The rear suspension features rising spring rate, pushrod suspension.
It’s all mocked up here with dummy push rods etc.
The idea being, the more the car rolls, the higher the spring rate seen by the wheel. It’s what the formula 1 boys do.
However, here are my problems:
- My current shock are huge
- I want to keep costs minimal
- I want to use a motorbike shock
- At normal ride I want a spring rate of around 220lbs/ft
- Most motorbike shocks have very limited travel (1.5 inch)
Most sports motorbikes have a spring exceeding 550lbs/in, for that reason I chose 220lbs/in shocks from an off-road DRZ400. Unfortunately with everything designed for off-road usage they are rather huge and butch. On paper these shocks looked perfect, about 3 – 5 inches travel, 2 of spring adjustment, 220lb springs, 2 different hydraulic adjustments. When the parcel arrived I thought ‘that’s way to be to be my shocks!’, “I hope they’ve used a lot of packaging”. Nope, it was all shocks. Big heavy duty shocks.
I buy most of my parts cheaply from auction sites and shocks for say a CBR600rr are advertised as that. The adverts do not mention spring diameter, poundage, eye to eye length or even weight. That makes things a little tricky. Brand name shocks carry a heavy premium, particularly true when you consider the first things a lot of sports riders do when purchasing a new bike is to change the shocks. These shocks; often with delivery mileage only; find there way onto the auction sites for a hand full of change. I have four new looking DRZ400 shocks, which cost me under £40 including delivery.
Aftermarket shocks are sold and advertised in a different way. A brand name shock is sold not with a particular vehicle in mind, rather it is sold by dimensions and performance. It has potentially hundreds of applications so adverts for secondhand items detail spring poundage, travel, adjust-ability, diameter, full length, compressed length etc.
With the cantilevers, I can change the length of rocker arms and the rising rate ratio. This can make the use of a small motorbike shock with it’s standard spring a possibility. At normal ride height the spring rate would start at 200lbs/in rising to the shocks’ full rate at maximum compression. If you were to stand in the car and jump up and down, similar amounts of movement would be seen, to one with conventionally mounted standard shocks, anything more than a light bounce will have a much harder spring rate.
Sharply rising spring rates are OK on super smooth F1 circuits but on a road, the car might simple crash into a bump and launch upward. Not good. Hence why I wanted to limit the rising rate ratio and use lower spring rates.
This area will need some physical experimentation. I can see me trying a few cheap shocks. – hopefully nice pretty small ones, that I can spend a whole evening watching as I jumped up and down. ☺