[ajf]

In the ongoing thread on countersteering, I'm disappointed that no one has addressed the following assertion directly, as far as I can tell from a quick read of the thread:

Next topic: How a motorcycle actually turns when leaned over. (Hint: it's because of the decreasing radius of your tires from center to edge)

Having read this statement in many magazine articles, I assumed that it was part of the working knowledge of magazine writers, who are supposed to be experts. This posting got me thinking about it for the first time, and I'm not convinced it's correct. I suppose that a proper analysis could be found (maybe on the site aschendel quoted - the descriptions of countersteering there are the best I've seen anywhere), but it's more fun to try to figure it out myself.

I'm looking for criticism of the following thought experiment, which has me nearly convinced that tire shape has little or nothing to do with causing a turning path once the bike is leaned over and stable, with no pressure on the bars.

For simplicity, imagine that instead of air in the tire, the tire is solid rubber and the treaded surface is flat and beveled at 10 degrees. (This is essentially the shape of the contact patch when a tire is leaned over 10 degrees.) If we roll this tire, obviously it will turn because one side travels farther than the other as the tire rolls.

Now imagine that we put two identical solid rubber tires on a motorcycle and fix the steering straight ahead, so no steering input is possible. Now lean the bike over 10 degrees so the tires are flat on the road (imagine a frictionless training wheel keeps it up, or a tractor beam from a hovering starship - whatever). Push the bike forward. Say the bevels on the tires have it leaning to the right. Clearly the front wheel wants to turn right. However, the rear wheel also wants to turn right, by exactly the same amount. But, because it's at the back, a right turning rear wheel will try to turn the motorcycle to the left. I think the two effects will cancel, and the bike will travel in a straight line. The line will be to the right of the direction the bike is pointing, and the two tires will not follow the same track, but there will be no curving path.

Now imagine that both the support holding the bike up and the steering are released. The bike will start to fall over, and the rider will have to steer into the turn (to the right) immediately to force a curving path to the right that generates the centrifugal force which balances gravity.

This pretty well convinces me that a motorcycle with identical front and rear tires follows a curving path only because the front tire is turned into the turn. The shape of the tires has nothing to do with it. (Bicycles often have identical tires and they feel remarkably like a motorcycle when you get them going really fast on downhill twisties - clearly they have no trouble turning.) Now, with different front and rear tires I imagine it's possible that tire shape can make a small contribution to maintaining a curving path, but the main cause is still likely to be that the front wheel is turned into the corner. (For completeness, the bike can also turn because the rear wheel is spinning under power and slipping sideways fast enough to the outside of the turn that the front wheel has to be turned outside as well to keep the bike from turning too quickly. But that's not relevant to turning when there's no wheelspin.)

Or, maybe I'm wrong . . .

 

[Woot!]

If the tires had knife edges and rode up on the edges then you'd be right I think (I couldn't picture your description without, well.... a picture). But since they don't, the inner side of tire's wheel will travel a shorter distance than the outer edge and the tire arcs towards the center.

 

[andyauger]

You've got it.

Bikes (motorcycle or bicycle) turn because the front tire is turned in to the curve. If you try it on a bicycle you can clearly see where the bars are. On a motorcycle it's not as easy, but with a simple pointer mounted on the bars extending over the tank and a piece of tape you can see it on a motorcycle. I would recommend doing this in a parking lot so you won't be in danger or endangering when you glance at the pointer.

I've never ridden flat tracks, but the three guys I know who did said that even with the rear end sliding way out the front tire still points in to the curve.

 

[ajf]

If the tires had knife edges and rode up on the edges then you'd be right I think (I couldn't picture your description without, well.... a picture). But since they don't, the inner side of tire's wheel will travel a shorter distance than the outer edge and the tire arcs towards the center.

The main point is the both tires want to turn in the same direction. But, if the bike is leaning to the right, the front tire tries to rotate the whole bike to the right while the rear tire tries to rotate the whole bike to the left (as it tries to turn itself to the right). The net effect is probably zero.

The knife edge idea is interesting. Thinking back to my hockey days, the way to make the tightest, hardest turn on skates is to put one foot in front of the other. The leading skate points into the turn, and the trailing skate points outwards. That's the opposite of what motorcycle tires try to do.

 

[JcTerra]

Here's a short (well that's relative I guess) article by Tony Foale who - if he didn't write "the book" on motorcycle chassis and handling - at least wrote "a book" that may as well have come to us on stone tablets carried by a beardy guy.

It speaks directly to the OP's question about the steering position versus the tire camber on bikes vs cars. Really interesting stuff ...

http://www.tonyfoale.com/Articles/Tyres/TYRES.htm

 

[Grounddevil]

The shape of the tire has nothing to do with it.
The physics works like this:
Two wheels are in a straight line and rotational force of the wheels against the pavement is allowing your bike to move forward. Unless a force acts to the right or the left of that straight line you are going, you do not move left or right (Newton's 1st law of motion).
Your wheels are curve but when you lean, that just makes another area of the wheels in contact with the ground, this does not produce a force to either side of you. Try it. There is absolutely no way you can make your bike turn if you hold your handlebars straight, no matter how much you wiggle.

As soon as you turn you front wheel at an angle, the original force that the front wheel exerts on the pavement to go straight is split into two force. one that is still straight and the other that is perpendicular. Every force can be split into its x and y component and that's what's happening when you turn your front wheel.

Notice that this must be done gradually. If your angle of turn is too much too fast then you begin to skit, this i because because you and the bike have mass, thus innertia you both want to continue traveling in a straight line but when you turn, your front wheel tells your bike to go right/left so you begin to skit.
Try it, whip your handlebar 45 degrees really fast and i guarantee you'll go down.

We lean into the turn to prevent from falling over. Imagine you are turning right. Inertia wants to keep you going straight but you are making your bike turn right thus you lean to the right to fight inertia. It's like when you're driving and you lean to the left when you turn right, on a bike you're just trying to fight that by leaning into the turn.

The only reason wheels are made the way they are is for turning. Imagine a tractor wheel. If you look at it straight on, it is a rectangle. It's possible for a tractor to turn on it because it has 4 wheels and none of the wheels have to lean to turn. but on a motorcycle, when you turn, you lean into the turn, if you have a tractor wheel, you would be now riding on the corner of the bottom and the side, this gives you no traction. A motorcycle wheel is rounded so no matter how you lean, you always have a good amount of contact between the wheel and the road.

 

[SV650R]

One little thing I would like to comment is the fact that to turn the bike with control one must counter steer... Now, when you are already leaned into the the corner...What makes you think the steering is straight? It is probably slightly pointed toward the turn.

I have a book, Motorcycle Dynamics, it explains that there are many different geometry angles at play... Also wheel/tire sizes... Throttle control and weight distribution... Even tire rubber compound sleepage...

The correct way to corner is to relax and let the bike figure out the smooth path. You can help traction and balance 60% rear and 40% front weight distribution with the throttle... You can make changes mid corner but the bike will have to settle again on is own... I do not think any of us could possibly force a smooth corner with steering inputs...

I like the theory of both wheels turning.... We often hear steering with the rear wheel... Have you seen those 4 wheelers with 4 wheel steering? imagine that set up on two wheels...

Luis

 

[Maviryk]

I like the theory of both wheels turning.... We often hear steering with the rear wheel... Have you seen those 4 wheelers with 4 wheel steering? imagine that set up on two wheels...

Luis

Aside from dirt bikes and parking lot maneuvering I don't see the practicality of the application in motorcycles.

Why not front wheel drive and rear wheel steering? Bikes would be maneuverable as forklifts!

 

[BKK Jack]

Wow, a sarcastic comment made at the end of an incredibly long post caused by an even more incredibly boring day at work is the genesis for a whole new thread. I'm flattered. Shockingly, my head still fits my helmet.

Let me see if I understand your position. If you role a single cone on a flat surface, the cone will eventually turn a circle. If you then take two identical cones and attach them with their axes fixed parallel to each other they won't turn, but will more or less crab in the direction of the tip of the cones. Is that about it?

The radius delta between the outside and the inside of the tire do have their part in how a bike turns. Since the inside edge of the contact patch is describing a slightly smaller arc than the outside edge of the contact patch, it does contribute to the turn, but the bar angle is the major reason, as you conclude. Hopefully, I never said in any of my posts that you can maintain a lean angle with the bars, and therefore the front wheel, straight ahead. I did mention equilibrium. When you are in equilibrium in a corner of constant radius, your front wheel will be pointed in the direction of the turn just enough to keep you turning without gravity plopping you to the ground.

At least that's how I see it.

Okay, next little bit of basically worthless knowledge is...how does a train turn? (This could become great sport, but I have a feeling this one will fall on deaf ears) ;D

 

[Woot!]

You could always just call it by the term we used in the Navy. PFM! Pure Frickin' Magic!! Works for me!! lol

Woot!

 

[DieTrying310]

Okay, next little bit of basically worthless knowledge is...how does a train turn?

Tracks, duh.

 

[JPL_SV]

Am I crazy, or maybe it's an illusion...but it seems like sometimes the tire is not facing the inside of the turn...(more like it stays strait or maybe out?)

 

[ajf]

Okay, next little bit of basically worthless knowledge

Worthless! Worthless??

Nearly 50 years of reading motorcycle magazines left me with three nagging questions about how motorcycles work. But, you can't argue with a piece of paper, and I was stuck with doubts but no motivation to think myself forwards.

!. Horsepower vs torque. This was answered in the long threads on this board about a year ago. (There's no fundamental difference, they're just slightly different ways of presenting the same data, and there's nothing you can estimate or calculate from one that you can't from the other. The way magazines talk about them as if they are different properties is just silly.)

2. Will motorcycle ABS protect you from locking up the front wheel while trail braking? If not, why not? Magazine writers ignore this obvious question, probably because they don't know the answer. It was answered on a thread I started on SportTouring.Net a few months ago. (The new ABS on the '09 CBR600RR might change the answer, and I might be turning my SV into a CBR because I really like trail braking.)

3. How does a motorcycle turn? Every magazine article that mentioned it said the main reason was the shape of the tires. If what most people are saying here is correct, then the magazine "experts" were wrong. (I haven't looked at the article by Tony Foale, yet.)

And, I'm a happy man because my nagging doubts about the answers I'd learned have been resolved, thanks to the participants of these forums. Thanks to all who contributed to these theoretical discussions.

 

[ajf]

Well, I just read the recommended article by Tony Foale. He clearly believes that most of the turning force comes from the shape of the tires when a motorcycle is leaned over (although he expresses it somewhat differently.) He describes in great detail how a unicycle turns. But he says nothing to me about how a motorcycle turns. He doesn't address what the rear wheel is doing, except when it is sliding/spinning. Maybe he's another of the "experts" who don't really understand it. Or, maybe it's me who doesn't understand.

I'm still leaning towards the "front wheel turned into the corner" explanation. Maybe someone will change my mind by coming up with an analysis of what both wheels are doing according to the "tire shape is everything" theory that doesn't result in them canceling each other out.

Or, maybe the right answer is a compromise. Maybe when the tires are identical, the "front wheel turned into the corner" theory is right. And, maybe when the tires are as different as they usually are on road racers, the turning force of the rear wheel is so much less than that of the front wheel that the tire shape of the front wheel can be the predominant driver of the turn.

 

[slattmattery]

Am I crazy, or maybe it's an illusion...but it seems like sometimes the tire is not facing the inside of the turn...(more like it stays strait or maybe out?)

If my own understanding is correct, the riders must counter-steer the bike at some point to not only lean the bike over, but to right itself again exiting the turn. If the wheel is indeed not pointing into the turn, I would assume that the bike is in the process of leaning over (or righting itself) via counter-steering techniques.

 

[Veee]

Am I crazy, or maybe it's an illusion...but it seems like sometimes the tire is not facing the inside of the turn...(more like it stays strait or maybe out?)

when you leaning in corner with throttle off wheel is pointing in, when you on throttle but still leaned wheel points out.
to keep turning while on throttle you have to countersteer to keep bike turning. if you picking bike up exiting corner you again countersteer.

 

[NTJordan]

when you leaning in corner with throttle off wheel is pointing in, when you on throttle but still leaned wheel points out.
to keep turning while on throttle you have to countersteer to keep bike turning. if you picking bike up exiting corner you again countersteer.

I feel this is a slightly disjointed but good description of what is going on. Makes sense to me anyway.

 

[Veee]

if you would do what they do it would be clear to you and not just making sense.
faster you go more clear and obvious this stuff becomes.
if you cruising down the road at 30 miles per hour you never experience it and harder to understand.

 

[SpeedisaVirus]

Tire shape and the decreasing radius play a role in how the bike turns but it doesn't make the bike turn. That reality becomes apparent as soon as you ride tires with different profiles. You could turn a bike with tires like the OP described but it would be much more work...I'm not sure if you would have to force the bars into the turn or not after the lean is in. Either way the bars must turn...

One little thing I would like to comment is the fact that to turn the bike with control one must counter steer... Now, when you are already leaned into the the corner...What makes you think the steering is straight? It is probably slightly pointed toward the turn.

It's interesting to see who does and doesn't understand motocycle steering. Surprised you do but you are still a douche...just an interesting one :nana:

 

[httc84]

So, when I am cornering my front wheel is turned toward the inside of the corner.

To stand the bike up in a corner, I can either gas it or . . . turn the wheel toward the inside of the corner.

Something is not making sense here.