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Minimum Sidewall/Wheel to Strut Clearance?

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Grant 302

basic and well known psychic
From the pic you posted:
Cornering distortion cropped and marked.jpg

I have the resting offset between the lip protector and the tread edge from the PSS 305 at ~0.8" based on Tire Rack data. The offset+deflection in the above picture is about 1.25" at the top and 1.84" at the bottom.

Almost 1/2" deflection at the top is very consistent with my findings and observations so far.

PSS 305.png
 

Norm Peterson

Corner Barstool Sitter
939
712
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
Super sports definitely move over 1mm at the top. This is my car with Super Sports 295/30-19 on 19x10 and 265/35-19 on 19x9 in front.
In post #5, they're 285/35-18's on 18x11 . . . about 20mm more tire on wheels a full 2" wider. Wheels a whole inch wider than what you're running 295's on.

I guess that means wheel width might be even more important of a factor than I've been giving it credit for.


Side note, in the very beginning I did have a tiny bit of rub when clearance on the RF was down around half a mm. Fabbing up a spacer from 0.025" sheet aluminum was all it took to make it go away.


Norm
 

Grant 302

basic and well known psychic
In post #5, they're 285/35-18's on 18x11 . . . about 20mm more tire on wheels a full 2" wider. Wheels a whole inch wider than what you're running 295's on.

The picture of my car on super sports is from Nov '2012.

I guess that means wheel width might be even more important of a factor than I've been giving it credit for.

That's what I thought.

There are 3 different wheel/tire combinations on my car that I've posted including 275 rivals on 18x11 apex.

You can see that they all are deflecting ~1/2" at the top. Rivals, Super Sports, R-s3. It's even worse with slicks.

I just think most don't understand how much the top of the tire moves.
 

Norm Peterson

Corner Barstool Sitter
939
712
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
There are 3 different wheel/tire combinations on my car that I've posted including 275 rivals on 18x11 apex.

You can see that they all are deflecting ~1/2" at the top. Rivals, Super Sports, R-s3. It's even worse with slicks.
I don't know what else to say, except that if I'd been getting anywhere near that much lateral movement at mid sidewall height - or only a quarter as much - after ten track days there'd be matching locations on the struts with no paint.

Though there is a little nick in one of them when the OE studs were in the process of letting go and the wheel wobbled enough to make contact. Story for another time, but the short version is that there aren't any OE studs anywhere on the car any more.


Norm
 

Grant 302

basic and well known psychic
I'm not saying that *yours* definitely are moving that much at the top tread. When I saw your pictures of your static setup, I was pretty sure it wouldn't contact the strut body like that. The section near the lip protector doesn't move much regardless, and it looks like way more than 1/2" of clearance at the tread edge. JMO, and why I 'liked' that post when I read it. Looked good to me.

And like I said before, near 0º dynamic camber there will be more travel at the top. If your setup has enough dynamic negative camber that would also explain having less travel than I have seen in my previous examples. Without pictures of your setup in action, I wouldn't know. I certainly believe you that you aren't having contact issues.

And to be clear, I'm not saying all tires and setups will move that much at the top. Just the ones I posted (and re-posted for the yellow 350). I have also used a set of 285/265 18 R-s3 that are XL sidewalls which do not deflect nearly as much based on track pictures.

The OP's RS4 275s, Super Sports, some SL and LL rated, and other soft sidewall tires can and will deflect depending on the setup.
 
I think the OP has all the info he needs on this subject.
 

Norm Peterson

Corner Barstool Sitter
939
712
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
I'm not saying that *yours* definitely are moving that much at the top tread. When I saw your pictures of your static setup, I was pretty sure it wouldn't contact the strut body like that. The section near the lip protector doesn't move much regardless, and it looks like way more than 1/2" of clearance at the tread edge. JMO, and why I 'liked' that post when I read it. Looked good to me.
Thanks, Grant.

Quick and dirty measurements give the shoulder at least 0.8" more static clearance (mid-sidewall height about 0.2" more).

And like I said before, near 0º dynamic camber there will be more travel at the top. If your setup has enough dynamic negative camber that would also explain having less travel than I have seen in my previous examples. Without pictures of your setup in action, I wouldn't know. I certainly believe you that you aren't having contact issues.
It's still a street-driver first, so I'm pretty sure the outside front camber goes slightly positive just based on static camber, roll rate, and differential vertical tire deformations. This is the best picture I have, where the front looks slightly more positive than the rear (which I'm estimating is at roughly +0.3 to maybe 0.5 degrees positive due to tire effects).

T3 between entry and apex.jpg


The OP's RS4 275s, Super Sports, some SL and LL rated, and other soft sidewall tires can and will deflect depending on the setup.
This discussion just might be what it takes for me to put a little more effort into something I started a few years ago for deflected shape.

2853518 deflected shape.jpg


Norm
 
Last edited:

Grant 302

basic and well known psychic
I thought we were done with this?

I was thinking this wasn't done either.

Not quite.

Most PSS tires are XL (for the matter of sidewall/lateral stiffness), and I'm not one to start a new thread for a thought so closely related to this one..


Norm

They are XL, but their construction uses a very soft sidewall. I do think this is why they are stretched in the OEM GT350 application.

One of the things to consider in your model is the amount of camber and the stiffness of the cap plies. The woven structure of the aramid polyamide in PSS helps maintain the cylindrical shape of the whole tread surface and resists 'ballooning' to some extent.
 

Norm Peterson

Corner Barstool Sitter
939
712
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
It's a very crude model, for illustration only at this stage. Plus, I was too lazy to remove the 0.2" vertical deformation.

FWIW, I've come to define 'stretched' and 'squeezed' or 'pinched' wheel width fitments relative to the tire size's measuring width wheel rather than by subjective appearance (for which there is no consistent, definable basis). Also, tires that claim relatively narrow tread widths tend to look more 'stretched' than tires with wider tread widths.


Norm
 

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