DIY Wheel Alignment

[jkdmag]

Need to know if the thrust angle is parallel to vehicle axis. If a two wheel only alignment is performed and the thrust angle is not parallel to the axis of the vehicle there will be a need to steer against the thrust angle. This will mean the tyres will toe-out when travelling in a straight line.

Measurements were made using a string square around the vehicle. A string line was run from behind the rear wheel to forward of the front wheel, on each side,and positioned such that it just made contact with the rear tyre sidewall at a point in front of the rear wheel centre and a point behind the rear wheel centre.

Then the distance from the string line to the centre of the front wheel on each side of the vehicle was measured. The distance from the string line to the front wheel centre on each side should be the same if the rear axle is true in relation to the vehicle axis, subject vehicle has a rear track wider than the front.

Driver's side front wheel centre is approx. 15 mm inside the string line, which is within a few mm of the expected result allowing for the front/rear track difference, while the passenger's side wheel centre is some 35 mm inside the string line.

Only explanation I can think of is that the passenger side rear axle housing is bent such that the projection of the string line diverges from the axis of the vehicle. This would give a thrust angle that is not parallel to the axis of the car. Can anyone see an error in the methodology.

 

[patprimmer]

I like to set the string line close to but not touching the wheels and measure from the string to the front and rear edges of the rim. I avoid actually contacting the string but work very close to it.

I set the string lines parallel to each other, then adjust until they are parallel to the rear axle horizontal perpendicular. I don't care to much about car body position over this.

I then check rear toe and front wheel location relative to the rear. The measurement in from the string to the front wheels should be the same both sides.

If the wheelbase is slightly different it is not so important as the axis being on centre.

Adjust rear end toe to desired toe and with the front is on centre relative to the rear. If the body is 1" out relative to chassis I would not worry to much so long as it is not impacting on corner weights.

Regards
Pat
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[NormPeterson]

Guessing that you have a solid axle.

That does not guarantee that either rear camber or rear toe will be zero.


Norm

 

[GregLocock]

" This will mean the tyres will toe-out when travelling in a straight line. "

Not too sure what you mean by that. Thrust angle is primarily an aesthetic requirement, that is to say, a visually horrible amount of thrust angle (say 1 degree) will have no perceptible effect on the behaviors of the car.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[jkdmag]

Patprimmer, thanks for your comments. I would emphasize that the vehicle has a solid rear axle and therefore the rear toe should be zero.

Have redone the measurements using your suggestion to space the string line away from the rear wheel rim edges. Happy to accept that this method reduces the uncertainty regarding “just” contacting the tyre sidewalls.

Results have not changed significantly as the distance from the passenger’s front wheel to the string line is still much greater than that measured for the driver’s.

Distance from driver’s wheel rim front edge to string line is 29.0 mm and distance from driver’s wheel rim rear edge to string line is 28.0 mm, where as the distance from passenger’s wheel rim front edge to string line is 51.0 mm and distance from driver’s wheel rim rear edge to string line is 49.5 mm

Again, allowing for the front/rear track difference, the driver’s side measurement results are close to the expected, while the passenger’s side are not. And perhaps more importantly they do not come close to matching the driver’s side measurements.

A factor I have not checked that should confirm a rear axle alignment problem is whether the string line projections from the rear wheel rim edges are parallel. If they are not then the only conclusion I can draw is that the rear axle housing is bent.

GregLocock, surely the thrust angle needs to factored into the front wheel alignment otherwise continual steering correction will be needed when driving straight, with resulting ackerman steering toe-out.

 

[GregLocock]

No, not for 1 degree or so, which would look terrible.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

I think a 1 deg steering angle will have negligible ackerman.

You absolutely need to set the strings parallel and measure rear end toe.

It is very possible that the beam rear axle is bent to give toe or camber. In fact a trick on old beam axle race cars was to deliberately bend the diff to get negative camber.



Regards
Pat
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[jkdmag]

Can now confirm that the string lines were parallel when the front wheel offset distances were measured and therefore the axle housing is straight. The question remains as to whey the differing offset distances. If the rear axle was offset, then the front wheel offset values should still total the front/rear track difference, it does not. The driver's side front wheel offset is equal to half the track difference, which is correct, while the passenger's side front wheel offset is some 15 mm more.

Is it normal for vehicle manufacturing tolerances to be so loose that the front/rear wheels are not positioned symmetrically to the vehicle axis.

Intend also to examine front wheel camber by measurements from wheel rim to vertical defined by plumb bob with vehicle levelled. I see no reason why such a methodology should be unsatisfactory. Any comments would be appreciated.

 

[NormPeterson]

Have you verified the track dimensions?

Are all of the wheels the same width and offset?

Are there any spacers behind any of them?

If the front end sits on a bolted-on subframe, has the subframe shifted?

Is the rear axle truly centered in the chassis?

Is there any chance that the rear axle was ever replaced?


Buy a digital angle finder that has a bubble level in it. Directly reading the angle to ±0.1° is much nicer than measuring and doing the math.


Norm

 

[Tmoose]

I check any datum surfaces I'm using on all the wheel rims for runout while mounted on the car. It's actually a wheel, axle/hub/flange runout check
Any steel wheels I ever checked were gruesome.
Sometimes you have to push the car forward or back about 3 feet so the same location on the rim is available for the back side measurement.

 

[GregLocock]

"Is it normal for vehicle manufacturing tolerances to be so loose that the front/rear wheels are not positioned symmetrically to the vehicle axis."

Sure. There is no such thing as a perfectly square car.

I can easily believe 10mm or so




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[jkdmag]

In an endeavour to explain the inconsistencies in my vehicle chassis that my measurements have raised, I repeated the whole exercise. The outcome confirms all the results with the exception of parallelism of the string lines.

It would appear that the string lines do diverge as they project forward from the rear wheels. As the driver’s side front wheel offset values are consistent with the front/rear track difference, I believe there is a rear axle alignment error on the passenger’s side and thus the reason why the passenger's side front wheel offset from the string line is greater than that of the driver's side wheel.

The divergence amounts to less than 1 degree, which I would agree is insignificant in terms of thrust angle, although it does appear quite horrifying when attempting to align the front wheels using the rear wheels as the reference.

To overcome the effect of the rear axle alignment error, the string lines should be simply set parallel and spaced equally from the centre of each rear wheel.

Front wheel toe adjustments can then be made with reference to the parallel string lines.