Unnerving braking experiences; sudden braking increase.

[Art Harris]

Johan Bornman wrote:
> > Check both the rim and pads for signs of unusual wear; specifically,
> > if the
> > rim is significantly worn, you can end up with a lip near the top and
> > bottom. As the brake pads wear, pressure is at first applied to the very
> > small surface area of the lip, after which it drops down into the main
> > section... which can suddenly increase the braking forces.

> Mike, I don't think this is the way it works. I have also
> experience this phenomena with KoolStop pads (I haven't experienced it
> with other pads purely because I have used Koostops for a while now
> and have not had the chance to try and repeat the phenomena with other
> pads) and the effect is repeatable. Ride the brake and suddenly brake
> force increases.

I've never noticed this with Kool Stop Salmon pads. The only reason I
can think of for braking power to increase over time would be if there
was water (or even more so, snow) on the rims. Snow on the rims will
keep the rims wet until all the snow is gone. When the water is
finally squeegied away, the braking power with increase.

> Cool it down a bit, ride it again and it does the
> same all over again. On inspection, you don't see any noticeable
> change in the pad appearance.
>

Could be dirt or grit getting trapped between the pad and rim. Pumping
the brakes a few times will usually clear that with Kool Stops.

Art Harris

 

[SYJ]

On Jan 24, 6:21 pm, Michael Press <[email protected]> wrote:
---snip---

At a later point in the brake
> application, the braking force suddenly increased
> sharply. This happened more than once.
---/snip---

I've noticed the same thing w/Kool Stop Salmons mounted on Dia Compe
Radius (BRS 200) single pivot calipers. Running the same pads/holders
on Tektro RX40 (dual pivot...more substantial) calipers did not reveal
the 'surge'. Because of this, I've come to wonder if the increase in
braking force is related to flex in the arms of the calipers -
specifically, as the brake force is applied, twisting force (from the
motion of the rim) causes the arms to flex 'out' at the fronts,
forcing the backs of the pads to 'dig in' to the rim - kind of a quasi
self energizing action. Just a theory, no data to back it up.

SYJ

 

[Michael Press]

In article
<[email protected]>,
"Johan Bornman" <[email protected]> wrote:

> Check both the rim and pads for signs of unusual wear; specifically,
> if the
> > rim is significantly worn, you can end up with a lip near the top and
> > bottom. As the brake pads wear, pressure is at first applied to the very
> > small surface area of the lip, after which it drops down into the main
> > section... which can suddenly increase the braking forces.
> >
> > --Mike Jacoubowsky
> > Chain Reaction Bicycleswww.ChainReaction.com
> > Redwood City & Los Altos, CA USA
>
> No Mike, I don't think this is the way it works. I have also
> experience this phenomena with KoolStop pads (I haven't experienced it
> with other pads purely because I have used Koostops for a while now
> and have not had the chance to try and repeat the phenomena with other
> pads) and the effect is repeatable. Ride the brake and suddenly brake
> force increases. Cool it down a bit, ride it again and it does the
> same all over again. On inspection, you don't see any noticeable
> change in the pad appearance.
>
> I believe it is temperature related and that something happens at a
> critical temperature at the interface of rim and pad.
>
> I also remember once that after applying using the brakes for a
> specific period of time, they suddenly blew off debris which landed on
> and stuck to on my thighs - little shavings of black rubber
> grindings. This was before I used Koolstop pads. I remember
> distinctly that this only happened a while into braking, not
> immediately. I'm pretty certain that I applied continuous pressure to
> the levers throughout, in both cases cited above.

I have not reproduced the phenomenon yet, as the
descents are not as steep since then. I did notice that
the KS pads were lipped over the outside edge of the
rim, so I adjusted that away. Now they squeal again!

The exact circumstance again is on a steep descent,
delay braking at corner approach, apply brake and
steadily increase brake pressure; then sudden increase
in braking with no increase in brake pressure.

--
Michael Press

 

[Michael Press]

In article
<[email protected]>,
"SYJ" <[email protected]> wrote:

> On Jan 24, 6:21 pm, Michael Press <[email protected]> wrote:
> ---snip---
>
> At a later point in the brake
> > application, the braking force suddenly increased
> > sharply. This happened more than once.
> ---/snip---
>
> I've noticed the same thing w/Kool Stop Salmons mounted on Dia Compe
> Radius (BRS 200) single pivot calipers. Running the same pads/holders
> on Tektro RX40 (dual pivot...more substantial) calipers did not reveal
> the 'surge'. Because of this, I've come to wonder if the increase in
> braking force is related to flex in the arms of the calipers -
> specifically, as the brake force is applied, twisting force (from the
> motion of the rim) causes the arms to flex 'out' at the fronts,
> forcing the backs of the pads to 'dig in' to the rim - kind of a quasi
> self energizing action. Just a theory, no data to back it up.

Thanks. The brakes are Ultegra dual pivot. No wet on
the road, but the air was damp and 50 F.

--
Michael Press