Front mech woes - incompatibility - or am I a duff diy mecha

[mcmoonter]

I'm try to adjust an Ultegra triple front mech fitted to my Roberts Audax frame. It worked perfectly before I swapped the crankset. I fitted a Middleburn crank onto an SKS Isis BB. The rings are Middleburn 50-34-28. Chain and cassette are used but unworn, cable and outer are new. Shifters are Ultegra Sti

The shift from the small ring to the middle requires pushing the lever past its first click and well over onto the second click to get the chain to move wen on the largest sprocket. The upshift onto the big from middle ring is hopeless. I have other triples on other bikes which work faultlessly.

I've tried adjusting the position of the mech clamp on the seat tube and rotating the mech relative to the chain rings. The improvements are only marginal.

There are two variables here. The crankset I replaced was an X type BB with a Bontrager triple 53 39 30.It was probably spaced out to 73mm. When I installed the SKS Isis BB I measured the BB shell. It was a 68 mm shell. There is a sticker on the BB which says you have to fit spacers to make up the difference to 73mm. I got two 2.5mm spacers. Are there other combinations of spacers adding up to 5mm that I could fit to space the axle in or out in either direction? Which side should I put them on?

Am I missing something obvious?

[thirdcrank]

I think it's all to do with the chainrings, or at least the steps between them. One of the effects of Shimano's attention to every small detail is that the user loses a lot of freedom to improvise. Afaik, a Shimano road triple is optimised for 52/42/30. When I lowered mine I successfully went down to 48/38/26 (it's been reported on here that current Shimano road stuff won't take a 38T) Other have mentioned using smaller inner rings, but I think those steps have to be preserved - the cage is specifically shaped to shift them, espcially middle up to big ring.

[meic]

I agree that probably your main problem is the 34 tooth middle ring.

[Edwards]

Try setting it up with the chain in a middle sprocket.
Sorry to ask but you have checked the settings for the stop screws on the mech and adjusted the cable having moved the mech down.

[mcmoonter]

Sorry to ask but you have checked the settings for the stop screws on the mech and adjusted the cable having moved the mech down.

.
Yep checked those too, and adjusted cable tension.

I'm thinking the mech is designed for a specific combination of rings, ie 52 39 30, I'll just refit the original crank just now to kep me going. I was just after some lower gears for a laden hilly tour.

[reohn2]

I too would say you're asking too much of the mech between 34 and 50t,a 16tooth difference is too much for the mech to handle IMHO.
I think you'd be OK with a 38t middle.

[Edwards]

I have been thinking about the 34 ring. This is a size normally found on a compact double. If it is for this it would not have the ramps and pins to lift the chain on the change up. From small to middle
This then led me to consider the 50 ring. If this is not from a compact. Maybe the ramps and pins are to far out to catch the chain properly.
I remember reading on The Byercycles web site about the smallest middle ring for a triple that they recommend is a 38.
Just something else to consider. I have gone down to a 48/38/26 Mtb set using a 105 front mech with no problems. So I also think it must be some thing to do with the 34 ring not the mech.
I have also replaced the 30 with 28 inner on 2 other bikes with no problems.

[mcmoonter]

Thanks Keith

I think your conclusion is correct. I've refitted the 52 39 30 chainset and the shift is flawless. The chainset you describe is listed as a treking chainset. I may consider that route.

Peter

[Edwards]

I am now curious to know about the chain rings in question. Have you checked them to find out if the theory is correct.
I like the idea of using a road triple with a 50 outer ring, so am interested to know why this set up does not work.
I just bought a Shimano Altus chain set from SJS to try and it worked.

At least you are back on the road.

[mcmoonter]

Keith

I was after the range of a compact double with a bail out granny.

The rings were all unramped Middleburns, the front mech a triple Ultegra.

[Edwards]

I seem to remember CJ writing something about adding the pins to help the chain climb to the next ring.
So is it possible to add rivets to the chain rings. Some thing I have never tried. It is so easy to forget about them. But a 38 middle might work. Do you have one that you could try.
I do not think the mech makes any difference in this.

[mcmoonter]

My feeling is the mech is designed specifically for a standard combination. I think if the smallest ring were a 30T the shift might be better.

[thirdcrank]

If you look at the cage you will see it has a sort of shoulder on the inner sideplate. Afaik the purpose of that shaping is to deal with the shift from the middle to the outer chainring. Unless you get the position of that spot on, there will be changing problems. In theory, you could move the mech up and down as much as you like to get it right, but you are also constrained to some extent by the need to have the cage the right distance above the outer and the bottom of the cage has to clear the chain at its lowest point.