I know this has been talked about before. It seems like most advise on increasing the rear tire pressures more than the front but my results speak differently. Also most say for autocross start with 40psi. My experience (limited) is giving me the conclusion that a stiff sidewall tire and low profile of the 205/45/17 wheel can use a lower pressure giving more grip.

At my last two events I have been running lower pressures in the rear than the front and I have been running quicker times, and winning the PAX.

I have been running 40 front (cold) and 36.5 rear (cold). I kept them there and then actually started lowering them a little at the end of the day. My times at the end of the day got better with the lower tire pressures.

With lower pressure you will have a greater contact patch and more grip. The car may feel loser but with more grip. At the next event I am going to try even lower in the front and rear and see what happens.

The ZIIs have a stiff sidewall. I am thinking 36psi front and 32psi rear. I will chalk the tires to make sure there is not too much rollover. Some discussion on an S2k forum has pressures down in the 32f 30r range, but thats a different type of car.

Suspension is JCW suspension with H-sport competition rear bar on medium.

Would love to hear thoughts before this Sundays event.

 

Every car is different, every tire is different.

Higher pressures can work with OEM runflats. I run lower pressures but how low can depend on-
Course surface- how smooth or worn is it?
Course conditions- is it clean or does it have loose gravel?
Climate conditions- is it hot or cooler?
Individual run conditions- are you in an early heat or a later one where rubber has been laid down on the course for you?
Do you have a co-driver?

I run higher pressures in front, I check tire temperatures and I chalk tires. I don't run high pressures. About 33-36 psi in front and 30-34 psi in back then adjust.

Each course is different, how you drive that day may be different. Start somewhere near the range you expect to work for you then adjust. You can even have the same pressure front and back if your course is not heavy on tight turns.

On a day with cooler weather you can run pressures that are a little higher, the tires will cool off easily so you want to race with tires in the warm pressure range. On a hotter day you can start with lower pressures and allow the tires to heat up to the warm range that suits you. Keep in mind that if your tires sit in the sun they will be warmer than in the shade (the other side of the car). A tire shade cover on the sunny side will help reduce that.

It is true that higher pressures allow for slightly easier handling while lower pressures can provide more grip.

 

What handling changes can I expect by dropping pressures down both front and back from the 41/36.5 settings to 34/30 settings?

Our surface is concrete and very rough in places. Gravel free as we clean the course that day very well.

 

also note that I am only running stock camber maxed out at -.9 left and right front and -1.7 left/ right in rear.

 

Your camber settings are OK.

see
http://www.tirerack.com/tires/tirete....jsp?techid=1&

When running tire pressures that are higher-
it can "provide an improvement in steering response and cornering stability up to a point. "

When running lower pressures-
Don't go too low when using a street tire, you don't have drag slicks.

This has a good general discussion on pressures-
http://farnorthracing.com/autocross_secrets12.html

Start with the fronts and see if you can find a pressure that allows a tire temperature that is as even as possible, it won't be perfect and will likely change for each course/event you drive. After you get that right then adjust the rear pressures to get the same even tire temperatures. It's very likely the front pressures will be higher than the rears and it is possible the tires facing the outside of the fastest turns will be hotter and run a higher pressure.

 

I tend to destroy sidewalls so I'd stay mid 30's.

I have ZII's on my Miata and usually run 35F 36R.

I'm a bit surprised lower rear than front works for you. In theory the stiffer sidewalls and less grip in rear would contribute to less understeer

 

See
http://www.tirerack.com/tires/tirete....jsp?techid=58

Front tire pressure higher- leads to decrease Understeer
Rear tire pressure lower- leads to decrease Understeer

 

wanted to bump this thread to see if anybody has any new suggestions.

Yesterday, I ran my (stock) MCS at 37 front 34 rear. Seemed to handle pretty neutral, if not a little "slippery". It was mostly the pavement I was on, I think.

 

i'll be playing with it a little - i did 34 all around at the first event, but i think i was getting some rollover - i'll probably give 36 all around a try for the next event

 

With 225/45-16 ZII tires I've been running 47/45.5 (cold) and they seem to be happy there.

This thread is getting a bit old now the ZII are being discontinued. I started a thread about that in the autocross forum: https://www.northamericanmotoring.co...continued.html

 

Wow! That's really high pressure. Almost at the max cold pressure on the sidewalls of my 205/50/16.

Do you think your pressure settings apply to all sizes? And how did you come up with those numbers?

Also, what wheels are you running? Why did you choose 225/45 over 205/50? Do you have any rubbing? Suspension mods?

 

I have to be honest I have no clue how people are getting away with <40psi on stock suspension. We used to run something like 42psi front and 46 rear. Fronts at the lowest pressure we could go before the tire started to roll over and set the rears to influence the "balance" of the car. And that was on the 215/40/17s which have a fairly small sidewall.

Even now with coilovers and -3.2 degrees of camber in the front the lowest we can go is about 38 before the tire starts rolling over bad and temps across the tire start to get all out of whack.

Lets remember here these cars have over 60% of the weight on the front tires. The last time we cornerbalanced the mini there was about 800lbs on each of the front tires. That's a lot of weight to keep supported. In contrast an NB miata would have something closer to 600lbs on each front wheel.

EDIT: All of the tires pressures I am talking about are set cold and pressure is bled off after each run so that the tires are "always" at the pressure I want them at.

 

Gotcha. I was under the impression that tire pressures should be lower in the rear to reduce understeer?

 

v10 climber is probably going to respond, but I'll chime in that - as far as setting tire pressures, IMO you want to get in the ballpark of where the tire isn't rolling over, good feedback/grip etc - and after that, you can start adjusting a little based on the balance of your car. If your car is a little loose, than you can decrease the rear tire pressure, and that should have some influence on the balance of the car.

 

Typically a lower rear tire pressure generates more grip and therefore a lower rear tire pressure would increase understeer. Eventually if you keep dropping the rear pressures you'll get to a point that the tire is rolling over so much that it's no longer generating as much grip as it could and so you'll start getting oversteer again. Although considering how little weight the rear of the mini has on it you'd have to drop the pressures pretty low I would think to get an appreciable amount of oversteer from having low rear pressures.

so an example on tire pressures (pressures are just for example purposes)...
F-40psi/R-47psi (oversteer)
F-40psi/R-40psi (neutral handling)
F-40psi/R-37psi (understeer)
F-40psi/R-34psi (understeer)
F-40psi/R-30psi (oversteer)

 

Thanks. I used the tire crayon method last season, but maybe I was still letting too much roll in. Do you use the arrow on the tire?

Thanks. I was looking at tire rack's guide and that's where I found the info on lower rear/higher front pressures:

http://www.tirerack.com/tires/tirete....jsp?techid=58

 

Its a high pressure, I like to keep under the max, but sometimes the tire seems to want it that high. I derrive a new pressure for each different tire model and each different size. That might make a good starting point for finding a good pressure for your tires. I'm using 16x7" wheels and I chose 225 because I subscribe to the theory that more tire is better. It'd be interesting to test 205 vs 225. No rubbing, I've got custom dampers and a stiffer rear bar.

The theory of finding the right pressures I subscribe to is getting good temperature profiles. The basic theory is that tires work best when evenly loaded. A tire is non linear, the more you load it, the worse it works, so you want to share the load as much as possible. That's the basic idea behind tuning oversteer/understeer, balancing the load among the tires.

Then you want to load the tire so all parts of the tire are doing equal work. The next theory is that the amount of work a tire does shows up as temperature. If you work a tire, or part of a tire harder, it heats up. So the idea you strive for is to get the tires to show a perfectly even temperature profile. Failing that, getting an even temperature gradient across the tire is second best.

If you plot these profiles on a graph you get a line, a straight line is what you're looking for. If the line is not straight, the pressure is wrong. If it bumps up in the middle, the tire is overinflated, the middle is doing more work that the edges. If it bumps up at the edges, the tire is under inflated, its riding on its sidewalls. If the line is straight you'd got the pressures about right. If the line is flat and straight your suspension is doing a good job. If the line is straight and at an angle, you need to change the camber, more negative brings the inside of the tire more into play, so if the line is straight but goes up towards the outside, you need more negative camber. Of course, Stock/Street MINIs can never get enough camber, so you have to settle for a straight line.

I tested this theory last year. I was using the pressures we used for the track with the autocross tires. At the track the tires heat up by about 100°F, so hot pressures are +10 psi or so over the cold. The pressures worked well on track, but turned out to be too low for autocross. I experimented putting in pressure and found I went faster on higher pressures. The pressure I ended up at actually gave the best tire temperature profile.

The method I use to find the pressures is to drive the car on the street, my back street commute to work is a good substitute for an autocross course. You want to work the tires, not just drive in a straight line like on a freeway. Then I take temperature profiles. Then I work out what the right pressure is on the street. On the street my ZIIs said they liked something like 50psi, which was worryingly high. Then I run those pressures at an event, and take some temperature profiles and adjust them accordingly. The ZIIs said they liked a slightly lower pressure actually at the event (47/45.5).

Below is a graph of the tire temperature profiles I took I think after my second run on Saturday at the national tour this weekend. I'm pretty pleased with the temperatures and I've been turing some of my best ever results at local events. (I screwed up at the tour, despite having perfect tire pressures.)

Below that is a profile of the tire I was finding the right pressure for last year. 35psi was the track pressure, 48psi was what they said they needed on the street. Below that is a bunch of profiles from an event (the sun was shining on the right side, hence the bump to the right outside.) And finally an IR thermal image of the tire when I got the pressure right.

Edit to add: The ZIIs show wear right down to the point of the wear bar triangle, which is another pointer, they about right.

 

awesome man, thanks!!
One quick question, are you keeping your tires at 47/45.5 the entire time, or letting them get to a higher pressure as they heat up?

 

For the tire pressure vs understeer/oversteer thing. The theory I subscribe to is to do with tire spring rates. As mentioned, the tires does better when they're evenly loaded. If a tire carries more of the load it doesn't grip as well, so will give up earlier than a lightly loaded tire.

The load on a particular tire out of your four tires is partly governed by the spring rates on that corner. So softer springs on a corner will unload a tire and make it grip more. You have to put the load somewhere, so all you can do is shift the load around. So if you soften up one end it'll grip more, if you tighten up one end it'll grip less. You've shifted the load, so you've shifted the grip.

The load on the tire is a governed by a combination of spring rates and bar rates. You change the bar to shift load, and thus grip around. In stock you can't change the springs, but the effective spring rate at a corner is a combination of the actual spring, and the tire spring rate. A higher pressure give you a higher tire spring rate and thus a higher effective spring rate for a corner.

So the temperature method I use is for finding the right tire pressure to get the maximum grip out of all the tires. If you change the pressure from the optimum pressure, either higher or lower, you get less grip. However changing the pressure also changes the spring rate, and I'm told the effect of the change in spring rate vastly outweighs the effect of changing the grip. So running a higher rear pressure increases the spring rate at the rear and shifts some grip to the front. It also makes the tire grip less overall, which increases the effect. More front grip, less rear grip is reduced understeer, or increased oversteer. Conversely lowering the rear pressures reduced the spring rates, shifts grip to the rear, but also lowers the rear grip slightly (probably a smaller effect that the spring rate effect). That would promote understeer.

 

I set them cold and forget them, so they heat up to the optimum running pressure.

I did find the pressures for these tires running them on track. I found that on track they liked about 49/46psi hot. I had no idea what the right cold pressure for autocross was, so I set them to 49/46psi cold, then bled air out of the tire to keep them at 49/46 hot as I ran. Then the next morning I measured them cold and used that as my cold pressure. After a run or two they're up to temperature and working well.

Constantly bleeding air out of a tire sounds like too much work to me.

 

Interesting that the rear tires only lost 0.5 psi after they cooled....are you running nitrogen-only?

And sorry to keep asking so many questions, but when I'm daily driving (which I do on my ZIIs), around what pressure will give me the most even wear? I've been keeping them around 38 cold, which is what the door says (for 17'' though).

Thanks

 

I just chalk/crayon/shoe polish/etc. the tire sidewalls in a few places to see how far the tire is rolling over. I also had the luxury of borrowing a temperature probe to see what the temps across the tire were doing. Don't use one of those IR temp guns. They're not accurate for measuring tire temps.

This part is pretty much correct. In the perfect world a car would have even load across all 4 tires and weight transition would be kept to a minimum.

I don't quite understand this part. If you stiffen up one end of the car (front or rear) you're doing a better job of keeping tires in their "happy place". This means they're are more evenly weighted and they are less likely to be in a "bad" part of the camber curve. Therefore that end of the car should grip more. This is of course provided you're not getting into crazy stiff springs where the car has lost all compliance and is now skittering over the surface. How does unloading a tire make it grip more? Geez this is super complicated to try and write down in a forum post...

The effective spring rate at a corner includes mostly spring rate and swaybar rates and to a small extend the "spring rate" of the tire. When you are changing the swaybar stiffness what you end up doing is moving more of that weight from the inside tire to the outside tire. Which effectively lowers the grip provided by both tires because the outside tire is overloaded and the inside tire is underloaded. This is different than changing spring rates where the weight would transition to the outside tire less and so the car would roll less transferring less weight and therefore increasing the amount of grip at that end of the car. So now you're probably asking why people in FWD cars run stiff springs in the rear (for example, 450f and 600r) to get the car to oversteer if more spring rate equals more even weight distribution which equals more grip. The stiff end of the car will transition weight and load up faster. In the case of the FWD car the rear loads up quickly to the maximum lateral grip the rear can provide while the front loads up more slowly keeping the tires more evenly weighted and providing more grip than the rear.

There are lots of books and articles on the subject if you really want to know more. Easiest way is to just find a codriver who is good with setting up cars and just do what they tell you

IDK about where you are but the mornings of our auto-x events can vary greatly in temp between early season and into the summer months. Not to mention depending on which way the car is facing the sun can change the pressures 3psi from side to side. Setting tire pressures to what you want and maintaining them is the only way to ensure consistent behavior across all your runs. What do you do at a National Tour when you only get 3 runs? Throw away the first 2 because the tires aren't up to temp yet and therefore the pressures are low and the tires are all squishy?

Constantly bleeding air out of the tires is the only way to go IMHO. Sure it can be a pain but I want to make sure my car is behaving the same from run to run unless I specifically change something. This also means airing the tires back up somehow if there is large time gap between say morning runs and afternoon runs.

 

The numbers may very well be wrong, and are for illustration purposes only, I don't have my notes with me. The rear tires run a lot cooler, so the drop would be less. If they heated up to 90-100°F that'd be 3-4psi over cold pressures. (The perfect gas law will estimate about 1psi per 10°F.) Its about 60°F in my garage. Though I'm measuring surface temperatures, that's what most important for autocross (hence the use of an IR pyrometer, I have a probe pyrometer for track use). I doubt the tire gets totally heated that much. It depends on how much of that surface heat makes its way into the gas inside the tire.

I use just normal air. Though around here, that's 30% humidity, so the partial pressure of the water vapor is only 0.1psi.

 

This is all pretty much black magic, but I've generally heard that softer springs lead to more mechanical grip.
Out by the bay (Oakland) the temperature tends to be pretty mild. Maybe 65 in the morning, 75 in the afternoon. Out by the ocean it can be 50°F all day in summer, winter might have a little more variation, 50°F in the morning, 70 in the afternoon. At San Diego it was 60-65 all day Saturday and 65-75 on Sunday. Pretty much only a 1 psi change due to ambient.

I suspect the temperature variation is an order of magnitude below my skill level.
As shown above, they were quite happy after my second run. There are two of us running, but I was surprised to find out how much variation there was in surface temperature. I need to check this data more thoroughly but I saw 130°F after one of my runs and 115°F after one of Cathy's runs. So I was trying to get a handle on how much the surface temperature changes. Again, this is all about surface temperatures, I pretty sure this doesn't make its way into the gas much.

That's one of the differences between autocross and track on track they get 20 mins at operating temperature to soak, so the gas gets up to the carcass temperature, and there's a big difference between hot and cold pressures. Like 10 psi. At autocross is only a couple of PSI, the difference between 49 and 47 for the fronts. That's hardly enough to notice.

 

Well that's interesting because that hasn't been my experience at all. This weekend we had a local event on asphalt (lower grip). I set the pressures to what I wanted them at before my first run. Then I got distracted talking to people between runs and kept forgetting to check my tire pressures. By my third run the car felt like absolute crap and after the third run my fronts had jumped 8psi and the rears around 3.