Tire Pressure for Autocross - 128i

[J312]

Anyone find anything that seems to work well for them?

[Artmasterx]

I am curious as well and don't have any experience. I was thinking something like 36psi front, 34 psi rear... but I don't know whats best.

[JB135i]

I would START at about 6-8 psi over "normal". Here's why I say start and not finish. There are WAY WAY too many variables to work with for anyone to conclusively say what's best, let's look at a list with some random thoughts-

Gross weight- Full tank and a passenger = more rolling for the same speed, 1/4 tank and a light driver = less tire rolling

Remaining tire tread- More grip will equal more rolling

Course design- More/sharper/faster corners = more rolling and more heat

Course surface quality- Grippy concrete = more rolling, freshly sealed asphalt = very little fun! Degraded/dirty/sandy course equals less grip

Air temp- 35 F on summer tires = LOWER pressure than stock, 100 F on summers = possible greasy tires or not, it depends

Driving skill (read speed)- newbie on a curvy course vs. a pro = will be very different pressures

Variations in gauges- if you say start at 37 PSI on the internet, will my 37 be the same as your 37, likely not

Moisture- less grip and less rolling = less pressure

Time between runs- Less time between runs = higher tire temps and higher pressures

Length of the course- More course usually means more heat and therefore more PSI

And probably a few more

First off don't be afraid to ask around, most events are loaded with people very willing to help you out!

Mark your tires with CHALK sticks or SHOE POLISH on the tread shoulders. I prefer chalk because it comes off easier and it's not a liquid that can spill in my autox box.

Then pick a PSI that's in the ball park (safe) and do your first run (practice) there.

Immediately after the run, look at your tires and see where and if the marks you have made have been removed by the run. Check your PSI right then to see where you are at.

Adjust from there, SLOWLY. If the marks have been worn away far down the shoulder of the tread, then add a couple pounds because your tire is rolling too much. If it hasn't worn away at all, the drop a couple PSI, because your tire isn't making full contact.

Always use the same gauge, all day and at different events. This is just like a DYNO reading, it's not the absolute number you need to measure, it's the difference in PSI you need to measure.

Eventually you will get a good feel for what works with all the vaiables. It may even be worth keeping a log.

Lastly, don't chase tire pressures all day. It's tempting to "blame" a bad run on tire pressures and adjust after every run. Things obviously change throughout the day, but as a rule, consistancy is key. Think "fine tune" rather than "big changes" when adjusting PSI.

Oh, one more thing, check EACH tire, you may be suprised at the variations you end up with depending, again on the variables.

Good luck, and go fast!

[TP 135i]

I don't Autocross or race in any manner, but I just have to comment on JB135i's post.
WOW...what a nice and thorough response. Well said. That's great info for the OP to get started with.

This is why I love this forum so much.

[vanbmw]

Nice reply JB135. If J312 can't figure that out...he never will! Good luck J312 with all the above ideas. Now go out to the AutoX and rip it up.

[J Tyler]

Originally Posted by JB135i

I would START at about 6-8 psi over "normal". Here's why I say start and not finish. There are WAY WAY too many variables to work with for anyone to conclusively say what's best, let's look at a list with some random thoughts-

Gross weight- Full tank and a passenger = more rolling for the same speed, 1/4 tank and a light driver = less tire rolling

Remaining tire tread- More grip will equal more rolling

Course design- More/sharper/faster corners = more rolling and more heat

Course surface quality- Grippy concrete = more rolling, freshly sealed asphalt = very little fun! Degraded/dirty/sandy course equals less grip

Air temp- 35 F on summer tires = LOWER pressure than stock, 100 F on summers = possible greasy tires or not, it depends

Driving skill (read speed)- newbie on a curvy course vs. a pro = will be very different pressures

Variations in gauges- if you say start at 37 PSI on the internet, will my 37 be the same as your 37, likely not

Moisture- less grip and less rolling = less pressure

Time between runs- Less time between runs = higher tire temps and higher pressures

Length of the course- More course usually means more heat and therefore more PSI

And probably a few more

First off don't be afraid to ask around, most events are loaded with people very willing to help you out!

Mark your tires with CHALK sticks or SHOE POLISH on the tread shoulders. I prefer chalk because it comes off easier and it's not a liquid that can spill in my autox box.

Then pick a PSI that's in the ball park (safe) and do your first run (practice) there.

Immediately after the run, look at your tires and see where and if the marks you have made have been removed by the run. Check your PSI right then to see where you are at.

Adjust from there, SLOWLY. If the marks have been worn away far down the shoulder of the tread, then add a couple pounds because your tire is rolling too much. If it hasn't worn away at all, the drop a couple PSI, because your tire isn't making full contact.

Always use the same gauge, all day and at different events. This is just like a DYNO reading, it's not the absolute number you need to measure, it's the difference in PSI you need to measure.

Eventually you will get a good feel for what works with all the vaiables. It may even be worth keeping a log.

Lastly, don't chase tire pressures all day. It's tempting to "blame" a bad run on tire pressures and adjust after every run. Things obviously change throughout the day, but as a rule, consistancy is key. Think "fine tune" rather than "big changes" when adjusting PSI.

Oh, one more thing, check EACH tire, you may be suprised at the variations you end up with depending, again on the variables.

Good luck, and go fast!

I didn't follow any of that ^, and I know a thing or two about tires. So I doubt the OP will make sense of all that...

So, I'm just going to offer my $.02 here:

First of all, what you should be concerned with is the hot pressure of the tires, not the cold pressure. This means taking note of the pressures as soon as the car comes in off the track, and making your adjustments then.

Second, different tire compounds will have different ideal "hot" pressures & temperatures. For example, racing slicks have a higher ideal "hot" temp than regular street tires. So it really depends on what particular tire you are running. Also, if you let the tire get hotter than it's ideal hot temp, it will lose grip (the rubber compound literally begins to melt).

Third, in general: Less Pressure = More Heat In Tire. More Pressure = Less Heat In Tire. This is because heat in a tire is generated when the tire carcass is deformed as it rolls over the "contact patch" area (the part that touches the ground). The weight of the car (and cornering/braking forces) deform the carcass as it passes underneath the wheel and touches the ground. These forces are huge, and although the tire itself is really stiff, it still gets deformed quite a bit. This deformation (twisting, squishing, etc) of the tire generates heat. If you think of the tire as a balloon (which it basically is), imagine what will happen if you add more pressure: it will expand, and it will deform less when a given force is applied to it. And, since deformation = heat, you can see that more pressure = less heat.

Fourth, in general: Less Pressure = More Grip. Less pressure makes the tire softer, more compliant, and better able to really conform to the tiny irregularities in the asphalt.


So, as you can see, setting the appropriate tire pressure becomes a compromise between heat and grip. You want to drop the pressure to increase grip, but that will also generate more heat in the tire. What I do is, start with what I think is a 'high' pressure (usually in the high 30's), and see how long it takes for the tire to get too hot and lose grip. If it takes a really long time, then I know the pressure is too high. If the tire gets hot and loses grip too soon, like after one lap, then I know the pressure is too low.

For Autocross, your run is what, 30 seconds long? - And you have a long cool-down period between runs for the tires to cool off. So autocrossers can get away with a much lower pressure than what you'd run on a real racetrack.


The most important thing for you is to understand the basics of how a tire works, and why it does what it does. That is what I tried to give you here. If you understand that, then you'll know how to adjust them for different tracks/weather/cars/tires/etc to really get the most out of your car


Hope that helps. If you have any more questions or need me to clarify something, just let me know.

[J312]

Thanks for all the info everyone! I agree, this is an excellent forum! I can't wait to try some of those variables out and see what works best! Thanks again!

[JB135i]

Originally Posted by J Tyler

First of all, what you should be concerned with is the hot pressure of the tires, not the cold pressure. This means taking note of the pressures as soon as the car comes in off the track, and making your adjustments then.

That's why I said- "Immediately after the run, look at your tires and see where and if the marks you have made have been removed by the run. Check your PSI right then to see where you are at."

Second, different tire compounds will have different ideal "hot" pressures & temperatures. For example, racing slicks have a higher ideal "hot" temp than regular street tires. So it really depends on what particular tire you are running. Also, if you let the tire get hotter than it's ideal hot temp, it will lose grip (the rubber compound literally begins to melt).

That's why I said "Air temp- 35 F on summer tires = LOWER pressure than stock, 100 F on summers = possible greasy tires or not, it depends." Maybe not as complete of a thought as yours, but I didn't have time to write a book about it.

Third, in general: Less Pressure = More Heat In Tire. More Pressure = Less Heat In Tire. This is because heat in a tire is generated when the tire carcass is deformed as it rolls over the "contact patch" area (the part that touches the ground). The weight of the car (and cornering/braking forces) deform the carcass as it passes underneath the wheel and touches the ground. These forces are huge, and although the tire itself is really stiff, it still gets deformed quite a bit. This deformation (twisting, squishing, etc) of the tire generates heat. If you think of the tire as a balloon (which it basically is), imagine what will happen if you add more pressure: it will expand, and it will deform less when a given force is applied to it. And, since deformation = heat, you can see that more pressure = less heat.

Fourth, in general: Less Pressure = More Grip. Less pressure makes the tire softer, more compliant, and better able to really conform to the tiny irregularities in the asphalt.

That's why I said- "Adjust from there, SLOWLY. If the marks have been worn away far down the shoulder of the tread, then add a couple pounds because your tire is rolling too much. If it hasn't worn away at all, the drop a couple PSI, because your tire isn't making full contact."

So, as you can see, setting the appropriate tire pressure becomes a compromise between heat and grip. You want to drop the pressure to increase grip, but that will also generate more heat in the tire. What I do is, start with what I think is a 'high' pressure (usually in the high 30's), and see how long it takes for the tire to get too hot and lose grip. If it takes a really long time, then I know the pressure is too high. If the tire gets hot and loses grip too soon, like after one lap, then I know the pressure is too low.

For Autocross, your run is what, 30 seconds long? - And you have a long cool-down period between runs for the tires to cool off. So autocrossers can get away with a much lower pressure than what you'd run on a real racetrack.

"what you'd run on a real racetrack" condescend much?, but anyway, I would call the average run 50-70 seconds and the cooldown period can of course vary depending on the number of entrants and other factors.

The most important thing for you is to understand the basics of how a tire works, and why it does what it does. That is what I tried to give you here. If you understand that, then you'll know how to adjust them for different tracks/weather/cars/tires/etc to really get the most out of your car

Hope that helps. If you have any more questions or need me to clarify something, just let me know.

Interesting......

[J Tyler]

OP, one thing to keep in mind since you seem new to this stuff: Autocross teaches bad habits. It teaches you to be jerky with the steering, to 'stab' at the pedals, to focus intently on a single spot in front of you (a cone); and it teaches you virtually nothing about gearshifting, trailbraking, the racing line, etc. I've instructed many autocrossers, and every single one of them had these bad habits formed. When they get on a real track, they immediately go into "cone-dodging" mode and try to be as aggressive as possible. This is not how to make a car go fast around a real track, and it's very hard for me as an instructor to get them out of that mental state. It's honestly easier for me to start with a 'clean slate' than most autocrossers.

If you really want to learn about performance driving, i.e. car control, weight transfer, trailbraking, the racing line, proper up- and down-shifting, etc etc, you want to go to a real racetrack. It will make you a better/safer street driver, too.

I'd also suggest picking up a copy of the Skip Barber book Going Faster. It's geared towards beginners and will point you in the right direction with all this stuff.

Have fun out there

[mechanicalmagic]

Thanks JT, I just bought the book on Amazon, $15 + shipping.

Do you use an IR thermometer, or hand to measure tire temps?

Dave J.

[AlienDNA]

J.T., I think it would be more fair to say that “Autocrossing badly teaches bad habits,” since most of the things you mention are almost equally applicable to slalom, and ArtmasterX and J312 should be aware of how they affect autocross perfomance. It sounds like the stab-n-steer crowd you’ve dealt with are mostly local club-crossers cursed with tight, technical courses where they can get away with horsing the car from corner to corner.

Smoothness is a benefit on flowing courses, and a good, flowing course will feel almost like a 5/8ths road course. Jerking the wheel or stabbing the pedals, especially the brakes, is just as likely to upset the car on a Solo II course as it is on a road course. The consequences are less physically catastrophic (lower speeds, not likely to hit anything solid), but more likely to make you slower (less room for error, each cone you hit while wagging your tail or plowing a wall adds two seconds). Car selection can mask some of this if you’ve got torque to burn, but all else being equal, a smooth driver is going to be more successful more often. Locked wheels and tire smoke under braking or acceleration is bad for times on either a road course or a slalom.

The racing line is also important in autocross, possibly even more so. On a road course, you have the pavement definition and usually kerbing and some sort of braking markers, and possibly even cones marking turn-in, apex and track-out. On a an autocross course, you have to create all that mentally in your mind, and you don’t get any practice.

For AtmasterX and J312, this means that one of your most important tasks is using the walkthroughs to decide which cones determine the racing line, which ones are there for racking up penalties and which ones are meaningless. Yes, crafty course designers have been known to put in extraneous cones just to sucker the inexperienced driver off the line and into a mess of trouble.

Take as many course walks as you can get, some with experienced people, some by yourself. Don’t just walk the course: pause, look at what is coming up and where you’ve come from. Mentally picture where you have to place the car and how you’ll be using the steering, throttle and brakes. If the club offers a rookie walk, go along.

Focusing intently on a single cone, even the next gate, is a recipe for disaster, or at least slow times. If you focus on the next cone or gate you will a) run over it b) be horribly surprised at how behind you are in your steering and braking when you get to the next gate or c) all of the above. Just like on a road course or the street, you should be keeping your eyes up and looking at where you will be going, not where you are going right now. And because the elements of the course will be coming up much more frequently than on a road course, you will never have time to look at your speedo or tach, so learn when to shift by sound and feel, and not when the rev limiter tells you to.

Trailbraking is a valuable skill to have on an autocross course, too. It will help you be smoother and carry more speed through corners. But J.T. is completely right about shifting - you’re not going to get much valuable practice on a Solo II course. You generally won’t have time to properly rev match with heel & toe, and you certainly won’t be able to use an autocross as a good place to practice the skill. In many cases, you’ll be driving virtually the entire course in one gear. Find a good instructor and spend some time on a road course to master proper heel-and-toe.

I echo J.T. and highly recommend Carl Lopez’s “Going Faster!”. It distils a lot of experience from Skip Barber instructors, former students and professional racers. While written with road racing in mind, the lessons you learn will be invaluable for autocross, too. Pay attention to the theory and think about what it means for car control. I bought mine years ago, along with the VHS tape. I’m not sure if the video was ever made available on DVD, but its well worth a watch if you can find it.

As for the original question about tire pressures, you’ve already been given a lot of good advice here. I’d stress the importance of chalking your tires and talking to your fellow competitors to find out what’s working on that particular day. You’ll be surprised at how eager to help most autocrossers are. After all, we tend to be a competitive lot and there’s nothing more satisfying than beating someone in their own car; helping them make their car as competitive as possible and then beating them is just a step down from that.

Finally, every once in a while, you’re going to run into a small, tight, technical course that will offer many, many chances to make mistakes and frustrate yourself. You can look at those as opportunities to learn patience and smoothness - or you can just have fun and use all the bad habits that J.T. mentions. You likely won’t be the fastest, but it will be a good workout and you’ll probably put on a good show for the fans.

Most important, have fun!

[lib]

Originally Posted by mechanicalmagic

Do you use an IR thermometer, or hand to measure tire temps?

I once stuck a hand on my tires after 2 or 3 autocross runs on a 100+ F day. I'm not sure how hot "Oh !#$%" is, but that's how hot they were.
Use an IR thermometer.

If you don't have one you could probably borrow one from someone else on the grid as long as it isn't too busy.

[J Tyler]

Originally Posted by mechanicalmagic

Thanks JT, I just bought the book on Amazon, $15 + shipping.

Do you use an IR thermometer, or hand to measure tire temps?

Dave J.

Use a pyrometer. You don't want to use an Infrared thermometer because that only gives you the rubber compound surface temp, which cools off and levels out as soon as you leave the track (heck, as you leave a corner and go down a straightaway).

A pyrometer sticks about .5-1cm into the tire compound, giving you the internal temp; and this temp stays pretty close to what it was when you were on track. It takes alot longer for the internal temps to cool off / level out.

If you are looking to buy a pyrometer, you want to get one that has a "predictive" algorithm in it; that is, one that will calculate the exact tire temp immediately after you stick it into the tire. You don't want one that requires you to wait for it to stabilize to the tire temp... you want one that will calculate the exact tire temp using a programmed-in formula quickly. This allows you to quickly take tire temps across all 4 tires before they cool too much.


-$.02

[mechanicalmagic]

JT,
Thanks for the info.

Since I'm a noob, I'll probably try my IR to get a hint. (I haven't been on a track/autox for a few decades). If I get semi-serious, then I'll make up a thermocouple probe, got stuff in the bin (somewhere). After reading your post, my first Google hit was:
http://www.coleparmer.com/techinfo/t...luke-track.htm
They have a helpful table for us noobs, about halfway down.

So much to learn. But then at my age, re-learning is a daily fact of life.

Dave J.

[J Tyler]

Originally Posted by mechanicalmagic

JT,
Thanks for the info.

Since I'm a noob, I'll probably try my IR to get a hint. (I haven't been on a track/autox for a few decades). If I get semi-serious, then I'll make up a thermocouple probe, got stuff in the bin (somewhere). After reading your post, my first Google hit was:
http://www.coleparmer.com/techinfo/t...luke-track.htm
They have a helpful table for us noobs, about halfway down.

So much to learn. But then at my age, re-learning is a daily fact of life.

Dave J.

Haha well, like AlienDNA said, the biggest thing is just to get out there and have fun None of us are going to win a World Championship.

I honestly don't use a pyrometer very much. I prefer to look at the tire: note how it's wearing, how the rubber balls/grains when it gets hot, etc to determine camber. If it's a powerful car, like our 135i, then you have to keep in mind the importance of putting power down out of slow corners, and that's something you go to the data logger for (or the seat of your pants if you don't have a data logger!).

[AlienDNA]

Oh, one other thing about tire pressures: be sure to re-check them before each run. You'd be surprised at some of the comical left-right differentials you can get just by parking with one side of your car sitting in the sun for an hour or so.

[BerkTechnology]

Originally Posted by AlienDNA

Oh, one other thing about tire pressures: be sure to re-check them before each run. You'd be surprised at some of the comical left-right differentials you can get just by parking with one side of your car sitting in the sun for an hour or so.

They will equalize when you go back on track and heat up the cool side

[J312]

Cool, I can't wait to try these out. Thanks!