Steering Dampers
Why is it that some outfits need steering dampers and some don’t?
To say that a well set-up outfit doesn’t need one really isn’t very helpful, so let’s start with the basics. Why forks oscillate A steering damper is there to prevent the steering from oscillating. It is, in a sense, an admission of failure but it may be necessary all the same. The first question is why the steering shakes in the first place and the answer is that it is inherent in the design of a motorcycle. A motorcycle is basically two castors joined at a hinge. A castor is a wheel whose contact with the ground is offset with respect to the steering axis. The castors on supermarket trolleys are famous for oscillation. If the steering is deflected for any reason, the "castor effect" will re-centre it. It may well overshoot once or more before coming to rest. If the overshoot keeps repeating it is an oscillation, which may decay on its own or may need deliberate damping. Two Manchester engineers named Roe and Thorpe did a lot of work on this back in the late 70s with linisher belts and rollers to help develop the mathematics and then put theory into practice with real motorcycles. I went to one of their lectures in Manchester and they showed clips of Honda 50s, Norton Commandos and BMW boxers with and without modifications. (You had to admire the test riders, putting machines into deliberate tank-slappers!) Consider a conventional solo motorcycle with telescopic forks. The front wheel contact patch trails the steering axis by about 110 mm and the rear wheel trails by about 1500 mm. Each of these castors has a natural resonance frequency, which is about 6 to 8 Hz for the front wheel and 2 to 3 Hz for the rear. These are known as wobble or flutter when the steering oscillates about the bike and weave when everything else (including the sidecar if fitted) oscillates about the front forks. The wobble typically occurs at around 30 mph and the weave at much higher speeds. Fitting a sidecar lowers the speed at which flutter occurs but it raises the speed at which weave occurs while reducing top speed, so that most outfits can’t reach the weave speed. You can ride through wobble as increasing speed moves you out of the resonant zone and the gyroscopic effect of the front wheel stabilises it. You won’t ride through weave! An oscillation can be initiated by anything that deflects the steering: white lines, potholes, camber, side winds, etc. Most of the time the oscillation decays very quickly, because it’s the wrong frequency, but occasionally it will hit a resonant frequency and if there is insufficient damping you get wobble or weave. It should be pointed out that all motorcycles have inbuilt damping: there is the friction in the steering system, the friction of the tyre on the road and a great big organic damper called the rider. Factors affecting oscillation Oscillation can be affected by the steering geometry (wheelbase, rake and trail), lateral stiffness of the motorcycle and steering system, speed, tyre choice and pressure, by weight of bike, sidecar, rider, passenger and luggage, by road surface, wheel balance, sidecar set-up, suspension, screens, wind and so on. The two dominant factors which affect the tendency to oscillate are trail, as is well known, and lateral stiffness, which is almost entirely ignored. Trail It is the effect of the contact patch trailing the steering axis that provides the self-centring action of a castor. The higher the value, the more difficult it is to deflect the castor. On a motorcycle this translates into greater stability and heavy steering, especially with a sidecar attached. Classic bikes often had trail values on the low side so that no modification was required when a sidecar was attached. They also had factory-fitted steering dampers. BMW, Norton, Panther, Royal Enfield, Vincent amongst others offered either alternative or adjustable forks for sidecar duty. As it leaves the factory, a motorcycle today typically has 90 to 125 mm of trail. Some of the heavy Japanese bikes in the 70s got up to 140 or more in an attempt to improve stability. The popularity of leading links is probably attributable to three things: They are easy to build in low numbers, the suspension still works when they are twisted by steering forces (unlike teles, which “sprag” or bind) and you can easily achieve any value of trail you desire. Lateral stiffness Roe and Thorpe’s work showed that the geometry makes surprisingly little difference, while the lateral stiffness is critical. They then experimented with real motorcycles. At the time, lateral stiffness of motorcycle front forks was found to be in the range of 70 – 120 N/mm (newton per millimetre). To convert this to more accessible units, if you applied 70 to 120 kg sideways on the wheel spindle, the forks would deflect about one centimetre. In general leading links were worse than telescopic forks. In particular Earles forks, which have the pivot behind the wheel had a stiffness below 70 N/mm. After a couple of attempts they came up with a short leading-link system with a stiffness of no less than 455 N/mm at the same weight as the original tele-forks! The results were astonishing – wobble and weave were completely absent. Induced deflections damped out immediately with no steering damper. (Picture the testers, hands off, at 100 mph thumping the end of the handlebars!) BMW were very impressed, but did not take it any further and as far as I am aware, the forks were never tried on an outfit. |
