[motorcyclistonline] - Conventional Vs. Telelever Forks | DRAWING THE LINE

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A look beneath the bodywork of the 2012 BMW HP2 Sport, the last true sportbike equipped with the Telelever fork design.



Lately I’ve fielded a few questions about BMW’s decision to use a conventional fork on the all-new R1200R (see the R1200R First Ride Video HERE), rather than the Telelever used on preceding models. Is it an issue of cost? Handling? Weight? Or is BMW admitting that there is something wrong with the Telelever design?

With BMW reverting to a conventional fork on some key models, I thought it a good time to look closely at the Telelever design. For my benchmark Telelever bike I chose the 2012 BMW HP2 Sport, which may be the ultimate Telelever-equipped BMW performance bike. There are a number of high-quality CAD illustrations of the HP2 available; these, plus published specs, made analysis of the HP2 Telelever relatively easy. Rake angle is listed at 24 degrees, a fairly conventional sporting number. Trail, on the other hand, is a very aggressive 3.38 inches. Compare that to Ducati’s 899 Panigale that has the same 24-degree rake but 3.8 inches of trail.

One might expect the Ducati to have the more aggressive number, but it doesn’t. That 3.38-inch number would likely give a bike with a conventional fork such quick steering that it would verge on instability. But the HP2 doesn’t have a conventional fork; it has a Telelever. While a conventional fork’s rake angle and trail decrease as the fork compresses, the Telelever’s rake angle and trail increase as the suspension moves from full extension toward full compression.

That’s because the end of the arm (or the “lever”) that braces the Telelever travels in an arc that actually moves that steering pivot forward. Since the top of the Telelever fork pivots in a spherical bearing directly beneath the handlebar plate, that movement causes the fork’s rake to increase, as the top remains fixed and the pivot above the wheel moves forward.

What surprised me as I drew up schematic illustrations of the Telelever at different points in its travel was just how much the rake angle and trail changed as the wheel moved through its 4.7 inches of travel: Rake increases about 5 degrees, and the gain in trail is close to 1.2 inches. In percentage terms, rake increases 20 percent and trail increases by 35 percent! Averaged over the length of the stroke, Telelever’s rake is approximately 26.5 degrees and trail is approximately 3.9 inches—much less aggressive than it seems at first glance.

The geometry of the Telelever is unique, but it obviously does its job well. So why change? There are a few possibilities: Because of its variable geometry, Telelever feedback may feel less precise even as it delivers improved ride quality. A conventional fork may cost less overall and may weigh less too, but I think the biggest reason for the change had, ironically, nothing to do with suspension. It’s the radiator. The latest version of BMW’s signature boxer engine is liquid-cooled and thus needs a radiator, and a radiator can’t be located in the conventional central location with the Telelever because the suspension arm swings through that area. On the GS model the solution was to fit two radiators, one on each side of the Telelever arm. Because the GS has a wide handlebar and dirt bike-like radiator shrouds, the wider, two-radiator setup worked in terms of styling and ergonomics. (Dual radiators work into the RT’s styling as well.) This wasn’t the case with the roadster; to allow a conventional radiator arrangement, the Telelever had to go.

I’m sure as BMW planned its new models there were entire lists of pros and cons for a conventional fork and for the Telelever that were discussed. The decision wasn’t made for just one reason, but the radiator concerns were probably near the top of any list.

MORE Drawing the Line by James Parker HERE

James Parker designed his first original motorcycle in 1971; his most recent design is the Mission R electric superbike. In between, he worked on multiple other motorcycle projects, including 30 years spent evolving the RADD front suspension system used on the Yamaha GTS1000 and various other prototypes.




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