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MG MGB Technical - TR3 body shake

I'm taking a short break from my B to work on a problem with my 56 TR3. I know; wrong board. But the people on this board are too good to not ask them for their thoughts on this.

At any kind of speed, the body shakes terribly. This is most evident through the cowl/dash. It gets worse as speed increases; it's real bad at 35mph+.

It has new, freshly balanced tires. I hoped that the new tires would help the problem, but they didn't have an impact. I would really appreciate any ideas on what to check out.

Thanks!
Chet

Steel wheels or wires?
Steve Merical

How about a bent driveshaft, or u joints that are either rusted solid or worn to the extreme?
Broken engine mounts?
Is there feedback in the steering wheel?
Andrew Blackley

Does't it have body insulators that could possibly have gone south?
George B.

You guys are all over it!

It has steel wheels. I've recently replaced both u-joints. Not much if anything coming through the steering wheel.

?????????????

Thanks again in advance for any and all ideas!
Chet

You might want to exchange your front and rear wheels just to see if that placed any vibration into the steering wheel. Cowl shake on open cars is a hazard of the way they are designed and to some extent, to be expected, particularly in older cars like your TR-3. However, severe cowl shake sounds more like a problem from out of round, out of line wheels, wobbling, and thereby causing a twisting motion in the frame, and hence, the body. Sometimes this is attributable to bad shocks as well, or a conjunction of the vibrations from bad wheels, engine vibration, and driveline problems all feeding each other.

Do you feel any similar vibrations when the car is standing still? If so, you may well have bad engine mountings. A badly oscillating/bucking engine has been known to cause terminal cowl shake. I have seen cars like these with "square cams" (competition cams) shake so bad that the windshield looked like it might fly off when sitting at idle!
Bob Muenchausen

I had a tire shop bend two rims mounting tires for me. That manifested in big cowl shake and vibration. If you trundle along at 5-10 mph, and can feel the car sway from side to side like a railroad car, suspect bent wheels. It was easier to notice if I let the car idle in neutral while doing this - use a small downgrade.

Bob nailed most of the best responses, but he left out loose wheel bearings (or loose lugnuts!). I'd take a serious look at the driveline. Not so much because it's the likely culprit. More because in the small chance that's where the problem lies, it can have serious consequences.

Matt Kulka

Don't they all do that?
;) sorry, couldn't resist!
Jim Blackwood

I was going to mention wheel bearings also.

You might try to find a shop that will road-force test and balance the tires/wheels (see http://www.gsp9700.com/ for information on how thats done). Several people at work have had weird vibration problems over the past two years and have been able to have vibration problems solved using this machine, one guy even found two BAD tires that were technically "balanced" but were out of spec.

I've had pretty good luck with it, but I've been told by a few shops that if the operator doesn't really know what they're doing they can mess it up badly too (although it looks pretty simple when they demoed it).
Scott K

Check out your shocks. If they are bad, the front wheels will dribble down the road like basketballs. TR's do tend to shake naturally...
Craig Cody

Thanks for all the input.

I would like to eliminate as many potential causes as possible before I start buying new parts. To diagnose the problem as either a drivetrain, wheel, shock, or whatever problem, would it make sense to put the rear of the car on stands, then with the engine running, work through the gears. It seems to me that building speed without the tires on the ground will help tell me where the problem is. If it still shakes, then it must be the drivetrain. If it doesn't, then it must be the wheels, tires, or shocks.

Does this make sense or do I have a death wish?
Chet

It makes sense AND you have a deathwish :-) Just make sure the car isnt pointing at anything dangerous and/or expensive.

Another idea while the car's up in the air would be to remove the rear wheels- that way you eliminate wheel imbalance as a source of vibration.

HTH
G
Growler

Chet;
I once had brand new Dunlops that made the car sway and shake as if it had bent wheels.This was immediate after pulling out from the tire shop.It did not do this with the old tires. The shop rotated, replaced and then told me I needed an alignment.It still did the shaking bit, and the steering wheel was off center. I finally made them take the Dunlops off and put Generals on,and the swaying/shaking stopped. The steering wheel was then right on center again. Weird, but true. If you do not have the shake with the tires removed while on stands, put them on and have someone run the car while you observe the tread pattern. Look for wobbling plies, even though they are new tires.
Pete Haburt
Pete Haburt

Chet:
Some things YOU can check -

1) out of round most any well anchored stick or other object placed near any wheel while it rotates can tell you if your wheels are out of round, off center, or wobble in and out. It is just a matter of placing the stick (or other object) near to the rim of the wheel and watching the relative motion of the wheel in relation to the stick.

2) You can check balance on the front axles. A grossly out of balance Tire/Wheel combo will quite readily drift the heavier part of the combo to the bottom providing your front wheel bearings are adjusted properly. A well balanced wheel will not drift readily or at all. You will need to exchange wheels front to rear to make a complete test for this.

3) look at the tires and rims for damage or other anomalies that can cause rough running on the pavement. A tread separating from the casing and causing a blister in the tire, will surely cause a great thump and out of round situation! A rim, badly damaged on the inside edge where you cannot see it easily, can also pull the tire out of shape and cause an out of round situation, and then there goes the cowl in response to the distorted wheel/tire as it wiggles down the road.

4) Check your shocks. If you have even one that is not working and the car just bounces at that corner, you have a potential twist of your frame with every swing of the suspension that is not dampened by the shock(s). In a case like this, the cowl shake will seem to be heightened each time that corner of the car hits a pot hole or bump. You will see these impacts as a trigger for a flurry of cowl shaking.

5) Driveline. If the drive shaft is not mounted correctly, it can shudder, and the shudder is translated by the body as cowl shake. If it is drive line related, you will note that the shaking increases in frequency as the car speeds up and slows as the car stops.

A freakish thing, but one that happened to me once, was the backing out of one of the pressure plate mounting bolts from the flywheel on another car. This upset the dynamic balance of the crank enough to induce a really noticable judder, again, in sync with the engine RPM.

I am sure there are some other considerations, but these are some simple tests you can run yourself to see if any of our guesses are anywhere near the mark.



Bob Muenchausen

Fortunately the GSP9700 I mentioned will take the out-of-round or off-center characteristic of slightly out of round rims and/or tires out of the equation - to a certain extent anyway.

The "out of spec" tires I spoke of earlier were actually brand new Goodyears that ended up to be completely bad. My buddy that bought them thought either he made a mistake with the brand, or something was seriously wrong with the car (after having them "balanced" several times). After one session with the '9700 they found 2 out of 4 of the tires were completely bad, the others were remounted in synch with the wheels (as were the other new tires).

If all else fails at least it's a relatively easy thing to have checked with decent precision.


>>RE:"1) out of round most any well anchored stick or other object placed near any wheel while it rotates can tell you if your wheels are out of round, off center, or wobble in and out. It is just a matter of placing the stick (or other object) near to the rim of the wheel and watching the relative motion of the wheel in relation to the stick."


Scott K

This thread was discussed between 10/06/2002 and 13/06/2002

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