Author Topic: Shimano MTB triple with STI shifter  (Read 809 times)

Shimano MTB triple with STI shifter
« on: January 02, 2020, 10:59:42 am »
One of my bikes has a Tiagra 4703 STI shifter and a 105 5703 front mech. I just switched the chainset from a road 5703 (50/39/30) to a Alivio MTB T4060 (48/36/26) and I can't get the front mech to work at all.

I can sort of get it shifting on the workstand but on the road the only shift that works is small ring to middle ring. The rest just mean give me chain clatter and no shifting, and the upshifts are incredibly stiff. I don't think the road mech likes the wider chainline of the MTB chainset.

Would obtaining an MTB front mech solve the problem? Are they compatible with road STI shifters? Cable pull data for either seems entirely absent from the internet.

quixoticgeek

  • Mostly Harmless
Re: Shimano MTB triple with STI shifter
« Reply #1 on: January 02, 2020, 11:10:06 am »

I had a 10speed MTB 2x mech working with a Tiagra 4700 2x front shifter. It was a little picky to make it work, but once set, it was flawless for thousands of km until I upgraded to di2.

Not exactly your setup, but it's another data point for you.

J
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Beer, bikes, and backpacking
http://b.42q.eu/

Re: Shimano MTB triple with STI shifter
« Reply #2 on: January 02, 2020, 04:05:39 pm »
One of my bikes has a Tiagra 4703 STI shifter and a 105 5703 front mech. I just switched the chainset from a road 5703 (50/39/30) to a Alivio MTB T4060 (48/36/26) and I can't get the front mech to work at all.

I can sort of get it shifting on the workstand but on the road the only shift that works is small ring to middle ring. The rest just mean give me chain clatter and no shifting, and the upshifts are incredibly stiff. I don't think the road mech likes the wider chainline of the MTB chainset.

Would obtaining an MTB front mech solve the problem? Are they compatible with road STI shifters? Cable pull data for either seems entirely absent from the internet.

Starting off with a daft question, but did you reset the limit screws?
The MTB chainset has a wider BB spacing doesn't it? This means that the chainrings are further outboard than the road chainrings.
It should work if the start point for the small chainring is aligned with where the mech "thinks" it should be.
Logically you should be able to do this by setting the lower limit screw and then winding out the upper limit screw to allow the cable to pull the front mech far enough across.
The stiffness you describe sounds like it's fighting against the limit screws.

If you already did this, then just ignore me!

Re: Shimano MTB triple with STI shifter
« Reply #3 on: January 03, 2020, 03:12:23 pm »
Would obtaining an MTB front mech solve the problem? Are they compatible with road STI shifters? Cable pull data for either seems entirely absent from the internet.
Shimano front road and MTB cable pulls are different.
The Jtek Shiftmate 7 will adjust the cable pull to suit.

I did once get an XT front mech to work with Ultegra STI (triple) by clamping the cable on the wrong side of the pinch bolt, but it was very fiddly (well over an hour) to get it working. Good once set up though. That was using a Stronglight Impact 48/38/24 (square taper) on a slightly short BB axle, and it was also quite a while ago (15 years?), so the front mechs may have changed since.

Re: Shimano MTB triple with STI shifter
« Reply #4 on: January 03, 2020, 06:43:18 pm »
Shimano front road and MTB cable pulls are different.
The Jtek Shiftmate 7 will adjust the cable pull to suit.

Thanks. That's the answer I was looking for.

I've done some more fettling and made it work:
1. Swapped one of the BB spacers to the NDS to shift the chainrings in a bit. Yes, I know this will give me rickets.
2. Replaced the cable inner with a skinnier one.
3. Pivoted the whole mech backwards a bit to give it move angle at the back.

Just done a test ride and had no problem accessing all 33 gears. I suspect the item 3 made the most difference.

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
Re: Shimano MTB triple with STI shifter
« Reply #5 on: January 04, 2020, 11:43:30 am »
Sounds very similar to the setup on my road bike, a very old Sora front triple shifter, newer 105 front mech and a MTB triple crankset.

I've found I can get it to work, with fiddling, but
- more spacers on the NDS than DS, for chainline.
- cable outers longer than you'd think
- fiddling with the limit screws

not perfect, but when road triples are getting harder to find, you take your choice
“Procrastination is the thief of time, collar him.” –Charles Dickens