Front roll centres and spring rates

[RaceM]

Has anyone properly modelled the Minor front end to get Caster, KPI and geometric roll centres?
Also has anyone actually measured the installed rate of the torsion bars at the wheel?
If so could they please post their figures?
I have used the search function but have not been able to find much.
Many thanks Neil

[Jefftav]

I'm afraid I have no idea what you are talking about but if you find jonothan on this site http://mog.myfreeforum.org/
he might be able to help?

[bmcecosse]

Caster is supposed to be 3 degrees and KPI 7.5 degrees. But then the camber is supposed to be zero - yet most standard Minors sit with fairly obvious positive camber...... I believe the front roll centre is somewhere over in the next carriageway...... But do ask Jono at JLH ( http://www.jlhmorrisminors.co.uk/ ) - he has done more work on the front suspension than most folks.........and perhaps you could come back with the findings ?

[MartinB]

I established where the roll centres are many years ago, it depends upon what ride height of the suspension you are using. When mine was lowered a lot so that the top trunnion was above the point at which the lower part of the top bump stop should have been, and with 20" OD tyres, the front roll centre was 1" below ground level. The front suspension had coil over shocks with 400lb springs and torsion bars fitted. Maximum suspension travel was about 3". Throughout that travel, the roll centre was very well located, barely moved 1/4" from max bump to max droop. What it is like at standard ride height I don't know.
The rear roll centre depends upon what axle location you have, mine had a panhard rod in line with the axle tubes so it was probably about diff centre height but obviously moved around a bit depending upon lean angle etc.
With the very low front roll centre and the high rear roll centre did make the car skew roll and had a huge tendancy to lift an inside rear wheel. I fitted a very stiff front anti roll bar to cure that.
Forget all the crap about stiff front anti roll bars creating understeer, this created an extremely balanced car that could be 4 wheel drifted on slick racing tyres in 4th gear (on race circuit of course), even with a heavy old Fiat engine up front! It even managed to lift an inside front wheel slightly occasionally doing that. The only problem I found was the front spindles flexing a bit due to the increased loads creating pad knock off on the brakes.

[RaceM]

Thanks for all the responses.
I should elaborate that my car uses the MM front suspension, but is not a MM. It is instead a Rochdale Olympic.
In the Rochdale the tie rods are replaced with a 5/8 sway bar (which is not really a good idea given the differing arcs of travel), so there are some differences. My interest is in finding out what works in a MM and then comparing the setup to how the suspension is mounted in the Olympic. I currently have koni telescopic dampers, which will be switched out for Bilsteins, with some additional coil-over springs (what is the rate of the torsion bars really?). I'll make an additional subframe to support a tie rod and an adjustable sway bar (made from a hollow torsion bar). Rear GRC is about 4" loaded. I too have a 150bhp fiat engine, but the water pump pulley is about 4" behind the front axle line, which may be a bit different to a MM?
The GRC below ground is interesting. As long as it stayed there I don't see a problem. KPI is a bit higher than I expected and caster about where I though it would be. Your 400lb springs - what do you think the installed rate would be, i.e. where did the spring act on the lower arm?
The pad knock off...do you think it was the pin flexing, axle flexing or the hub/bearing moving? I have one of OB's marina brake conversions ATM and well, it works most of the time.

[MartinB]

The engine in mine then was atmo 16valve 2 litre - 190bhp at the wheels.

The lower damper mount was as close to the lower swivel as you can get without it rubbing, really close, I think the wheel rate from the spring was in the region of 370lb/in. The anti roll bar was made fron a shortened torsion bar and was acted on by long drop links that attached to extended upper trunnion nuts where the lever arm damper attaches, therefore the anti roll bar saw the wheel movement 1:1. It was so stiff, when the car was jacked up on one side, if I jumped up and down on the front of the car that was on the ground, the one in the air went up and down a bit too!
I used modified escort hubs on mine, my own conversion, with vented discs etc. I had experienced pad knock off before that set up with different brakes/hubs and softer suspension but that was when going over curbs/rumble strips aggressively, with the stiffer set up it would happen just through corner loads. I really don't think it was the hubs. When I later changed the front suspension for Cortina based uprights etc, I used the same discs/calipers and have never had it since (the stub axle is much bigger and one piece with the uprght).

Please note, I established the roll centre at the suspension height my car ran, it will very likely be much higher than this at a higher ride height and more than likely won't be as well located, you really need to measure it all out for your particular settings.

[RaceM]

Good to know. I have a 16V head and forged pistons, but will not use them. It has enough power and the added heat from having more power would not be welcome in warmer months (fibreglass burns!)
I currently have the damper mount at 60% of wheel rate, attaching roughly where the original tie rod attached. I'm keen on moving it out further and getting more damper piston area for the lower wheel mvt.
I'm also keen on adding another arm to assist the lever arm at the top - it hardly seems an ideal arrangement - hes this been tried on a MM?.
I have Susprog, so I can model the suspension well once measured, but I am still interested in the MM experience given the component use. Pad knockoff occurs in some cars locally. The fix is to often increase the size of the bearings in the hub and sleeve the axle to stiffen it. However I'm not planning to run 13" wide slicks, so I doubt this will be required.
I am not a great believer in GRC's, but they are useful in predicting rate of weight transfer. I currently have neg one degree of camber - did you try more with the MM parts?

[MartinB]

I ran about 1.5 degrees of negative camber but that was for cross ply racing slicks. Initially I did this in the usual fashion of packing out the eyebolt but with the last incarnation of that suspension I made longer lower arms in order to put the lower pivot back where it should be, I did this using 2 sets of lower arms cut them in the appropiate places and welded the longer bits together, one weld on each arm (TIG welding of course).
On the MM, the engine is pretty much fully ahead of the front axle line so the suspension settings will be quite different to yours with it set back in the chassis.

When I established where the roll centres were, it became blindingly obvious why my car was doing what it was doing and allowed me to develop it properly at that stage.
Mine had and has got fibreglass bonnet, wings, doors etc, the bonnet hasn't caught light even with the turbo glowing close to it! (engine spec has been well over 400 bhp for the last 7 years or so), the heat generated from the atmo 16 valve will not be any more of an issue than your current 8 valve engine and the head is so much better than the 8 valve!

[RaceM]

I too have packed out the eye bolts. Modifying a lower arm is out of the question however as the car is used on public roads and such modifications are illegal here.
I have 2" of clearance between the headers and the side of the engine bay, where the FG is very much structural. Despite two layers of silver backed insulation it was routinely at 60 deg C - where FG loses more than 50% of it's strength. Alloy heat shield helped a lot, but the Olympic engine bay is sealed from the Rd and thus has no native airflow. - this has been rectified and continually flushing the area with air has helped most of all, except when the ambient temp is over 30.
150bhp is enough, I have built other cars to go faster in, the Olympic needs to retain some of it's original charm!