Disc Brakes - Thermal Expansion Drag

"Vincent J. Souki" <[email hidden]> wrote in message
"]news:[email hidden]... I recently purchased
a Giant 2002 NRS1 with Hayes disc brakes. On the long
downhills where the brakes need to be applied regularly to
attempt to keep the speed a little in control ( 3 clavical
fx in 12 years) , the hot pads or rotor expands enough to
cause a serious drag like I need to pedal hard to continue
on slight downhill grades. Once the brakes cool off a bit
the expansion goes back down and the brake drag lessens
enough as to not be noticed. Is this just a phenom with the
Hayes brakes or is this thermal expansion problem inherent
in all the dics brake systems. Any knowledge, experiences or
remedy with this issue would be greatly appreciated. I
really love riding this full suspension machine but those
brakes suck real bad so far. Previously riding a Specialized
Extreme FSR with LX V-brakes - no problems...........
Help,,,,,,,,,,,,,,

Thanks for your time.!

It's not actually the disc rotors or the pads expanding, its
the heat from friction being channeled through the caliper
into the break fluid, causing the fluid to expand, and apply
the piston, as if you were squeesing the lever a little.
Effect is called pump-up, and is well known. I have never
owned Hayes, so i can't comment as to whether the problem is
worse with this brand. I know that Hope use stainless steel
cylinder inserts specifically to reduce the heat transfer.
The only solution i know of is to pulse the brakes on and
off on the downhill sections, in order to give them a chance
to cool. Road racers have always had to do this to avoid
overheating the small surface area of their rims and melting
the innertubes on long decents.

--
Mark

In "]news:[email hidden],
Mark (UK) <[email hidden]> typed:

Quoted message said:

"Vincent J. Souki" <[email hidden]> wrote in message
"]news:[email hidden]... I recently
purchased a Giant 2002 NRS1 with Hayes disc brakes. On the
long downhills where the brakes need to be applied
regularly to attempt to keep the speed a little in control
( 3 clavical fx in 12 years) , the hot pads or rotor
expands enough to cause a serious drag like I need to
pedal hard to continue on slight downhill grades. Once the
brakes cool off a bit the expansion goes back down and the
brake drag lessens enough as to not be noticed. Is this
just a phenom with the Hayes brakes or is this thermal
expansion problem inherent in all the dics brake systems.
Any knowledge, experiences or remedy with this issue would
be greatly appreciated. I really love riding this full
suspension machine but those brakes suck real bad so far.
Previously riding a Specialized Extreme FSR with LX V-
brakes - no problems........... Help,,,,,,,,,,,,,,

Thanks for your time.!

It's not actually the disc rotors or the pads expanding,
its the heat from friction being channeled through the
caliper into the break fluid, causing the fluid to expand,
and apply the piston, as if you were squeesing the lever a
little. Effect is called pump-up, and is well known. I
have never owned Hayes, so i can't comment as to whether
the problem is worse with this brand. I know that Hope use
stainless steel cylinder inserts specifically to reduce
the heat transfer. The only solution i know of is to pulse
the brakes on and off on the downhill sections, in order
to give them a chance to cool. Road racers have always had
to do this to avoid overheating the small surface area of
their rims and melting the innertubes on long decents.

What Mark said and...

With Hayes if you pull the lever the least little bit it
will close off the hole in the master cylinder and prevent
the expansion of fluid into the bladder. This will instead
close the caliper. Lay off the levers every now and then to
allow expansion of the fluid to go into the bladder. Pump up
is generally not a problem with a open system like Hayes but
if you nurse the lever the system will become closed and the
fluid will have nowhere to go except to close the caliper.

Mike