Worn Front Mech?

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Mick F
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Worn Front Mech?

Postby Mick F » 27 Oct 2020, 2:44pm

How worn does the front mech cage need to be before you replace the whole thing?

Never worn one out before .......... ever :shock: ............ but this one on Moulton at 8,000miles has had a hard life.
61/48/34 triple and is beyond the Shimano Tiagra total capaity.

Should I buy another .............. not many around of the triple Tiagra braze-on units ................ or buy something more beefy to take its place?
IMG_0123.jpg
Mick F. Cornwall

Brucey
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Re: Worn Front Mech?

Postby Brucey » 27 Oct 2020, 2:51pm

I wonder if part of the problem is that the FD is meant for a smaller interval (middle to big) than you have been using it for?

Modern triple FDs have shoulders and shapes on the inner plate which are meant to help with the middle to big shift. However if the FD is wrong for the chainring interval then they may perhaps hinder as much as they might otherwise help.

cheers
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Re: Worn Front Mech?

Postby PDQ Mobile » 27 Oct 2020, 2:53pm

Mick F wrote:How worn does the front mech cage need to be before you replace the whole thing?

Never worn on out before .......... ever :shock: ............ but this one on Moulton at 8,000miles has had a hard life.
61/48/34 triple and is beyond the Shimano Tiagra total capaity.

Should I buy another .............. not many around of the triple Tiagra braze-on units ................ or buy something more beefy to take its place?IMG_0123.jpg


How worn before...?
I would say when it not longer shifts well.
I have one on a mountain bike (previous owner didn't adjust things well!) that has some of the cage worn right away but it still shifts well and I continue to use it without issue.
Pivots are quite worn too.

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Mick F
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Re: Worn Front Mech?

Postby Mick F » 27 Oct 2020, 3:04pm

Brucey wrote:I wonder if part of the problem is that the FD is meant for a smaller interval (middle to big) than you have been using it for?
Exactly.
Like wot I said.

PDQ Mobile wrote:I would say when it not longer shifts well.
I have one on a mountain bike ............but it still shifts well and I continue to use it without issue.
Pivots are quite worn too.
Pivots are fine, and so is the shifting considering the huge range this thing has to cope with.

Up from inner of 34t to middle of 48t ............. seems ok eh?
Then from middle of 48t up to outer of 61t.

When the bike was original, the outer was (only) 58t ......... so not much different diameter to 61t.

My wonderment/questions, are how worn does it need to be before it is scrap?
Is there a better unit available?
Mick F. Cornwall

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Mick F
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Re: Worn Front Mech?

Postby Mick F » 27 Oct 2020, 3:26pm

This is the one.
IMG_0124.jpg
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bgnukem
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Re: Worn Front Mech?

Postby bgnukem » 27 Oct 2020, 7:20pm

Regarding the effect of wear, provided the cage has not actually been perforated by the wear, I'd think the slop in the pivots caused by wear would be more of an issue affecting shifting, as the movement at the cage caused by sloppy pivots could be far greater than the 1mm-or-less of material worn off the inner face of the cage.

Of course, if the cage is always rubbing on the chain in certain gears you need to adjust the mech, or avoid the gears causing the contact.

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Re: Worn Front Mech?

Postby peetee » 27 Oct 2020, 7:26pm

Brucey wrote:I wonder if part of the problem is that the FD is meant for a smaller interval (middle to big) than you have been using it for?

Modern triple FDs have shoulders and shapes on the inner plate which are meant to help with the middle to big shift. However if the FD is wrong for the chainring interval then they may perhaps hinder as much as they might otherwise help.

cheers


I have had this happen after attempting to replace the middle ring with another with two less teeth. The chain became jammed under the ramp and no amount of careful shift action would assist it up to the big ring.
Winter had arrived in the land of Kernow. Along with it came wet roads and cool winds.
“Oh, my wheels and coupling rods!” Peetee exclaimed.

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Re: Worn Front Mech?

Postby foxyrider » 27 Oct 2020, 7:39pm

Looking at the pics i'd hope to get a lot more use out of that mech. There certainly looks like theres plenty of material left on that back plate, when its worn through its dead :)

Heavier duty? doubt you'll find anything more solid than that to do the job you need it to do, I've had the cage break on Ultegra after a couple of years but had old school mechs last decades. The crisper, cleaner the shifts, the less damage incurred so avoiding low rev upchanges is a good thing for fm longevity.
Convention? what's that then?
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Mick F
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Re: Worn Front Mech?

Postby Mick F » 28 Oct 2020, 8:08am

Thanks guys!
No worries then. :D
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peetee
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Re: Worn Front Mech?

Postby peetee » 28 Oct 2020, 9:53am

On this photo you provided you can see the back plate has a inward fold that does not in any way lie parallel to the lower edge of the front plate.
E19F8AE8-96FE-46A2-A4F5-CCA3C93ADE96.jpeg
E19F8AE8-96FE-46A2-A4F5-CCA3C93ADE96.jpeg (29.6 KiB) Viewed 173 times
The way I understand it is that this ‘ramp’ encourages the section of chain at the rear of the derailleur to lift up AND across to the large chainring. The lift action there is crucial as it needs to be done before the chain fully engages with the middle chainring teeth. Once that has started the angle of that ramp lifts the chain from the upper engaged chain wheel teeth and the shift to the outer ring goes well. As described earlier I had issues when changing the middle ring tooth count. The inner plate ramp was slightly too high and moving the mech just tucked the chain up to the flat section below the ramp where the angle change was too great and it wouldn’t move up. In reality the mech back plates are a bit more sophisticated than I have described and I think your mech cage back plate was coping with the chainring discrepancy but now it’s sufficiently worn such that there is now a sharp change between that flat section and the ramp and the chain won’t lift.
Winter had arrived in the land of Kernow. Along with it came wet roads and cool winds.
“Oh, my wheels and coupling rods!” Peetee exclaimed.

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Re: Worn Front Mech?

Postby pwa » 28 Oct 2020, 9:57am

I have had some ancient and very tired looking front mechs that still worked okay, so definitely keep using it until it no longer works.

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Re: Worn Front Mech?

Postby Brucey » 28 Oct 2020, 10:11am

pwa wrote:I have had some ancient and very tired looking front mechs that still worked okay, so definitely keep using it until it no longer works.


so have I. But those mechs were correctly matched to start with and they didn't wear anything like as quickly as Mick's appears to have done. I remember being surprised that it worked acceptably well to begin with.

cheers
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Re: Worn Front Mech?

Postby Jamesh » 28 Oct 2020, 10:21am

Suggestion from tight Yorkshire ...

How about a sliver of uhmwpe epoxied on?

Then when it wears out you simply heat and replace??

Cheers James

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Mick F
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Re: Worn Front Mech?

Postby Mick F » 28 Oct 2020, 10:24am

As I said, the mech is being used way out of its comfort zone.
It's a 9sp mech to start with, let alone coping with a HUGE difference in chainwheels ........... 34/48/61 = 27t difference when it's supposed to have a 22t max. Also, I understand the max chainwheel should be 50t for it.

The mech was as original with my TRS30 which had a 58t outer, so it was way out of spec from new.
Coupled with the fact that the braze-on on the rear triangle is far too low even for the 58t outer, so it was right at the top of the braze-on.
In order to fit a 61t, I designed a boss to fit the mech to which could be bolted to the braze-on so the mech could be higher and adjusted up and down too.

Our very own Ferrit Worrier on here built the boss for me to my design. :D

Going back to my TSR30 as was, it needed a far bigger range of gearing as far as I was concerned. First, I changed the cassette for a 11-28 instead of the 11-25 and fitted the 61t outer. This STILL wasn't enough for me, so I fitted a 3sp SA which incidentally gave me a TSR90 ......... but gave me a big BIG range ........ too big at the top end maybe.

Therefore, if and when the chainrings all wear out, I'll fit a "reasonable" one, but still keep the SA 3sp. That way, a standard triple front mech will work as per spec.

This is how it was from new.
The front mech looks to be too high up, but put it lower and the inner cage-plate caught on the middle 48t ring.
Consequently, it didn't change sweetly even from Day One.
2016-03-03 08.59.05.jpg
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Brucey
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Re: Worn Front Mech?

Postby Brucey » 28 Oct 2020, 10:52am

the big issue is not necessarily the total capacity, it is almost certainly the individual intervals, specifically the middle to big interval. 13T M-B is more than the mech is intended for and IME this will result in baulky shifts with many modern FDs. It is also usually the case that larger than normal chainrings lack shifting aids too.

If anyone wishes to simulate this arrangement, just for experimental purposes, you can try fitting an oversized chainring from an old chainset (with no shifting aids) and using that with a larger than intended M-B interval. You can get some idea just by raising the FD so that is set higher than normal and the ramps on this inner plate don't line up any more.

Even modern FDs handle the L-M shift rather differently; normally there is just a radius on the rear edge of the inner plate and the lifting action required for the shift is encouraged by the angle of the plate (as viewed from the side). This is a very 'old school' approach. This also means that the L-M interval is not fixed by the mech design; provided the chain doesn't collide with the FD unintentionally almost any L-M interval is usually OK.

Actually, looking at your latest photo, the FD is set too high I would say, and the worst wear on the FD coincides with the position of the chain when it is already on the big ring (rather than during the shift); when I see this I immediately suspect rubbing in use. As a serial FD rubber myself, I am always keeping an eye out (or more accurately an ear out) for this, but others can ride seemingly oblivious of it.

cheers
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