Triumph Repair: Wider Wheels, fender lip, spitfire 1500

Question
Hi Howard,

I seem to finally have the A/F issue fixed, thanks for the advice with that one!

This question should be relatively simple, however I have been searching forums and message boards to no avail. I am prepping my 1976 Spitfire 1500 for autocross, and have found that Diamond racing wheels offers a 13" x 7" wheel. My hopes are to put a lower profile sport/autocross tire instead of our current OEM style skinny tires. Do you think this is too wide of a wheel? I have heard of people plus sizing to 14 x 6 or 6.5, but I am not looking to roll the fenders or change body panel shape. Ideally I would halve the size of the current OEM spec tire sidewall(can't recall the exact sizing right now) to allow for the wider wheels.

So is this feasible? Any recommendations or suggestions? I know offset will come into play, but I had hoped for some practical advice before calling DRW. Thanks so much and hope your summer is off to a good start!

Answer
The width of the rim determines how wide a tire can be used. You can get away with one size wider tire on a stock rim but when you go further then that, you start deforming the tire so that you get no benefit from the wider tire. When you put on a wide rim,(like you are considering) you can use a wide tire and get the benefit from it. You have to watch the amount of offset in a new rim so as not to foul on the fender lip like you are concerned about, but that is not the only problem with a wrong offset in a rim. The steering and suspension geometry is normally set so that if you drew a line from the upper ball joint down through the lower ball joint, that line should contact the road (with the car loaded)roughly in the middle of the tire foot print on the road. If you don't pay attention to that, you can get some handling problems and bump steer.

On the rear you need to watch not to get the rim and tire out too far for the fender lip can get fouled but in addition to that when the wheel if offset to the out side you again change the suspension geometry and the spring will usually sag.

If you are going to autocross the car you need to stiffen up the suspension anyway so you may not foul the fender anyway.

Other then talking to someone with a Spitfire who has wide rims and runs autocross. The only way I know of to check all of this is to remove the front springs and with the wide rims and tires on, move the suspension to it's full travel in different turned positions. That can't be done on the rear because of the design of the suspension. But you can just measure the clearances with the stock wheel with the spring compressed (a couple of friends sitting on the rear fender will usually work.) Then measure how much difference between the stock wheel and the new ones.

Howard