| Author | Message | ||
     Werewulf |
the letter to the editor, was from a cyclist who was arguing the countersteer theory. evidently david hough wrote an article from his superbike school, in which he stated that you cant steer a bike by leaning. he has a special bike made up that the handlebars are not connected to the frame. he invites people to steer the bike by leaning, which evidently cant be done! i believe its been the topic of discussion for a couple of issues. ill have to dig it out and look. | ||
| Blake |
Leaning will steer a bike, but VERY slugishly. It is not possible to control a bike for any length of time just by leaning. One more comment to further clarify that precession is NOT helping us turn our bikes. Proponents of precession believe that flicking the front wheel a degree or two supposedly flops the bike over into an aggressive lean. If that is true, how is it that when the more powerful gyros constituted by the flywheel and the rear wheel flop over 45 degrees, their precession has no discernable effect? Believers of the gyro speal need to explain that to me. Blake (onamissiontoenlightentheconfused)  | ||
| Hans |
Counter steering is a mere psychological issue. The biker wants to keep a controlable equilibrium at any moment and making a real sharp turn to right he makes even a short inititial turn to right, yes right, then a second to left (the initial turn) before the final big sweeper to right. The bike is kept under control at any stage of the turn. Counter steering is nothing as the short deliberate steering mouvement to the outside of the turn to ride the bike from under you away: AGAINST the reflexes. The bike is steered far out of control for a moment. The immediate fall into the turn is the WANTED result and the normal reflexes are picked up during the sharp resulting leaning angle again. Hans | ||
| Hans |
Blake wrote, "Leaning will steer a bike, but VERY slugishly. It is not possible to control a bike for any length of time just by leaning." Disagree: Saw the instructor making sharp slaloms with his hands acorossed his chest while he instructed to let have your body as much contact with the bike as possible. To make his point more clear he lifted his legs from the pegs and made a pirouette on his butt on the straight riding bike. After reading the theories of Keith I was wondering why you could make no turns on his instruction bike with the handles fixed on the frame. Every experienced pushbiker, as we all are in Holland, can manoevre without the hands at the steering bar. The secret lies in the placing of the Keith`s fixed handles: LOW. Riding a bike sitting upright with your point of gravity high is easy the peasy. Easier with lighter bike. The same instructor showed us the effect of counter steering, without naming it as such, by pulling a string fixed on the right clipon which resulted of course in making a sharp turn to left. Hans. | ||
| Blake |
Hans. Of course you are correct and have a very convincing argument. But... Was the instructor you mentioned in a parking lot or other large paved area where he was not required to navigate within narrow boundaries, like on a street? Was he piloting a 200 Kg motorcycle? Was he traveling at normal roadway speeds? Was he working the throttle somehow? The handlebar position only affects the height of the cg, not its lateral shift. You CAN turn Keith Code's fixed handlebar demonstration bike by leaning, just not well enough at a normal speed to stay on the track. I've tried steering by only leaning, hands off the bars while going downhill on some curvy roads. I'm lucky if I can make the second curve without having to touch the handlebars to keep the bike on the road. Gyroscopic characteristics of the wheels definitely cause a motorcycle to respond more slugishly as speed increases. Consider that without holding onto the handlebars as you lean left, gyroscopic precession of the front wheel will tend to turn the front wheel to the left tending to counter your leaning. On a small lightweight motorcycle it might be possible as it is on a bicycle at moderate speeds to navigate roads effectively solely by leaning, but I strongly doubt it. You would have to really practice to be proficient enough at control through leaning only. Gyroscopic precession is a too much a hinderance to turning. It's interesting, but the point I just made above is probably the most convincing argument against precession being a major factor in turning a motorcycle. When only leaning to get a bike to turn (no input to the handlebars) imagine the massive forces of precession trying to turn the wheel. I mean if, as the precessionists claim, only 1.5 degrees of turning on the bars results in 30+ degrees of lean, imagine how much 30 degrees of lean will would turn the bars. There you have it. The myths of gyroscopic counter-steering debunked once and for all. Blake (a nal y tic think er) | ||
| Jima4media |
This article appeared in the local City Bike motorcycle magazine, and I retyped it and posted it to SacBORG when the subject of steering came up. The NO BS Bike Ride by Don Jenson The notion of countersteering has been with us since it was first identified by an obscure bicycle mechanic named Wilbur Wright, who then went on to make a name for himself and brother Orville in some other field of endeavor. The subject lay dormant for most of a century, probably because most riders were self taught. There was no analysis of how you got your bike to corner. You either managed to figure it out on a practical level, and lived, or you didn't - truely the school of hard knocks. Nevertheless, countersteering remained a viable principal with wide acceptance in the general motorcycle population. As a friend puts it, "Who doesn't believe in Countersteering? The Flat Earth Society?" But now we have track schools that put riding techniques under intense scrutiny and after you've plunked down your hard earned dosh for said school, you may be surprised to hear instructors with impeccable credentials tell you that you steer a bike by shifting your body weight. So who is right? Expert A? Expert B? Both? Nove of the above? Why do you feel like you're back taking your SAT's? Keith Code, of California Superbike School fame, has been fanning the flames of the controversy lately with his No Body Steer- or No BS- bike, which is a Kawi ZX-6 with a second set of bars welded to the fairing subframe. The stock bars are free to move as usual. Keith claims that countersteering is what makes a bike steer and that body weight shift - as espoused mainly by Reg Pridmore at his CLASS school- does not, but how do you separate the two techniques to find out which is the real deal? The idea behind the No BS bike is to take any possibility of countersteering out of the equation to see if a bike can indeed be body steered. It seemed like a good idea, so I turned up at his Laguna Seca school to see what all the commotion was about. But first, what seems to be missing from this discussion is that while most everyone knows how to countersteer, nobody gets into why it works, which is why it may seem like so much voodoo. I'll confess up front that I regard it as a real phenomonon, maily because I actually happened to show up- awake AND sober- the day we studied gyroscopes in physics class. And fear not, gyros are perfectly understandable without the kind of mathematical jibber-jabber that makes your eyeballs glaze over like you just burned some Mexican dirtweed. But let's clarify one thing. We're talking about initiating the turn by making the bike lean, not the act of turning itself. That's governed by a different set of rules that we can talk about some other time. Also slides are a special case, so we'll leave that out for simplicity's sake. A gyroscope is a spinning disc that is held in place by some sort of cage or other structure, which pretty much describes a motorcycle wheel. The rule for the amount of gyroscopic force is rotational speed times the distance from the center of mass from the axis, and since motorcycle wheels and tires concentrate their mass so far from their axles, they make effective gyros. Gyros are commonly used in navigation and stability devices because they have the property that they want to maintain a fixed orientation in space and that's because they have a form of inertia that works differently from objects that move in straight lines. You remember inertia: an object will stay in motion unless an outside force acts to make it stop, change direction, etc. Consider a bullet resting inside your favorite Glock 9. It's not going anywhere until the explosive fore of the power pushes it on it's merry way. Now, the bullet will fly along until air resistance, or some unfortunate body, applies a counter force to stop it. All good, except for the unfortunate body, however this is the case for linear inertia. The bullet is still free to move around another axis of motion. Gun makers discovered ages ago that smoothbore firearms allowed the bullet to tumble in flight which would affect its accuracy. Flintlock dueling pistols were said to be so inaccurate that a combatant would be extremely lucky to hit a blanket stretched out on a clothes line at ten paces, which explains why otherwise rational persons were willing to take the risk. In fact, Aaron Burr was said to be astounded that he had actually managed to hit Alexander Hamilton during their famous duel, but he could have taken solace in that the act spun off into a rather clever "Got Milk" commercial. By cutting spiral grooves in the gun barrel, gunsmiths forced the bullet to spin and gyroscopic force (rotational inertia) then made the bullet fly true, vastly increasing the odds of actually being shot and putting an end to the "romantic" age of dueling. Well, what's good for Mr. Glock is good for us because the same gyroscopic force that stabilizes a bullet also acts on the spinning wheels of a bike, providing the force that keeps the bike upright and stable once it's in motion. Otherwise, the bike would always want to fall over no matter what its speed and would probably be unridable. The spinning crank and gears also add some gyroscopic force and it's more effective than you might think. I know dirt bike riders who swear that keeping their motors revved up while picking their way through the slow stuff makes the bike more stable. Ah, but what about when you come to a turn? Doesn't that stability work against us? Four cylinder GP bikes have used counter-rotating cranks, which cancel out gyro forces, to make turn-in easier for the rider, but for the rest of another quirk of gyro behavior, precession, allows us to redirect our wheel's gyro effect to our benefit. Before we go any further, go out to your garage and take the front wheel off of your bicycle. I'm not kidding - go now! Hold the axle between your arms as if they were the front forks and turn (yaw) the wheel to the left and right. You find that nothing unusual happens. The wheel easily turns in the direction you point it and that's all. Now get the wheel spinning in the forward direction and then try to turn the axle as before. You'll notice two things: first the axle strongly resists being deflected from its position - an important point, as we shall see - and second, the wheel banks (rolls) to the opposite side with surprising force. Congratulations! You have just demonstrated the principle of gyroscopic precession. Play with it until you've satisfied yourself that the phenomenon is real and don't dismiss the idea unless you really have tried this out. Now consider your bike's front wheel. It's spinning along, its rotational inertia keeping you and the bike upright and happy, but then you upset the apple cart by applying a steering input to the left. In other words, you've added a counterclockwise torque around the wheel's vertical axis. The wheel can't respond instantaneously because of its inertia but it still has to react to the force somehow. It does this by allowing the wheel to change its orientation, but later in time which also means at some point later in the wheel's rotation and that happens to be at 90 degrees from the point where the torque is applied. That is, you've twisted the bar to the left in the vertical axis but the force doesn't appear until the wheel has rotated 90 degrees, so the torque acts upon the wheel's horizontal axis instead, causing the wheel to roll to the right and since the wheel is held captive in the fork assembly, it transfers the force to the frame and the bike leans. So let's take this out on the road and make a few other observations. At very low speeds gyro effect is minimal, so you have to steer your bike like a car. That is, you have to steer into the turn with a lot of wheel deflection and not very much lean angle. The bike is steered by what is known as slip angle. As your speed rises, however, gyroscopic force increases and you find yourself taking turns at high lean angles with very little bar deflection. You'll recall from our bicycle wheel experiment that the axle resisted turning and that is why you don't observe much, if any, bar movement when turning your bike and it's probably this absence of movement that fools riders into thinking that they're not actually steering with the bars. But you do notice that pushing on the inside bar causes the bike to roll to that side and it does so RIGHT NOW. The well established rules for gyroscopic precession fit the reactions we see when we countersteer. On the surface, body-steering would appear to be a case of destabilizing a balanced object, and the rule for that is pretty simple. When the object's center of gravity passes beyond the outermost point of it's base, its going down. A motorcycle is somewhat akin to balancing a pencil on its point because your bike's CG is high in relation to its size and the base is only as wide as the contact patch. Tip it just a little and it should all right over. Keep in mind that the overall CG is actually the bike chassis' fixed CG combined with your body's movable variety. When you ride staight down the road, the two CGs fall on the vertical centerline and this is therefore no lateral force to encourage the bike to lean. But if you hang off in the style of your favorite Superbike star, you've moved the combined CG toward the inside of your intended turn. It would seem that you would now have enough leverage to pull the bike over to your desired lean angle, but we've forgotten about the stability that the gyro effect supplies. Remember we said that without gyroscopic force our bike would be just as difficult to keep upright at speed as it would be back at the stop light, but since our bikes track straight and true almost by themselves, we know this force exists. The question is, is the CG shift we induce by body weight sufficient to overcome this force? Going back to the rule for gyroscopic force, let's remember that it increases with speed and with the mass of the wheel, brake rotors and tire. You could make the case that small bikes with very light wheels and operated at lower speeds would have less gyro effect. They would be easier to muscle around and the way you ride dirtbikes seems to bear that out. A Goldwing, on the other hand, will produce a prodigious amount of gyro effect and it seems unlikely that weight shift will have any meaningful influence in turning the bike... unless you're a sumo wrestler. With all this in mind, I arrived at Keith's school with the lingering thought that body-steering might still be an important ingredient, if not the prime mover in bike control. I jumped on the bike and ran it up and down the paddock a few times to get the feel of how it responded to normal rider inputs...no surprises here. Coming out of my last U-turn, I switched to the fixed bars and the bike ran uncontrollablly off to the right. By the time I got back on the regular bars the bike was weaving around like a poorly designed Bonneville streamliner, ending up in a low speed tip over. I had at least expected the bike to be stable in a straight line, but no such luck. It had caught me completely unaware. When I sheepishly rode back to Keith, he said, "You didn't tell me you were going to try the fixed bars yet. There is a procedure you have to follow." It seems you have to transition to the fixed bar's throttle first to stabilize the bike before you switch your left hand. I had done the opposite so that the off/on effect of the throttle upset the bike. Doh! I sallied forth for a few more laps but the result was nearly the same. Given more time to practice and I might have been able to coax hte bike through a wide sweeping turn, but I wouldn't want to take the bike on the road under any circumstances. I consoled myself with the knowledge that Roadracing World editor, John Ulrich, had also crashed it and Motorcyclist's John Burns regularly crashes bikes without the benefit of the No BS bars, so I figure I'm in company. Motorcycle author, David L. Hough, rode the No BS bike and observed in the July issue of MCN that it always wanted to run off to the right as it did for me, so I think the bike is an old track mule that's been pranged more than once and was to be put out to pasture before it saw new life as a demo. Therefore, it's entirely likely that it's been bent by a number of off track excursions, which could account for the pulling. One thing that surprised me was the reaction the bike had to the throttle. It sure aggravated the bike's change of direction but it wasn't anything like real steering. A CityBike reader wrote in recently toask, "If body steering doesn't work then how come when my girlfriend squirms around on the back it just about puts us into the side of a bus?" As Keith's instructor, Paul Dyall from the Australian SB School said, "You can ride a bike hands off and if you jerk the bike around hard enough it'll move off line, but can you call that steering?" And Keith responded with, "Destabilizing a bike is not the same as steering it" With Keith Code's invention of the "No BS" bike, it seems Code and the countersteerers have won their longstanding debate with Reg Pridmore and the body-steerers. But Code's victory raises interesting questions. Amoung them: Did Reg Pridmore become a three-time AMA Superbike champ without knowing how to turn a motorcycle at speed? If one cannot body-steer a motorcycle effectively, just what is Pridmore teaching at his CLASS schools? As a student at both Code's Superbike School and Pridmore's CLASS school, I think I can shed some light on these questions. Code teaches that the way to turn a motorcycle at speed is to make deliberate countersteering inputs to the bars. Importantly, however, Code teaches that, to maximize stability, the genesis of these countersteering inputs should be outside footpeg. The technique involves using the calf muscle to lock the outside knee into the gas tank, then flexing the outside quadriceps muscle just enough to begin to unweight the outside butt cheek. If one keeps the upper body relaxed and the shoulders square throughout (which Code strongly counsels), flexing the outside thigh in this fashion translates an opposing forece across the torso and through the inside arm and hand. This creates a countersteering input (a push) to the inside handlebar (try it). Thus, one is countersteering, but the countersteering input is generated with the lower body and not the shoulders, chest or arms. Code calls this technique "cross-steering" Pridmore similarly recognizes that it is potentially unstable to steer a motorcycle simply by pushing and pulling on the bars with the arms, chest and shoulders. But instead of teaching how to generate countersteering inputs to the bars through specific lower-body actions, Pridmore encourages experimentation with a variety of lower-body inputs to the bike (pressing the feet, calves, knees, thighs, and butt against the pegs, side panels, tank, seat, etc.). But in suggesting that these actions can and do steer the motorcycle in and of themselves, Pridmore is quilty of, at best, imprecision, and, at worst, misunderstanding. Code has shown these lower-body actions do not "steer" the motorcycle per se. Rather, these actions translate forces across the upper body and into the arms and hands- ultimately resulting in traditional countersteering inputs at the bars. These are what steer the motorcycle, whether one is conscious of it or not. I believe Code's and Pridmore's approaches can be harmonized, but we need to define body-steering not as a steering technique in and of itself, but rather as a means of generating steering inputs to the bars. Once we agree to this definition, it would appear these gentlemen are talking about much the same thing. I think Mr. Code can rightfully say that he has the better understanding of the way in which lower-body-English gets a motorcycle to turn (by applying a countersteering input to the inside bars). So too, he can take pride in having brought the debate to a head with his clever invention, the "No BS" bike. But rest assured that Reg Pridmore knows how to steer a motorcycle. He was, and is, blazingly fast and eerily smooth, and he's a three-time AMA Superbike champion to boot. He need feel no shame, and I'd like to believe he's spent the last year rethinking his ideas about the way in which his lower-body gymnastics are causing the machine to change direction. | ||
| Jima4media |
Here is another article by Tony Foale on steering which I find educational... You can view the diagrams at the website at http://www.tonyfoale.com - Miscellaneous Articles BALANCING ACT. © Tony Foale 1986 -- 1997 Let's return to basics and look at the mechanisms of stability and steering, as they relate to single track vehicles (motorcycles in other words). BALANCE. As a single track vehicle, a motorcycle lacks inherent static balance, i.e. it falls over, if left to its own devices when stationary. Once moving above a certain speed however even the most uncoordinated riders find that the machine seems to support itself. So it is obvious that there are two aspects of the balance process, the low speed case and that in the higher speed ranges. There have always been clever sods who can balance indefinitely on a stationary bike, but for most of us we need a minimum of forward motion before this is possible. However, at these low speeds it is necessary to move the handlebars from side to side to stay upright, and as all trials riders know, it is easier if we stand on the footrests instead of sitting down. Let's examine why. Fig.1 shows the rear and top views of a bike and rider. Now, if the combined centre of gravity (C.of G.) is vertically above the line joining the front and rear tyre contact patches, then balance is achieved, but this is an unstable situation, any small distubance such as a light breeze will be enough to start a topple over, i.e. the C.of G. moves sideways. This can be prevented by either of two methods or a combination of both, one is to move the tyre contact patch line to under the new position of the C.of G. If the bike is stationary this can only be done to a limited extent by moving the bars, however once under way we can steer the bike to place the position of the tyre line wherever we need it, and this is why it is easier to balance when moving. The other way to maintain low speed balance is by moving the combined C.of G. of both the rider and machine to above the line joining the tyre contact patches. This is what trials riders are doing when moving their bodies from side to side whilst standing up. The high C.of G. of the rider has more effect on the toppling over moment and also gives more control over the position of the bike's C.of G. Thus to a great extent the process of low speed balance is dependent on the individual skill of the rider. In addition, some bike parameters can also affect the ease of remaining upright, the main ones being: 1. A low C.of G height helps. 2. A large trail changes the position of the tyre line more for a given handlebar movement. 3. A small rake angle reduces the fall of the steering head when the bars are turned away from the straight ahead position, assisting with the balance process. The balance mechanism at higher speeds is more complex, but at least is largely automatic and independent of rider ability. To understand the action it is necessary to look at a few properties of gyroscopes, which is another way of describing spinning motorcycle wheels. A spinning wheel has a very stable axis of rotation, i.e. a strong tendency to maintain its plane of rotation. In other words, while it can easily be moved laterally along the axis of spin, it resists tilting about any other axis, and more importantly, when it is tilted it automatically causes a strong twisting moment about an axis at 90 degrees to that of the original tilt. This twisting effect increases as the speed of the wheel rises, this is known as gyroscopic precession. When you have finished reading this, I expect you to go and remove the front wheel from your brother's mountain-bike, if you then obey the following intructions you will get a graphic practical demonstration of the strength of these precessional forces, which are so vital to the balance and steering of any bike. Firstly hold the wheel upright, as in fig.2, get your young brother (well he won't be out riding, will he?) to spin it so that the top of the wheel is moving away from you, as if it were the front wheel of a machine you were riding. If you then try to tilt the spindle to the LEFT (equivalent to banking your machine) you will find that the wheel _turns instantly and strongly to the LEFT, as if steered by an invisible hand. In other words, your attempt to tilt the wheel about its fore-and-aft axis has produced a torque swivelling it about its vertical axis. Now start again but this time turn the wheel to the LEFT about a vertical axis, just as sharply and strongly it will bank to the RIGHT. Try both these manoeuvres again, but do it at different wheel speeds and tilting speeds, you will see that the precessional forces depend strongly on these factors. Note particularly, the directions in which these forces operate as this is important for the automatic retention of balance. Let us now see how these forces keep the machine balanced and on a relatively straight path without assistance from the rider. Suppose the bike, whilst travelling along at a normal speed, starts to fall to the left under the action of some extraneous influence. As we have just seen, gyroscopic precession of the front wheel immediately turns it to the left. This sets the machine on a curved path (to the left), so creating a centrifugal force (to the right), which counters the lean and tends to restore the machine to the vertical, the precessional forces are thus reversed tending to restore the steering to the straight ahead position. In practice, that which we regard as riding in a straight line, is really a series of balance correcting wobbles, if we could look at the actual paths taken by the centre-lines of the wheels, we should see that the front wheel path continually crosses that of the rear. In the explanation above, I have only described the effects on the front wheel, precessional forces are at work on the rear also, but it is much harder to steer the rear wheel independently, as the whole bike must yaw, rather than just the wheel and forks, as on the front. Hence, only a small contribution is made to the auto-balance mechanism by the rear. We have now considered balance in a straight line, but as we lean when cornering, there must be other factors at work to maintain equilibrium under these conditions. STEERING (CORNERING). To analyse this, we can divide it into two phases;- 1. Initiating the turn, 2. Maintaining the turn. Since the second phase is easier to analyse, let's look at it first. It is not feasible to steer a motorcycle through a corner in a substantially upright position, as in a car or side-car outfit, because the centrifugal force generated would cause it to fall outward. Hence we must bank the bike inward so that this tendency is counteracted by the machines weight tending to make it fall inward. See fig.3. Equilibrium is achieved when the angle of lean is such as to balance the two opposing moments, the one due to centrifugal force acting outward, and the other to gravitational force acting downward (both acting through the C.of G.). The actual angle, which depends on the radius of the turn and the speed of the machine, is that at which the resultant of the two forces passes through a line joining the front and rear tyre contact patches. This is the steady-state roll axis. But how do we actually initiate the turn - do we lean or do we steer first? Let's see what happens with each method. If we turn the handle-bar in the direction in which we want to go, both centrifugal force and the front wheel precession would cause the bike to topple outward, and that leads to gravel rash. But if we momentarily try to turn the bar quickly in the opposite direction, (sometimes known as counter steering) then these two forces will combine to bank the machine to the correct side. Gravity will then augment the banking effect and this, in turn, will give rise to gyroscopic forces helping to steer the front wheel into the curve, whereupon the processes for maintaining balance as described above take over and keep the bike on our chosen path. This is all very well, I hear you say, but if this is the way to corner, how come we can steer a bike no-hands. Well, it certainly is possible to do so, but only with a lot more difficulty. Precise control and tight turns are difficult to accomplish without handle-bar manipulation. Just try it! Let's consider the no-hands situation. As we saw earlier, simply banking the bike steers the front wheel in the correct direction automatically, through precession. But how do we make the bike lean in the first place, what do we have to push against? There is nothing solid to push against and so the only way to apply bank (without the facility of steering), is to push against the machine with the inertia of our own body. This means in practice, that in order to lean the bike to the right, we must initially move our body to the left. So now we have two possible methods of initiating a turn, and it is interesting to note that in both of them (banking and reverse handle-bar torque) our physical effort is in the opposite sense to that which might be thought natural, but when learning we adapt quickly and the required action becomes subconsciously automatic. It is these reverse actions that require us to learn to ride in the first place, when learning most of us wobble about out of control until our brain latches on to the fact that counter-steering and counter-leaning is the way to do it. Once the brain has switched into reverse gear, it becomes instinctive and is usually with us for life, and we could return to riding after a long layoff with no need to relearn the art of balancing or steering. So which of these two possible methods of initiating a turn do we use in practice? We probably subconsciously combine both methods, and the pressure on the inner handgrip is partly forward (counter-steering) and partly downward (banking). Remember though, that the actual counter-steering movement is very small, since gyroscopic precession depends for its strength on the speed of movement not on the amount of movement. If you still don't believe that steering to the opposite side works, then next time you are out riding, try jerking the bars quickly to one side, and see what happens. Leave yourself plenty of road if your reactions are a bit on the slow side. Do this at about 40 mph., and don't blame me if you fall off. The relative proportions with which we combine the two methods depend partly on riding style but also on speed and machine characteristics. For example, a heavy machine with light wheels at low speeds demands a different technique from that applicable to a light weight machine with heavy wheels at high speeds, and hence the two machines will have a different feel. But humans adapt quickly and the correct technique soon becomes second nature. It may seem strange that in the above discussion no mention has been made of such important parameters as, steering geometry, wheel and tyre size, wheelbase, frame stiffness and so on. This is simply because, balance and the ability to start and maintain a turn can be achieved within a wide range of these parameters. That is not to say that these factors are unimportant. We shall now look a little more closely at one of the more important parameters that come under the heading of steering geometry, i.e. TRAIL. Consider first fig.4, which shows the basics of steering geometry. TRAIL. The primary function of this, it is often said, is to build in a certain amount of straight line stability, in addition to that obtained by precessional effects as described above. But trail also introduces other effects which are vital to the feel and handling of the motorcycle. Fig.4 shows that both the front and rear wheels contact the road behind the point at which the steering axis meets intersects the ground, this gives rise to a self-centering effect on both wheels, rather like the castors on a super-market trolley. The measurement of this castor is called the trail. The mechanism by which trail produces a self-centering force can be understood by reference to fig.5, If the wheel gets displaced from the straight ahead position, i.e. the wheel is at an angle to the direction of travel (slip angle is the technical term), a force at right angles to the tyre is generated. Since the contact patch is behind the steering axis (positive trail) then this force acts on a lever arm (approximately equal to the trail) to provide a correcting torque to the angled wheel. That is to say, if the steering is deflected by some cause e.g. uneven road surface, then positive trail automatically counter-acts the displacement and gives a measure of directional stability. However, as shown earlier, we cannot just consider any steering effect in isolation, gyroscopic forces must be considered also, suffice to say, at this stage, that in this case trail and precession work in harmony to keep us on the straight and narrow. One may be forgiven for initially thinking, that because the rear wheel trail is much greater than that of the front, the rear wheel is the more important in this respect. The reverse is actually the case for several reasons. See fig.6. Imagine that the contact patch of each wheel is, in turn, displaced sideways by the same amount (say * inch.). The front wheel will then be turned by approximately 7-10 degrees (depending on the value of trail) about the steering axis, this gives rise to a slip angle of the same amount and generates a sideways force that has only the relatively small inertia of the front wheel and forks to accelerate back to the straight-ahead position. But the slip angle of the displaced rear wheel will be much less (about * degree) and so the restoring force will be reduced accordingly, but this also has to act on the inertia of a major proportion of the machine and rider, hence the response is much slower than is the case with the front wheel. From this, we can see that increasing the trail as a means of increasing the restoring tendency on the wheels is subject to the law of diminishing returns. It must also be emphasized that the distubance to a machine's direction of travel, due to a sideways displacement of the tyre contact patch, is less from the rear wheel than the front because of the much smaller angle to the direction of travel that the displacement causes. To summerize, while the large trail of the rear wheel has a relatively small restoring effect, the effect of rear wheel displacement on directional stability is also small, and hence compensates. As mentioned before, trail has effects other than directional stability, let's look at a couple of the more important ones. STEERING EFFECT. If we lean a stationary machine to one side and then turn the handlebars, we find that the steering head rises and falls depending on the position of the steering. In motion, the effective weight of the bike and rider supported by the steering head, is reacted to the ground through the tyre contact patch. This force tends to turn the steering to the position where the steering head is lowest (i.e. the position of minimum potential energy). For a given amount of trail, this steering angle is affected by rake angle and wheel diameter, one reason why different size wheels feel different, if all else remains the same. As long as we have positive trail, as is normal, then this turning effect is into the corner. Thus the amount of front wheel trail affects the amount of steering torque that the rider must apply (hence the feel of the steering) to maintain the correct steering angle consistant with the radius of the turn and the bike's speed. Some bikes seem to need to be held down into a corner, whilst others need the opposite approach. This is also influenced heavily by tyre characteristics, but that will have to wait for another occasion. STRAIGHT LINE FEEL. As we all know, even when we are riding straight ahead the steering feels lighter on wet and slippery roads than on dry. This is because as we have seen, our seemingly straight line is actually a series of balance correcting curves, with the handlebars turning minutely from side to side all the time. Also as we have seen, a small steering displacement produces a slip angle, which causes a restoring torque. For a given slip angle, this torque depends on tyre properties, surface adhesion and trail. On slippery surfaces the correcting torque is less, thus through the handlebars, we get a feedback (dependent on trail) for the amount of grip available. A bike with only a small trail value may give too much of a sense of slipperiness in the wet, and give the rider a certain degree of apprehension, whereas on the other hand, a large trail, under these conditions, may give out a feeling of security, which can easily engender overconfidence with predicable results. | ||
| Blake |
Jim: Thanks, I had read the Jenson article. It is infortunate that so many ply their ignorance in type. His statements... "the same gyroscopic force that stabilizes a bullet also acts on the spinning wheels of a bike, providing the force that keeps the bike upright and stable once it's in motion. Otherwise, the bike would always want to fall over no matter what its speed and would probably be unridable." A motorcycle would be unridable without gyroscopic forces?! How ignorant is that?! Riding at 5 or 10 mph won't produce any significant gyroscopic action. Ain't it a wonder how we can keep upright on a motorcycle at low speeds, and yes, even with the engine turned off. Gyroscopic action DOES stabilize a bike at high speeds. I agree with that. "At very low speeds gyro effect is minimal, so you have to steer your bike like a car. That is, you have to steer into the turn with a lot of wheel deflection and not very much lean angle. " WRONG! No matter what the speed, in order to turn, the bike/rider cg must lean into the curve.  The FBD above applies to all cases of a bike turning. At low speeds the lean angle is just very small, but the bike/rider MUST lean before it can turn. Even at low speeds, counter-steering is taking place. It is just so slight and intuitive to be unnoticeable. The reason the low speed turns generally require "a lot of wheel deflection" (steering deflection) is that the turns are VERY sharp (small radius). Ride a motorcycle just as slowly around a high speed curve (large radius), and the bars won't be turned any more than they would be at high speed. "As your speed rises, however, gyroscopic force increases and you find yourself taking turns at high lean angles with very little bar deflection. Yes, but the small bar deflection has nothing to do with gyroscopic action. The radius of the curve dictates the amount of turn-in steering required. Lean angle is dictated by the need to balance centrifugal force with gravitational force per the above diagram. But I thought he was discussing countersteering?  He got the countersteering idea half right, just throw out the gyroscope BS and it's pretty much correct. Wow, you typed all that! Thanks Jim! As to Mr. Foale's article But how do we make the bike lean in the first place, what do we have to push against? There is nothing solid to push against and so the only way to apply bank (without the facility of steering), is to push against the machine with the inertia of our own body. This means in practice, that in order to lean the bike to the right, we must initially move our body to the left." I respectfully disagree. To use our inertia to get a bike to lean is a self defeating proposition since as we stop moving, our inertia acts in the opposite direction with as much net effect as when initiating such a movement. Rather, it is by shifting our weight/cg in the direction of the curve that will help (albiet negligibly) to get a bike turning/leaning) "...the actual counter-steering movement is very small, since gyroscopic precession depends for its strength on the speed of movement not on the amount of movement. Wrong. The forces translated by gyroscopic precession depend on one thing and one thing only, the work done (moment applied times angle acted through) in acting to deflect the gyroscope in the first place. I guess until someone like Keith Code builds a zero momentum front end, one that includes a counter-rotating gyro to cancel the gyro effect of the front wheel itself, the precessionists will continue to believe that gyroscopic forces are responsible for leaning a bike. Or maybe I'll have to beat it out of 'em with some real physics.  Arvel will help me with that. One other point comes to mind. I'm a avid slalom skier (water skiing). The feel of leaning and turning on a slalom ski is EXACTLY like the feeling of turning a motorcycle at speed. Last time I looked, a slalom ski didn't have a gyro. In closing I would say that gyroscopic precession aids the stability, but hinders the agility of a motorcycle. | ||
| Hans |
"gyroscopic precession aids the stability, but hinders the agility"  "Things are soon becoming complex, aren`t they ?" | ||
| Blake |
Hans, is that you?! Great illustration of no gyro steering! LOL! | ||
| S320002 |
Blake, I respectfuly but strongly disagree with you. I won't get into a long detailed discusion but; go back to your own gyro sketch and look what happens when you push on the left end of the axle. The top of the wheel tilts to the left thus initiating the turn. I think if you reread the artilce with respect to gyro progression you will find that you and the author are saying the same thing but in different words. I also think you also misunderstand the importance of the gyro effect on stability, without it a motorcycle would be unstable to the point of being impossible to ride at speed. Greg | ||
| S320002 |
Jim X2.5, The "flickability" of the Firebolt is probably more the result of front end geometry than any reduction in front wheel gyro effect. Even though the front wheel assembly is significantly lighter, more of the mass is at the outer circumference. The more mass at the outer circumference the greater the gyro effect. The lighter wheel assembly does however reduce unsprung weight. Greg | ||
| Rick_A |
I agree. Reports of those dual perimeter brake systems state that they significantly increase steering effort. In regards to the Firebolt...also don't forget a 3" shorter wheelbase and significantly lighter overall weight, as well. | ||
| Tripper |
Greg, I locked up the front wheel once trying to avoid small dog. Bike did indeed become very unstable, and my arms filled with small stones. Last time I went without jacket. | ||
| Werewulf |
gyros, whats all this about greek sandwiches? |
