Racing Sidecar Design
RACING SIDECAR DESIGN
HEINZ SCHNEIDER
IN THE OLD DAYS, when motorcycles were a "poor man's transport," somebody had the idea of fitting a third wheel, a sort of second track to his bike to make it carry more payload. Thus the sidecar outfit, as we still know it, was born. It has not changed much of its looks since, it has not changed much of its reputation as an unpleasant mongrel, and, most unfortunately, it has not changed much of its ancient engineering approach.
This kind of vehicle still is referred to as "motorcycle with a sidecar," and the expression perfectly describes what everyday outfits are: some chair bolted to some motorcycle in a most uneasy way, regardless of what engineers have discovered about stressing out a design. A combination is a combination; it belongs to the independent category of asymmetric threewheelers, a category that has its own problems, its advantages and drawbacks. For a long time these facts have escaped even racing engineers' attention. As FIM-type racing becomes increasingly popular in the U.S., let us hope that the thrills of sidecar racing might become a regular feature in American events. Therefore we need to know what is expected from a sidecar frame and how designers of leading European combinations approached the task of building up-to-date frames. Conclusions drawn from these examples will be of more interest to a special builder than to any factory, because an enthusiast-designer can start his road-going outfit from scratch and take a good look at successful racers, while a mass-producing factory cannot afford to make the comparatively few out-and-out racers enthusiasts want to buy. A solo's frame has to cope with engine torque, with the pull of chains, with braking forces and the thrust from the rear wheel, and the direct battering from rough roads. Sidecar outfits suffer similar treatment, but bad roads impose even greater stress, since there are two tracks and one additional wheel to catch the bumps.
Let's assemble the facts a designer has to study to make an efficient frame for a solo motorcycle: solos have to be heeled through bends; therefore they need ample ground clearance, if footrests and megaphones are to be kept off the road, and the design must also allow for fast cornering on bumpy ground, when the suspension may be considerably compressed. Sidecar outfits, on the other hand, are usually vertical. So there is no need to provide as much clearance.
The 1963 Scheidegger outfit.
And there are other powerful forces in operation. Power is applied to the rear wheel, the bike accelerates, inertia tries to hold the sidecar back. One result: distortion of the tubes connecting sidecar to machine. Another: the outfit will begin to veer, and so the front wheel will have to be turned to hold the desired direction. The same forces apply when the driver brakes, but working the other way round. Only a brake on the sidecar wheel eliminates this trouble.
But a three-wheeler has to be steered, The front wheel is turned in the direction one wants to go, as in a car. Any mass objects to changes of velocity, which not only covers straight ahead movement but applies also to changes of direction A sidecar outfit, which has a distinct mass, does not want to be forced into a bend; it wants to go straight. The principal victims of this argument between front wheel and the remainder of the vehicle are the front fork and steering head, which tend to be well and truly twisted.
In a bend, centrifugal forces pull the outfit's center of gravity to the outside. Think of the wheels as running in rails. Centrifugal forces work on fork, steering head, rear pivoted fork and bearings, as well as on the framework between these points. And (the wheels again regarded as fixed points) centrifugal forces attempt to throw the chair over the bike or vice versa. These forces, counteracted by the passenger, stress the machine-sidecar connections. Distorted or fractured frames often result from hard driving with a frame which has not been designed with the stress points constantly in mind.
We have only considered forces until now, but what really stresses the frame are moments. Moment, or torque, is the product of a straight working force and the length of the lever to which it is applied. Example: the forces working on the steering head are centrifugal force multiplied by the distance between steering head and the road. The power which wants to tip the outfit over is the product of centrifugal force and the height of the center of gravity. A moment vanishes when you rob it of its lever; the shorter the lever, the smaller the moment.
It is an old engineer's trick to eliminate levers and so convert dangerous torques to simple pushing or pulling forces, which are easier to control. In this lies the main reason why successful outfits are ultralow, kneelers or not. The improved maximum speed gained from the smaller section of a low outfit is a free gift, valuable on the straights. A lower center of gravity gives higher cornering speeds and results in a stiffer vehicle. That helps in every department.
This general lever-shortening also results in outfits with extremely narrow tracks. They have a smaller frontal area, which is good, but the torque generated from acceleration and braking is also cut, and this is probably the more important gain. Another reason for favoring narrow-track outfits is the asymmetric drive of the device. Air resistance slows the undriven part of the outfit (sidecar) in relation to the driven one (bike). Therefore the motorcycle tends to be twisted towards the chair. And remember, the longer the lever — the track in this case — the more notable the effect, resulting in higher road resistance, more tire wear, lower top speed and decreased acceleration. Therefore, and because of reduced wind resistance, narrow track outfits are faster. And easier to control as a bonus.
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The objection might be raised that a narrow combination will more easily topple over the bike in a sidecar-side curve, but the outfit should be so low that the sidecar wheel is not likely to lift at cornering speeds which are possible today. There is one insurmountable limit to cornering speeds: tire adhesion. Until now we have assumed that the wheels are fixed in their lines, which they definitely are not. A tire may start to slide sideways. Just when it does so depends on the load it carries and on the coefficient of friction between road and rubber.
If highest cornering speeds are known, maximum forces can be worked out. Now it is time to go low, to reduce the levers. Since one does not need the stabilizing effects of wheels, as on a solo, the first and easiest step is to reduce wheel diameter. Sixteen inches has proved to be the popular dimension. Smaller wheels offer too little resistance to driver's shock actions. Sidecar tires are available down to 12 inches (reduced air resistance is the main factor here). Small diameter tires and today's soft high-hysteresis rubber mean a high rate of wear, especially when outfits are drifted through bends. When wheel diameters are reduced, the engine must be lowered. BMW power plants are predestined for sidecar operation, because they are of low overall height. Fitting this engine in a sidecar frame, one can bring the bulky, protruding cylinders close to the ground, much closer than on a solo which has to be heeled over. But this operation calls for frame alterations, and preferably the design of a new frame, because the BMW RS was made for solo work (and was so originally laid down in 1954). A new, low frame should be longer than the original one, to accommodate the driver between engine and rear wheel, not on top of it. The steering head should be as low as possible, just high enough to give the wheel sufficient free movement. The lower the steering head, the shorter the lever at which centrifugal forces pull. When starting a sidecar chassis from scratch it is natural to build it in one piece. The design provides the best and lightest way to make the chair attachment points at locations where stresses can be absorbed best, and where connections can be reinforced by gusset plates or stiffening tubes. But a one-piece development makes it difficult to get to the engine — thus designs that feature removable tubes for easier accessibility.
When designing the frame, a triangular layout should be incorporated if possible. A triangle gives the stiffest of all constructions, and the wider the base of the triangle, the stiffer it is. "Spaceframe" is the popular name for a chassis built up of triangles and pyramids. Other items to keep an eye on when building the frame are the steering head and front fork. The fork should be as short and as strong as possible. A leading link type is easier built as a one-off than a short telescopic fork which requires special dampers, while the Earles-type fork is controlled by standard suspension units.
Coming to the driver's position, the Camathias three-wheeler is a cross between a true kneeler 'and the ordinary type. Although the driver lies on his stomach, there is no support for the lower part of his legs. Ordinary footrests take the driver's feet. On the Continent there is some discussion about the desirability of kneelers. The conventional "sit on your behind and put your feet on the rests" type is considered safer, for reasons I don't quite understand. Some people would even like to see ultra-low machines forbidden by the FIM. As I think I have demonstrated, a low outfit is the only answer to the technical demands these vehicles have set up. A low seating position results in an uncomfortable crouch, while kneeling looks more natural and also gives the driver enough freedom to move when he wants to lean in on a bend.
While on the subject, we come to the question of whether it is necessary for a driver to lean in at all. No driver has lately tried to do away with these spectacular acrobatics, but sometimes I wonder if the same lap-times could not be achieved with the driver sitting tight. If he could keep relatively still, a full enclosure could be built, which would result in an even higher top speed. And, there is this point: the full fairing could be welded up from stressed steel or even made of large diameter thin-wall tube, thus giving a very strong body which could replace most of the framework. As there is no weight limit for motorcycles, this layout could result in an extremely light, well streamlined and, obviously, very rigid sort of vehicle with only one drawback: spectators would not be able to see the driver. Max Deubel's outfit comes nearest to the old idea of a motorcycle with sidecar attached. The reason might be the factory's conservative approach to technical progress; it might originate from an attempt to keep racing machinery more closely allied to production motorcycles; or it might come from the belief that the Camathias, Scheidegger and Vincent designs are "unprofessional," therefore dangerous degenerations.
It is true that they are not developed by a team of lettered engineers. But Camathias, at least, has a sure knack in these matters; he has the whole business of frame design and engine preparation at his finger tips. Scheidegger's vehicle admittedly looks as if it has been thrown together by an erratic genius. The Deubel racer is a spotless piece of Schoolbook engineering, though perhaps an old school book. Nobody has taken a chance in trying for a sophisticated design. It is the average brought to perfection, reliable but home spun; admittedly it lacks the brilliancy one likes to admire in a racing motorcycle or sidecar.
However, the world championships have proved that a good, solid design with a driver who treats his job as a good craftsman is successful in the end. As usual the works outfit steering head is braced with tubes and sheet material which distributes stresses over wider areas. But the rear end is just like a solo's. As a spectator one has the feeling that other outfits are much stiffer and thus better behaved through the bends. The bolted-on sidecar seems to be one of the reasons for this, too. A simple nut-and-bolt connection is not so rigid as a good welded job, and you cannot forget to tighten up a weld, which is easily overlooked with nuts.
Deubel's outfit is the only top-class one which still has a real driver's seat, although small and hard. And it is the only top one with a righthand sidecar, since Swiss drivers very early adopted the F.nglish pattern. •
