Getting to the hub of Bimota's 'funny front end'
Your bike’s forks offer certain advantages: they’re light, neatly packaged, provide lots of feel and have ample stiffness. But forks are a compromise.
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Their many functions interfere with each other: suspension loads pass to the steering, thrust from acceleration and braking goes through the suspension, and their leverage tries to snap off the steering head.
They also deflect, hampering suspension movement. So bikes end up with huge fork legs, massive headstocks and steering that doesn’t work if you’re braking over bumps.
The solution is a front end with separate, direct load paths for each force – thrust, suspension, steering – so the functions work independently. Like the new Bimota Tesi H2’s front swingarm with hub-centre steering.
The Bimota’s swingarm replaces the forks and passes suspension movement to a shock like you’d normally find at the rear. In fact, the unit sits next to the rear one, operated via a link on the left of the bike. Steering input travels from the handlebars through by a series of links and joints, with the front wheel pivoting about its hub.
The design means steering isn’t affected by suspension movement or braking. With forks the steering geometry alters as the suspension compresses or extends, but Bimota’s parallelogram design keeps rake and trail constant – so the steering has consistent weight and feel.
The suspension’s action isn’t affected by braking or steering, and weight transfer under braking can be reduced if required, too. You can also alter ride height without upsetting geometry.
With no suspension forces going through the steering there’s no need for a chunky headstock… or a frame at all. With the engine as a load-carrying chassis part, the new Tesi H2 only needs a collection of plates and brackets to tie it together. No frame offsets the additional weight of the swingarm and linkages. And with the H2 engine, it helps counter the weight of the supercharger plumbing, too.
There are, of course, disadvantages, too – mainly complexity and reduced feel due to play in all those joints. However, having spent 30 years developing the Tesi road bike, Bimota believe the benefits are worth it. And you can notice it on the road. Big bumps seems to magically disappear.
The only other problem is cost. Forks are a bit of a lash-up from an engineering perspective, but they’re effective and are comparatively cheap. Bimota don’t care, of course. When you’re building desirable, premium motorcycles then the additional cost of an exotic front end is actually a positive.
Bimota hub steering at a glance
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