Gear Cable fatigue failures in STIs/Ergos

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Brucey
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Gear Cable fatigue failures in STIs/Ergos

Postby Brucey » 20 Sep 2012, 9:32pm

One of my chums uses a Shimano STI-equipped bike for commuting on; several times in the last two and a half years he has experienced a cable failure inside the STI unit. On average his rear shifter cable has lasted no more than nine months. At first I was inclined to blame poor quality cables for the problem, but now I'm not so sure.

First a note on cable inners;

Campagnolo gear cable inners are slightly larger diameter than Shimano gear cable inners (in contrast to the nipples). The cable diameter for all gear cables runs from as thin as 1.1mm up to 1.6mm. Few currently available have more than about twenty strands in them. [At one time 'braided' cables with more, finer strands were popular, e.g. from Campagnolo and others. These were significantly more flexible and probably last at lot longer, but do not seem to be available any longer. Probably they are not desirable for index shifting because they are not as smooth on the outside].

Wire ropes (cables) have quite predictable fatigue performance in bending (given fixed lubrication and abrasion conditions) and there are design rules for what diameter pulley (sheave) should be used for good fatigue performance.

There is more detail e.g. here;
http://www.wireropeworks.com/product_pdfs/EL_TB_09.pdf
http://www.unols.org/publications/winch_wire_handbook__3rd_ed/08_oper_char_ropes_cables.PDF

but to cut a long story short, a 40:1 pulley diameter:cable diameter is typically recommended for applications where premature cable fatigue 'would be bad', e.g. in lift (elevator) suspension cables :shock: .

[edit; more info here toohttp://books.google.co.uk/books?id=Pk9mZYT0G80C&lpg=PT203&ots=qi-aLffBzQ&dq=friction%20in%20bowden%20cables&pg=PT203#v=onepage&q=friction%20in%20bowden%20cables&f=false which is the link I was thinking of when I found the other one.... :roll: ]

If pulleys of this recommended size were used on gear levers and inside STI's /Ergos then the cables would be wrapped round drums of ~44 to 64mm diameter. Needless to say this is not what we find; old-style gear levers have ~15-20mm diameter. STIs and Ergos are similar. MTB-style 'pods' are sometimes a slightly larger diameter, as are some SRAM-pull gripshifts etc. However many of these shifters also send the cable round tight bends en route, as well round a small diameter drum. Suffice it to say that none of these devices would pass muster as designs for elevator suspension cables, and we must anticipate that the cables will suffer fatigue failure at some point.

Oddly enough, an old-style Sturmey-Archer trigger shifter comes closer than most to meeting 'good design for fatigue' rules; the cable is skinny (~1mm) and the pulley diameter inside the shifter is about 30mm.

By contrast a typical shimano rear mech requires the cable to undergo a very sharp bend next to the pinch bolt; cables very often fail here, too.

My chum tells me that he estimates his gearchanges as running to about 20-30, 000 per month. This is rather a lot, but does explain why his cables don't last!

On a related point, gear and brake systems that use extra pullies ('travel agents', 'rollamajigs', 'shift mates', pulley-style brake hangers etc) must also render cables susceptible to similar fatigue. (One MTB I owned had a hidden pulley in the stem, and only a braided cable would render it remotely reliable).

Since removing broken cables from STIs is awkward, and broken brake cables are dangerous, I would suggest that regular inspection of cables wherever they are likely to flex and/or be wrapped round pulleys is a very good idea. Slotted cable guides make such inspections very easy to do, without using any tools.

cheers
Last edited by Brucey on 2 Oct 2012, 10:55pm, edited 1 time in total.
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reohn2
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Re: Gear Cable fatigue failures in STIs/Ergos

Postby reohn2 » 20 Sep 2012, 10:00pm

I've never had a gearwire break in a road STI and I've been using them since 1997 on solos and for the last 6years on tandems.
What I tend to do(in fact I fitted one this afternoon)is dip the nipple and first 2" of cable into grease before threading into the STI unit,then with grease between thumb and forefinger coat where the cable is inside the outer(s) before threading,to stop water ingress.
The worst bit for water ingress IME is the short outer on the rear mech so I keep a careful eye on that bit,especially in winter,since I once had one freeze solid and had to resort to warm body fluid :shock: to get it moving again.
I take your point about cable wires and minimum pulley radii but as I've no choice,I try to make things as slippery as possible.
I also find it best to use pre-stretched stainless gearwires,it also helps with slick shifting to change the outers every 12months or so IME.
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Brucey
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Re: Gear Cable fatigue failures in STIs/Ergos

Postby Brucey » 20 Sep 2012, 10:09pm

that is very interesting. I don't think I've ever had more than a couple of gear cables fray round the lever itself, but then I probably don't change gear as much as some folk; also, like you, I try to keep my cables well lubricated, too.

Any idea how long you let your inners go before changing them?

cheers
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mattsccm
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Re: Gear Cable fatigue failures in STIs/Ergos

Postby mattsccm » 20 Sep 2012, 10:24pm

I can't remember ever having a cable break.
They go tatty and eventually flatten and split if you repeatedly undo the clamp bolt. Other wise . Never.

reohn2
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Re: Gear Cable fatigue failures in STIs/Ergos

Postby reohn2 » 20 Sep 2012, 10:30pm

Brucey wrote:Any idea how long you let your inners go before changing them?
cheers

I tend to inspect them when I change the outers and if they'll go through the outer after being flattened at the pinch bolt on the mech I reuse them or I may cut a rear short and use it as a front.
I'd say I can get 3years out of an inner.
I've once had a single wire breakage near the nipple cause mayhem on rear changing.

PS,reading that,it sounds tight fisted of me not to change them when I change the outers,which come to think of it I probably am :oops:
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Mick F
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Re: Gear Cable fatigue failures in STIs/Ergos

Postby Mick F » 21 Sep 2012, 7:38am

I've never had an Ergo gear cable break either.
(except once at the front mech where the pinch bolt is because I had it laced incorrectly)

I'm willing to bet that I do well in excess of anyone in the amount of gear changes I do - both front and rear. I rarely remain in the same gear for more than a few minutes.

I'm on my second set of complete cables since I went to Ergos in 2004. I changed them because I thought they were due and would be better renewed. I've decided to leave well alone now and only do breakdown maintenance on them.
Mick F. Cornwall

Brucey
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Re: Gear Cable fatigue failures in STIs/Ergos

Postby Brucey » 21 Sep 2012, 7:58am

Mick F wrote: ...I'm willing to bet that I do well in excess of anyone in the amount of gear changes I do - both front and rear. I rarely remain in the same gear for more than a few minutes...


I wonder; my chum's estimate was, frankly, unbelieveable to me. But then I took notice of what he was actually doing as he rode along, and he genuinely changes gear very often; once or more every hundred yards would be a very fair estimate. I doubt he very often stays in one gear for even one whole minute....

Re. cable replacement/maintenance; for most uses I reckon lube, inspect, replace if necessary is a good way to go. It is what I do, (mind you, I suppose my 'derailleur gear miles' are spread thin over several bikes). But for my chum I am inclined to advise a 'replace inners' policy once every six months; the inners are cheap enough and replacing them once they have actually broken is a total ball-ache.

cheers
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Ayesha
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Re: Gear Cable fatigue failures in STIs/Ergos

Postby Ayesha » 21 Sep 2012, 8:03am

Ask your chum...

When you move the chain to the largest sprocket, do you give the shifter a real hard push to make sure the chain climbs?

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Mick F
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Re: Gear Cable fatigue failures in STIs/Ergos

Postby Mick F » 21 Sep 2012, 8:50am

I once tried to count the gear changes on my commute. For the life of me I cannot remember how many times. Back then I had a 6sp double (latterly went to 7sp). I tried to count how many times I moved the shifters. Hundreds, seems to ring a bell with me. I commuted 17miles each way and that ride took less than an hour in and a bit more than an hour home.

Say a minimum of 100 per journey of 1hr = 1 change per 1.2mins or even more frequently.

I was out yesterday, and rode up to Yelverton. There's a long climb along the A386 from Bedford Bridge over the River Walkham, up through Horrabridge to Yelverton. About 2.5miles - not steep, just a long climb.

I shifted to the middle ring not far from the bridge, then clicked up the block as the hill began to bite and found that eventually I was in middle/big. By Horrabridge, I was on the inner ring and perhaps half way up the block. The hill tends to be concave from then on, and slowly I clicked away down the block one cog at a time. By the time I'd neared the top, I'd reached small/small!

It just shows you that you can click away without thinking- almost subconsciously - and the number of gear changes mounts and mounts and mounts.

I must start counting again and see how many per mile I do.
Mick F. Cornwall

reohn2
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Re: Gear Cable fatigue failures in STIs/Ergos

Postby reohn2 » 21 Sep 2012, 9:33am

I must start counting again and see how many per mile I do.

MickF
You're a serious nerd :)
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Mick F
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Re: Gear Cable fatigue failures in STIs/Ergos

Postby Mick F » 21 Sep 2012, 1:45pm

No, not a nerd. Serious maybe! :D

Back in the old days with DT shifters, it was a noticeable thing to do - changing gear. These days with the convenience of Ergo/STI, you can change gear without even thinking about it. Also, with a 6sp block (14-28) the jumps were bigger. Now I have 10sp 13-29 so the jumps are tiny by comparison, so basically I'm clicking away all the time to keep in a comfortable cadence. In the old days, my cadence must have varied much more.

So, the subject that Brucey has raised is a valid one. I don't believe it's making much difference to the cables I use, but it makes me realise how reliable the whole system is - cables, Ergos, mechs, chain - the lot!

One gearshift per minute over a six hour ride is 360 gear changes, most of them many-multiple clicks. It says a great deal about the reliability of the technology.
Mick F. Cornwall

David Cox
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Re: Gear Cable fatigue failures in STIs/Ergos

Postby David Cox » 21 Sep 2012, 5:10pm

I had rear derailleur STI cable go at nipple end after 4 years on "best" bike. I now know it was going a bit the week before probably frayed because indexing wasn't quite right. Campag Ergo cables lasted until respray 10+years. I really ought to get into a maintenance/replacement schedule.

mig
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Re: Gear Cable fatigue failures in STIs/Ergos

Postby mig » 21 Sep 2012, 9:08pm

Brucey wrote:
Mick F wrote: ...I'm willing to bet that I do well in excess of anyone in the amount of gear changes I do - both front and rear. I rarely remain in the same gear for more than a few minutes...


I wonder; my chum's estimate was, frankly, unbelieveable to me. But then I took notice of what he was actually doing as he rode along, and he genuinely changes gear very often; once or more every hundred yards would be a very fair estimate. I doubt he very often stays in one gear for even one whole minute....

Re. cable replacement/maintenance; for most uses I reckon lube, inspect, replace if necessary is a good way to go. It is what I do, (mind you, I suppose my 'derailleur gear miles' are spread thin over several bikes). But for my chum I am inclined to advise a 'replace inners' policy once every six months; the inners are cheap enough and replacing them once they have actually broken is a total ball-ache.

cheers


:shock: is this on a flat road?

Brucey
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Re: Gear Cable fatigue failures in STIs/Ergos

Postby Brucey » 21 Sep 2012, 9:36pm

mig wrote: :shock: is this on a flat road?

-almost, yes. I genuinely have not seen anyone change gear quite as much as this before. Maybe I've led a very sheltered life... :roll:

cheers
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mig
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Re: Gear Cable fatigue failures in STIs/Ergos

Postby mig » 21 Sep 2012, 9:41pm

what reason does he give for changing gear?!