The myth of anti-dive front ends

Why they don't work on the track

Innovative design is something that should be applauded. It's the way we evolve, the way we improve on what we already have. Over the years there have been many clever designers and engineers who sought to improve a motorcycle's handling characteristics. It's an ongoing process of trial and error, particularly in racing, and with racing, if something works it's used and if it doesn't work it's not, it's as simple as that. And we've come along way. We started with no suspension at all and progressed to having a sprung front end and later, the rear was suspended by twin shocks that featured damping as well. This was probably the single biggest breakthrough.

The progress since then has been one of refinement rather than revolutionary breakthroughs. The twin spar aluminium beam frame was a big step forward and now is almost universally used on performance bikes. There are some notable exceptions though. KTM uses a steel trellis frame on their MotoGP bikes. Time will tell if this layout is good enough for the top level of competition. It was first made popular by Ducati who used it on their street bikes as well as the Desmosedici MotoGP bike. They have since switched to an aluminium beam frame for their performance bikes, retaining the easier to make trellis frame on some of the street bikes.

Another big step has been the single shock rising rate rear end with variations now used on almost everything except cruisers and retro styled bikes. There have been many ways in which designers have sought to improve what is basically a 100 year old design, one of them has been the centre hub steering front end.

The latest version of the Bimota Tesi

High hopes were held for the Bimota Tesi when it was released in 1992. Bimota had always been at the cutting edge of handling technology and many people were expecting it to set a new trend in chassis design. Hub-center steering is characterized by the steering pivot points being inside the hub of the wheel, rather than above the wheel in the headstock as in the traditional layout. Most hub-center arrangements employ a swingarm that extends from the bottom of the engine/frame to the centre of the front wheel.

Yamaha introduced the GTS1000 based on James Parker's RADD design in 1993 but nobody wanted it. It was quickly scrapped. One wonders if sometimes technology is applied for technology's sake. To be seen as an innovator

Yamaha's GTS1000
Image: Wikipedia

Trying to fix the mousetrap

It is thought that the inherent diving of the front forks going into a corner is a problem that needs to be solved. The thinking behind it says that using a hub-center steering system instead of a more conventional fork separates the steering, braking, and suspension functions. With a fork the braking forces are put through the suspension, a situation that leads to the suspension being compressed, using up a large amount of suspension travel that makes dealing with bumps and other road irregularities more difficult.

These 'solutions' have been well and truly tested on the racetrack where competitors are continually looking for something to give them an edge. To my knowledge no one has ever been able to improve their lap times using a centre- hub steering anti-dive front end in a back to back comparison using a conventionally forked machine. They have in fact gone slower. Although, depending on the track layout, this could possibly be achieved. Journalist/Racer Alan Cathcart did win the BOT race at Daytona, twice in fact, on a Bimota Tesi but Daytona is a track that would be well suited to the anti dive layout and I doubt if a comparison test would have been done with a more conventional Bimota. Had this been the solution, racers everywhere would have been quickly fitting a similar set up to their own bikes, but it didn't happen. Testers have said that it "just didn't feel right" probably not knowing why. I think there is more to this than just unfamiliarity with the unconventional system.

The problem here is that something has been identified as a liability when in fact it is an asset. As a rider applies the brakes and starts to turn into a corner the front fork compresses considerably and the rear of the bike rises as the weight is transferred to the front. The compression of the front forks shortens the wheelbase and the rising of the rear end changes the steering angle, not relative to the bike of course, but relative to the ground. The angle becomes steeper. These two attributes are highly desirable and allow the rider to turn into the corner easier. Racing bikes will generally have a shorter wheelbase and steeper steering than road bikes for this reason but it can be taken only so far. Take it too far and the bike will be unstable on fast corners and even in a straight line. When the front dives under braking it produces these desirable characteristics when you need them.

When the power is applied before the apex and harder afterwards the opposite happens. The front lifts more than the back squats and increases the length of the wheelbase and the fork angle is raked out further than it would be relative to the ground, compared to the bike at a standstill. This adds to the stability of the bike when the power is being fed on hard coming out of the corner. It also gives the rider more ground clearance allowing him to lay the bike over further in the initial stages of the exit from the corner. This is the key to going fast but some do it better than others.

Development never stops and someone will surely come up with a solution that allows for the full amount of suspension to be available after the brakes are applied and still give the rider the feeling he needs to go fast. The solution may well be a variation on the centre- hub steering anti-dive front end, but we're not there yet.

Written by Paul Harmon. 19-1-18

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