Quote:
Originally Posted by qoncept
I didn't mean you. The real world results show you're right. But according to everyone else that's impossible because the stock brakes can already lock up the wheels which means the car can't possibly stop any faster. So, anyone else, please explain how this impossible thing happened.
|
If you watched that video, you'd see that they're running starspec tires. The motortrend article is on the stock tires. That most likely accounts for a HUGE part of the difference.
Performance pads also tend to improve modulation and the linearity of the bite, so it's feasible that they improved the driver's ability to get right up to the limit of the tire's adhesion.
That said, all your talk about coefficients of friction show a flawed understanding of the system as a whole. The wheel locks not because of some magical transition between sliding and static friction. That sudden drop in force caused by switching to kinetic friction from static? That NEVER EVER happens in a car's breaking system, and the phenomenon does NOT work in reverse. Once sliding, it takes a ton of force to lock, and there is no sudden drop off or rise in the needed force like you'd see when dragging a brick from a standstill.
When dragging something on a flat surface, it goes static-force rises-static-force rises past threshold-kinetic. With brakes, the pads area always stopped (attached to the car) and the rotor is [almost] always spinning. That's kinetic friction right off the bat. The force output by the brakes is linear (or close to it) right up until the time they lock.
What DOES happen, is the tire, who's friction coefficient can be fudged and represented by static friction for the purposes of our example, loads up. That "static" friction force applied to the tire by the road, resisting the forward momentum of the car and slowing it down (Basically, the force of the car is translated through the brake system from a linear force to a moment, which is then converted back into a linear force by the wheel/ground interface) increases as the braking force is increased, until it reaches the threshold of the tire's grip, at which point the TIRE transitions into full kinetic friction mode, at which point the tire's desire to continue rolling drops off significantly (Since the brake wants to stop it, and the force the road was previously exerting on the tire to keep it rolling is suddenly decreased), at which point the force the brakes are exerting on the tire, which were previously doing all the work to negatively accelerate the car, now only have to resist the [small] force needed to keep the wheel from spinning since it's already sliding.
Now with all the variables involved, it is possible that given a good driver who can threshold brake well, on the same tires an improved braking system would result in a shorter stopping distance. But, with tires that are likely going to slide and engage the ABS system regardless of peak braking torque (because in either case peak braking torque exceeds the amount needed to lock the tires and make that static==>kinetic transition), the difference will be much smaller, if anything, because it's highly unlikely that the ABS system is tuned well enough to take advantage of the "feel" and linearity improvements uprated brakes provide.
So again, from a road safety, single panic stop perspective, as long as the brakes are strong
enough to lock the tires, and the driver isn't Lewis Hamilton, it's likely that the stock brakes, on the stock tires, will stop the car in approximately the same distance as any other brakes on those same tires.
Cheers
Nathan