Guys and Gals....PLEASE HELP! Here is my dilema. Last year I installed a Moser 12-bolt rear end. When I install my ET Streets, they rub on the rear of the fender well on the passenger side of the vehilcle. There are no clearance issues on the drivers side. Even though I know the body isn't necessarily perfect, this variance led me to believe that my rear end was crooked in the car, causing a positive thrust angle. After taking multiple measurements in the wheel well, I discovered that the rear end sits more than an inch further back on the passengerside (as referenced to the front opening in the wheel well) than it does on the drivers side. Next I needed to deterimine why this was the case. I made the assumption that the control mounting locations on the body were consistent (car has never been wrecked). I also verified that the control arms were the same length (non-adjustable BMR tubulars). I then put an angle finder on the LCA mounting brackets on the rear end. I found the bracket on the drivers side to be welded on at a 5 degree shallower angle (ie. mounting hole located further back) than the bracket on the passenger side. Since the mount on the drivers side of the car is effectively closer to the front of the car, this would cause the rear end to need to be pushed back further on the drivers in order for the LCA to mount up to it, causing my suspected positive thrust angle. In order confirm my suspision, I took the car today and had it put on an alignment machine. The alignment machine confirmed that I have a +15 degree thrust angle. As a note, the factory tollerance for thrust angle is +/- 15 degrees, so I'm just barely within the high side of the spec.

OK, now on to my proposed fix. Next weekend, I will be installing BMR LCA relocation brackets and I figure this is the perfect time to correct my thrust angle. The plan is to correct the LCA mounting bracket angle in order to properly align the rear end. In order to know how far forward my rearend needs to come on the drivers side to make up 15 degrees, I need to determine the gap (line to line) in a 15 degree angle, starting from the centerline of the vehicle and ending at the wheel mounting location. This is very roughly about 31 inches (I haven't measured precisely yet). So, here is how I am going about figuring this out:

1. On a sheet of paper, I drew a 6 inch straight line.
2. Next to it, I drew another 6 inch line, but 15 degrees off of it (both lines originate from the same point, obviously).
3. I then measured the gap between the two lines, 6 inches out from the angle's origin. This measured 7.5cms, or 2.95 inches.
4. Next I did a little math, assuming that the angle gap will incrementally double with each six inches of length. (ie at 12 inches from its origin the gap should measure 5.9in). That is the part that disturbs me, considering the angle that I am measuring is 15 degrees off at 31 inches. By my calculations that means I'm off by 15.24 inches and that is DAMN impossible.

It is now very obvious to me that I don't understand the reference points here, so here is what my current understaning is:

1. To have a 0 degree thrust angle, the rear end should be perpendicular to the centerline of the vehicle. (Essentially forming an upside-down 'T'; see attached picture).

2. If the angle measured is positive, that means that the rear wheels are pointing to the right of the vehicle centerline by the given amount of degrees.

3. What I don't know is where these measurements are referenced from. Given what I THOUGHT I knew, I'm off by 15.24 inches which is impossible.

Someone with some experience please tell me where my thinking is F'ed up. I know I won't be able to get this thing aligned perfectly, but I want to compensate as much as I can when I put on the LCA relocation brackets. to anyone who can help my dumb ***. Thanks!

 

OK, writing that all down helped me to think it through. I think where my error lies is that the thrust angle has to do with where the rear wheels are pointing in reference to the vehicles centerline, not necessarily how the rear end is oriented. It has the same effect, but would be measured much differently than how I'm measuring it. Maybe I should ask a much simpler question. Does anyone know how far forward I should move the drivers side of my rear end to correct for a +15 degree thrust angle? Thanks.

 

I hear what you're saying...the tangent angle is something that needs to be aligned, depending on car etc.

What reference point you are using is going to make a huge difference...try to find a point at least 1/3 the length of the width ahead of the center point, then locate and mark the perpendicular on the axle with a chalk string and exact same ref point from both sides...ie the centerline of the rotor...if that's accurate.

Hope this helps, as would adj trailing arms, sounds like you have a good method lined up for putting all the markings together and reducing the separation angle.

 

Thanks. I spent this weekend installing the relocation brackets. I ended up moving the rear end forward 1/4in and everything measured out perfectly. I haven't had a chance to get it back on an alignment rack yet, which will tell the real truth.

 

I was just thinking the simple solution would be to get some adjustable LCA's and then get a 4 wheel alignment. Would that work or am I over simplifying the problem?

 

No, you're absolutely correct. That would be the easiest way to fix the problem. The reason I didn't go that route is because I already have BMR chromoly non-adjustables and I don't want to sell them to buy new adjustable ones. That coupled with the fact that I was installing the lowering brackets, I figured that was a great time (and way) to correct my problem. Which, BTW, I installed this past weekend. I ended up moving the passenger side of the rearend forward 1/4 inch. I haven't had the alignment rechecked, but I am pleased with the initial results. The wheels now sit more evenly (front to rear) in the wheel wells and my measurements from the front wheel to the rear are within 1/16th of an inch side to side. A true test would be to cross measure the car, but I didn't feel it was necessary. When I get some free time I'll have the car put back on the alignment rack and post the results.