By Ken Gillanders
The old vintage Triumph seems to have its share of front end vibration, shimmy and shaking, some of which is the original design and some of which appears to be lack of knowledge as to its cause. Those of us who have owned a shaker seem to go through the usual process of looking for loose tie-rod ends, bad front end bushings and worn ball joints without ever finding the cause. Fortunately, the usual causes are no mystery; it’s just that we’ve been looking in the wrong places. So with the aid of a technical paper from the DOT on this problem, along with 25 years of personal experience, I have developed a checklist that might help.
If you are going to have the wheels balanced dynamically (on the car), it will be necessary to mark the wheel and the hub so that if the wheel is removed for some reason later, you can replace it in exactly the same location and not lose the balance. Usually, a center punch mark on the end of one wheel stud and another mark next to the corresponding stud hole on the wheel works fine. For wire wheels, mark the edge of the wheel hub, then make a matching mark on the edge of the wheel center. Generally, off-the-car balances don’t work too well because they do not compensate for the weight of other rotating parts such as brake discs (or drums).
Would you believe the most common cause of shake and wobble is excessive tire pressures? All tires have a normal cushion effect as they are running down the road which absorbs bumps and irregularities in the roadway up to its natural limit without transferring them to the suspension. This limit is directly related to tire pressure. Furthermore, the load and frequency of road deviation transmitted to the suspension and shock absorbers varies not only with tire pressure, but also with tire design, size and conditions. The most persistent shaker that I ever saw was a TR6 whose problems we chased for two years. The problem turned out to be about 5psi too much air in the front tires and shocks that were worn out. The DOT tests found that one model of car was completely unmanageable when the front tire pressures were inflated to 30psi, but was perfectly normal at 28psi!
TIRE OR WHEEL OUT OF ROUND
Sometimes a tire or wheel may be out of round, and this will create a shake that your suspension system cannot handle. Jack up the wheel in question and use a block under the wheel and tire, but make sure the block does not touch the wheel. Then spin the wheel and see if the space between the spinning tire and the block changes, indicating a wheel/tire out of round. This is also a good time to check for a wheel or a tire with a run-out problem. Position yourself in front of the wheel and see if it moves back and forth or left and right as it spins. This indicates there is run-out in the tire or wheel.
As time goes by, many of these variables change, such as the tires that become available, their particular road characteristics, the shocks available to us and their capacity and valving. Fortunately, with a little experimentation, most of the problems that arise can usually be overcome.