Physics Question - BMW S1000RR Forums: BMW Sportbike Forum
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post #1 of 54 (permalink) Old 07-24-2019, 01:40 PM Thread Starter
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Physics Question

I'm wondering how fast an S1000RR or similar will slow down from wind resistance alone at different speeds from just rolling the throttle off.

Typically VERY hard braking can reduce speed by 22mph/sec. I'm thinking that VERY hard braking at very high speeds should yield more deceleration due to wind resistance. To understand that, I'm wondering what the wind resistance alone would do in speed reduction/sec from 30mph, 60mph, 120mh, 180mph, and 210mph.

I'm aware that there are many variables not the least of which are wind, engine braking, gear, rpm, rider position, downhill/uphill, etc.

I became curious after riding the HP4 Race at The Ridge which has a long flat 6th gear straight into a fast sweeping first turn. So easy to over-slow for the corner. Another one very similar is turn 1 at Phillip Island.
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post #2 of 54 (permalink) Old 07-24-2019, 01:49 PM
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Its too much math for me

Air Resistance Formula

I am sure NASCAR calculates this all the time.

When you said ride position, I was thinking SIZE of rider.

Lois B pops up from behind the windscreen is like a huge wall compared to Danny P popping up in the wind.
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post #3 of 54 (permalink) Old 07-24-2019, 01:51 PM
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It's probably easier to work this out using telemetry off the bike instead of calculating drag.
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post #4 of 54 (permalink) Old 07-24-2019, 10:13 PM
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Factors such as the coefficient of friction between the tires and the surface play a large roll in traffic collision reconstruction. I have never calculated drag in that manner as wind resistance doesn't usually play a roll in crashes on streets (sub 100MPH) most of the time. You would have to know multiple things to calculate this. The hardest one to pin down is the actual drag coefficient of the motorcycle. Keeping all things equal... flat roadway, consistent roadway friction coefficients, no wind, consistent rider position (upright likely to create the most drag while under hard braking), an engine braking coefficient on a motorcycle is typically around .3 but the new s1000 can vary that to some degree. I would venture to estimate that it's negligible in the totality of the equations though. To be anywhere near the mark you would have to accurately know the drag coefficient of the bike rider combination. Taking information from standards set for by NHTSA they "assume" a drag factor of .45 at 60MPH. Now I can tell you that this must be an AVERAGE between all bikes currently on the roadway... GOLDWING vs S1000RR is much different. In basic drag modeling for motorcycles, rider position can play a factor of around 25% of the total drag factor.

You have my attention I will work on this tomorrow at work and see if I can't get an expert or two to assist.
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post #5 of 54 (permalink) Old 07-25-2019, 12:23 AM Thread Starter
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Originally Posted by wes4895 View Post
You have my attention I will work on this tomorrow at work and see if I can't get an expert or two to assist.
Thanks a lot. Look forward to anything you come up with.
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post #6 of 54 (permalink) Old 07-25-2019, 09:23 AM
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Actually not so much a physics question as an aerodynamics question. The winglets in MotoGP may actually help under braking as it changes the angle of attack. The fairing on a bike might help a little, but aero drag and effects aren't linear, so they're a lot more effective at higher speed. I remember reading that an F1 car decelerated over 1g simply due to the drag caused by the aero downforce.

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post #7 of 54 (permalink) Old 07-25-2019, 02:38 PM
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Originally Posted by CYCLE_MONKEY View Post
.... I remember reading that an F1 car decelerated over 1g simply due to the drag caused by the aero downforce.
I brake HARD and have 1.1g deceleration and they have 1g just by downforce...
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post #8 of 54 (permalink) Old 07-25-2019, 03:35 PM
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I brake HARD and have 1.1g deceleration and they have 1g just by downforce...
Yup, that's what giant aero wings and undertray aero will get ya! F1 cars are designed using rolling windtunnels, extremely expensive, and they pay almost as much attention to aero as NASA does, whereas bike aerodynamics are by comparison crude and about as aerodynamic as a barn door. Bikes are at an extreme disadvantage in having an extremely large frontal area compared to their length. It's extremely important to be able to have the length needed to "close" the air behind you, and bikes (without the old "dustbin" fairings) are simply too short to do it.

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post #9 of 54 (permalink) Old 07-29-2019, 08:25 AM
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Quote:
Originally Posted by Dylan Code View Post
I'm wondering how fast an S1000RR or similar will slow down from wind resistance alone at different speeds from just rolling the throttle off.

Typically VERY hard braking can reduce speed by 22mph/sec. I'm thinking that VERY hard braking at very high speeds should yield more deceleration due to wind resistance. To understand that, I'm wondering what the wind resistance alone would do in speed reduction/sec from 30mph, 60mph, 120mh, 180mph, and 210mph.

I'm aware that there are many variables not the least of which are wind, engine braking, gear, rpm, rider position, downhill/uphill, etc.

I became curious after riding the HP4 Race at The Ridge which has a long flat 6th gear straight into a fast sweeping first turn. So easy to over-slow for the corner. Another one very similar is turn 1 at Phillip Island.
Some old measurements what I have done:

Clutch egaged, air drag + rolling resitance:

Tucked down, lower body in the middle
180 kmh -0,200 G
150 kmh -0,155 G
120 kmh -0,100 G

Tucked down, lower body on the side (for the nex turn)
160 kmh -0.215 G
150 kmh -0.180 G
140 kmh -0.165 G
130 kmh -0.140 G
120 kmh -0.125 G

Braking body position (head up, knee on the side)
190 kmh -0,400 G
180 kmh -0,350 G
170 kmh -0,315 G
160 kmh -0,275 G
150 kmh -0,225 G
140 kmh -0,215 G
130 kmh -0,205 G
120 kmh -0,175 G
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post #10 of 54 (permalink) Old 07-29-2019, 08:33 AM
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Originally Posted by DrIoannis View Post
I brake HARD and have 1.1g deceleration and they have 1g just by downforce...
It is easy to brake max 1.4-1.6G if your bikes front end can handle the load transfer and brake force, and speed is +200 kmh, because air drag gives you an extra 0.4-0.5 G deceleration.
But one problem is also your mindset, if you are used to brake 1.1 G at speeds below 100 kmh and you senses are tuned for that. You must understand that you can brake much harder with the air drag aid and then you must tune your senses to allow much greater deceleration than 1.1 G when speed is +200 kmh.

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