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" wrote in message ...
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