The Bum Steer

[Deano]

Just because you don't notice it, doesn't mean it's not happening.

And on that note - just because you can't see the GP stars doing it, doesn't mean they aren't.

I would go so far as to say that in the tight flip flop chicanes, these guys are literally wrenching the bars to change direction in time.

[Ocean1]

You're pulling my leg. I see

At what speed do you consider a bike to behave “normally” with regard to control inputs. In other words using other than lateral body movement alone to maintain ballance and affect course?

How much vertically stabilising force do you suppose gyroscopic precession is contributing at that speed?

And on that note - just because you can't see the GP stars doing it, doesn't mean they aren't.

I would go so far as to say that in the tight flip flop chicanes, these guys are literally wrenching the bars to change direction in time.

And taking advantage of the vertical kick between turns to shift their arse? Bloody difficult to do once you’re into the corner with under much higher G.

[James Deuce]

You can't turn a bike without a counter steering input at the bars.

"Body English" for steering is pure and simple rubbish. Next time you want to steer into a turn without using counter steering, take your hands off the bars, shift your bum in the seat and we'll record your howls of dismay as Rotational Momentum and inertia keep the bike going straight ahead. As your speed decays, there will be a point where gravity overwhelms rotational momentum and inertia and the bike will flop from side to side if you move your weight and you'll turn just before you fall over.

[Steam]

You can't turn a bike without a counter steering input at the bars.
"Body English" for steering is pure and simple rubbish.

What??? Try it next time you are in a safe deserted place, take your hands off the handlebars and just shift your weight slightly. I swear it works. Doesn't it?

[Forest]

I am about to deal to another sacred cow.
I too believed that countersteering worked because of gyroscopic precession.
However, about 18 months ago there was a thread in here that argued this quite vehemently (can't find it at the moment). Anyway, the whole thing was settled when someone quoted an article concerning a guy in the states a couple of years back, who built a bike that had counter-rotating wheels within the normal ones. This, of course, had the effect of cancelling out any gyroscopic effects at all.
He found that the bike handled pretty much as any bike does, i.e. that counter steering still worked and so gyroscopic forces (which do explain the phenomenon) are not the major reason for it.
The conclusion reached was that if (for example) one wishes to turn left and so counter steers to the right. The tyre contact patch moves right and the bike rolls about its own centre of mass and so leans to the left.
Further, for reasons that I can't remember, precession does make some contribution to standing the bike up again when exiting the corner. I presume it has something to do with acceleration.
I shall continue hunting for the thread that I mentioned.

I think the web-site you are looking for is:

http://www.reverserotatingrotors.com/

Here is a link to the patent:

http://www.google.com/patents?vid=USPAT6918467

[MSTRS:Me]

Exactly, Jim2. All the weight changing in the world will only make the process smoother and more stable, it is the bar work that really counts.
Try riding in a straight line when you've been working the bar heavily

[Sully60]

You can't turn a bike without a counter steering input at the bars.

"Body English" for steering is pure and simple rubbish. Next time you want to steer into a turn without using counter steering, take your hands off the bars. shift your bum in the seat and we'll record your howls of dismay as Rotational Momentum and inertia keep the bike going straight ahead. As your speed decays, there will be a point where gravity overwhelms rotational momentum and inertia and the bike will flop from side to side if you move your weight and you'll turn just before you fall over.

How does that explain wheelies around corners or circular wheelies were the front wheel has no contact with the ground?
Off the general direction of topic but I'd like to hear your explanation.

Edit: sorry for crappy picture, but usefull to expalin my point

[Forest]

How does that explain wheelies around corners or circular wheelies were the front wheel has no contact with the ground?
Off the general direction of topic but I'd like to hear your explanation.

Edit: sorry for crappy picture, but usefull to expalin my point

When both wheels are on the ground, the stability comes from the trail of the front wheel.

Basically the friction of the front tyre behind the steering axis causes the front tyre to "self correct".

Here is an article that explains things much better than I can.

http://wellyouneednot.blogspot.com/2...intuitive.html

When you lift the front tyre during a wheelie there is no longer any trail or correcting forces on the front wheel. So the only way you can steer is by changing your center of gravity i.e. moving your body weight.

[vifferman]

How does that explain wheelies around corners or circular wheelies were the front wheel has no contact with the ground?
Off the general direction of topic but I'd like to hear your explanation.

That's a little different - the steering is accomplished by a controlled fall (and recovery) in the direction you're steering in. You're moving the C of G towards the intended direction of travel.

[Sully60]

When you lift the front tyre during a wheelie there is no longer any trail or correcting forces on the front wheel. So the only way you can steer is by changing your center of gravity i.e. moving your body weight.

You're moving the C of G towards the intended direction of travel.

Which was exactly my point, I cannot countersteer with no front wheel contact but I can alter the direction of the bike.

"Body English" for steering is pure and simple rubbish

For steering/turning alone with no counter steering I agree, but the simple physics are that the rider must make up on average at least a third of the total weight of both rider and machine, and being perched on top of the "lever" moving your weight will influence the direction of the bike.

I think Grubs reference to the racers is maybe a little misguided in this instance, He has made no mention of how the riders distribute their weight with their feet. Although they have there body mass weighted to the inside of the turn most of their weight will be on the outside footpeg, just ask #34 (Kevin Schwantz)

[James Deuce]

Keith Code built a ZX6R to refute the leaning/body weight movement theory of motorcycle steering once and for all.

http://www.superbikeschool.com/news/...ke_release.php

http://www.superbikeschool.com/machi...bs-machine.php

There are a huge range of influences on motorcycle dynamics and they aren't understood very well as a whole. Moments of inertia shift as a bike steers, the effect of gravity on a leaning object changes how it behaves in a corner, the geometry of the forks dictate at what speed counter steering takes over from "normal" steering, braking and acceleration change a bike's chassis geometry in relation to the surface it is traveling on, and so on. But to overcome gyroscopic precession for a two wheeled, tandem in-line vehicle you HAVE to turn a wheel opposite to the direction you want to turn to initiate a lean in that direction.

Distributing your weight changes how the chassis responds to input. To steer you HAVE to countersteer once you are traveling at more than 10-25 kph depending on motorcycle geometry. The faster you go and the more your bike weighs, the greater the force required to initiate a turn. Once the bike has turned the motorcycle will maintain that attitude until another force acts on it to change its attitude.

You need to learn what force you need to make a bike lean to a particular angle when countersteering. This will take a huge amount of stress and guesswork out of your Sunday blats if you can precisely dial up a particular angle of dangle for a given speed and a given turn radius. Racers get really good at this because they get to practice the same corners over and over on a race track. They turn harder and later and spend less time loading suspension up on the way into a turn and as a consequence don't have to lean as far for a give speed.

[Deano]

What??? Try it next time you are in a safe deserted place, take your hands off the handlebars and just shift your weight slightly. I swear it works. Doesn't it?

Try going around anything other than a very shallow bend without your hands on the bars.

I can do it down Oakleigh but it takes practice and a lot of body shift - and it is a very shallow bend.

[Deano]

And taking advantage of the vertical kick between turns to shift their arse? Bloody difficult to do once you’re into the corner with under much higher G.

Cycosis beat White Trash at Manfeild (just) - ask Jimmy how much Cycosis moved on his seat.

He will no doubt have moved slightly, but not to the same degree as Jimmy, yet still managed to go as quick, if not quicker through the corners...how did he do this ?

Counter steering.

[Deano]

So the only way you can steer is by changing your center of gravity i.e. moving your body weight.

Pretty sure the guys I know use body weight shift AND turning the bars, especially while the front is still spinning.

[James Deuce]

Exactly. Why waffle about with vague and minor angles of lean for a lot of body weight shift when you can precisely dial up the angle of lean you want through your bars? The faster you go the lesser the amount of lean you will generate for a larger and larger amount of weight shift.