Board logo

Naive (probably) question about suspension geometry.
plentywahalla - 27/2/11 at 06:56 PM

I have a problem installing the engine in my Ron Champion type Locost chassis.

The engine is a GM (Lotus Europa S) Z20LER. It was always my intention to dry sump the engine to facilitate installing it in a traditional low profile bonnet line but I have only recently sourced the dry sump kit. Unfortunately I have already finished the front chassis.

I have found that there is a serious problem with the location of the pump as the pulley is right where the offside rear lower wishbone bracket wants to be. I have no alternative but to move the bracket outboard by about 25mm.

I could of course, move the whole suspension assembly outboard by the same amount, but that would involve quite a lot of work. Also it would look all wrong as the upper wishbone brackets and shock mounts would be exposed outside the nose cone flanges.

So I checked out how the geometry would be changed by moving just the lower wishbone outboard and using shorter bones. Using the 'racing aspirations' website I worked out the it would only have a 0.4 degree effect on camber over the whole (100 mm) theoretical suspension travel. As the effective travel in normal driving is probably only about 50 mm the end result appears to be negligible.

So can I do that without effecting handling? I know that it will alter the roll centre but I am able to lower the engine by 75mm in the process.

All opinions, even abusive ones, are welcome.

James - 27/2/11 at 07:10 PM

I'm not qualified to comment on the suspension TBH.

But! Thinking laterally is there no way you could get the engine further back so the pump misses the bracket? By widening the entrance of the transmission tunnel I got my Pinto about 12" further back than The Book would have it.

HTH,
James

plentywahalla - 27/2/11 at 07:27 PM

quote:
Originally posted by James
I'm not qualified to comment on the suspension TBH.

But! Thinking laterally is there no way you could get the engine further back so the pump misses the bracket? By widening the entrance of the transmission tunnel I got my Pinto about 12" further back than The Book would have it.

HTH,
James


Good idea ... but I have looked into that. The clutch release arm would move into the space occupied by the clutch pedal mechanism as my pedaL box is bottom pivot type.

britishtrident - 27/2/11 at 07:42 PM

The main effect on roll centre is the RC will move about more on roll and bump, only real way I know to counter that is stiffen the car in roll &/or bump.

[Edited on 27/2/11 by britishtrident]

Chippy - 27/2/11 at 10:40 PM

A picture of where the problems lay would possably help with anyone being of assistance. Cheers Ray

plentywahalla - 27/2/11 at 10:51 PM

quote:
Originally posted by Chippy
A picture of where the problems lay would possably help with anyone being of assistance. Cheers Ray


Thanks Ray but a picture wouldn't really help. I have eliminated all other options of moving components.

My question was to the Suspension Guru's on this site as to the the practical effects of slightly shortening the unequal length of the wishbones. I know that it will raise the front roll centre, but roll centre is a bit theoretical whereas CofG is a fact and if in the process I can lower the 120kg mass of the engine by 75mm it must be a better trade off.

James - 27/2/11 at 11:36 PM

quote:
Originally posted by plentywahalla
quote:
Originally posted by James
I'm not qualified to comment on the suspension TBH.

But! Thinking laterally is there no way you could get the engine further back so the pump misses the bracket? By widening the entrance of the transmission tunnel I got my Pinto about 12" further back than The Book would have it.

HTH,
James


Good idea ... but I have looked into that. The clutch release arm would move into the space occupied by the clutch pedal mechanism as my pedaL box is bottom pivot type.



Could you fit a hydraulic clutch actuator? I dont know what gearbox you have but Flak Monkey only had to lightly modify a Saab one to fit his type9 gearbox I seem to remember.

Alternatively, what about moving the wishbone mount laterally back (or forward) along the line of the chassis?

This wouldn't affect the geommetry (as long as you lengthened/shortened the arms appropriately) but might mean the w/bone mount clears the chassis?


Am sure someone who knows more about suspension will say your original plan is all fine soon!


Cheers,
James

JC - 28/2/11 at 08:48 AM

I would lengthen the chassis in order to make it fit, rather than shorten the wishbone. If for no other reason, if you 'bend' it slightly, you can then replace with standard parts, rather than have to make something special!

I kept standard wishbones but widened the chassis on my midi to plus 4 dimensions. Haven't driven it yet though......