Wheel backspacing

[TC Cobra]

Hey Don!

Trying to get a handle on the correct backspacing requirements. Did a search and came up with a few inconsistencies/differences of opinion.

I'm planning to use 245/45-17's on 17x8's on the front and 275/40-17's on 17x9 1/2's on the rear.

I'd like to keep the backspacing to a minimim to maximize the track width and appearance.

Other guys have reported from 3 1/4 - 4 on the front and 4 1/4 - 6 on the rear.

What would you do? (WWDD ;-) )

Thanks!
Dwain

[Jack21]

Front: using 235/45ZR17's on 17 x 8 wheels with 4" backspacing.

Rear: using 275/40ZR17's on 17 x 9 1/2" wheels with 5" backspacing.

Works well, looks good, no rubbing or wear problems.

Derivation. Original front MII suspension dimensions and geometry are retained. Just using aftermarket components. Original factory wheels were Cragar 15 x 8 with 4" (split)backspacing front and rear. Kept the same backspacing for front.

Rear is mid-'80's 8.8 Mustang. As rear wheel was widened from 8" to 9 1/2", ideal backspacing was 4 3/4". Since wheel positioning in - out might be necessary to correct tire rubbing, and it would be easier to move tire out with shims than in, aded 1/4" additional backspacing to rear wheels (5"). Turns out this was not necessary, and 4 3/4" would have worked fine. If you're using a wider diff, you'll need to adjust rear backspacing.

17's have no sidewall bulge like 15's, so pose less clearance problems.

[dscott]

TC, Jack's spec's will work fine. The front should be 3 3/4 to 4" with the 8" wheel. The rear should be 4 1/2 to 5" on the 9 1/2 wheel.
Don

[TC Cobra]

Thanks, Jack and Don!

That helps!

Jack - Hope you're doing better these days.

Dwain

[sirelesar]

TC,
You may want to calculate your scrub radius and add that to your backspacing considerations on the front. Minimizing the scrub radius will help your car perform better.
Good luck,
Darren

[TC Cobra]

Thanks, Darren!

...what's a scrub radius and how is it calculated?

Dwain

[Jack21]

Hello TC,

Doing much better, thanks. Not driving yet, but walking a lot better. Just might make it to DVSF this year.

Jack

[jdean]

Hey Jack,

I tried to click to your photos to see if you had any showing your wheels, but none showed up. I thinK you had some before though, because I'm sure I've seen them.

John

[TC Cobra]

So, anybody know about scrub radii?

[Jack21]

Sorry, I haven't uploaded any photos. Guess I should. Lots of different stuff, particularly interior.

Wheels are Team III 17x8 front, 17x9 1/2 rear. Spinners are from Image wheels in U.K. with adapters to fit the TIII's.

I was somewhat skeptical of the TIII's at first. They're very lightweight. But the tires mounted and balanced perfectly, and require no maintenance.

Was also somewhat skeptical of using 17's at all, thinking the car will look like it has fork lift truck tires, and ride like one as well. Yet all the tire guys were telling me, go 17" for a performance car.

Well, the 17's look all business, and like they were factory equipped. The tires I'm using are traction rated AA, and they mean it. Like Super Glue in tire compound.

The trick to getting a comfortable ride with 17's in a light car like a Cobra is don't inflate them to 44# like a heavier car. Start at 32#, check tire contact patch and wear, and work down to 28# or so.

With 16,000 miles, tires still look brand new, and ride terriffic.

[Mohuska]

What tires (and sizes) are you using?

[Double Venom]

Here's a picture of one of the DV II's with 17's. Very effective on the street or track. Jack has a good point on tire pressure.

Jack, drop me a line fella if you have time...I most certainly have missed something

DV

[392cobra]

...what's a scrub radius and how is it calculated?

I would like to know too.

[Jack21]

The lower tire pressure trick came from Falken Tire tech support. I didn't invent this myself, but it does work in the ride and wear dept. At the rate I'm going, I expect well in excess of 40,000 miles before even thinking about replacement.

Keeping in mind that the Classic Roadsters Cobra is about 2,450#, front tires are 235/45ZR17, rears are 275/40ZR17. Using 45's in front, instead of 40's kept the tire diameters more closely equal without sacraficing treadwidth. These sizes seemed to be a good compromise between traction and hydroplaneing resistance on wet roads. And it has worked out just that way in driving.

With a heavier West Coast car with a big block, you might look into upsizing the tires such as used on Vipers, and late Vettes.

The tires, BTW, are Falkens. They are Japanese made, and are comparible to some of the more popular Jap brands. Two things prompted me to go Falken as opposed to Bridgestones from Tire Rack. The Bridgestones were about $680, the Falkens $460 (not apiece!, set of 4!). Falken sent me a reprint of a German car entheusiast publication where these tires were track tested against the "High Priced Spread", Mich Pilots, etc., and the Falkens stayed with them. After 16,000 miles, I'm more than happy and would buy another set.

Got 'em on-line from vulcantires.com. Located in Scranton, PA, the trip took 3 days by UPS Ground, and cost $35 in freight.

Mounting and balancing was another exercise in exasperation. Tires shops wanted over $100. Costco did it for $35, and did it right the first time without scratching the T3's.

[dscott]

The scrub radius is the portion of the tire that you put on the tire shine product.

I have no clue what scrub radius is, never heard of it. I wish someone would respond.
Don

[mickmate]

OK from the front (or back) draw a straight line between the upper and lower ball joints and project it down to where it hits the pavement. Now figure out where the center of your tire is on the pavement. The difference between those two points is the scrub radius. Next question is what effect does it have, right!

[dscott]

mickmate, thanks, I knew someone would answer the question. Now, what effect does it have?
Don

[cobra427mnsi]

Don Scott

Will these wheels and tires fit under my Classic Roadster with the back set listed. Here is the listing in the classified adds section.

" have a set of brand new 17 inch wheels for sale. I bought them and the backspacing doesn't work on my car. I had my axles shorten to much! They are image "Halibrand" type wheels. These are an ultrra light wieght three piece wheel. Polished rims with black centers. The rear rims are 11.5 inches wide with a 6 in backspacing and the fronts are 8 inches with a 4 inch backspacing. They have 4.5 bolt spacing with lug nuts and spinners. The tires are Nitto 555R tires that have been machine heat cycled and look great. I will seperate if someone is interested. I paid $1000 for the rims and $800 for the tires plus the heat cycling! I would like to have $800 for the rims and $700 for the tires."

Paul

[dscott]

Paul. the rims are to wide for the rear and the backspacing would be a problem.
Don

[cobra427mnsi]

Thanks Don !!