THE FEDERATION OF SIDECAR CLUBS
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Steering Dampers

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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 are unlikely to be able to accelerate through a 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.

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Since the seventies, forks have got stiffer. I don’t have any figures, but the diameters of stanchions and wheel spindles have increased. The stiffness of a tube goes as the cube of the diameter – that means if you increase a member from 25 mm to 50 mm diameter, the stiffness will increase by a factor of eight. The weight increase can be compensated by using thinner walled tube. Upside-down forks are hugely superior in this respect.
 
 









Decay time of oscillation for telescopic and leading link designs of fork. Roe and Thorpe 1979


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Everything else
You can experiment easily with tyre pressures, weight distribution, suspension and alignment, etc. and if you’re lucky it will make enough difference to reduce flutter. It’s a fact that heavier riders are better dampers than light ones. Trail (unless it’s adjustable) and lateral stiffness may be difficult to change, so you fit a steering damper.
 
Steering dampers
A lot of bikes come out of the factory with steering dampers already fitted by the manufacturer, so it’s not just fitting sidecars that causes the problem – it just makes it more obvious. The original solo item, if fitted by the factory is rarely effective with a chair and the very fact that the manufacturer had to fit one warns you to expect problems.
A given outfit may have forks with high lateral stiffness and still be stable with very little trail, while another similar outfit shakes its head like a wet dog. If it’s a classic bike, Ariel to Zündapp, you tighten up the factory-fitted steering damper! If it’s a Japanese machine from the 70s or 80s you have no such luxury, but it probably has so much trail that you can get away with it. It’s just heavy to steer.
Otherwise, you fit a steering damper and the unit of choice in the sidecar world is from the Volkswagen Beetle. (Ask yourself why VW couldn’t make inherently stable steering!) They are cheap and effective. 
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Wasp leading link forks. No steering damper visible.
​​Types of damper
The rider is a significant damper. That’s why the bars often don’t wobble until you let go. Heavy riders are better dampers than light ones.
Most modern dampers are hydraulic, although there are some cheap ones that are cylindrical, but rely on friction on the rod. Most classic dampers are of the friction plate variety. These have the advantage of being neat and adjustable. The damping rate is constant lock-to-lock, except for the stiction in the middle, which is handy, because that’s just where you want it and it drops off as soon as you move it.
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Friction damper (Jawa)                      Hydraulic damper (Volkswagen) 
Fitting a damper
If your bike was made with a friction damper, you are in clover. If it has a hydraulic unit for a solo, throw it away and fit one from a VW.
 
It can be really tricky to find the right position for a telescopic hydraulic steering damper. It must allow full lock-to-lock movement before bottoming or topping out and it must not touch forks, fairings or fittings (other than at the mounting points, of course). It must be fitted to a part of the forks which does not go up and down. The damping rate may vary considerably throughout the stroke as the angle to the forks changes, but this doesn’t really matter so long as you have effective damping near the middle. The damper does not have to be horizontal: the one on my EML is nearly vertical. If it makes the steering too heavy, move the fork end closer to the steering axis.  The rubber bush provided is usually fine for the fixed end and a 10 x 10 rod end (Rose joint) will be needed for the moving end.
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Correctly fitted damper                                                      Incorrectly fitted damper 
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As the forks compress, the damper resists and deflects the forks to the left (in this case).
My own experience
My first outfit was a 1960 Triumph Thunderbird, Monza chair, 90 mm trail, lousy forks, standard friction damper. Shook its head violently at 25 mph unless the damper was screwed down. I still have the bike and I have now reduced the trail to 35 mm and it just shakes its head a bit at 10 mph. The same steering damper is just barely applied to make it sweet. The awful Triumph forks remain.
A 1978 Triumph Tiger with a Garrard GP followed. Much better forks with a trail value of about 115 mm. No steering damper was fitted or required, probably because of the high trail and stiffer forks.
 
My Yamaha XS 750 / Watsonian GP Sports had twistier forks and lots of trail. I lived with it until I fitted some BMW Earles forks with the necessary VW steering damper. It handled very well. Having a solid bottom yoke, the BMW forks were about as good as an Earles type can be and I never noticed any lack of stiffness.
 
I never used my Kawasaki 1100 with telescopic forks. A set of Unit forks went on straight away. I did have problems with lack of stiffness here and the front end would break away on hairpins. The Units were replaced with Wasp forks and the difference was enormous. The forks from Hedingham were undersized with sleeves inserted into the yokes to make up the difference. The Wasp forks were full diameter and just looking down at the front wheel, I could see everything was moving around much less. If I hammered it round hairpins now, the back end would eventually start drifting while the front tyre remained glued to the tarmac.
 
My trials, enduro and grasstrack outfits have all had leading links with steering dampers. The steering on all of them was very light with trail values under 10 mm. The steering damper wasn’t really to stop oscillation so much as to put some kind of resistance into the steering. I have an EML motocrosser housing an Africa Twin motor. One-piece frame, very little trail and doesn’t shake its head at any speed. A steering damper is fitted simply to stop it wandering on tarmac.
 
My sister had a Virago/Hedingham SS with Unit forks and it was just about the sweetest handling outfit I’ve ever ridden. The forks were of much larger diameter than on my Kwacker and thus stiffer. A couple of times it developed a shake and each time it was cured by replacing the pivot bearings. I think this is because the forks lose rigidity when the bearings are worn.
When the Virago was swapped for a Triumph Speedmaster, I nailed an EZS Rally to the Virago and that was (and still is) a pretty good handler too.
 
The last outfit I built with forks was the BMW K1200S with EZS Rally and the so-called EML-Solid forks. I was keen to do this because I wanted to know how Hossack-type  forks would work with a chair. These forks are cost effective because you keep the original front suspension system (hence the “Solid” bit). The unsprung weight is enormous, but the BMW suspension coped with it perfectly. The original solo damper was left in situ, and I have no idea if it was needed. Trail was reduced to about 35 mm but the steering at low speeds was still very heavy, because with a wide tyre and a 60⁰ steering angle you are lifting the front of the outfit about 15 or 20 mm as you go from centre to full lock. Apart from that it worked very well indeed.
 
I converted the BMW to a Lefèvre hub-centre-steering system which is simply different from any kind of fork. It’s my second hub-centre-steered outfit, both without steering damper and never a hint of wobble or weave under any conditions. Why? Because of the extremely high lateral stiffness inherent in the design. Forks of any type are based on a three-foot lever mounted in bearings six inches apart! The corresponding lever with HCS only amounts to the radius of the wheel while the bearings are nearly half a metre apart. Do your own sums! 
​Thoughts on fork design for sidecar combinations
The only real contender is the leading link and I have a huge respect for the Wasp design. Choose large diameter tube and don’t sleeve down the yokes to accommodate a smaller tube. I am always disappointed when I see small wheel spindles used in links because the manufacturer wants to standardise the wheel and bearings. An integral bottom yoke is stiffer than a clamped one and a third yoke below the standard bottom one is really effective. (Lefèvre did this in 1980 and claimed it needed no steering damper.) Make sure the pivot bearings are large enough and in good condition. If all these points are ignored, the resulting forks are unlikely to be as stiff as the originals.
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Lefèvre CX500/Squire ST2 (still with steering damper)
​References:
Roe G E, Thorpe TE, Motorcycle Stability and Front Fork Design in The Manchester Engineer 123 No. 4
Foale T, Willoughby V, Motorcycle Chassis Design Osprey, 1984
Bradley J, The Racing Motorcycle, Vol 1 Broadland Leisure Publications, 1996
https://www.heritagepartscentre.com/uk/volkswagen/beetle/steering/steering-dampers.html
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