PedalFaster

It would be more accurate to say that ABS doesn't do much for stopping distances on dry pavement when compared to a competent driver threshold braking. Locking up the brakes, as the Viper's driver did, does not qualify as a competent driver threshold braking.

Mr.E.G.

I think your understanding of ABS is off a bit. ABS wont make for better stopping distances on dry pavement compared to stopping the vehicle CORRECTLY.

Assume that a car can stop from 60-0 in 100 feet on dry pavement when a professional driver applies the perfect amount of pressure. He gets it to slow down as fast as possible without locking the wheels.

If you have someone try it in the same car with ABS activated, then yes, the braking distance will be more than the 100 feet.

BUT if you have someone lock the brakes up, the stopping distance is going to be considerably more than 100 feet.

They way you should look at it is that ABS will result in the mid point. It wont be as good as a perfectly executed braking maneuver, but it will certainly result in less stopping distance than a poorly executed braking maneuver.

Since this guy delivered a worst case scenario braking maneuver, I think it is safe to say that ABS would have helped shave off more speed and result in less of an impact.

PS. The guy driving that Viper is a douche bag who made his own bed. Had he not been showing off for camera guy, he would have been OK. I get it, I like driving fast too, but that's why there are race tracks or at least open roads.

Mr.E.G.

Whoops. You beat me to it. lol.

I guess I should have read a bit further.

ZDan

Regarding my prior comments regarding ABS vs. locked, and hitting the stopped van at xx mph vs. (xx-0.5mph), I couldn't have been more wrong. As someone above observed, the brakes were locked for something like 3 seconds. I ran the numbers assuming 0.9g decel for ABS and 0.72g for locked brakes, and a 60mph initial speed. After three seconds with the wheels locked, final speed is 12.5mph. With modulated threshold braking (whether via ABS or a talented and unrealistically reliable panic-stricken driver), final speed would be less than 1mph.

So I retract my earlier assessment, ABS (or talented driver threshold braking) may well indeed have prevented this particular accident, and certainly would have significantly reduced the impact speed.

But it's probably safe to say that a talented experienced driver wouldn't have been driving like an idiot in traffic in the first place...

ZDan

Actually, I screwed up even worse than I thought, the difference in velocity at the point of the back of the van is MUCH greater than 12.5mph., because it is speed vs. DISTANCE that's important, not speed vs. time.

If the Viper hit the van at 12.5mph after braking from 60mph at .72g, distance covered is 160ft. In the modulated-braking case at .9g, the car could have come to a dead stop in 134 ft, 26 ft short!

So, yeah, I was WRONG.

tarheel91

Your also off in terms of the difference in applied forces. The coefficient of static friction between rubber and asphalt is 1.2. The coefficient of dynamic friction is around .8 depending on conditions. Of course, that's for rubber in general, and doesn't take into consideration the type of tires or anything, but it's about as accurate as we're going to get. Still, using these, braking while maintaining static friction should yield around 1.5 times the force compared to braking while sliding. Assuming .9g is correct while under static friction, .6g would be closer to the real value while sliding.

ZDan

For fully warmed up serious-performance street tires I'll give you 1.2 braking coefficient of friction. For cold tires, I bet 0.9 is closer.

Regarding sliding vs. non-sliding coefficients, it will depend heavily on the tires used and surface conditions. But I thought that in dry conditions for good modern high-performance street rubber, the dry sliding coefficient is closer to 80% than to 67% of non-sliding, but I'm open to any data resources you have

dammitjim

Drivers can be talented, experienced and stupid all at the same time.

PedalFaster

One more thing on ABS versus competent drivers -- a competent driver who puts his mind to it can match or even exceed the performance of your average ABS system. However, it's one thing to threshold brake in a braking exercise on a closed track -- it's quite another thing to be zooming through traffic, listening to the radio, having a good time, and keeping one eye on your friend in the chase car, then be surprised and have an "oh sh*t" moment. It would take great presence of mind to threshold brake perfectly in the split second that would separate a close call from disaster -- I imagine even most skilled drivers would struggle to do it. On the other hand, almost anyone can stand on the brakes and let the ABS sort it out.

In summary: yay ABS.

Not Sure

Dude, just say "ABS > locked wheels".