WHEEL WOBBLE –CAUSES AND CURES

by Brian Loveridge

Wheel wobble can happen when something isn’t right in the front suspension or steering. These comments pertain to beam axle cars such as the vintage Bentley. It manifests itself by the steering wheel starting to shake rotationally and can be a nasty thing. Indeed, an extreme case was recorded of a 4 ½ Bentley in, I think, the 1990s, when at high speed (70+mph) the whole of the front of the car started to shake uncontrollably and the panicking driver managed to bring the car to rest. Major repairs were needed as a result including re-riveting of the front of the chassis. This was an extreme case, but illustrates that it is a good idea to take preventative measures.

Let’s look at the main causes, which are firstly, out of balance wheels, secondly, loose steering joints, thirdly, suspension defects and fourthly, wrong settings.

Balancing the wheels is the first thing to do. This is detailed in the latest edition of the Technical Facts at 2.12.3. 4). Vintage size wheels can be balanced on a machine, and usually static balancing will suffice. More modern smaller wheels rotate faster and are wide so need static and dynamic balancing because they can be out of balance in a ‘dumbell’ sense. Large vintage wheels are usually ‘flat’ enough to be balanced in your garage, on the car, but really the original pattern balance weights need to be fitted. These are the 3 or 4 weights arranged symmetrically around the rim and radially on the rim, and comprise a stack of lead and fibre washers. The trade are not likely to use these weights to balance as it is too time consuming. They will stick weights onto the inner flat area of the rim. The snag here is that the ‘stick’ is only as good as the paint adherence, so the weights can be lost. In the ‘good old days’, it was common practice to wrap lead wire around spokes. Cheap and cheerful but unsightly and liable to slide up and down the spoke after a while.

Loose steering joints or worn king-pins mean that the stub axles are not held firmly, so the front wheels are more prone to do their own thing. Any tendency to wobble is exacerbated. Loose steering joints will make the steering sloppy and reduce the pleasure of driving. And an MOT failure. Excess play in the steering box is part of this problem and difficult to rectify as there is no adjustment provided. Happily, most steering boxes seem to have survived the decades of use quite well.

Suspension defects are likely to be a broken leaf spring or too much play in the shackle pins and spring bushes. Road springs were fairly flat originally under static load, and many cars nowadays have the springs set too high, to the detriment of cornering ability, and also can alter the castor angle. Close inspection and repairs are necessary.

The last defect is settings. The toe-in must be about right, (1/8” to 3/16” at the tread) though this is a minor cause of wobble, and probably can be discounted, but must be right to maximise tyre wear and general steering feel. A major cause of wobble is a wrongly set castor angle on the beam axle. The original setting of about 4 degrees can be upset by spring wear or breakage but can be corrected by the use of a wedge between the spring and axle bed. The angle can be measured easily these days as there are Apps for your phone which will measure the angle of the phone edge, which can be just about rested on the axle bed alongside the spring bolts. This works because the axle bed is at right angles to the axis of the steering (king) pins in the side elevation. The wedge ideally should be steel, but aluminium or even hard wood will work. Making them is tricky as the correction angle is probably only a few degrees. If the castor angle is too big, this can veer towards causing the so-called ‘hospital trolley’ effect where the trail of the wheels is so great that they flap from side to side uncontrollably. Strangely, the steering will feel at its best with the correct castor angle! And the correct tyre pressure. Wheel wobble. Don’t put up with it! Enjoy your Bentley as it was intended.

Edward Way comments:

Interestingly our Three Litre : Chassis 1060, has an earlier steering box (No 70) ; I believe this is higher geared than later cars. It had impossibly heavy steering even on my preferred choice of beaded edge wheels and tyres with only one turn lock to lock. Also on our trip to spectate on the VSCC Lakeland Trial a year ago it started developing quite violent wheel patter.

The BDC “technical manual” does have a brief note mentioning reducing castor angle which I recently tried. What a transformation. No patter even on our awful pot holed roads in Sussex and MUCH lighter steering. I milled up a wedge of about 1½ degrees and inserted this ‘twixt spring and axle plate so reducing castor by moving the top of the kingpin forwards. I think the wheel wobble with excessive castor was probably caused by the inherent caster return fighting to pull the wheels to the straight position With the lighter high geared steering on those lovely fast A and B roads with sweeping bends you can almost think it around those corners……….