Timing belt conversion Yamaha XJR 1300 West
The Yamaha XJR 1300 from Motokram is driven by a toothed belt. The future belongs to the alternative drive to cardan and chain?
The fact that the XJR 1300 from Motokram is an eye-catcher is due, on the one hand, to its unusual paintwork, which was modeled on the western cigarette advertising, as well as to various exquisite components: Upside-down fork from Showa, struts from Technoflex, strikingly styled bench and lots polished precious metal. On the other hand, also on the unusual drive: Instead of a chain, 106 HP trot over a 37 millimeter wide toothed belt to the rear wheel.
And this type of power transmission leaves a consistently positive impression. The toothed belt acts like a shock absorber and transmits the power more smoothly than a chain and cardan. Load changes are hardly noticeable. In addition, the belt drive runs very quietly. Its tensile strength is comparable to that of an O-ring chain, it is also lighter and, once correctly adjusted, does not have to be permanently checked, tensioned or greased. No cleaning actions because of thrown off chain grease or even oil changes as with the cardan. The only disadvantage: it is more susceptible to extreme pollution.
In practice, a number of models, mainly cruisers, have proven that the timing belt is not only socially acceptable, but also viable. That he has not yet been able to assert himself is due to ?? besides the ignorance of the motorcycle manufacturers ?? also on two requirements for its perfect function: Firstly, the toothed belt must not be excessively kinked, because it consists of a nylon belt with internal, high-strength Kevlar threads and reacts sensitively from a certain curve. Second, it should always have a constant tension. The ideal prerequisites for this are tight spring travel, a long swing arm and a short distance between the pinion and the swing arm pivot point. Curiously, this is not the case with Buell’s timing belt-driven models and proves that theory and practice can differ. When fully extended, a Buell X1, for example, has a belt sag of 50 millimeters. A sufficient angle of contact of the toothed belt around the pinion and belt pulley prevents it from skipping.
Graduate engineer Hans Helms, owner of the company VH in Oldenburg and manufacturer of the conversion kit for the tested XJR, has been converting from chain to toothed belt drives for more than ten years. Due to the design, this fails in many models for reasons of space: either because of the wider belt in relation to the chain, or the diameter of the pinion required for the secondary transmission cannot be achieved.
Helms is currently offering four models (see box) that can be converted to toothed belt drives. Encouraged by the empirical values of the Buell models, there will be even more in the future. In addition, Hans Helms hopes that the patented timing belts that Harley has been using since this year will soon be available for purchase. Instead of Kevlar, carbon fibers are used, which increase tensile strength by 30 percent. This enables a narrower belt, so further modifications can be carried out.
Including TÜV entry, a minimum of two days are to be estimated for a conversion, the costs amount to about 2200 marks. Depending on the driving style, the belt lasts between 30,000 and 50,000 kilometers and costs as much as a good chain set: 400 marks.
A.owing to progress in the materials sector on the one hand and increasingly strict noise regulations on the other, it is to be expected that manufacturers will increasingly rely on toothed belt drives in the future. Because there are neither logical nor technological reasons that speak against it. BMW is taking on the pioneering role with the new F 650 ST and will also make the belt socially acceptable in the mid-range segment. A clean affair.
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