Chain slipping Between Chainrings

[Mick F]

Mine has one of those Tiagra front mechs, but what you say isn't quite right regarding the seat tube angle.

With a Moulton TSR, the bike doesn't have the BB below the chainstays, but above it
Bikes, generally, are defined by "BB Drop" as in that the BB is lower than the centres of the wheels.
Moulton TSRs have the BB above the wheel centres and as the outer chainwheel is larger than "normal" bikes, the angle of the chain to the cassette, and the front mech angle too, isn't "normal" by any means.

[Mick F]

Compare that to my Mercian.

[slowster]

I'm running a Shimano 10 speed chain.

Apologies for probably teaching granny to suck eggs, but is the chain on the right way round, i.e. with the lettering on the side plates facing outwards?

[slowster]

Mine has one of those Tiagra front mechs

I think you have the previous model, FD-4603, which does not have the channel in the outer part of the cage that the OP's FD-4703 has. Most of the online photographs of the FD-4703 on retailers' websites show a model with a very distinct channel in the outer side of the cage, although Shimano's own website shows a cage without a channel in the outer side (I suspect that might be an earlier version or pre-production model).

Incidentally, according to the Bike Components listing for the FD-4703, the derailleur is supplied with the Shimano cable setting tool, which is for front derailleurs with a choice of two settings depending upon the angle of the cable, but I cannot see the corresponding protruding pin on the back of the derailleur body.

https://www.bike-components.de/en/Shima ... ur-p43860/

FD-4603

[iandusud]

Slowster, I had considered your theory about the chainstay angle (well spotted) but the correct part of the front mech is making contact with the chain and dropping it off the big ring. The real problem is not enough movement in the mech for the chainset, so if I set it up to make clean shifts from the outer to middle and middle to inner then I get chain rub when on the outer and on the smaller sprockets. If I set it up to get no chain rub on the outer then I get poor shifts from outer to middle (with the chain struggling to engage) and difficulty engaging the inner ring because I've run out of cable travel (not mech adjustment). As you have noticed the FD-4703 front mech doesn't have the pin giving a choice of settings depending on cable angle that the FD-4700 has. But why?

[iandusud]

Mine has one of those Tiagra front mechs, but what you say isn't quite right regarding the seat tube angle.

With a Moulton TSR, the bike doesn't have the BB below the chainstays, but above it
Bikes, generally, are defined by "BB Drop" as in that the BB is lower than the centres of the wheels.
Moulton TSRs have the BB above the wheel centres and as the outer chainwheel is larger than "normal" bikes, the angle of the chain to the cassette, and the front mech angle too, isn't "normal" by any means.

Triple Chainset.jpeg

But the front mech is angled back much more to compensate.

[iandusud]

Last night I put my previous chainset on and there are no issues whatsoever with the chain failing to engage with the middle chainring when shifting from the outer. Although it isn't easy to measure, visually the spacing between the rings is less on the previous chainset. When I get a moment I will carefully take a file to the spider and report back.

[roubaixtuesday]

Last night I put my previous chainset on and there are no issues whatsoever with the chain failing to engage with the middle chainring when shifting from the outer. Although it isn't easy to measure, visually the spacing between the rings is less on the previous chainset. When I get a moment I will carefully take a file to the spider and report back.

The focus on mechs here was bemusing to me, always seemed more likely to be chainset related.

[peetee]

Last night I put my previous chainset on and there are no issues whatsoever with the chain failing to engage with the middle chainring when shifting from the outer. Although it isn't easy to measure, visually the spacing between the rings is less on the previous chainset. When I get a moment I will carefully take a file to the spider and report back.

Hmm. That will then create a bigger gap between middle and small chainrings.

[iandusud]

No, I'll take the material off the outer face.

[531colin]

Last night I put my previous chainset on and there are no issues whatsoever with the chain failing to engage with the middle chainring when shifting from the outer. Although it isn't easy to measure, visually the spacing between the rings is less on the previous chainset. When I get a moment I will carefully take a file to the spider and report back.

How does chainline compare with the two chainsets?
Chainring spacing is best compared by putting the big ring of one against the granny ring of the other, that way the rings line up better. (maybe thats obvious?)

[zenitb]

No, I'll take the material off the outer face.

I have a micrometer and, when I have a chance I will measure the amount of aluminium between the two faces Ian.

Its easier I guess if I measure the spare Thorn and Spa spiders I have that are not on the tandem ATM although ideally I should measure the spider that is currently installed on the tandem and generating issues (a Thorn one - tolerances?)

Hoping we can share measurements...

[iandusud]

Ok, I measured the lugs on the spider at 3.5'm and the spacing between the teeth on the outer and inner chain rings at 4mm, giving a spacing of 7.5'm. Shimano give the spacing on their road triples at 7.1 mm, so I filled 0.5 mm of the outer face of the lugs, reassembled and it has been transformed! Works perfectly.

[roubaixtuesday]

Ok, I measured the lugs on the spider at 3.5'm and the spacing between the teeth on the outer and inner chain rings at 4mm, giving a spacing of 7.5'm. Shimano give the spacing on their road triples at 7.1 mm, so I filled 0.5 mm of the outer face of the lugs, reassembled and it has been transformed! Works perfectly.

Nice one. And thanks for sharing.

[zenitb]

Ok, I measured the lugs on the spider at 3.5'm and the spacing between the teeth on the outer and inner chain rings at 4mm, giving a spacing of 7.5'm. Shimano give the spacing on their road triples at 7.1 mm, so I filled 0.5 mm of the outer face of the lugs, reassembled and it has been transformed! Works perfectly.

Whoa! Thats a result. Ok ..time to measure and (probably) file mine. Many thanks for sharing this Ian.