Re: Emergency braking and bird anatomy [was: Re: DVD copying and CSS]

[Richard Lyons <richard@the-place.net>]

On Wednesday 25 February 2004 20:15, Monique Y. Herman wrote:
> On 2004-02-25, Paul Johnson penned:
> > On Wed, Feb 25, 2004 at 02:16:32PM +0200, Micha Feigin wrote:
> >> As for down hill, I ride quite a bit of free ride, not so much
> >> downhill since unfortunately I don't have the money for the big hit
> >> bikes, but the steeper the terrain the more I use the front break
> >> since the rear wheel has almost no traction on steep terrain.
> >
> > Because you're leaning forward already!
>
> Yeah, I'm a bit confused by this rear wheel traction statement.  When
> I'm going downhill, I move my body back on the bike -- afaik, this is
> why "good" bike saddles are quite narrow -- to allow you the freedom to
> move front to back as necessary.  Depending on how steep it gets, my
> whole pelvis could be behind the saddle.
>
> (This is somewhat from memory, as I'm a big wuss when it comes to cold +
> biking and haven't biked in a few months.)

THe problem is not only one of weight distibution.  Braking force is applied 
well below the centre of gravity by either wheel, so the resulting torque is 
in the same direction for both wheels - if you look from the side at a bike 
going left to right, the torque is clockwise.  This means that the more the 
rear wheel tries to slow the bike, the more it is lifted off the pavement, 
reducing the available friction.  So the power f the rear brake is 
self-limiting.  The front brake on the other hand causes greater force to be 
applied to the pavement as it operates, increasing the braking effect -- and 
the tendency to cause rotation of the whole system (bike and rider) around 
the point of contact on the ground.

It's not too difficult to work out the maximum breaking force that can be 
applied before you get thrown onto the road.

-- 
richard