Why does a bike turn

[MajBach]

I have no clue as to how my anatomy works when I walk, I just walk. Same w/ ride'in; if you want to learn how the bike works, go ride!

Ya notice how you never really meet too many pilots that say "I learned how to fly by just gettin' in da plane and going!"?

I remember when I was about 9 years old learning the hard way on my 85 cc Kawasaki that you don't hit the brakes in a turn.

 

[MajBach]

MajBacH can't solve the conundrum of how a motorcycle goes around corners.

Au contraire. I stated I disagreed with the "expert" on how it turns.

 

[Brodie]

Ya notice how you never really meet too many pilots that say "I learned how to fly by just gettin' in da plane and going!"?
I remember when I was about 9 years old learning the hard way on my 85 cc Kawasaki that you don't hit the brakes in a turn.

And don't ever grab a fist full of front brake in a turn while driving on gravel.

damhik

What a wonderful way to spend 15 minutes and walk away with a headache. I should know better than to click on a thread like this.

Brodie

 

[ionbeam]

MajBacH can't solve the conundrum of how a motorcycle goes around corners.

Au contraire. I stated I disagreed with the "expert" on how it turns.

But, if you don't follow what has been suggested, at no $$$ expense and not much personal effort you have NO ROOM to continue to "disagree with the "expert". Revelation is at your hands

 

[wheatonFJR]

...Because it can.

 

[Lowcountry Joe]

It's all about the necessity of counter steering required to turn (at speed) created by the gyroscopic forces at the front end of the motorcycle (or any other two-wheeled device with a front and rear wheel (at speed). If you research it enough, sooner or later the light will go off in your brain and you will say....Hmmmm, now I understand.

 

[N4HHE]

Interesting article here it focusses on hanging off but does a good job of explaining steering/ counter steering IMO

That article somewhat validates what I was implying. The author states that 40 years ago, bike tires were round (meaning cross-sectionally symmetrical) which would have the result of the contact patch not changing in shape regardless of how much lean there is.

That's trivially easy to prove wrong. Radius of motorcycle axle to tread is greater to the center of tire than to either side, today and 40 years ago. The inside edge of contact patch is in a smaller rolling diameter than the outside edge.

 

[thewrenchbender]

I'm pretty much with banjoboy. I would rather not cogitate much on why (or how) a bike turns.

Riding, it pretty much has the same effect cogitating too much on how wing-shooting works has on my clays score-and it ain't a positive effect.

I do know from previous riding experience that when a young teenager decides to commit suicide by darting out in front of you from right-to-left, quickly shoving forward as hard as you can on the LH handlebar of your motorcycle immediately induces an instant hard lean to the left, and an immediate left turn-allowing you hit him on about a 45 degree angle launching him back to the right rather than running him under the wheels (like I did with his bicycle) or cutting him in half.

I always figured it was the whole 90 degrees spinward precession deal, all I know for sure is it worked. Only a broken thumb on the kid's part. And bad dreams on my part occasionally even 25+ years later...

 

[MajBach]

Well, thanks everyone. I more or less got what I was looking for. Seems it's no simple answer and a reminder not to believe everything you read.

Stay tuned for my next inquiry: the taller the bike, the tighter you turn....

 

[wfooshee]

Gyroscopic forces have nothing to do with steering a bike, and absolutely nothing to do with it staying upright.

The bike turns because the steering is turned towards the direction you want to turn. That's not anything you did at the handlebars, it's the result of the rake and trail. You push the bar on the side you want to turn to, which induces the lean, and during the lean the rake/trail puts the wheel into the turn. Look down at the triple tree while in a 40-degree lean and you'll see. OK, maybe point the video camera at the triple tree..... The lean also induces an effect from the rake/trail combination that forces the front sideways more than the steering angle alone.

Counter-steering doesn't stay in place. It induces lean, then the rake/trail effect puts the wheel at a certain angle. You want to turn harder, you need more lean, so you push the inside bar again. Even as you hold pressure on the inside bar, the steering angle changes with the lean. If you pushed the bar right 5 degrees and then held it 5 degrees to the right, you'd lowside to the left pretty hard.

As for two wheels staying upright, it ain't gyroscopes. The bike always wants to fall one way or the other. As it leans, the front wheel steers into the lean to put the center of gravity back between the wheels rather than to one side or the other. The bike stays upright by steering into the fall. When you want the bike to turn, you push the bar to keep the lean going, but the bike is still turning towards the fall, keeping itself off the ground. When you're through turning, you remove pressure from the bar and it stands up. By itself. If bicycles and motorcycles didn't do that (stay up by self-steering into the fall and forcing the center of gravity over the line between the wheels) then riding no-hands would not be possible.

 

[mcatrophy]

Agree with wfooshee 100%.

Many moons ago I read about an experiment done with a bicycle where a second wheel was fixed beside the front wheel, geared to it so as to rotate on the opposite direction, and raised so as not to touch the ground. There was then no gyroscopic action from the front wheel. Riding and steering were apparently unaffected apart from the additional weight.

I've also taken some videos with a camera mounted on top of the brake fluid reservoir. The top of the front fairing and screen mount is visible in the frame. I was very surprised at how little movement of the bars is involved in normal riding, it is almost indiscernible, even the initial counter-steering. Except for the bike's lean, it would be impossible to say whether you were in a straight line or cornering from the video.

 

[eflyguy]

Yeah, I've noticed having my phone on a mount fixed to the frame (bolts to tank hold-down) positioned right between the bars, that they rarely seem to move relative to it.

 

[Denver_FJR]

Some of us know enough about physics and formulas to be dangerous, and some of us know little at all, I'm in that category me thinks. It seems to me that Lee is using an analogy incorrectly to arrive at a desired conclusion. For instance, a motorcycle tire has a round left to right profile, if properly inflated, it is only slightly flattened under weight onto the road surface. The contact patch location on the front tire moves to the side the bike is leaning toward, which induces more rearward force on the inside fork and less on the outside fork. I think that's how a bike can be steered simply by leaning it with no hands on the bars. \

If what you say is true then a slight imbalance in fork spring rates left/right will cause a motorcycle to "pull" to one side when upright like an automobile with poor wheel alignment. However, we know that is not what happens. Dirtbikers commonly mix spring rates they have on hand to get a rate they do not. Some motorcycles have only one spring in one fork leg. Many do compression damping in one leg, rebound in the other.

The typical fork assy is rigid enough to overcome those imbalances which forces both forks to move at the same rate with each other. With a rigid enough assembly, one heavy enough spring and damper assy. on one side or the other would work just as well. In a balanced fork assy there is less induced drag on the bushings which act to keep the fork sliders lined up.

Hear, hear! As also proven by bikes with a single disk on one side (ahem, rear FJR) or even driven from one side (ahem, FJR).

 

[Denver_FJR]

....
Now... the real question is: Would a stationary motorcycle on a conveyor belt turn with no forward momentum?

Based on my use of one of these (with bicycle), I would say "yes!"

You can even crash on these things.

 

[NTXFJR]

^^^ It's not a conveyor, and the bike's wheels are not stationary.........

 

[Denver_FJR]

It's essentially the same as a conveyor belt as the front and rear rollers are connected by a belt.

Bike's wheels are not stationary? Well if they were it would just be a parked bike.

 

[wheatonFJR]

Yawn.

<Turns key on, starts bike, rides twisty toads, smile appears on face, obliviously ignoring the physics involved>

Whoa...dozed off for a minute...did I miss anything?

 

[wil780]

wfooshee wrote:

Gyroscopic forces have nothing to do with steering a bike, and absolutely nothing to do with it staying upright.

The bike turns because the steering is turned towards the direction you want to turn. That's not anything you did at the handlebars, it's the result of the rake and trail. You push the bar on the side you want to turn to, which induces the lean, and during the lean the rake/trail puts the wheel into the turn. Look down at the triple tree while in a 40-degree lean and you'll see. OK, maybe point the video camera at the triple tree.....

The lean also induces an effect from the rake/trail combination that forces the front sideways more than the steering angle alone.

Counter-steering doesn't stay in place. It induces lean, then the rake/trail effect puts the wheel at a certain angle. You want to turn harder, you need more lean, so you push the inside bar again. Even as you hold pressure on the inside bar, the steering angle changes with the lean. If you pushed the bar right 5 degrees and then held it 5 degrees to the right, you'd lowside to the left pretty hard.
As for two wheels staying upright, it ain't gyroscopes. The bike always wants to fall one way or the other. As it leans, the front wheel steers into the lean to put the center of gravity back between the wheels rather than to one side or the other. The bike stays upright by steering into the fall. When you want the bike to turn, you push the bar to keep the lean going, but the bike is still turning towards the fall, keeping itself off the ground. When you're through turning, you remove pressure from the bar and it stands up. By itself. If bicycles and motorcycles didn't do that (stay up by self-steering into the fall and forcing the center of gravity over the line between the wheels) then riding no-hands would not be possible.

The overall picture given here is correct. It's a big advance over the thinking you see in the motorcycle press to realize that the bike turns because the front wheel is pointed into the corner. However, the details are not correct. It's far more complicated. The biggest issue with this simple picture is that the effect of rake/trail is backwards. Once you're leaned over, the effect of rake/trail is to steer the bars away from the turn, not into it.

In an established turn to the right, the contact patch of the front tire generates a force to the right, which is transmitted up the forks to the steering head, and through the frame to the entire bike. That's (mainly) what turns the bike to the right. Unfortunately for wfooshee's picture, the contact patch is behind the steering axis (that's basically the definition of trail). The force the contact patch generates to the right has a component created by trail that turns the bars to the left (because of trail). So it can't be the force that steers the front tire into the turn. Rather, it's camber thrust that does that job. Along with a bunch of other minor effects that in total give you either a neutral handling bike, or one which needs constant steering force to hold it down in the turn. (It could be tuned to have to hold the bike up, but that's pretty rare.)

A minor point: It seems to be accepted in the community researching these topics that 20 - 30% of the stability of a bike comes from the gyroscopic effect. So it's not very important, but it's not zero. This is part of the reason why the faster you go the harder you have to push on the bars to lean the bike - the gyroscopic effect increases with speed. I suspect the rest of the increase in push required is related to trail, but I can't see exactly how that happens. Understanding these issues is near the top of my bucket list, so I'd be grateful for any suggestions.

 

[wfooshee]

I think what I said about the rake/trail is what you said, when I said the initial countersteer doesn't stay there. What I said was the rake/trail combination steers the bike, not that it steers the wheel.

Camber thrust is a better description of what I was trying to say, but that in itself is a result of the rake/trail geometry, I think. Compare a huge chopper with the front wheel 5 feet out there, and it can only lean 15 degrees. How hard is that going to corner? There's an ideal rake in there somewhere, you certainly don't want vertical forks, and there's the right amount of trail, too. How those affect the bike in physics is a little over my head, but I'm thinking that what I said is not quite what you thought I said, so we're on the same page.

What you described, as the rake/trail combination trying to turn the wheel out of the turn is what I described when I said that you let off the handlebar pressure and the bike stands up by itself. You do maintain pressure on the bar on the side you want to turn towards, but you don't actually maintain counter-steer (wheel actually pointed the other way) as many people seem to think. The pressure keeps the wheel from straightening, but it's still steered very slightly towards to turn direction. Only on the initial tip-in is the steering head actually pointed the other way, and once lean is induced the wheel steers back into the turn. It does want to center so you hold pressure, but you don't hold an opposite angle on the steering head. That leads to road rash.

 

[Old Guy]

Actually, you turn a bike the same way at any speed, it is just more pronounced at higher speeds.
You turn the bike by getting it leaned in the direction you want it to go. You do this by steering the front wheel in the opposite direction you want to go. Once the bike has some lean angle the front wheel wants to turn itself in the direction that it is leaned due to the steering angle trail force and gyroscopic force of the tilting wheel (try tilting a horizontally spinning gyroscope some time). That is why you have to keep pushing on the inside bar (or pulling on ionbeam's outside bar) to stay in the turn.

Even at slow speeds, if the wheel didn't turn itself (or you didn't help it) back into the turn after the initial counter-steering, the bike would just fall on its side. You could demonstrate that by locking the wheel straight forward on a bicycle. You can get it to lean one way or the other by shifting the weight but it won't turn and you will fall on your side.

I like your point in the first sentence. I've heard it said often that countersteering doesn't work at less than some slow speed -- people usually say 5 mph. But I always try to put my left foot down and leave the right on the peg. I know I'll invariably come to a stop and just as I stop I'll cock the front wheel right very slightly to make it lean left so I can get that left foot down. Countersteering will lean my bikes even stopped.