Cars use front-wheel drive – so why don’t bikes?
Some ways of doing things are so universal that people don’t even think to ask if there’s any other option. Until, on occasion, some lateral-thinking genius comes along, turns convention and accepted wisdom on its head and makes a major breakthrough in the process. Or a complete idiot of himself instead...
Let’s apply this theory to two of the most fundamental design features of every modern production motorcycle – without exception, they all drive through the back wheel and steer with the front. But do they have to? Yamaha recently showed a two-wheel drive prototype motocrosser. And in cars, every possible permutation of front, rear or all-wheel drive is commonplace.
Several even have some form of rear-wheel steering to improve agility, either passive where the cornering forces cause the wheels to steer, or active where they’re steered positively at the same time as the fronts. In light of this, it does look as if rear-wheel drive on two-wheelers is just a relic of the days when motorcycles were no more than bicycles with engines fitted. But there are some good technical reasons why front-wheel drive alone won’t happen.
The first is the difficulty of actually getting the power to the front wheel. Drive from the engine comes from a rotating output shaft which will either turn a chain and sprocket or another shaft to the back wheel via one or more sets of bevel gears. This is simple because the path from the engine’s output is a straight line, the only complication being the rear wheel’s movement on its suspension. That’s taken up by slack in a drive chain, or universal joints and splines when there is shaft-drive.
Getting the drive to a front wheel poses a whole new set of problems. The engine could be redesigned so its output shaft is at the front, which would make life a little easier. But, even then, it wouldn’t be possible to simply put a sprocket on the front wheel and connect a chain, or even a bevel drive box with a drive shaft, because the wheel has to be steered. Come the first corner and the chain would jump off.
The solutions are either to set the sprocket or bevel drive well to one side, clear of the wheel even at full steering lock, with another shaft passing across to the centre of the wheel which it then drives through a swivelling universal joint, as on front-wheel drive cars.
But this adds unacceptable width to a bike and lots of extra weight on the structure needed to suspend the drive train over to the side, enough to make the bike not only far heavier but also lopsided.
The alternative is to drive the wheel from above, via the headstock. With a conventional mechanical system (chain or shaft) this demands a very complicated, tortuous drive train with several changes of direction, again adding lots of weight and expense to the machine. In addition, if a drive chain runs down to the front wheel from the headstock, opening the throttle would pull tight on the chain, which in turn would severely compress the suspension. If a drive shaft were used instead, there would be a powerful torque reaction which would cause the bike to steer to one side, making the idea a non-starter.
The main difficulty seems to be due to the fact the bike is steered at the front, so couldn’t the rear wheel be steered to solve this problem? The answer is an emphatic NO! We’re familiar with the term " trail " , referring to the distance between where a front wheel touches the ground and where a line through the steering axis meets the ground, usually somewhere between 90 and 110mm in front. It’s this which gives the front wheel its self-centring property. More trail and the wheel is harder to deflect, adding stability, but detracting from agility.
But the same principle applies to the bike as a whole. The dynamics of a turning motorcycle are extremely complex and, surprisingly, not even fully understood. But it is known that there is a second trail effect between the front and rear wheels.
The greater the distance between them (the wheelbase) the more stable a machine is. But this does depend on the front wheel being steered and the rear being fixed. If it’s the other way around, the trail figure becomes negative and the bike would be inherently unstable. Jump off a bike at speed (not recommended) and it will continue in a straight line until something stops it or it slows down and falls over.
But a rear-wheel steering bike is only inherently stable when going backwards. Jump off that at speed and it will immediately crash. If you could ride it, you ‘d have to make constant corrections through the bars just to keep it going in a straight line – lose concentration for the merest moment and it’d spit you off.
Pilots of tail-dragging aircraft (those with tiny tailwheels and two big front wheels) know this very well. The tail wheel steers (either passively or actively) while the front ones don’t, and as a consequence these aircraft are very difficult to handle on the ground.
So the front wheel must be the steering one, leaving only one other front drive solution. Yamaha has solved it with hydraulic drive, where the engine drives a pump which sends fluid at high pressure through flexible hoses to a turbine attached to the front wheel. As the fluid is circulated by the engine, so it drives the front wheel around. The hydraulic hoses can be routed up to the headstock and down to the wheel without a significant loss of efficiency, and their flexibility means the suspension and steering movement doesn’t present a problem either. It also does away with any torque reaction difficulties.
But we’ll only see this used in conjunction with rear-wheel drive, as front-wheel drive alone won’t work as well on bikes as it does with cars.
The attraction seems obvious, as a bike ought to be able to turn more sharply by being pulled into a corner, but on cars it’s front-wheel drive’s safe handling which is its biggest appeal. When the limit of grip is reached, the car simply understeers. Most drivers react to this by backing off the throttle. But on a bike, understeer of this sort is followed by the rider falling off, as bikes don’t slide their tyres like cars.
Another problem is weight transfer. As you accelerate, more weight is carried on the rear and less on the front. Accelerate hard enough and the front wheel starts to lift clear of the ground. With rear-wheel drive the increase in weight on the tyre helps prevent wheelspin and you can accelerate hard until you wheelie. With front-wheel drive, the weight transfer still goes to the back wheel exactly as it does with rear-wheel drive. So the front wheel would start spinning as the weight on it decreased, which would happen at much lower levels of acceleration. And you wouldn’t be able to wheelie either!
So front-wheel drive is complex, expensive and has no obvious advantages, while rear-wheel steering would make a bike throw you off the moment you stopped concentrating. Which makes you wonder what Yamaha is up to.
Well, two-wheel drive is quite different. You lose none of the usual beneficial characteristics of rear-wheel drive, but in addition you can put more power on the road. Let’s say that you can drive a bike out of a corner at a certain speed, using 50bhp until the rear tyre starts to spin. You simply can’t get any faster because you’ve reached the limits of your grip. But if you can put an additional 30bhp on to the road through the front wheel, you’ll now be driving out of the corner with 80bhp, which will be much faster. AND you’ll have the advantage of the front wheel pulling you.
Acceleration won’t be affected as you can still drive the bike forward hard enough to lift the front off the road, but this doesn’t matter because you haven’t lost anything, and in slippery conditions the bike will be able to go faster as you have more rubber through which to put the power on the road.
All-wheel drive transformed road and rally cars once it was proven in mass production with the 1980 Audi Quattro. There’s a good chance it will do the same for road and race motorcycles in the next 10 years, assuming the hydraulic drive system can be perfected. But front-wheel drive only, or rear-wheel steering?
Not a chance!