Hey there.... need some help!

[Doofus]

OK so I probably didnt phrase my question well... But what makes it go round a corner...? . What makes the bike corner (as opposed to what initiates a turn)...

So far I see two answers

This seems to contradict itself... How

Cool But is it different for high speeds?

I always thought it was more to do with the tyre profile and the fact that there is a larger circumference at the centre of the tyre and a smaller on the outside... Therefore, a bit like putting a cotton reel on a pencil and rolling it over a desk, it will go round a corner due to the difference in diameters....

Please help me out cos this is really bugging me now

Hi

The force that makes you turn when leaning is gravity. We need to think about what is happening to the centre of gravity/mass of the bike (PLUS rider!), not what is happening to the wheels.

Ignore the fact the bike is moving forward for now. When leaning, the bike's centre of mass wants to drop because of gravity. The centre of mass cannot fall directly down, it needs to rotate down about where the wheels touch the road. This means at any given instant gravity (combined with the reaction force at the tyres on the road) is urging the centre of mass to move diagonally downwards (perpendicular to a line joiing the cetre of mass to a line connecting the contact points for the two wheels) - i.e. there is both a force component straight down, and a force component sideways. It is the sideways component that pulls you over.

However, now imagine the bike is going forward and leaning. We have a sideways force applied to a centre of mass that is moving forward - this continued sideways force is what maintains the turn. You don't fall over as it is balanced by the centrifugal force (basically an invented force associated with rotational frames of reference). In reality, the bike is always falling, but being "caught" by the turning motion. The same sort of thing as with orbiting spacecraft - they are allways falling (hence feeling of no gravity) but stay at the same altitude due to their forward motion providing for a centrifugal force.

In summary, it is the inertial forces on the centre of mass that cause the turn, not forces from the tyres on the road. In fact the tyres on the road are forced to do something they don't naturally want to do by the inerial forces on the centre of mass - this puts stress on the tyres (i.e. applies forces to them), and if the forces are high enough the tyres give up and you skid.

The tyre profie is relevant to the extent it is a tangent to the profile at the point of contact that defines the natural lean angle for that contact point. This is why a squared off tyre feels bad as there is a sudeen switch in natural lean angle when you roll over sideways. For a given constant lean, the profile is irrelevant as the tyres only contact at one place,

Again, this is my understanding.

Darren

 

[Wapping]

But... What makes the bike corner (as opposed to what initiates a turn)...

Once the bike is leant over in the turn, it is going along (at least as far as the bike, it's geometry, and physics sees it) no differently than it does whilst upright, it's just at a different angle. If you don't wonder about the 'Reasons why...' when its upright, why wonder about them when its trundling along, leant over?

To quote Keith Code's excellent book:

As long as you continue pushing or pulling on the bars, the bike will continue to lean over and turn more sharply. When you ease the pressure on the bars, the bike will stay at the lean angle you reached when you eased up the pressure. You don't have to hold the bike into the turn with any great amount of force, if at all. On most bikes, once you have the machine at the desired lean angle, you no longer need to hold onto the bars. Actually, if you were riding at 60 mph on most bikes with a cruise cotrol in a huge open parking lot with no bumps, you could take your hands off the bars once you had it leaned over and the bike would continue going around in a circle until it ran out of gas.

When you find yourself holding on very tightly in turns, you're doing a whole lot of unnecesary work. Bumps and other irregularities can change the situation of course, so you do have to hold on to make steering corrections.

At the end of the bend, the bike needs another countersteer action, to bring it out of the turn, from leant over to upright. Similarly, if the bend tightens, you need to countersteer again, increasing the bike's lean and sharpening its angle.

 

[GSmonkey]

Countersteering: You all know how to do it, if you didn't, you'd fall off. The theory is simple, the practice is simple. Never understood why people bang on about it so much.

Rear wheel lockup: I had a fuel pump failure which caused a rear lockup going into a corner. I was accelerating when fuel pump started to throw a wobbly. Lost a lot of power so aborted the overtake which was against an oncoming double decker.

Decided to put the bike into second for the corner ahead. Upon gearchange the engine stalled (due to fuel pump) and rear wheel locked. Took me half a second or so to work out what was going on and disengaged the clutch and coasted around the corner.

A more nervous rider may have crashed. Not all lockups are pilot error.

That said, the boxer engine does provide a lot of back-torque and rear slides on imperfect changes are common. Premeditated induced slides can also be a bit of fun The IL4 sportsbikes are becoming more sanitised in this respect. My gixxer had virtually no back torque and had a slipper clutch as well. I tested it be dumping the bike into first at 80mph with no blipping and it just gradually slowed down.

 

[Doofus]

Countersteering: You all know how to do it, if you didn't, you'd fall off. The theory is simple, the practice is simple. Never understood why people bang on about it so much.

Hi

I'd agree with all of this - except that the theory is simple.

I would hazzard the theory of countersteering is far from simple to anyone who hasn't had an applied mathematics or physics education.

Darren

 

[GSmonkey]

I would hazzard the theory of countersteering is far from simple to anyone who hasn't had an applied mathematics or physics education.

.....you're probably right. Seems like common sense to me though.

 

[Packer]

Four points here:-

1) Counter steering is really very simple, you steer the bike out from underneath you to allow gravity to start pulling you down, i.e. making it lean in the desired direction. The lean can be induced in other ways as long as you have enough time for the smaller forces concerned to take effect. We all know how to ride a bicycle around a corner without holding the handlebars and you could do the same with a motorcycle but due to it being much heavier it takes much longer for the desired result. At higher speeds you don't have enough time.

2) Once you have the desired angle of lean you have to maintain that by steering into the corner, you only steer opposite to the direction of turn if the back is sliding out or the turn is tightening and you need more lean.

3) The gyroscopic effect of anything rotating does not try and do anything except resist change. If you are upright the gyro tries to maintain this situation and if you are cranked over it tries to maintain that as well. Buy a toy gyroscope and play.

4) Apart form gravity, centrifugal force and gyroscopic forces every bike is different due to it's design, especially steering geometry. My 1150GS has very neutral handling at low speed, it doesn't seem to need much to force it into the corner and it doesn't need much to pull it back out. However when I had a Rockster as a loan bike I found that once I had tipped it into a slow speed turn it was trying to "fall in" and to tighten the turn. I had to concsiouly steer more tightly into the turn to pull the front wheel back under the bike or it would have gone down.

 

[Doofus]

Four points here:-

2) Once you have the desired angle of lean you have to maintain that by steering into the corner, you only steer opposite to the direction of turn if the back is sliding out or the turn is tightening and you need more lean.

3) The gyroscopic effect of anything rotating does not try and do anything except resist change. If you are upright the gyro tries to maintain this situation and if you are cranked over it tries to maintain that as well. Buy a toy gyroscope and play.

Hello

I am not sure I agree with (2) or (3).

For (3) a gyroscope does not simply attempt to resist change - it will precess. A gyroscope responds to an attempt to twist its spinning axis by applying a reaction-like force about an axis that is perpendicular both the gyroscope axis and the axis about which one attempts to twist it. Otherwise how could countersteering induce a lean? E.g., see http://www.exploratorium.edu/snacks/bicycle_wheel_gyro.html

For (2) once the lean is induced by countersteering we keep the wheel in the same place - we do not change it to steer into the corner - any attempt to change the wheel dirction would apply a force and change the lean. We only steer the wheel back to straight when we want to get out of the lean at the end of the turn. (The action of pulling the wheel back to straight applies an equal an opposite force to that than induced the lean in the first place and lifts us up.) If you actively think about what you're doing when you next come out of a sweeping lefthander you'll note you push with your right hand (or pull with your left).

regards
Darren

 

[Nate]

why wonder about them when its trundling along, leant over?

To quote Keith Code's excellent book:

Wapping, I wonder because I have a Phd in Physics (atmospheric) and I like to understand the physics behind things and understand how they work. I understand that it won't change how I ride or how I corner, but I will have understood it, therefore I will be content.

This Keith Code book, do you have shares in it?

Dazlewis, thanks mate... I will have a think about the vectors and which way things are being pulled... At the moment though, it all seems to be in balance, which it cant be, otherwise the bike would only go in a straight line... Here is my quick analysis...

Two sets of forces, one running through the Centre of Mass, other running through the contact patch of the tyre...

CoM
horizontal force towards the outside of the corner... (i.e. gyroscopic)
Vertical force downwards (i.e. gravity)

Tyre patch
Horizontal towards the inside of the corner (i.e. mechanical grip from tyre)
Vertical force upwards (i.e. reaction force from mass of bike)

so, turning must come from the difference of the forces between the front and rear of the motorcycle... Yeah

 

[GSmonkey]

Just consider the bike as a mass, CofG some way above the ground. Going in a straight line, CofG directly above and between contact patch of both tyres.

To go round a corner i.e. change velocity, you must apply a force. The force is applied either left or right by leaning the bike either left or right and moving the CofG away from the contact patches of the tyres. To lean the bike you countersteer i.e. turning the bars to the right will put the bike off balance and it will lean to the left. All very simple.

This is the general theory of how a bike goes round corners. Things that modify the overall effect are action of the suspension, profile of the tyres, rotational inertia of the wheels, etc. etc.

 

[Doofus]

Wapping, I wonder because I have a Phd in Physics (atmospheric) and I like to understand the physics behind things and understand how they work. I understand that it won't change how I ride or how I corner, but I will have understood it, therefore I will be content.

Dazlewis, thanks mate... I will have a think about the vectors and which way things are being pulled... At the moment though, it all seems to be in balance, which it cant be, otherwise the bike would only go in a straight line... Here is my quick analysis...

Hi Nate - small world - I did some post-doc work on solar physics / space-weather which links in with you atmospheric chaps nicely.

Regarding your concern about it all being in balance so you'd expect a straight line - don't forget that one of the forces needed for the balance is the centrifugal force, and this force only arises because you are going in a circle. There would be an imbalance if you had all the other forces and weren't going in a circle!

Darren

 

[Nate]

Hi Nate - small world - I did some post-doc work on solar physics / space-weather which links in with you atmospheric chaps nicely.

Regarding your concern about it all being in balance so you'd expect a straight line - don't forget that one of the forces needed for the balance is the centrifugal force, and this force only arises because you are going in a circle. There would be an imbalance if you had all the other forces and weren't going in a circle!

Darren

of course

Nice one on the space weather!!! Would have been useful to talk a few years ago. My original research was Radio Occultation within the Troposphere... Turned out to be a nightmare with signal drop out from the attenuation...

Ended up working on T Sky Brightness and equipment calibration... With a few new ideas on radio occultation but with a twist

So what is your field now?

 

[Deleted account 210609001]

This cornering business all sounds far too complicated for me. I think I'll stick to straight roads in the future.

 

[Wapping]

This Keith Code book, do you have shares in it?

Not at all. But have never found a better book for explaining the best way to ride a motorcycle (what does and doesn't happen) - but without all the, Half an MV squared = 2 pies arse cubed.

 

[Nate]

Half an MV squared = Square Root of 2 pies arse cubed.

fixed it

 

[Doofus]

of course

Nice one on the space weather!!! Would have been useful to talk a few years ago. My original research was Radio Occultation within the Troposphere... Turned out to be a nightmare with signal drop out from the attenuation...

Ended up working on T Sky Brightness and equipment calibration... With a few new ideas on radio occultation but with a twist

So what is your field now?

Hi Nate

Glad to see you stayed in the reserch field - I am afraid I sold out a few years ago and got a job in the real world (but I claim my job still makes me primarily a scientist so not a complete sell out).

My post-doc work related to cornal mass ejections / solar wind type stuff.

regards
Darren

 

[Packer]

Hello

I am not sure I agree with (2) or (3).

For (3) a gyroscope does not simply attempt to resist change - it will precess. A gyroscope responds to an attempt to twist its spinning axis by applying a reaction-like force about an axis that is perpendicular both the gyroscope axis and the axis about which one attempts to twist it. Otherwise how could countersteering induce a lean? E.g., see http://www.exploratorium.edu/snacks/bicycle_wheel_gyro.html

For (2) once the lean is induced by countersteering we keep the wheel in the same place - we do not change it to steer into the corner - any attempt to change the wheel dirction would apply a force and change the lean. We only steer the wheel back to straight when we want to get out of the lean at the end of the turn. (The action of pulling the wheel back to straight applies an equal an opposite force to that than induced the lean in the first place and lifts us up.) If you actively think about what you're doing when you next come out of a sweeping lefthander you'll note you push with your right hand (or pull with your left).

regards
Darren

Hi Darren, take a look again at the videos posted above, you will see clearly that with regard to (2) the front wheel moves clearly in one direction until the desired amount of lean has been induced and then the wheel is turned in the opposit direction so that the lean is maintained. If this didn't happpen the wheels would simply track out from below the bike and down you go. At higher speeds on wider radii corners the effect is less pronounced but it is still there.

With respect to (3) what I was pointing out was that you can spin up a gyroscope and place it at any angle to the plane of rotation that you choose and it will stay there as long as it has sufficient angular velocity. I do not dispute that when you try and move it forces become vectored differently. The comment was in repsonse to the statement that the gyroscopic effect of the wheels would try to stand the bike back upright.

 

[Hookster]

Hi there all,

new to the site as just recently bought a 1200 GS.

To cut a long story short i crashed it last night. Going into aq corner, the rear locked up after a gear change and i ended up going into a gate post. Got a broken wrist, but keen to know if anyone has heard of similar issues before? The road was dry (for a change) and I was doing about 40mph max.

Any thoughst would be great!

Cheers

Dave

First thought is glad you're (mostly) OK and get well soon BTW

Second; in any similar experiences I've been fortunate to recover from, I've accepted or convinced myself, rider error is the main factor. I find this helps enormously when i next swing my leg over a bike because the alternative (unless an obvious mechanical fault exists) is that the bike has a potentially lethal fault that may only manifest at the least welcome moment.

Prudence says get it checked out to allay any fears (she's always right) but don't dwell on it. I adopt the above because I need to be concentrating not thinking about what might be going wrong next.

On the old 1150, it used to regularly stall when cold and sometimes warm, as I'm want to blip the throttle for down changes. At least with that scenario you get a milliseconds warning of a stall as the engine gives an asthmatic cough, instead of a lively bark, thus giving time to rethink the whole thing.

Whatever happens, hope you sort it and get back on soon.

 

[Doofus]

Hi Darren, take a look again at the videos posted above, you will see clearly that with regard to (2) the front wheel moves clearly in one direction until the desired amount of lean has been induced and then the wheel is turned in the opposit direction so that the lean is maintained. If this didn't happpen the wheels would simply track out from below the bike and down you go. At higher speeds on wider radii corners the effect is less pronounced but it is still there.

With respect to (3) what I was pointing out was that you can spin up a gyroscope and place it at any angle to the plane of rotation that you choose and it will stay there as long as it has sufficient angular velocity. I do not dispute that when you try and move it forces become vectored differently. The comment was in repsonse to the statement that the gyroscopic effect of the wheels would try to stand the bike back upright.

Hi Packer - r.e. (3) - I see - sorry I didn't see what you were getting at.

r.e. (2) I maintain my view! Conservation laws mean you can't turn the wheel one way to induce a lean and then turn it the other way without undoing the lean (and indeed going on to lean the other way). Honestly - if you actively think about it when you next take a bend you will realise for a left hander on approach you pull with your right hand (or push with your left) to get a lean. (This pushing / pulling will probably be nothing more than a very gentle pressure for any speed above 20 mph or so). You then do not apply any force until the end of the bend when you want to get out of the lean. At that time you push with your right hand (or pull left). The wheels do not run out from you because the centrifugal force through your centre of mass combined with the tyre's friction in a sideways direction provides a pivot force that keeps you up. The actual amount of turn of the steering is minimal and the tyre just scrubs.

If the front wheel were pointing into the turn during the bend - how would it ever get straight? If you were to try and turn it back, you would simply lean more because this is the way you need to turn it start the lean in the first place.

I think what the video shows is three distinct steps:

1 - rider attepts to countersteer. However, he finds he is not going fast enough (i.e. the inertial forces associated with lean cornering are not going to be enough to scrub the tyre sideway - primarily because he has had to turn the wheel too far to get the lean as there is too little angular momentum in the wheel). If he continues the bike will indeed begin to run out from him.

2 - in order to correct, the rider turns back, this brings the bike straight up and back to where we were before step one.

3 - the rider goes on (admittedly in one smooth move) to use "normal" low speed turning (i.e. based on front wheel firction) to make the turn.

Steps 1 and 2 are not necessary. The position after step 2 is the same as at the begining and the rider could simply do step 3 without steps 1 and 2. My view is that the rider has simply performed a normal low speed left turn, with an unnecessary quick flick-right and recovery before doing so.

However, I think this is something we really need to look at together with some paper and pens in front of us if one of us is ever going to persuade the other!

Darren

 

[Wapping]

Prob go straight to BMW and confront them with the issue! Really concerned and unsure if I wanna stick to the GS if this is a potential problem again. Once bitten and all that!
Dave

What's the news from BuMW?

 

[Wapping]

What's the news from BuMW?

No news?