Why do manufacturers, specials builders and crackpot inventors have to try and re-invent the wheel: or – in this case – the humble front fork?
Until we see something weird-looking at the front of a MotoGP machine, we ‘ere at CB-Net reckon that a pair of finely-fettled forks can’t be beat – but god knows people have tried. OK so they’ve tried with good reason.
Look at anything with what has often been referred to as a ‘funny front-end.’ We’re talking about any number of DiFazio specials, the Vyrus, Yamaha’s GTS1000A, BMW’s Telelever or the likes of the Bimota Tesi….
The thing with forks is that when you hit the brakes hard, you get ‘dive’. This is where the weight transfers to the front of the bike. Too much weight transfer and the forks bottom out over bumps and the front wheel skips or jumps over the Tarmac. This isn’t good as the front tyre can’t do what it is supposed to do (steer/brake) as it’s not in contact with the road itself.
Yes, decent, adjustable suspension can help – but only so much. Set up your suspension too stiff and you’ll get a hard ride… Also, when forks bottomed out, this would lead to changes in steering geometry of the bike itself, which would be another factor in trying to get a front suspension system that wouldn’t be so affected by hard braking and its associated ‘dive’.
So, anti-dive tech was required to help a bike handle and it came from the race-tracks. Early anti-dive systems were fitted to the likes of the square-four Suzuki RG500 and Yamaha’s OW 500cc race machines. Sometimes it worked, sometimes it didn’t: apparently King Kenny Roberts wasn’t much of a fan. With manufacturers hating to waste development dollar/yen, anti-dive would find itself on road bikes of the late 1970s/early 1980s.
Honda’s mechanical anti-dive system was first seen in the 1970s, courtesy of Honda’s TRAC system. TRAC, stood for ‘Torque Reactive Anti-dive Control’ and this was seen on a number of Hondas of the era, including CB1100s, CB900s and even the VFR750F of 1986. It worked by using a brake caliper, hinged behind the fork leg on a pivot-link. When you pulled the brake lever, the caliper wanted to drag when the pads began to bite into the brake disc. Normally the caliper is fixed in place, but with TRAC this movement was used to press against a valve inside the fork leg, which altered the flow of oil, restricting it and therefore making the suspension stiffen up.
Other manufacturers soon jumped onto the bandwagon. Kawasaki had a system called AVDS or ‘Automatic Variable Damping System’ and this was fitted to a number of Kawasakis of the early to mid-1980s, most noticeably the GPz900R and GPZ600R. This system used a secondary, smaller, hydraulic system mounted at the front of the fork legs. It was connected to the hydraulic brake lines and the fluid inside the forks themselves. It was a simpler system than Honda’s TRAC: when you pulled the brake lever, the pressure in the brake hoses pushed a plunger to close a valve. The valve restricted the flow of oil in the forks making the suspension stiffer. Kawasaki’s AVDS system used a three-position adjuster to allow for the road conditions and to how much anti-dive you wanted.
Suzuki had both PDF (Positive Damping Fork) and NEAS (New Electrically Activated Suspension) while some smaller machines like the RG250 had a system called ANDF or (Anti Nose Dive Forks.) Suzuki’s system was similar to Kawasaki’s AVDS, apart from the fact that the valves had a return spring so that if you hit a bump with the suspension stiffened the valve could return and restore oil flow to the forks to absorb the bumps. Kawasaki’s system wasn’t so subtle and suffered as a result. Other systems included TCS or ‘Travel Control System’ used by some home market Yamahas in Japan.
As good as all this was, the evils of brake dive was never truly eliminated and – having a further system on the bike – meant it was just something else that could go wrong. Today you’ll find the classic market split between those that carefully strip and reassemble the delicate anti-dive systems on their restored machines or those that use blanking plates to cut them off. Perhaps the latter folk are right as anti-dive pretty much took a dive (ahem) in the late 1980s – the Kawasaki GPz900R had it deleted from their forks from 1990. We like to think bikes of the 1990s didn’t need it so much, thanks to better forks/suspension, brakes and tyres.
