is a camera at 60-120fps...looking at the front wheel/fork.

 

[speedfinn]

No, I dont understand that its ridiculous. If the o-ring is still 3-4mm from bottom mark then it means none of your 20-30mm bumps I came across had a large enough force (vertical acceleration) to get through the tire flex and put enough load into the fork to compress the fork further against compression damping and spring force. But you are equally correct that if the force of the bump was enough to cause the fork to compress I would have the o-ring at the bottom. If the o-ring was at bottom I would add maybe some compression damping and next time around maybe it is now 2mm from the bottom due to the same forces (I hit the same bumps). Or maybe its 5mm from bottom because I missed the most square of your 25mm bumps. A 25mm bump does not match to 25mm fork movement (I assume we can agree on that?). A 25mm sharp square edge bump has significantly more vertical acceleration vs a 25mm bump that ramps up over a distance (a hunk of fairing that is angled ?) and that ramped bump would not impart the same vertical force. We are not dealing with a solid wheel, so the tire composition contributes a lot to how much force is transmitted to the suspension.

Tyre has flex, but typical 120/70 tyre is typically 6-times or so, stiffer than front suspension.
And when you are braking hard, and weight is on the front wheel, you already have used most of the normal tyre flex, and most of the suspension compression travel.
So when you hit a bump, while braking, most of the additional compression travel is first done by suspension.
That is why it is very easy to momentarily bottom out your front end while braking hard.
Of course when you have used all of your compression travel, tyre flexes a bit more, but as said it requires that all of the suspension travel is used first before that happens.

If your speed is 200 kmh at brake mark and you start braking, and you manage achieve your max brake force 1.5 sec later (typical track day rider) at 150 kmh, you are traveling 41,7 m/s. If that small 20-30 mm bump is 8.34 meters long, it takes 0.1 sec to reach bump top, and suspension compression speed is 200-300 mm/s at most.
That is why it is very easy to momentarily bottom out your front end while braking hard.
I would guess that it is very hard to see that kind of bump, even if you take a track walk.

 

[J_D_W]

Tyre has flex, but typical 120/70 tyre is typically 6-times or so, stiffer than front suspension.
And when you are braking hard, and weight is on the front wheel, you already have used most of the normal tyre flex, and most of the suspension compression travel.
So when you hit a bump, while braking, most of the additional compression travel is first done by suspension.
That is why it is very easy to momentarily bottom out your front end while braking hard.
Of course when you have used all of your compression travel, tyre flexes a bit more, but as said it requires that all of the suspension travel is used first before that happens.

If your speed is 200 kmh at brake mark and you start braking, and you manage achieve your max brake force 1.5 sec later (typical track day rider) at 150 kmh, you are traveling 41,7 m/s. If that small 20-30 mm bump is 8.34 meters long, it takes 0.1 sec to reach bump top, and suspension compression speed is 200-300 mm/s at most.
That is why it is very easy to momentarily bottom out your front end while braking hard.
I would guess that it is very hard to see that kind of bump, even if you take a track walk.

Can you please show your data logging graph of your suspension (F and R), speed and brake force/% (F & R). Lean angle would be nice as well. I would really like to see the plots of something like this.

So I looked at my speed vs time for a session I know my zip-tie was low in travel and picked the hardest braking corner (long straight with relatively slow corner entry, ~100kph for me). Turn 5 Calabogie.
226.4kph @ the 590.6s mark of log (looking at data from Android phone and GoPro 7 data, GoPro providing GPS speed and time in this case)

1.5s later I am at 195.3kph @ the 592.1 mark of log

I do not have a calculated rate of change for my speed or brake pressure but I approximated the angle of the graph in different locations and seems my hardest braking is just starting at the 1.5s mark.

• yes I know this is not how it should be but like I said, I am slow
• I can see in video that I am letting off the throttle before my marker that I think I am so I assume there is a coasting aspect
• at the steeper part of the graph I did look at a starting point of 200kph and in 1.5s I got down to 158.7kph so I am somewhat off your hypothetical but also somewhat close

As for your other numbers I dont have the calculations (or willingness to simulate/calculate them) to confirm or deny so OK, will take your word for it.
Is 200-300 mm/s the rate of change at the wheel surface (ie the bum itself)? That rate of change it would then impart a force onto the tire, I would expect different tires and pressures to pass this force at different rates to the wheel even if the tire is mostly collapsed by braking. The rate of change would then also be effected by the weight of the unsprung mass, fair, most bikes the wheel weight are probably close enough for this discussion. But that force eventually gets to the forks and if too small, your spring rate + compression damping takes higher to move, then really the forks will not compress and the bump will be fed up to the hands of the rider. Maybe that is your point? That by keeping 3-4mm on the o-ring it means I am missing that last optimal 3-4mm of travel and I could soften my forks even more? I guess we could do the math to determine the amount additional reduction of ground trail I get with that additional 4mm of fork travel but I dont think that trail reduction is really going to help me much. And maybe that is the difference here, you are explaining based on lap record pace theory and I am thinking most (me and friends) trackday rider.

Taking the same hypothetical 200kph to 150kph braking over the initial 1.5s of braking, I could also have too soft a front end to support the load and blow through my travel and bottom 100m from corner turn in. 100% agree zip-tie does not differentiate optimal 'just bottoming at the right time' vs 'way to soft and bottoming when I sneeze' cases. But after a few sessions adjusting compression and preload and the zip-tie is still slammed I would probably get the idea that my front end doesn't have the support it needs for the pace I am going. Is this not a valid use of zip-tie information?

Again, I am trying to understand your hypothesis and conclusion. You now seem to be hung up on what a zip-tie/o-ring does or does not tell you. I agree, its not perfect, but its cheap and tells you something and after a suspension change it can tell you did that something change (within context). We agree to disagree if that 'something' is useful to riders. I think we all agree a full data logging system with suspension is best if you are willing to look at the logs. I know fast riders who just ride and have data loggers they have never use other than GPS based lap timing (because they came on the bike from a race team). I will also agree that bottoming your forks under hard braking can happen, (I think you think it must happen), but without data logging you do not know if it is happening under 'safe' conditions.(maybe you believe there is no difference between conditions when it happens so safe is irrelevant?) It seems to me there are conditions where you would not want to bottom out your forks for long periods of time (as it puts additional stress on tires). Maybe that too is your point, if I have 3-4mm of o-ring travel left then I am still putting the loads through the tire as the suspension is not compressing more so something is taking the force? And the optimization is allow for the extra 4mm of travel and maybe the tire will have less to deal with until I hit the case where it needs 6mm of additional travel vs 4mm, at which point we are back into the same loop.

None of this (data logger better than zip-tie, ability to bottom in some cases) I argued against. The only difference in opinion I can see is you must bottom out vs leaving some travel implies some level of safety for people without data loggers and willingness to look at the log.
You started in the other thread that I dont understand how suspension works. That I should learn the basics. In this thread you state that zip-ties are worse than bottoming out <period>. As far as I can tell the conclusion from you so far is 'you can always bottom your forks as you can always hit a bump large enough to do it' and 'zip-ties can't tell you where the bottoming happened'. Both true.

Jeff

 

[speedfinn]

I will also agree that bottoming your forks under hard braking can happen, (I think you think it must happen), but without data logging you do not know if it is happening under 'safe' conditions.(maybe you believe there is no difference between conditions when it happens so safe is irrelevant?) It seems to me there are conditions where you would not want to bottom out your forks for long periods of time (as it puts additional stress on tires). Maybe that too is your point,

So lets assume you have S1000RR track bike.

It is very safe to assume that, typically:
Front fork can bottom out only when you are braking very hard.
You can brake very hard only when you are upright.
If bottom out happens, it is typically a very short time period, and because you are upright, it does not cause any problems.
That is what you can/ should expect from a front fork which has adequate front springs (and typical oil level/ air spring).

If front fork springs are a little bit too soft, and you brake very hard, and front fork bottoms out for longer time period, bike front end starts to squirm.
You do not want to sacrifice you overall front tyre grip by adding compression damping.
You do not want to mess up you sags and bike balance by changing preload.
You do not want to raise front fork oil level, because then front end becomes more non-linear.
Correct options are; brake earlier to be able to use smaller brake force (if you brake earlier you will also lose in lap time), or change stiffer front springs.

You can also assume, when you are trail braking and leaning in:
Front can bottom out, at moderate lean angles, only if you have totally f'd up your sag settings (way too much sag) and spring rates (way too soft springs).
Your brake force (and weight transfer) is getting smaller, so bike does not use that much of compression travel. Centrifugal force does not add compression travel nearly as much as hard braking did.

It is also safe to assume that:
Front or back can bottom out in apex, at full lean angle, only if you have totally f'd up your sag settings (way too much sag) and spring rates (way too soft springs). Centrifugal force does not add compression travel nearly as much as hard braking did, typically about as half as much or less.
Exception to this is long sweeping corners with huge amount of positive track camber. Then centrifugal force plays a major role, and can additionally compress suspension significantly, BUT only if you have maximum lean angle at the same time.
If you are riding long sweeping corners with huge amount of positive track camber, but your bike is perpendicular or so, to track surface, then centrifugal force does not add compression travel nearly as much as hard braking did.
You would need a ridiculous amount of track camber to be able to bottom out your suspension with centrifugal force "only".

Typical crashes are:
Just a little bit too much trail braking before apex.
Just a little bit too much throttle after apex.

I have never seen a crash at or after brake mark when front end bottoms out (momentarily).
I have never seen a crash at or after brake mark when front end bottoms out and starts to squirm.
I have seen multiple crashes at or after brake mark when there has been oil, coolant or water, etc, at braking zone.


And if we mix in typical street bikes (including S1000RR with original suspension) with hydraulic bottom out stoppers, those are bottoming out (hydraulically) typically at every hard brake, no matter how smooth they try to be, and riders do not even know that they are bottoming out.
Yes it is not the same as hard mechanical bottom out, but the situation form tyres point of view is the same: Suspension does not flex, so it is the tyre which must flex.

 

[speedfinn]

Is 200-300 mm/s the rate of change at the wheel surface (ie the bum itself)? That rate of change it would then impart a force onto the tire, I would expect different tires and pressures to pass this force at different rates to the wheel even if the tire is mostly collapsed by braking. The rate of change would then also be effected by the weight of the unsprung mass, fair, most bikes the wheel weight are probably close enough for this discussion. But that force eventually gets to the forks and if too small, your spring rate + compression damping takes higher to move, then really the forks will not compress and the bump will be fed up to the hands of the rider.

Lets put it this way.
Have your bike on paddock stands, front end supported from steering stem. Grab the front tyre and push it up, it typically compresses front forks a couple of millimeters.
Take the bike off from the stands, grab the handle bars, apply front brakes, and push very lightly down at the handle bars, it typically compresses front forks a couple of millimeters.
Put a rider on the bike, grab the handle bars, apply front brakes, and push very lightly down at the handle bars, it typically compresses front forks a couple of millimeters.
Put a additional (second) rider on the bike, grab the handle bars, apply front brakes, and push very lightly down at the handle bars, it typically compresses front forks a couple of millimeters.
The point is, under any load/force, front end compresses very easily (and fast), when any additional load is applied.

For example if you have 10.0 N/mm front springs and 25 deg caster angle, then actual spring rate at front axle/tyre is 2.3 kg/mm. Additionally you have (progressive) air spring which is typically about 2.5 N/mm at the first 100 mm of the total 120 mm front fork suspension travel, and about 5 N/mm at the last 10 mm. That will add 0.6-1.2 kg/mm to actual front wheel spring rate.
The point is, you need very small load/force to move/ compress font forks at any stage of the suspension travel. 2.9-3.5 kg load to compress one millimeter.

One thing which may mislead a lot is the name "shock absorber". In reality the spring(s) is the shock absorber. Spring is the component which (mainly) stores the energy impulse. And comp and reb damping is only used to control the energy going in and out from the spring(s). So the actual damper is a spring movement damper.
Spring (and air spring) is also the component which carries the actual weight/ weight transfer/ load /braking force, that is why you can not use compression damping if you want your front end to be able to handle more braking force/ harder braking.

Typically compression damping force is relatively small, because you want the energy go to the spring, not to the bike chassis/mass (and to riders hands). Because you want to keep the bikes mass as steady as possible. You do not want to ride a rodeo bull because it is slow when measured with lap time.
Typically compression damping force is two times or so smaller than rebound damping force. Because you want to fade away the suspension (unsprung mass) movement, and you can not do it at compression stage, you must do it in rebound stage. That is why you must have more spring rebound damping force than spring compression damping force.

And you are not damping bike mass/ sprung mass, you are only damping unsprung mass, that is why damping forces are relatively small and it is easy to compress/move front fork with a very small additional force as mentioned above. Front unsprung mass is typically 15 kg and rear 20 kg, vs bike total mass 200 kg or so.

So, at any circumstances, with any front tyres, bump impact is mainly absorbed by front springs as long as there is compression travel available. As I said earlier, any 120/70 front tyre is typically 6-times stiffer than front springs.

 

[speedfinn]

@J_D_W you want to smack the pony...?

 

[J_D_W]

@J_D_W you want to smack the pony...?

Nope .... you are happy with your understanding. I am happy with mine and willingness to generally keep my o-ring off of bottom by a bit.