HELP!! TREAD width vs SECTION width on Cup 2s

[Dinosgt]

Folks:

I have a very perplexing problem with some Michelin Cup 2s. I had a set of forged Rotiform wheels made for my car (20 x 9.5 FR and 20X11 RR), with a rear offset of 55 mm. My desire was to also try the Cup 2s, and the “engineers” speced out a 305/30/20 to go on the rear. Ergo – my problem:

Once installed, and the car on the ground, the tire stick out of the fenders about 4 mm (FWIW the rear is centered with the panhard). I don’t want the tires to stick out of the fender, and more so, no rubbing or other interference.

Taking a closer look at the tire spec - The tread pattern on the 305 N01 Porsche tire that I have is 12.2 inches on a 12.3 inch section width. So - its very “square” in its profile. Interestingly, the Cup 2s also come in three “305/30” sizes with differing TREAD widths. They are:

• The Audi tire at 11.2” TREAD WIDTH
• The Ferrari tire at 11.7” TREAD WIDTH
• The Porsche tire at 12.1 TREAD WIDTH

Again - They ALL have the same section width of 12.3”. So it seemed obvious to me that I should just spec the Audi tire with the 11.2 inch TREAD Width and get the tire inside the fender.

HOWEVER, since they all have the same section width at 12.3”, will it really come inside the fender? I also looked at the 295/30 size, with a section width of 11.9” and a TREAD of 11”, but I am afraid it will look like a rubber band on the rear. (Its only 27 inches tall, vs the 285/35 on the car originally at 27.9”).

Most frustratingly, my tire guy has been useless (i.e. no help), I called Michelin directly and got nothing; Talked to Tire Rack and Discount tire, and basically got no help either.

My question is simple, does the profile of a tire with a 11.2 inch tread have different “look” than one with a 12.1 inch TREAD? I.e. is it more “rounded” than the 12.1 inch tread?

I can’t find anyone that has a set of these to compare the two.

Help! I am beyond frustrated with this, especially in the fact that the guys who claim to be in the tire business can’t seem to help. I have seen lots of threads about 305s on an inch wheel, and in many cases with less Offset – so - how the heck is that happening without the tires sticking out?

Many thanks for any help that the folks here can provide

 

[Catmonkey]

Let me say this, I have never seen Michelin specify a tread width. Anything you get from TireRack is based on some other basis that's explained on their website, which to me is kind of hokey because it's based on a curved measurement. I'd take those measurements with a grain of salt. I'm running SVTPP wheels widened to 11". With the added width, they're at a +69mm offset. I run a 1/4" spacer which brings the offset to +63mm. I' have a 325/30-20 Michelin Sport Cup 2 mounted on the car now and the cross section is even with the outer fender. I've verified this with a straight edge.

Prior to the 325s, I was running a 315/35-20 on the same rim with a custom 8mm spacer. The 315's were 1" taller. Interestingly enough, both sets of tires measure 12.6" at the cross section mounted on the same 11" rim. The tread width is definitely wider on the 325s. I'm running a watts link which is less forgiving than the panhard. Using your specs overlayed with my specs, it seems to push your cross section only 2mm out further than mine. Unless your car is really low, I can't see them rubbing on the outer fender.

Here's how you can verify whether the fender can contact the tire. What I have done with mine, is to remove the rear springs and jack the axle up to where it's literally sitting on the bump stops. In doing so, you can see just how much clearance you have before any contact. It's not going to get any closer than what you see by doing this.

 

[Dinosgt]

Let me say this, I have never seen Michelin specify a tread width. Anything you get from TireRack is based on some other basis that's explained on their website, which to me is kind of hokey because it's based on a curved measurement. I'd take those measurements with a grain of salt. I'm running SVTPP wheels widened to 11". With the added width, they're at a +69mm offset. I run a 1/4" spacer which brings the offset to +63mm. I' have a 325/30-20 Michelin Sport Cup 2 mounted on the car now and the cross section is even with the outer fender. I've verified this with a straight edge.

Prior to the 325s, I was running a 315/35-20 on the same rim with a custom 8mm spacer. The 315's were 1" taller. Interestingly enough, both sets of tires measure 12.6" at the cross section mounted on the same 11" rim. The tread width is definitely wider on the 325s. I'm running a watts link which is less forgiving than the panhard. Using your specs overlayed with my specs, it seems to push your cross section only 2mm out further than mine. Unless your car is really low, I can't see them rubbing on the outer fender.

Here's how you can verify whether the fender can contact the tire. What I have done with mine, is to remove the rear springs and jack the axle up to where it's literally sitting on the bump stops. In doing so, you can see just how much clearance you have before any contact. It's not going to get any closer than what you see by doing this.

Catmonkey:

Really appreciate the reply. I have read all of your past posts/threads on this, and in fact, your wheel exercise was an inspiration for my game plan. Armed with your experience and some fitment measurements, I estimated somewhere between 56-60 mm offset for the rear, but the "expert" I was dealing with assured me that the 55mm offset was perfect, and that there would be no interference. Sigh....

So now I am stuck with boat anchors - OR - I have to find a tire that works.

Thanks for the input on the tire rack data - even Michelin told me they didn't have that data, and so I wondered about the tread measurements too. I am guessing my theory about the Audi tire at 11.2 TREAD width isn't a real solution, because the Section width means it will still stick out of the fender. I really dislike that look (not knocking anyone else's preference mind you), so I gotta find a tire that works.

Sounds like I am down to a 295/30 or the 285/35 in the Cup 2s. I am loathe to give up the tread width (I measured 12" on the 305s vs. 9.5-.7" on the stockers with a straight edge), but according to the tire rack team, I get another .5 inches of tread with Cup 2s even in the 285 size (dubious, I know...).

Regarding clearances - I have about 2 inches of clearance with my dropped stance from tire to fender (vertical clearance). I have the Eibach pro kit that drops the rear about 7/8 and the front less so on the TP cars), so I guess I have got try your method to look for interference.

My gut is to go with the 295s for the added TREAD, but fear the rubber band/4X4 look. Still looking at pics on the net to see if I can live with that look.

Sorry for the rambling, just really frustrated - but again, thanks...

 

[Catmonkey]

Unless you've cut your bump stops or installed shorter ones, I bet you don't have a lot of downward travel. Two inches is a lot of clearance even, with shorter bump stops. I bet you'd be surprised how much clearance you have. A 30 aspect ratio makes for a fairly short tire. If you've changed out your springs, you know how easy it is to swap them out. Just unbolt the bottom of the shock to remove them. Also check your bump stop clearance at ride height. With stock bump stops and being lowered, anything less than .5" will make for a harsh ride. Maximum Motorsports has shorter bump stops. You can also trim the stockers.

 

[Dinosgt]

Unless you've cut your bump stops or installed shorter ones, I bet you don't have a lot of downward travel. Two inches is a lot of clearance even, with shorter bump stops. I bet you'd be surprised how much clearance you have. A 30 aspect ratio makes for a fairly short tire. If you've changed out your springs, you know how easy it is to swap them out. Just unbolt the bottom of the shock to remove them. Also check your bump stop clearance at ride height. With stock bump stops and being lowered, anything less than .5" will make for a harsh ride. Maximum Motorsports has shorter bump stops. You can also trim the stockers.

Again - many thanks for the reply.

Although my geriatric arse cant do the springs this evening (maybe a weekend deal), I climbed under there and made a few measurements. My rough eyeballing (couldn't get calipers in there) shows about 18 mm clearance for the bump stop before contact. I also have about 52 mm of fender clearance before I hit the outer tread area of the tire. My question - do you know roughly how much compression will I get from the stock bumpers?

Also, it looks like these suckers are REAL close to the inside wheelwell area (the carpeted part).

Anyway - I guess I have three options:

1) Try the Audi tire with the 11.2" tread (but you pointed out the measurements of TREAD width are probably nonsense - however I measured my tread and it came in as advertised)

2) Go to the 295/30 with a 11 inch tread and a 11.9" section width (would bulge less and fit nicely I think) - but probably too short?

3) Go back to the 285/35 and get my tire HEIGHT back, but get little extra tread over the Goodyears

Thoughts?

Final question - If I chose (3), I wonder if the compound would make up for the TREAD width I didn't get over the Goodyears?

THANKS again sir.
Dino

 

[Catmonkey]

My bump stop compressed to about half it's length. I would remove that inner liner. Mine is long gone. Are the 305s already mounted on the rims?

 

[Dinosgt]

My bump stop compressed to about half it's length. I would remove that inner liner. Mine is long gone. Are the 305s already mounted on the rims?

Yes - mounted and balanced. They appear to bulge about 6 mm from the rim face. With a straight edge I measured about 5 mm to the fender edge. There appears to be about 4mm if thickness in the fender - so, if I wanted it tucked (for a future Airlift potentially) I need another 9 mm.

I told these guys all of that, but here we are...

I don't understand how some folks get away with 305s on these cars, especially with the Alcoa and others with 44mm offset ... I am perplexed.

If the bumper compresses half its length, that means it will go another 25mm or so, which I think will clear without rubbing.

Just am not sure if I should take the chance.

Thanks again catmonkey.

 

[Catmonkey]

I don't understand how some folks get away with 305s on these cars, especially with the Alcoa and others with 44mm offset ... I am perplexed.

If you widen an Alcoa to 11.75" or 12", you end up with about the same offset I'm running with spacers. On an unaltered wheel, there's no way it won't rub. In fact Shelby recommended a 275 for the rear with those wheels for that reason. The offset on the Alcoas on a 20x10 was +37mm. I understand the replicas have the same offset.

Now if you're running really short tires, like 20s with a 30 aspect ratio or 19s with a 35 aspect ratio, you can probably get away with a wheel/tire combo that extends beyond the fender lip because the bump stop prevents contact between tire and fender. That's the main reason I recommend removing the springs to find out. Otherwise, I have to wonder how hard others use their cars.

Our roads are crap. I can't get away with it. Let us know how it turns out.

 

[Dinosgt]

If you widen an Alcoa to 11.75" or 12", you end up with about the same offset I'm running with spacers. On an unaltered wheel, there's no way it won't rub. In fact Shelby recommended a 275 for the rear with those wheels for that reason. The offset on the Alcoas on a 20x10 was +37mm. I understand the replicas have the same offset.

Now if you're running really short tires, like 20s with a 30 aspect ratio or 19s with a 35 aspect ratio, you can probably get away with a wheel/tire combo that extends beyond the fender lip because the bump stop prevents contact between tire and fender. That's the main reason I recommend removing the springs to find out. Otherwise, I have to wonder how hard others use their cars.

Our roads are crap. I can't get away with it. Let us know how it turns out.

Again - thank you for all the feedback.

I was an debating whether or not to live with it. I doubt they will rub, but not because they are a proper fit, but because the bump stops save the day.
I REALLY wanted to run the cup 2s - I had them on a Lexus GS F and loved them.

I just really dislike the look w a tire that isn't tucked. A different profile of sidewall to Tread transition might work better, but then I give up on the cup 2s. Or I go to 295s.

BTW How do like your cup2s?

Thx sir

 

[Catmonkey]

They look nice. I should have my car back on the road very soon. I attempted to make a set of Grigg's coil overs work out back, but there just isn't enough room, so I had to regroup. That and a bunch of other cooling and engine related upgrades.

 

[V8Flexin]

I'm confused on this post. Are you trying to get the tire or the wheel to line with the fender?

Sent from my PURE XL using the svtperformance.com mobile app

 

[Dinosgt]

I'm confused on this post. Are you trying to get the tire or the wheel to line with the fender?

Sent from my PURE XL using the svtperformance.com mobile app

Just trying to get the tread profile to tuck inside the fender. I think the design of the cup2s, with a square profile, makes them tougher than other brands to tuck with a 305, even with proper offset.

The tread sticks out past the fender edge by about 4-5mm.


Sent from my iPhone using the svtperformance.com mobile app

 

[Robert M]

The thing that most people don't realize is that tires of the exact same numerical size are different between the different brands. The reason is because the method of "tread" measurement is not standardized in the tire industry. Some manufacturers measure their tread all the way around the corner where the tread ends (partially on the sidewall inside and outside) and some measure their tread as only what contacts the surface. 1" or more difference can be a result of these two different areas of tread measurement on a tire................and then the sidewall is a percentage of that, so tires are not all going to be equal in width and height when the tread width measurement is not standardized.

The internet tire calculators?...............a very general guideline, but useless for custom fitting a tire/wheel combination to a car.




R

 

[Dinosgt]

The thing that most people don't realize is that tires of the exact same numerical size are different between the different brands. The reason is because the method of "tread" measurement is not standardized in the tire industry. Some manufacturers measure their tread all the way around the corner where the tread ends (partially on the sidewall inside and outside) and some measure their tread as only what contacts the surface. 1" or more difference can be a result of these two different areas of tread measurement on a tire................and then the sidewall is a percentage of that, so tires are not all going to be equal in width and height when the tread width measurement is not standardized.

The internet tire calculators?...............a very general guideline, but useless for custom fitting a tire/wheel combination to a car.




R

Agree. Even Michelin couldnt/wouldn't give me a tread width - Only section width.

Ultimately, although I've seen folks with 305s that fit under the car with my offset, the Michelin are too "square"; in that the tread basically runs all the way across almost horizontally, with little rounding. Other brands, as you point out, are rounder, and although their measurements deceptive, mean they will tuck better.

I just lament the fact that I trusted these guys to make me a custom wheel/tire combo that worked for my spec - and they failed, expensively...

And I get to do the R&D now to fix it.

...Will probably go with the 295s and see how they work out.




Sent from my iPhone using the svtperformance.com mobile app

 

[Dinosgt]

Agree. Even Michelin couldnt/wouldn't give me a tread width - Only section width.

Ultimately, although I've seen folks with 305s that fit under the car with my offset, the Michelin are too "square"; in that the tread basically runs all the way across almost horizontally, with little rounding. Other brands, as you point out, are rounder, and although their measurements deceptive, mean they will tuck better.

I just lament the fact that I trusted these guys to make me a custom wheel/tire combo that worked for my spec - and they failed, expensively...

And I get to do the R&D now to fix it.

...Will probably go with the 295s and see how they work out.




Sent from my iPhone using the svtperformance.com mobile app

Update - put the car up in the air and contorted the axle with weird loadings (one wheel loaded, etc.).

It APPEARS that the tire finds it way under the fender due to the rotation of the axle - however I cant rotate the wheel because its loaded, to see if I have contact inside the fender with the bump stop bracket.

Also, it appears that the bump stops wont let the wheel contact the fender - they have about 3/4 inch gap (resting) and the OEM stops dont seem to allow much travel past that (.5 inch maybe?)

So, it APPEARS that they wont rub on the fender - but only because the bump stops prevent it.

Questions for the collective wisdom:

1) Does anyone know if an 11 inch rim with a 55 mm offset is pretty much guaranteed to rub on the bump stop bracket once unloaded? I know that's a weird loading, but if the rear unloads at speed due to a undulation, will it rub?

2) is it the RIM that rubs or the TIRE (in folks' experience)

3) Would an 10.5 inch wheel been preferable for someone who doesn't want to modify anything out back?

4) How crazy is it to try the 295/30?

5) Do Michelins run wide on TREAD, which is why others can get 305s on the back and I cant?

Again - many thanks to those who have engaged on this. Appreciate it.

 

[V8Flexin]

Agree. Even Michelin couldnt/wouldn't give me a tread width - Only section width.

Ultimately, although I've seen folks with 305s that fit under the car with my offset, the Michelin are too "square"; in that the tread basically runs all the way across almost horizontally, with little rounding. Other brands, as you point out, are rounder, and although their measurements deceptive, mean they will tuck better.

I just lament the fact that I trusted these guys to make me a custom wheel/tire combo that worked for my spec - and they failed, expensively...

And I get to do the R&D now to fix it.

...Will probably go with the 295s and see how they work out.




Sent from my iPhone using the svtperformance.com mobile app

The cup2's are pretty much a track/race tire for the street. A square tire is design important for minimized side wall deflection during cornering. But you will suffer in ride quality due deflection of the sidewall is necessary to absorb road shock just like a thick sidewall vs thin. Anyway, if you're looking for a tire that's more rounded you want to look at a more comfort biased tire like the Continental extreme contact's. The MPSS are somewhat in between with a more square design. You could like you said go with a thinner tread and let the bead stretch the tire to the rim (think stance style) to get the cup2's to tuck some. But the cornering performance and stability while cornering will suffer. You can try mounting the bare rim on the car with it supported by the rear axle to simulate the rear axle at ride height and take a straight edge and check the rim to fender clearance before mounting the tire. You can add shims to get the desired clearance. I'm not sure if you going for looks or actual track performance but the first number of the tire size is the tread width usually measured from where the tread grooves start and end (inner/outer) in millimeters. You can convert that to inches and compare that to actual rim width. The wider the tire vs the rim, the more square/concave the tire will look on the rim and vice versa.

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[Pribilof]

20x11 +56 here. I didn't test but it appeared that the rim would hit the bracket while unloaded. I cut that bugger out and used the relocating bracket for the bump stops. Took like 15 minutes with a grinder and some Rust-Oleum.

Based on what I've read in prior threads on here, you'll have to do some modifying with an 11" rear wheel. That's a lot of additional width to jam under there.

 

[Catmonkey]

I don't think an 11" wheel will make contact with bump stop bracket, but I'm not sure how much wider you could go before it becomes a possibility. I know when I first mounted my 315/35-20s, I still had the bump stop brackets in place. I had the wheels powder coated and the powder coating caused a very tight fit between the wheel and the axle flange to where I couldn't take the wheel off. I had to let the axle droop, to pop them off and the bump stop bracket only contacted the tire's sidewall. Considering my wheels would be 14mm closer to the bump stop bracket, I don't see an issue. The sidewall on the other hand is likely to rub on a 305, but it should be easy to determine. Jack up the rear end and let the axle drop down. It might not with a 295 and shouldn't with a 285.

 

[Dinosgt]

The cup2's are pretty much a track/race tire for the street. A square tire is design important for minimized side wall deflection during cornering. But you will suffer in ride quality due deflection of the sidewall is necessary to absorb road shock just like a thick sidewall vs thin. Anyway, if you're looking for a tire that's more rounded you want to look at a more comfort biased tire like the Continental extreme contact's. The MPSS are somewhat in between with a more square design. You could like you said go with a thinner tread and let the bead stretch the tire to the rim (think stance style) to get the cup2's to tuck some. But the cornering performance and stability while cornering will suffer. You can try mounting the bare rim on the car with it supported by the rear axle to simulate the rear axle at ride height and take a straight edge and check the rim to fender clearance before mounting the tire. You can add shims to get the desired clearance. I'm not sure if you going for looks or actual track performance but the first number of the tire size is the tread width usually measured from where the tread grooves start and end (inner/outer) in millimeters. You can convert that to inches and compare that to actual rim width. The wider the tire vs the rim, the more square/concave the tire will look on the rim and vice versa.

Sent from my PURE XL using the svtperformance.com mobile app

Thanks - I was going for some improved looks (a tire more flush to the wheel well, but I specifically asked for no rubbing, interference or mods necessary ), BUT more importantly, I wanted more grip, as do all GT500 owners... ;-)

Regarding the TREAD width, are you saying that all OEs measure from the actual contact patch or it varies? Oddly, the Goodyears have only about 9.7 inch or usable TREAD (from what I measured), but they tread goes over the edge a bit. Tire rack seems to measure it the same way - but earlier posts point out I cant rely on the posted data for comparison

20x11 +56 here. I didn't test but it appeared that the rim would hit the bracket while unloaded. I cut that bugger out and used the relocating bracket for the bump stops. Took like 15 minutes with a grinder and some Rust-Oleum.

Based on what I've read in prior threads on here, you'll have to do some modifying with an 11" rear wheel. That's a lot of additional width to jam under there.

I am going to dismount the tire and check it out, but Catmonkeys experience is that with my offset the RIM probably wont rub. I am most concerned with metal to metal contact, which I can't change since there is no room for spacers - the wheel face is almost exactly flush with the wheel well. And I really don't want to remove the brackets. Had the folks who sold me this combination had a clue what they were doing, they would have recommended a different size/width/offset given my expectations.

I would post pictures but I don't have a clue how to do it.

I don't think an 11" wheel will make contact with bump stop bracket, but I'm not sure how much wider you could go before it becomes a possibility. I know when I first mounted my 315/35-20s, I still had the bump stop brackets in place. I had the wheels powder coated and the powder coating caused a very tight fit between the wheel and the axle flange to where I couldn't take the wheel off. I had to let the axle droop, to pop them off and the bump stop bracket only contacted the tire's sidewall. Considering my wheels would be 14mm closer to the bump stop bracket, I don't see an issue. The sidewall on the other hand is likely to rub on a 305, but it should be easy to determine. Jack up the rear end and let the axle drop down. It might not with a 295 and shouldn't with a 285.

Thanks again - but I don't follow completely. Are you saying that because of the width the wheel, it couldn't be mounted on the hub unless it was in full droop?

I noticed that under full droop, there is probably 2 mm of clearance or so (e.g. I could fit a tie wrap back there between the tire and the body). However, with weird loads (one side loaded), the axle seemed to move enough to cause contact.

Now I am debating going to a decent tire shop that is willing to work with me by trying several fittings - Or this may end up being a never ending do loop...

Thanks to all - really debating now the 295/30 or the 285/35. Problem is that the 295 is VERY short (will it look silly?), and the 285 is on the widest recommended rim (will it look too stretched? AND I got very little extra TREAD width form the OEM goodyear (9.7 vs 10.1 according to Tire racks method)

THANKS

 

[Catmonkey]

Thanks again - but I don't follow completely. Are you saying that because of the width the wheel, it couldn't be mounted on the hub unless it was in full droop?

No, just the opposite. It's only when the axle is at full droop that the bump stop bracket and the wheel and/or sidewall are on the same plane

I noticed that under full droop, there is probably 2 mm of clearance or so (e.g. I could fit a tie wrap back there between the tire and the body). However, with weird loads (one side loaded), the axle seemed to move enough to cause contact.

That's not bad. In order to have the axle droop to that point in an actual driving experience, there shouldn't be any load on the axle, so I wouldn't think it would get much closer. It's not like the bracket has any sharp edges.