After a solid day’s worth of googling chassis diagrams, measuring, rusty maths and general head-scratching I am done! In order to work out the best place to mount a custom lower wishbone to get a decent camber curve as the suspension moves this is a to-scale plan view of a rotoflex GT6’s rear suspension :)
The idea’s cribbed off a chap called GT6 M over on Club Triumph (who pioneered the adjustable lower a-arm for Spits/GT6s using repurposed front tie rods), but with a few improvements based on his experience. Primarily, he used another piece of wood for the upper link, but as the upper link on a Spitfire is a leaf spring it bends as it arcs upwards, pulling in the top of the wheel more than it otherwise would do with a solid wishbone.
I contemplated working out the maths to calculate the curve and quickly decided that my decade-old education is thoroughly inadequate for that sort of complication. So, I thought I’d make a leaf using some thin-wall PVC oval tube I found lying around :) works rather well!
First impressions are that the camber change seems quite large (although I’m yet to work out what the actual suspension travel is yet). Next is that surprisingly it’s in the right direction. From a brief look at suspension location, you’d know that having a much shorter lower link will cause positive camber under compression as it describes a tighter arc, pulling the bottom of the wheel inwards more.
That’s counteracted here by mounting it at an angle, meaning that it pushes the bottom of the wheel out under compression, resulting in negative camber. The trade-off for this is increased positive camber under droop.
Now the fun bit :) the image below is the main chassis rail (the red box) and 4 potential new wishbone locations :) (the F, R and M are how far the chassis drops down from the main plane at the rear)
Using some new scraps of wood, I can do this!
Much, much reduced camber change for the same suspension travel :)
Next up is to read up a load and do some more head scratching about what the camber curve should be, and experiment to try and find a location that fits that best.
Then, I’ve got some more head-scratching to do to make sure that my chosen location has an acceptable roll centre.
Then then, I’ve got a couple of popular aftermarket wishbone locations (and a few homebrews) to test out as well.
Then then then, I get to do it all over again with a new diagram overlaid in blue to simulate a 3/4" lowering block under the spring.
Then then then then, I can actually make a jig for the brackets, make the brackets and weld them in place.
Then then then then then, I’ve got a cunning plan for making adjustable lower links... ;)
Not much to do then ;)