Which end of the car needs air pressure to reduce understeer?

[Attack Eagle]

which end of the car do you up the tire pressure to reduce understeer? think its the rear but I am not sure.

My rear tires I believe are too soft of a side wall and is oversteering under power (you can feel the rear tires "lay down" ) but the front is pushing too, just not as bad.

I looked here and on the web and can't seem to come across the matrix...

it had a list of thingds to do to reduce under steer and vice versa for oversteer.

Bar/ tire pressure/ springs/ tire size etc...

 

[wret]

You're right. The rear. I wouldn't say it reduces understeer so much as it increases oversteer by allowing less grip at the rear. Playing with camber and spring rates have an effect as well.

 

[DSMeclipse4G63]

I would say lower psi in the front and higher in the rear would induce over steer and keep the front planted a little better. The car wont over steer if the rear has considerably more grip then the front. the front tires in that case will always give out before the rear cause an understeer. just my .02 cents
Andrew

 

[jtmcinder]

There is no simple answer to this question. All tires have a sweet-spot in pressure (or, rather, a sweet-range) where they get the most lateral grip. And this spot depends on what specific tire you have and how much your pig weighs, plus a bunch of other things.

I could give you my favorites pressures for a half-dozen tires, but I really suggest that you learn to feel this for yourself. Go to an autoX with the express plan of playing with tire pressures ... not to win (or lose less badly) ... to learn tire pressures. Make large changes to really get an idea of how the inverted-U function for pressure-grip works.

- Jtoby

ps. Those who are giving the one-size-fits-all answer that lower pressure gives more grip (and, conversely) higher pressure gives less are maybe thinking of longitudinal grip (i.e., drag racing). The same rules do not apply to lateral grip.

 

[HALO1]

Also, another important note (and I think this has been said before):

The secret to decreasing understeer should not be to increase oversteer. To semi-quote, it will do little more than make your car "dance around" more, which does nothing to achieve what should be your goal of getting power to the ground. There is an incredibly helpful post on this subject by DG (I think that was his name?) in the FAQ at the top of this page. It mostly discusses spring rates, but it will give you a better overall idea of what to do.

So, I'd say that JToby pretty much answered your question, but in the big picture there's a lot more to it.

 

[Attack Eagle]

thanks
I am somewhat familiar with playing with pressures as I used to autocross an MGB, and that is a pressure dependent setup with no other adustments possible.

I know my rear pressure is too low but I could not remember If I had to up the front to keep the same ratio/feel. Since I have a bit of push and power oversteer I'm going to try upping the rear pressure to 35-37 from the current 30 which was cushy on the highway.

The front is pretty well hooking (for stock) so I have it somewhat close.

I jsut did not want to tamper with the front unless I had too to keep the same proportion.
Either way I have to air up the rear.

Thanks to all.

btw I used to use water on the driveway technique to look at the tire footprint and make sure I wasn't over or under inflated... anoyone else ever done this?

 

[jtmcinder]

I know my rear pressure is too low but I could not remember If I had to up the front to keep the same ratio/feel. Since I have a bit of push and power oversteer I'm going to try upping the rear pressure to 35-37 from the current 30 which was cushy on the highway.

If you've got power-on oversteer in a DSM then you either have a funky center diff, spring rates from hell, or your current tire pressures are too screwed up to be believed.

Also, 30 psi is way too low, even for the rear. Is it possible that you're rolling the rears over and mis-reading this as power-on oversteer?

- Jtoby

 

[Attack Eagle]

that is exactly what I believe is happening. I can actually feel the tire laying over because the pressure is to low... Exiting a corner under full boost is terribly fun with the butt hanging about 1 -2 feet out... I'm going to miss it...


when I got the car back the fronts were at 45psi and the rear at 30 psi, which did not seem right...


I dropped the fronts down to 36 and they seem to be ok...

what psi is everyone running front and rear or just a ball park figure.

My parents lost my manual and key fob.... so I can't look it up.

I'm thinking i'll probably up the rear to about 45 and then drop psi a couple of pounds at a time until I am satisfied, but if someone has a magic number like "stat at about 40 bc you should only need 5psi or less extra in the rear" would be good to know

 

[jtmcinder]

I run my V700s at 41/37 (hot ... as in: when I'm just about to close the hood and get in line for my next run); these are 245/45/16s on 16x8.5s. When I was in STX, I ran my MXs at 43/39; these were 245/40/17s on 17x8s.

Here's my advice. Start at about 45/40 and then lower the fronts after each run until they are just about to roll over. (Obviously, bring some white chalk or white shoe polish to the event.) My experience is that you get maximum front grip from a tire that is just about to roll over. However, I should point out that I only have experience with pretty good tires (MXs, Azenis, etc.) and tires with softer sidewalls might be different. Furthermore, I have pretty good front camber (more than 2 degrees); if you have less than this, then you might have to try a different approach. In any event, Step One is to maximize front grip.

Then, when you're happy with the fronts, use the rear pressure to balance the car. I prefer to run high pressures to reduce rear-end grip, because I don't like a soggy rear end. Other people run the rears too low on purpose, to give the feel of oversteer. If you're relatively new to autoXing, then I strongly suggest taking the high-pressure approach, because it's crtitical that you have a good feel for what is happening and a soggy rear-end does not give this. Also: if you haven't shimmed the rear upper control arms to get the rear camber at or below two-thirds of what you have in the front, then this is the next thing on your list. My experience is that high rear pressures and low rear camber is the easiest way to get the car to turn while still being stable at higher speeds.

Another complication to all this concerns the design of the course. If it is tight, then you might want to go to extremes to get your pig to turn, upping the rears to almost the same as the fronts. In contrast, if the course is wide open, then you might want to keep more rear grip, because our cars shift towards oversteer at higher speeds and can snap-spin must faster than more humans can react. I do this adjusting for the course using only the rear pressures. When the guy with the Civic designs the course, my rears go up about 2 psi; when the guy with the BSP 'Vette is the designer, the rears drop 1 psi.

- Jtoby

ps. I'm quite aware that some people argue against the idea of giving up rear-end grip to balance a car. I tried maximizing both ends and it didn't work for me. Maybe it will work for you. Take the above advice with this pile of salt in mind.

pps. I'm assuming stock boxes-of-goo. If you have Quaife and/or Kaaz and/or Cusco diffs, then definitely don't give up as much rear grip.

 

[Attack Eagle]

I run my V700s at 41/37 (hot ... as in: when I'm just about to close the hood and get in line for my next run); these are 245/45/16s on 16x8.5s. When I was in STX, I ran my MXs at 43/39; these were 245/40/17s on 17x8s.

Here's my advice. Start at about 45/40 and then lower the fronts after each run until they are just about to roll over. (Obviously, bring some white chalk or white shoe polish to the event.) My experience is that you get maximum front grip from a tire that is just about to roll over. However, I should point out that I only have experience with pretty good tires (MXs, Azenis, etc.) and tires with softer sidewalls might be different. Furthermore, I have pretty good front camber (more than 2 degrees); if you have less than this, then you might have to try a different approach. In any event, Step One is to maximize front grip.

Then, when you're happy with the fronts, use the rear pressure to balance the car. I prefer to run high pressures to reduce rear-end grip, because I don't like a soggy rear end. Other people run the rears too low on purpose, to give the feel of oversteer. If you're relatively new to autoXing, then I strongly suggest taking the high-pressure approach, because it's crtitical that you have a good feel for what is happening and a soggy rear-end does not give this. Also: if you haven't shimmed the rear upper control arms to get the rear camber at or below two-thirds of what you have in the front, then this is the next thing on your list. My experience is that high rear pressures and low rear camber is the easiest way to get the car to turn while still being stable at higher speeds.

Another complication to all this concerns the design of the course. If it is tight, then you might want to go to extremes to get your pig to turn, upping the rears to almost the same as the fronts. In contrast, if the course is wide open, then you might want to keep more rear grip, because our cars shift towards oversteer at higher speeds and can snap-spin must faster than more humans can react. I do this adjusting for the course using only the rear pressures. When the guy with the Civic designs the course, my rears go up about 2 psi; when the guy with the BSP 'Vette is the designer, the rears drop 1 psi.

- Jtoby

ps. I'm quite aware that some people argue against the idea of giving up rear-end grip to balance a car. I tried maximizing both ends and it didn't work for me. Maybe it will work for you. Take the above advice with this pile of salt in mind.

pps. I'm assuming stock boxes-of-goo. If you have Quaife and/or Kaaz and/or Cusco diffs, then definitely don't give up as much rear grip.

Thaks brother.
exactly what I am trying to do. firm up those rears that were folding over when they shouldn't and making the rear soft AND twitchy while maximizing grip w/o losing balance.
Grip is no good if you can't use it because the car is unbalanced. (and I don't need max grip more on that in a second)
I just upped the presures to 47 -42 (from about 37-31 hot temp right off freeway) `cause its easier to lower pressure than add it (don't have an inflator around the house). noticed a considerable diffference in the rear "feel" already... got some understeer which is where it should be for a car with stock suspension/no aftermarket diffs, just rear limited slip... pretty tight feeling in the rear (good as new or a little better) under no power or constant state cornering from what I can tell at reasonable street velocities(30-40 mph) and going around the right hand corners. It seems to barely break loose in the rear first under excessive! acceleration and almost starts "drifting?" laterally with just a bit of push, but rears are older than fronts so should be that way, and thats the way i like it cause I can recover that easily by backing off a little... I'll play with lowering the pressures a little at a time until I get the car's grip in the front the best it can be, and then adjust the rears for balance... think i am close enough for a street driver now anyway.

IF I had the ability to go to azenis sports or something like that I am sure I would be all over modding the suspension and improving grip all around but on my compromise tires (toyo proxes fz4s) I just want good balance... Once my wife finally gets a wagon or suv... then I might be able to get back into the performance and handling upgrade game again.
I don't autox anymore, but once you've had a tight good handling well balanced car ( i miss my auto x MGB!)... well you can't be satisfied with cruddy steering response or crummy shifty feeling in the tires or suspension, even on a daily driver family car (yes the talon is my family car!)

I cant wait until I can get new rims and tires on it to really start going out to Autox again and hone my skills... only thing I will never be able to do (ankle /knee problems/can't physically do it) is heel toe so I'll never be competitive, I have to drive Italian style as best i can... besides I always went there to practice,learn, and compete against my self (and have a butt load of fun!)

thanks again

 

[ACM]

Somewhat off-topic, but I share your mangled ankle problem, and also can't heel'n'toe in a conventional manner.

Unless you're running a Stock class, you can add 'pedal covers'. Experiment with lateral extension to the accelerator pedal above and below the brake pedal pad, and adjust the height of the brake pedal pad so that it is just higher than the accelerator under heavy braking. Depending on how your movement is limited, you can then blip the throttle with either your toes or your heel.

Having said all that, one aspect I love about DSMs is that with left foot braking I have never really needed to drop a gear, at least at autocrosses.

Ditto everything JT said about tyre pressures - dial the front in, run the rears high, not low. On a stock centre diff I ran 40f/48r on 245 Hoosiers, 44f/4?(8?) on VictoRacers.


Charles

 

[jtmcinder]

I love tangents.

My approach to "heel-and-toe" downshifts is to not do it with the heel and toe. (I think I got this from the Bondourant book, but can't be sure.) In any event, when I see a corner that will require a downshift (such that I won't be left-foot braking, which is something that ACM convinced me to learn), I roll my right ankle to the right and then slide my foot sideways so that the center of the bottom of the ball of my foot hits the side of the brake pedal. Then I roll my ankle back to straight to press the brake, which takes the right edge of my foot off the gas. When I need the blip, I roll my ankle back to the right again while still pressing the brake with the bottom on my foot. In other words, my ankle joint does the blip while my leg (compensating for my ankle) does the brake. Works pretty well for me.

Whatever you do decide to use, do it all the time. Don't take breaks from doing every downshift correctly just because you're only going to get groceries. You want it to be automatic ... as it were.

- Jtoby

 

[wret]

...adjust the height of the brake pedal pad so that it is just higher than the accelerator under heavy braking.

I agree that raising the brake pedal is highly beneficial in accomplishing the ankle roll brake/throttle technique. I accomplished this by fabricating a spacer mounted under an aftermarket pedal. Is there an easier way?

 

[Tevenor]

I must playing mine way to low then. I've been running my V700's in the 34-38 range.

 

[jtmcinder]

[NABR mode] There are three reasons why you do this, Sean. First, I've seen you eat and you seem to like things sloppy. Second, you're a recovering drag-racer and so you seem to think that "turn-in" means that it's time to go to sleep. And, third, rumor has it that you have some "issues" with stiffness, sidewalls and otherwise. [/NABR mode]

More seriously, if you don't need precise feedback because you turn early enough and know what your car is going to do, then probably softer is better. But I'm not ready for that and I made the assumption that the guy asking the question wasn't, either.

- J "not ready for Hoosiers" toby

 

[Tevenor]

[NABR mode] There are three reasons why you do this, Sean. First, I've seen you eat and you seem to like things sloppy. Second, you're a recovering drag-racer and so you seem to think that "turn-in" means that it's time to go to sleep. And, third, rumor has it that you have some "issues" with stiffness, sidewalls and otherwise. [/NABR mode]

1) The sloppier the better. I will bring you a garbage plate next time I am in town.
2) Sleep is a precious commodity to me as I get so little of it but I definitely not a recovering drag racer.
3) Chassis stiffness, yes. The rest of my 'issues' are for young ladies 18-29 to verify.

More seriously, if you don't need precise feedback because you turn early enough and know what your car is going to do, then probably softer is better. But I'm not ready for that and I made the assumption that the guy asking the question wasn't, either.

- J "not ready for Hoosiers" toby

I agree whole heartedly. It's easier to come down off a high pressure setup than it is to add more and feel where the sweet range is. I chalk my lower pressures to the fact that I treat the steering wheel like a fine wine, not a shot: sip it smooth, not shoot it.

Where is Fedja when you need him?

 

[jtmcinder]

I chalk my lower pressures to the fact that I treat the steering wheel like a fine wine, not a shot: sip it smooth, not shoot it.

I don't drink fine wine (since it's wasted on a nicotine-stained palate), but I did learn how to handle a steering wheel from watching what better drivers do to that bubbly crap ... they shake from side to side as hard as they can. This skill is useful, by the way, because I often have to spray down my V700s after a white-knuckle, red-haze run.

Where is Fedja when you need him?

He's over on SCCAForums still trying to argue that the Evo VIII belongs in ESP.

- Jtoby

 

[DG-FNR]

This is modified HEAVILY by the actual construction of a given tire, but the general rule of thumb on tire pressures is:

1) Increasing tire pressure INCREASES grip

2) The line between "I've got it" and "it's got me" gets much finer, and the grip level once that line is crossed may fall off more rapidly.

When you decrease air pressure, the tire looses grip, but it gets less abrupt.

So the real answer to the question "Which end of the car needs air pressure to reduce understeer?" depends on the answer to "Why is the car understeering?"

And like all rules of thumb, observation trumps the rule.

DG