FLYNAVY30

So the factory wheels are 15x8 with 4" backspacing.....with the factory trailing arms, can I step up to a 15x9 with 4.5" backspacing without any major clearance issues (short of the e-brake bracket) in the rear? My understanding (and I may be mistaken) is that by going with a 9 inch wide wheel and 4.5 inch backspacing, I will evenly divide the additional inch of width between the face and the back side of the wheel. Doing some measurements on the car, it looks like that will make the wheel face flush with the rear fender lip, and still provide enough clearance to the rear control arms. Any input/advice is much appreciated

rdroket

You need to be aware of the clearance between the tire you choose and the end of the rear spring. I have stock 8" rally wheels with 275/60/15 on the rear and only have about 3/8" between the sidewall and end of spring.

FLYNAVY30

thanks

CV62Air

I had about an inch clearance with 255/60s. The brake cable had evidence of rubbing with weight on car, about 1.5 mm clearance with the wheels hanging. Must have been the tire bulge with the weight.

I needed work on my trailing arms and differential anyway, so I ended up getting 2" offset trailing arms from Gary at Vtech Corvette, and a new monospring from Van Steel.

I haven't measured it, but there is substantial clearance now.

That is the expensive option!!

Is there someone near you with the wheel/tire combo that you could try the fit?

FLYNAVY30

yea, my plan was to run 255/60/15s on 15x9 TQ Thrust II with 4.5 in backspace on the rears

C3s4me

I got this link from one of the other members and it has been very helpful to me....

http://www.rimsntires.com/specspro.jsp

TheSkunkWorks

At first glance that calculator is a pretty cool tool. Problem is that the installed section width doesn't vary with rim width changes, only reflecting that on the TRA standardized design wheel width (a meager 7" in the case of a 255/60R15). Also, I don't care for converting between offsets and BS.

One can WAG installed section widths on high-aspect ratio passenger tires with a fair degree of accuracy by assuming ~0.5" growth for every inch wider than the TRA model is the proposed wheel. For the application in question, this would result in an installed section width of 11.04" on a 9" rim vs. 10.54" on an 8". Since going from an 8" w/4" BS up to a 9" w/4.5" BS we're moving both the inner and outer beads by the same amount relative to offset, we can simply divide that extra 1/2" of section width by 2 for a rough estimate of how much wider each sidewall (inner & outer) would likely be relative to if they were mounted on the stock wheel; in this case ~1/4" each way.

As it happens, I have a set of 255/60R15s on 8.5" rims (w/4" BS) in storage that measure right at 10.8", which would appear to support the above method, but this is definitely something in regards to which YMMV. And, yes, this latter configuration fit my lowered '78 w/o mods. In any event, tho any given tire will tend to do by a lesser amount on a wider rim, don't underestimate the amount of sidewall deflection that can occur under heavy cornering loads. In addition to moving the parking brake bracket, I'd also consider a narrowed rear main leaf any time you go with a wider than stock setup. (Yep, have one on my shark too.) Anyway, I do hope that helps more than confuses the issue.



TSW

jboweriii

Van Steel did my chassis and put reverse trailing arms and Danny said i can put a 295 on a 9.5 wheel

FLYNAVY30

much appreciated guys....Ill let you know what I come up with

MrForce

Have you considered going to an 18" wheel? Tire selection is much better at this point and handling is greatly improved.

TheSkunkWorks

The higher priority is performance, the less you'll want to leave on the table due to tires.

FLYNAVY30

I may eventually go that route....at this point Im just waffling on what direction, if any, I want to go with with car but in the mean time, Im not a huge fan of the factory wheels and have always lover the old school look of the 15" TT2

techspaces

I've been researching a lot and this is what I'm coming up with. Can someone confirm this works on a 1978 with stock suspension lowered 1" all around?

Rear - 18x10 with 5.25" backspace - 275/40/18 tire
Front - 18x8 with 4" backspace - 245/45/18 tire

TheSkunkWorks

FWIW, 18x9 w/5" BS fronts would put the outer lip at the same point as stock 8" rim w/4" BS. The big thing to watch for with increased BS up front is limited lock due to rubbing the inners (assuming you don't go so far as to interfere with the steering or suspension itself). Having long run 8.5" fronts w/4" BS on the front my '78 lowered to Chevy Power D & Z heights without issue, I'm looking real hard at 9.5x18 w/5" BS. Out back, many have succeeded with 18x10 w/5.5" BS. There, I'm thinking about modding the frame just enough to sufficiently clear 10.5-11" w/6-6.5" BS.


DISCLAIMER: Not all sharks are identical. There are examples of fitments proven on one C3 of a given year not clearing on another.

techspaces

The alternative is to go all around with:

18x9 with 4.25" backspace and 275/35/18 tires.