Mystery Thumps -- 12/25 Ultegra with Dura-Ace Derailluer

I just slapped a 12/25 Ultegra cassette on a 650c HED DEEP wheel for an upcoming race. Other than
the cassette, the drivetrain is Dura Ace. Chainwheels are 39/53. Shifting on all cogs in both rings
is flawless. However, in the 43/25, there are two very audible thumps for every rotation of the rear
wheel. The thumps sound like they are coming from some kind of contact between the 25T cassette cog,
chain, and/or pulley cage wheel, but I can't really tell for sure. The wheel is close to, but not
exactly in, the same position when the thumps occur each rotation. The pulley cage seems to "jump"
just a little in conjunction with the thumps. The thumps and jump still occur, but are much reduced,
in the 43/23 combo (next cog down). The problem doesn't occur in the smaller cogs. The drivetrain is
also dead silent with my training wheels and Dura Ace 11/21. Can anyone shed some light on this
problem? I'll road test it when I get to the race site tomorrow, but I'd like to take care of the
problem, especially if the chain is going to jump when I use the 43/25 on the relentless hills in
this race.

Thanks group!

Herm

A shy person wrote:
> I just slapped a 12/25 Ultegra cassette on...the drivetrain is Dura Ace. Chainwheels are 39/53.
> Shifting on all cogs in both rings is flawless. However, in the 43/25, there are two very audible
> thumps for every rotation of the rear wheel. The thumps sound like they are coming from some kind
> of contact between the 25T cassette cog, chain, and/or pulley cage wheel, but I can't really tell
> for sure. The wheel is close to, but not exactly in, the same position when the thumps occur each
> rotation. The pulley cage seems to "jump" just a little in conjunction with the thumps. The thumps
> and jump still occur, but are much reduced, in the 43/23 combo (next cog down).

That's actually "up" ;-)

> The problem doesn't occur in the smaller cogs. The drivetrain is also dead silent with my training
> wheels and Dura Ace 11/21.

My guess is that the pulley is bumping into the large sprocket. Modern derailers have an adjustment
for the tension of the upper spring, for the purpose of regulating this gap according to the size of
the largest rear sprocket. Shimano calls this the "B-tension" adjustment. When you go to a cluster
with a larger sprocket, it is usual to have to tighten this screw to push the cage farther downward.

This adjustment is also somewhat affected by chain length.

See also http://sheldonbrown.com/derailer-adjustment.html

Sheldon "Springs" Brown +-----------------------------------------------------------------+
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"The Herminator" wrote:

> I just slapped a 12/25 Ultegra cassette on a 650c HED DEEP wheel for an upcoming race.

> Chainwheels are 39/53.

> Shifting on all cogs in both rings is flawless. However, in the 43/25, there are two very audible
> thumps for every rotation of the rear wheel.

You mean 53/25, right?

> The thumps sound like they are coming from some kind of contact between the 25T cassette cog,
> chain, and/or pulley cage wheel, but I can't really tell for sure. The wheel is close to, but not
> exactly in, the same position when the thumps occur each rotation. The pulley cage seems to "jump"
> just a little in conjunction with the thumps. The thumps and jump still occur, but are much
> reduced, in the 43/23 combo (next cog down).

There you go again. 53/25 right?

> The problem doesn't occur in the smaller cogs. The drivetrain is also dead silent with my training
> wheels and Dura Ace 11/21. Can anyone shed some light on this problem? I'll road test it when I
> get to the race site tomorrow, but I'd like to take care of the problem, especially if the chain
> is going to jump when I use the 43/25 on the relentless hills in this race.

53/25!!! You shouldn't be using that combination anyway. The gear police will get you.

Art Harris

Does your wheel have one of those round spoke protectors to prevent the chain from touching the
spokes? My Bontrager Race Lite wheel has one, and it is warped enough to scrape the rear derailleur
at every rotation in the largest gear. I think most racing wheels don't have protectors anyway.
Another possibility is something under the cassette which might be preventing it from remaining flat
against the hub, or maybe the hub is not perfectly flat. Is your chain older than the cassette?
Maybe the chain is stretched to the point that it is climbing up the teeth of the larger gears.

r.b.

"The Herminator" <ulloa@provide.net> wrote in message news:<xpCcnSHbhK0WR8iiXTWJjg@provide.net>...
> I just slapped a 12/25 Ultegra cassette on a 650c HED DEEP wheel for an upcoming race. Other than
> the cassette, the drivetrain is Dura Ace. Chainwheels are 39/53. Shifting on all cogs in both
> rings is flawless. However, in the 43/25, there are two very audible thumps for every rotation of
> the rear wheel. The thumps sound like they are coming from some kind of contact between the 25T
> cassette cog, chain, and/or pulley cage wheel, but I can't really tell for sure. The wheel is
> close to, but not exactly in, the same position when the thumps occur each rotation. The pulley
> cage seems to "jump" just a little in conjunction with the thumps. The thumps and jump still
> occur, but are much reduced, in the 43/23 combo (next cog down). The problem doesn't occur in the
> smaller cogs. The drivetrain is also dead silent with my training wheels and Dura Ace 11/21. Can
> anyone shed some light on this problem? I'll road test it when I get to the race site tomorrow,
> but I'd like to take care of the problem, especially if the chain is going to jump when I use the
> 43/25 on the relentless hills in this race.
>
> Thanks group!
>
> Herm