THSCC Forums

....moving from my gas tank question.....

I had posted a note stating that I now suffer from over steer on the 328 after installing a larger 28mm front bar. My question is to Keith, Mike and any other bimmer folks out there. Since the car is now oversteering, what advice to you have to make it more neutral? The car felt great with the front end planted, just un-nerving to have to fight the rear end from coming around on you.

Any advice? Is the fix as easy as tire pressure adjustments? Everything on the front end is new (from a bushing standpoint) while the rear components have 111k on it. Since there is technically very little I can do for the rear, what solutions do I have?

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Rodney

'08 Bullitt mustang, CAM 7
Autox VP '09-'10, President '11-'12, interim President 2nd half of ‘14
proud recipient of the Bowie Grey service award '12
Now just a guy driving a mustang....

1. Learn how to drive it.

Most people that have driven my car think it is terribly loose. Every time I have made it a little wilder, I think it's undriveble for the next few events. Then I adjust to it and get faster. And I am about to make it a *little* looser by going up in rear rates 50#.

Having not driven your car, I can't say whether it's beyond uncontrollable. I would doubt it with the stock suspension and just a bar upgrade. I'd love to do a fun run and give you some feedback though

So give it a few events first.

2. Make sure there is NO toe out, front or rear. I'd suggest 0 toe all around. But don't change it until #1 is done! Yes, it's adjustable in the rear.

3. Dial in some rear camber. Yes, it's adjustable. Same comment about #1.

4. Finally, check tire temps. If it's "really" oversteering, rear temps would be much higher. That could be from excessive wheelspin though, but that takes a different approach to correct (ie fix the driver).

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Mike Whitney
whit32@gmail.com, 919-454-5445
V10, V8, V8t, I6, I6, V6, F4t, I4, I4, I4, I4, I2, 1, 1

Thanks Mike, here's my feed back....


1. Learn how to drive it.

Most people that have driven my car think it is terribly loose. Every time I have made it a little wilder, I think it's undriveble for the next few events. Then I adjust to it and get faster. And I am about to make it a *little* looser by going up in rear rates 50#.

Having not driven your car, I can't say whether it's beyond uncontrollable. I would doubt it with the stock suspension and just a bar upgrade. I'd love to do a fun run and give you some feedback though

So give it a few events first.

OK, I'll give it a shot. That was the first event I drove w/ the new set up. The car is not uncontrollable, just a little unsettling when the rear end wanted to come around so quickly in the slalom. You're welcome to drive the car. I'd appreciate your feedback.

2. Make sure there is NO toe out, front or rear. I'd suggest 0 toe all around. But don't change it until #1 is done! Yes, it's adjustable in the rear.

Done before I hit the track. Just had the car aligned. The rear is dead on spec w/ about .4 degree toe in and .2 toe in for the front.

3. Dial in some rear camber. Yes, it's adjustable. Same comment about #1.

Done. After the alignment, it's sitting at -2 degree negitive camber.

4. Finally, check tire temps. If it's "really" oversteering, rear temps would be much higher. That could be from excessive wheelspin though, but that takes a different approach to correct (ie fix the driver).

I agree the driver needs to be fixed. I adjusted the tire pressure on my last run at 40psi all around and had my fastest run.

I think an Evolution school is in my future...

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Rodney

'08 Bullitt mustang, CAM 7
Autox VP '09-'10, President '11-'12, interim President 2nd half of ‘14
proud recipient of the Bowie Grey service award '12
Now just a guy driving a mustang....

Rodney,

My initial thought was maybe you had too little rear toe (or toe out), but that's definitely not the case if you're at the stock setting. Based on your e-mail, I take it you're staying in Stock class, and, as you said, there's nothing more you can do to the chassis that's Stock legal. Do you have access to a tire pyrometer? If so, you might try adjusting rear tire pressure for more grip, assuming it's not already optimized. If you're running on OEM shocks, a change to adjustables (Koni yellow or Tokico Illumina - beware the Koni front "struts" are inserts which require gutting the OEM strut bodies) might give you enough adjustability to "fix" the oversteer. The other approach is to sell that new front sway bar and replace it with one that's adjustable - it's hard to say whether a stiffer or softer front bar would reduce oversteer at this point, depends on how much of your front suspension travel you're using at max turning load.

Keith

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Keith Q.
2008 Top Gun
http://www.youtube.com/watch?v=KmX8XuZ_DCo&NR=1

I agree with Mike. Loose is good in autocross . . . if you can detect pending oversteer in time to adjust your speed adequately. Loose in a slalom is preferable to tight elsewhere, especially turns longer than 90 degrees. Loose is easy to deal with and frequently is actually faster (Mike certainly demonstrates this). If you are used to tossing a car then a little loose can certainly feel like Very loose. Mike's stiffer rear springs may not actually loosen the car, by the way, if the change in suspension control compensates for the increased rear roll stiffness. A big improvement may be reduction in any corner exit "power push" due to reduced rear squat and subsequent raising of the front. Stiffer rear springs may also improve rear acceleration grip by keeping the rear tires "flatter". Conversely, if Mike uses rear "squat" while accelerating to improve rear grip after turning into a corner (due to weight transfer or more outside tire negative camber), then the car may actually become too loose . . . more from a suspension dynamics perspective than from a roll stiffness distribution perspective. Lots of interrelated factors . . . especially with a street based suspension.

Dick

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Dick Rasmussen

FS 50 2018 Mustang GT

I suspect that I will be truly changing LLTD more than any of those other factors, considering that I will be going from 600# -> 650#. So I firmly expect the same change in car characteristics that I had when I went from 550# -> 600#. Slightly more oversteer in transitions, less understeer in tight stuff, and more toward neutral under power in sweepers.

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Mike Whitney
whit32@gmail.com, 919-454-5445
V10, V8, V8t, I6, I6, V6, F4t, I4, I4, I4, I4, I2, 1, 1

Rodney,
I used to run a stock Datsun 510 with a very similar suspension design to your BMW. Back then I put Bilstien shocks on it. They helped somewhat with trailing throttle oversteer. But the biggest help was to be smooth with steering inputs when turning into a corner. I also ran about 5 lbs. less air pressure in the rear than the front tires. As Dick said loose is good at an auto-x. A quick stab of the brakes and a flick of the wheel and my old car would dance through a Chcago box very quickly. At Rocky Mt. the workeres would be running for the box thinking that I was sure to eat the center cone and I did most times. It also caused a few entertainig spin outs too.

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George Bright Member # 141
One of the youngest of the old farts.
Yes I voted against you joining the club.

Mike,

What does LLTD stand for? (I suspect I should know ). Also, when you say more neutral under power in sweepers, was it too tight or too loose? If it was too tight then your car certainly isn't too loose from my perspective The only car I ever had that pushed in sweeper was my Stock Lotus Europa. I fixed that when I went to Street Prepared!!!!

Obviously Mike has a really good understanding of handling variables in general and of his car in particular. For you "rookies" at this stuff, another thing that stiffer springs permit is tuning with ride height since the car can stay off the bump stops at a lower ride height with stiffer springs. FYI with cars like my Formula Car, the issue with lower is keeping the underside of the car off the ground.

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Dick Rasmussen

FS 50 2018 Mustang GT

All that said, you put a bigger front swaybar on a Toyota Spyder and you end up with more understeer. I suspect that's a bit of a characteristic of mid-engine cars (or any car with a bad rear weight bias like the Spyder, anyway, which is 60/40 rear), though.

Also, as for "loose being fast", well, my Miata tends to push if you don't trail brake into corners and *make* it loose. Of course, we're talking about stock class where you can't monkey with springs. It's probably a "fast" car in that configuration given that a clone of it won two national championships and then *it* won three more, all in a row.

Yours in devils advocacy,


--Donnie

Donnie,

Actually given the constraints and tradeoffs of Stock moderate trail brake oversteer is what I was able to get with my 260Z (much crappier suspension than the Miata but the same goal). Like the Miata (I think) a big front bar was key for the Z for me so that I could actually do some trail braking without being too loose and could get the power down better. Worked very well. With my Europa the car pushed so much with the super soft rear springs that I didn't even consider a bigger front bar. When it first went to ASP all I did was change to stiffer rear springs and add a relatively soft rear bar. (ASP was a lot less "developed" in 1986 than it is now )

For you "rookies" out there, there are no magic answers . . . no matter what the class. You need to experiment and test. Make sure when you test that you also try changing your driving style and line to adjust to the changes to the car's balance and "needs". Also, consider the course conditions. I suspect that a "Sanford" setup may not be appropriate for a high grip site.

Yours in agreeing . . .

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Dick Rasmussen

FS 50 2018 Mustang GT

Lateral Load Transfer Distribution. Yeah, I thought it was some dumb acronym too when I first heard some Saturn guy talking about it, but now I agree it's the best way to embody the main variable in chassis tuning.

In English for the rest of you out there It's the proportion of how much weight transfer happens at the front versus the rear. If a lot of weight transfer happens at the rear, you overload a tire and oversteer. If a lot of weight transfer happens at the front, vice versa.

(Side note - this is the soapbox I get on when people talk about "fixing" handling problems with tire pressures or stagger. Note that when you change LLTD with stiffness at one end, you actually increase grip at the other. All other chassis tuning techniques decrease overall grip.)



I should have clarified. The car is power-oversteer loose in 2nd gear in sweepers (ie autocross). It tends toward slight understeer in full throttle 3rd gear corners (ie track). I am hoping to make the car neutral in 3rd gear high grip sweepers (track) and I am hoping to still be able to control the car at an autocross under power! We'll see ... it's an experiment

Anyone have 650lb 6" 2.5" springs to sell?

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Mike Whitney
whit32@gmail.com, 919-454-5445
V10, V8, V8t, I6, I6, V6, F4t, I4, I4, I4, I4, I2, 1, 1

Mike,

Thanks for the definition of LLTD. Yes I should have figured out what it meant. In "my day" the phrase "roll couple distribution" seemed to be more common.

Regarding the 3rd gear push problem . . . sounds like you need front downforce and/or lift cancellation . . . or a bunch more 3rd gear power

I've got a pair of 6" 650 springs . . . however they are holding up the front of my FF . . . and they are 2.25 inch, not 2.5 inch diameter

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Dick Rasmussen

FS 50 2018 Mustang GT