The Geometry Of Traction

[atomant]

From my humble position of watching riders, especially watching Ross in practice and at events, he appears to virtually climb slippy banks with virtually no throttle at all. It appears that he applies loads of revs then when the bike is moving forward, backs off and the pure momentum of the wheel spinning and decellerating gets grip and up he goes.

Watch this vid to see what I mean -

At the beginning he climbs the first part with in 2nd gear with low revs where as a novice, I couldnt get up that bit at all from a standing start . The last step you will notice that hes off the revs when he goes up it as well.

Its a mystery

[dabnabit]

Preload

Damping

Tire (tyre) pressure

Tire construction

Throttle

Clutch

Momentum

Weighting (old guys have better weighting depending if you are a apple or a pear! Center of gravity)

Pressure

Rolling off the throttle

Connecting the mind to the rear wheel

Put it all together and you have TRACTION (GRIP)!

Edited November 14, 2006 by Dabnabit

[ham2]

So if you believe you know what makes the geometry of one bike out grip another and/or know all the pros and cons that are associated with that, please lets stop the guess work and discuss this in open forum?

Your opinion would be greatly appreciated

Best of balance.

Neo

Ok...TC people..this was the actual question.

Only a handful of us tried to answer this.

The lad wanted a technical response not the obvious '' just use some skill'' type of reply.

I was looking forward to a lot more good engineering/scientific theories here!

[slapshot 3]

All,

granted my initial response was a little tongue in cheek however reading throught the thread many have highlighted the variables involved in creating traction/Grip

So let's discuss:

Grip - a colloquial term for the frictional interaction between two surfaces

Traction - the dynamic process in which a force is applied directionally to a surface inorder to propel the body applying the force.

Dynamic Grip - directional application of a friction force to create propulsion in the opposite direction (newton's third law)

If you read through my first post, all those factors I noted are the dynamic functions necessary to create the propulsion elements in the puzzle, the creation of the force that moves us forward.

The element of the TYRE is far more important than suggested, otherwise tyre companies would not go through this process to create the best possible tyre and compound for the job whether it be trials, motocross, F1, Rally, you name it. Let's assume that all trials tread patterns are the same the only difference is rubber compound and wall construction, more physics and engineering. The tyre companies have done this work so assume again that the Michelin X11 (tubeless ) is the best tyre available and in all instance we assume this is used. We use a constant tyre pressure as well say 7psi.

Back end construction is fairly similar these days wheel, tyre, mudguard, swinging arm therefore weight is similar as well. What makes them different is wether they are Linkless or have a linkage. We assume every test is done on a Beta (linkless) suspension and again assume that Beta have done their homework on the bike to create the best balance between weight of the bike and the ability of the suspension to keep the wheel on the ground.

We have created the experimental control, the same bike and the same tyre all the time.

How do we vary this, stick a rider on the bike and instantly the variables change but the emphasis is always the same, keep the maximum contact patch in touch with the surface and its here that our rider comes in. Unless we apply this force in a certain direction (thrust) the bike won't move (Newtons First Law of Dynamics) and to do this we use the contact patch of the trye to create a patch of FRICTION on the surface we attepmt to ride over.

Take fat useless git rider, me, I get shed loads of fives because i can't keep the Centre of Gravity balanced so the bike and I topple over or my feet go down to help regain the balance. I lose the contact patch too easily or I create a patch that I can't control.

Now take skinny genius, call him Lamkpin if you like, He has an inane ability to balance the forces that control the bike in any terrain thrown at him he can subconsiously control the balanced centre of gravity of the bike at whatever angle it's at by altering the way his weight (weight = mass x the force of gravity(assumed 10m/s/s)) applies force through the contact patch of the tyre on the surface, using his weight and it's position over the bike and throttle control (controling the propulsion available).

So ultimately we prove that what creates traction or grip call it what you will, is the rider and his relative ability to balance the forces acting on him and the bike and how he uses the power available to propel him over the obstacles presented.

There's lots more to it but it's too many years since I studied physics in depth and I now have a headache, I prefer the simple answer....because it does

either that or 42........

[arun1664]

To explore the problem from a different perspective, grip is the ability to exploit the coefficient of friction that exists between the tyre and the section surface at any given time. The higher that coefficient the easier (say) the climb becomes.

The coefficient will vary according to:

1) Tyre construction. Which goes some way to supporting the argument that some brands

have better performance than others in different conditions.

2) The section surface e.g. mud, wet rocks, tree roots or other riders

3) The ability to maintain consistent contact between the tyre and section surface. This is a

function of suspension i.e. the wheel not bouncing up and down; a wheel in the air has

no grip, and. The rear wheel having the correct angular velocity,. a spinning wheel represents a lack of grip.

So you need the right tyre, correctly set up suspension and good throttle control, bet you didn’t know that ….

Which leads me to my question, seeing as I know the stuff above to a reasonable degree of scientific depth, why do I still ride like a t**t?

Edited November 14, 2006 by arun1664

[neo]

Hi "Slapshot 3"...after reading your point 14 I was convinced that you are a jerk ...but your last post proved me totally wrong...so many thanks for your detailed reply....great information.

distance between gearbox sprocket and end of swinging arm is one issue (effect of engine weight)

back pressure through exhaust ( that popping noise when your engine is slowing down after being revved, similar to flywheel effect ) just two stroke ???

Interesting factors...many thanks

The better riders use a taller gears, converting the power, and superior clutch control, to allow the maximum amount of power to the rear wheel, without loosing traction.

"superior clutch control" seems to come out time and time again. But I just wanted to focus on the dynamics of the bike for now. And regarding this....30 year sounds like a long time...but why give up now?!?

Thanks "AtomAnt" awsome video.

Great summary "Dabnabit" but again, I just wanted to focus on the dynamics of the bike for now.

Thanks "HAM2" for bring this back into focus.

Which leads me to my question, seeing as I know the stuff above to a reasonable degree of scientific depth, why do I still ride like a t**t?

Because we all have to earn a living and only occasionally ride trials...I think we can can only get out, what we put in.

For me, understanding what happens to the bike, while I ride, should help me "put in" just that bit more.

Please keep em coming....PLEASE

Best of balance.

Neo

[trialsrfun]

Graham Jarvis on the Sherco could ride up a vertical ice rink floor (if we could find him one) but I think Neo is right not enough riding time, not enough forward motion & too little determination after all on a good day if you think you can clean something you will.

It is all in the head ???

Graham Jarvis on the Sherco could ride up a vertical ice rink floor (if we could find him one) but I think Neo is right not enough riding time, not enough forward motion & too little determination after all on a good day if you think you can clean something you will.

It is all in the head ???

[scottt]

stroke of the engine is another issue ( a longer stroke engine will be less likely to break traction due to the number of times the piston goes past TDC per rotation of the wheel )

Rotation of the wheel will depend on engine revs and/or gearing, the stroke length doesnt make any difference. A piston goes past TDC once every revolution no matter how long the stroke is.

Scott.

[baldilocks]

Rotation of the wheel will depend on engine revs and/or gearing, the stroke length doesnt make any difference. A piston goes past TDC once every revolution no matter how long the stroke is.

true, a piston goes past tdc every revolution of the crank but its how far the rear wheel goes for that revolution of the crank i'm referring to.

shorter stoke = higher revving = less torque (ability to pull higher gear).

gearing will effect the ratio of wheel and crank but rpm wont (unless the crank length or gearing changes with rpm the ratio wil always be the same) . If you had the same gearing on two engines with different strokes side by side then the short stroke must turn the wheel a shorter distance for one turn of the crank.

[atomant]

If you had the same gearing on two engines with different strokes side by side then the short stroke must turn the wheel a shorter distance for one turn of the crank.

I beg to differ here. The crank in either example will do 1 revolution. The primary gearing being the same will rotate the wheel the same distance in both cases. What will be different is the way the power is delivered with different torque characteristics for either design

IMHO

[scottt]

AtomAnt, thats spot on, thats what I was trying to say. Baldilocks can you see what we mean now?

As far as grip goes, in my opinion its all down to weight distribution and throttle control, a better rider moving his body weight and using his throttle will always find grip where the rest struggle.

So to answer the original question, most bikes feel different because of there weight distribution and the way there power is delivered.

Scott.

Edited November 19, 2006 by ScottT

[baldilocks]

Doh

ok i get it.

Short stoke may not grip as well but because of the associated power charecteristics rather than the revolutions of the wheel idea. Must admit I got this idea from HRCs big bang 500cc GP road race bike. HRC changed the firing sequence of a V4 so that the interval between cylinders firing was greater, MCNs (Motor Cycle News) theory was that this provided greater traction.

I'll leave MCN and HRC to argue over that one !