FRONT Tire Cupping

[Mustang]

http://will.mylanders.com/outdoors/motorcycle/notes/read.pl?file=144

Found an interesting article on why tires cup ...............

here's the re-post
interesting reading , the bit about 'tread squirm' makes sense to me as most of the tires we run on tiggers have big tread blocks which are prone to 'tread squirm'


Here is an interesting technical analysis of head shake, weave and tire
cupping. Credit this information not to me, but to Charles Cherry, a member
of another email group I belong to. I post it with his permission.

Michael "islemann" Pierce

>From: "Charles Cherry"
>To: Northwest Bikers Social Mailing List
>Subject: RE: Tankslappers
>X-Comment: Northwest Bikers Social Mailing List
>
>I don't claim to understand how a motorbike steers or fails to steer, but I
>read somewhere that on a single track vehicle the front wheel always steers
>to one side and then over-corrects to the other side, then over-corrects to
>the original side, etc. This is called "weave." It is normal and is always
>present, but usually it is too small to be perceived by the rider. Under
>good conditions, friction in the steering system (tire friction on the road
>and steering head bearing friction) damps out the weave and it remains too
>small to be felt.
>
>If weave is not sufficiently damped, it grows larger, although still too
>small to be felt, and it causes "cupping" wear on the front tire. Under
this
>theory, the reason that some brands of tires cup more than others is that
>they are not as good at damping the weave, i.e., they have less friction
>resisting the twisting, maybe because of tread squirm.
>
>Anyway, given the right combination of inadequate damping and resonant
>frequency, the weave can grow into a headshake and a headshake can grow
into
>a tankslapper. The reason that you get a headshake at the top of a rise is
>that weight comes off the front tire, reducing the tire's friction against
>the road and the stem bearing's internal friction, thereby reducing the
>damping so the weave can grow. (However, there is a feedback loop here in
>that taking weight off the front wheel also reduces the weave forces.)
>
>Clearly, many other factors, like energy being stored in a twisting frame,
>input from the rear tire and rear suspension, chassis geometry, etc. can
all
>add to the mix. But it starts with the weave.
>
>Rider input may also be a major factor. There's a guy writing in MCN who
>says that a bike will never do a headshake or tankslapper on it's own. It
is
>always initiated by rider input. This happens when the weave is still too
>small for the rider to perceive, but the rider counters forces felt in the
>handlebars and, in so doing, hurts rather than helps the situation. No one
>can react fast enough to catch the weave when they feel it. So, by the time
>the rider pushes back on the bar, the bike is already weaving the other
>direction and the rider's input amplifies the new weave, instead of damping
>the previous weave. This guy says the best way to deal with a headshake is
>to relax your grip and let the bike straighten itself out. I think Gordon
>Jennings also said the same thing.
>
>Damping out the weave when it is imperceptible is not difficult because the
>forces are small. By the time it reaches the tankslapper stage, the forces
>are huge. Nevertheless, it is a combination of weave forces, damping, and
>resonance that makes it happen and changing any of them can make it die
>down.
>
>Even if you can hold onto the throttle during a tankslapper, you usually
>can't accelerate out of the problem. Acceleration forces tend to be
>converted into more weave, so you don't speed up and you don't change the
>resonate frequency; you just increase the weave forces. If you can
>accelerate hard enough to lift the front wheel, or even lighten it
>significantly, you can change the balance of the equation, and that might
>help. That is the Kenny Roberts solution.
>
>But it is usually easiest and most effective to apply the rear brake. This
>slows the vehicle, changing the resonate frequency, and it weights the
front
>wheel, adding tire and stem bearing friction and thereby increasing the
>damping forces. (Although increasing weight on the front wheel also
>increases weave forces, the damping forces usually increase faster.) This
>was SSPaul's solution, but as he said, it hurts lap times. I think it is
the
>right thing to do on the street.
>
>The only tankslapper (lock to lock) I personally experienced was initiated
>by a side wind gust on the Richmond-San Rafael bridge at about 90 mph on a
>Kawasaki H1 500 triple. I gently applied the rear brake and rode it out
>safely. To my way of thinking, the rear brake works. Or maybe I was just
>lucky
>
>My Sabre suffered from pretty bad front tire cupping even though the
>steering head bearings were properly snug. I added the tiniest bit of
>preload, about 1/8 turn, and the cupping almost disappeared. Changing to
>radial tires (more tire friction) made it vanish.
>
>Personally, I think that all bikes should come with a steering damper,
>standard. Weave is normal behavior and bikes should have something to cope
>with it, not just hoping for the proper combination of steering head
preload
>and sticky tires to deal with it. You don't need expensive hydraulic
>dampers, the simple friction dampers of a bygone age worked just fine.
>
>Understanding the weave is normal changed my whole perspective on
>weave-related problems like cupping and headshakes. I am no longer looking
>for what causes the cupping and headshakes. I am looking for why the weave
>is not being damped out, trying to understand what amplifies it and what
>damps it.
>
>Now, I'm not sure anybody really knows what goes on in a single track
>vehicle. But I like this explanation, and it has worked for me so far.
YTSMV
>(your tankslap may vary).
>

[iansoady]

Interesting reading. I think he's probably right that the bike is in a constantly unstable state - falling either to right or left then being corrected by the trail acting on one side of the tyre.

However, I incline more to the view (posted here by someone IIRC) that cupping is caused by braking forces. I don't think the forces involved in the self-correcting mechanism are anywhere near powerful enough to wear the tyre significantly - the "squirm" he mentions is an inherent part of tyre behaviour. In any case if that was the cause I'd expect more side to side wear rather than longitudinal.

And as someone who has experienced many friction dampers, never again thanks*. Their only function was to disguise poor handling. My Velo has a friction damper but being properly designed never needs it.....


*Except of course to counter horrendous steering flutter on badly set up sidecar outfits.

[2004Tiger]

http://will.mylanders.com/outdoors/motorcycle/notes/read.pl?file=144


>Now, I'm not sure anybody really knows what goes on in a single track
>vehicle. But I like this explanation, and it has worked for me so far.
YTSMV
>(your tankslap may vary).
>

When you hear hoofbeats do you assume zebras or horses? Braking forces must be the logical explanation for cupping.

[Mustang]

When you hear hoofbeats do you assume zebras or horses?

Wait for it .............................





















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