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View Full Version : Cobra R 17X7.5" 4.25pattern



Michael Mattix
05-12-2003, 03:55 PM
Guys/gals,
Will the 13" brake upgrade fit within the Cobra R 17X7.5" 4.25 bolt pattern? Thanks,

Mike

Duffy Floyd
05-12-2003, 05:25 PM
Yes...the issue for the 13" rotors is the diameter of the "hole" in the wheel where the rotor and calipers sit. Any width will work as far as brake upgrade clearance.

David Neibert
05-12-2003, 06:05 PM
Mike,

I would suggest 9" width on those wheels, because 7.5" won't accept a very wide tire.

David

Michael Mattix
05-13-2003, 09:26 AM
Thanks David. But, I thought that I read (somewhere) that you could get up to a 245/45 R16 on a stock SC (oh yeah, it was on tirerack.com and listed 26 different tires). So, I was assuming that I didn't necessarily need to go with a 9" rim, unless I wanted to go HUGE in the rears, but I don't think I really want to. And I also heard that the 9" wide rim sometimes has an interference (in the fronts) with the wheel well plastic. Looking at mine when the wheels were off, I can attest that there doesn't seem to be a pattern to it's design, and plus I have some chunks ripped off of the bottom (must have been something big that I hit).

The REAL question I have is how do I find out what the inside diameter is of the 17X7.5's to see if the 13" brake package will fit? Does anyone know? I don't think anyone sells these close as I would just go in there with a tape measure. lol
Any answers would be appreciated.

XR7 Dave
05-13-2003, 10:52 AM
Far as I know the issue is not with the inside diameter of the rim, rather it is clearance on the back of the face of the wheel. When I got my 17X8's custom made, they told me that I had to have 4 3/8" backspacing to clear the 13" calipers. (I had asked for 5" backspacing.

On some of the 17x9's, there is not enough backspacing to clear the big calipers. I believe that is what triggered GDiii to get custom wheels since the Cobra R's he had would not clear the big brakes without a spacer.

There are two different back spacings available on 17x9 Cobra R's. One I think is 4.75" and one is 4.25". I think the one is a Mustang offset and one is a Tbird offset. You need the Mustang (4.25") backspacing I think.

Hopefully I have muddied the waters enough that someone who actually knows will post and tell me I am right or wrong.......

As for rim width, that I do know about. The lower profile the tire, the less accomodating the tire is going to be regarding rim width. Also the acceptable rim width ranges listed on the tire manufacturer's listings don't always work out for best handling and/or wear. For example a 255/50/16 can go on a 7" rim according to the manufacture's listings. However, I can tell you from experience that most high performance 255/50's will wear really bad in the middle on a 7" rim due to casing distortion. 255/50/16's typically fit best on a 9.5" rim - the tire and rim combination that it was originally designed for - the Corvette.

Now not all tires are created equal! Even in the same size. I have sitting in my garage a 245/50/16 Pirelli tire that fits the 7" stock rim almost perfectly, and a 245/50/16 Firestone that is all bowed and messed up looking on the same rim. The Firestone is a FULL inch wider than the Pirelli! Using this same aproach I have some Michelin 255/50's on my stock rim and they look and perfect.

So getting to the point, on a 7.5" rim I see no reason why *most* 245/45/17's or even 255/45's wouldn't fit fine. However, it would be of benefit to compare different brands side by side. You will see the differences I am talking about. It is also usefull to check tire rack.com and go through the manufacture's specs and look at tread width's before choosing a tire. You do not want the tread width more than the rim width for good wear and handling characteristics.

Funny last note. Michelin does not provide the tread width specs for many of their tires. One of the reasons they do this is that if you have a high performance car, and you want the best possible traction, right? So you might shop for a wider footprint, right? They want you to trust them that even though Michelins tend to be narrower, they feel that they will handle just as well and ride better (what Michelins are known for!).

Another note: I originally purchased 245/45/17 Michelin's for my 17x8's. This is a perfect size match according to the books. The tire shop couldn't even get the tires to air up on the rims! They were just too narrow. So after trying for about 1/2 hour to get one tire to mount, we said screw it and put on 255/45's. Perfect fit! But like I said, Michelin's tend to be narrow, especially the Pilot XGT series.

Ok, I'm done now. :)

Michael Mattix
05-13-2003, 01:11 PM
XR7 Dave...take a breath...here's some gatorade! No really, THANKS for the info! Being on a tight budget and having quite a bit of time, I have been scoping ebay and parts BBS's for a "h#ll of a deal". And because my family is on one budget, I pretty much HAVE to wait for a "h#ll of a deal". Considering that I want to go with the 13" front brake upgrade, I want to find out what WILL fit, so that when I see it, I will try to get it. I don't want to purchase a set of wheels (new or not) now only to find out when I get the rotors and get them on (probably too far off), to find out they don't fit! I would be furious! So, what I'm basically hearing is that I need to know ALL specifics (backspacing, offset, inside diameter,etc.). If anyone knows the absolute minimum for all these "must know" items, I would appreciate it!

gldiii
05-13-2003, 10:18 PM
The issue with the Cobra calipers is the size of the caliper and the diameter of the rotor. I have not heard of any issues with Cobra "R" or Cobra "R" replica wheels in any bolt pattern, rim width or rim diameter having a problem going over the Cobra caliper. That is what they were made for!

Fitting over the caliper/rotor combo has nothing to do with backspacing. It takes at least a 17" rim to go over it. Additionally, you need clearance between the spokes of your wheel and the caliper itself. Not all 17" rims have enough clearance in this area and the spokes will hit the caliper!!!

I remember on the Baer Brake site they used to have a template that you could print, cut out and fit to a wheel to see if their calipers would clear. The Cobra calipers, Bullit calipers and even some Vette calipers are just fancy castings of the PBR 2-piston calipers.

But if you are looking at 17" Cobra "R" replicas in any bolt pattern or rim width you will be OK.

XR7 Dave
05-13-2003, 11:16 PM
George I thought you had some issue with your Cobra R wheels. Am I hallucinating?

gldiii
05-14-2003, 12:46 AM
David, the problem I had with the fitment was more with the tires than the rims. I had the 17x9 real Cobra "R" wheels with the 6.42" backspacing. When I ran 255/50/17 tires on those rims on late model spindles the tires actually rubbed the spindle until I put in a 3/16" spacer.