Chalk Marks

[ausgezeichnet]

Ran my 2nd Autocross today. Did much better. Here is a picture of the chalk mark on one of my front tires after my fastest run. I had just lowered the tire pressure 2lbs down to to 31lbs. Should I go lower?

[HarryD]

Mike,

you can if you wish. I typically do 5 lb increments one way and 2 lb the other to fine tune.

[euro911sc]

I would stop there given that mark. YMMV. Was the tire warm when you ran that run? How many runs previously? I like to go for a 20 minute drive prior to my runs so that my tires are fairly warm when I start. That way I do not have so much variance in air pressures across the day and event to event. Though... I have not autocrossed in about a decade now...

[ausgezeichnet]

This was my 5th run in a row but there was about 15 minutes between runs.

The tire pressure only increased by about 2 pounds from cold. The hot tire pressure stayed about the same after the first run.

The reason I am asking, according to the chalk marks, I would have to go below the recommended 29lbs front and 36lbs rear cold tire pressures to get the chalk mark to wear down to the top of the triangle on the sidewall. I am running the new Bridgestone Potenza RE-11a's and they have very stiff side walls so maybe that is OK.

[msterling]

You might go lower next time and see if you go faster. That tire is not rolling over too much. RE11a is a new tire and suggested pressures may not be well known.

Another thing to consider is how the car is handling. Is it understeering too much? Is it tail-happy? Tire pressure may the only way to adjust these handling issues at an event. Not everyone has adjustable swaybars etc.

[Cajundaddy]

Not using all of the tread yet so there is a little more grip available at lower pressures.

[ausgezeichnet]

When I dropped 2 PSI for my 5th run. I did pick up 7/10th of a second and that was with no understeer.

Next time I am going to experiment with lower PSI to get closer to the tip of the triangle. I agree that it looks like there is tread to support a lower PSI.

Perhaps I also need to be more aggressive in the corners

[winders]

Originally Posted by Cajundaddy

Not using all of the tread yet so there is a little more grip available at lower pressures.

There is more to tire performance than how much tread you use as shown by chalk. The chalk method is way overrated and probably an anachronism today.

Tires have pressures and temperature ranges they run best at. Find out what those are from the guys that are fast and running the same tire on similar vehicles.

Scott

[Cajundaddy]

Originally Posted by winders

There is more to tire performance than how much tread you use as shown by chalk. The chalk method is way overrated and probably an anachronism today.

Tires have pressures and temperature ranges they run best at. Find out what those are from the guys that are fast and running the same tire on similar vehicles.

Scott

The chalk method is simply one tool in the toolbox but a good place to start for a new driver after doing only 2 AX. Those of us doing competitive driving for 8-10 years consider chalk, tire mfg recommendations based on vehicle weight, top driver preferences, toe and camber, tire temp zones, tire wear characteristics, lap time data, skidpad data, and personal driving styles.

After all that, the chalked tread marker still yields pretty useful information. YMMV

[Jeremy Cottrell]

See if you can borrow a pyrometer. There's usually one floating around in the pit area.

[winders]

Depending on tire construction, setting pressure based on what the chalk tells you may have the tire at far to low of a pressure.

Let's go back in time when BFG R1 tires wanted 18 psi and Hoosier autocross tires wanted 44 psi (mid '90's). Neither tire could be setup using the chalk method. The Hoosier would have been filled with too little air and the BFG with too much. Consider cars with improper camber settings and the situation gets worse.

I haven't looked at autocross tires lately, but I bet the really fast guys don't use chalk. I know we don't for road racing. That's why I suggested finding someone fast using the same tires. They will know what pressures work for both speed and wear.

Scott

[DaddyGlenn]

As a novice myself, I find there is more speed to be found in my driving ability than any tweak I could make to the car. Maybe that is just me.

[HarryD]

The chalk gets you close. The timer tells the story. If you are finding better times as pressures fall, keep going until it stops being of benefit. I would go in 5 psi increments as 2 psi may be too small to provide a meaningful change.

[Dave at Pelican Parts]

You can also change the pressure on one end of the car and feel how the balance changes. If that end seems to stick better, chances are your change improved the grip. If that end doesn't stick as well, the change was probably in the wrong direction.

--DD

[ausgezeichnet]

Thanks Guys. Looks like I will just need to try a few different pressures to dial it in a little better. I probably will not play with it until I get a little more consistent with my times.

For those of you that are experienced autocrossers, how much do your times vary from the first to the last run?

In both the first two autocrosses I ran, I improved about 7 seconds going from about 54 seconds to about 47 seconds between the 1st and 6th run.

[msterling]

It is normal to run faster times with almost every run. As you get more proficient the first couple of runs will be much closer to your best run. There is always the run where you spin or collect a bunch of cones. If you never do that then you are not exploring the limits. Autocross lets you do that without damaging you car or yourself.

I just introduced my son to Autocross yesterday. He had a blast and he did spin the car in the slalom section. He came back beaming and saying that he smelled the tire smoke. He may be hooked already. He wanted to know when the list of run times would be emailed to him!

[Tobra]

I was surprised how much dropping the rear tire pressure helped with grip. The advertized balance is like 52:48, front to rear, but the recommended tire pressures are 29 F, 32 R. I have been using 30 on all four, and it seems to be working out. We shall see what happens auto crossing on the new tires, but they seem pretty sticky, especially compared to what I had. I am glad I read this thread, will stick a piece of sidewalk chalk in the trunk tomorrow morning. Oh, it is a miata.

I need to get a pyrometer, will the HF laser one work, or do I need one that has a probe that actually touches the tire?

[HarryD]

Originally Posted by Tobra

I was surprised how much dropping the rear tire pressure helped with grip. The advertized balance is like 52:48, front to rear, but the recommended tire pressures are 29 F, 32 R. I have been using 30 on all four, and it seems to be working out. We shall see what happens auto crossing on the new tires, but they seem pretty sticky, especially compared to what I had. I am glad I read this thread, will stick a piece of sidewalk chalk in the trunk tomorrow morning. Oh, it is a miata.

I need to get a pyrometer, will the HF laser one work, or do I need one that has a probe that actually touches the tire?

The laser one will give you a clue but the surface cools quickly so you got to be fast to get the readings. You are looking for an even temperature across the tire tread.

[winders]

Originally Posted by HarryD

You are looking for an even temperature across the tire tread.

This is not normally the case for a radial tire. Because of the static negative camber and toe settings you generally want to see the inner section of the tire 10 degrees (+/- 3 degrees) hotter than the center or outer sections of the tread.

Generally, if the center section of the tire is hotter than either the inner or outer section, you have too much pressure. If the center is cooler than both the inner and the outer sections, the tire has too little pressure.

Scott

[winders]

Originally Posted by Tobra

I need to get a pyrometer, will the HF laser one work, or do I need one that has a probe that actually touches the tire?

Instead of reinventing the wheel, see if you can find out what camber settings and tire pressures the fast running your tires and car are using. Start there then experiment with different pressures.

Don't bother with a pyrometer if you don't get one with a probe. Reading the surface temperature is a waste of time.

Scott