2 Wheel Drive/Steering
(c) 1986 -- 1997 Tony Foale
Those of you with long memories will recall that in the previous issue I discussed some of the features incorporated on Suzuki's new 'future bike', the Rusty Falcon. Well, since that was written some further details have flown in regarding their new fledgling.
Instead of having a fully automatic transmission it seems that some human intervention is required. The drawings of the hand controls show that on the left hand grip there are two buttons marked SIFT UP and SIFT DOWN. Perhaps rashly, I assume that this is Japanese English for SHIFT UP/DOWN as in changing gear. But is this the only way to change ratios or is it just a facility for manual override? Another question to which I do not have the answer, concerns the method of operation. Does the gearing change in discreet steps each time a button is pressed, like a conventional gearbox, or should we hold the button down until the desired ratio is smoothly selected by an infinitely variable mechanism? There is certainly no clutch to operate but a manual change seems somehow to be out of the technological context of this machine. Last month I suggested that perhaps the front brake hydraulic pressure might be derived from the transmission pump, however this is not relevant because Suzuki claim the front brake to be electro-magnetic. As with many features suggested for a future generation of motorcycles this system is used in other areas and the principles have been long known. But this does not detract from any benefits that may be on offer. If Suzuki have successfully developed the system for bike use then anti-lock becomes incredibly simple and existing hydraulic systems should become obsolete overnight. But does a flat battery or blown fuse mean a return to the time honoured 'foot-in-the-spokes' retardation mechanism ( What? --- No spokes! )? The suspension is claimed to be electronically controlled, but that could mean anything. It could be a pump powered self levelling design or just variable damping to accomodate different loads. Or perhaps they intend to go all the way with a bike version of the Lotus "Active Suspension" with computer controlled, interactive, suspension movement. Well, now that last month's story has been finished let's get on with this month's.
As progress in any one field often comes by the adaption and then adoption of ideas from other areas, it seems only reasonable to keep our eyes on developements in the car world and see if they can be of any use to us. Two such topical features that may be worth a look are 4 wheel drive and 4 wheel steering, or in our case 2WD and 2WS. But before reading further it may be wise to dig out your December 85. issue and re-read the article on balance and steering.
Let's try 2WS first:- With car 4WS the rear wheels may be steered in one of two ways, i.e. in the same direction as the front or in the opposite direction.
The sketch shows the benefits of steering all wheels in the same sense when parking in a tight space, but this is hardly likely to be of much use with a bike ( unless you own some gross overweight touring monstrosity ). At low speeds this agile lateral performance may actually make it easier to balance. But if we were to steer our 2WS bike like this at higher speeds then in the steady state it could happen that we travel in a straight line, crabwise, with the wheels out of true.
Hence, the tendency to keep the wheels aligned with the rest of the bike would be quite weak. Although as the handle-bars are being turned the gyroscopic and centrifugal forces would still act to produce leaning moments ( see December issue ). Because these forces now act at both ends in harmony the leaning response would be very rapid. So at speeds above 20mph. or so I feel that this would be a very strange beast to ride. The antics of the machine when trying to correct a slide would probably be very interesting. Any volunteers? Strangeness does not mean impossible, the mechanisms for automatic balance are still operating and so, practice should be all that is necessary to master the device.
Any of you who have been to the main motorcycle shows ( British ) over the past few years will have seen 'Rodeo Bill' ( or whatever his name is ) with his reversed handle-bar BMX bike. If the bars are turned right the front wheel will turn left, for an investment of 50p. he offers the chance of winning £10.00. All you are asked to do is ride this bike along a straight course of only a few yards in length. I have watched maybe 100 people try this and have not seen any contestants get farther than one machine length. On the basis of that it would be very easy to erroneously conclude that it is unridable, that is until one sees old Bill himself get on it and ride around turning tight turns and figures of eight with complete confidence and competence. The point of this tale is to show that, despite the weird feel, the basic bike was still stable it only needed reversed hand/eye/brain coordination to operate. Just changing the handlebar action did nothing to alter the inbuilt auto-stability mechanisms. In the meantime the ever smiling cowboy will still be collecting his 50p.s and every now and then will pay out the odd £10.00 to a drunk granny who couldn't normally even ride a bike but managed to conquer this one.
Let's see if opposite sense 2WS has anything to offer. Back in the sixties ( surely before I was born ), in an attempt to break out from the tunnel vision that aflicts anybody enthusiastically engulfed in their own interests, I actually considered building a rear wheel steered bike. Well, some fork lift trucks use it, don't they? Fortunately for me, I spent the time thinking about the consequences and decided against it. If only the rear is steered then it must turn the "wrong" way, i.e. to go right the wheel must turn to the left.
Don't confuse this with the momentary counter steering necessary to initiate a turn, with 'normal' steering. If I had built it then, I may not have been around now to write about the effects because the automatic balancing mechanisms described in the December 85 issue would operate to upset the balance. Imagine riding along in a straight line when some extraneous unbalancing force causes the bike to lean to the left. Precessional forces will then steer the rear wheel to the left causing the machine to turn right. The centrifugal force thus generated will reinforce the original disturbance, rather than correct it as with front wheel steering. Assuming that by the application of much skill and strength we could still manage to ride such an animal, then another interesting effect would manifest itself. Viz;- Counter steering to initiate a turn MAY not be necessary, because the rear wheel is steering to the opposite side anyway. I deliberately use the word MAY because whether or this is the correct technique depends on various parameters. Unlike the case with front wheel steering where centrifugal and gyroscopic forces act in unison, with rear wheel steering these forces oppose. The dominant one depends on such factors as;- Weight, C.of.G height, and rear wheel size and weight. e.g. A light bike with a very low C.of.G and a heavy rear wheel probably would not require counter steering, whereas a high and mighty machine with push bike wheels might. The reason for looking at rear wheel steering separately was so that we can now clearly combine the effects of steering both ends into opposite sense 2WS.
Assume for the moment that our machine has identical wheels at each end, and that the steering is arranged so that both wheels steer through equal angles relative to the bulk of the machine ,( and just for the purists ) further assume that the weight distribution is even. Thus any precessional forces from the wheels will be of similar magnitude at each end, but they will act in opposite directions. Hence, the self balancing effect of the front wheel steering will be cancelled out by the UNbalancing tendency from the rear. But does this mean that the bike could not be balanced at all, at those higher speeds when we normally rely on auto-balance? Well no, --- because we could use the centrifugal force generated from steering to the left ( this force then acting to the right ), to counteract any unbalancing tendency to the left, and vice- versa ( naturally ). This is similar to our present method of maintaining low speed balance, and so should not present too many problems except those caused by wobbling all over the road. Another problem with balance becomes apparent when we consider the very slow speed case. Normally if the machine starts to topple over to the right, say, we would steer to the right in order to bring the line joining the two tyre contact patches under the C.of.G and restore equilibrium. Unfortunately with this 2WS as we steer to the right the back end will move outwards to the left as the front moves in to the right. The result being that the tyre line will just twist around and will not be brought under the C.of.G. The only way then to remain aboard is with footing or a large amount of body lean. Let's forget about the problems of balance and consider the act of cornering. We have seen that the gyroscopic forces are cancelled out end to end, so these cannot be relied on to help. Consider that we wish to negotiate a left hand bend, if as we approach we apply some left hand steering the bike will then obviously veer to the left but this will set up a centrifugal force making the machine lean to the right, just like a car. But unlike a car, which has a double track to maintain balance, we would simply fall off. After dusting ourselves off, we can try again. This time try a bit of opposite steering, the bike will lean to the left OK. but we will have to bring the bars back to prevent turning right and to enable the bike to steer left. So, with some difficulty the machine with opposite sense 2WS may be ridable but the feel would be very different from that to which we are used. Another disadvantage with this setup, is that as the precessional forces from the two wheels oppose each other, the chassis is subject to quite strong twisting moments. So if the frame is none too rigid all sorts of gyrations may be set in motion, and this is the basis of the high speed weave that afflicts some motorcycles. The reason that this concerns a conventional bike is that it is really just a special case of the 2WS genre. Both wheels are able to steer about the head stock or steering axis. To look at it in another way, imagine that the two separate steering axis for the two wheels coincide at the position of the front axis. This has the effect that for the rear wheel to steer, the bulk of the machine must move too. The inertia of this, limits the extent of the rear's influence, under normal circumstances, and so we usually only consider our bikes as front wheel steered.
Well, full 2WS doesn't seem to have much to offer us, so let's move on and check out 2WD. To my knowledge very few attempts have ever been made to build 2WD bikes. Perhaps this is due to the mechanical difficulties involved in driving the steered wheel. Imagine the multiplicity of chains and sprockets necessary. However, this problem is easily solved if we adopt hydrostatic hydraulic drive, as discussed last month. If we do this, are there any benefits? The most obvious is that of increased traction, driving through both tyres should enable more power to be put down, but let's examine this in a bit more detail. The SuperBike ultimate street bike binges seem to be an applica- tion for any device that gives more traction, but how can we get any drive from the front when it is most likely two feet in the air anyway? This highlights a major difference between a car and a bike when considering all-wheel drive, --- weight transfer. The car suffers from this too, but to a lesser extent because of it's more favourable 'wheelbase to C.of.G height ratio'. After all, when did you last see a car doing a wheelie? So, not unexpectedly any traction benefits would make more impact under slippery conditions, when the limited acceleration possible, limits the weight transfer. Anyone who has ridden in mud or sand can easily imagine the help that a tug from the front end could provide. Some years ago I seem to remember that the Canadian firm 'Rokon' made a 2WD, ballon tyred, go anywhere vehicle. I think that it was intended for forestry commisson use or similar. I suspect that riding over logs etc. became much easier. Of course, skilled trials riders can do this with ease anyway, but the average tree feller would be stumped much easier. Despite the advantages for this type of use, there is a problem that I feel would render 2WD unusable and positively dangerous for normal road or racing use. That is, the interference that the driving forces would have on the steering. These forces act on a car to a certain extent but it is our need to lean whilst cornering, and our dependence on the steering for balance that puts any two wheeled single track vechicle ( a bike ) in a class of its own. To illustrate the problem to which I refer, just think of the effects when the front brake is applied during cornering. The machine usually tends to straighten up and this often happens to a lesser extent when the throttle is closed. If we were to drive the front wheel then the application of power would tend to make the machine lean over farther. The roads would soon bordered by holes punched into hedges on the inside of bends. To understand why this is so, I'm afraid its back to those gyroscopic forces again. As we lean over in a curve the tyre contact patch moves around the tyre toward the inside of the bend, out of alignment with the steering axis. Any forces on the tyre will now create a torque acting about the steering axis, these torques act on both wheels but with front wheel steering only the effects on the front are much greater, and so we will just consider these. The torque produced by the driving force will act in a way to straighten the steering. For example if we increase the power whilst turning left this torque will act to turn the handle-bars to the right, now, both the centrifugal and the precessional forces will act to make the bike lean over more to the left while the steering is trying to go right. I leave it up to you to imagine how well such a machine would handle. Not only will these effects be evident when the throttle opening is changed, but a steady torque will be felt even on a constant power setting. We will be required to apply a compensating force to the handle-bars. This force will vary depending on the level of power being used. On the overrun a torque steering out of the bend would be needed, with the opposite action necessary for the power-on situation. The magnitude of these upsetting torques may be less under power that under braking, but we can usually avoid the need to brake in a corner. However, we can not avoid the use of varying amounts of power in the bends and so the feel of the bike will be inconsistant. The modern trend toward ever wider tyres would aggravate this problem, because the contact patch can move even farther from the steering axis, so producing higher unsettling steering torques. Imagine the problems if 2WS and 2WD were combined together, the traction induced steering forces could then act on both wheels with quite dramatic effect.
So to conclude, it is evident that not all avenues of progress in the automotive world are likely to be of benefit to a future generation of motorcycles. Of the two ideas featured here, 2WD would seem to have some worthwhile application in the limited field of all-terrain vehicles. The handling characteristics rendering normal use quite dangerous. I can see no application for 2WS at all. Opposite sense 2WS removes the normal auto-balance ability and same sense 2WS is unlikely to provide accurate directional stability. The effect on the weave and wobble characteristics is not likely to be beneficial either. Just think of the wobble that might ensue if back and front steering systems had similar resonance periods. I can see no compensating advantages to offset these problems, in any case 2WS or 2WD is bound to increase weight and I just hate heavy bikes.
( 1997 note ) I have recently heard about an offroad bike built by Ian Drysdale in Australia, which features hydrostatic 2WD. and hydraulically controlled 2WS. By all accounts it seems to work very well. This very cleverly designed bike used some features not considered in the previous text. The 2WS system was of the "opposite sense" type and got around some of the concerns mentioned above by allowing the front wheel steering to have +/- 5 degrees of movement before bringing in any RWS. Thus for normal riding where large steering angles are not used, it simply behaved as a normal FWS machine, but in sandy terrain when the bike would start to lose cornering ability, the 2WS and 2WD would work together and keep the bike on track. During several tests it was shown to be able to maintain tighter turns and good drive beyond that of other offroad machines.
( 1999 note ) The above machine has recently been purchased for the Donnington Collection.