Tiagra - Temperamental rear derailleur?

[cyberknight]

@mickle
What are your thoughts on cable stretch ?

 

[Yellow Saddle]

Just my observations:

On my most recent cable installation I used PTFE coated cable, and aside from some adjustments needed after the first 60km of riding the shifting has been entirely reliable over the last two years. A similar installation using plain jagwire cable needed adjustment three times over the same two year period, but I only replaced the inner cables. I find it hard to draw any meaningful conclusions from this, although I may purchase the PTFE coated cable kit next time I overhaul the gear cables.

PTFE is for frying pans, not cables. It is too soft to resist the fretting motion of a gear or brake cable and quickly crumbles off, jamming (or alt least hindering) the inner's free movement through the outer. This doesn't mean that PTFE coated cables have less friction than naked steel cables, it just means that the effect doesn't last and when it fails, it goes pear-shaped.

This doesn't give you any insight into your experience. However, I will always argue that the PTFE cable coatings are a waste of money and are counter-productive. A good quality stainless steel cable does not have a daisy shape in cross section, but the outer wires have been flattened after weaving to create a smoother, less fretting shape. This is good enough and extremely durable. Most of us think that the amount of force required to shift on a bike with cables in good condition think that enough is enough. Smoother won't make us happier. A bit of feedback is nice.

 

[User6179]

The reason for this is cable housing wear

That is what I said !

 

[PpPete]

Pre-stretch is also a myth. This gives the impression that steel is soft until it has been stretched. Steel strains perfectly linearly in range before yield. To turn the phrase around (as I've heard before) and say that a "pre-stretch" isn't really a stretch but just a settling in of the winds in the cable is also wrong. For wires wound helically around each other to settle in a different position after a bit of tension would imply that they have yielded (taken on a permanent set), and this is not possible, as I've explained earlier.

Whilst you are right on your basic metallurgy I think you may have misunderstood the forces acting on helically-wound cable. I don't expect any of the individual strands to stretch but real world experience suggests that minor adjustments to the indexing are often necessary after a few tens or hundreds of miles .... and thereafter things don't change much for a few thousand. I can't see this being explained by the nylon inner starting to wear and then suddenly becoming hard enough to resist the wear.

I agree about the PTFE coated inners though - waste of time.

 

[Yellow Saddle]

Whilst you are right on your basic metallurgy I think you may have misunderstood the forces acting on helically-wound cable. I don't expect any of the individual strands to stretch but real world experience suggests that minor adjustments to the indexing are often necessary after a few tens or hundreds of miles .... and thereafter things don't change much for a few thousand. I can't see this being explained by the nylon inner starting to wear and then suddenly becoming hard enough to resist the wear.

I agree about the PTFE coated inners though - waste of time.

I often hear this but this is not my experience. Although I never logged exactly when I re-adjust my own gears and my customers rarely (I feel confident saying never, but there may have been the odd occasion) came back for such initial adjustments or mention them, I think most initial adjustments are down to poor road testing of adjusted gears. Often the bike is fine in the stand but sub-optimal on the road and I think few mechanics take enough time testing before letting the bike go.

Without dismissing your experience, I would like to find another reason for that. Cable technology is mature and well understood. There is no settling of the weave for reasons I have explained but I'll reiterate: for settling to happen we're talking about the coiling settling into a tighter position. This scenario can only happen if the individual wires yield and settle into a new position. They're hard steel and just don't yield under the small forces we apply.

What I think may happen is that the rate of wear of the housing's liner changes as the wear path progresses. For instance, if you consider the inner cable 1.2mm thick and the ID of the housing inner to be 1.5mm thick and you picture the wear as the inner "sinking" into the liner, the picture looks like this: The contact line of a cable on a lining bigger than it (I.e. new) is narrow. As the cable wears into the lining and sinks deeper, the contact line increases until the maximum point where it is exactly as wide as the inner cable. The rate of such wear would be fast initially and then slowing down and then stabilizing.

Nylon doesn't work harden, so it remains soft throughout its wear life. However, it does absorb quite a bit of water which makes it swell and become softer, but I am not a polymer scientist and cannot speculate how that would affect wear. I'm not even convinced that enough water gets in there and to the right places.

 

[Levo-Lon]

Not read all.
heat, cold and wear..cable adjuster compensates for all these .
I dont think cables stretch but a cold day makes my gears clack, so half a turn in on adjuster resets the indexing,same in a warm spell but half turn out instead..
cable adjusters are for on the move tweeks

 

[mickle]

Also not, all shifters I know of has a metal (usually zinc die-case) cable end receiver. However, even plastics won't yield on gear mechanisms.

I'm kinda delaying the answer because I'm waiting for the peanut gallery to retort. But I'll let the cat out of the bag.

Gear mechanism do go out of tune over time. The reason for this is cable housing wear. Wherever the housing makes a turn, the moving inner cable tends to file and fret away at the soft nylon inner lining and take a shortcut through the corner. It's the exact opposite of a river that erodes the outer radius on the bends. When this happens to a cable, the inner cable becomes relatively longer to the outer cable and the settings go out. However, the cable never stretched.

On brake cables there is sufficient force involved for the inner cable to show signs of wear at those sharp turns. This is visible upon inspection as shiny bits. If you rearrange the little bits of housing on a table, the shiny bits corresponds with the sharp bends, especially the one where the rear brake cable makes a transition from top tube to seat stay.

Nah.

 

[mickle]

@mickle
What are your thoughts on cable stretch ?

Sh!t stirrer!

 

[PaulSecteur]

Just my observation... my bikes that have Shimano SP41 cable (ptfe) always seem to shift more consistently and for longer than those without.

Im not gonna argue about it, that's just how it seems to me, so much so that I brought a 10 metre coil of sp41.

 

[Yellow Saddle]

Just my observation... my bikes that have Shimano SP41 cable (ptfe) always seem to shift more consistently and for longer than those without.

Im not gonna argue about it, that's just how it seems to me, so much so that I brought a 10 metre coil of sp41.

No argument about that, other than the confusion between cable and cable housing. The Teflon is great as a liner, but poor as a coating on the steel cable.

 

[cyberknight]

Sh!t stirrer!

And maybe i value your opinion , bike related of course

 

[mickle]

And maybe i value your opinion , bike related of course

Thank you for your kind words.

When we (cycle mechanics) assemble a new bike or fit new cables we give the gear lever/shifter/brifter a good hard yank to take up any slack in the system. It's not 'cable stretch' but that's what we call it. Because it's easier than trying to invent a word or phrase which explains that under the compressive load of the good hard yank the cable barrel-end settles into it's home inside the lever, the multiple ends of the freshly cut cable outers settle in to their ferrules and the ferrules settle in to their new homes in the frame stops, shifter body and derailleur. If all of these tiny settles in add up to just one millimeter then the rear derailleur will require an adjust at the cable barrel. And that's the whole story.

'Wear' of the nylon liner of the cable outer casing has nothing to do with it.

 

[mickle]

Tiagra rear derailleurs are no more or less likely to go out of adjustment than any other broadly comparible derailleur. Because it aint the deraiileur it's the cables (outers, ferrules &c).

Cycling journalist write an awful lot of made up bullsh!t.

 

[Richard A Thackeray]

I have Tiagra on my (2013) Ribble 'work'/'general' bike (one of their blue 'winter' framed offering), & the shifters appear to have an odd habit, on a intermediate 'click'/'stop', as sometimes a one-gear movement, doesn't happen, & it has to be pushed again
if you follow..

If I was a new rider, I'd probably not question it, but have been commuting for close to 30 years, & using STI since 1994, so it just seems odd

The difference comes in with the finish - DA for instance is incredibly beautifully finished - as well as durability and of course, weight. I doubt there is a lower end product that is more durable than its most expensive counterpart though.
.
If you compare to derailleurs - one say a Dura Ace and the other a Tiagra, you'll notice that the Dura ace one has almost no steel anywhere. Even the bolts are titanium. The cages are carbon or aluminium.

The difference in weight is quite phenomenal as well. It isn't all that obvious when you handle only the high-end product but if you handle both, the difference is startling.
.

Agreed, 'Pounds off = £££'
The difference in weights is substantial

Eg;
10 speed rear-derailleur
Dura-Ace = 166g
Tiagra = 265g

10 speed chainset
Ultegra = 670g
Tiagra = 943g (standard, but crank-length, not stated)

10 speed cassette
Dura-ace = 163g (11-23)
Tiagra = 261g (11-25)


All weights, taken from the 'Ribble' website (didn't look at 'Weight-Weenies')

Not quite. Bedding in at both ends suggests some sort of yield in the steel, which I've explained is not possible, particularly on gear mechanisms where the tension is very, very low.

Could it be the softer metal of the nipple distorting?

 

[Yellow Saddle]

Could it be the softer metal of the nipple distorting?

By the nipple I take it you mean the blob of metal at the one end of the cable where it anchors in the brake lever or shifter.

No. That metal is zinc and its yield point is higher than clay. If you examine a used one you'll notice that it is still in its original shape. Besides, it fits so snugly that should it distort, it wont come out.

However, the mechanism was already explained, it isn't yield in any component.