Front Suspension – Marks Cobra Build

Setting the bumpsteer was interesting I firstly tried clamping straight edges and using line of sight to keep them parallel. This did identify how far out it was to begin with (quite a bit) but I didn’t feel confident about achieving the best position this way since it was difficult to perceive very small changes.

Out with the trusty Hilti Laser (this is a self-levelling type of laser so I clamped that on the wheel disk fired straight ahead, in the direction of travel onto the garage door a few feet away. After some plotting of curves it quickly becomes obvious that you can’t really do it this way since the observed travel is a combination of bumpsteer together with scrub (the in/out movement of the wheels caused by the wishbone radiuses) and camber change effect (caused by the unequal wishbone lengths). Geometry-Model wishbones

You could use the above method by calculating the amount of scrub then plotting the curve that is created by the radius arm travel Then whilst measuring bumpsteer the best line that could be achieved is the compound curve shown above right. But even then you would also need to account for the camber change that is happening throughout this movement.

The method of shooting the laser perpendicular to the wheel disc is better because this eliminates the scrub effect since the laser will not move “off line” due to scrub, it moves off line only due to bumpsteer. ( plus a bit of vertical change due to camber change).

You could use a mirror clamped flat to the disk and fire the laser at the mirror as has been suggested before but I’m not convinced what the mirror brings to the party, I feel it just complicates the setup.

With a couple of simple clamps fixing the laser approximately perpendicular to the disc at any point will allow exceptionally accurate readings to be made very swiftly – horizontal movement of the laser line only represents the element being induced by the steering arm…. Job done.

The further away you can put the target the better since any slight movement is exaggerated, in my case I was able to get an almost vertical line at 4m distance that’s definitely zero bumpsteer (well near enough). Again well done GD for excellent geometry around the steering rack chassis elements. This would not be possible unless everything was in excellent geometric harmony.

Ok that’s the end of this boring bit (Civil Engineers white helmet off – petrol head back in place…..)

Did I mention I was impressed with GD’s chassis geometry J?

As you can see I have been rather milking this part of the build simply because I have no other parts… However things should start getting real busy soon. Just had confirmation my engine gearbox clutch atc are all arriving next week then i can start with engine installation wiring etc. plus body is still on schedule for early May so lots of action coming shortly..

Found the short Drive Shafts and Wishbones, these aren’t essential as I already have the standard ones but I really prefer the deep dish on the rear Wheels – Keith A has used these same shafts on his build and has recently acquired his wheels – they look just right.

On Cue received the Diff and hubs back just before the Easter break so that gives me plenty to go at Easter Weekend.

I have identified a small problem with the front wishbone bushes. I used Superflex bushes after and could not get them to close up properly. In the end I had to dismantle them and take a closer look. Turns out the internal crush bushes as supplied by superflex measured 37mm. This just wasn’t allowing the bushes to compress and i worked out 2mm needed to be turned off the internal bush length. Just to be certain Keith A sent me some close up pics and measurements of his set up and these confirmed the bushes need to be 35mm. I called Superflex and they were helpful but too slow. By the time they sent me revised bushes I have already turned the originals down to size and everything fits fine now…

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Rear Axle put together on the Saturday and chose to lift it in since I already had the front end built up so it was getting a bit heavy to lift the chassis – seemed easier to lift the axle with the help of the engine lift and a jack to control the angle it went together superbly. Compliments to GD jig work as these bolts around the diff are pretty tied up little room for error when the weld the frame up.