aryork

Some time back did some research after spinning at VIR on a wet track and going into the grass (and ultimately armco). Turns out the range of friction coefficients for wet grass overlaps that of the "stickiest" ice! That's why it literally felt like my speed increased as the car transitioned from asphalt to grass while I was sliding.

Here's a plot of theoretical stopping distance (assumed skidding) of a 3000 lb car on various surfaces. I validated the calcs with published data at lower speeds. Given a real (non-smooth) surface, air resistance, and miscellaneous energy absorption mechanisms (like rotation, suspension, etc.) it is fair to say these distances are an upper bound. Realistic distances at higher speeds may be 10-20% less (just a guess) than these here. But, they are still large - larger than I would have guessed. Go to Google maps and bring up your favorite track and do a few measurements of the distance from the end of the straight to where where the car will be stopped by armco, tires, trees, etc. in the event you have an uncontrolled off. Yikes!

The velocity-squared effect is huge.

Matt Romanowski

There area lot of influences on the coefficient of friction of an asphalt surface. My learning has indicated a dry road is 1-1.3, with a damp road being down to .1-..2 (severely polished).

I would say your distances are reasonable based on my experience.

TXE36

A few years ago I had an off on the outside of T7 going CW at TWS. A friend was ahead of me and got sideways at the exit of T7. I drove off track left to avoid him at about 100MPH. The track was dry, but the grass was soaking wet. Managed to not spin but the car just didn't seem to slow down. I even looked down to make sure my foot was on the brake. I went down the entire length of the back stretch in the grass and didn't get back on until T6.

-Mike

PorscheAddict

Both informative and scary, thanks. It always looks pretty far away from the cockpit (Road Atlanta not included).

aryork

Thanks. So, I think you are saying, if we think about track/race tires the compounds can achieve higher coefficients of friction - good point. I added a higher friction case (1.0)...

mark kibort

Just to add some complexity and accuracy/clarity to the discussion... Are you talking lock up comparisons? %slip ?

aryork

Yes, it is assumed to be rubber sliding on the surface - skidding. No rolling or partial rolling.

That brings up another thought...what is ABS doing during a spin/slide while you are trying to press the brake pedal through the floor (when the direction of skid/slide is trying to turn the tires in the normal direction)? During my spin the Porsche Track Precision App decided not to save the partial lap, so I got no data.

mmuller

Maybe I don't get what's being discussed here because I am not that smart overall, but I am confused. You didn't understand that wet grass is more slippery than wet pavement?

aryork

I think the "so what" idea is the magnitude of distances involved. Like TXE36 talks about taking a huge distance to stop. I would not have guessed that it could take around 1000 ft (> 3 football fields) to stop on dry grass when going 100 mph. I don't know of many tracks with 1000 ft of runoff room - maybe some of those out west might have that much in some areas.

Some of the other takeaways:

• What looks like a lot of runoff might not be; IOW a lot of innocent looking turns that seem to have decent runoff may not be innocent
• If you only have, say, 200-300 feet of runoff until armco or tires, expect to take a pretty hard hit unless you are going highway speeds (say, <= 80 mph)
• Hope (or try) to stay on asphalt as long as possible because you slow down *much* more (of course traffic is a consideration)
• Makes sense how critical it is not to hang a couple tires off the course on dirt or grass when turning (duh!) - too easy to lose control

mark kibort

well, this creates all sorts of variables that skew the conclusions. for example... a racing slick can be locked up and you can skid further down the track than with a locked up street tire. also, there are differences that favor the street tire.. like , on grass or wet pavement.
when you look at the mu potentials of all tires and conditions, you start taking things like slip % into account.
ABS can hurt or help a spin depending on when you might not want the tire spinning in a spin, or visa versa.

zedcat

Interesting plot. Just a comment- in my experience the "delta mu" situation (2 wheels off) is survivable and generally better than pinching the exit trying to stay on track.

2BWise

Well, if the wheel is not spinning then it doesn't really matter what ABS is doing. In the case that the car is sideways and sliding across the track the wheels are likely to no longer be rotating. Whether it gives you 100% requested pressure on 0 pressure a sliding tire (no rotation) produces the same amount of grip. The interesting part is what happens if the tire has a chance to recover and start spinning again as in the case of a 360 degree spin when the tires start pointing in the direction of travel. The response is then very dependent on the surface mu to how ABS then responds; asphalt, grass, dirt/sand.

cosm3os

Shhh. Just let the engineers play.

ace37

chancecasey

Classic Rennlist. Detailed professional charts and discussions that reduce to....
Grass = Bad

Glad you weren't hurt!