Christini-equipped bikes have a second drive chain, running from the countershaft to a point high on the frame. From there, a shaft transmits power under the fuel tank. A pair of counter-rotating bevel gears in the steering head transfers power down the steering stem to the lower triple clamp. Two small drive chains in the clamp transfer power out to a pair of telescoping shafts that run parallel to the fork legs, down to the front hub. The key feature of the Christini system is those twin shafts, which spin in opposite directions.
At the front hub, a Sprague clutch–similar to the freewheel mechanism in a bicycle’s rear hub–transfers power to the front wheel when rear-wheel speed exceeds front-wheel speed by more than a prescribed ratio.
Any system that applies positive torque to the front wheel in a turn makes the bike lean in. That ‘steer-torque’ complicated handling enough, so Christini decided to minimize the torque effects of his own system by using counter-rotating shafts that canceled each other out. Those two shafts also provide a second, unexpected benefit: they act as gyroscopic steering dampers, dramatically reducing the ‘bump-steer’ effects produced by other front-hub motors.
The first 450 SuperSingle prototypes were fitted with forks from street bikes. But it turns out that the stock motocross forks can be converted (revalved and set up with reduced travel) with results every bit as good. The one catch is that since the stock MX fork legs carry the front axle ahead of the fork axis (instead of directly under it as in street bikes) the converted bikes have almost no trail. This results in poor front grip.
The solution, if you’re building a conventional RWD SuperSingle, is to fabricate triple clamps with a negative offset. That would be a major machining and fabrication job for a bike fitted with the Christini system, since the lower triple clamp contains two chain drives. The last time I spoke with him (which was years ago) Rodney Aguiar told me he was going to convert his 2WD SuperSingle into a street bike, since he’s tired of losing the front on the track. (No, I don’t mean he’d rather lose the front on the street, I mean he won’t push it as hard on the street!)
Battle of the Boffins...
Despite the fact that the ‘hydraulic’ versus ‘mechanical’ 2WD system camps are quick to point out each other’s disadvantages, it was interesting to hear Öhlins’ R&D Chief Lars Jansson and Steve Christini agree, almost word-for-word, on some things.
For example, both of them realize that one of their problems is that all manufacturers’ development riders are experts and ex-racers. “A lot of new motorcycle technology is geared towards the top riders,” Christini lamented. “Our technology is different; the worse rider you are, the more you benefit. There’s no reason to think that it will be different on the street.” And both men concluded that in road bikes, 2WD must be marketed as a safety feature, not a way to boost performance.
In a performance-driven marketplace, where most of the people with media access are expert riders, that’s a problem. Back when I was actively researching this, I called a handful of respected experts and asked them what potential they think there is for 2WD in road bikes. It would be an understatement to say that, on balance, they’re skeptical. Here’s what they had to say…
Tony Foale is the author of ‘Motorcycle Handling and Chassis Design.’ When I last spoke to him, he was working as an R&D consultant to the Segway Corporation.
Tony Foale told me, “I can’t see it being much of an advantage in a racing situation, since under acceleration, when the extra grip would help, there’s no weight on the front wheel anyway.
“Now in the dirt, that’s totally different; there, I can see immediate advantages, because in sand or mud, only a little power, maybe 15%, would need to be transferred to dramatically improve handling. All you need is enough power to prevent the front wheel from ploughing; all it needs to do is lift itself up out of the sand or mud.
“On tarmac, there would need to be a lot more power transmitted to the front to make a difference. Maybe in the context of much longer, lower machines, something like Gurney’s ‘Alligator,’ there’d be an advantage.”
Kevin 'I'm not as angry as I look' Cameron said, “It’s especially well-adapted to any situation where there’s significant weight on the front wheel; either when it’s too slippery for a motorcycle to gather all its weight onto the rear wheel, or in some longer-wheelbase design. A longer wheelbase could be tolerated with two-wheel steering, but even at that... where traction is good, I just don’t see the advantage.
“If I were going to market this I’d go straight to BMW. Their buyers are that funny-hat crowd; they love to have features no one else gives a shit about. Put me down as a stick in the mud.”
James Parker is an independent industrial designer. He created the RADD suspension system used on the Yamaha GTS1000, and has continued to develop it. He is currently working on a RADD Moto2 proof-of-concept.
James Parker, who once actually patented a 2WD system for motorcycles said, “It’s a lot easier to get drive the front wheel of a hub-center steering system. If the GTS had caught on, we’d have more two-wheel drive motorcycles–not a lot of them, but more of them.
“If you look at really good ABS, like the Hondas now have, or traction control like some of the new Ducatis–those things can offer significant safety advantages on the street, but they’re trying to eliminate them on the race track. So if racing is the way we introduce new technology, does that mean consumers won’t want those things on street bikes?
“I live in Santa Fe, at 7,000 feet. We get a lot of snow here, and people who have four-wheel drive cars have confidence in situations where they shouldn’t. That might happen with two-wheel drive motorcycles, too.
“One of the great strengths of motorcycles is their simplicity. When you make them more complex, they become a different animal. That said, my new suspension system has less weight, less unsprung weight, less steered mass and lower steering effort than either a conventional fork or earlier RADD prototypes. I suppose if I find a manufacturer that wants to use it, I might revisit my 2WD system and see if I can make corresponding improvements.”
Damian Harty is a vehicle dynamics specialist at Prodrive, and an expert on the advantages of all-wheel drive in automobiles, he’s also an avid rider and has studied motorcycle dynamics.
Damian Harty told me, “At less the 20 degrees of lean angle, in a racing situation a motorcycle is basically a unicycle, but between 60 degrees and 20 degrees, in the ‘early corner exit’ phase there’s an appreciable amount of weight on the front wheel, and any advantage in acceleration you get in the early part of corner exit is carried all the way through the acceleration zone; all the way down the next straight.
“Bear in mind that in racing, at the end of 30 laps if you’re half a bike-length ahead, you win. As long as it was allowed under the rules [highly unlikely at a time when all sanctioning bodies are desperate to cut costs–MG] advantages wouldn’t have to be that great for everyone to adopt it, and soon afterward, manufacturers would be offering it on road bikes whether it was an advantage on the road or not, since it would be something they could charge for.
“In rallying, where driving is very unrehearsed and reactive and where grip is unpredictable, all-wheel drive is a huge advantage, and I think it’s got a rosy future in road bikes for many of the same reasons.”
Neil Spalding, the author of MotoGP Technology, said, “Everyone says it’s marvelous in the wet, no one says it’s marvelous in the dry. Reduced grip is the obvious thing about wet conditions, but everything changes–rider inputs, for example, are much less violent.
“What hamstrings motorcycle development is, you develop some great new two-wheel drive system, or radical new suspension and it’s got to be completely transparent; it’s got to feel exactly like a conventional front fork or whatever. That’s why I think it’s toast, unless it’s an electric motor in the front hub, as part of a KERS system. Then, you’ve got the thing sitting there, you might as well use it.”