Sticking the knife in forks

by Simon Hargreaves

Funny front ends? You must be joking

Twenty one years ago BMW launched the R1100RS with a Telelever front end. It's gone on to be fitted to the most popular bike of the 21st Century, BMW's GS. Yet still – apart from BMW's other forkless system, Duolever – conventional telescopic forks survive on all other bikes, including MotoGP. Why? On paper, forks are an inelegant engineering solution to steering and suspending a bike. They have advantages – simplicity, tradition – but lots of disadvantages, such as an indirect load path and allowing those loads to influence steering.

Imagine a side-on view of your bike. The forks are one leg of a Λ shape, the headstock at its apex, the frame spars are the other leg. The peak – the headstock – is the weakest point, yet the forks and the frame direct their loads through it.

The headstock, which is short compared to the levers trying to bend it, has to resist rear tyre thrust pushing the front forward under acceleration, and resist the opposite force under braking. At over 1g, it's equivalent to the weight of the bike. Then there's the actual weight of the bike – all of it, under braking – as it tries to squash the legs of the Λ apart. Meanwhile bump force is trying to do the same thing in the opposite direction (bump force is tremendous. Rough calculations suggest a 1in bump with a 12in duration will generate a force many times the bikes' weight). At maximum braking a headstock could have over 1000ftlb of torque trying to twist it from the frame. It won't, but resisting flex and keeping the steering stem straight is why headstocks are massive (big enough to have to hollow out to let air flow through to the airbox).

Meanwhile the rider steers the bike using the same components transmitting the forces trying to pull it in half. What you feel at the bars is the sum of these forces, plus cornering force (camber thrust, weight and rolling drag torques). On one hand this is good because you're informed about grip. On the other it's bad because when, for example, overcooking a bend and rolling off, weight transfer makes the bike understeer out of the turn and a self-righting torque at the bars convinces the rider, who already thinks he has no grip, he cannot steer any harder. The result is called 'running out of road'.

Alternatives to forks, that a) pass loads directly into the chassis and b) separate them from steering, include the hub centre steered Bimota Tesi and Yamaha GTS1000. But while they look futuristic (or did) and obviate the need for a headstock (or a frame), they're bulky, complex and aesthetically unpleasing. Which leaves BMW's single wishbone Telelever and double wishbone Duolever. Telelever has a better load path, but Duolever completely decouples steering from braking and suspension. As a result, the K series BMWs have consistently weighted steering even on the brakes and do not understeer – perfect for overenthusiastic riders who corner too fast. But what Duolever and Telelever gain for the clumsy rider, they lose in grip feel at extreme lean. By removing the loads you'd normally feel through your hands, you don't get total feedback at maximum lean; part of the information is missing.

Which is why MotoGP bikes still have forks.

Navy Boy
Offline
Joined: 12/02/2009

I suppose that this is a similar argument to the use of a chain rather than shaft drive. Chains are cheap and work well - In other words the investment vs reward ratio remains high.

Having ridden both K and R-series BMWs I appreciated the 'Different' front end feeling that they offered but I don't ride along on my Triumph Sprint GT wishing that it had a different front end.