Rear toe: spacers to degrees

[soldave]

My front wheels face in the same direction! Had this replacement track rod cut a tiny bit and adjusted so I have the right amount of toe in. The car doesn't want to crab any more so I'm happy. An alignment was done on the rear but no adjustments, so both are toeing out; I have -0°0.' toe on one side and -0°11' on the other. I know that aligned correctly it's got 0°25' of toe in, plus or minus 0°06'.

Do we think that moving a single spacer from the rear radius arms on both sides would bring them into alignment a little better, or would it be multiple spacers? I know the best bet is to get it aligned once more but I'm just wondering if I move one or two spacers how much it's going to bring in the toe at the rear.

[EXCEL V8]

Can you not remember your trigonometry from school Dave!!??😄.  I think you're mixing up your units - it's 0.25 inch overall toe (+/- 1/16").  You should be able to calculate a rough figure by measuring the thickness of a shim and the length of the arm:  The change in angle would be (approximately) the inverse Tan of the shim thickness divided by the length of the arm.  It would probably be easier to estimate it from the position of the wheel rim in relation to the mounting point - for example; if the measurement point on the wheel was halfway "along" the arm, removing a 2mm shim would move the wheel rim in 1mm. 

Don't forget to load up the car to get the required ride height for geometry measurements.

Pete

[soldave]

Trig always destroyed me. Was much more of a statistics man . So I'm trying to get around 1/4" or 6.4mm or toe in no each side, give or take a bit.

That kind of makes sense though. Had a reply from someone on Facebook who said similar, but who said it would be ArcTan of shim thickness divided by distance between trailing arm mount to body and center of the inner U-joint cross (ie, center of differential output shaft). Whereas you seem to be saying it's length of the arm itself. Can you confirm which one to use?

May use the estimation option you gave though. Basically calculate the front to back distance between centre of wheel and mounting bolt for arm, and then measuring the rim (excluding tyre) from centre to outer edge, and using that to give me my denominator. Won't get me perfect, but should get me closer than I am now.

[EXCEL V8]

1/8" on each side - 1/4" overall.  Thinking about this again I'm not sure how easy it would be to calculate at all, and the trig calculations still leave you with an answer in degrees which then needs transcribing into a linear measurement at the wheel rim.  The problem is that as you remove shims the arm pivots about the outer lower link bush, but the wheel is tilted (less negative/more positive camber) at the same time by the fixed length of the drive shaft, making it a compound motion at the wheel rim.  Very complicated.  But - if you assume that the wheel is tilted around a horizontal plane by the drive shaft so that there is only a very small effect at the rim at the point of measurement then you can probably make some estimates.  If the wheel needs 1/8" of toe, that leaves it at an angle of 0.477 degrees to the axis of the car.  Your wheels appear to be in the straight-ahead position - 11 seconds of arc is pretty small!  Since the arm is going to pivot around the outer lower link bush (and ignoring the change in camber - something else to be checked afterwards!), if you move remove 7mm of shims (arctan 7mm divided by the length of the arm - about 860mm) you end up with a change in angle of 0.466 degrees - probably close enough.  This is just theory though!!

Pete

[soldave]

2 minutes ago, EXCEL V8 said:

1/8" on each side - 1/4" overall.  Thinking about this again I'm not sure how easy it would be to calculate at all, and the trig calculations still leave you with an answer in degrees which then needs transcribing into a linear measurement at the wheel rim.  The problem is that as you remove shims the arm pivots about the outer lower link bush, but the wheel is tilted (less negative/more positive camber) at the same time by the fixed length of the drive shaft, making it a compound motion at the wheel rim.  Very complicated.  But - if you assume that the wheel is tilted around a horizontal plane by the drive shaft so that there is only a very small effect at the rim at the point of measurement then you can probably make some estimates.  If the wheel needs 1/8" of toe, that leaves it at an angle of 0.477 degrees to the axis of the car.  Your wheels appear to be in the straight-ahead position - 11 seconds of arc is pretty small!  Since the arm is going to pivot around the outer lower link bush (and ignoring the change in camber - something else to be checked afterwards!), if you move remove 7mm of shims (arctan 7mm divided by the length of the arm - about 860mm) you end up with a change in angle of 0.466 degrees - probably close enough.  This is just theory though!!

Pete

Much appreciated. Await the inevitable post this weekend saying "there's only one bush on the inside of the arm"!!!

[soldave]

Well I actually just had a quick lie down on the garage floor and a feel of the radius arm. The driver's side doesn't seem to have many washers between the radius arm and the mounting snubber washer - maybe 5mm worth. Even stranger, the passenger side radius arm seems to be flat against the snubber washer with no spacers inbetween. That would mean that there's no more toe in I can get on that side.

Hopefully that's not the case; if so I might leave them as is so at least both sides have about the same amount of toe. Will report back on Saturday.

[soldave]

How thoroughly predictable! The passenger side doesn't have any spacers between the radius arm and snubber washer on the mount. So unless I'm mistaking, there's no more toe in I can get unless other radius arm mountings have a thinner rubber/poly mount.

If that's the case, it's going to have to be something I live with for a while. Whilst the mountings are probably due for a refresh as most of the older rubber bushings are, I'm not even sure they would be any thinner and fix my problem.

[andydclements]

The mount can be placed on the other side of the subframe plate. That get you some movement, spacers would give more.

[EXCEL V8]

Are the lower rear links the same length?  Do they look like they may have been damaged/repaired?  If one was shorter than the other it would cause problems.

Pete

[soldave]

1 hour ago, andydclements said:

The mount can be placed on the other side of the subframe plate. That get you some movement, spacers would give more.

Isn't the mount (6) going inside the subframe plate (3) normally (see pic below) or am I misunderstanding? Putting it on the outside of the reinforcing plate would give me more toe out.

39 minutes ago, EXCEL V8 said:

Are the lower rear links the same length?  Do they look like they may have been damaged/repaired?  If one was shorter than the other it would cause problems.

Pete

Take the back end apart before and nothing seemed untoward.

Edited December 4, 2020 by soldave

[soldave]

Had the radius arm off the mounting but still a bit of a mystery to me. The snubber washer doesn't appear to be sitting off the flats (and therefore not as far "in" as it should be, mount is behind reinforcing plate as it should be etc. Took a couple of shocking quality photos but nothing is jumping out to my untrained eye

Edited December 5, 2020 by soldave