Alternative Steering Mechanisms for Motorcycles By Rohit Chaturvedi
Conventional Steering Mechanism in Two Wheelers The Telescopic forks A telescopic fork uses fork tubes which contain the suspension components (coil springs and damper) internally. The main advantages of the telescopic fork are that • it is simple in design and relatively cheap to manufacture and assemble; • it is lighter than older designs using external components and linkage systems; and • it has a clean and simple appearance that bikers find attractive.
On some modern sport bikes and most off-road bikes, this system is inverted.. This is done • to reduce unsprung weight by having the lighter components suspended, and • to improve the strength and rigidity of the assembly by having the strong large-diameter "sliders" clamped in the yokes. The inverted system is referred to as an upside-down fork. A disadvantage of this USD design is that the entire reservoir of damping oil is above the slider seal so that, if the slider seal were to leak, the oil could drain out, rendering any damping ineffective.
Conventional Forks Upside Down Forks
Need for Alternative Steering Mechanisms There are problems with normal forks, and these begin to show up more as modern bikes get heavier and more powerful. The major problems that arise are: • Lateral flex, and • Leverage
Any lateral flex in the fork legs allows the tyre contact patch to move away from the steering axis. Wobbles can be caused or greatly increased by this misalignment. The Lateral Flex Consider what happens when the wheel glances a bump to one side, the forks deflect away from this bump and if still deflected when the next bump is hit, the force of this will produce a torque about the steering axis that will cause the forks to steer to one side.
Another disadvantage is the large amount of leverage that can be exerted on the steering head itself. This results in very large forces that have to be resisted by a strong and hence heavy frame. • The Leverage In addition, telescopic forks are well known for their propensity to nosedive under the influence of braking forces, encouraging the current solution of increasing the damping to slow the rate of dive. Poor response to small bumps, such as expansion joints in the road, which may be due to stiction in the sliders is another frequent complaint aimed at telescopics.
Alternative Steering Mechanisms The most commonly used alternative steering mechanisms for motorcycles are: • BMW’s Telelever, • BMW’s Duolever, and • Hub-Center Steering
The "telelever" system is easiest described as an “A” arm or "wishbone" that's been mounted to the sliders just above the wheel. BMW Telelever This system not only lowers unsprung weight as well as decouples wheel placement function of the forks from the shock absorption function - eliminating brake dive while providing superior traction during hard-braking situations.This system also improves rider stability and comfort while still delivering excellent sportive handling.
The system separates the two functions of wheel guidance and damping / suspension in a simple and effective manner, thus significantly improving ride comfort. • The function of wheel guidance is still performed by the actual forks. • A trailing link attached at the front of the frame supports the fork and front wheel. A central strut is for suspension and damping. • Unlike a conventional fork, the fork of the telelever system reacts with low levels of bending when braking or going over bumps, therefore the fixed and sliding tubes cannot jam against each other.
Other benefits of this construction with a lower diameter of the telelever stanchions as compared to conventional forks are the weight advantage and an extremely responsive performance. • The low unsprung masses and the quick reaction of the suspension make for excellent road contact over bumpy surfaces. • The telelever system allows the geometry to be designed so that the fork bars and thus the front section only respond with minimal dive upon sudden braking, giving the rider improved feedback.
BMW Duolever Unlike telelever, in duolever the pivoting links and springs are not steered. But with duolever, the physical link from the handlebars to the suspension involves a hinged link. The front suspension is completely independent of the steering, with the two only being connected by the hinged link up top.
Hub-Center Steering A large diameter, steerable but non-rotating hub is mounted on a king-pin located within it. Another hub, of larger diameter, and forming part of the wheel, is mounted onto the first hub via large ball races.
The advantages of using a hub-center steering system instead of a more conventional motorcycle fork are that hub-center steering separates the steering, braking, and suspension functions. • Hub-center steering systems use an arm, or arms, on bearings to allow upward wheel deflection, meaning that there is no stiction, even under braking. Braking forces can be redirected horizontally along these arms, or tie rods, away from the vertical suspension forces, and can even be put to good use to counteract weight shift. • Finally, the arms typically form some form of parallelogram which maintains steering geometry over the full range of wheel travel, allowing agility and consistency of steering that forks currently cannot get close to attaining.
The hub center steering's Achilles heel, however, has been steering feel and turning radius . Complex linkages tend to be involved in the steering process, and this can lead to slack, vague, or inconsistent handlebar movement across its range. • After so many years of telescopic forks, people are used to riding a bike that handles in a specific way, and almost expect the limitations, and compensation is part of the experience. • Also there is a depth of knowledge known about fork based chassis design that attempts each year to get around the limitations through technological advances on the current system.
Conclusions At present several steering mechanisms for motorcycles exist and several new are to be developed in the near future. All these mechanisms promise a great deal of improvement over the conventional telescopic forks. However most of these mechanisms are premature and need deep engineering analysis. Some of the mechanisms which are well adopted by many manufacturers are not accepted by us as we are used to ride bikes with conventional telescopic forks. Hence, these mechanisms did not see the light of mass production and hence are very expensive to incorporate. The development of new steering mechanisms will curtail the cons of the conventional teleforks and will result in new frame designs. This will result in better performance and mileage for the motorcycles