Rear spring and shock towers

[corazon]

Hi all, been thinking around this for a while and wanted to see what anyone else thought, maybe one of you was even on the production line at some point..

So the rear spring cup towers are drilled for the shocks off centre. Mine some 17mm difference side to side.
I’m guessing this was done on a jig after the crossmember, trailing arms and diff were all fitted? Then the resting position arc of the trailing arm with spring and shock was used to align the holes..

The thing that’s puzzling me a bit is why they couldn’t be centred if the trailing arms are constantly moving in and out sideways with the dedion tube expanding and contracting anyway?

The reason I’m interested is that I’m modifying this area and plan to fit coilovers into spherical mounts that bolt into a new structural tube design.

My thinking is that I can centre them now since there is no large separate road spring to locate just the coilover top and bottom mounts, further still the lower mounting also has flexibility as it will be custom.

Thoughts?



Jim

 

[SydneyRoverP6B]

Hi Jim,

One thing you can be sure of, the mechanical engineer/s that designed the rear suspension did everything for a reason. If the holes could have been placed centrally, then they would have done so. A draftsman (draftsperson these days) will have produced a drawing based upon the engineer's calculations and design. A working model would then have been produced to ensure that it functions as per the design. If you were to contact an engineer within the school of mechancial engineering at a university of your choice, they may be open to taking a look and providing professional advice for a fee.

Ron.

 

[cobraboy]

When I fitted coilovers I measured the stepped part of the new top bushes, this was much larger than the holes that were in the tower. In order to get the coilover up into the turret the hole needed to be central, so I enlarged the hole to come centre and to fit the bush.
There was no issue at all, as there is plenty of compliance in the bushes.
I did struggle however with the OD of the spring contacting the inside of the tower at the top most narrow part. As I progressively upped the rear spring rate, the spring got fatter, I ended up shaving a little off the top coil OD of my chosen spring.

It was a satisfying conversion to do, and I thought it worked well. The bushes however due to their small size and the loading they were subjected to did take a pounding and I did change them out for fresh ones quite soon, they needed looking at regularly even though they were red poly.
On the top bushes I seem to recall fitting a 6mm nylon spacer washer in order to drop the shock down a bit in order to get the spring to clear the tower.

 

[corazon]

Yes I remember you saying that now Mark, great.

I shouldn’t have the clearance issue as I’m using a new section of structural steel tube to replace the tapered cones.

These will have laser cut steel rings welded to the top which the aluminium spherical mounts will bolt to. Time will tell how long they will last, they are standard rally offerings but I believe made by Compbrake..

Also a brace between the two new tubes planned

Jim

 

[cobraboy]

Its gonna be horses for courses, if you want a fast GT I would be wary of putting in too much solid bushing, if track focused, then no problem.
A lot of sound deadening in that area will help cancel out the harshness. I must say that my old thing kept quite a reasonable cabin environment, and I nylon bushed the bell crank bushes ! Getting those to keep squeak free was a challenge though, I would use impregnated plastic next time........... ha

 

[mikecoombs]

I've owned p6s built in both Solihull and NZ some of which were Home market cars and some colonial export cars (with tougher suspension). All of them had the hole more or less central but there are significant differences in the strength of the turret and diameter of the hole. The NZ built cars have a very thick cup on top and normally come with a large hole to fit a sheave that goes over the shock shaft and the factory rubbers are much bigger - 2" instead of 1". The shaft can also wear the hole to much much bigger and non round when the rubbers go. My current car has the tops made from at least 3/16" sheet. The bottom one is 1/4" plate.

I doubt that the position of the hole is important as they do not match or reflect each other and suspect that multiple top pieces were probably stacked and drilled with very wide tolerances given its only purpose is so the post can pass through the body. The shock is held in place by flexible rubber clamp which allow movement anyway. Minor deviations from centre aren't going to affect the shocks operation.
M

 

[SydneyRoverP6B]

I doubt that the position of the hole is important as they do not match or reflect each other and suspect that multiple top pieces were probably stacked and drilled with very wide tolerances given its only purpose is so the post can pass through the body. The shock is held in place by flexible rubber clamp which allow movement anyway. Minor deviations from centre aren't going to affect the shocks operation.

Hi Mike,

A professional engineer will never do anything for no apparent reason, everything has a logical purpose. There needs to be justification for having made a decision, in case something goes wrong later. Given the significance difference in the location of the hole, all I can say is that is down to poor quality control.

Ron.

 

[mikecoombs]

Yes Ron that is pretty much it...The Engineers probably specified it be within + or - 1/16" or so but the guys on the line would have put on whatever part they had at hand unless it was way way out knowing that the inspectors wouldn't think it was worth correcting if indeed they picked it up at all. The earlier built frames were much better put together than the later ones. My 1976 frame has some abysmal miss alignments in it but it's still square in all the right places.
M