So a guy in my club running a front wheel drive Dodge stealth says that he runs his front tires at 50psi and the rears at 28-29 psi. We are both running Dunlop Direzza ZII tires. My tires are 205/45 and his are 215/60.

He says he tried running below 40psi in the front and his times dropped significantly. I have been running 37-38psi in the front and 29 psi in the back.

First, would his taller side wall require a drastically higher pressure? Second, I am very comfortable with how my car is handling except in the slalom. My back end really seems to get tail happy if I push too hard. My suspension is the JCW springs/shocks and an H-sport competition rear swaybar on medium. When I slowed down through the slalom my overall time dropped 1 second but it just felt slower. I don't really know if I was faster through the slalom or just faster through the other parts of the course because the entire run felt good.

How should the car feel in side to side slaloms? Is tail happy fast if I slow down and keep a tighter line or should I consider going to soft on the swaybar?

 

The reason that Stealth is having to run such high tire pressures is because he's got a lot more weight on the front tires than the mini. Any softer and he's probably rolling his tires over real bad and losing grip. The taller sidewalls aren't helping either. And sweet baby jesus who would want to auto-x a dodge stealth?

FWIW we run 38-39psi front (depending on the surface) and 41-43psi rear (depending on surface and course design). That doesn't really do you much good though since we run with coilovers.

I think your tire pressures are a little odd. When we used to run with stock suspension and the Z1s we used to run 45psi front and something like 40psi rear. Any lower than 45 and we were rolling the tires over. Perhaps the lots you run at have lower grip levels?

The problems in the slalom are probably caused by two things. First I would say your rear tire pressures are way too low. You probably don't need that much of a delta between your front and rear tire pressures. I would run 38 square and see how you like it. Then adjust rear pressure from there to get it handling how you want. Second, that big rear swaybar isn't helping. I know the people on this forum looooove their big rear bars but I was never a fan. IMO all they do is introduce snap-oversteer at the auto-x. They make the car better to drive on the street and make the car "feel" sportier but they're typically not a great option for a serious auto-x car.

Your problem could also be driving technique. I don't know how long you've been auto-xing but typically when people get loose in the slaloms it's because they're late on their turn-ins and not attacking the backs of the cones like they should. It's a rare occurrence when I can't take a slalom flat footed in the mini provided I enter the slalom correctly and it's not like 9 cones long. I would suggest a local auto-x school.

Generally with regards to auto-x the concept is that loose is fast as long as you can handle it. A car that rotates well doesn't do you any good if you can't catch it when it steps out. The way our mini is set up is fairly loose but not all that crazy. Keep in mind that street tire cars tend to be driven with a lot of slip angle. Most race tire cars are driven a lot more "tidy" than a street tire car.

Our car rotates well in the slaloms but never feels like it's going to get away from me unless I do something stupid. There is definitely some looseness though as the car has to rotate around the slalom cones.

 

The reason for low tire pressures in the rear is because when I chalk the tires it takes that low of a pressure to start getting any wear on the edges of the tread. From my reading and limited experience it seems that lowering the tires pressures in the rear will give more grip. I am currently leading in the points going into our last autocross. If going back up to 38psi in the rear and 42 in the front will give me better times then I would definitely consider but my results throughout the season have been good where I am at. But like I said, I could be way off and if I can do better I would like to.

I am sure I am not attacking the slaloms fast enough as they have never been my strong suite. Here is a video of my best run this past sunday. You can see where I am not close to backsiding the cones. I was 3rd fastest raw time with an SSM car running 4.5 seconds faster than me and a B Mod car running 8 seconds faster. Indexed it should be very close, I am hoping to squeak by with the top points.

Should I consider going to the softest setting on the swaybar?

The slalom is at the end of the run.

 

Tire pressures can vary, there are many factors.

The best way to check how you are doing is - right after your second run, check tire temperatures and see where you are on each tire, inner middle and outer treads using a probe pyrometer.

Tire temperatures are telling you how much heat you are generating on your tires in various locations. The hotter the temperature the more you are using that portion of the tire.

The goal is to keep the temperatures as close as possible inner to outer thread areas for each tire. Tire temperatures increase relative to many factors, especially how you drive (your style) for the given course for that event.

For a given event you may sometimes have to adjust pressures on one side of the car or fronts over rears due to the higher speed areas.

Bottomline is the you want to keep your tires in their optimal performance zone (known as happy place). In some areas the tires will be overworked while in others, hardly worked. You want to adjust tire pressures a little to get the most out of your tires and increase traction.

The problem with chalking of tires is it is not a replacement for checking tire temperatures. I do both chalking and check temps to determine my tire pressures. When I see the temps running very close across the treads for a given tire I leave it alone. I almost never see all three readings the same across any one tire.

If I see the temp too high in the middle I lower pressure, if too low in the middle I increase pressure. Once I get the middle temp to equal either the outer or inner tread temp then I stop adjusting.

Sometimes I will see inner tread at 38, middle at 37, outer at 35. I'd be happy with that reading vs the outer tread to be 30.

As for your video, your slalom is spaced far apart enough that if you did backside each cone you could have driven faster through the whole thing assuming they were evenly spaced. In any slower parts of your course the best line to take is the shortest one rather than the classic line.

For your pressures, see
http://www.tirerack.com/tires/tirete....jsp?techid=58

You might want to increase rear pressure a little, 30-31 psi cold.
Adjust front pressures to reduce understeer given your driving style, higher pressure usually better. If you are getting more understeer then increase front pressure.

 

Why would increasing front pressure help with understeer? I would think that a higher tire pressure would give less grip and make the front end push (more understeer). A lower front tire pressure (within reason), would give more rubber on the ground in the front, more grip, and decrease the tendancy to push the front end, right?

Any comments on the middle vs soft setting of the H-sport competition rear swaybar?

 

Lower pressure also makes the side wall softer. As an instructor I had to get several students to trust me to increase their pressures so they could learn to drive faster in the corners. This takes a few events to start paying dividends so I wouldn't recommend it for your last event of the season.

 

Thats what my thoughts are... Finish out the season and then start fresh next season. I will do an evo school again at the start of next season. What do you recommend for your students as far as pressures?

 

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

and discussion at-
http://forums.rennlist.com/rennforum...ndersteer.html

The goal is to make the most use of your tires and to reduce understeer without getting excessive oversteer.

"Increasing" front tire pressure can give you relatively more or less grip (depends on other variables), adjusting tire pressure can allow your tire to work better up to a point. Going past optimum would not be of benefit so you have to consider where you are starting from.

If your current race pressures are already on the high side for you then increasing front tire pressure may be of relative limited value. The only way to tell if you are close to optimum is to check tire temperatures and for you to observe if you are getting more or less understeer.


see
http://www.thesmokingtire.com/2012/m...teeroversteer/
http://www.northursalia.com/techdocs...ver-under.html
http://www.nsxprime.com/forum/showth...-pressure-bias

For swaybar settings-
Do you detect understeer when you drive?
If it is minimal then your rear bar settings are OK, learn to drive with it.
No need to make it stiffer to the point of oversteer which is more difficult to control.

Normally you can set a rear bar to softest or a middle setting. If you have a test and tune day you can try out both using the same tires/pressures, etc.
See if you can tell the difference.

 

Pressures vary wildly by car and tire so I recommend that people ask someone who is driving their model car faster than they are. Otherwise it is the chalk method to determine a minimum followed by experimentation and a repeat of chalking as the driver becomes faster. Then it is time for the pyrometer as minihune explained above.

I needed 49 fronts 44-45 rears on my Star Specs on my Justa. But then I really lean on my tires and only had factory camber.

 

For autocross you may be better off taking surface temperatures with an IR pyrometer instead of a probe.

I've been using a cheap(ish) thermal IR camera to check the temperature: http://www.rhworkshop.com/pages/product-ir-blue-dm

 

Low tire pressures are great as long as you're not rolling the tire over and using a bunch of the sidewall to get around the course on. Sidewalls aren't very sticky Chalk the sidewalls to get a "minimum" pressure and then go up from there as needed. Don't try too hard to find minimum pressure on the rears. The rear tires hardly do anything on these cars anyways.

I would go to the soft setting on the swaybar and see how you like it. I'm sure you'll find that as your driving level changes you'll be able to make those decisions on your own as you figure out what you like.

FWIW I've not had good luck with the IR pyrometer. Always borrowed a probe and seemed to get more consistent temps.

 

38 square has been working for me on my CMS All4 running PS4s. Was thinkning about running cups next season. and wow i just realized how old this thread is.