Moving Front Mech

Quoted message said:

I have a problem which I think I know the answer to but would value
the learned experience of all here.

I have a very old (about 20 years) Aluminium road frame which I got
from my brother for free. It's been working fine until last weekend
when I did a ride from Skipton to Hawes (great ride for those in the
area) and whenever I got up out of the saddle the front mech would
start moving from the left to the right on the 'power stroke' if you
know what I mean. It didn't move enough to change chainrings, but the
chain did jump off a couple of times when the mech went back again.

Also, it would often jump down a gear on the back too.

Is this casued by the frame flexing a bit too much around the BB and
pulling on the gear cables? Any help greatly appreciated.
Thanks
Phil

I'd have a good look - a _really_ good look - for cracks in the frame.

A lesser worry would be a loose crank axle (which might make it appear
the the front mech was moving relative to the chainrings), but you
would probably have noticed other symptoms as well, were that the
case.

in message <[email hidden]>,

(') said:

I have a problem which I think I know the answer to but would value
the learned experience of all here.

I have a very old (about 20 years) Aluminium road frame which I got
from my brother for free. It's been working fine until last weekend
when I did a ride from Skipton to Hawes (great ride for those in the
area) and whenever I got up out of the saddle the front mech would
start moving from the left to the right on the 'power stroke' if you
know what I mean. It didn't move enough to change chainrings, but the
chain did jump off a couple of times when the mech went back again.

Also, it would often jump down a gear on the back too.

Is this casued by the frame flexing a bit too much around the BB and
pulling on the gear cables? Any help greatly appreciated.

If an aluminium frame is flexing that much expect it to fail imminently -
typically aluminium frames don't flex much unless quite badly cracked.

However, I think it's more likely that your chainset is flexing. This is a
common problem - chainsets with pressed steel spiders always flex, and
some aluminium chainsets flex. Also, if the chainset is not sufficiently
well mated to the bottom bracket axle, either because it's the wrong
bottom bracket for the chainset or because the mating surfaces of the
chainset are worn, you get a movement that looks a lot like flex.

So - my advice is investigate urgently, but don't panic. It's most likely a
chainset problem. If you do find cracks in the frame (most commonly where
tubes attach to the bottom bracket shell, but also possibly where the
front derailleur clamps on), scrap the frame. An cracked aluminium frame
will fail, and is not economic to repair.

--
[email hidden] (Simon Brooke) http://www.jasmine.org.uk/~simon/

;; part time troll.

Quoted message said:

Quoted message said:

I have a problem which I think I know the answer to but would value
the learned experience of all here.

I have a very old (about 20 years) Aluminium road frame which I got
from my brother for free. It's been working fine until last weekend
when I did a ride from Skipton to Hawes (great ride for those in the
area) and whenever I got up out of the saddle the front mech would
start moving from the left to the right on the 'power stroke' if you
know what I mean. It didn't move enough to change chainrings, but the
chain did jump off a couple of times when the mech went back again.

Also, it would often jump down a gear on the back too.

Is this casued by the frame flexing a bit too much around the BB and
pulling on the gear cables? Any help greatly appreciated.
Thanks
Phil

I'd have a good look - a _really_ good look - for cracks in the frame.

or a loose gluejoint?
--
---
Marten Gerritsen

INFOapestaartjeM-GINEERINGpuntNL
www.m-gineering.nl

[email hidden] was thinking very hard :

Quoted message said:

I have a problem which I think I know the answer to but would value
the learned experience of all here.

I have a very old (about 20 years) Aluminium road frame which I got
from my brother for free. It's been working fine until last weekend
when I did a ride from Skipton to Hawes (great ride for those in the
area) and whenever I got up out of the saddle the front mech would
start moving from the left to the right on the 'power stroke' if you
know what I mean. It didn't move enough to change chainrings, but the
chain did jump off a couple of times when the mech went back again.

Also, it would often jump down a gear on the back too.

Is this casued by the frame flexing a bit too much around the BB and
pulling on the gear cables? Any help greatly appreciated.

Are the components also 20 years old? If so, the gears probably won't
be indexed, and it may simply be that your gear levers have worked
loose. They usually used to have a fixing bolt which could be tightened
by hand, which was important, because they used to work loose all the
time!

Or the "cracked frame" prophets of doom could be right, of course.

--
Simon

Thanks for the advice guys.

I had a quick look for cracks at the time and now I've had an even
more thourough look, but can't find any. The chainset is not loose
either, but it certainly could be flexing under pressure, I don't
think the compinents are original, but they are quite old too,
certainly more than 10 years.

Looks like the best option is to finally cough up some dosh and buy a
new one. This bike was my introduction to road bikes a couple of
years ago and it has given me the cycling bug. Guess it's a good time
of year to buy one as I'll be out and about a bit more now.

Thanks again.
Phil

Simon Brooke said:

If an aluminium frame is flexing that much expect it to fail imminently -
typically aluminium frames don't flex much unless quite badly cracked.

If the frame is an old Vitus or Alan, then it may be of the screwed and

glued lug type. These do losen slightly over time and become a little

flexible. Nothing you can do about it except take apart and rebuild.

Probably better to sell on eBay

Pete Whelan said:

Simon Brooke said:

If an aluminium frame is flexing that much expect it to fail imminently -
typically aluminium frames don't flex much unless quite badly cracked.

If the frame is an old Vitus or Alan, then it may be of the screwed and

glued lug type. These do losen slightly over time and become a little

flexible. Nothing you can do about it except take apart and rebuild.

Probably better to sell on eBay

Thanks for the tip Pete. It is indeed an old Vitus. Perhaps I should
have a go at stripping it down first then. I've got quite a good LBS
here who may be able to help if I get stuck.

If successful I could rebuild it with a better groupset with the money
saved!

Thanks guys. What a wealth of information there is here.
Phil

Just had a much more detailed investigation and found the problem.
Basically the seat tube is pulling out of the bottom bracket assembly
by about 5mm when under pressure. Looks like the glue has indeed
gone. This may sound silly, but would just supergluing fix it? Seems
a bit too easy to me, has anyone tried that?

Cheers
Phil

[email hidden] wrote on 24/03/2007 11:52 +0100:

Quoted message said:

Just had a much more detailed investigation and found the problem.
Basically the seat tube is pulling out of the bottom bracket assembly
by about 5mm when under pressure. Looks like the glue has indeed
gone. This may sound silly, but would just supergluing fix it? Seems
a bit too easy to me, has anyone tried that?

Cheers
Phil

The success of gluing aluminium is 90% in the surface preparation which
will be difficult/impossible without disassembling the frame to access,
clean and prepare the surfaces. If you take out the BB you could try
wicking phosphoric acid into the joint to prep it and then flush it
through with plenty of water to remove all the acid. Not sure how you
are going to clean out the old adhesive though.

You should also use a high performance structural epoxy but I doubt you
will get that into the joint either so a high performance cyanoacrylate
might be your only option.

--
Tony

"The most savage controversies are those about matters as to which there
is no good evidence either way."
- Bertrand Russell

Quoted message said:

Just had a much more detailed investigation and found the problem.
Basically the seat tube is pulling out of the bottom bracket assembly
by about 5mm when under pressure. Looks like the glue has indeed
gone. This may sound silly, but would just supergluing fix it? Seems
a bit too easy to me, has anyone tried that?

a) no

b) yes, probably.