Dalema

Hi all. I'm super excited to be doing my first autocross this weekend - looking forward to the instruction part of it / learning to control the car before moving onto the track at some later date.

I had dialed in my tire pressure to where it was comfortable for daily driving, but just got the car back from its first service and the the tire pressure was set to F42 / R49 - wow! When cold the next day they were at F36 / R44. Apparently in CA they are legally required to fill to the car placard. If I had known I would have noted where I had them set to before. But I digress.

I will be driving up to the event. What would be a reasonable to set as a cold tire pressure? Running stock 20s with PZeros on a C2.

John's 991

31/34 is a good cold starting pressure. It is unlikely they will increase too much given the relatively short runs at an autocross. Have a great time!

chuck911

Cold doesn't matter. Cold pressures will be all over the place. What matters is the pressure when driving. Regardless of what you had cold, if its hot and you're driving hard your pressures will rise, usually requiring you to let some out in order to avoid losing traction as the tires get too hard. I don't know what you want exactly, but 42/49 is too high.

The approach I find works well with novices the first time out is pick some pressure- 36/40 say, or 32/36, something around there- and adjust just before each run. All day long you'll be hit with new things, trying to get a handle on the cars handling. To the extent your pressures vary throughout the day this will tend to mask some of what you'll be trying to accomplish with throttle, steering and brake. So remove this variable by holding it constant all day. Then if you want to try something different later (and you will) you will have a solid baseline of experience for comparison.

Dalema

Thanks - yes the 42/49 is way too high - no idea what they were thinking! Not having done autocross before, I'm unsure as to how much the tire pressure will increase from cold (at around 65oF outside) in the morning - so was trying to back into what may be a reasonable starting pressure in the morning when I leave the house.

Chuck - the tire pressure you note to try for is when running the course - correct?

Really looking forward to it!

chuck911

Yes, when running the course.
John's991 is correct in saying it won't change much during a short autocross run. But this begs the question, how much off are you willing to be? Even more to the point, how much off are you willing to be without knowing?

This is the problem with relying on cold pressure. You simply have no idea where you'll be when you're driving.

What typically happens is you drive out there with whatever you have. Then you check just before your first run and bleed out as needed to get to where you want to be. Lets just say 32/36. Right after the run lets say you show 34/38. Do you let air out? Not unless your next run is right away. More likely its half an hour away, by which time the tires cool a bit and you're starting your next run with 31/35 instead of the 32/36 you want. So wait and check just before the run.

Does it really matter? Well if you leave it alone then usually what happens is everything continues getting hotter as the day goes along, until you wind up driving 42/49 and the car is skating around. So yes it matters.

What matters even more though is that at this point you are entering a whole new realm of driving. Most people have never applied their brakes full power so much as even once. In autocross you will be doing this 10, 15, 20 or more times per run. Same for cornering. The question then is no longer how hard can I corner or brake, but how hard will my tires let me corner or brake? How much traction do I have?

What your instructors will (or, should) be helping you learn is the way your throttle, steering and braking inputs alter weight balance and hence traction. This is a whole lot easier to learn when the car is handling consistently from run to run, something that can only be accomplished when starting each run with the same tire pressures.

lunarx

Go -1 from Comfort Part Load pressure.
You wont get the low pressure warning and your hot temps should end up fine.

drcollie

First time autocross? You'll have a lot of fun! Clear out all the loose stuff from the car interior and remove the floor mats. Tire pressure isn't going to matter all that much for you - not at this stage of the game, just take some of the recommendations here, set it and forget it. Don't fuss with it during the day.

What i would recommend you do is three things to have a successful first day.

1. Walk the Course.
2. Walk the Course looking backwards through the turn you just came through
3. Walk it one more time to memorize it.

Most first timers get lost on the course unless your event organizer is putting out so many cones that you can't get outside them. You'll actually get lost probably the first dozen times! Autocross is different from track days, its more like a game of Chess than flat out driving. You have to know what you are going to do on each segment before you even get in the car to turn on the ignition. It's very much mental strategy and you have to decide where your turn-in points, braking spots and exits are going to be before you actually get there.

When you get there in the morning, you will see a lot of people walking the course, shooting the breeze, laughing and carrying on. They don't even know WHY they are walking it, just that they were told to do so. You will see their results later, mid-pack or last. When I go Auto-X, even though I have lots of friends at some of them, I always walk the course alone and try to do so out of the pack of walkers. I'm setting up my turns in my mind and linking the course together. Exit speed is everything - you do each corner to maximize exit speed to the next one. Watch the technique of the fast guys when they run, they get the car slowed to each segment- turn in and rocket out. That's the trick and its harder to do than it might sound. Tire pressures really are not all that important until you're hunting for tenths of a second. You can have the best set-up car at the event and an old time Auto-Xer's like me can jump in your car and take three seconds off your time, just because I know how to run the course and you haven't learned how to do it yet, and that includes not over-driving the tires.

Technique is everything in Auto-cross - and not getting lost. lol

chuck911

Good one. Reminds me of early days. Somehow managed to do quite well the first time. Second event went out and at some point looked up at a sea of cones. Blew by a whole slalom. People were like what happened? Figured I better get an instructor. Greg Fordahl gets in. Did not know he was 5 time SCCA national champion. Just as we are about to start he asks, "what's the first turn?" Huh? Somehow in spite of the course maps everyone gets, walking the course several times, it still had not even occurred to me to memorize the course!

There's a really good thread on the autocross forum with posts by myself and others seconding the remarks about learning the course when you walk it. Autocross looks easy, but it is a MUCH bigger challenge than the track. Imagine if people went to the track and were allowed only 4 laps to turn in their best time. Then when they come back next month they find the tracks been torn up and now its a whole new track to learn. Oh, and there's five times as many turns. That's autocross.

John's 991

Hi Chuck, I understand you comment about measuring hot tire pressures throughout an event. However, for somebody's first few events, I think it is better that they don't pay much attention to tire pressures unless they are way off. It is really unlikely that a change of a couple psi is going to make any difference for quite a while.

By my experience with those tires, they will go up a 2-3 psi between the driveway and arriving at the autocross course, so will probably be 32 or 33 front and 35-36 rear at the start. The PO's are good until 42-44 (higher than the MSPSS which don't like >40). It would be a pretty unusual autocross that would increase tire pressures more than 3-4 psi. Even if one tire was 3-4 psi higher than another at the end of the run, it would not matter for the next run.

I think Dr. Collie's advice is great. Learning the course and the line and working on technique is what is important. Chasing tire pressures is not important at all for awhile, assuming they are in an acceptable range.

Hammer911

At my first ever track day, I did my first session without adjusting my tires but I did check my starting pressures. I figured the first session wouldn't be running as hard and loss of some grip due to higher pressures late in the session wouldn't be much of an issue. Measured again immediately after the session, my pressures were 8 pounds higher in front, 10 pounds higher in back. I bled off slightly less than that amount from each tire, and found they remained consistent the remainder of the day.

Chuck, John, Dr. - was that an appropriate process?

Dalema

Thanks guys for all the advice. Really looking forward to it and hopefully not being overwhelmed enough to learn something

rpilot

Why Porsche does not have PATPM (Porsche automatic tire pressure mangement), I will never know

If they can figure out variable springs (air susp), sway bars (PDCC), dampers (PASM) , someone can figure out a way to put a air compressor/bleeder built onto the wheel to adjust pressure to a target and keep it there. In typical Porsche fashion, modes like comfort & sport will come standard with PATPM, but you will have to upgrade to Sports Chrono to get the Autocross mode.

John's 991

Hi Hammer, I would agree that it is more important to manage hot tire pressures during a 20-25 minute track session than a 2-3 minute autocross. Checking them before the first session then again after, and making an adjustment makes sense.

My general comment is that I have seen driver's who are starting out focusing on tire pressures to their detriment, whereas there are more important things to focus, such as learning the line, smoothness, proper vision techniques and learning the track. As long as the tires are in an acceptable range when hot (40-42 max is probably a good general number for most street tires) I would leave them alone.

As you get more experience and refining your driving, then going back to tire pressures is more appropriate. It is similar to the progression of focusing on the line then exit speed before you worry about braking at the last second and mid corner speed. Eventually you want it all, but there are bigger gains to be had by focusing on the most important things first and not being overly distracted by the rest.

Dalema, have a great time, and if your car is ready before hand (all the lose stuff removed, tire pressures set, etc.) then you will be more free to focus on learning the course and practicing good technique. In between sessions try closing your eyes and driving the course 2-3 times. Check the map if you are uncertain in one area. The more you can do before you are actually driving, the more you can get out of the actual driving.

chuck911

The thing about tire pressures, there's so much variation due to weather (ambient and track temps), course and driving styles, its impossible for any one recipe to be right for everyone in every situation. The situation only gets more complicated when you consider one person may be critically overloaded just trying to learn a few basics like John's991 mentions, while at the other end another may be soaking everything in like a sponge that's always dry.

That said, I do think this is something everyone should be getting into the habit of checking consistently and often from day 1. But for the same reasons I was just saying no one recipe will always work, I think its essential that everyone learn not just the what but the why. The only way I know of doing this is keep explaining the basics of tire pressures so the info is out there and people stand the best chance of understanding how to use this for themselves. As someone a lot wiser than me said, I can tell you something and you can remember it but it means nothing until you realize it for yourself. In other words when it becomes real in your own mind then it takes on a power much greater than anything anyone can say.

All tires work the same with regard to pressure. Within a range of a few pounds they give max traction. Towards the low end of the range they continue to stick but are softer so yield a smoother ride but also are a bit less responsive. Towards the high end of the range they continue to stick but are harder so yield a firmer ride and are more responsive. Within this range we can tune our tires to give the feel and balance we want. Outside this range things get predictably worse. Going lower the tire will wallow around with lousy response and increased risk of rim damage. Going higher the tire will skate around with super fast response but much less traction.

Now granted, someone starting out isn't going have a very good idea just where that "max traction" range is at. Even finding a successful and highly skilled driver with the exact same car/tire setup and copying that cannot be guaranteed to work. Its a much better starting point than the door jamb numbers, but the odds are still high that you will eventually over time develop your own style and preference that has you settling on different values. So here we come back to it, there's no escape, its just something each driver has to learn for himself.

So how best to learn? I can only say what worked best for me, and has worked best for all the good drivers I personally know, and that is to start checking early and often and keep at it consistently.

Yes its true your pressure won't change much in one autocross run. But it could easily rise 2 psi. So lets go with that. These things usually start early and run into the afternoon. So both tires and track are heating up throughout the day. Four runs, 8 psi, and we haven't got to fun runs nor are we talking about a hot sunny day that could raise tire pressures another 5 psi just from that alone.

Now it just so happens I had a day exactly like that back during the summer of my first year of autocross. Just as I was finally getting the course figured out and getting a handle on my traction limits suddenly my car is skating around all over the place and my last two fun run times went a couple seconds the wrong way. All I can do is leave it to each person to decide for themselves- do you want to be the newbie skating around clueless as to why? Or can you find time to check and bleed and learn? I sure know which one I would have chosen back then- if only someone had been offering this information to me!

Dalema

Just wanted to say thanks for all the advice. I had a blast yesterday!