Center Line Convo Pro Fitment

[Jack Evers]

I'm looking at buying a set of four (4) Center Line Convo Pros and have a fitment question. The 15 x 8.5" 5 x 4.75 wheel is only available in 3 3/8" and 5" backspacing. The 5" won't fit for sure, but how about the 3 3/8"? Stock suspension. What tire size will I get away with?

 

[tom h]

Stock GN wheel is 7" wide, 4" backspace.
For ANY size tire on the rear,

Going from 7" to 8.5 wide wheel,

1) inboard side of tire, moves OUTWARD 0.75" (= 1.5" ÷ 2).
2) outboard edge of tire, also moves OUTWARD 0.75" (= 1.5" ÷ 2).


Now, decrease backspace from stock 4" to 3.4 (rounding off),

3) inboard side of tire, moves OUTWARD another 0.6" (= 4" - 3.4").
4) outboard edge of tire, moves OUTWARD 0.6" (= 4" - 3.4").


Total movement of tire sidewalls relative to old wheel is:

5) inboard side of tire, moved TOTAL OUTWARD of #1 + #3 = 0.75 + 0.6 = 1.35" total.

6) outboard side of tire, moved TOTAL OUTWARD of #2 + #4 = 0.75 + 0.6 = 1.35" total.

From #5 you can see the Inboard clearance has INCREASED an additional 1.35". Thus, you could go to a Jumbo tire Section Width up to 2.7" (= 2 x 1.35) wider than original, and still maintain original inboard clearance.

Problem is more likely to be on outboard edge of tire. From #6, the *stock* tire's outside edge will move outward 1.35". This might be enough to snag the outer tire edge on the fender lip.

Add in the hypothetical Jumbo tire (2.7" wider SW), and the outboard edge will have moved outward a grand total nearly 3".

Long story short, may need airbag to keep rear suspension from bottoming out and ripping tire tread, possibly even on a stock tire but more likely so on a 275-60x15 rear tire.

 

[matt87]

mr. evers, the 3 3/8 will fit very nicely. the gn i just sold to Russ Merritt had that exact rim setup. no rubbing in or out with 28x12.50 et streets. the car did have a very nice suspension too.

 

[murphster]

Pics of my Centerlines with the same rim in this thread:

http://www.turbobuick.com/forums/showthread.php?s=&threadid=114629

I went to a 28" tire and it would have rubbed without adding airbags and/or new springs. The outer sidewall on the passenger side will rub. Gives the car more of a "jacked up" stance though with the 28" tires plus springs and airbags. With a 26" tire I would think you'd be ok, but even then you may need new springs to be safe if the current ones are shot, depending on how fat a tire you have. Inside should not be a problem, just mainly the outside on the passenger side.

 

[tom h]

Originally posted by murphster
Pics of my Centerlines with the same rim in this thread:
I went to a 28" tire and it would have rubbed without adding airbags and/or new springs. The outer sidewall on the passenger side will rub. ... Inside should not be a problem, just mainly the outside on the passenger side.

Do you mean, during hard launch (body twisted to P/S), or just in normal driving around?

Users of the new HR sway bar have reported nice, flat, level hard launches, possibly that would mitigate any rubbing on P/S fender lip (?).

 

[murphster]

Before the Centerlines, I had some 28x11.5 ET Streets on a 15" rim with a similar backspace. Would rub a lot, mostly on turns in and out of a driveway, etc. The Nitto Drag radials (275/60) I have now look bigger than the ET Streets.

Car has no front sway bar either, so that doesn't help. The springs were really shot, so I doubt the car would have been driveable with the 28" tires on without doing anything to the suspension. I don't even want to let the air out of the airbags now all the way to see if it rubs. Sorry I couldn't test everything before changing the suspension, but I didn't want to risk the $ spent on the rims and tires. Better safe than sorry. Would have made me very sad if I cut a tire and damaged a rim right after I bought everything.

I didn't want to go the HR sway bar route because I think the oversteer will become a lot worse for daily driving with that bar. I can still take the air out of my bags and all I've done is put stiffer springs in and it won't worsen the ride that much. With the HR bar I've read that you don't even need air bags so I can only imagine how stiff the back end is for daily driving. You'd probably have to put the front sway bar back or you'd be doing doughnuts around every turn.

Also keep in mind that the HR bar will not physically increase the rear ride height, so clearance may still be tight without new springs or air bags. I don't doubt that the HR bar is better for the track though, as airbags are just a limited fix. I would think most people with an HR bar already have stiffer springs, airbags, and replaced control arms too, so hopefully tire clearance has already been addressed.