Why does a bike turn

[wfooshee]

First off, the gyroscopic effect. It has been stated that it plays a major part, and that it plays no part what so ever. I think the truth falls into the middle. The gyroscopic effect plays a primary roll in the stability of the bike; the faster you go the more stable it is. Try riding without any hands on the handle bars at 10 mph, and then try at 50. The bike can wander all over at 10 mph, but stays fairly true to line at 50 mph. Also look at stunts that super cross racers do; the one in particular where they kick the back of dirt bike out about 90 degrees and then bring it back straight before landing. They can do that by manipulating the gyroscopic effect (as mentioned before with the 90 degree reaction force bit). The gyroscopic effect does not cause a bike to turn (unless you're airborne), quite the opposite, it resists the change in direction (hence giving the bike straight line stability at speed and at a given lean angle when cornering). Another way to practically see this in the real world is to weave between the dotted lane marker lines. Try it at 30 mph and then at 60 mph. Try to keep the tire close to the lines so you're side to side distance traveled is about the same. You'll find it takes a lot more input from the rider to weave back and forth rapidly at 60 mph then it does at 30 mph. I've done this myself and it can be hard to do it quick enough (even on a light weight dirt bike) at speed do you don't overshoot a line.

The steering effects you describe would more properly be attributed to trail, not gyroscopic effects. As for the jump trick, there's definitely not enough speed in the rear wheel of an idling dirt bike to be gyroscopic. And yes, while airborne, they close the throttle.

Think about how fast gyroscopes spin before they actually become, well, gyroscopes. I'm not sure a couple hundred RPM (wheel speed at 60 mph) makes enough of a gyroscope to actually affect the bike's physics.

 

[Constant Mesh]

Mount a car tire on the front wheel of a bike and see how the steering is affected.

 

[Fred W]

As for the jump trick, there's definitely not enough speed in the rear wheel of an idling dirt bike to be gyroscopic. And yes, while airborne, they close the throttle.

Usually, but not always. This is going off track a little from how the bike steers, but it does talk to how momentum can affect the bike's attitude. Once a bike has launched off a jump and is airborne it is difficult for a rider to alter its trajectory since there is no ground contact to be used as a fulcrum to lever against.

If an MX rider feels he is going to land with the front wheel too high he will hit the rear brake in mid jump. The momentum of the rear wheel will be transferred into the rear brake caliper and lever the front end down a little. Likewise to raise the front in mid air he'll whack the throttle open. Newtonian inertial physics are what is in play.

So there is some small amount inertial mass to the wheels, both rotational and gyroscopic, but not nearly enough to be of any significance compared to the influence of turning the steering slightly and changing the tire's path on the ground.

 

[Northwoods Snowman]

First off, the gyroscopic effect. It has been stated that it plays a major part, and that it plays no part what so ever. I think the truth falls into the middle. The gyroscopic effect plays a primary roll in the stability of the bike; the faster you go the more stable it is. Try riding without any hands on the handle bars at 10 mph, and then try at 50. The bike can wander all over at 10 mph, but stays fairly true to line at 50 mph. Also look at stunts that super cross racers do; the one in particular where they kick the back of dirt bike out about 90 degrees and then bring it back straight before landing. They can do that by manipulating the gyroscopic effect (as mentioned before with the 90 degree reaction force bit). The gyroscopic effect does not cause a bike to turn (unless you're airborne), quite the opposite, it resists the change in direction (hence giving the bike straight line stability at speed and at a given lean angle when cornering). Another way to practically see this in the real world is to weave between the dotted lane marker lines. Try it at 30 mph and then at 60 mph. Try to keep the tire close to the lines so you're side to side distance traveled is about the same. You'll find it takes a lot more input from the rider to weave back and forth rapidly at 60 mph then it does at 30 mph. I've done this myself and it can be hard to do it quick enough (even on a light weight dirt bike) at speed do you don't overshoot a line.

The steering effects you describe would more properly be attributed to trail, not gyroscopic effects. As for the jump trick, there's definitely not enough speed in the rear wheel of an idling dirt bike to be gyroscopic. And yes, while airborne, they close the throttle.

Think about how fast gyroscopes spin before they actually become, well, gyroscopes. I'm not sure a couple hundred RPM (wheel speed at 60 mph) makes enough of a gyroscope to actually affect the bike's physics.

Wfooshee, do you balance your own tires? If you do I want you to try a cool experiment the next time you do. Heck you could just do it the next time you have a wheel off for something. Take an air blow gun and blast the tire with it on the balancing stand to spin it up to speed (doesn't have to be crazy fast either). Then grab the axle of the balancer, pick it up, and and try to walk around with the wheel spinning. After it about breaks your arms off, you'll see just how much of a gyroscope a wheel can really be! It's quite impressive. I motocross rider may let off the throttle after he leaves the jump, but those wheels are still spinning at probably 30-40 mph (or faster?) in order to clear that huge tripple. Small gyroscopes you see as toys or as a demo have to spin very fast because their fulcrum is small and they have little mass. Relatively speaking a motorcycle has a lot of mass (which is nearly all at the outer ring) and a much longer fulcrum so it does not need to spin as fast to generate the same inertia. You're on the right track in your statement; that's why at slow speed the wheels don't do much to stabilize, but at high speed they are very stabilizing and I don't think you realize just how much inertia really builds up. That's what keeps you from falling over riding in a straight line. When you're balanced, it doesn't take a whole heck of a lot to keep you there when there isn't some other force acting on you (think of how much a wind gust can blow you across a lane). That's a point I forgot in my original post. People mentioned that the bike steers back and forth to keep you upright. Well, that would mean the bike would oscillate back and forth and an oscillating motion is also called a vibration. If the bike was oscillating back and forth while riding at high speed the vibration would definitely be noticeable. Riding slower like around town, you have enough room and time you can correct for any imbalance and it will be almost invisible. Traveling at high speed that imbalance would be much more pronounced, but you don't have to correct for it because the wheels help to stabilize the bike. You still must make corrections for wind, road crown etc, but it's less at high speed. I think I learned this best riding dirt bikes growing up; momentum is your friend and it's a lot easier to ride though some nasty stuff with some speed, which seems counter-intuitive, but it works. Once I got over the fear, my riding skills greatly improved.

As for the jump trick, there's definitely not enough speed in the rear wheel of an idling dirt bike to be gyroscopic. And yes, while airborne, they close the throttle.

Usually, but not always. This is going off track a little from how the bike steers, but it does talk to how momentum can affect the bike's attitude. Once a bike has launched off a jump and is airborne it is difficult for a rider to alter its trajectory since there is no ground contact to be used as a fulcrum to lever against.

If an MX rider feels he is going to land with the front wheel too high he will hit the rear brake in mid jump. The momentum of the rear wheel will be transferred into the rear brake caliper and lever the front end down a little. Likewise to raise the front in mid air he'll whack the throttle open. Newtonian inertial physics are what is in play.

So there is some small amount inertial mass to the wheels, both rotational and gyroscopic, but not nearly enough to be of any significance compared to the influence of turning the steering slightly and changing the tire's path on the ground.

Fred is right on here. The wheels act as a stabilizing force, but the forces involved in turning due to the mass of the bike and friction of the road easily overcome it when turning. Another thing I learned with dirt bikes that goes with what Fred said, is that you do NOT want to touch the front brake in mid air. It's a good way to "fall out of the sky" or in other words pile drive the front end of the bike into the ground when you land instead of landing on the wheels; the rotational inertia of the front wheel is transferred to the bike rotating the front end down with no way to get the wheel spinning again.

 

[Eagle Six]

According to an online calculator, a spinning tire at 60 mph is up around 800 rpm, and creates about 230 G's at the surfact of the tire. That's enough force, I'm not grabbing it!!

 

[wfooshee]

Yes, a motorcycle wheel turning on its axle and separated from the bike will demonstrate gyroscopic effects. It will NOT demonstrate effects strong enough to work for or against an actual motorcycle being ridden and steered.

i still insist that gyroscopic effects have nothing to do with a motorcycle's stability. Steering towards the lean (or the fall) keeps the bike upright. The fact that a bike is more stable with more speed is still not gyroscopic, but geometric. When crawling, if the bike leans 4 or 5 degrees, the rider may have difficulty keeping it upright simply because it's moving too slowly to bring the front wheel back underneath the bike. Think about it: when riding slow, what do you do to catch the bike if it's on its way over? You give it some clutch and gas, and speed up. Now you've got time to bring the front wheel under the bike by steering into the fall. If the bike's already moving along, that task of keeping the front wheel under it is much easier, simply because the bike moves further in a given amount of time. It's not so much work to keep it upright. Again, nothing gyroscopic about that.

Nothing to do with why it turns, though. But you can't turn a non-leaning bike. OK, you can, but not unless you're hanging off way in the direction of the turn to put the C.G. off to the side, or you'r willing to fall off in the direction opposite the turn.

 

[Old Guy]

Who here remembers the early 80s and the sudden revelation that a smaller front wheel turns more easily? I bought an 83 Suzuki 550, the first year it had a 16" front, and steering input was scary quick compared to my friend's '82 with a 19" wheel.

 

[Northwoods Snowman]

Yes, a motorcycle wheel turning on its axle and separated from the bike will demonstrate gyroscopic effects. It will NOT demonstrate effects strong enough to work for or against an actual motorcycle being ridden and steered.
i still insist that gyroscopic effects have nothing to do with a motorcycle's stability. Steering towards the lean (or the fall) keeps the bike upright. The fact that a bike is more stable with more speed is still not gyroscopic, but geometric. When crawling, if the bike leans 4 or 5 degrees, the rider may have difficulty keeping it upright simply because it's moving too slowly to bring the front wheel back underneath the bike. Think about it: when riding slow, what do you do to catch the bike if it's on its way over? You give it some clutch and gas, and speed up. Now you've got time to bring the front wheel under the bike by steering into the fall. If the bike's already moving along, that task of keeping the front wheel under it is much easier, simply because the bike moves further in a given amount of time. It's not so much work to keep it upright. Again, nothing gyroscopic about that.

Nothing to do with why it turns, though. But you can't turn a non-leaning bike. OK, you can, but not unless you're hanging off way in the direction of the turn to put the C.G. off to the side, or you'r willing to fall off in the direction opposite the turn.

So if the wheels don't have any stabilizing effect on the motorcycle and it requires constant correction from the front wheel to stay upright (implying it is NOT inherently stable), then how is this bike able to not only able to keep traveling without its rider over uneven surfaces, but TURN at the same time? Note how the slower the bike goes, the more it is able to fall over and hence turn tighter.


Sort of separate, but anyone ever notice how large front wheels like 21" wheels on dirt bikes have the pinch bolts on the bottom of the forks that hold the axle in front of the centerline of the forks?

 

[Old Guy]

Sort of separate, but anyone ever notice how large front wheels like 21" wheels on dirt bikes have the pinch bolts on the bottom of the forks that hold the axle in front of the centerline of the forks?

Dirt bikes have way more suspension travel than street bikes. The axle on front allows for longer tubes.

I'm assuming that.

 

[wfooshee]

So if the wheels don't have any stabilizing effect on the motorcycle and it requires constant correction from the front wheel to stay upright (implying it is NOT inherently stable), then how is this bike able to not only able to keep traveling without its rider over uneven surfaces, but TURN at the same time? Note how the slower the bike goes, the more it is able to fall over and hence turn tighter.

Trail does keep the front wheel in line with the bike, but as you pointed out, it's going over just a bit more with each passing second as it slows down, with the lean inducing the turn it goes through. trail imparts a lower force the slower the bike goes, and the wheel goes further and further off center, the bike leans more and more, and turns tighter and tighter. Then it gets bumped almost upright when it crosses back onto the pavement that last time. When the front wheel encounters the edge of the pavement it is forced back left far enough by the trail geometry to right the bike temporarily. That bump kicks the wheel back towards center. If anything, that video proves that trail is the dominant factor, not gyroscopic force, just from the behavior of the bike at that bump. Trail imparts the caster-like self-centering force on the front wheel that you want to attribute to gyroscopic stability.

The trail geometry giving the self-centering force on the front wheel is the constant correction that you mentioned, and the source of the bike's stability. It's also the reason the bike is more stable at speed, as I explained in my previous post. A bike without trail would be effectively unrideable, and would certainly not do a ghost ride.

 

[HotRodZilla]

 

[Fred W]

That video does a great job of showing how a motorcycle steers. It's two parts: First you lean it, then you steer into the lean to go around the curve. If you need to turn sharper you have to lean it further to be able to turn into the corner harder.

With no input on the handlebars on a bike with neutral steering the wheel naturally turns into the lean, so when you are in mid corner and press on the inside bar it creates the additional lean, but the wheel is still pointing into the lean and around the corner, just less so than what made it stable.

The steering geometry (trail) is what determines what the steering will do on its own when the bike is leaned over. That geometry can be affected by many things including the tire profile and shape. As an example, a nice new tire tends to be stable when cornering and turn itself into the turn just enough to maintain the established lean angle. But when a front tire has worn into a trapezoidal shape it no longer is self stabilizing when going around the corner and requires input on the handlebars to either maintain the lean angle (continuous counter steering) or to keep the bike from leaning too far. (It's usually the former) I'm sure you have all experienced this. It's what makes riding on new tires such a joy.

 

[Eagle Six]

This is a link to a simplified explanation of counter steering and turn effects. It doesn't go into centrifugal force or gyro effects, tire shape, friction, rake/trail, geometrics and other aspects, but does include some chalk board and video that simplifies the process of turns......

 

[wil780]

There is a way to test for yourself how important gyroscopic forces might be while leaning a bike. Any gyroscopic force increases in proportion to the rpm. So, flick the bike from side to side at 40 mph - it takes almost no effort. Then try to get the same side-to-side motion at 80 mph. It takes way more than twice the effort required at 40 mph. If gyroscopic force were all that resists leaning it should take exactly twice the coutersteering force on the bars to get the same rate of lean at 80 vs 40 mph.

My subjective impression from this test agrees with what scientists researching motorcycle dynamics have concluded, that gyroscopic forces account for roughly 30% of the inherent stability of a motorcycle.

 

[Eagle Six]

My subjective impression from this test agrees with what scientists researching motorcycle dynamics have concluded, that gyroscopic forces account for roughly 30% of the inherent stability of a motorcycle.

Hi wil780, would you please post a reference link to the research report of these scientist, substantiating the "roughly 30%" results? Thank You.

 

[Uncle Hud]

Why does MY motorcycle turn? <shrug> Because I want it to.

 

[Eagle Six]

Why does MY motorcycle turn? <shrug> Because I want it to.

Out of all the comments I like this explanation the most, now I'm going out to ride, just because I want to!

 

[wil780]

My subjective impression from this test agrees with what scientists researching motorcycle dynamics have concluded, that gyroscopic forces account for roughly 30% of the inherent stability of a motorcycle.

Hi wil780, would you please post a reference link to the research report of these scientist, substantiating the "roughly 30%" results? Thank You.

I can't claim to have done an exhaustive search, but here are the highlights of what I've run across over the years in my amateur efforts to understand motorcycle dynamics.

An explicit claim that gyroscopic forces are not dominant: J Fajans, 2000. Steering in bicycles and motorcycles. Am. J. Phys. 68, 654 - 659. As far as I know, no one has pointed out any errors in the conclusions of this paper, despite the use of a simplified theory of motorcycle steering that allowed a specific focus on gyroscopic effects.

A report that stability does not require gyroscopic effects (because they were engineered to not exist): JDG Kooijman, JP Meijaard, JM Papadopoulos, A Ruina, AL Schwab, 2011. A bicycle can be self-stable without gyroscopic or caster effects. Science, March 20, 2011. (I got a preprint from the authors which doesn't have the full reference and don't have it handy. A quick search should find it for you if you need it.)

The most comprehensive theory currently available: JP Meijaard, JM Papadopoulos, A Ruina, AL Schwab, 2007. Linearized dynamics equations for the balance and steer of a bicycle: a benchmark and review. Proc. R. Soc. A. 463, 1955 - 1982. This shows that there are many effects that govern motorcycle stability, not just gyroscopic effects.

I've corresponded with the some of the authors of the second and third papers regarding the "cone effect" questioned in the original post of this thread (it's usually called camber thrust), and they confirmed my conclusion that it has almost nothing to do with turning a motorcycle. (Repeating what I posted early in this thread, it is one of many forces that try to turn the handlebars while a bike is established in a stable turn. These forces must balance if the bike is to go around a corner without needing constant pressure on the bars by the rider, but none of those forces cause the motorcycle to follow a curving path. That turning force comes from the front tire slipping slightly sideways. I say almost nothing, because I think that camber thrust from the rear wheel does apply a very small force that acts in the same direction as the slipping force from the front wheel. I have not run this idea past any experts.)

 

[Fred W]

^^^ excellent post.

I would agree with everything except the term "slipping" sideways. I think a better word would be "rolling" sideways to the chassis. The slipping infers tread slip with the ground, which does happen but isn't the force that causes the bike to turn. It's the fact that the wheel is (even slightly) off axis to the bike that actually causes the bike's direction to change and turn.

 

[HotRodZilla]

I'm with Uncle Hud. You fellas need girlfriends.