Bmw's 450 Revolution

BMW'S 450 REVOLUTION

ROUNDUP

ONE REASON THE rumor mill grinds out so many tales about BMW is that this is a company which could do just about anything. But what it usually does do is continue its established traditions, peppered with attractive novelties such as Telelever front suspension.

Now come fresh tales from the patent office. Drawings filed by BMW earlier this year illustrate how the company might build an unusually compact, racing-oriented offroad Single. With necessary fork angle combined with current long suspension trav el, very little room is left where

the front wheel and the top of the engine both might wish to reside. BMW's scheme to place a smallish clutch on the crankshaft (so the swingarm pivot can be made concen tric with the gearbox output shaft) might nicely ease the space pinch.

Both ideas have a history. Adler, a German maker who supplied information to Yamaha many years ago, used a crank-mounted clutch-and so did early Yamaha TD 1, TD 1-A and TD1-B roadracers. A glance at the patent drawing shows BMW knows the potential for failure in vibration of the shaft connecting crank and clutch; in BMW's version, this shaft is large, hollow and stiff. Can a crank-mounted clutch be lighter because, turning faster than a gear box-mounted clutch, it needs less torque capacity? Not quite, because the lighter the clutch, the higher its temper ature rises during the vigor ous slipping of a fast start. Therefore a clutch needs a certain minimum mass in the same way brake discs do. My 1965 TD1-B needed clutch service after every quick start.

Placing the swingarm pivot on the same axis as the gearbox sprocket looks attractive and many have tried it. It does accomplish constant chain tension through full suspension movement, and this may be enough reason to adopt it on a dirtbike. On very powerful pavement machines, the ability to locate the swingarm pivot higher than this has proved useful because the jacking force that results can cancel rearward weight shift-and the front-end push that comes with it. Yet a word of caution: When Cal Rayborn rode for Harley, they had one chassis whose swingarm pivot position was vertically adjustable over a

1-inch range. Rayborn could not tell any difference! What was different? Spring rate, power and traction.

Why a telescopic fork, instead of something a little... edgier? It’s hard to beat a tele when long travel is your goal. Levers and pivots take up a lot more room. At the rear, our drawings show a direct, linkless connection of swingarm to suspension unit. Why no rockers, pivots and linkages? Simplicity brings reliability, and a moderate rising suspension rate can still be designed into such a simple system. All that is necessary is to arrange for the angle between damper and swingarm to increase from a small value toward 90 degrees as suspension compresses. Rather than immediately tackle the non-trivial problem of making an aluminum chassis whose welds and elements don’t crack in 100 hours, multiple steel tubes are pictured. Large aircraft still have landing-gear struts made of steel for a reason.

BMW’s major expertise relative to such an off-road bike is engine technology. But

aren’t the established makers already masters at this?

Think again. When fourstrokes returned to motocross, people’s response to them was,

“Oh, great! Reliable four-strokes like Dad used to have. They’re like lawnmowersnever need maintenance. If there’s oil on the stick (never mind if it’s black or a bit low), we can go dig deep in the dunes and ride all day.”

The result is-too often-

galled cam lobes and tappets, scuffed pistons, wiped rings and 10-hour crank life.

These are really miniature

F-l engines-something in which BMW has particular “core competencies.” (Don’t you just love modern managementspeak?)

BMW raced a 450cc prototype in the German enduro championship this spring, while another bike is said to be testing stateside currently.

Couple this information with the announcement by Buell that it intends to enter the competition dirt market within the next two years. No further information was released but insiders say this will be a bike meant to compete directly with offerings from moto/enduroclass stalwarts.

Serious dirt contenders from America and Germany?

It sure looks like it’s going to happen. -Kevin Cameron