RSR rear trailing arms

[tar356]

Has anyone converted standard steel arms to RSR spec "short" arms?

Is it possible?

Thanks -

[ret9191]

Yes I did and i kept one set for a future project. My fab shop also has the jigs I had them make. Takes a lot of time and the bearing holder has to be replaced. Cost is around $1500 per side. I also have RSR center feed cam oil fittings and and had RSR sway bar mounts made. check it out here.

http://www.pbase.com/slidevalve911rs...ished&page=all

[tar356]

Thanks for the reply...

Any photos of the arms during the modification process?

[Flieger]

Would it be easier to use Turbo arms if one was not concerned about originality?

[chris_seven]

We have converted several sets of steel arms to RSR spec and they work reasonably well.

The 3.3 Turbo arm increases the track more that the steel RSR arm which means it may use different torsion bar tube mountings.

[tar356]

I would really like to see some photos of the conversion.

Also, is the trailing arm mounting point on the RSR in the same location as the 930?

[chris_seven]

Also, is the trailing arm mounting point on the RSR in the same location as the 930?

I am not 100% sure what you mean by a 930 comparred to an RSR but I will try to explain what I know.

We originally shortened some 1972 Arms to fit to a pre-1968 SWB car. These arm have better bearings and the vented discs are much lower cost than genuine SWB parts.

Using this layout of arm gives a track increase of about 12.5mm per side as the spring plate to hub face is longer on this arm.

(We also converted a 1982 SC shell to SWB including fitting narrow wheel arches with 1969 type flares, so we made another set of converted arms)

If you took the aluminium 3.3 Turbo Arm and fitted it to the SWB car the track increase would be about 25mm per side as the length from the spring plate mounting face to hub face is greater.

The RSR had an arm which is the same basic geometery as the SWB arm and if you want to maintain the LWB geometry with the short arm the trailing arm mounting on the torsion bar tube must be extended and moved outwards.

It is a simple matter of the radius of the arm being constant but the arc has effectively shortened.

This leads me to believe that the tariling arm pick ups on the 3.3 Turbo with the short alloy arms must be in a similar location to the RSR. Some production RSs' also had short steel arms but I don't know

The track of the car is dominated by the spring plate position which obviously can't change, hence the variation in axial length from spring plate mounting to hub face being the ruling dimension.

The other final small detail is that if you convert a post 72 shell to SWB then the shocker mounting bracket that you need to add to the arm will have to be increased in length as the rear shock tower moved backwards in these later shells.

I hope this isn't too muddled.

[Flieger]

From reading Anderson's book it seems both 930 and RSR had similar pivot locations, moved up and back. I think the 930 may have been a little bit higher for more anti-dive and anti-squat.

[tar356]

The track of the car is dominated by the spring plate position which obviously can't change, hence the variation in axial length from spring plate mounting to hub face being the ruling dimension.

That makes sense....take a long arm and shorten it, bolt it to the spring plate, and your mounting position on the torsion tube would be determined automatically (at least for the lateral location).

[chris_seven]

From reading Anderson's book it seems both 930 and RSR had similar pivot locations, moved up and back. I think the 930 may have been a little bit higher for more anti-dive and anti-squat.

Max,

Very interesting and I had a quick look at Bruce's book and am now a bit confused.

Bruce states that the pivots on the RSR on moved toward the centre of the car by 15mm and backwards by 47.5mm.

This has several implications.

If the arms are shortened then the pivots must move outwards unless the curvature of the arm or the spring plate location moved.

If the pivot is moved backwards by 47.5mm then either the arm is not shortened to the SWB dimension or the wheelbase of the RSR is 5mm-ish less than the LWB car.

I have always believed that the RSR Arm had the same Geometry as the SWB car in terms of arm length and curvature and that the Aluminium Turbo Arm was identical in this respect.

We made our jig from a SWB Arm and we have checked it using a Turbo Aluminium Arm which fits perfectly hence my confusion.

Can anyone shed any light on this - please.