why do we lose balance in a bike when at a standstill?

GOD__AM

Yes tires are bigger on a motorcycle than a bike....wheelbase is different, rake angle is different, weight and many other things too. They turn "pretty much the same" because all the numbers add up to stability in the given aplication. Try getting that bicycle up to about 100mph and see how the steering compares to a nice stable motorcycle designed to handle at those speeds.

You can bring up all kinds of scientific words to try to describe how it turns, but in the end the bike leans over on a contact patch that produces camber thrust. All the forces and effects that the bike feels from inertia wanting to push the tire straight, or trying to flip it over and to the outside are all overshadowed by the inside of the tire traveling a shorter path than the middle. Doesn't matter if the tire is skinny or fat, it will still turn the same way. The bike could even be rolling on spheres with an axle and it would still turn "pretty much the same".

To get back on topic I think you were on the right track as to why the bike doesnt fall over at speed until you started talking about gyroscopic effect. We know when the bike is still and the bike is perfectly balanced that gravity pulls equal on both sides of the bike so it stays upright. Now if it leans even a small degree to the left gravity pulls harder on that side and the bike falls. The more foward momentum the bike has the less effect gravity has at pulling the bike to the left or right. This allows the rider to use less pressure on the bars to make corrections and keep himself balanced (hence less wobbling at speed). As I said the bike is always falling to the left or right, the rider has to make small corrections constantly to remain upright. I'm sure I'm missing some good scientific words here such as an object in motion tends to stay in motion or something like that, maybe someone else can help with that part.

BobG

I can buy that. But, both phase 1 and 2 are pretty unstable. It's basically a process of constantly catching your fall. Phase 3 is very stable and the stability comes from gyroscopic stability.

The counter-rotating wheel experiment is kind of interesting, though. What you really have are two equally large angular momentum vectors that are pointing opposite directions. Each has its own stability, so you would think overall stability is either 'twice as good' or, since each angular momentum vector is pointing in the opposite direction, they cancel each other out and you have no gyroscopic stability.

I would tend to think that overall stability would be increased.

If you hang one end of the axle of a spinning bicycle tire from a rope loop, the tire won't just sit there in one place spinning away. Gravity will result in torque and a second angular momentum vector. This doesn't reduce the stability of the tire relative to the ground (the tire stays perpendicular to the ground). But it does cause the tire to precess about the rope in the direction of the weaker angular momentum vector provided by gravity. (The tire's been taken off of the bicycle, by the way).

Having two tires whose angular momentum vectors are 180 degrees apart should result in increased stability with none of the undesirable side effects such as precession.

Loren Booda

why do we maintain balance on a bike when, e.g., the earth's rotation gives it an apparent forward motion of 20 km/hr, but otherwise (with zero torque applied to the petals) it would seem relatively at a standstill?

Ach Mach!

krab

Just remembered I wrote something about steering a while ago:

Motorcycle Dynamics:
There's been lots of controversy and dim-witted flame wars on rec.motorcycles concerning how a motorcycle steers. I myself have given this lots of thought, done many calculations, tried some experiments, and here in layman's terms is what I think.

When you initiate a turn by pressing the bars in the opposite direction (i.e. countersteering), the bike responds by leaning. This is due to the `outtracking' effect. Basically, you steer the bottom part of the bike out from under the centre of mass.

As the bike leans, gyroscopic forces cause the forks to turn into the proper direction. You don't notice this because the handlebar movement is very small. All you notice is that a constant pressure to the left causes a lean to the right and subsequently a turn to the right. In effect, the handlebars end up turning in the direction opposite to that in which you are pushing them. But this depends upon the spinning of the front wheel. That's why it doesn't happen in the same way when driving very slowly. You can verify this by rigging up some kind of long pointer from the handlebar centre. Others have done this. The gyroscopic effect can be verified by playing with spinning bicycle wheels.

Rake and Trail:
To understand the importance of motorcycle/bicycle steering geometry, a good place to begin is with a simple caster wheel as found for example on grocery carts. This is a system that has trail but no rake. (Trail is the distance that the point of contact of the tire with the road trails the intersection of the steering axis with the road.) Basically, the wheel pivots to the necessary direction to follow the direction of motion. A similar system does not work on a single-track vehicle because such a vehicle leans when it corners. Try the following. Take such a wheel (say from an office chair), and lean it to one side, pushing it down to mimic the effect of the vehicle's weight. You will notice that it tries to flip outward, turning into the turn. In other words, if a motorcycle had such a geometry, you'd have to counter-steer very strongly just to keep it leaned over in a turn. Now orient the caster the 'wrong' way so that the trail is negative. Lean it and weight it. You'll notice that it tries to turn out of the turn. This suggests that steering would be neutral if trail were zero. This is correct, but zero trail is not an option since we want some of the self-stabilizing caster effect. The way around this is to tilt the steering axis back from vertical. The angle of tilt is called rake. Take a wheel on a steering axis with no trail. Now rake it backwards. Immediately you have trail equal to the tangent of the rake angle multiplied by the wheel's radius. It turns out that for neutral steering, this is too much trail. The optimum is about 2/3 this amount. On a motorcycle, this is achieved by moving the forks ahead of the steering axis. On a bicycle, the forks are curved forward.

Example: My motorcycle has a rake of 27.5 degrees, and a wheel radius of 13.5 inches. With no fork offset, the trail would be 13.5" x tan 27.5 = 7". A 2.5" offset reduces the trail to 4.5".

rcgldr

Krab has this mostly right. A motorcycle is a uni-trak vehicle. Just like balancing a broom stick on your hand, you have the lean the broom stick first before you can accelerate it sideways.

Consciously or not, you have to lean a bicycle, or motorcycle by steering the wheels out from underneath you in order to turn, motorcyclist call this counter steering, you twist the bars right to roll (lean) left and vice versa. Gyroscopic forces, if anything, just make this more difficult, as they resist any effort to adjust lean (both downwards and upwards, more on this below), or turn the front wheel.

Next is the stability question. The steering geometry is setup so the contact patch is behind where the steering axis line would reach the ground. Hold a bicycle by the rear seat and lean it over. The front wheel will fall into the direction of the lean because gravity is pulling down on the bike, the contact patch is pushing up, and multiplied by the offset from the steering axis, this creates a torque force that turns the front wheel into the direction of the lean. The steering geometry is setup so that within a speed range (more on this later) it self corrects, so that while a two wheeled vehicle is in motion, any lean is countered by the front wheel falling inwards and straightening the vehicle back up.

Most bikes reduce the amount of self correction, to reduce steering effort. On bicycles, the forks are curved forward reducing the trail as mentioned by Krab. On motorcycles, the there are two triple clamps. The middle clamp is the part that pivots. The outer two clamps that hold the fork are forward of the middle clamp, again to reduce the trail. Move this too far forward and you can get stability issues. One case was the first year Honda 900RR sport bike, it would wobble a bit at racing speeds when encountering irregularities on race tracks. The fix was to move the forks 3/8" back closer to the pivot point with a new set of triple clamps, the result was a more stable bike, but it takes more effort to steer it.

Unlike the previous posts talking about pushing a bicycle and letting it free run, the speed that it falls at has to do with the amount of trail and the amount of inertial yaw (turn) resitance in the front wheel. If you have a bicycle that you can turn the wheel backwards, so that the forks curve backwards, which greatly increases the trail, the bicycle will pratically come to a stop before falling over. It's almost unbelievable the first time you see this. No gyroscopic forces in this case.

Maintaining a lean requires constant counter steering pressure (or the rider hanging off to one side, unbalancing the system). Because the steering is setup to self-correct, the front wheel "wants" to turn inwards enough to straighten up a bike. To hold a lean, a bit of opposite pressure on the handle bars is required to overcome the self correction force. This is more apparent on a motorcyle than it is on a bicycle.

So why can you ride a bike with no hands? This works because the bike counter steers for you if you lean to one side or the other. Say you lean to the right, the bike leans to the left, and the steering geometry self corrects to straighten up the bike. However, at this point, the center of mass is off to one side of the wheels, the system is out of balance, so the bike now falls to the side your leaning on. Fortuantely, the self correcting geometry keeps the bike from falling over as long as you reduce your lean with respect to the bike soon enough. This works on a motorcyle as well, unfortunately, this can be an issue if the driver doesn't understand what's going on, and at high speeds, it doesn't work. There's a limited speed range where the no hands method works; too slow and there's too much yaw interita of the front wheel to react in time; too fast and gyroscopic forces resist any change in the yaw axis at the front tire, or in the roll axis on the bike. You'll lean over, but not the bike, and not much else is going to happen. More on this next.

At high speeds, gyroscopics forces resist any change in lean, virtually eliminating any self correction. Up to about 70mph or so, this isn't much of an issue, but a motorcyclist taking turns at 100+ mph is going to be in for a thrilling experience if he doesn't know about counster steering (steering left to roll right and vise versa). It takes a lot of force on the handle bars to lean a bike at high speed, and it take almost as much force to straighten up as it does to lean over at high speeds. For high speed control, it's better to think ot the bike like an airplane that you roll by steering the other way. Actually there's almost no perceptible movement of the handle bars, you're really just applying pressure (and a lot of it at high speeds). A comment once made by a motrocyle magazine tester regarding riding a race bike at daytona, where one of the banked turns is exited at close to 180mph, "either the rider knows about counter steering, or he ends up in the infield". He was impressed by the large amount of force on the handle bars it took to straighten up the bike.

If you're new to motorcycling, you can get acclimated to counter steering by leaning a bike side to side while weaving a bit within a single lane, by twisting the handlebars side to side just a bit. It's best to do this on a freeway, as there's almost no resitance to counter steering at speeds below 50mph. At 65mph or so, there's enough resistance that you'll get the sensation of applying a force instead of actually moving the handlebars.

rcgldr

No, its a combination of gravity and trail that cause the wheel to turn inwards when a bike is leanded. Gravity pulls down on the bike, the ground pushes up at the contact points, and in the case of the front wheel the contact point is "behind" the pivot axis, and the resulting torque force turns the wheel inwards. You can hold a non-moving bicycle (no gyroscopic forces), and lean it side to side, the front wheel will "fall" into the direction of the lean.

There are some gyroscopic forces that turn the wheel inward when leaned, but in most cases this is insignificant, with the exception of some bonneville type (they look like torpedos with 2 wheels) bikes with very low center's of mass (some of these transition from counter steering to steering like a tricycle (at around 100mph) back to counter steering again (at around 200mph) as the vehicles go through speed transitions - note very high speeds invovled here combined with very low center of mass, lots of trail for stability, not a normal case). You're dealing with the gyroscopic forces of both front and rear tires with these special bikes. Also the driver is just trying to follow a straight line and keep the bike vertical.

rcgldr

Actually when stopped, because the front wheel contact patch is not in line with the pivot point, you can hold the rear brake and steer with the front to move the bike around underneath you (the contact patch moves a little, but it's mostly a rolling (leaning) torque force that's being created), to balance yourself. Velodrome track racers can stand still (no forward or backward motion) for long periods of time without moving and maintain balance this way. The reason they do this is that they want the other rider to take off first and follow them, their equivalent of a psych job. Some rock back and forth, but this is more like showing off, these guys don't need to do this.

Trials riders on lightweight motorcycles swing one leg side to side, sort of like a high wire act, to remain balanced on a stationary motorcyle.

rcgldr

Race bikes weren't setup like this, but as you mentioned, Suzuki made a few bikes like this with the idea that the smaller wheel would take less effort to turn. It would also reduced unsprung weight. I had one of these bike (GS 1150ES), and didn't really notice much difference. It did have more of a tendecy to straighten up when braking while leaning (the contact patch is on the side of the tire which contributes to this). I'm not sure if the smaller front wheel was the cause of this.

rcgldr

The bike turns because the front tire points in a slightly differnt direction than the rear. There's some camber thrust effect, but not much. Motorcycle tires don't all have circular cross sections, some older ones (Dunlop K81's) had high narrow domes with nearly flat and very steep sides. Modern Dunlop tires generally have near flat centers (for longer wear), and rounded sides. Bridgestones are pretty close to a circrular arc. You feel the difference. The Dunlops flatter profile resist the initial lean on a motorcycle a bit, transition from a Dunlop to a Bridgestone and it feels like the back end is slipping out from underneath you (no lean resistance, so it takes less countersteering). Racing tires are generally like the Bridgestones, close to a circular arc.

The tires also have a bit of a overhanging lip at the edge of the tread above the sidewall, that allows the contact patch to flex a bit on the edge for better grip (only racers probably notice this).

rcgldr

This is a matter of how much trail there is. Record speeds for a bicyle following wind breakers are over 150mph. The bikes have more trail than a typical bike.

At high cornering forces, tire squirm and slippage eliminate most of the camber thrust effect. Bikes turn because the front tire points in a different direction then the rear.

Safely ignoring distances that are significatly realtive to the radius of the earth, gravity pulls equally on the bike at all times. The ground is pushing up at the points where the tires meet the ground (contact patches). If the bike leans, then the center of gravity is off to one side of the contact patches. The ground pushes up at the contact patches, gravity pulls downwards on the center of gravity, and these mis-alinged forces create a torque force, twisting the bike to make it lean and fall.

Gravity pulls the same on the bike, regardless of the bikes speed.

Less movement, but more pressure is required, the gyroscopic forces in the front resist turning forces applied at the handlebars.

The steering geometry's trail effect do these small corrections for you (see my other posts). You can push a bike and let it coast by itself, and it will remain upright within a speed range (too slow and there's not enough correction force, too fast (100+mph probably) and other issues come into play, like underdamped overcorrection (speed wobble).

rcgldr

Huge loads on the axis of each wheel, but they counter each other, and the net result is no gyroscopic resistance to a rotation of the common axis. Counter rotating props on planes eliminate pitch - yaw coupling.

GOD__AM

The bike turns because of camber thrust my friend, period. The camber thrust is a result of the the bars being turned in a direction, or as you say both wheels pointing different directions. So the result of having both weheels pointing a different direction is camber thrust which results in the bike turning.

Here is one page that describes the physics behind it http://www.tonyfoale.com/Articles/Tyres/TYRES.htm . Even a tire with a more square profile like a car will deform as the bike is leaned and produce the same effect, ie; the contact patch still has a longer distance toward the middle.

Oh and suzuki wasn't the only ones to use a smaller front wheel. A lot of the ninjas, including the very popular ninja 900 had 16in front and 18in rear wheels. Possibly some of the honda interceptors had them too. I also know for a fact that some of these bikes were used in racing.

rcgldr

Read that article again, it turns due to a combination of slip angle and camber thrust. It maybe mostly camber thrust, but not all camber thrust. There will be combinations of grip, speed, and cornering radius where it can be all camber thrust, but this not the typical case.

The article also assumes that slip angles are the result of tire slippage, which is incorrect. Tires flex when cornering, and it's this flexing that accounts for most of the slip angle, with only a bit of slippage with typical cornering forces. The slippage component comes into play with high g cornering. Both flexing and slippage reduce the amount of camber thrust, requiring more slip angle to compensate for the reduction in camber thrust.

Here's a quote from that ariticle:
The camber thrust is trying to produce a radius to match the cone radius of 1.5 ft, however the actual radius is 327 ft at 70mph, 82 ft at 35mph, so camber thrust may be helping, but there's a huge difference between 1.5 ft and 327 ft, obviously camber thrust alone isn't doing much in these cases. At higher g forces corners, it's the slip angle than counts.

Yes it was a "fad" at the time, and street bikes have been used for racing. However if the F1 (500c 2 stroke GP) racing bikes used this, it wasn't for very long.

rcgldr

The profile of a tire reduces the camber thrust affect. Because of the circular shape, when leaned over at a certain angle, there's virtually no camber thrust, because the inside (higher up on the tire profile, smaller radius) is moving slightly slower than the outside (lower down on the tire profile, with a larger radius). For a range of specific speeds, there is an lean angle where no camber thrust effect occurs at all.

Dunlop made some high dome tires (K81, K181) with steeply sloped almost flat sides on the tire profile. The goal was to provide a larger contact patch when cornering, and a skinnier one when going straight. This is a profile that reduces camber thrust (it may be negative camber thrust at some lean angles).

Dunlop also makes tires with the opposite profile for most of it's street tires, a bit more contact patch while going straight to reduce tire wear. Bridgestones and most racing tires are near circular arcs. For racing bikes tire size is used to adjust contact patch area. The soft compounds flex enough to increase contact patch area and also reduce camber thrust, but the camber thrust reduction wasn't a goal, just a consequence of a stickier, softer tire that flexes more.

rcgldr

This thread didn't last very long.

rcgldr

The article attributes balance to steering corrections, steer inwards to reduce lean, (and implying steer outwards to increase lean).

rcgldr

Someone asked about this, so I thought I'd bring it up again with this thread.