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Pucker up! Dani Pedrosa does a stoppie at 280km/h!

is there a performance benefit from that move? is maximum loading of the front better than what braking you can get from the rear (assuming you have the talent to hold it)?
 
OVerLoRDI said:
Wait.... I think you have it backwards?
Click to expand...

tortillasoup said:
He does have it backwards.
Click to expand...

Le sigh. Here we go again.

I said that tires were more EFFICIENT at creating grip when there was less load on it. Not that a tire created more grip with less load.

Basically, take any basic fizzicks you learned and forget them. The coefficient of friction of a tire is not constant, it is related to the normal force on that tire. Specifically: the more load that's on the tire the lower its coefficient of friction.

tire_load_curve.jpg


See how grip is generated very rapidly when the loads on a tire are light? And then how less and less grip is generated per unit load as the load increases? The two gray bars are the same width (i.e. same additional load), when the tire is lightly loaded this additional weight results in a large amount of additional grip, when the tire is heavily loaded this same weight addition results in much less additional grip.

By putting all the motorcycle's weight on one tire the total available grip is lowered. The rear tire contributes 0 and the front tire contributes (for the sake of argument) a grip value near the top of the chart. If both tires were weighted equally their total combined grip (say, half-way along the x-axis in the above chart, where grip is ~2/3 of maximum) is 2*2/3=1.33 times that of the single heavily-loaded tire.

TL;DR: the physics mechanics most of us learned in HS and college doesn't really apply to tires (or polymers). Why this is the case is a whole other ball of wax we can dive into if you'd like.
 
i'll assume the graph is reasonably accurate.

of course grip rises with an increase in load, just not linearly.

as for efficiency, it completely depends on how efficiency is defined. with diminishing returns, grip/load might not be so great but grip/tire size will be near maximum.
 
The Boston Dangler said:
i'll assume the graph is reasonably accurate.

of course grip rises with an increase in load, just not linearly.

as for efficiency, it completely depends on how efficiency is defined. with diminishing returns, grip/load might not be so great but grip/tire size will be near maximum.
Click to expand...

Yes, the individual tire's grip capacity will be saturated, but the vehicle's grip will be diminished and thus performance will be diminished. I think you're confusing efficiency and utilization. In some cases fully utilizing something's capacity is efficient, but this isn't one of those times.
 
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JCH13 said:
Le sigh. Here we go again.

I said that tires were more EFFICIENT at creating grip when there was less load on it. Not that a tire created more grip with less load.

Basically, take any basic fizzicks you learned and forget them. The coefficient of friction of a tire is not constant, it is related to the normal force on that tire. Specifically: the more load that's on the tire the lower its coefficient of friction.

tire_load_curve.jpg


See how grip is generated very rapidly when the loads on a tire are light? And then how less and less grip is generated per unit load as the load increases? The two gray bars are the same width (i.e. same additional load), when the tire is lightly loaded this additional weight results in a large amount of additional grip, when the tire is heavily loaded this same weight addition results in much less additional grip.

By putting all the motorcycle's weight on one tire the total available grip is lowered. The rear tire contributes 0 and the front tire contributes (for the sake of argument) a grip value near the top of the chart. If both tires were weighted equally their total combined grip (say, half-way along the x-axis in the above chart, where grip is ~2/3 of maximum) is 2*2/3=1.33 times that of the single heavily-loaded tire.

TL;DR: the physics mechanics most of us learned in HS and college doesn't really apply to tires (or polymers). Why this is the case is a whole other ball of wax we can dive into if you'd like.
Click to expand...

Interesting, good to know. Like most things related to speed and force, it is exponential. A little extra weight makes the biggest difference, but once you have that tire "loaded" each extra bit of load does less and less as far as generating more grip.

So the question is, could Pedrosa stopped faster if he kept his rear wheel on the ground and used it to generate some braking force? I might be off, but here is what I think once you take into account the decreasing exponential load/grip function. When a rider brakes, the center of mass tips forward and the front tire is loaded and the rear tire is unloaded, how much or suddenly you brake creates stoppies of varying degrees. If loading the front tire so heavily that the rear wheel leaves the ground is a good idea depends on how the tire compounds behave under load. It maybe be possible, depending on the particular tires behavior that the extra load is beneficial enough to offset the increasingly minimal amount of brake lost by unloading the rear. Yeah?

Essentially what I'm trying to understand is did Pedrosa make the best decision as far as braking or would he have braked more effectively (and thus been able to keep the throttle open longer) if he didn't stoppie his way towards the corner.
 
OVerLoRDI said:
Interesting, good to know. Like most things related to speed and force, it is exponential. A little extra weight makes the biggest difference, but once you have that tire "loaded" each extra bit of load does less and less as far as generating more grip.

So the question is, could Pedrosa stopped faster if he kept his rear wheel on the ground and used it to generate some braking force? I might be off, but here is what I think once you take into account the decreasing exponential load/grip function. When a rider brakes, the center of mass tips forward and the front tire is loaded and the rear tire is unloaded, how much or suddenly you brake creates stoppies of varying degrees. If loading the front tire so heavily that the rear wheel leaves the ground is a good idea depends on how the tire compounds behave under load. It maybe be possible, depending on the particular tires behavior that the extra load is beneficial enough to offset the increasingly minimal amount of brake lost by unloading the rear. Yeah?

Essentially what I'm trying to understand is did Pedrosa make the best decision as far as braking or would he have braked more effectively (and thus been able to keep the throttle open longer) if he didn't stoppie his way towards the corner? [ftfy]
Click to expand...

To question 1: no. Braking hard enough to lift the rear wheel is unstable, that is to say that if he continued to brake as hard as he started to the bike would flip over. To keep from flipping over he had to back off on the braking force, increasing stopping distance. I could sketch out the FBD if you would like, but doing it yourself might be more instructional. The best braking comes from riding the edge of locking up and/or lifting a wheel. The moment the brakes lock or a wheel lifts (car or bike) the total braking capacity of the vehicle is reduced.

To question 2: I very much doubt he 'decided' to stoppie his way to the corner. What he did was probably way better than releasing the brakes too quickly, slamming the bike down, and upsetting the suspension. In that sense it was the right decision - he didn't fight what was happening, he guided the bike as best as he could given the situation. But I very much doubt it was the fastest way to brake for that corner. If it was we'd see every MotoGP driver doing it because no doubt they have the skills to make that happen.
 
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