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Lean Angle == Turn Radius?


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Here's another nugget I missed from page 60 of Twist II:

 

When the front has just the right amount of weight, it feels planted and follows through with the direction the rear of the bike is giving it.

 

This discussion has definitely given me a better understanding of this chapter and the significance of the load on the front wheel. Due to the length of the wheelbase and the C/G, after a certain lean angle, the amount of cornering force will not allow you to lift the front, ie. the front will wash out (go wider) with too much or too little weight. So, the front is holding the line, but, it is still only following the line dictated by the rear of the bike. Ultimately, it is responsible for handling its part of the cornering force or load. Not the direction the bike is traveling.

 

So, in that respect, I think I now see what Leftlaner meant when he said the front wheel is "commanded" by the rear. The bike is traveling where the front is pointed. But, the salient point is that even though the front wheel is holding the front end to the pavement and the bike is going where it is pointed, the angle it is turned is an effect of the direction the bike is traveling. Not a cause. The direction the bike is going is dictated by the rear. The front is merely handling its share of the cornering load.

 

Does this make sense to you guys?

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Returning to the question of the mechanics of why a motorcycle runs a wider arc or radius at higher speed... or back to square one.

 

If the rear is responsible for direction under standard throttle and acceleration (60/40), then it follows that the mechanics of how the bike runs a wider radius at higher velocity must be happening or at least beginning at the rear. And it follows that the front wheel must turn out more to follow.

 

So, merely increased centrifugal force at the rear making the main mass behind the steering head "want to run wider" and the increased centripetal force applied behind the steering angle at the front contact patch turning the front wheel wider?

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Here's another nugget I missed from page 60 of Twist II:

 

When the front has just the right amount of weight, it feels planted and follows through with the direction the rear of the bike is giving it.

 

This discussion has definitely given me a better understanding of this chapter and the significance of the load on the front wheel. Due to the length of the wheelbase and the C/G, after a certain lean angle, the amount of cornering force will not allow you to lift the front, ie. the front will wash out (go wider) with too much or too little weight. So, the front is holding the line, but, it is still only following the line dictated by the rear of the bike. Ultimately, it is responsible for handling its part of the cornering force or load. Not the direction the bike is traveling.

 

So, in that respect, I think I now see what Leftlaner meant when he said the front wheel is "commanded" by the rear. The bike is traveling where the front is pointed. But, the salient point is that even though the front wheel is holding the front end to the pavement and the bike is going where it is pointed, the angle it is turned is an effect of the direction the bike is traveling. Not a cause. The direction the bike is going is dictated by the rear. The front is merely handling its share of the cornering load.

 

Does this make sense to you guys?

 

It does indeed. Once the bike is on "autopilot" (stabile) at a constant lean angle, the rear is telling the front wheel where to point, which essentially means that the rear is actually controlling the whole bike: velocity, lean angle, corner line, you name it! Of course the front wheel also serves it's purpose. Without it, the rear would be carrying too much load (it would be sliding at a much less aggressive lean angle).

 

But the main purpose of the front wheel is to STEER the bike (i.e change the lean angle). First, it leans the bike over by STEERING "away from" the corner. And as the bike approaches the exit, the front helps standing the bike up by STEERING the front wheel further into the corner. Speaking of autopilots, I believe TOTW I compares this phenomenon with controlling an airplane. When the airplane is set at a certain roll angle, the plane will stay at a constant arc until you STEER it out of the arc. On a bike, the rear wheel will keep the bike stabile and settled mid-turn at a constant lean angle/radius (at 60/40 load), and the bike won't break away from the cornering line before you STEER it (front wheel) out of the turn.

 

So, very simplified; once the first part of the hook-turn is done (lean angle is set), the rear wheel is telling the bike where to go.

 

I really do feel that the rear wheel serves as an "autopilot" mid-turn. I've noticed that it's a lot easier to stay on the intended line and to stay relaxed once I'm loose on the bars, letting the rear do it's job. It makes the bike a lot "happier" and less stressed out than if I constantly use the front to steer/correct lean angle through the turn. Which proves what Keith says in TOTW I&II, and what we all (hopefully) now agree on..

 

This has been a great, guys. Like you said racer, the chapter on "steering for the rear" in TOTW II makes a lot more sense now! :)

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  • 2 weeks later...

As I said at this point I need more information at my disposal to understand the logic behind this accurately, and will come back to this thread when I have more from reasonably reliable sources. Just quick poking around it seems racer and I both are correct........ to a point....... and the truth may even be some combination of both of us.

 

In the meantime, NOT as an authoritative source, but more as a look at the results of someone even more obsessed with pushing bicycles in the garage than either racer or I, take a look at all of this guys web pages and his conclusions on the subject. It's an interesting enough read with a couple of interesting experiments even though with bicycles. I've got a line on some better sources than this for sure related to motorcycles, that I'll go over in future including Tony Foale, but some of what this guy has come up with makes sense to me.

 

His experiments with cones lashed together, bars wired straight on turning and "tracking" are important and they do bolster my front wheel most largely determines changes in lean angle and radius, while rear adds stability theory. They do not discount racers assertion the rear steers with conical steering and the front follows either, better sources will have to be used to settle that out. At the outset of my looking around it appears conical steering (most often labeled "camber thrust") tries to turn a tighter radius than I previously thought possible, and it seems slip (not slide) angles are involved as well.

 

here's the website http://www.terrycolon.com/1features/bike1.html

 

As to why lean angle changes with speed here is what the guy on the above website came up with, stated more simply than the way I tried.

 

" Circular motion, including turning a bike, is from a combination of momentum and centripetal force, a force deflecting your forward motion to the side. This creates a centrifugal "effect" (my own note here: centrifugal effect coming from the bike trying to continue going straight) directly opposite the centripetal force (here I think he's wrong about directly opposite, more like at a right angle).

 

Gravity is a constant. The centripetal force is determined by the angle the wheel is turned, the size of the circle you're turning in. The centrifugal effect is determined by your speed, i.e. momentum, plus the centripetal force. The faster you go, the tighter your turn, the greater this effect. That is why you must lean more the faster you're going for the same size turn. " (my note: if you lean more for the same size turn, you have a larger radius for the same lean angle with the increase in speed).

 

I would use more words to clarify it better, but basically that's about right.

 

Anyway I'll be back with whatever extra information I have found (however it turns out), but it will be likely be a long while, there are books to order and read, since I've gone as far as my own logic could take me with the limited information I have at my disposal right now.

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Welcome back Carl,

 

It seems there may be some misunderstanding of what I (and Keith Code) are saying or perhaps some confusion over terminology here. I'm not sure exactly what "conical steering" means as someone else made it up and I don't know that it is a very good term in any case. I certainly don't understand what it means when I am being attributed with using it as a reason why the rear wheel is responsible for the direction a bike moves while at stable lean angle. And I think the phrase may be leading to some miscommunication. That said, if you want to clarify that "conical steering" means the same thing as "camber thrust", that is a phrase I am familiar with.

 

In any case, there are directions (or types) of consistent motion (including circular) and there are changes in direction. Camber thrust or "conical steering" is not really, in this case, about changing direction. It is about a direction or type of consistent motion. This is why I think the word "steering" in "conical steering" may be leading to confusion as it seems it might imply an active change in direction or type of motion. While traveling in a circle is a constant change in direction, in this case, I think the key word is constant, which denotes a constant "direction" to me.

 

That said...

 

Rider input at the handlebar turning the front wheel will change lean angle. And, when changes in radius are due to a change in lean angle, again, ultimately, rider input at the bars moving the front wheel will be responsible. Everyone seems pretty clear on that.

 

Once the lean angle is established and the bike is accelerating, the direction the bike moves is determined by the rear wheel. You can use the word "stabilized" rather than "determined", but, at the end of the day, it effectively means the same thing when the lean and radius are stable. Both through personal experience and the research of others like Keith Code, the evidence for this seems overwhelming to me.

 

In any case, the question I am interested in is about the mechanics of a bike running wider with more cornering force when the lean angle remains consistent. I think I mentioned "slip" angles before and that that is where I left it. The critical or salient point being that the "slip" that is responsible for the radius is happening at the rear wheel while the front wheel actually changes the direction it is pointed as a result of that "slip" angle happening at the rear.

 

Thanks for the link. I will check it out.

 

racer

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The most telling aspect of the rear steering or maintaining the bikes attitude to the road surface is during a leaned-over tank slapper. The bike continues on its merry way with no lean angle changes that are perceptible. It is easy to notice that the bike maintains its line over bumpy pavement despite the fact the front end is rotating side to side. Then we have the turn exit wheelie, where the bike, without any changes in throttle, will maintain its lean angle. In the end, the only practical reason for looking at this aspect of riding is to discover how the rider can assist or spoil the bike's inherent stability and how too much misplaced rider input affects its line.

 

Keith

 

Also, the lean angle vs speed issue is pretty obvious when you see 45 degrees of lean in a 50 mph corner or the same lean in a 125mph corner where the riding radius of the turn is far greater. That is the empirical observation. The Physics behind it I'll leave up to you gents.

 

KC

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The most telling aspect of the rear steering or maintaining the bikes attitude to the road surface is during a leaned-over tank slapper. The bike continues on its merry way with no lean angle changes that are perceptible. It is easy to notice that the bike maintains its line over bumpy pavement despite the fact the front end is rotating side to side.

 

What a coincidence... I swear I was just about to say the same thing. ;)

 

Thanks for your input, Keith.

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Well, I've slept on this for a while now and I feel pretty confident that the critical issue is the the slip angle and camber thrust at the rear wheel determing the radius for a specific lean angle. Varying speeds which cause more or less "cornering force" alter the slip angle(s) and camber thrust at the rear wheel causing a proportional change in radius.

 

Thanks for eveyone's input so far. I am still interested in the gyroscopic forces and there is still significant disagreement between scientists of note just how much gyro forces contribute to the motorcycle. I think that the experiments with bicycles have limited value considering how much heavier motorcycle wheels are, especially when combined with the significantly higher speeds that are typical of the motorcycle in general.

 

Someone suggested a couple of months ago that precessional "gyro" forces are always at play due to the constant "angular acceleration" or circular motion of the bike as it moves through a corner.

 

Any thoughts about this idea?

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I am still interested in the gyroscopic forces and there is still significant disagreement between scientists of note just how much gyro forces contribute to the motorcycle. I think that the experiments with bicycles have limited value considering how much heavier motorcycle wheels are, especially when combined with the significantly higher speeds that are typical of the motorcycle in general.

 

Any thoughts about this idea?

 

I'm not a scientist. And, I admit I do not understand all I know about bikes or gyroscopes.

Yes, there is significant disagreement between scientists about precession from what I have read. My friend who is both an experienced motorcyclist and aeronautical engineer and physicist assures me it is only marginally relevant. (He also dismisses Bernoulli’s Principle as having little to do with flight.) :)

 

Also, motorcyclists disagree about the relevance of precession in motorcycle steering (often vehemently).

 

To my mind, precession provides a good description of what is happening while riding a motorcycle. That is a force operates 90 degrees in the direction of rotation. In the absence of an outside force another gyroscopic property controls the bike. That is rigidity in space. Of course, when this rigidity is lost, a gyroscope tumbles, i.e., the bike crashes.

 

But, with regard to the difference between the experiment with a bicycle wheel and a motorcycle in operation I think the higher speed and mass would make precession more forceful in a motorcycle.

 

It seems to me the experiment with a bicycle wheel using handle grips attached to the axle is only half the turn input. That replicates the push on the handlebar. That gets lean.

 

The other half is the resulting diagonal force applied by the road to the contact patch - that gets the turn.

 

The force seems to be there, however we may interpret it.

 

Just some thoughts.

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An instructor at a local riding school recently told me that a rider must apply slight pressure to the bars to maintain lean angle under acceleration (to prevent the bike from standing up?) and that hanging off with your upper body just the right amount will create equilibrium without applying pressure to the bars. I have not had the opportunity to play with this. And I don't know if it has any bearing on the question at hand.

 

 

Lean angle is subject to speed and radius of turn. If you accelerate and want to maintain the radius, you need to increase your lean angle.

 

I think the only thing that happens when you are leaned over and accelerate is speed up, not stand up the bike.

 

Is this wrong?

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Hi samuidave,

 

The first part of your post appears to be quoting me, so, I'll try to explain in more detail what I was on about there. The point is merely that due to several variable factors, not all bikes are always "hands-off" stable at lean angle. This can be caused by tire profile or the inter-play of front geometry and degree of acceleration. My posting this here was an attempt to gain more info from others with more experience or awareness of this.

 

 

PS - There are several ways to offset "quotes" from another post. First is to simply click on the "reply button at the lower right of any post. You should be signed in first to post a reply. Another way is to use BBCode without the name tag

[/quote ][/b]. Delete the last space to make it work and there is more BBCode available from the BBCode help button again below right side of posting field. Last would be to simply use quotation marks. Just a friendly suggestion.

 

racer

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An instructor at a local riding school recently told me that a rider must apply slight pressure to the bars to maintain lean angle under acceleration (to prevent the bike from standing up?) and that hanging off with your upper body just the right amount will create equilibrium without applying pressure to the bars. I have not had the opportunity to play with this. And I don't know if it has any bearing on the question at hand.

 

 

Lean angle is subject to speed and radius of turn. If you accelerate and want to maintain the radius, you need to increase your lean angle.

 

I think the only thing that happens when you are leaned over and accelerate is speed up, not stand up the bike.

 

Is this wrong?

 

samuidave,

 

It is a common misperception and misunderstanding, most riders will tell you that the bike comes up (more towards vertical) as throttle is applied but, NO, they don't, with some exceptions. The V MAX, which has a shaft and strange handling has a tendency to rise on throttle, there may be other bikes but I've never been on a sport bike that actually came up on throttle.

 

Similarly, the pressure on the bars to hold a line is another false perception and contributes to poor handling. There are many reasons why riders have this perception but all of them are other errors that they are making while riding and the pressure on the bar through the turn is a "solution"

to them.

 

This is stuff we go over in Level 1 and is all over the A Twist of the Wrist, Vol II book. What's up, did you guys forget???

 

Keith

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I think the only thing that happens when you are leaned over and accelerate is speed up, not stand up the bike.

 

Is this wrong?

 

The bke doesn't stand up under acceleration. It will run wide, but won't stand up. Here is a tease for you guys: we did some fun and interesting stuff in Twist 2 vidoe shoot...I can hardly wait to see it myself!

 

C

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C

 

When is this video coming out and where can I get it from?

 

Sorry for going off topic

 

We don't have the release date yet, its quite a project, massive amount of work going into it.

 

Best,

C

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This is stuff we go over in Level 1 and is all over the A Twist of the Wrist, Vol II book. What's up, did you guys forget???

 

Keith

 

*chagrined*

 

No, Keith, I didn't forget. I'm merely repeating what I've heard/read from other supposedly knowledgeable sources in an attempt to clear this up. My own experience supports what I've read in TOTW. And I was surprised to hear an instructor from another school say different.

 

Are there other excpetions besides a shaft drive bike? Can tire profile affect it? I've found worn tires, especially "squared off" from street riding, can seem to cause instability.

 

Thanks,

 

Bill

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