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MG MGB Technical - 50 to 60 MPH Vibration

I have a question concerning vibration. My 77 B has of late developed a slight to medium vibration the first 5 miles of driving from a cold stand still.
Generally this occurs between 50 and 60 MPH most noticeable to me at 60. Once beyond 60 MPH it rapidly diminishes and smooths out. After the first 5 miles when the car has warmed up it also rapidly smooths out to the point of not being noticed.

Everything front end wise appears to be ok. The car is driven primarily on weekends only. Tire wise I have Toyo tires with less than 3000 miles on them that were balanced when installed.

Any ideas as to cause? Is it at all possible because the car sits all week until the weekend that the tires could develope a "flat" spot that smooths out 5 miles or so up the road? Maybe I should rotate tires? Should I be looking closer at the front end?
JCH Hibbard

JCH,

Are you close to the car during the week so that you could roll it just a little bit everyday of the week? Then go out on the weekend and see if it still does it.

I think I remember hearing of flat spotting, even in a short period of time, when steel belted tires started becoming common. I didn't think it was much of a problem these days, but it kind of sounds like your symptoms fit.

Charley
C R Huff

It's probably the tires flat spotting when sitting. My Protege 5 did that with the OE Dunlops and does with the recently installed Khumos. The Michelin 185/70/14 tires on the my MG don't flat spot when sitting.

Clifton
Clifton Gordon

I've known of flat-spotting occuring when the car is left standing for several weeks, but not for just five days, which mine regularly are. If they *are* flat spotted then you should feel this as an up and down movement of the car at slow speeds - something like a circus clown car with the axles not in the centre of the wheels. This is how mine felt after being left several weeks. I didn't notice any vibration at higher speeds, and a couple of dozen miles cleared it. Nevertheless Charley's suggestion of rolling it 1/4-turn every day (and not forward one day and back the next!) should resolve it one way or the other.
Paul Hunt 2

Thanks CR, Clifton and Paul. I will try giving it a roll but I somewhat tend to agree with you Paul, seems like a short period to be flat spotting. Any suggestions as to where I might look in the front end. Everything seems ok though.
JCH Hibbard

My tires flat spot after a few days, I notice it when I drive away from my house at 20-30 MPH, and it goes away within 5 mins also.
Erick Vesterback

JCH,

The other thing you could do, which would be more definitive but more trouble than rolling, is to put it up on jack stands or blocks right after driving it home. Then see how it is the next weekend when you let it down.

As for condition of the front end, if your shocks are weak or out of oil, it would probably make a slight flat spot apparent when it would otherwise not be noticed.

Charley
C R Huff

The tires on my Mazda Protege 5 will flat spot in 3-4 days, but they clear up in about 1-2 miles.

Clifton
Clifton Gordon

Had the same problem with my 79 MGB that you are describing. Check your motor mounts! Mine were "broke" allowing the engine to "rub" on the rod that goes to the steering box. At rev's low, then when driving at about 50-60 mph, then smoothed out.

The PO said he always had problems.... once I replaced the mounts it aligned the engine correctly, (raised it off the rod preventing the rub).

Very easy to tell by looking at your exhaust/header to see if it is rubbing against the steering rod... mine had a gouge where it was occuring. My mounts broke where it bolts to the engine.

I drove myself crazy with tires/pressure/alingment etc. until I saw the problem with the mounts...

Best of luck!
Don 1979 MGB
Don

Sometimes wheel bearings on the way out will grumble for a few miles then smooth out. I had this with an Opel Rekord once, but never with the MGB.
Stan Best

Sounds to me more like universal joints and when they warm up they expand just enough to eliminate the vibration. Check by grasping both halves of the joints and try to counter rotate them. Any play whatsoever is a scrap joint.
Iain MacKintosh

Thanks. I hadn't thought to check the universal joints. I will do so tomorrow. Thanks again.
JCH Hibbard

This thread was discussed between 09/03/2008 and 16/03/2008

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