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MG MGB GT V8 Factory Originals Technical - Unstable and control problem
|Would anyone care to make suggestions as to what is causing this problem?|
I have recently finished a 3.4 V6 conversion for a customer on an original 100% factory stock 1977 rubber bumper MGB roadster with 63,000 miles. This engine is the strongest one I have produced for sale to this date. I estimate it to be making about 225 HP. I have not experienced this problem on any other MGB car with 200 or less HP. I do know that all the other converted cars did not have sway bars in the rear and none of them had the "high water" suspension.
I have never had any experience with this type problem. I am a go fast, stop fast man and my handling knowledge is fair.
I have heard that the factory sway bar was installed because of the handling problems on these "high water" MGB models. I had a friend who has raced a variety of British cars drive the car and he thought that the torque of the engine was causing the problem by trying to lift the front wheels off the ground and changing the camber. Sounds logical but then not because the other converted cars had no problems like that and they lifted the front end somewhat also.
On hard acceleration in any of the four gears the rear end of the car feels like it kicks to the left and the front goes to the right. Between the gears, as the front comes down suddenly it feels like the front moves to the left or it is just trying to get back to it's normal track. The car feels like it twists or twitches but it always has excellent traction and no burning of tires. It will do this also when I aggressively kick it in any of the first four gears. It lessens a little as I progressively go to the next higher gear. It also does it somewhat when I am in fifth gear when I am in the power band.
The owner doesn't want me to alter the suspension unless I use original parts. He doesn't want me to lower it in any way . He wants a sleeper. It sets a little higher in the front because of lesser weight. All suspension parts appear to be working properly. I suspected the rear shocks might be a little soft after I detached them and tested them for resistance in both directions. The right one is a little softer then the left but they both appear to soft for my liking. I don't know what a new/good lever shock should act like but they sure don't act like any tube shock I've ever used. I also hear that tube shocks can cause rear end jutter and ride very harsh in some set ups. The car has the exact sway bar in the back as fitted by the factory and all bushings are good and the mechanicals are good. I tried removing the sway bar and it did lessen the problem a little or it was my imagination. Any suggestions? Thanks, Dann
|Try getting a 4 wheel alignment check & have the thrust angle checked.|
While there is almost no adjustment for the MGB, a wreck in the car's history could have upset the geometry. The body could have been repaired, but few shops until recently could straighten a unibody. Somewhere, in the piles of stuff I have is a diagram of the MGB body & measurements to see if the body is true. You may also find such in one of the service manuals like Benchley.
|Sounds like torque steer being created by unbalanced axle movement due to either tracking as Jim suggested or a weak shock absorber. Also check the sway bar is properly centralised on the axle and you could uprate the sway bar bushes to polyurethane. It is also possible that the left hand rear spring is weaker than the right.|
|Had the same problem in my car under the EXACT same cirumcstances. Check the rear suspension and make sure everything is in order. Especially the u-bolts, the pads in the springs, and the shocks.|
|I agree with foregoing and Jim's suggestion. My money is on the rear suspension ( spring and shackle bushes)but the following will also make the MGB unstable:|
More than recommended tyre pressure in front.
A faulty shocker (front as well as rear)
Bad tracking (steering geometry)
Higher profile tyres than 70s
and Some makes of cheap road tyres
Dan - the 1962 UK cars came with a front sway bar - most of us have uprated it to a 3/4 or 1" bar with urethane bushes - its the best value improvement for this car.
Many conversions have an engine steady bar in additions to very stiff engine and gearbox mounts - the gearbox mounts sometime get overlooked in conversions ?
Finally when you are satisfied that the rest of the suspension is as it should be - how are the pads between the chassis arms and the front crossmember.
keep us posted
|If the rear is twitching on acceleration and coming back on decelleration I think that the spring pads on the rear leafs are worn out and the U bolts may be loose. Is this a stock rear axle and suspension set-up?|
This is a common problem on stock Bs.
I am running a 302 V8 with the GM rear axle set-up that D & D make with posi. My suspension set up is lowered rear springs with urethane bushings and the stock rear sway bar and have not experienced any of this twitching. Jims suggestion of an accident related deviation of the geometry could be dead on too.
|I've had a sheared leaf spring center bolt act like that|
|I agree with most that it sounds like something a little loose in the rear suspension. My 2.8 Midget is also starting to exhibit some of this after a few years on the bushings. You might also check for chassis flex by placing some masking tape tightly across the door gaps before you test drive. If the chassis is flexing or twisting it will either tear or wrinkle the tape.|
|I have a B GT with a well massage 302 which it has been stroked to 331 ci no problems due to the reinforce chassis and well mounted rear axle. |
Had similar problem with my 4 banger B Roaster with a blower. The front bushings on the rear spring were worn and would cause the car to feel as you have declare in your message.
Replace bushings with hard bushings and torque bolts as per specs.
BTW camber does not change that drastic in the B.
You may also have some problems in your differential, but this is rare. What you have experience is called rear axle torque steer. Check your rear axle mountings, 200 hp is not going to affect the B's attitude under accelaration.
|Fast car Larry|
|Thank you everyone for your input. I have not answered until I checked out the suggestions. I had already checked out some of the suggestions before hand. I suspected a sheared center bolt but they are OK. No wreck because the paint is original and there is no frame damage underneath. Anyway I will follow all the suggestions and hope I find the trouble. Meanwhile I am taking it back to the alignment shop and ask them to do a four wheel check. I'll post the cure here in case you might be curious to the outcome. Thanks again.|
My only thought is that its not happening under braking, which is a more dominant force. I have 300lb of torque but with anti tramp bars with no problems.
|Well enthusiasts, I'm back days later after taking the B to 2 more reputable alignment shops. I will have to conclude that the tape suggestion by Bill Young showed that the body is twisting on acceleration. I am still confused as to why this is happening to this particular car, especially since it is a real original cherry. I do not find any broken welds or split seams any where. I guess this is one of those things you have to live with since re-enforcement of the unibody is not in the owners budget. I heartily thank everyone for their help.|
This puts you in a very difficult situation of handing back a car which could be dangerous.
Like you, I find it difficult to accept the car is flexing - while the rear suspension is the obvious culprit you probably know the front can also be problematic - wear on the shocker spindle will allow involuntary changes to the geometry for example.
The only sure thing is the reason for the handling aberration will become apparent at some time - hopefully not in a forensic situation.
I doubt you've thought of this, but check the integrity of the front crossmember, as it serves a very important role in preventing body twist. The crossmember can rust from the inside out and I've heard reports of poorly done welds from the factory at the seams. Optimal chassis design dictates one or more large diameter tubular crossmembers rigidly tied into the side rails. This prevents flexing of the side rails by resisting torque through the tube. Picture a pair of rails walking like stilts then add a rigid tube at the top and you get the picture. In the unibody the concept is equally important at the front crossmember location because there is inadequate sheetmetal available to serve the same function at the front of the car. So if the attachment bolts are loose, the rubber pads have deteriorated, there is significant rust, or the welds are bad this could be causing your handling problem.
|I had similar problems caused by wrongly mounted engine steady bar. It was mounted as per the Rover sedan set up. LH head to LH fender wall, almost horizontally. On accelleration the weight shifted to the RH side & tended to lift the RH wheel off the ground. I remounted the bar LH head to LH chassis rail,vertically. This cured the fault. You don't mention a steady bar. May you need one. Does the engine move much under accelleration ? I suspect it does,if your problems are only when accellerating. It sounds like the balance of the car is upset. Barrie E|
|Dann, Have you removed the rear springs and checked all the leaves as well as the bushes in the spring eyes, the clamping bolts and the shackle mountings? If you have pronounced rearend steering on acceleration; it probably indicates the rear axle is not remaining at right angles to the car centerline as power is applied for some reason. If there is nothing obvious, are the mounting holes worn allowing the axle to move? If the rear steering movement on drive is pronounced, as to be scary, there must be something pretty fundamental wrong. Is the static centre to centre measurement front to rear axle the same on both sides? I think that whatever else may be wrong, the power on/ power off rear steering effect is likely to be mainly confined to the proper location of the rear axle. Bob.|
|Bob, one of the attempts to find the trouble left me so desperate that I had Acme Spring in Dayton, Ohio remove every piece of rear suspension. They took apart and inspected, tested the springs and replaced all parts except the springs, including new U bolts and put new anti friction dividers between the springs, declared the shocks to be OK and uprated all bushings. This car looks about 2 years old so wear on anything would be surprising. I had still another four wheel alignment check also and all seemed well. Still the same problem. I am going to put it on a dyno and watch for any shifting. After that I give up. Thanks, Dann|
What you describe souds about right for a stock Rubber bumper suspension trying to handle a lot of torque. It is the cumulative result of many small transient forces. The car is going to squish those puny springs on the right rear (with little resistance from the stock shocks), and try to lift the left front (also with very little resistance from the shocks). The front droop is goint to give you some camber change and thus toe change. If the tire is lifted enough, the opposite tyre may even be able to determine vehicle direction independently of the lifted corner. The right rear spring is going to do a little bit of winding up. By doing so, it changes lenght, and is no longer perpendicular to the centre line of the chassis. That the car is hooking up, just shows how effective weight transfer is with a loose front suspension and little compresion resistance in the rear shocks. The problem is that now your body is headed in one direction, and your suspension in the other. Keep in mind the mass of those bumpers that are trying to turn the thing in to a rocking horse as weight shifts from one end to the other. When the car comes back down, it is up to the driver to point it all back in the proper direction.
A simple and cheap trick we used on drag cars many years ago was to limit the front suspension "droop" by using a piece of cable or chain to keep it from drooping too much.
There is still a lot that can be done without losing the "sleeper" nature of the car. Shocks and springs , or just heavier fluid, would go a long way to reducing weight shift.
|Pete, very interesting. Not having ever driven a stock RB car would leave me without much experience with this situation. I follow your logic very well. This information might just lead me out of this dilema. I did put a lowering kit in the back which seemed to help with the handling but not the torque problem so what you say makes a lot of sense. With that big bumper hanging on the back it seems to get knocked around in high crosswinds also.I'm trying to get it to Grand Rapids.Maybe we will see you there? Thanks for the help. |
This thread was discussed between 07/07/2004 and 18/07/2004
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