You don't need to apologise, Johno, this is clearly on-topic. From your photo it does look like the wheel axis is a bit further to the front than the kingpin axis, and I assume Andy left the caster angle the same as in the Sierra-based wishbones.

Actually, it's easy to test-self centering in the stage you are in, just put it on the wheels, turn the wheel to the lock, and push the chassis a couple of feet and back. My steering wheel stays where it was on the stroke forward and unwinds a bit when pulled back.

Johno, robo posted an amazing link about suspension geometry a while back:
http://www.haynes.co.uk/forums/showthread.php?t=7371

Hi Alga good site,
Still trying to get my head round it all (too much info:eek: :eek: ).
I'm going to remake my top wishbones soon and need to readup on this before I fab them.
But I actually found out what "bump steer was this weeend".:D
I fitted my new wishbones and moved them throughout their travel with the shock on and noticed the wheel moved in and out...LOL

Johno

Dont panic the steering rack has to centralised mm perfect before you can run that check.

Edit ,another usefull site http://www.optimumg.com/OptimumGWebS...me3_legacy.htm

Bob

Alga, how much caster have you calculated for the original setup?
After a bit of work on CAD, i came up with this rough figure.
I assumed a vertical distance of 200.5mm between ball joints (according to this drawing) and a slight divergence between the upper and lower wishbones towards their outer ends (ball joint ends).

According to my calculations, by offseting the wishbone chassis mount by 2.5mm you increase caster by 0.7deg and by making a new wishbone with 10mm more offset you get 2.8deg of caster! For 5mm offset wishbone you will gain 1.4deg

Original setup: 5.73deg
2.5mm offset chassis mount: 6.43deg
new wishbone with 5mm more offset: 7.14deg
new wishbone with 10mm more offset: 8.54deg

Reading a geometry article by Nigel Dean handed out by Track Developments at a Kit Car show he recommends 3 to 3.5 deg for a 7, 5 to 7 degrees for a Cobra
How much are you guys looking for ultimately?

Self-centering is a safety and stability feature.
If you don't get any sufficient self-centering effect with 5.5deg then you have to go for more.
I think that the best solution would be a new top wishbone with 10mm more offset and rose jointed so you will be able to fine-tune the caster angle according to your specific needs.
I will seriously think of making a pair for my car as soon as i finish building it. Maybe earlier than that..

Thats exactly what I intend to do. The book bones needed an ammendment yonks ago to at least put the caster situation in the picture. Its like the rack being 27mm to long, why have the chassis brackets not been altered 13.5mm on each side to put the bump right. Surely just a few bits like this is going to make a huge difference. Improve the breed with a tinker:)

bob

you may as well ditch the sierra upright and go for some light cortina rep front uprights and redo the whole lot in one go

you cant polish a turd and rolling it in glitter is messy :rolleyes:

A first mock-up: