Suspension and cornering forces

[Darkness]

It would be interesting to see the build up to that slide?

[emzedder]

I'm a relic in same camp as Dave Croxford and Mike Hailwood " just get on it, screw it on and ride around the faults". With knees tucked in.

[Mr Gazza]

Originally Posted by 350TSS

He's just waving to a fellow rider with his foot...Could be dangerous to take your hands off the bars whilst cornering...Although a simple head nod would have done.

I'm in the same school as emzedder...Come to think of it we did go the same school..!

[utopia]

I still disagree that the bike rotates about the centre of gravity when cornering.
Apart from the requirement for the contact patch to move sideways to allow this to happen, the tyre would also have to lift off the ground in order for the C of G to remain at the same height when the bike leans into a corner, and this clearly doesn't happen.
I don't believe that the contact patch moves sideways due to cornering forces either. Instead I think the lateral displacement of the tyre track after going through a puddle is generated by the trail of the front wheel. This initially shifts the front wheel contact patch sideways, and the castor effect then pulls it back in line, giving the straight-line steering stability observed to extreme in choppers with long forks and therefore increased trail.

Furthermore, I don't think it is entirely correct to state that the centrifugal/centripetal forces act solely through the C of G.
Every single element of mass contained in the motorcycle, from the sidestand at the bottom to the camera on top of the riders helmet experiences these forces. It is merely an expediency for the sake of calculation that they can be resolved into a single resultant that can be considered to act through the C of G.

There has been some talk about countersteering, but this is not really relevant to the discussion about the forces acting during the turn, it merely relates to the forces which initiate the turn in the first place.
However, while we're on the subject, countersteering does not actually involve turning the bars in the opposite direction at all. That is to say, there is no actual contra rotation of the steering. Instead, a force is applied in the contra direction but this causes the front wheel to lean.
The front wheel doesn't first turn the wrong way and then react to its "mistake" by leaning. It is one "holistic" process and involves no actual turning of the steering at all.

Finally in response to the "just get on and ride the thing" attitude .. yes, I kinda agree in many respects.
However I don't think the two ways of doing it are mutually exclusive.
That is to say, whether or not one claims to ride by feel alone, the physics is still going on and an analysis of how it operates can only compliment and enhance the riders' experience.

Oh and just to nit-pick slightly .. strictly speaking, a vector has direction and magnitude, where that magnitude is proportional to its length. Sorry, I am being a bit pedantic there, but science sometimes demands such pedantry.
Similarly, Newton said that a body will continue in a state of rest or uniform motion IN A STRAIGHT LINE unless acted upon by an external force.

If all the above sounds like I'm a know-it-all, I apologise and assure you that this is not the case.
I'm just as likely to sometimes talk cobblers as the next bloke.
But I hope my two-pennorth furthers the discussion.

[Darkness]

A question for you Utopia:
You and your bike are vertical whilst travelling forwards on a flat level road in a straight line at a constant speed.
Resolving forces and moments about the centre of the contact patch, what force or moment do you think causes the bike to begin to rotate? Gravity alone doesn’t do it because the weight is vertical through the contact patch so doesn’t create a moment.

[Darkness]

Another set of words about cornering here:

https://eatsleepride.com/c/121369/th...orcycles_leans

[Luddite]

I'm with Darkness on this one - not because I understand all the forces involved but because...Tony Foale says so!



(Don't ask me to explain it though

[slob]

Originally Posted by utopia

...It is merely an expediency for the sake of calculation that they can be resolved into a single resultant that can be considered to act through the C of G...

duh...that’s pretty much the definition of CoG

[utopia]

I guess I would be well advised to fully study the examples provided before responding to the question, but instead I have decided that an immediate answer might have its merits too ... so here goes.



I think that there is more than one mechanism which can initiate a turn but that by far the main one is countersteering.
This causes gyroscopic precession of the front wheel with the result that it wants to tip into the turn by rotating about a horizontal axis at 90deg to the wheel spindle.
However, the front wheel cannot rotate in this manner because it is constrained by the tyre's contact with the ground and therefore it rotates about the contact patch instead.
Other inputs to the bike, like weight shifting or weighting one peg (essentially the same outcome via different methods) also contribute to the initiation of a turn, but these are supplementary to the main, countersteering input.

I guess what I'm saying is that although forces may want to act about their centres of rotation (spindle) or of mass (C of G) etc, the resultant outcome of these forces is that rotation must occur about the tyre's contact patch, because that is the only available option (as long as the wheels are on the ground).


Right ...... now I'll study the Tony Foale stuff and see where I end up after that.
I do very much respect the man and I reckon it will probably turn out that we're actually saying much the same thing in different words .. or else any error will be mine.
But for the sake of the discussion, I've decided to shoot from the hip in the first instance (and did likewise in my original post too) ... I'm hoping that it will make for a better learning process all round, myself included.


ps. actually, gyroscopic precession forces wont be about a horizontal axis but one which is mutually at 90deg to both the wheel spindle and the steering axis .. but it would have got too wordy to include that bit in the above text)

Dare I press "submit reply" now, I ask myself ?
Yeah, bugger it.

[utopia]

Originally Posted by slob

duh...that’s pretty much the definition of CoG

Yes it is .. that was my intention.

[Darkness]

Originally Posted by utopia

.....I think that there is more than one mechanism which can initiate a turn but that by far the main one is countersteering.
This causes gyroscopic precession of the front wheel with the result that it wants to tip into the turn by rotating about a horizontal axis at 90deg to the wheel spindle.
However, the front wheel cannot rotate in this manner because it is constrained by the tyre's contact with the ground and therefore it rotates about the contact patch instead.....

....ps. actually, gyroscopic precession forces wont be about a horizontal axis but one which is mutually at 90deg to both the wheel spindle and the steering axis .. but it would have got too wordy to include that bit in the above text)

I’m struggling with your description of countersteering that produces gyroscopic precession of the front wheel rather than turning of the forks?

I think in terms of steady state, then action and corresponding reaction with imbalance of forces or moments producing accelerations (Change of speed and/or direction of travel).

Can you expand on that please?

Originally Posted by utopia

Other inputs to the bike, like weight shifting or weighting one peg (essentially the same outcome via different methods) also contribute to the initiation of a turn, but these are supplementary to the main, countersteering input.

I guess what I'm saying is that although forces may want to act about their centres of rotation (spindle) or of mass (C of G) etc, the resultant outcome of these forces is that rotation must occur about the tyre's contact patch, because that is the only available option (as long as the wheels are on the ground.

Shifting weight can also create a turn because a bike isn’t rigid, it has a hinge behind the front forks. Movement of the rider generates equal and opposite forces in the bike as it’s a closed system.

If you jerk your body weight sideways it causes the steering to turn slightly because it pulls the headstock to left or right (Slightly) relative to the contact patch. That is easily demonstrated when pushing a bicycle. If you couldn’t do this you couldn’t ride a bicycle “no hands”.

If you had vertical forks with no trail you wouldn’t be able to do this, but the bike would be pretty unrideable as there’d be no self centring of the steering either.

[Darkness]

It’s worth mentioning that when considering a contact patch, that isn’t a fixed point on the ground: it’s moving forwards along the ground at whatever speed the bike is doing, so moving 6” to the left is perfectly possible whilst the wheel is rotating and moving forwards.

[Mr Gazza]

Jeff, if you want a real world experiment to help understand counter steering, just get a bicycle wheel and spin it up whilst holding the spindle each side. (yes a third hand is required!.)
This will be a gyro, as you know. Now try and push the left side forwards as if to steer it right. (or pull back with the right.)
It will try and lean to the left... Seriously, try it, the force cannot be resisted.
This is what initiates a turn, after that moment the counter steering effort ceases and the wheel becomes stable in that lean without further input.

The bike now describes a curved course because the curved section (profile) of the tyre has a greater circumference in the centre, to that of the outer edges. In other words it acts like a cone... You can't roll a cone in a straight line! it just goes round in circles.

In this hypothetical model the wheel would be the circular base of the cone, with an imaginary extra long spindle reaching the pointed end.

That's just the principle in a simplified model, as the "cone" in reality would be changing in a complex way with varying lean angles and tyre profiles.

[utopia]

Originally Posted by Darkness

I’m struggling with your description of countersteering that produces gyroscopic precession of the front wheel rather than turning of the forks?

I think in terms of steady state, then action and corresponding reaction with imbalance of forces or moments producing accelerations (Change of speed and/or direction of travel).

Can you expand on that please?

What I'm trying to say here is that my understanding of countersteering is that it occurs when a turning moment is applied to the forks, but that the outcome of that turning moment is not that the forks actually turn at all, but instead the wheel banks into the turn ... instantaneously.
I don't see this as being divisible into a separate "cause" and a subsequent "consequence" but rather as one single mechanism.
In other words, the wheel doesn't first turn, and then bank as a consequence of that initial turn. It banks as a direct and immediate consequence of the input which attempts, but fails, to turn the steering.

Though reading my earlier post, that's pretty much what I said last time so I dunno whether it helps.

I still haven't studied the other stuff yet ... sorry.
I'm felling a giant Leylandii in my garden and it demands full attention.
These are teabreak responses, but I will certainly study everything in detail at some stage.

[Mr Gazza]

Originally Posted by Darkness

If you had vertical forks with no trail you wouldn’t be able to do this, but the bike would be pretty unrideable as there’d be no self centring of the steering either.

So a Penny Farthing or uni-cycle cannot turn?