How To: Fit Big Ass Tires & Rims on the Stock Frame

[motorhead]

I get asked a lot about how I fit 315/35R17s in the back of my Buick on an 17x11 wheel without modifying the frame. 

So to begin I will quote myself:

For starters I have no idea, or no working memory, of what the backspacing of my rims is (I think it was 5.5").

What I can tell you is the method to fitting the biggest rim possible, using a modular 3-pc rim; because every car is different the final section width of tire/wheel assembly is more important than wheel width alone.
 
 For starters you need to determine if your car's body is offset on the frame, every 1/16" is going to count against the final total.  Next you have to determine if the axle is centred under the frame again a 1/16" counts. You are going to want to measure from the axle to wheel mounting surface in toward the frame (A) and out toward the body at the wheel well (B). You are going to end up with A + B = C, C being the total available space for your tire/wheel combination, or what section width you can fit. You may find that the dimensions are equal, or limited on one side or the other.
 
 You now have to determine what amount of the total section width you want to (or can) fit between the axle flange and the frame. I recommend subtracting 3/8" for axle articulation from the furthest most inside point at the chassis/frame.  This will then translate into the physical backspacing on the wheel. If you have deep offset wheels like I did you can install long wheel studs and use washers to get a workable final dimension.
 
 I can tell you that when I was done I had to have two custom machined wheel spacers made to get the tire/wheel combo to fit just right. I think one was 3/8" and the other 5/16".  This is a painful procedure to go through - I hate math.

And now I will field questions as required in this thread.

[Jeff]

Woww.sweet...

[motorhead]

This is the work I did to fit oversized tires to our TrailBlazer SS, tires I was told would nevvvveeerrr fit (http://www.tbssowners.com/forums/showthread.php?t=87231). 

So now I need to decide on tires. Since I feel strongly against lowering an AWD truck, potentially screwing up the suspension geometry and generally hate wheel gap - taller tires are in order. It'll make the family war wagon look a little more truckish too. So rather than doing a 1" drop, I am going to add 1" to the radius of the tires. The stock tires measure out to about 30" and the ones I am about to list are at 32".
 
 Stock
 Goodyear RSAs:
 P255/50R20 - 10.9" section width on an 8" rim, tread width 8.6", OD 30.1”
 
 Aftermarket Staggered
 Nitto NT420S:
 275/55R20 - 10.82” section width on 8” rim, tread width unknown, OD 31.97”
 305/50R20  - 12.64” section width on 10” rim, tread width unknown, OD 32.05”
 
 Kumho Ecsta STX:
 275/55R20 - 11” section width on 8” rim, tread width unknown, OD 31.9”
 305/50R20  - 12.6” section width on 10” rim, tread width unknown, OD 32”
 
 Falken Ziex S/TZ04:
 275/55R20 – 10.8” section width on 8” rim, tread width 9.4”, OD 32”
 305/50R20  - 12.3” section width on 10” rim, tread width 10.5”, OD 32”
 
 Based on aesthetics it would seem that Nitto offers the best option in a staggered fitment (take the section width, subtract rim width and divide by two) as there is the least variation in section width front to rear (about 0.09” in front bias).
 
 As far as fitment goes (and I am certain there are some strong opinions on this) using the Nittos with a 1” spacer/adapter on the front there should be no complications with the upper balljoint mount on the spindle. This is because the 275/55s have a narrower section width than the stock 255/50s on the same size rim, and with the spacer/adapter pushing out the wheel to overcome the extra 1” in tire radius interfering with the spindle.

[motorhead]

Handy link: http://corvettecentral.com/wheel-offset/

[phil_long]

How do you determine if the body is offset from the frame? Is this a newbie question?  Lol

[motorhead]

Measure from the axle flange out... that is the easiest way.

[phil_long]

thanks