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MG MGB Technical - Roll centre

Continuing along the saga of constructing a Panhard rod. Can someone explain to me about roll centres? quote; 'one needs to take roll centres into account when fitting a Panhard rod to any car.If rear roll centre is too high the car will not roll enough resulting in oversteer. if too low you could have understeer.' Is it simply a case of how high the axle is off the ground, or the rest of the body, or the centre of gravity? Mike
J.M. Doust

Hi, I have found this, I hope is helps.

http://thecartech.com/subjects/auto_eng2/Roll_Center.htm
J Wale

Just theory on an MGB, surely. You can only attach the rod to the axle and body such that the rod is more or less horizontal, and live with the results.
PaulH Solihull

At the risk of sounding negative...If you really want to improve the (rear) handling you will need to start again. (Hoyle or maybe Frontline). Wikipedia points out that Panhard Rods were seldom used for leaf springs which have more inherent lateral stability that coil springs. Just how much lateral movement can there be? (The bushes?) I can see that increased power can move the springs/axle longitudinally by lozenging the spring, but not so sure laterally. Just my two penn'orth.
Michael Beswick

I have actually experimented with different panhard rod heights and it does make a big difference to handling. When I made and fitted mine, I was advised to make it parallel to, and in the centre of the axle.
I have found that the optimum height for my car is approx 2 inches below the centreline.
If you fit one, make it as long as possible.
Lowering the rod definateley reduces understeer.
This is mine cornering with some Lotus boys.
http://www.youtube.com/watch?v=muIYiftiflg
Mark
M Rawlins

Mike,

If you are going to do this seriously, the 'thecartech' article above is where you must start. A chap in our local club undertook this on his A some years ago. He has a unique rear suspension supplemented with stainless wire torsion lines. Works a treat in sport applications.

He did all his own research and development and wrote a 13 part article series for the club magazine on the project. I should point out that this chap 'devours' physics and advanced mathematics as a hobby interest. And by research I mean the serious scientific/mechanical literature sources.

I've helped him out on measuring his vehicle weight at the four wheels, and he did once give me a demo of the structures he has installed - together with an overview of the theory involved. We settled for a beer and a chat about the weather when he noticed my eyes glazed over.

Other than that approach I think Paul, Michael and Mark offer the practical alternatives.
Roger T

Mike
A car with leaf springs has a theoretical roll centre height built into it.
This height can be measured by-------
Having your car at normal ride height and running a string line from the centre of the rear spring's front springeye through to the rear springeye-
The point at which the stringline passes the centreline of the axle housing is deemed as being the rollcentre height-----Usually on an MGB this is a couple of inches below the centre of the housing
As Mark has found above
In my opinion this is the height your panhard bar should be
The theory is that the car already has it's rollcentre built into it at this height and if you fit a bar at any other height your car will then have two mechanical roll centres fighting against each other causing your rear suspension to bind up as the body leans

Your bar should be parrallel to the ground, as long as you can possibly make it and mounted to the body on the drivers side and to the axle on the passengers side

IF the car has a very firm suspension and negligable bodyroll the bar could be crept down just a little to suit driving style but the bottom of the bulge in the banjo housing has been proven to be the absolute limit, but ONLY on a very stiff suspension car as mentioned
Willy
William Revit

Willy
As I understand it you are correct the natural roll centre is the height as you describe which on my car puts it around the centre of the axle. By fitting a lowered panhard rod you are lowering the roll centre, hence the improvement in handling which was very noticeable. I would like to experiment with lowering it further.

Mark
M Rawlins

Michael-
A Panhard Rod that has a center-to-center length of 36" with a vertical movement of the suspension of +/- 3" will produce a lateral movement of 0.125" (1/8”), and a vertical movement of the suspension of +/- 4” will produce a lateral movement of 0.22” (less than 1/4"). This is a far greater restriction of lateral movement than that which is attainable with even the hardest rear leaf spring bushings. With the Panhard Rod maintaining the lateral position of the axle, you need not install hard polyurethane bushings into the leaf springs in an attempt to limit the fulcrum effect of a swaying rear axle. All that harder bushings would do in such a case would be to transmit more wheel vibration, noise, and road shock. With a Panhard Rod, you can retain soft bushings and have the best of both worlds. Before you mount a Panhard Rod, you should do a four-wheel alignment.
Stephen Strange

Mark
I have been studying your youtube clip and yes I agree that you could possibly try dropping your bar down a little more which should give you a bit more drive out of the corner and as you have found, help reduce understeer as well
A side benefit of this if it further reduces understeer is that then you can go up a size with your front bar and gain some extra drive out of the turns , which appears to be a little problem still
Cheers Willy
William Revit

Willy
I have in fact found that reducing the panhard rod height reduces oversteer. I have never really had a problem with understeer. The following clip is when the Panhard rod was centre height of the axle and you can see the excessive oversteer on some of the tighter corners.
http://www.youtube.com/watch?v=n5wGQC0zM14

Mark
M Rawlins

Stephen
Thanks for that-I will store it away as a potential "next stage". My 69 B Roadster is supercharged and I have fitted parabolic springs with Koni Classic shocks (+ polybushes). The first set of springs lasted about 3 years and 12k miles before the left side "lozenged". The replacements started to lozenge after 3 months.The best the MGOC could suggest was to fit rubber bumper parabolics which are also supplied as "suitable" for V8. I have had the original parabolics repaired, re-tempered, and a third half leaf added form the centre forward, as a sort of anti-tramp bar. I have not had much chance to try it since I fitted them. The original reason for fitting paras was to get a more compliant ride -the car is a tourer not a racer. The ride was always most comfortable with 2 up with luggage. (Permanent holidays was one suggestion!)
But I have always felt the rear end was a little "hoppy", just for normal use on smaller roads with the usual lumps and bumps at modest speeds. I followed the original advice of having the shocks as soft as possible. After some more trials I intend to firm up the shocks a bit to see the effect. This is not based on much more than trying to alter one thing at a time, rather then an understanding of suspension systems, (as you have no doubt deduced!)
Michael Beswick

Mark
I was using what you had said earlier as a base for comment
You stated that
"lowering the rod definatley reduces understeer"
I read your post and mistakingly wrote understeer instead of oversteer -but in my head I was always thinking oversteer as I am sure that is what you meant as well----------my appologies there
Even if you do lower your bar again which would definatley be worth a try,I would still be tempted to try a bigger front bar . Just looking at the pics, that to me is what it needs now
Again - my appologies for the confusion there
Willy
William Revit

OOps
Sorry always meant oversteer.
I am using the 1" race front ARB
Have fitted different (stiffer) front springs as an experiment.

Mark
M Rawlins

Cheers - Sounds like you're heading in the right direction
What was the result of stiffer springs , How stiff are we talking 700 ????
Willy
William Revit

This thread was discussed between 08/02/2012 and 14/02/2012

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