I searched old threads and came across posts by "Bluewasp" and "Rome" that cover a very similar issue but I am not quite sure I am reading and measuring correctly. So I think I need some clarification.

I have a 90 C4 toy. A local guy has a nearly new set of CCW Classics in 18 x 11.5 (8.25" of backspacing) and 17 x 10.5 (7.25" of backspacing). The price is right. If they had tires on them this would be a piece of cake, but they don't. By my rough calculations the fronts will have around 51 mm of offset and the rears will have a whopping 82 mm of offset. I want to run 285/40/17 in front and 315/30/18 in the rear. The car will be lowered 1"-2". In my screwy little mind the front should be fine but the backs will require a small spacer to keep from rubbing the trailing arms. Do I have this right? How much of a spacer would I need to keep them from rubbing? Am I setting my sites too big?

 

I believe your front wheels are maybe something closer to 38 mm offset and your rears seem to be something closer to 50 mm.

Did you measure the "back spacing" yourself?


Offset from "back spacing" is calculated by subtracting the "total wheel width" divided by 2 from the back spacing and multiplying the result by 25.4 to get mm.

math for front: 7.25 - (11.5/2=5.75) X 25.4 7.25 - 5.75 = 1.5 X 25.4 = 38mm


math for rear: 8.25 - (12.5/2=6.25) X 25.4 8.25 - 6.25 = 2.0 X 2534 = 50mm

The rears are a very good fit when compared to an 11" 50mm 17" with a 315 tire!

The fronts will provide "more" clearance to suspension but will push the tire/wheel nearly 1" further "out". You will need to check clearance to the "hood edge" for these.

 

i think the question would be easier to answer if we knew what the original application was for (for the ccw's). From what i know (very little weight here, lol) the c5 wheels (for example) fit right onto the c4, but require a 1/4" spacer in the back (newer c4's, i.e. 89'-up). I would get a hold of CCW directly and let them know what you've got

 

The wheels have "NO tires mounted" and if he can measure the two required dimensions (total wheel width and back spacing) the offset can be calculated to 1mm. The math is "fool proof"!

You can use some on-line comparison calculators by adding the tire to the wheel and comparing each "side by side" to the tire/wheel combination on the car now.

I used the "advertised width" + 1" which is a generally accepted calculator for "total wheel width". My 11" GM wheels are just a little short of the 1" but "very close".

If you've got a wheel with out a tire that you believe to be a "specific" offset get a straight edge and a tape measure check the math. You can "tailor" a tire/wheel fitment using these calculations and if spacers are "desired" you can get the fitment to +/- a mm or so.

Here's math for an 11" ZR-1 36 mm using the "accepted rule of +1":

7.4375 - (11 + 1)/2 6.0 = 1.4375 X 25.4 = 36.51 mm. Pretty close to the advertised 36 mm.

 

You could get an idea by maybe test fitting them on your vehicle.

 

I did the math based on the owners measurements. I do not have immediate access to the wheels. I wanted to make sure we were close before I drove an hour to see them. I rounded as I assumed the owner measurements to be inacurrate to some minor degree. I did assume he could read a ruler to within a half inch.

I am not sure I understand your +1 logic if the wheel was actually measured. I sort of understand if it is an "advertised" dimension and not actual, but assuming the guy could run a ruler (not always a safe assumption) the plus one method would seem to add error where none existed.

My mistake was that I did the math in my head and left out a step on the rear. My front calculations were close. The rear was obviously wrong. That is why my brain wasn't liking that 82 mm offset I came up with. Duhhh. When I saw your math I instantly recognized my mistake. My new calculation came up to 63.5 mm rear offset. In your plus one methodology you got 50 mm. Assume that we may be over/under that gives us an average of 56.75 mm. Similarly, in your plus one method you came up with 36 mm for the front and stated the wheel might stick out a bit. With my calculated 50 mm, we get an average calculated offset of 43mm so it should be just about right. Assuming, the seller was in the ball park and so are we, this might work. 82 mm seemed ridiculous. It was.

Odd, I actually teach this stuff for a living. I fry my students for not showing their work. I'll give myself an "F" and wear a dunce cap to bed tonight.

I'll drive up tomorrow and measure them myself. If his measurements are close then I will just throw my money down and spring for some tires. If they don't work I'll have some nice paperweights!

Thanks guys

 

You might want to re-think the front Tire choise; a 285/40/17 is taller and has a bigger side-wall heigth than the rear Tire.

 

Using the measurements provided by the "current" owner use this technical information from CCW that seems to confirm my math! This was a "first visit" to their site and it seems an interesting calculator!

The first section of their technical information if you read "deep enough" they don't mention the math but they explain an 8" wheel is actually 9" "TWW". They it seems rely on the +1 I used in my calculations for you since their calculator seems to agree!

The rears seem fine for you. You could drop a string line (plumb bob) from the hood edge at the hub center, measure the present "side wall" to string. If you've got 1" plus clearance and you've got 275's on a 9.5 wheel now mounted the fronts will be good. I wouldn't think you will have.

http://www.ccwheel.com/files/technical.php

 

Here is a link if it works to a C4 ZR1 with 40mm offset wheels from the front of a C6 Z06. These wheels are 9.5 wide I believe. Your tire/wheel combination of the front would stick out an additional maybe .3 of an inch.

http://www.zr1netregistry.com/forum/...hlight=locobob

 

Why 315s on the 11.5 wheel...shouldn't you be looking at 335s? Also, I'm going to start a thread on my issue(s) with this tonight... with lots of pics and specifics...I'm in the process of trying to fit (under the fenders) 18 x 11.5 CCW classics

 

A 335 tire on the 50mm 11.5 that is used in this thread would put the 335 tire "side-wall" something near 1/2" closer to the rear suspension and chassis, it would also "boost" the "side-wall" outward something near 1/2" in comparison to a 315 on an 11" 50 mm wheel. In what "increments" will CCW build the wheel you're interested in? You mentioned fenders, I assumed you meant quarters on the rear. Correct?

I just read your comment again and it seems maybe you have wheels already? What offset or backspacing?

 

Yes, I have the wheels 18 x 11.5 and 335 tires already. And yes, tucked under the rear quarters without using GS flares. I don't remember the specific offset/BS...I'll dig out my CCW build sheet, and post lots of pics in my thread tonight.

 

As per WVZR-1, I think the 315 will provide the clearance I need without rubbing under hard cornering. If you go back and read posts by "Rome" you will find that he had to buy new trailing arms and a custom bracket to fit a 335/30/18 on CCW Classics safely under his C4. I would like to avoid that if possible.

 

The ROME route is the way the I'm trying to go. Jon @ CCW even pulled ROME's build sheet for me. I have the new trailing arms, swaybar end links, etc....but they still don't fit "right"...anyway, that's my issue, not yours. You "should" be fine using 315s with the proper offset/bs (I'm not sure what that is)...but that's with 11 inch wheels...not sure about 315s on 11.5 inch wheels?

 

Your problem with the wheels your contemplating buying isn't with the rear it's the front! Did you look at the link I posted for the C4 ZR-1 with a "similar" fitment on the front? Did you check the CCW site and enter the information you have?