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Fitting Larger Wheels/Tires

A common misconception is that bigger tires will only rub if a car is lowered.

The fact is: A given tire will either rub or not. The ride height has nothing to do with it. Only if you cut or remove your bump stops, or make other changes like those in the list below, does this not hold true. The only thing that is different is that if you lower a car the tires are closer to the wheel well already, so if they rub they might rub more often. The same tires with standard ride height might only rub occasionally on bigger bumps or in turns or something, but they'd still rub.

A shorter spring simply starts out shorter than a standard spring. It does not change the actual suspension geometry (that is, its range of motion, the axes it swivels around, etc.). Therefore, clearance is the same as a standard spring throughout the suspension's range of motion, regardless of which length springs you install. It is true that the prevailing clearance may be affected, in other words, that lowering the car can change the clearance typically experienced, since the "midpoint" of the suspension's travel is now different. However, whenever your suspension drops back down to the standard ride height (like for the inside tires in a turn, or the wheel diagonally opposite the wheel that rises when hitting a bump) your clearance issue is the same.

The only things that will typically change your actual tire clearance are things like:

  1. Wheel and tire sizes (including wheel offset, width and diameter).
  2. Wheel alignment (including camber and toe).
  3. Modifications to the suspension (including the use of smaller diameter springs, different spring perches, modified suspension geometry, modified wheel travel).
  4. Modifications to the body (fender flares, collisions, etc.).

I once figured out a really cool way to determine the biggest tire/wheel combo that would fit any given car:

Get a bare wheel (preferably... will work with a tire mounted too, but it's harder to see what's going on) that fits your hub. Get some cardboard and cut it into the shape of a tire cross-section with the "bead" fitting snugly against both bead areas of the wheel. Make sure the sidewall and tread areas are substantially larger than those areas would be for the largest tire you would consider installing. What I used to do is find the tire specs for the tires I was considering and calculate the dimensions this "template" should have, based on the tire's section width, tread width, profile and diameter. The accuracy of your template is important, so spend some extra time getting it right. Find a sample of the tire somewhere if you can and measure it installed on a rim if possible. Should look something like this if you're using the same rims but just a larger tire:

If I was also changing rim width/diameter/offest I'd add material in the appropriate areas. Sort of like this one, which assumes a wider rim, and a bigger diameter, and a different offset (the blue area in the first shows the original tire size, the black shows the new wheel and tire with the new bead areas shown as a "shoulder"):

 

The "bead" area remains the same width so it will still fit the bare rim. We've just added cardboard to the sides to simulate the effects of a wider rim/tire section, and to the height to simulate the effects of a taller rim/tire. This little trick can also be used to allow you to substitute some non-standard rim in place of a standard rim for the testing. Say you can only find a bare space-saver spare. You could add material to the template to compensate for its reduced size so you don't have to find a bare standard rim, or remove a tire from one of yours.

In both cases slide this "template" around the bead area of the bare rim and determine if the template hits anything. If it does there'll be interference (rubbing) between the tire and your car... not good. If it rubs, or even gets too close to something (within about .25" in the radial direction, perhaps .5" or more in the sidewall area due to tire distortion in corners), select the next smallest sized tire and trim your template to match the specs of that tire and try again. Do this with the front wheels turned in all directions too. OK, to do a 100% job you really need to remove the spring too so you can easily compress the suspension to see if it rubs at full bump too. You might be able to get by without doing this if you place the floorjack under the suspension of the wheel you're measuring for, and the suspension is soft enough to compress nearly all the way. Just estimate the clearance loss if the suspension moved all the way up. And you might be OK if there is just a tiny amount of rubbing that happens only at full bump with the wheel turned all the way to one side or something equally rare. Just don't come crying to me if your wheel wells and/or tires get damaged! :-)

Do I expect anybody to actually try this? Not really... only fanatics like me put ourselves through this sort of thing! ;-)
 

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