My MKV is painfully close to being back on the road but it seems to have a vibration issue. My specialist said after a test drive today, “The car drives and performs very well, however all gears give a lot of vehicle vibration. Fourth gear has significantly less vibration.” He has suggested excessive “end-float”. Has anyone had experience of such a phenomenon or might suggest other possibilities?
The car is just stunning after so much remedial work and all I want to do is drive it and enjoy it. There is always something as many of you will know only too well.
It is important that the universal joints are aligned correctly. The two must fold in the same plane as each other so if the one on the front folds up and down then the one on the rear must also fold up and down and not at some angle. This misalignment can come about if the splined section has been removed and then reassembled at a different angle.
In the picture below see how the axes of the pivots at each end must be parallel.
You can see that it would be possible to remove one end and rotate it one or more splines and end up with the red axis lines not parallel to each other. This would cause vibration.
Audible vibration like a chatter sound, or physical vibration? Mine had the chatter and yes it was found to have twice the specified acceptable max end float on both main shaft and countershaft. it’s back within tolerances now but haven’t been able to test it yet to see if it’s better.
Peter is pointing to the fact that the Hooke type joint is not a constant velocity joint. If those two flanges are not all in one line, for example as the car bounces up and down, and assuming the rear axle rotational velocity is more or less constant, the center shaft actually speeds up and slows down twice for every revolution. The amount is dependent on the angle, or how much bounce in the car. The yokes are assembled parallel, and the gearbox shaft is more or less parallel to the axle shaft, so this non-constant rotation is cancelled out. If the front flange is not assembled with the yoke parallel to the rear, the engine will be forced to turn at a non-constant speed, i.e. speeding up and slowing down twice in every revolution. Thus you get vibration.
Failure to observe and follow these parallelism requirements is why some off-roaders and jacked-up pickup trucks go through a lot of u-joints.
Notice the two yokes are parallel in this picture from the Mark V Service Manual. Mine had a couple of arrows for alignment marks.
A Mark V has three u-joints.
This is the front shaft of the first type.
It has splines on the rear so it is possible to assemble the rear flange wrong. They tell you to mark it before taking it apart, but if you didn’t do that, you can figure out the correct way by looking at the second type.
On the second type, also used on Mark VII, the flange can be put on only two ways, both of which are correct. The yoke on the front drive shaft ends up parallel to the yokes of the rear drive shaft.
I wish I had taken better pictures of this feature before putting the body on, but they were all taken at the same time and the driveshaft was not turned. You can see the pairs of bolts close to each other, all 3 pairs in the same angular relation.
I also wrote a short piece on the non-constant velocity UJ here that shows the graph of the change in velocity against joint angle. In other words, there is no problem if the shafts are totally in line but if not then parallel axes are essential.
Checking the parts catalogue, the first type front drive shaft with the splines was only used on 46 cars, 620001-620028 and 627001-627018, so it is unlikely you have that type. You probably have the second or third type drive shaft, both of which use Woodruff keys in the front shaft.
So your problem is most likely mis-assembled rear drive shaft. The yokes need to be parallel. Look for arrow marks.
Here is an example of a Mark V rear driveshaft put together wrong, poor sad pink 628003, the white lines showing the yokes not parallel. This car is no longer in Beverly Hill’s inventory, so I hope the new owner realizes this or he is going to have the same problem.
To add to the good information above, also see page G.6 of the service manual which has two places where alignment is discussed and emphasized.
As well, the engine is not mounted parallel to the car frame. Rather, the engine is higher in front than back, my memory is a 3.5 degree downward tilt going from front to rear (but I can’t find a quick reference point in the service manual).
Good point, and that also requires a similar tilt back-to-front, or the rear axle, usually accomplished by wedge spacers between the diff and spring perches.
Let’s not confuse Tim on the issue with any talk of wedge spacers. That was only used on some (but not all) XK120s which had certain spring saddles on the rear axle. It does not apply to Mark V if you are using all the original springs, spring shackles and axle; no wedges required.
The idea is as I said before, the engine crankshaft should be parallel to the rear axle input shaft with the car in normal driving condition, or at rest with an average passenger and luggage load. The car was designed for this. Deviations from parallel with bouncing and/or light or heavy loads are small enough that vibration is not detectable.
However, the engine is not co-linear with the axle, there is an offset, which is why angular alignment is important, the yokes must be parallel.
Thanks to one and all for the helpful, well considered suggestions.
As of today,
The propeller shaft is correctly installed and in balance.
The gearbox is fine.
The engine seems to have the vibration issue from 1800 rpm to 2800 rpm. Below 1800rpm the engine is smooth.
On the engine the harmonic balancer has been checked and is satisfactory, so what next?
I was wondering if it might be carburettors or some kind of fuel starvation as the ignition system has been overhauled / refurbished by a specialist and the timing set by Ivan Stevens? I was also wondering if it could be the flywheel or clutch assembly being slightly loose?
Any suggestions would be most welcome at this time.
Tim
If vibration magnitude is well above normal for this engine type and also is present with transmission in neutral and car stationary, several possible sources of the vibration may include (assuming engine is firing smoothly and evenly on all cylinders at all rpms):
1.) imbalance among pistons and rods set, improperly matched set
2.) bent harmonic balancer
3.) unbalanced crankshaft, flywheel, clutch
4.) insecure engine mounts
If the engine was overhauled/refurbished by a specialist, did they witness the vibration on testbed run-in or when timing was set? Has timing , mechanical advance, and vacuum advance been measured across rpm range? Is distributor vacuum, mechanical, and total advance correct and smooth across rpm range, not jittering in mid-range?
The car will go on the oscilloscope tomorrow to perform that diagnosis and eliminate ignition issues.
What’s mysterious is that there is no vibration below 1800rpm and above 2800rpm. The compressions are all good, the valve timing and valve gaps are all correct. My suspicion at this stage is the Harmonic Balancer. If the rubber has become hardened over time would this possibly cause the vibration issue? Sadly I don’t know the history of the motor only that it runs very sweetly aside from the vibration within the rev band. If it was an out of balance crankshaft or piston and connecting rod issue then one would assume that it would become progressively worse as the revs increased and not simply stop at 2800rpm? The same might also apply to clutch, pressure plate and flywheel I would assume?
I wonder if others have experienced issues with the harmonic balancer?
I have just come to this interesting discussion. My IV 3.5 also has a noticable balance/vibration problem in the 2300 +/- 300 rpm, gradually coming into the range and out the other side. I do not like cruising in this rev range so it limits the desire to run in the 70 to 90 kph (40 to 55 mph) zones. It seems to be engine related, but I should test this phenomenon down a gear when I get the car back on the road in the New Year, to check if it could be drive-line related. I don’t recall it being noticeable in third through this range, but it needs a more dedicated test. I generally only use 2nd and top gears for most occasions, the engine having such low down tractable torque. I would use third more for an overtaking move or the like and would be concentrating more on the other things.
Looking at my photo of running the engine with the gearbox removed in my previous post I see that I did not have the engine stabiliser bolted on. With this configuration I couldn’t detect any vibration. I wonder if these mystery resonances are caused by a lack of compliance at the chassis mounting of the stabiliser arm?
I certainly found a big difference in vibration between using my soft gearbox rear mounting and a rather solid modern XK120 mounting.
