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Understeer: Relationship between tire pressures and swaybar settings

Discussion in 'Autocross' started by Brian A, May 21, 2009.

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    Brian A

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    My 1991 318i is a nicely balanced autocross car. I like the way it handles and other (much better) drivers like it too.

    Through chalking the sidewalls, I've settled on 40 psi front and 37.5 psi back tire pressures for 195/65R14 M/S rated tires. I have settled on these pressures solely focused on limiting roll-over and evening out tire wear over the cross-section.

    I have adjustable swaybars. They are currently set at the "middle" setting both front and back. (All upgrades to the car are documented here: http://www.justracing.com/homepage/BrianA)

    Shouldn't the tire pressures be the same front and back if the car truly has neutral handling? Can I generate more grip by equalizing tire pressure in all 4 corners (raise rear tire pressures?) and adjusting the swaybars to compensate?
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    CRKrieger

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    'Neutral' handling comes from a number of different factors including tire pressures. It does not mean that all the suspension stresses are equal front & rear. You generate larger slip angles in front both steering and braking than you do in the rear. In fact, when doing any track or autocross driving, the tires will probably wear more in the front than in the rear.

    DO NOT adjust the antiroll bars to compensate for tire wear. That is not what they're for; at least not directly. Your tires will wear excessively when you have too much over- or understeer. Dialing that out will change the tire wear for the better.

    What you need to do is determine if your car is understeering (more likely than not) or oversteering at the cornering limit. What you would ideally like to see is a neutral breakaway that you can control with the throttle (for power-induced oversteer). The antiroll bars can be adjusted to produce, or at least get closer to, that dynamic state.

    One thing you want to remember when making any adjustments is to make only one at a time and then test it. In this way, you will know whether you're going the way you want or not. If not, then put the adjustment back where you started before changing something else.

    FWIW, those tire pressures sound about right for an E30.

    32002 guest

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    If your car handles good with the pressures your using then leave them where there at. If the car has any bad traits then I would adjust accordingly. There are too many different conditions that you may have to adjust for to list them all here. The higher the pressure the less flex there will be in the tire. More pressure makes the tires more responsive but too much pressure can adversely affect grip. Same thing with the sway bars. The bars limit body roll and allow the chassis to respond quicker. If your sway bars are too tight though they can limit overall grip. the stickier the tires, the tighter the sway bars can be. If you make your front bar too stiff then you will have understeer. If your rear bar is too stiff then you will have oversteer.

    There is no magic setup. Everyone's driving style demands a different setup. Trying different things at an autocross is half the fun. You may find that an understeering car is easier to drive fast. Or maybe since your car is relatively lower powered then you may want a more neutral or oversteering car. Either way, as long as your comfortable driving it then that's all that matters.
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    Brian A

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    Resurrecting this thread is very timely, thank you for the response.

    I just returned earlier this evening from our monthly autocross event. I am now running Spec E30 wheels (15"x7") with 205/50R15 Kumho Escta XS tires. We ran a new venue with a surface with a lot more grip and, to my surprise, I had to push front tire pressures up to 42 psi (with 37 rear) to control roll-under. The grip is massive.

    Having said that, the nice neutral easy-to-rotate handling I used to have has gotten unstable. I can't just set the car and hold the corner the way I could before. More dynamic steering input is needed as is judicious use of the gas.

    I find the 5 psi difference between front and back alarming since my rears should have less grip at that pressure than I seem to be getting from them. This is based on "chalking" the tires and trying to restrict roll-under to the point of those little indicator triangles molded on the sidewall. At those pressures, roll-under is acceptable.

    I am thinking about stiffening my rear sway bar. I guess I could fiddle some more with tire pressures, but I'd hate to lower the pressures for sidewall wear reasons, likewise, I hate to raise them since max tire pressure rating is 51 psi and the tires already rise to the mid 40s when hot.

    Basically I feel bamboozled. The good news is that the car is fast as-is. I just wonder if it could be faster with pressure/swaybar adjustments. Likewise the effortless gliding-sideways-on-ice feeling is gone when cornering. Its now work.
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    conechaser

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    Do you also have adjustable shocks?
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    Brian A

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    Do not: I have a Dinan "Stage 1" suspension (with Dinan/Bilstein non-adjustable shocks) and Eibach Adjustable 20mm/16mm swaybars.

    32002 guest

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    Sounds like you need to tighten your rear sway bar. Just remember that when you make a slight adjustment to one part of the car, you may need to adjust everything else again to work with the new settings.
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    conechaser

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    After re-reading your posts it looks like you tuned your suspension to an all season tire and then switched to a very high performance summer tire (I think TireRack calls it an extreme performance summer tire) as well as changing sizes (lower aspect ratio) and probably on a wider rim. That's a pretty good change that I don't doubt you are having to change your settings.

    What are your alignment settings? What was the surface you were running on before? (what is is now?)

    Explain what you are stating as "dynamic steering" Is the car loose? Does it push? Do you need to add a round of wedge and adjust the track bar? (sorry, just watched a NASCAR race.
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    Brian A

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    I stiffened the rear swaybar this morning. Not sure why I am such a Hamlet about it; I can easiliy change it back if I want to.

    Yes. I used to use All-Season tires (Bridgestone Turanzas) which I really liked and allowed me to compete in a lower class in Autocross. The tires were discontinued in 14" size so I bit the bullet, changed wheels/tires and jumped up to a higher classification. Stock E30 rims for the 318i are 14"x6" (bottlecaps); I changed to 15"x7" (Team Dynamics Pro Race 1.3).

    I am surprised at this. I would have thought I the wheel/tire change would have had added the same amount of grip in all 4 corners, so vehicle dynamics would not have changed. Note that I am inquiring about optimizing; the vehicle dynamics are still very good; I am just trying to optimize.

    Plain stock. (F&R: -1.2 camber, F&R: 0.38 total toe ... front castor is 9 degrees because I have solid M3 offset control arm bushings). I suppose a next step would be installing adjustable camber plates; I can't bear the thought of taking my whole suspension apart again to put them in and it would bump me up yet another class.

    We usually run on dead flat concrete. Yesterday we ran on gritty undulating asphalt.

    By "dynamic", I just meant that it seems like I'm just moving the steering wheel a lot more while in the corner itself. The car is pretty close to neutral (sorry, Bubba, don't know the NASCAR term.) with probably a slight tendency toward understeer. With the grippier rubber, it seems like either end is equally eager to break free. It is a remarkably easy car to "catch" when the rear breaks loose, which make it fun. Wedge???? ... I had to look that one up; nope; not buying coilovers. Track bar???? ... we only serve water and soda at our venues.

    I am mostly trying to figure out how to optimize my tire pressures. All I have been doing to them so far is just pumping them up until they stop rolling over onto their sidewalls. Are those the optimum tire pressures?
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    CSBM5

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    Don't be surprised. When you change to a tire with a lot more grip, and it appears you made a pretty radical change from all-seasons, it's easy to significantly affect the handling characteristics of the car. My experience is that, "in general", the more sticky the tire and/or surface the more the car will understeer given no changes in setup. This is particularly true in transient load situations (as opposed to pure steady state cornering which is short-lived in most autocross corners). With less overall grip tires/surface, it is easier to rotate the tail on most cars in a highly dynamic situation (rapid turn-in, slalom, etc).

    To really see the point, consider driving on a gravel road and think about how with just a quick steering input how easily you can create rotation of the tail (one reason why the "Scandinavian flick" works so well). This is an example in the extreme of what I'm referring to. It's easier to dynamically overload the rear tires given a lower level of grip. Up the grip level significantly, and all of the sudden that car that liked to rotate and had nice balance is now stuck hard feeling and when really pushed may just plow instead of rotate.

    A real-life example in my recent experience is with my 03 330i ZHP that is setup for SCCA D-stock autocross. With the race tires on the car (245 Hoosier A6 or BFG R1), it drives COMPLETELY differently than with ultra-high performance street tires (Dunlop Star Specs). With the street tires, if feels great on an autox course: good rotation, excellent turn-in, lively and responsive feeling. With the race tires, it is much less rotation-able. You have to drive it in a different fashion. It's faster on the race rubber, significantly, of course, but you have to seriously adapt your inputs to account for their higher grip and the lack of a more easily rotate-able tail.
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    CRKrieger

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    Not in the rear. The rear tires usually do not roll over as much as the fronts. To optimize tire pressures, I do what you are doing in the front. For the rear, I adjust for over- and understeer and ease of rotation. In the rear, there are two possible extremes that can achieve the same end result. A very low pressure will allow the car to rotate by deforming the sidewalls. A very high pressure can do the same thing by simply limiting rear traction and allowing the rear to slide. With the latter approach, you get no rollover. I would not worry much about 50+ psi in competition. Your tires can handle it. Just bleed it off before you leave for home.
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    Brian A

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    This is all starting to make a lot of sense. I relate to your explanation; I feel like you are describing exactly how my car has changed with the grippier rubber.

    Thanks for this. FINALLY I am feeling comfortable with my tire pressures. I suppose a "third way" to rotate the rear would be to stiffen the rear swaybar (which I did yesterday). In the future, I will raise the pressure of my rear tires more confidently to try to reduce grip (and as an experiment, I may even overdo it, just to see what it handles/feels like). I think I have the fronts optimized.
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    wiley

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    You know, its been a couple of decades but I remember I always had between four and six PSI less in the back then the front. It seems right to me. Just keep driving. I think if you had more negative camber in front, they make get closer but likely never equal.
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    EuroWerkz1

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    Short and long of it

    Yes, the short answer is increase tire pressure in the front until the rolling is limited to just touching the indicators. The rears should be hard enough to be stable yet soft enough to lay the power down.

    The long answer in my opinion is this. You have added a much stickier tire to the mix and you are now seeing the limitations of your suspension. Remember even this middle of the pack extreme performance tire probably has more dry grip than racing tires of your cars vintage. What you are feeling is the car trying to steer itself, very common on the semi-trailing arm rear suspensions of the E30, and especially when you start introducing more grip. I find that folks who race E30's or track them at a high level use VERY stiff rear springs to help reduce the toe change of the rear suspension to a minimum. Solutions for a dual purpose street car are limited. Larger sway bars on E30's are an okay upgrade but just murder the ride quality and will not be stiff enough to be the complete answer, the same goes for very stiff springs as the car will be undrivable on the street when you finally get it hard enough. So maybe the real answer for you situation is to make some minor tire pressure changes and re-learn your car. At least that will be the most fun solution,
    Enjoy!

    Arash2002tii guest

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    Don't know if this has been answered already but the max pressure rating is for when the tires are COLD, it is ok to see higher pressures than that as they gather heat.

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