Rear toe angles

Everything that keeps your car in contact with the road, and from contacting other road users.
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david_lall
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Rear toe angles

#1 Post by david_lall » Fri Jan 01, 2021 1:09 pm

I understand how the rear toe angle is supposed to be adjusted using shims between the rear subframe and the trailing arm brackets but the fitting to the body of the subframe mount mystifies me...

... Why is the bolt hole in the floor oversized? Presumably so that if a mount has to be changed one doesn't have to struggle vainly getting everything to line up.

However, does it mean that if a subframe mount is changed, even if the body of the car and also the subframe are supported, that the rear toe-in needs to be remeasured and probably adjusted?
Last edited by david_lall on Fri Jan 01, 2021 6:02 pm, edited 1 time in total.
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Clifford Pope
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Re: Rear toe angles

#2 Post by Clifford Pope » Fri Jan 01, 2021 5:04 pm

The shims behind the trailing arm brackets adjust the toe-in between the wheels, and nothing apart from damage will alter that once set.
The two sub-frame arms are locked together at the diff nose piece, so the two wheels on the trailing arms form an independent unit.
This unit is attached to the car at the rear of the diff, and also at the ends of the two arms through the large holes you describe.
So it is theoretically possible for the toe in to be correct but the entire unit misaligned with respect to the body and the front wheels, because of the oversize mounting holes.

I don't know how one is meant to ensure that the alignment is correct - in practice there is not a lot of choice because the position the whole assembly adopts will be determined by the interaction of the 4 mounting points.
My practice has been to set the toe-in correctly, and then check that the assembly is aligned by sighting along the edges of the front and rear tyres and confirming by eye that the offset is the same front and rear.
That's always seemed good enough, but I suppose a full laser optical alignment would do the job properly.

johnnydog
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Re: Rear toe angles

#3 Post by johnnydog » Fri Jan 01, 2021 5:51 pm

I usually change a rear subframe mount one at at time, which reduces any possible movement of the whole unit, especially if the opposite wheel is kept on the ground. I fortunately have a pit, so the car doesn't have to be raised as much for access, and the vertical movement on each side of the subframe is minimal, as the mountings are fitted from the bottom. If you are refitting the original anti vibration straps, that is a good indication of their original fitted position. Usually, there are also marks left in any underseal or paintwork on the body above the mounting, and in the grey body sealant under the seat base, that give a good indication and guide to their original fitted position.
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Alec
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Re: Rear toe angles

#4 Post by Alec » Fri Jan 01, 2021 6:03 pm

Hello all,

The oversize mount hole is something I decided to correct. I turned up a spacer to fit accurately in the floor with a central hole to suit the attachment bolt.

That done I then set the toe in.

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