Wheelbase & Alignment question

[lee]

Took my SVX in for a 4-wheel alignment job as my back tires are wearing badly. Turns out the toe-in was pretty far off and the rear struts are dying too (just ordered them thru Mike @ Kerven).

Anyway, they give me a printout of of the specs, before & after settings, and a few other points.

The one I'm questioning is the wheelbase. Their printout shows a difference of 0.3 inch. I asked the tech, who said not to worry, that is was normal for one side to be longer than the other and helped with running straight on a crowned road.

I've had the trim off just in front and back, and the car shows no signs of ever having been in an accident. So, can anybody confirm/deny this?

[svxistentialist]

Lee

Without being a complete sceptic, it is hard to accept what they have told you.

First, how accurate is their method of measuring? Do you trust it? Is it one measurement, or the mean of two or three?

The owner's manual lists the wheelbase as 102.8 inches. Even taking into account the small difference toe-in would make [toe in is +/- 0.1 inches, and this would translate to minimal forward or back displacement] Check the measurements yourself. Measure from the rearmost part of the front rim to the rearmost part of the back rim, each side, and while the wheels are pointing dead ahead.

You can also check by measuring the distance between the front tire and the rear part of the arch. If the body is straight as you say, any difference here will indicate the front suspension having had a knock, or some loose positioning bolt allowing it to trail. In case this is it, check what condition are the bushes on the front subframe.

0.3 inches is a fair bit to be off, and I would expect uneven front tire wear if this is so. In theory, the rear suspension could be trailing one side either, but this is unlikely, usually the front takes the hit. Anything is possible though.

It is probably a misreading, and I hope it is something simple and easy to fix.

Joe

[mbtoloczko]

As joe suggested, I'd suggest measuring the wheelbase yourself. Another important number in this area is the thrust angle. If you draw a line between the two rear wheels and then draw a line perpendicular to that line, the angle between that perpendicular line and the centerline down the car is the thrust angle. It describes the angle that your car is being pushed by the rear wheels. What's that number?

BTW, what is the rear camber both before and after. Its not related to your problem, but I'm curious about it for something else.

[lee]

here's the numbers for the rear before/after (all in degrees unless so stated - that's the way their machine prints it out):

toe left: .4/.0
toe right: -.35/.05
total toe: .05/.05 so that was a big part of tire noise/wear

camber left: -.4/-.4
camber right: -.5/-.5

Thrust angle: .4/.0

I also had a total toe on the front of .5, now .05. I'm not a spirited driver, and the roads are very flat here, but the car never "pulled" and I might not have gone in if I hadn't notice inside rear tire wear that was getting ugly.

Joe/mbtoloczko: They pull the car on the rack and put some sensors on the wheel and then seem to let the computer guide them as to what to do for adjustments. It was only done once, I have no idea if their machine has enough sensitivity to determine wheelbase accurately, or even if the way they mount the sensors to the wheels can affect the result. I did notice that when I stepped in front of a sensor (when he was showing me the final numbers) that the display dropped off - he said I was blocking the laser return - it came back on when I moved out of the way ???

edit: paragraph that was here is now deleted, questions asked before doing an internet search. shame on me

[mbtoloczko]

The "after" numbers look pretty good to me. The camber numbers are just on the edge of the specified range, but they should be fine if you don't wail around corners too much.

I've found the laser-based alignment systems to spit out very reproduceable numbers if the mechanic does everything properly. I had my car on the rack for an alignment in January, and then a month later, I had it in again to fine tune the alignment. The "before" numbers for the second alignment were essentially the same as the "after" numbers for the first alignment. I also kept the alignment stats for every alignment I've every had done on my Toyota P/U (probably about 6 alignments total), and the alignment numbers are all very similar.