The Bum Steer

[Mikkel]

I am lagging behind the discussion yet again. However, being a pedantic SOB, I think the following is worth a reply

No worries

Forest has it right.
I have never argued that precession was not a factor in countersteering. What I have contended is that it is NOT THE MAJOR FACTOR.

Well, to me it sounded like you guys were arguing that the gyroscopic forces were not a factor...

As evidence, I made mention of a bike that had been created with contra-rotating counterweights (which would negate gyroscopic forces) but which reportedly countersteered just fine.

And I just said that making something like that is bloody difficult since the counter rotating weights would have to have exactly the same rotational inertia AND rotate around the same axis as the tyre in order to counter the rotational momentum of said tyre. I'm not saying it's impossible to make such a machine - but it's pretty fucking difficult though! So chances are that you could get it wrong... And my argument was that if you didn't notice any significant change in the behaviour of the bike (especially if you cancel both front and rear tyres) - then I'm 100% sure you haven't succeeded in countering the rotational momentum properly.

You contend that such a bike is unlikely because it is Gyroscopic stabilisation which makes it easy to ride a bicycle and so getting rid of it would make the bike difficult/impossible to ride.

My comment on this was that one only needs to annul the angular momentum of the front wheel to investigate the part that precession plays in countersteering. This leaves in play, the gyroscopic forces generated by the rear wheel and the motor and so such a bike would not be particularly difficult to ride at all.

Indeed - with no balancing from any rotational momentum it would be very difficult (i.e. countering both front and rear).
And yes - the balancing from the rear wheel would be adequate to stay upright. However, riding at very slow speeds ought to be more difficult than usually.
I agree that the trail would stabilise the front wheel and prevent it from changing direction.
However, I'm confident that steering input would be very much different than usual. I'm not going to say that you couldn't use countersteering to lean the bike over - changing the geometry of the bike will have an effect on what the bike does... So much is obvious.

If you watch the videos I posted earlier you can see how even small gyroscopic forces seemingly can suspend gravity... If that does not clearly illustrate how powerful these forces are then I don't know what will. Also, the video with the double gyro illustrates exactly what happens when you counter steer - you apply a small torsional force upon the rotating mass and it tilts along the third axis.

As for the engine helping with balancing the bike - I doubt very much that the effect of the crankshaft would be measurable compared to the flywheel (I assume bikes have flywheels here...).

[Bass]

It seems I spoke too soon. Just seemed logical at the time. What you say makes a lot of sense.

If you want to test the effect the engine has on your stability, next time you are being knocked around by a cross wind, change down a gear of two but maintain your speed. You may be surprised.

No worries
And I just said that making something like that is bloody difficult since the counter rotating weights would have to have exactly the same rotational inertia AND rotate around the same axis as the tyre in order to counter the rotational momentum of said tyre. I'm not saying it's impossible to make such a machine - but it's pretty fucking difficult though! So chances are that you could get it wrong... And my argument was that if you didn't notice any significant change in the behaviour of the bike (especially if you cancel both front and rear tyres) - then I'm 100% sure you haven't succeeded in countering the rotational momentum properly.

Firstly, I agree that it is difficult, although I can think of several ways in which it could be achieved - the radial aero engine in the front of our aeroplane does all the things you describe as far as concentric motion goes. Further, it seems to me that if someone was prepared to put in the work to do it, they would put in the time to do it right.
Don't you think that your claim that because it did not perform the way you expected then they got it wrong, sounds just a bit arrogant?
Having said that, it took me by surprise too and I was just as difficult to get to derail my train of thought.
Whether the guy got it right or not quite right, it still seems to me that if the rider was able to countersteer the machine AT ALL, then my point is proved. Instead, the report was that it READILY countersteered and so surely my point is doubly proved
Any handling difficulties encountered seem to me to be completely irrelevant to the question of what the predominant reason for the countersteering phenomenon is.
It could be a total bastard to ride but if it readily countersteered with little or no nett angular momentum at the front wheel, then the major reason for countersteering happening cannot be gyroscopic precession.
I would still like to be convinced otherwise however, because I like neat and tidy theory.

If you watch the videos I posted earlier you can see how even small gyroscopic forces seemingly can suspend gravity... If that does not clearly illustrate how powerful these forces are then I don't know what will. Also, the video with the double gyro illustrates exactly what happens when you counter steer - you apply a small torsional force upon the rotating mass and it tilts along the third axis.

I disagree.
Surely, within the scope of this discussion, these machines obey Newtonian mechanics with the proviso that because of the rotation, the output is perpendicular to the input.
Consider the bike wheel suspended by string at the end of the axle. It rotates slowly around the string.
Why only slowly?
Only small frictional forces retard it.
If the force were large it would rotate rapidly around the string.

In this case, because the axle is short, the string takes most of the weight and the precessive force is small.

You get out what you put in - the gyroscopic forces are as strong as you make them.

I enjoyed your videos largely because I remember carrying out ALL those experiments in the physics lab at high school. The double disc gyro that we used was actually the master gyro out of a Lancaster bomber.

As for the engine helping with balancing the bike - I doubt very much that the effect of the crankshaft would be measurable compared to the flywheel (I assume bikes have flywheels here...).

Yes most bikes have a flywheel but it is usually quite low mass
The cranks are usually a disc and pin arrangement to reduce the width of the engine. The do not have a huge mass but they are solid enough and at the speeds the modern engines spin, the crank has a quite noticeable effect.

[Ixion]

Instead of sitting behingd a friggin puter screen why not get on ya bike go for a ride and find out?
find a quiet bit of road and experiment ?

Because, when I was much younger, and know-it-all even younger types started spouting on about this new counter steering that had just been invented, I did indeed get on my bike and perform quite a few experiments on a quiet bit of road (off road actually) . Such conclusions as I reached were at the expense of no little pain and bruising. I prefer to stick to theory nowdays.

[Ixion]

Think that's what he was getting at, the bars need to move AND the bike to lean.

Actually, just to chuck in a wee red fishy thing, lean angle is only relevant at countersteer speeds, you can turn a trials bike the opposite way to the lean, simply by compensating with body weight to maintain the CG over the contct patch.

Edit: I think that's be possible at higher speeds too, if you could get enough mass inside, but I ain't trying it.

No I think (but my head as asploded from all this so I haven't really thought it through), that you only need the bars to move (and thus the wheel. When that happens, gyroscopic effect will cause the lean. But the lean on its own , with the bars kept straight ahead does nothing - my example of pushing your bike to the gas station , leaning it inward toward you to make the pushing easier.

I think that is why the counter rotating thing doesn't work , too. The lean on a bike doesn't CAUSE the bike to corner. It's a RESULT of the bike cornering , OR the thing that initiates the cornering (by causing the wheel to counter steer). So , I think, in theory, fixing counter rotating weights to both wheels (needs to be both) , you could still counter steer the bike, and doing so would make it corner , but it would stay upright (you could still cause it to lean, though, by hanging off etc). In fact , didn't Honda make a little bike that had a built in gyro that did exactly that ? Called, in fact, the Honda Gyro ?

[GSVR]

Ever missed a downchange and hit a false neutral on the track going into a high speed corner where you engine would pull high revs on the downchange.

I don't know if its the sudden loss of revs and consequent loss of motor gyro effect or just the dissengagement of the rear wheel to the motor but things feel very different and you usually end up running wide or in some cases off the track if you had a deep line.

[Sully60]

In fact , didn't Honda make a little bike that had a built in gyro that did exactly that ? Called, in fact, the Honda Gyro ?

What?!

Do you mean those three wheeled things with roof's the Hell Pizza guys ride have got gyros? Is that to keep pizza level to stop the toppings form coming off?

[Deano]

I don't know if its the sudden loss of revs and consequent loss of motor gyro effect or just the dissengagement of the rear wheel to the motor but things feel very different and you usually end up running wide or in some cases off the track if you had a deep line.

It's all in ya head.

There's no such thing as countersteering or weight shift - just don't think about it and you'll be sweet.

[Ixion]

Actually, after thinking a little more ( one beer's worth more, actually), I think the major effect of gyro force on a motorcycle , is not to enable it to corner, but to prevent it cornering. Or, more exactly, to stabilise it so it doesn't fall over.

Stationary, or at low speeds, a bike falls over. We all know that, those of us with short legs more than most. Go fast, it doesn't fall over. Or, not as easily. Except for Gixxers of course.

The reason being that gyro stuff. If the bike tries to fall over to one side or other, gyro force then wants to move the handlebars and make the bike go in a curve. But this force is resisted, and controllably so, firstly by the stability effect of the trail (as someone mentioned) and if the is not enough (eg, strong winds gusts) by you, the rider, pushing on the bars to prevent it.

Incidentally the 'pushed over by strong winds, feel it through the bars' effect is an evidence of the point I made earlier, that lean causes steering, not just the other way round.

[Sully60]

What about Segways?

Apart from them being extremely ghey!

[pete376403]

10 - 20 kilos of crank spinning at 8 -10,000 or better is going to have a LOT of gyroscopic effect.
One of the reasons GP bikes (Yamaha OW70, Cagiva, went to twin cranks was that, as they were contra-rotating, they cancelled each others gyro effect.
Quote:".. Ever since '84 there had been dark mutterings about 'single-crank voodoo' and a growing belief that the crank's gyro effect was responsible for the bike's wayward handling. In other words, crankshaft inertia made it hard work to steer the bike from its current course. Yamaha ran contra-rotating cranks, which canceled out any gyro effect, HRC thought that was the reason the Yamaha handled better." http://www.superbikeplanet.com/NSR500.htm

[RearWheel]

From my limited experience (39yrs riding) with only 1 binning to my name (should i feel ashamed?) I have found from personal experience that its simply a combination of body language and counter steer that gets you round the corners. For myself going into a corner i very subtley use a combo of both at the same time and once fully in i use body weight and position to keep it where i want it..... except where a sudden line change is called for say in mid corner and then counter steer is really your only option for a quick reaction.... think about it , its quicker to tweek the bars mid corner than it is to move ur body to change your line in a hurry when you want to avoid that rock or possum laying right in your line. All the theries about gyro forces dont mean squat when ur 1 second from disaster, you just do what ya gota do to avoid bining your baby and busting up ur arse. In a word just do what comes naturally which may be different things for different riding levels , but you sure cant get to the higher levels without having a few momments and those "close calls" are what gives you the experience to go on and be a more aware more skilled rider...... live in peace ride in safety and learn from your mistakes.
Oh and one more thing .... dont let anyone ever goad you into riding past ur experience level...... I have much respect for one young fella ( Hornetboy) whom i had the pleasure of riding with last wednesday and i was razzing him a wee bit about riding like a nana (just kiddin with ya of course) but he was riding at a level he was comfortable with..... and thats the key to it ... keep ya head up and wheels down..... cheers

[007XX]

And this from an ex dirtygirl. *sigh*

I figure you already know, lean the bike and keep your body weight over the tyre's contact patch. Helps if you load up the uphill peg too, same as in the dirt. In fact I'm embarrased to say I sometimes do it standing up Just seems easier to pretend the Buell's a KT250 No, don't do that, but there might be a clue there about weight transfer eh? And throttle control.

Appart from that, it's just an afternoon in a carpark, making with the little wee circles.

yeah, yeah I know...

It all seemed so much simpler in the dirt...*sigh* Oh well, definitely will get my arse in a carpark practicing then.

Mind you, the only time I did try a tight U turn, I was exhausted after 4 days pretty much solid of riding, so it could have had something to do with it.

[Mikkel]

Don't you think that your claim that because it did not perform the way you expected then they got it wrong, sounds just a bit arrogant?

Arrogant - perhaps. But I do happen to have a good friend who's both a genius and very much into doing what he calls a "Science Show" wherein a lot of the more freaky and showworthy physical phenomenon are put on display. Anyway, this fella once rode a bicycle where they had fixed a set of counter rotating wheels that matched the wheels on the bike 100%.
He said it was quite impossible to ride - and you would be all over the place with no stability at all.

Whether the guy got it right or not quite right, it still seems to me that if the rider was able to countersteer the machine AT ALL, then my point is proved. Instead, the report was that it READILY countersteered and so surely my point is doubly proved
Any handling difficulties encountered seem to me to be completely irrelevant to the question of what the predominant reason for the countersteering phenomenon is.
It could be a total bastard to ride but if it readily countersteered with little or no nett angular momentum at the front wheel, then the major reason for countersteering happening cannot be gyroscopic precession.
I would still like to be convinced otherwise however, because I like neat and tidy theory.

Not really. Unless he got it exactly spot on there would still be gyroscopic forces at play, albeit weaker. And then you could just as well argue that it just illustrates how important these are.

Please note that I haven't at any point said that countersteering is ONLY due to gyroscopic forces. However, I'd have to see something more substantial than a vague claim, with no references, that someone has proven gyroscopic forces not to be a major factor in the mechanics of motorcycles before I'm willing to accept it.
Because, as you say, the theory is neat and tidy - not easily understood in a compound 3D environment - but still it is pretty much bullet proof.

I disagree.
Surely, within the scope of this discussion, these machines obey Newtonian mechanics with the proviso that because of the rotation, the output is perpendicular to the input.
Consider the bike wheel suspended by string at the end of the axle. It rotates slowly around the string.
Why only slowly?
Only small frictional forces retard it.
If the force were large it would rotate rapidly around the string.

In this case, because the axle is short, the string takes most of the weight and the precessive force is small.

You get out what you put in - the gyroscopic forces are as strong as you make them.

Still, as you say the axles are short which means that gravity working on the COM will only provide a small amount of torque compared to the rotational inertia of a bike wheel with a fairly large radius.

I'll even expand and go in utter nerd mode and throw equations at you just to prove my point. Neglecting the mass of the axis itself and assuming the weight of the bicycle wheel, m, can be described as a toroidal mass we have an rotational inertia, I, equal to m*R^2 where R is the radius of the wheel. Given that l is the length of the axle and the gravitational acceleration is g we have the following expression for the torque, t, around the point at which the string is affixed:

t = l/2 x m*g

The change in rotational momentum, L, is given by:

dL/dt = t = l/2 x m*g

If we let F describe the angular frequency around the vertical axis and f decribes the, constant, angular frequency of the wheels rotation around the axle we get the following:

dL/dt = F*f*I = l/2 x m*g
<=>
F = l*m*g/(2*f*I) = l*g/(2*f*R^2)

The last formula illustrates the motion - note that the weight of the wheel cancels out due to our assumptions and the fact that it's gravity providing the torque.
The rate of precession grows linearly with the applied torque - e.g. axle length and gravitational pull - however there's an inverse relation with the angular frequency and an inverse square relation with the radii of the wheel.

So, yes you get what you put in (linear relationship) - but the bigger the gyroscopic forces the harder you have to work to achieve the same result. And so, in the case of the video where R is much larger than l it's no wonder that the rate of precession is small.

Sorry for being long winded - and even more sorry I couldn't be arsed making a sketch...

[GSVR]

Arrogant - perhaps. But I do happen to have a good friend who's both a genius and very much into doing what he calls a "Science Show" wherein a lot of the more freaky and showworthy physical phenomenon are put on display. Anyway, this fella once rode a bicycle where they had fixed a set of counter rotating wheels that matched the wheels on the bike 100%.
He said it was quite impossible to ride - and you would be all over the place with no stability at all.

This is interesting and had me think of a different way to reverse the steering inputs. What if the steering wheel was the one on the back of the bike?
Then to get a gyroscopic induced lean you would have to turn into the corner if you get the idea. Still away but the away movement would be turning the corner.

[Badjelly]

What if the steering wheel was the one on the back of the bike? Then to get a gyroscopic induced lean you would have to turn into the corner.

Many years ago I made a skateboard with a castor wheel, the sort they use on furniture. (I'd already worn out the one I made earlier with my sister's roller skates.) I put it at the top of the hill with the castor wheel at the back and stood on it--it swapped ends on me. There was no way that thing would go down the hill with the castor on the back, but it was perfectly happy with the castor at the front.

So, I think the answer is that a bike with the steered wheel at the back would insist on swapping ends to bring the steered wheel to the front.