Motorcycle frame stiffness
MAY 3rd, 2020
What allows a vehicle to remain in contact with the ground are the tires. In order to take advantage of maximum grip when cornering, they must work in the optimum manner: they should not lift off the ground, and they should remain at the correct working temperature. A tire that lifts, overheats, or stays too cold will lose grip, leading to loss of grip for the entire vehicle.
The suspensions are that element that allows the tires to work in the best way, copying the roughness of the ground and thus maximizing the grip. This is the reason why car chassis must be as rigid as possible: to leave to the suspensions alone the task of controlling the movement of the wheels, without introducing unforeseen movements that would affect the handling.
In motorcycles, the situation is different: a motorcycle chassis must be flexible. The reason for this is quickly stated: you can imagine the car as a vehicle that moves on a plane, while the motorcycle moves in space.
When a car drives around a corner, the roughness of the ground is absorbed by the suspension. A motorcycle, on the other hand, must lean in order to negotiate a curve. Encountering an asperity, part of the force is absorbed by the suspension, part acts laterally, stressing the chassis. If the frame were too rigid, this force would not be absorbed at all, causing the loss of grip of the tire.
It is clear that the greater the lean angle, the greater this lateral force will be, making the effect of the lateral stiffness of the frame more important. For this reason, in motocross still resist the single beam steel frames, particularly flexible: the ground (full of bumps) and the high angle of lean reached by the bike (which is lowered to the ground) make the frame work hard. Honda realized this when, in 1997, it put on the market the first motocross bike with an aluminum perimeter frame. This first version turned out to be too rigid, making riding rather tiring and unprofitable. All subsequent aluminum frames were produced with smaller sections to increase their flexibility.
Rider in action: note the high lean angle achieved, it is very important that the frame is able to absorb lateral vibrations.
Previously we mentioned the term "lateral stiffness". There is no single value of stiffness for a frame, but this varies with the direction of the applied force. The stiffnesses of interest in a motorcycle frame are:
As noted above, the lateral stiffness must be low. Longitudinal and torsional stiffness, on the other hand, must be high. If the longitudinal stiffness were low, the bike would feel inaccurate when braking. If the torsional stiffness were low, the bike would feel slow and heavy, because every command from the rider would result in a deformation of the frame, before the intended maneuver.
Ideally, the torsional stiffness should be infinite. The perimeter frame succeeds better than others in meeting the requirements of high torsional and longitudinal stiffness and low lateral stiffness.
Let's compare, for example, a perimeter frame with a classic motocross double cradle. In the perimeter frame, a torque is transformed into two vertical forces, while in the single beam the same torque is transformed into two lateral forces. So, by torsionally stiffening the double cradle frame, it will inevitably stiffen laterally as well. In the perimeter, however, the forces generated by torsion are orthogonal to the lateral force, so the two stiffnesses can be (within certain limits) independent of each other.
However rigid it may be, the perimeter frame will still be far from the ideal infinite-torsion stiffness. Much stiffer are monocoque type frames, which are essentially made up of a box. Their stiffness is so high that they can be made with very thin wall thicknesses, often resulting in lighter weight than a normal perimeter frame. Moreover, often the airbox, the tank or both are obtained directly inside the frame itself, leading to a further weight reduction, due to the elimination of these components. An example of a frame of this type can be found in the Ossa 250 grand prix of 1967. In this bike the lower weight of about 20 kg and the exceptional handling qualities (due to the high stiffness of the frame) allowed it to compete against much more powerful opponents, especially in the most tortuous tracks, so much so that it won the Spanish championship in 1967 and 1968, and came seventh in the 1968 world championship.
Ossa 250 monocoque
This frame has very good handling qualities, but is also extremely rigid laterally. The problems of lack of feeling of the Ducati Desmosedici designed by Eng. Filippo Preziosi, with carbon monocoque frame and load-bearing engine, are well known.
It is the writer's opinion that the older bikes (such as the Ossa) were still competitive, both because the tires of the time did not allow to reach the bending angles of modern ones, making the lateral stiffness less important, and because they used small diameter straight forks, much more flexible than modern upside-down forks.
It would be interesting to test a motorcycle with a monocoque frame, and equipped with a lateral suspension system that allows the absorption of bumps while leaning. It would probably combine the advantages of the high stiffness of the monocoque, with the feeling when leaning guaranteed by the lateral flexion, which in this case would also be adjustable.
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