280zx and minimizing oversteer

[Afshin]

My current set up includes a 1983 280zx turbo with eibach’s, tokico’s (nonadjustable), 17†wheels with 225/45/17 Z summer tires and new urethane sway bar bushings.

Car handles quite well except for a steady oversteer which seems to limit my cornering ability prematurely. The car clearly handles much much better than stock, but it feels like the oversteer is less predictably than before and is a little more difficult to straighten out with the accelerator once I have exceeded the limit (which is quite often ). My goal is to maximize handling for serious twisties and mountain roads, which in this case is to minimize the inherent oversteer in the car.

I would greatly appreciate others input/experience with the ZX suspension set up

I am considering these options:

-Loosen rear sway bar (since stiffening front would require new larger bar)

-Have front tire pressure slightly lower than rear

-New control arm bushings. I expect that my current bushings deform under load resulting in rear toe (and camber) changes.

Before I get new bushings, does anyone have any experience with the motorsport bushings that allow for camber change? And if so has anyone found the amount of negative camber that can be dialed in without causing too much premature tire wear (some is OK) ?

I would greatly appreciate all input regarding decreasing the amount of oversteer.

Thanks.

[260DET]

While I'm not familiar with the ZX the usual principles apply. As a prerequisite the suspension has to be in good condition throughout whatever bushings you decide to use.

 

Then, as an experiment, I would remove the rear sway bar completely and go for a test drive. Take it easy at first until you become familiar with the different setup. Too much understeer? Consider fitting a lighter than stock rear bar. Too much body roll? Consider using a heavier front bar, perhaps in conjunction with a light rear bar.

 

Then again there may be someone around here who gone through the process and can suggest a definate setup.

[Drax240z]

Well lowering your front tire pressure will have the opposite effect to what you are looking for.

 

Try without the rear bar and see. If that creates too much understeer, you may be able to make the rear bar adjustable.

 

Also have you checked your tire wear/rear toe/etc. since you've lowered the car and put bigger wheels/tires on it? It might be worthwhile to measure your toe and see where you are. If you are toed out in the rear, that would be an issue... as would too much negative camber if you've lowered a lot. I've also noticed that when semi-trailing arm cars start to wear out their rear bushings, they really start to pile up on negative camber.

 

Anyway, take some measurements and then go from there...

[Synlubes]

IMO, I would not loosen or remove rear sway bar.

 

What brand of tire ? Cheap or high dollar ? The tread wear #`s and compound come into play.

Low pro 17" tires need to ride "flat", meaning all of the tread face (contact patch) need to be on the road.

 

Try playing with tire pressures come more and see if it gets any better ???

[Afshin]

thanks for all the input.

In terms of tires I am running Kumho 712 which are relatively new (about 3000 miles) and are strong peformers. to date the tire wear is even. the car was aligned a year ago and the rear toe was within specs (but under no load off course) I can't remember the camber but it did not raise any alarms at the time and my tire wear remains even.

My understanding was if the rear tire pressure is optimal, then having slightly low pressure in the front will decrease the effective front contact patch size under load reducing grip and hence in this scenario getting the car closer to neutral handling. Am i missing something?

Also has anyone played around with the rear camber and have any suggestions for a range to set when I get the new bushings and allignment?

[Dan Baldwin]

My understanding was if the rear tire pressure is optimal, then having slightly low pressure in the front will decrease the effective front contact patch size under load reducing grip and hence in this scenario getting the car closer to neutral handling. Am i missing something?

 

Reducing tire pressure INcreases contact area (contact patch area is roughly the load on the tire divided by the pressure). But that doesn't mean cornering grip is increased, usually the opposite with street tires at street-normal pressures. Maybe if you tried something like 32 or 34 in back and 30 or 28 up front, rather than reducing front pressures. Removing the rear sway bar might be a good thing to do if oversteeriness persists. Are the Eibachs progressive-rate springs? You might be better off, handlingwise, with linear rate springs, as that would reduce your initial roll rate in cornering, and reduce your total roll for a given level of cornering gs.

 

Don't be shy to remove the rear bar if other methods fail to fix your understeer condition.

[Afshin]

Dan, I know that under static condition (no cornering load) decreasing tire presure increase contact patch size, however I thought that a underinflated tire under load when cornering would role more and provide less pressure in the middle and inner side of the tire (eg low pressure tire wear more on the side and less in the middle).. which in effect would reduce the effective contact patch. If I remember correctly there is only about a 3-4 psi range for maximal tire grip/effective contact patch. So if one where to either overinflate or underinflate the tire grip would be reduced.

If effective tire patch size was to increase with lower pressure, then lower pressure would result in better handling and grip which is not the case .

In other words this is how I thought it thru :

less pressure=softer tire sidewall = more tire roll = less effective contact patch = less grip. (=better handling balance) Do you agree?

[Guest cbrunberg]

The link below has a good overview on handling characteristics. It may help you diagnose your problem and give you some input on treating it.

 

 

http://thundervalleyracing.com/features/tech/wolfsteer.html

 

Carl

[Afshin]

thanks for the link Carl. nice summary.

I also did some more homework by reading the section on oversteer in Bob Alexandre book "performance handling". According to him and the web site link lowering the front tire pressure as I thought could definitely help and not worsen the oversteer.

At this point I think I will loosen the rear sway bar first and give her a run, then I will give it a second run with lowering the front tire pressure a tad.

I will post an update after I get the chance to take her back into the twisties

[80LS1T]

Hey what kind of 17" Konig's do you have Afshin?

 

 

Guy

[Afshin]

Hi Guy

I got the Konig Imagines and I absolutely LOVE the way the look on the car (nice red new paint, similar to Cody's 8) ). I also put in the black and red aerospeed seats and an isotta steering wheel which look perfect in the car. I will try to get some pictures up soon. DO you have pics with the Konig Monsoon Rims (i looked at your web site) fully mounted?

[Drax240z]

Dan, I know that under static condition (no cornering load) decreasing tire presure increase contact patch size, however I thought that a underinflated tire under load when cornering would role more and provide less pressure in the middle and inner side of the tire (eg low pressure tire wear more on the side and less in the middle).. which in effect would reduce the effective contact patch. If I remember correctly there is only about a 3-4 psi range for maximal tire grip/effective contact patch. So if one where to either overinflate or underinflate the tire grip would be reduced.

 

Well you are right, there is an optimum pressure for each tire, and it is probably a range of about 10psi. This optimum range is when the tire sits flat on the pavement. This is best measured on a hot tire with a pyrometer.

 

Tires will lose grip faster as you lower the pressure from their optimum range, than they will by raising them the same amount from their optimum range. Lower pressures lead to higher slip angles, and more deformation as well.

 

If effective tire patch size was to increase with lower pressure, then lower pressure would result in better handling and grip which is not the case .

In other words this is how I thought it thru :

less pressure=softer tire sidewall = more tire roll = less effective contact patch = less grip. (=better handling balance) Do you agree?

 

I don't agree.

 

Lower contact pressure between the rubber and the road increases the effective "friction coefficient" by increasing the tire contact patch. (to a point) The reason I put friction coefficient in quotations is because it really isn't the correct term, as the grip of the tire is more due to the deformation of the rubber into the pavement, and the associated shear resistance of the rubber. (a cogging effect of the tire with the road)

 

I guess my point is that lowering your front pressure may or may not help. It depends where you are allready relative to the max grip. You might solve your oversteer problem by losing overall front grip, and then the car becomes balanced, but slower. Really, the best way to tune this is with a pyrometer/thermometer after a hot lap, and its what I recommed doing if you want to dial in the tire pressure.

 

However, loosening the rear sway bar is much simpler and predictible.

[Afshin]

I agree that loosening the sway bar is a better aproach than lowering tire pressure which is why I will do that first and then try playing with the air pressure. I just wanted to clarify that lowering front tire pressure would not worsen my oversteer as was stated earlier. I fully agree that it may or may not help (need to start somewhere).

You bring up a very valid point about using a pyrometer to fine tune the suspension since knowing which tires and what section of the tire is heating more or less... can provide all the info one needs to fine tune the suspension correctly. Unfortunately I don't have one, and as usual I'm already overbudget.

But then that never stoped me before , so does anyone know of a CHEAP (less than $100) pyrometer that works ? (I don't need the best, just cheap and moderately accurate/sensitive)

Thanks

[johnc]

Are you seeing this oversteer at an autocross, a racetrack, or on the street? Semi-trailing arm rear suspensions normally exhibit lift throttle oversteer which is a trait that is beneficial to faster track and autocross times. Sometimes this behavior is sudden so controllability is more difficult.

[80LS1T]

No I dont have any pictures yet because I am still trying to get my car driveable! (see my story in "I'm Telling Ya") After I get all the bugs out and it drives nice, stays cool, and doesnt give me too many problems......Im going to buy the ground effects from MSA and then paint the car. Then I will be buying tires for my new rims. Unlike most guys rims are my last prority(not talking about anyone here, Im talking about ricers! )

 

 

Guy

[TimZ]

I agree that loosening the sway bar is a better aproach than lowering tire pressure which is why I will do that first and then try playing with the air pressure. I just wanted to clarify that lowering front tire pressure would not worsen my oversteer as was stated earlier. I fully agree that it may or may not help (need to start somewhere).

You bring up a very valid point about using a pyrometer to fine tune the suspension since knowing which tires and what section of the tire is heating more or less... can provide all the info one needs to fine tune the suspension correctly. Unfortunately I don't have one' date=' and as usual I'm already overbudget.

[/quote']

 

I agree with Drax (possibly others) that there is a range of values that are optimim for tire pressure balance. Because of this, it kind of depends on more factors than we can accurately predict (i.e., where is your starting point - what pressures are you running currently, how does this particular tire react to the present loading conditions, how bad is the oversteer, etc). Really, I don't think that tire pressure is a bad way to fine tune your handling - I just wouldn't try to rely on it for making big changes in your oversteer/understeer balance.

 

Additionally, I would not recommend going too low on tire pressure, as bad things can start to happen if the pressure gets too low. For one thing, the tire will tend to heat up more quickly, and you could get some weird swings in performance under different conditions. For another, lower pressures can make it easier for the tire do de-bead under high lateral loads. Probably not an issue if you keep the pressures above say 28psi, but something to keep in mind. As Dan suggested, maybe try keeping the pressures the same on the front and increase the pressure in the rear a bit.

 

The question I have right now is how bad is the understeer currently? Do you need a big change or a little change? If it's just a little more oversteer than you would like, then I would think that completely removing the rear bar would be way too big of a change, unless your rear bar was really tiny (as in smaller than the stock bar). This would most likely get rid of your oversteer, but I'm guessing it would be a pretty sizeable jump into the understeer realm, especially if the front bar is larger than stock.

 

Also, where does the oversteer occur? Does it start when you are braking into a turn, or when you are powering out of the turn? There can be very different causes/cures for the two cases. Also, is it a 'snap' oversteer, or does the tail just start drifting out?

 

Finally, assuming you are looking for a smallish change and you have a safe place to test, is there some reason you can't go try some different tire pressure combos? This would be a much better approach than trying to get us to guess on the right combo, and it's free and only takes a few minutes to change. Just go 2psi or so at a time, and always evaluate in the same repeatable maneuver (same corner, same entry point/speed, etc). If you take a disciplined approach, you might be surprised at how little a pressure difference you can discern.

[TimZ]

One more thing - if you are running a larger front bar than stock, you do want to check the condition of the frame rails around the sway bar mounting points. On the early Z's these were just weld nuts on the inside of the bottom of the frame rails, and significantly stiffer bars tended to tear the frame rails apart after extended hard useage. I would imagine that the zx setup is the same in this respect. This would definitely reduce the effectiveness of the front bar.

 

If you find this to be the case, all is not lost - it is fixable. Not trivial, but fixable...

[Afshin]

thanks for all the input (and the very comprehensive response from Tim).

The oversteer that I am experience is mostly felt from entry in the turn and most pronounced at the apex of a sharp turn. the bothersome part has been that prior to the apex it has become more difficult to modulate the slide since it tends to give out a little unpredictably while maintaining the throtle. In terms of off throtle oversteer, it feel s no different than these cars always have and remains reasonably predictable.

The dynamics with braking in the turns is not very different than expected.

Actually one can even feel the rear of the car overturning as opposed to nicely following the front in moderate turns at slow speed with no sliding and without the suspension being maxed out.

In essence I am trying to fine tune a slight turn in oversteer (the car handles quite well, but it's still not "fully right".

I won't take the rear sway bar off just loosen it (stock bars, with good mounting points). I 'm thinking about buying adjustable end links for them.

For tire pressure I was going to start with 32 psi all around and then try 32 rear and 28 up front. I'm also thinking about getting the pyrometer. Can any of you share any practical experience/use with the pyrometers?

Also what are you experiences with the control arm bushings that allow for camber adjustement, do they do agood job of maintaining whatever setting you have or do they deform,slip...?

[Drax240z]

Pyrometers are pretty straightforward, just point and shoot.

 

Come in hot, and try to get the pyrometer on the tires asap. Record all values. Measure outer, center, and inner edge of tires. High temps in center, mean too much pressure. Low temps in center, mean too little pressure. If you have a hot inner edge, too much negative camber, hot outer edge, too little negative camber. Thats about the extent of my tire temp knowledge but I am sure a guy like John Coffey could fill in some more details. (Hint hint!)

[Afshin]

I did some more homework (OK, OK, I really did this during work) and found this company called long acre racing (I have never dealt with them). They have some very nice article on using pyrometers and tunning suspensions. They also provide a lot of information on they pyrometers (infrared vs probe tip), and sell both types of pyrometers ranging from $80 and up. Here is the link:

 

http://www.longacreracing.com/articles/art.asp?ARTID=16

 

Does anyone have a good reason (ie negative experience) not to get the cheapest model ? they all seem to accurate and perform the same function (there is a difference in temperature range)