To provide brief background, i am doing a highly daft and convoluted build, turning a Chapman chassis into a Haynes Chassis, then further modifying it to run MX5 gear. Long story short, i have my rear modified now to accept a Haynes IRS set up, and ultimately a sierra diff.

This is where it gets messy. Part of me wants to keep the Sierra cage, as made below:



...but the MX5 diff will need to be mounted right on the tubes where the top wishbones locate. Now, rather than modify the top wishbones to be shorter and locate further out towards the wheel, can i make them wider to take off further towards the rear of the chassis without any adverse effects on suspension geometry? My theory is widening will keep the original take-off point but keep the lengths the same, allowing me less geometry trial and error....

Have you considered modifying the MX5 diff to suit the Sierra chassis, rather than the other way round? I can't send you the link at the moment as my internet access is limited, but someone on here has mocked up a new diff casing that gets rid of the MX5 mounting and allows a more Sierra-like suspension layout. It's definitely got my vote for ease of use, even if it's an extra cost.

Edit: Found it;

http://www.locostbuilders.co.uk/viewthread.php?tid=172866

[Edited on 24/4/13 by Slimy38]

If I'm understanding you correctly (you're moving the wishbone pickups longitudinally, not transversely, so when you look at the car directly from the rear, the geometry would look the same - it's the plan view that would change?), then yes, that's fine so long as you gave no anti-squat geometry... in fact a wider wishbone 'base' (greater distance between the fornt and rear wishbone pickup) will make them stiffer and less prone to deflection under hard braking, if you design them right.

If you do have anti-squat geometry, then you'd need to adjust the heights of the pickups slightly to maintain the same percentage anti-squat, but it's no biggie if you do your sums right.

Hi Sam,

That's *exactly* the plan. As far as i am aware there is no anti squat designed into the Haynes plans, so no worries there.

Thanks for the advice!