Castor change

[Graham Perry]

As mentioned on another thread I am converting my 1990 de-dion car to that later narrow track suspension, with the more modern lower wishbones and uprights . Something that is imediately apparent is that the later suspension has a significantly different caster angle the original Trunnion style set up. By my reckoning the new spherical joint sits about 5mm further to the rear than the original and makes the Caster angle on the upright more ...... well 'upright'. I would say that it now has about 3 degrees whereas before it was about 6-8. Is this more or less caster ?

 

Can anyone remember if Caterham ever advised why this was changed ? Was there any advantage gained in this change apart from less self centreing and I guess less negative camber on full lock on the outer wheel and a little more on the inner wheel. Has anyone ever noticed any real difference ?

 

 

 

Edited by - Graham Perry on 29 Dec 2002 09:28:12

[Swiss_Tony]

I seem to remember reading on bc a while ago that the later (97 on) chassis has more castor built into it. Not sure why the change though. Maybe something to do with racing and not enough available adjustment etc.

Castor is only really relevant when its measured up to about 20 degrees ie you probably wont be turning the steering more than this on a race circuit anyway.

 

James

 

 

[Chelspeed]

Cars with this suspension, mine had it and that was 1993 so I guess it was introduced in 1991 or 92, had provision to adjust castor by moving washers from in front of the wishbone bush to behind it. This is the bush at the back of the lower wishbone, the one that fits through the skin of the car and is held by a bolt thro the big square vertical chassis tube. Cars built later than 1993 also had an extra bracket on the chassis behind the bush so that the bolt is in double shear rather than just single shear.

 

There is space between the chassis leg and the bracket for the bush and 4 washers. 2 washers in front and two behind gives you the mid point that Caterham recommend as a start point. To get this setting you also need two washers between the front wishbone bush and the chassis. Moving to 3 in front, one behind on the back bush and one washer on the front bush brings the wishbone back and so gives you more negative castor which is what a lot of sprinters run.

 

None of this will give you 5mm of movement though, maybe 3mm max (from 2 washers to none). I can only suggest you build it and then get the geometry checked.

[Graham Perry]

Chelspeed , thanks

 

G

[Graham Perry]

I have finally finished my rebuild of the front suspension, and the difference between the old and the new is really noticeable. When I embarked upon fitting the later narrow suspension on a 1990 car a few people advised against it as it would apparentlty be quite twitchy. However this has proved not to be the case with the two washer either side of the rear mounting setting. The car feels positively enlivened and now has much lighter steering that seems to turn in better at low speeds (not tried fast stuff yet) even though it has the 22% quick rack. The visual appearance of the suspension is much better although the springs sit at a much flatter angle than previously which will no doubt accentuate the falling rate of the set up. Whole thing seems well worth doing if only just to get rid of the trunnions.

 

Why did it take so long ? Well my car has hand made ali cycle wing stays, and I discovered that with the new set up the old ones wouldn't work so they had to be remade and recoated. Don't ever attempt to make your own stays its an absolute nightmare as there are so many things to consider and even I began to lose patience as I removed the stay for the fifty odth time due to something else fouling a bracket.

 

Next thing is to test it at a sprint to see if I can set it up properly. Thanks to those of you who offerred advice.