For a bike, you would just need a G meter. The complication comes from the bike leaning over, but with the knowledge that in a straight line, you have 1 G straight down, you can find the lateral G of the bike very easily. You take the squared sum of up and down forces of the G meter (which are really diagonal forces since the bike is leaning over), and subtract 1.0 and take the square root. Pretty sure you can do it with your iphone.
Yes, this sounds correct. From the bike's perspective, all G loads are always directly down through the centerline of the bike. This is the reason we, as riders, feel no side-to-side lateral G's at all as we corner - just down. So a G-meter mounted on the bike will feel nothing but G load straight down. (As an aside, I find this the main reason I prefer to sport ride a bike vs. a car - not being throw from side-to-side and having to brace myself against the car structure.)
I think this explains some reports of G meters on bikes reporting G loads of 1.4-1.7 G's. As said above, you can compete the lateral G by the equation:
Lateral G = Sqrt (Total G^2 -1)
Plug a total G of 1.5 into such an equation and you get exactly the upper limit for lateral G I mentioned earlier - about 1.1G. (Also note that if you plug a total G of 1.0 into this equation, you get a lateral G of 0 - exactly as you'd expect for a bike traveling in a straight line.)
There have been numerous car vs. bike articles articles published over the years and while there are a lot of variables, the car typically wins. If the track is tight enough, a bike may be able to pull the advantage because of its super power-to-weight down straights and out of corners, but in the corners, a car will usually has a significant cornering advantage due to having four contact patches and more mechanical grip. And because you can put a car very near its limits of adhesion a lot more safely. Also, most cars have some aero advantages and racing cars with wings have a huge advantage. With most tracks, corner speed trumps acceleration and the car is quite a bit faster.
This is reflected in, for example, Nurburgring lap times where cars are typically faster.
The last few years, I've taken a few trips where we have mixed groups of cars/bikes. I can say from personal experience that keeping with a well-driven sports car on anything but the tightest roads is very difficult if not impossible.