P1800S -vibration at speed

[Groundes-Peace]

My car has developed vibration at speed from 65mph+ ,upwards.
However it does not feel like a wheel balance issue which one can feel through the steering.
The feel is not like propshaft in-balance, but seems to be coming up through the floor of the car.
Although there does not seem to be excess wear in lateral movement of the rear wheels, could there still be an issue with worn wheel bearings?
Thoughts please.....

[sleek lemur]

Check out a recent question I sent out in the forum "Wheel bearing shot?". There were some really good responses regarding how to check.

Originally Posted by blueosprey90

Grab wheel at 3 and 9 and try to rock in and out. Should be zero motion. Also stand next to car and lean against top of tire with knee. Should be no movement. I had both fronts fail at same time on my MGA. Enough wobble to knock back disc brake pads. So my case started as a brake problem.

[Derek UK]

Swap the wheels front to back. Some people don't bother with balancing the rear wheels but it's just as important as the fronts. Perhaps get someone to drive behind you and a bit off set so they can check to see what your wheels are doing. If a rear shock is poor it can also give you continuous patter if the wheel is out of balance.
Can you drive through the vibration period? i.e. How is it at 75?

[norustplease]

Still worth checking the prop shaft for vertical movement at the joints, also the centre bearing. Any clonks whilst pulling away?

[Groundes-Peace]

No clonks when pulling away.
Below 60 mph nothing feels unusual

[Burdekin]

Sounds like a resonance problem so something out of balance; wheels or drive shaft but if they are fine next things for me would be engine and gearbox mounts.

[142 Guy]

Originally Posted by norustplease

Still worth checking the prop shaft for vertical movement at the joints, also the centre bearing. Any clonks whilst pulling away?

I agree with this.

Since the noise appears to be originating from the floor pan, in addition to checking the condition of the center support bearing check the condition of the rubber mount for the bearing to see if it has collapsed allowing for more movement of the drive shaft (drive shaft balances are rarely perfect).

If checking for vertical movement of the joints means checking for wear in the U joint bearings that is also an excellent idea. Wear at one of the bearings may not be sufficient to cause a clunk while driving; but, may be sufficient to cause vibration at higher speeds.

Finally, is your transmission mount in good condition? If the transmission mount has collapsed the angle of the front driveshaft flange may have changed. U joints are not constant velocity joints. The front and rear U joints are clocked 90 deg apart so that the angular velocity changes that occur during rotation cancel each other out. However, that cancelation is only perfect if the mounting angles of the front and rear flanges are identical. Velocity changes / vibration caused by differences in the mounting angles become more noticeable as the speed increases. Changes to the rear suspension or a collapse of rear axle mounting bushings can also cause a change in the angle of the rear driveshaft flange leading to the same problem.

[Ron Kwas]

GP;

With roadspeed related vibrations, the first place to look is always tires (although the driveshaft is obviously also roadspeed dependent)...it could be that you simply threw a balancing weight (I always mark their locations, so a missing weight, since last balancing, is pretty obvious upon inspection)...and don't forget rotational imbalance goes up with the square of rotational speed, so the imbalance may not be noticeable until the higher speeds. I've also had someone drive the car in question, as I drove along side in formation, carefully looking at each corner for anomalies.

Good Hunting!

[sleek lemur]

Does sound like wheel balancing. I replaced tyres once and didn't get them balanced. Seemed fine, until I took the car to the continent. The better quality roads immediately showed up vibration at >60mph. Had seemed fine in UK!

Might as well get wheels balanced. Only costs a few quid.

[Vintagewrench]

Have noticed a similar issue with my '67 1800s at the same speed range 65 + mph on our interstate highways here in the US. The car has been run on three different sets of wheels and tires, the suspension system is totally rebuilt and as new. The drive shaft balance has been checked and the u-joints and center bearing and support have all been replaced.

Narrowed it down to in this case to it being caused by poor road surfaces and possibly the short 96.5" 1800 wheelbase (an Amazon is 102") contributes to it.

On a freshly paved and smooth highway or on the race track during a pace laps there is no vibration at all in this speed range. It may not be the case with your car, but pay attention to how it feels on a freshly paved and smooth highway if you can find one, you may notice a difference.