Did you know that?

I just checked out the Pettit racing website, and looked at their alignment specs. According to them going from a 16" wheel to a 17" wheel, the rear camber should be adjusted from -1.1 to -0.3
That's a huge difference and alot less negative camber in the rears I'm assuming. Perhaps that's why the R-compounds had such excessive inner edge wear?????

PS. I just realized today, that I probably hold the world's record for running a R compound tire! Almost 19K miles...ran it right down to the belt... eek.

Nobody ever mentioned that the alignment should be adjusted when you get larger wheels...first time I heard it.

Thanks.

 

Why os that I wonder? I just purchased some 17" and the tires are actually just about the same size as what came stock as far as physical height goes(set the new wheels and mounted tires and the stock wheels and tires on the ground together and the height is the same), they just have a lower profile and they are a tiny bit wider, not much at all.

 

In this case wheel tire size does not make any difference. if you size the tires correctly you will not have any change of overall ride height. And in turn you will not change anything. Even so you wont change the Axis of the suspension anyway. Even if you didnt plus size corretly. The geometery of the suspension wouldnt be effected in any way.

 

Are you sure about that?

Why does Pettit recommend such a HUGE change in rear camber going from a 16" to 17" wheel then?

With 225/45/17 yokohama R-compound tires on the rear, and 235/40/17s normal tires on the front, I noticed that the R-compounds had excessive inner wear, consisten on BOTH rear tires (not just one side), suggesting too much negative camber. The overall diameter with the 225/45/17 is fairly close to stock diameter.

Using the stock tire & wheel, there isn't this kind of inner wear.

Mind you R-compounds wear 10 times faster than normal tires, so they'll show this kind of thing much much quicker.

 

Hmmm, those tires are quite a bit larger than stock. 215/45/17 is what I run and they are sized the same as stock. But I still don't see how tire size would effect the geometry at all. Your suspension is still going to have the same amount of travel with any wheel size. Perhaps this is to keep the tire from rubbing on the fender?

 

I calculated the overall diameter and it turns out as follows:

Stock rx-7 tire/wheels 225/50/16: 24.86"
225/45/17 on rear: 24.97"
235/40/17 on the front: 24.4"

And for fat tires:
255/40/17s on the rear: 25.03"

 

I would imagine the camber is changed since there will be less sidewall deformation when cornering because of the lower profile on the 17s. Imagine the contact patch sticking to the road as the force of cornering is applied through the rim to the inside circumference of the tire. The contact patch is forced to roll under the car and does so because of the soft sidewall. Negative camber will keep the tread more flat on the road as contact patch is rolled under. Less sidewall=less neg. camber needed to keep contact patch flat as it doesn't roll under as much.

 

I assume that the Pettit numbers are for stock suspension.

I have a very modified suspension and that alignment number they had sucked ****. I was better off with somewhat of a stock setup since I had a more agressive rear sway bar and springs. that menat less lean which meant it was easier to slide out the rear unless I gave it more negative chamber.

 

This shouldn't matter. Now if you get radically wider wheels and tires, then the camber settings might change, but 16, 17, 18-inch diameter wheels shouldn't matter if the tire widths and overall diameters remain close to stock! For example, I had my alignment done using 8 x 16, 38 mm offset wheels with 225/50-16 tires on them, knowing full well that I would be switching to 8.5 x 17/245-45-17 and 9.5 x 17/275-40-17 track wheels and tires for track duty. I had no issues with having the alignment done with the 16-inch wheels. In fact the alignment was fabulous. I insisted the tech align the wheels with my 235-lbs of mass sitting in the driver's seat. This also ensured, that I got to straighten the wheel, not the tech. My weight in the car made a big difference in camber settings.