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MG MGB Technical - Reducing snap oversteer?

Ok... My car has negative camber A-arms, 15" Panasports, 195-60-15 tires, 550lb front springs, race rear leafs (can't remember the rate) stock '80 front and rear anti-roll bars, stiffer valves in the Armstrong shocks, and slightly heavier weight motorcylce for oil.

On a couple of occasions, I've run into "sneaky oversteer"... so I have 2 questions, first is, how to perhaps minimize oversteer, without inducing understeer. Stiffer front anti-roll bar? Softer rear?

Also recovering from oversteer seems to be a little extra-tricky versus some other cars Ive driven. It tends to "snap" back, or swing back the other way quite easily. And the car remains upset for a short time after recovery. It has to kind of "settle back into line".

Im looking for better track / autocross performance, so in limiting these traits, Im looking to make the car more manageable at its limits. Not so much add in safety margins, although that is a consideration.

I have constructed parts for a Panhard bar, but after reading online a bit, im not sure if i want to finish construcking it, as ive read it tends to upset MG handling, or at least, make the handling in right vs left turns different... And I'm not sure it would have any effect on the oversteer anyhow.

Thanks in advance for the suggestions.

-D. Bach
Dylan Bach

Dylan I have not had experience with this myself but I know most of the MBG racers here prefer to run without a rear anti-roll bar for this reason. There is a good NZ sight on http://www.mgparts.co.nz/menue_technical_notes.html that explains the reasons.
DENIS4

That does not seem to work so try typing in.
paul walbran motors technical notes NZ
DENIS4

http://www.mgparts.co.nz/index.html

Clifton
Clifton Gordon

I am not a track expert, but I am planning some radical suspension changes and have done my homework.
The summary-
Big antisway bar and uprated springs in the front to control roll. Because my car was a rubber bumper, the shorter uprated springs have produced some negative camber for free.
Soft springs in the back to allow the live axle and its tyres to stay as much as possible on the road
No rear antisway bar for the above reason.
I am about to fit an a panhard rod. Lots of research says;
it should be parrallel to the axle and dead level at ride hight and as symetrical and as long as possible. LH vs RH corner differences otherwise.
How high or low you set this level will determine how much body roll at the rear you get. Low = more roll, High= less roll and more oversteer.
I intend on starting at spring height and then adjusting to suit. The main reason for fitting is to stop the axle moving around on its shackles. I have noticed that there are faint tyre marks on the inside of my RHS wheel arch (15 by 6 inch wheel, 195 by 60 Michelin xm1 tyres). There is at about one inch of clearance when the car is stationary/straight line. That means there is an awful lot of movement going on back there and stopping it can only be good.
Bump stops-make sure you have at least 2.5 inches of upward (from ride height) suspension movement available. If you don’t have this then in corners the outside bump stop may contact and leverage weight off the inside wheel thereby causing sudden horrible surges of oversteer. I found this out by lowering my rubber bumper (now a chrome bumper) car to chrome bumper height and not cutting a bit (20mm approx’) off the bumpstops.
When I did cut the bumpstop short, the horrible surges of over steer went away.
On one occasion this snap oversteer caused a 7 end for end spinnout! This event promted some serious thought as to the cause, and a solution.
Peter Sherman

Dylan, I'm certainly no expert in racing suspension, but it sounds like your car is snapping when the rear spring internal leaf friction is overcome. I am assuming that you are still using the leaf springs on the rear. My experience is that adding some type of plastic strips between the leaves of the spring makes the spring rate more progressive and less prone to the quick movement experienced when inner leaf friction is overcome. To some extent the same applies when returning to a 'normal' ride position, the spring doesn't fully rebound because of internal friction and usually comes up to a point then 'inches' up a little at a time as the spring works over normal road irregularities. Suitable material is available from suppliers of street rod suspension parts such as Speedway Motors, although it's a bit wide for the MG springs it can easily be trimmed to fit. This is the type material I used. http://www.speedwaymotors.com/p/2079,323_Spring-Liners-with-Lip.html

Photo of material attached.

Bill Young

Thanks for the info guys... Now that you mention it, I have about 1.75" (or less) of travel before my rear axle hits the bump stops. (it too, is a RB car converted to chrome... or Sebring, without the chrome bumpers.) I guess I'll start by cutting the bump stops down for the next event, see how that goes, and try without a rear antiroll bar the one after, so as not to change too many viariables at one time. I guess I'll look into finishing my panhard too... I probably wouldnt hurt to try it. I was also having some rubbing, but I rolled my rear fenders, and pulled them out about 5MM, which seems to have eliminated that problem.

I have a buddy who always removes his rear antiroll bar in an E-type prior to autocrossing, and he mentioned that was something a lot of people who have track-prepped E-types have done. I just didnt think the same rule would apply to our cars.
Ill also look into the plastic shim idea if I'm still having problems... Thanks a bunch for the quick replies guys!


Dylan Bach

Dylan,
I would leave the rear bar alone and upgrade the front bar to at least 7/8". I run 1" on my B. I plan having my yellow 64 B #13 at Pacific Raceway (formely SIR) 29/30 Sept. Drop by if you get a chance.
Lee Bradley

I added a rear bar to my 75B (oem size front sway installed) and found that the sudden rear break-away in a tight turn to be a little scary.

The car was indeed flatter going into the turn up until the break-away. Then the rear end suddenly (like a light switch) looses traction and tries to skip forward of the front wheels.

My experience was exactly as described in this link at http://www.mgparts.co.nz/performance2.html where they recommend only having a larger than oem front sway bar.
Steve Rechter

'Snap oversteer' is usually the result of suspension running out of travel. Too big of front bar gives you understeer and too big in the rear causes oversteer. You could go with no bars but the body roll becomes extreme during hard driving. This could be addressed by high rate shocks (see go-karts for very hard suspension) but this reduces compliance on any but the smoothest roads.

I use 1" bar in front with 9/16" bar in rear and no rubber anywhere in the suspension. I have a panhard bar mounted from a drop bracket to a plate on the bottom of the leaf springs and a locker differential. This setup along with very hard shocks (filled with castor bean oil) gives the a slight understeer. Oversteer can be induced via braking or throttle to rotate the car as required.
Lee Bradley

So woult the ideal sway bar setup be a small bar in the rear, and bigger than stock in the front? Or absolutly no bar in the rear, and a bigger than stock one up front?

The impression i got throm that article was, with a solid rear axle, a rear sway bar will only have adverse effects on rear grip. I can imagine how this is acurate... Im asuming the sway bar on the rear simply transmits more of the load to the outside tire.

So basically what you are trying to acheive, is even loading of the rear wheels, and upgrading the front bar to counteract the loss of body roll control by removing the rear?

Also, how much of an axle-locating dudty does the rear bar serve? Would it be a bad idea to remove it without first installing my panhard?
Dylan Bach

Please excuse my spelling errors, I accidentally hit submit before proof-reading!
Dylan Bach

Anti-roll bars make the handling worse on the end of the car where they are located. A just right sized bar in the front would reduce bodyroll and balance the handling front to rear. However if you want more reduction in bodyroll, you go to a bigger bar. Now you have a car that is flatter but understeers so you add a rear bar to add oversteer to rebalance the car. It is easier to adjust the smaller rear bar than the bar; moving the link on a 9/16" bar 1/2" doesn't have near the effect as moving the link 1/2" on a 1" bar. When it rains it much easier to disconnect a rear bar link to make the car easier to drive in the wet.

Yes the rear bar has adverse effects on rear grip just as the front bar has adverse effects on front grip that is what anti-roll bars do, they resist body roll by unweighting the inside tire.

The anti-roll bar should not location the rear-end at all. My links have hiem-joints top and bottom.
Lee Bradley

Dylan The anti roll bar has no part to play in axle location.
DENIS4

With a 9/16in. front sway and an 11/16in. rear sway, my car was downright scary to drive in wet weather.

I took off the rear and can live with the extra body roll for more predictable handling.
Steve Rechter

Dylan

May want to put an e-mail to Killerbeez in Wisconsin. Racing B's is all they do.

http://www.mgbracing.com/
BEC Cunha

Dylan-
I can confirm Lee's comment on the origin of "Snap Oversteer." Due to the design of the rear spring shackles, as one spring flattens upward and the other arcs downward, the spring that is compressing lengthens rearward on its shackle and the attached end of the axle moves to the rear along with it while the opposite spring is that arcing downwards shortens on its shackle, moving the other attached end of the axle forward. As the rear axle becomes diagonal to the chassis, the resulting thrust angle of the axle worsens both understeer and torque steer. When the lateral forces are high (as during hard cornering), both the lateral and directional misalignment combine, resulting in increasingly serious torque steer which, when combined with the serious misalignment caused by lateral movement of the axle, results in what is called “Snap Oversteer” when the rear axle contacts its bump stop. This should not be confused with “Spin”, which is usually caused by a loss of traction resulting from the lifting of a rear wheel. A rear stabilizer bar will not only help to reduce body roll, but will also help to reduce this compound misalignment by virtue of its resistance to movement. Adding a Panhard rod will all but eliminate it. 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.

By configuring the Panhard rod with the body mount on the left-hand side of the car, the effects of engine torque on corner exit grip are minimized, allowing you to put the power down earlier in the curve and get a jump on the competition down the straight-away, no matter what direction the corner is. If the Panhard rod were to be mounted with the body mount on the right there would be a much bigger disparity between left and right hand cornering ability. This would effectively make you choose between setting the car up to exit either left-hand corners or right-hand corners, and live with an under-performing car in the other direction.

One thing that you will notice after installation of the Panhard rod is that the steering response will seem to quicken and the rear end of the car will seem to be lighter on roads that offer reduced traction. This is due to the fact that there will no longer be any delay caused by the shifting of the body over the axle. You also may notice that the rear end of the car has a greater tendency to break loose during hard cornering. If this occurs, you will need to upgrade your tires as this type of suspension system is intended for performance, and 60,000 mile treadwear family sedan tires just will not do! If you need a Panhard Rod that is adjustable in order to accommodate mounting onto either wire wheel or solid wheel Salisbury tube-type rear axles, you can get one from the MG Owners Club over in the UK. They have a website at http://www.mgocspares.com/
Steve S.

If you are running 550lb springs, 'uprated' dampers and a 7/8" anti-roll bar, and assuming that the springs are lowered (I don't suppose you can buy 550lb springs at rubber bumper ride height!!!) then you are not going to get any roll even without a rear bar. If you are then (i) the springs are not 550lb and/or the dampers are not uprated or (ii) the rear spring is too stiff which is causing traction problems (ii) it must be the effect of the rear anti-roll bar. On the track, this set-up will result in oversteer if you push too hard. The problem with a Panhard Rod is the price - in the UK at least.
Neil
Neil22

If you check the bump stop for the rear axle on the rubber bumper cars, you will find the bump stop is about 2" lower than the chrome bumper cars. You can cut it out and with a little fabrication raise the bump stop to the chrome bumper position. This will make a difference as to when you bottom out and should reduce the sudden loss of traction when the rear end bottoms out
Barry
Barry Parkinson

This thread was discussed between 30/08/2007 and 10/09/2007

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