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Stiffening the Suspension

by

Ade Colmar

This article describes an alternative approach to reducing the travel and increasing the stiffness of the suspension on a Pembleton by, somewhat conversely, increasing the travel of the bits that do all the work.

Most cars use a suspension ratio of in between 1:1 and 2:1 (that is the ratio of wheel travel to spring/damper travel). The 2CV donor uses a suspension ratio of around 3.5:1, which puts it in a different league, but then the 2CV is an unusual car! It was specifically designed to have very supple, long travel suspension. The high suspension ratio creates large forces in the components, but the Citroen engineers ingeniously cancelled most of these out by placing the front and rear springs in the same cylinder. This leaves only the difference between the front and rear suspension forces being transmitted to the chassis.

Due to the layout, the Pembleton design has to do away with this bit of engineering subtlety and requires all the suspension spring forces to be taken directly by the chassis. Consider that a sprung corner weight of 150Kg translates to 3.5 times this amount (525Kg force) on the chassis and the demands start to become obvious. Steel is fantastically strong in tension and the 2CV spring cylinders are only 1mm thick and the forces are perfectly aligned. In the Pembleton three wheeler, these forces are taken through a mixture of compression and bending in the chassis outriggers and tension in the outrigger strap and inner longitudinal member. Another salient fact is that the load on the rear wheel of a limper is actually greater than that on the each of the two rear wheels of a 2CV when carrying a driver and passenger. So it is not surprising that there have been challenges leading to reinforcement of the spring hangers over the years.

This is only part of the thinking, the main objective is to get stiffer suspension. If this can be done at the same time as reducing the forces on the chassis, then so much the better. As an alternative to oversize springs or spring assisters another option is the increase the length of the fulcrum arm (the triangular part with the knife edges in). This is around 115mm for the front and 125mm for the rear suspension legs (it is hard to give an exact measurement because the knife edges are over centre and so the fulcrum length varies with the attitude of the suspension arm). There are practical limits on how much the arm can be lengthened due to the risk of grounding and tie-rod angle, however, if the lower edge of the spring/spring hanger are used as a guide for grounding, then the fulcrum arm can be increased to 160-170mm reducing the suspension ratio to 2.5:1 and giving 35% stiffer suspension. Just as important, this also gives 35% more travel for the dampers making it easier for them to do a decent job without cranking up the settings to the limit.

I lengthened the fulcrum arm on my car with cuts through the steel triangle 10mm from the base of the main arm and welded 3x 50mm steel strip over the gap as shown in the pictures. This necessitates access to welding facilities, but it is a lot easier the get a suspension arm welded than a chassis repaired!

suspension arm
suspension arm

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