Someone school me on indexed triple cranksets.

uart
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Someone school me on indexed triple cranksets.

Postby uart » Fri Oct 12, 2018 8:32 pm

Believe it or not, I've had almost no exposure to indexed (as opposed to friction shift) triple cranks. :shock: I haven't actually got back into the MTB side of things since returning to cycling, so the only triple I've really had any experience with is my ancient old Apollo MTB that is so old (like prolly early 90s) that it uses friction shift front and friction or indexed rear.

Anyway, last week I dug out said ancient MTB to get it running again, just to do a little bit of light trail touring, nothing too serious. No problems, old and crappy as it is I had it running perfectly after a little bit of tuning. However, I also decided to fix up another old one (that I've had lying around here for ages) for my wife.

This second one is also very old, but it must be a little bit newer as it is index only - front and rear. This one also tuned up pretty well except for one thing, I can't get the indexed front derailleur to centre properly on the middle ring. In just about every rear position it makes an annoying little scrape-scrape-scrape noise as I pedal down the road. It's only a tiny rub, and I know that it probably has only a minuscule impact on efficiency, but it's as annoying as hell.

TLDR: With all crank sets you can adjust both of the end stops so that neither the inner most or the outermost chainrings rub (at least in most gears). But what do you do if the alignment is a bit out on the middle ring on a triple setup?
Last edited by uart on Fri Oct 12, 2018 11:23 pm, edited 1 time in total.

ironhanglider
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Re: Someone school my on indexed triple cranksets.

Postby ironhanglider » Fri Oct 12, 2018 10:02 pm

The limit screws (by definition) should have no effect when the chain is on the middle chainring, because it is nowhere near either the lower or upper limit. the position on the middle ring is entirely determined by cable position/tension.

The quick and dirty method: If the derailler is resting against the lower limit (and the chain runs close to but not rubbing the inside plate in the lowest cog) and there is minimal tension on the cable then you are probably pretty close.

There are more complete answers out there if you go looking.


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Duck!
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Re: Someone school my on indexed triple cranksets.

Postby Duck! » Fri Oct 12, 2018 10:17 pm

I'll start by pointing out that there's no such thing as an indexed crank; it's the shift action that is indexed.

With that out of the way, derailleur setup is fairly critical for achieving properly aligned indexing. Start with setting the derailleur height; the bottom edge of the outer cage plate should be no more than 2mm clear of the tips of the tallest teeth on the big ring (look closely at the ring, you'll see that there are teeth of differing heights and shapes, in repeated patterns around the ring; these aid shifting). Next set the fore & aft alignment so the cage is parallel with the rings.

Shift the rear to Low gear, the largest sprocket, so the chainline is at its most inboard, and adjust the front derailleur's Low limit so there is about 1mm clearance between the inner cage plate and the chain. Ensure the barrel adjuster on the shifter is wound OUT a few turns (so you have some room to ease cable tension if necessary), and connect the cable to the derailleur, being careful to note correct routing over any leverage tabs, not under them. Don't pull the cable too firmly, many MTB front derailleurs like a bit of slack in the low position.

Shift up to the middle ring. If it's slow to shift, drop back to Low gear & add tension. When on the middle ring shift across the cassette to High gear (small sprocket), and shift down to the small ring; if it doesn't shift, ease the FD Low limit until the chain drops to the small ring. Shift back to the middle ring. You should be able to cover the full width of the cassette with no chain rub on the FD. Play with cable tension to adjust derailleur/chain clearance in the extreme gears.

Shift to the big ring and small rear sprocket, and adjust the FD High limit so there's about 1mm clearance between the outer cage plate and the chain. If thre limit is set correctly but the derailleur drops away from it after shifting and causes chain rub, add cable tension.

It is almost inevitable that triples will have some chain rub in the cross-chained gears (small ring/small sprocket and big ring/big sprocket), but on the middle ring you should be able to cover the whole cassette without FD rub, especially on 5,6 or 7-sp. systems which have narrower clusters than 8+ (8, 9 & 10-sp. are near enough to the same overall width, 11 is wider again). The key factors are derailleur alignment and cable tension.
I had a thought, but it got run over as it crossed my mind.

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Duck!
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Re: Someone school my on indexed triple cranksets.

Postby Duck! » Fri Oct 12, 2018 10:22 pm

Oh, and also check the bottom bracket is running properly. If the crank is wobbling around in it, you'll have noise in every gear because the rings aren't tracking straight.
I had a thought, but it got run over as it crossed my mind.

uart
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Re: Someone school my on indexed triple cranksets.

Postby uart » Fri Oct 12, 2018 10:40 pm

ironhanglider wrote:The limit screws (by definition) should have no effect when the chain is on the middle chainring, because it is nowhere near either the lower or upper limit.

LOL, that was exactly my point. I can adjust the two limits no problems so that the derailleur is positioned correctly in relation to the outer and inner rings. I did that right from the get go, but the middle one remains slightly out of centre causing rubbing (the position it clicks into is consistently a few mm too far outboard).

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Re: Someone school my on indexed triple cranksets.

Postby Duck! » Fri Oct 12, 2018 10:48 pm

That means you have too much cable tension.

Although you may need to very slightly ease the Low limit, as that dertermines the reference point for the indexing, and then tweak the tension. Just make sure that it can shift from middle to small ring while on the large rear sprocket without overshifting & dropping off the inside.
I had a thought, but it got run over as it crossed my mind.

uart
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Re: Someone school my on indexed triple cranksets.

Postby uart » Fri Oct 12, 2018 10:59 pm

Duck! wrote:I'll start by pointing out that there's no such thing as an indexed crank; it's the shift action that is indexed.

Seriously, you don't need to tell me that. :?

With that out of the way, derailleur setup is fairly critical for achieving properly aligned indexing. Start with setting the derailleur height; the bottom edge of the outer cage plate should be no more than 2mm clear of the tips of the tallest teeth on the big ring (look closely at the ring, you'll see that there are teeth of differing heights and shapes, in repeated patterns around the ring; these aid shifting). Next set the fore & aft alignment so the cage is parallel with the rings....


Yeah I had already adjusted the height and everything reasonably well. I'll take another look tomorrow in better light as I was rushing it a bit late this arvo.

You know it might just be that this old POS doesn't have the original FD and it's simply incompatible with the index levers. I really don't know much about this old bike, it was just an old junker that someone gave me years ago on the chance that I might want to get it running again. It was one of those things that I just never got around to doing, so it's sat here for god knows how long and I've never even ridden it before. It could be any mix of old junk for all I know.

Anyway, it's such an old junker that I'll probably just take to it's pressed steel FD with a pair of pliers and a pair of multigrips and see if I can bend that sucker a little bit to stop it rubbing (after double checking that the height and alignment are already as good as I can make them). If that doesn't work I start looking through my parts box for another triple compatible FD.

uart
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Re: Someone school my on indexed triple cranksets.

Postby uart » Fri Oct 12, 2018 11:12 pm

Duck! wrote:That means you have too much cable tension.

Although you may need to very slightly ease the Low limit, as that dertermines the reference point for the indexing, and then tweak the tension. Just make sure that it can shift from middle to small ring while on the large rear sprocket without overshifting & dropping off the inside.


Yeah thanks duck ( I hadn't read this reply before making my other comment above, so ignore that).

I knew about the inner limit being a reference point, and I had already set that as far inboard as I possibly could (any more and I lose chain off the inner ring on downshift) to try and alleviate this issue.

You could be right about the cable tension though. I would normally have a adjusted that a little better but it's got a frayed cable there at the moment, and that made setting the tension a little more difficult (and I was trying to keep the frayed ends under the clamp screw so that may have caused me to leave a little more tension there than I'd normally do). I'll double check that tomorrow.

BTW. I know that I have to get some new cables and a few minor bits and bobs for this bike. I was just doing a quick and nasty fix/tune today to make sure it was otherwise rideable and therefore worth the effort.

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