Wheel tracking width differences
 

Have just replaced aftermarket wheels with stock on a 2001 ZR2. The previous owner put tubular upper A arms on and it turns out my factory rims rub. I need to put on about a 3/4" spacer. Now I am reading that there is a difference between the front and rear wheel tracking. Why would this be? Why would the factory do this? Is this a cause for any kind of handling difficulties? Seems like an engineering miscalculation. So I guess I would need to put 2" spacers on the rear to help alleviate the difference.

In the same vein, speaking hubcentrically, I have read that the hub O.D is 70.3mm and also have read 70.5mm. Not that it seems like too much to worry about but anybody know what it is FOR SURE? I have also heard hearsay that the front and rear hubs may be different. I have also been told that 120mm lug spacing will work as well as 4.75" even though it is a mm+ off. True or false? My rig is in the shop at the moment so I cannot go out with a ruler in person.

One of these days I'll actually get to go out and drive this thing...

Thanks ahead of time.

The rear axles were a bit narrower than required to make the rear tires have the same track width as the front tires. The track widths, front and back, depended on the configuration of the vehicle.

Here are the specs from 1999 (from 1999 ordering guide)

type: frt/rear
2wd: 55.0"/54.6"
4wd: 57.2"/55.1"
ZR2: 61.1"/59.0"

So without going out and actually measuring the outside tire edges to see, am I hearing from you that the axle lengths are different but the tire outside measurement is the same?

If I understand your question correctly, the answer is no. In the stock configuration, all four wheel rims and all four tires will be the identical so any difference in track width will be due to the geometry of the front suspension and the width of the rear axle. For example; for the 4wd and ZR2 Blazers, the center-line of the rear tires will be 1 inch closer to the center-line of the vehicle than those of the front tires.

OK so if I understand you correctly the rear wheels need 1" spacers to equal the tracking of the front tires. Yes?

And any info on the hub diameter?

I installed 1.25" spacers that were hub-centric. I used 1.25" because I did not want to have to cut down the original lug bolts and I got hub-centric so that they would be additionally supported a bit by the lip on the axle and guide the wheel into place. There might be other reasons too for the hub-centric but I cannot think of them off the top of my head. :)

These are the ones I installed: https://www.ebay.com/itm/2-Hub-Centr....c10#vi-ilComp

Be sure to double check the fitment though!

https://cimg8.ibsrv.net/gimg/blazerf...3f2a8a716e.jpg
https://cimg9.ibsrv.net/gimg/blazerf...71d61d6c29.jpg

OK so you put spacers on the back only to mach the front. I get that now. I still don't get why it is configured this way from the factory.
I have a clearance problem up front and want to keep the tires tucked in so am going to try 20mm spacers and shorten the bolts. Then on the back add 1 1/2 to 2" spacers to make up the difference. The wheels I took off had 4" back spacing VS factory 6" and what a difference it made in looks. With the spacers it should be somewhere in between...
I included photos of the original wheel profiles. I'll send a few when I get it back... https://cimg3.ibsrv.net/gimg/blazerf...672a5e6042.jpg
https://cimg7.ibsrv.net/gimg/blazerf...e349389cd6.jpg
https://cimg8.ibsrv.net/gimg/blazerf...b955e578d3.jpg

If I understand the problem the tube uppers are thicker than stock and rubbing on the inside of the rim and not the inner face, if that is the case and a spacer will move it to the taper of the rim and not contact you will need to install the same depth spacer on front and back to keep factory differences in width for handling.

OK so back to the factory differences. So then it IS an engineered thing and it DOES have effects on handling so whatever spacer I add to the front I need to add to the back. Correct? Is this common on some vehicles. I never really paid attention to this before in all my years of wrenching. What would be the advantage to handling?

Oh yeah, It is the a arms hitting inside the rim...

So can you post a picture of the upper control arms? You say they are tubular. Are they the ones made by Rough Country? https://www.roughcountry.com/gm-susp...kit-242n2.html

I ask because I installed these upper control arms and noticed that my stock rims will hit the side of the controls when the steering is full over to one side. If money was no object, I'd get new 16" rims and new tires. :)
https://www.roughcountry.com/media/c...ontrol-arm.jpg