Alignment of subframe when fitting?

[Nuvo]

When fitting a subframe to a RHD car how should it be positioned?


I seem to recall someone saying in the dim and distant past that the subframe needs to moved as far forward as it will go on the driver's side to compensate for camber on the left hand side of the road. I may have got that completely wrong, I'm hoping someone has a definitive answer to this.

No reference is made to it in any of the workshop manuals I have.

As always, thanks in advance.

[LHD_Kid]

I am also VERY interested in this, but for a LHD car... Front and rear frames..

[Mikes2]

front right bolt is fixed and the others can be moved within the chassis by small amounts to compensate for all the tolerance differences so it rotates slightly. It's better to get all the bolts attached and subframe in place before moving it. And if you haven't disturbed the captive nuts, I'd just put it back the way it was. I measured from the centre of the bolt heads left and right to a fixed point under the car and made them the same.

[Nuvo]

Hi Mike, thanks for the quick reply.

This is for a b3 cabriolet, the rear left bolt is fixed on this paticular car.

So is it correct to say that the movement of the subframe is soley to allow for tolerance differences and has no bearing on road handling and or chassis aligment?

The captive nuts have been disturbed from their original positions which makes that impossible. Measurements from fixed points on the chassis would have been the way to go if the subframe was still attached.

I think the only thing I can do is do it by eye and try to have the bushes sitting symmetrically on the chassis leg mounting points. Not very precise I know, but that seems to be the only choice I have.

[piotr]

I did try this before on my car and I dont think the subframes are adjustable .there is no movement at all .once you start tightening the bolts the frame will only fit in one position. If I'm wrong i would like to see some workshop procedures /manual or alignment tools for doing this job .

[steve briance]

Well, it is adjustable a small amount, it rotates around the fixed mounting point. There is a small amount of float with the other 3 captive nuts and theoretically it could be adjusted. I've just had my subframes off and to be honest I think the captive nuts are there more to accommodate tolerances... Once I'd got all 4 bolts in and had them screwed in but not clamping; all the float/adjustment had gone. I just torqued up the bolts and left it like that. 4 wheel alignment was still possible.

[Lago Blue]

Nuvo,

- If your rear left mount bolt is fixed, is that on the rear sub-frame?

- Did you perhaps see this particular post:

https://www.s2forum.com/forum/technical/suspension-steering/14519-wierd-thing?p=259052#post259052

[Nuvo]

Lago Blue this is a front wheel drive B3 Chassis, so no rear subframe. The rear left bolt is the fixed pivot point as viewed from the position of the RH driver's seat to clarify.

Thanks for the helpful link.

I've not read it through fully just yet, but I see it mentions taking measurements between the rear edges of the wheel arch and the wheel centre. This will be useful to determine if the is any skewing between each side.

[Lago Blue]

Nuvo

It may be slightly better instead that measurements between actual accessable wheel features be taken, for example: with a helper and a tape held horizontal on the wheel center-line, measure from the trailing edge of the front wheel center hole or wheel edge, to the leading edge of rear wheel center hole or wheel edge, then compare side to side), as this may help by-pass / eliminate any tolerances of the chassis which may exist that others have mentioned above, and may help to "square" the theoretical front axle to the rear one; - if- you haven't moved the rear one, if you see what I mean.

[Mikes2]

lhd / rhd doesn't make a difference. The adjustment is very small. You basically want it to be straight. You will see a castor measurement when you get it aligned - it's usually that value that tells you how straight it is

[Error404]

Assuming you will do the 4 wheel alignment then subframe alignment is pretty much irrelevant.
If course you might have a very tiny different in wheel base side to side, but if the wheel geometry is correct then it will be negligible and not even worth measuring. Certainly measuring off the wheel arch will be completely pointless, I bet they move 5mm when the car is on a lift.
You can correct minor castor differences by slewing the ball joint slightly.

The build tolerances on these cars is loose by modern standards. An example of this: I machined some custom alloy subframe bushes recently and the brief was to make the chassis as stiff as possible, so I reamed the bolt holes to 12.02mm for maximum install rigidity. When it can to fitting, the subframe wouldn't go on the car - I had to open the bolt holes out to 12.20mm before all 4 subframe bolts would engage the chassis pickups.

[Nuvo]

Thanks chaps.

Sounds straightforward enough, get it straight and then get the car geometry aligned properly by a professional afterwards.