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Countering understeer with front wheel spacers: How much mm are necessary?

Old 3/19/19, 06:54 AM
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Countering understeer with front wheel spacers: How much mm are necessary?

Dear forum,

I have the following issue:

I have a Boss 302 MY 2012 and I track it regularily.
If possible, I am tracking it with a square setup of semi slicks, 265/40 R18 because I get these for free. With these tires, the car handles quite well considering the balance between understeer/oversteer.

The problem is that when weather conditions are changing between rain and dry (or if i cannot find anyone to haul my semi slick wheels), I usually use the original rims and tires, which is 255/40 R19 front and 285/35 R19 rear.
With this non-square setup, understeer is quite extreme, partly because I then run Good Year Eagle F1, but also because the setup is not square I believe.
The thing is: I do not want to optimize camber and adjustable sway bars for the original 19" set up because then I will be in trouble when using the square 265/40 R18 semi slick set of wheels.
Now, my idea is to put wheel spacers on the front wheels with the 19" rims because that's supposed to help counter understeer, right?

My question now is this:

How much width will I have to add until I see an effect? Or in what range? Should I start with 5 mm per side? Or will more track width be necessary to see any effect considering understeer?

Not only do I not want to buy all available wheel spacers from 5 mm to 40 mm. I also would have to change the wheel studs and I do not want to install wheel studs that are longer than necessary.
Using wheel spacers to adjust the balance between understeer and oversteer, I could leave the rest of the setup untouched and would not have the readjust everything with every tire change.

Thank you very much for any input from any fellow track day drivers out there!

Regards

Oliver aka Tomcat0815
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Old 3/19/19, 11:05 AM
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hmmm . . . I am really not sure that widening the track by using spacers will achieve the result you are hoping for; I am not familiar with that so hopefully someone who is, will chime in

A more direct way to address it, would be to use wider tires on the front; or narrower on the back. You mentioned that you have the "original wheels and tires" but I don't think those are the original tire sizes; I thought the Boss cars came with a square set-up; I am not sure but I think the original Boss wheels were 9" wide and they had 245 tires on all four.

If you were interested to consider changing tires, first take a look at the back of your wheels to confirm how wide they are. Then you can look up tires to see if they correctly fit that rim width. For a 9" wide wheel, the widest tire is generally about 255; for a 9.5" wheel, 275. If you currently have 285 tires on 9" wheels on the back, that might cause some undesirable handling.
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Old 3/19/19, 11:47 AM
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I am rather sure these are the original tire dimensions for a Boss 302. I checked on the car and double checked on the internet.

Copied from the Boss 302 manual:

Ą19 in. x 9.0 in. front, 19 in. x 9.5 in. rear cast aluminum wheels
255/40ZR19 front, 285/35ZR19 rear tiresď

Every handbook of how to adjust and optimize a cars suspension and handling states the influence of track width on oversteer/understeer balance as one factor.
The question is just how much width is necessary to feel a difference: 10 mm or 100 mm?

And like I said, finding road legal tires for road (not teack) use is close to impossible for a Mustang here in Germany. The car hasnít officially bern sold here, so the legal papers necessary for any different tires than the original ones donít exist.
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Old 3/19/19, 11:55 AM
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OK, that answers the wheels/tires question and I guess that won't help.

Originally Posted by Tomcat0815 View Post
. . . Every handbook of how to adjust and optimize a cars suspension and handling states the influence of track width on oversteer/understeer balance as one factor.
The question is just how much width is necessary to feel a difference: 10 mm or 100 mm?

hopefully someone else on here can answer that

Originally Posted by Tomcat0815 View Post
finding road legal tires ... is close to impossible for a Mustang here in Germany.... the legal papers necessary for any different tires than the original ones don’t exist.

Wait, you mean you have to get official permission from the government to change the tires on your car?

Last edited by Bert; 3/19/19 at 11:56 AM. Reason: correction
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Old 3/19/19, 12:14 PM
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Changing tires would definitely help and I appreciate your input.
But itís very difficult to legalize a tire/wheel combination you donít have the necessary papers for, usually provided by the carĎs or at least the tiresĎ or wheels manufacturer.
There is a mandatory bi-annual inspection which you wonít pass if you donít have the necessary papers. And if the cops catch you with these tires youíll be fined and have to turn up later with legal wheels and tires to prove your car is in a road legal condition again.
With other, less obvious mods I donít bother but tires ad wheels is the first I check.

Once the tires are worn out I might try going to 265 45 R19 on all four corners but not yet. The whole set would cost me about 1.000$.
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Old 3/25/19, 08:45 AM
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As I recall go kart racers often increase the front track in an effort to decrease understeer so in theory using spacers might work. The problem as I see it is that 5 mm wouldn't be anywhere near enough to make any real difference [ 10 mm as a percentage of the normal front track of most cars isn't much ] and going any thicker would IMO require longer high strength studs and open ended lug nuts for safety, and you said you didn't want to do that. Using spacers thick enough to make any real difference will also increase the scrub radius and put extra load on the front wheel bearings especially at high speeds with semi slick tires. If you have adjustable anti-roll bars and castor/camber plates then you may be able to get the balance you want and it would still be easily and quickly changed back again.
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Old 3/25/19, 09:10 AM
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@SilrBult thank you for your input.

I found out that removing and replacing the front wheel studs is pretty simple, also I already got a pair of 5 mm plates for free. So I am definitely going to try that, just for fun.

But maybe you are right and adjusting sway bars is the easiest since they are pretty accessible when one is changing the wheels anyway.
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Old 3/25/19, 12:35 PM
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Nothing ventured, nothing gained, so trying the 5 mm spacers is worth trying. In so far as installing longer studs is concerned, as you have found, it's not all that difficult. I've done it a few times now, both for front and rear. A hydraulic press will make things go much faster and easier, but it can be done without it. Good luck.
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