Expert advice on choice of front derailleur, please.

[Chris Jeggo]

I am building a touring bike with a 3x9 transmission. Hubs are Shimano 105 and chainset is Spa TD-2 on its recommended 113mm BB so chainline is 45mm. Downtube is 28.6mm. Chainrings are 40-33-24t to get the gear ratios I want. MTB FDs are designed for smaller chainrings than road FDs, but I think their 50mm chainline would be likely to make my inner ring unreachable. Also, my chainstay angle is about 62deg while MTB FDs are spec'd for larger angles, but this is probably not so critical. So what would the shifting be like using a road mech with its larger-radius outer cage plate? And with the mech mounted high because of outer-middle minimum tooth difference requirements? For example, Shimano Sora FD-R3030-B needs an 11t difference so I would have to mount the mech as if I had a 44t outer, or thereabouts; total capacity is 20t so that's OK. That calculation is much the same for a triple MTB mech such as Shimano Deore FD-M591 with 12t outer-middle difference and 22t total capacity. I am using a down-tube friction LH lever so indexing is not an issue; I am happy to overshift and then trim if necessary. So what FD should I buy? I am trying to do something Mr Shimano doesn't want me to do, so what is the least worse compromise? Would it help to consider 8-sp or 10-sp mechs too? Or obsolete FDs readily obtainable on eBay?

[rotavator]

FWIW after much experimentation in my quest for lower gears, I am using a Campag Centaur 10 sp FD with a Spa TD-2 24-36-46T triple crankset (and Campag Ergo shifter) and it is working well. I may change the middle ring to a 34T to help the middle to inner shift which is not quite as smooth as the middle to/from outer shift.

[Brucey]

one FD I have used with this kind of setup is FD-M700. This is going back a bit (6s.... ) but it works fine with 8s chain and the front can be 'tweaked inwards' to make it narrower if the chain unships off the big ring too often.



https://si.shimano.com/pdfs/ev/EV-FD-M700-0552A.pdf

https://si.shimano.com/pdfs/si/SI-N-1B-000-00-ENG.pdf

this FD will cope with tight chainlines reasonably well, and is designed for middle-big intervals as small as 8T. I've used it on 6T and obviously it needs to be set slightly high.
However I am concerned that
- your 'big ring' is a bit small so the curvature won't match perfectly
- your big ring is a bit small and the tail of the FD may clout the chainstay
- your chainline is too tight even for this FD and you won't get the inner chainring perfectly

Shimano made some (a few) FDs in the 7s/8s era which had similar characteristics, such as FD-1057

https://si.shimano.com/pdfs/si/SI-5B70D-000-00-ENG.pdf

which allows 10T minimum interval.

Later shimano FDs eg FD-M751 (9s) are made in high band bottom pull format and are almost correct but fail on the chainring intervals; they 'expect' at least 12T interval between middle and big ring.

Note that if you use a braze-on mech with a band-on converter, some modification of the fitment can be made (by filing one or the other) so that the FD fits better to a smaller big ring

cheers

[Chris Jeggo]

Gentlemen, thank you for your replies so far.

A further thought: 20 years ago I used a half-step-and-granny set-up. I haven't kept that front mech, nor a precise record of what model it was, but IIRC it was a double, not a triple. So might a double work better than a triple for the set-up I currently desire (which is close to a 'one-step-and-granny') because I would not have to set the mech high to avoid the inner cage plate interfering with the middle ring when the outer ring is selected?

[Brucey]

yes it might, and the less complex inner cage plate design in older double FDs (and older triples such as FD-M700) mean more consistent shifting using different chainring sizes, albeit it probably isn't ever going to be quite as slick as combinations that 'match' perfectly.

IIRC campag triple FDs are meant to shift a 10T interval, so they can work very well as rotovator suggests. I happen to favour a FD with a wide cage and I've used FD-M700 quite happily for many years. NB there are two versions; FD-M700 (as pictured) and FD-M700-GS which is different; IIRC it has a longer cage (so more likely to hit the chainstay?) and has a cutout in the outer cage plate so it is easy to spot which is which.

cheers

[peterh11]

Just picking up on your comment about the BB length: I have recently fitted a Spa TD-2 on my tourer, and with a 116mm First Components DX-30. The chainline is spot on (between 44 and 45mm) when tightened up to 35Nm with a torque wrench. I had ordered a 113mm BB initially but this was definitely too short on my bike (see my earlier posts on the subject). May be OK as long you would be happy with a closer chainline on a frame with narrow chainstays but mine has wide ones to take up to a 50mm tyre.

FWIW I have 24/34/46 chainrings on that chainset. A Shimano Tiagra triple (FR4703) shifts very well on this using the Tiagra brifter. Better than on the original Tiagra chainset. I have another bike with the XD-2 chainset, same chainring size and same FD but friction lever, with 9-speed chain. Again shifts very well, little trimming needed. It also worked well with 26/36/46 chainrings before I changed them, though I had to have it a bit raised up for best performance. I don’t know what it would be like with a smaller middle-outer difference but worth asking around perhaps - this is IMO an excellent triple FD.

I’m curious about your gearing choices, can you share a bit more about that? Sounds interesting.

Peter H

[slowster]

SJS occasionally have some New Old Stock MTB front derailleurs. Currently they have this one, which might be worth trying given the low price.

https://www.sjscycles.co.uk/derailleurs-front/shimano-ty30-triple-front-derailleur-286-mm/

http://www.sjscycles.com/Instructions/Shimano/Shimano_Tourney_TY30_Front_Derailleur_Instructions.pdf

[mcshroom]

If you want to use a MTB front derailleur, you could get a wider BB and space the chainset to 50mm chainline. I have a Stronglight impact triple on a 122.5mm bb matched with an Alivio front derailleur. I haven't measured the chainline, but it all works fine.

[Chris Jeggo]

SJS occasionally have some New Old Stock MTB front derailleurs. Currently they have this one, which might be worth trying given the low price.

Thanks for that. It's cheap enough and easy enough to do the experiment, so I shall.

[Brucey]

I just looked on ebay and the going rate for 'deer head' FD-M700 appears to be about £35 (*cough*....) used or about double that NOS.

There are other (less expensive) shimano FDs which might do though...

cheers

[Chris Jeggo]

Just picking up on your comment about the BB length: ...

I’m curious about your gearing choices, can you share a bit more about that? Sounds interesting.

Peter H

Yes, the chainline is tight. With my 113mm axle it is 44-45mm but there seems to be enough clearance so I will stick with that.

Re gearing. Lately I've been using two bikes. The first is a Hewitt Cheviot SE (2010) which came as a complete bike with mainly Shimano components including STI levers, 44-32-22 chainset and 11-34 9sp cassette. I later tweaked the gearing by changing the outer ring to 42t and the cassette to 12-36t and have been very happy with that. The second bike is a 1984 Roberts fitted with Stronglight Impact triple, 40-24t with no third outer ring, so the 40t ring is on the 45mm chainline which goes with the 105 rear hub in the dropouts which at some stage were spread to 130mm to accommodate more modern equipment. Thus the Roberts has a similar range of gear ratios, and I have been able to compare double chainset with triple. The double shifts well, of course, but so does the triple - I have had no problems with the Hewitt. However, on the Roberts I do not like that huge shift from 40t to 24t, equivalent to changing 3-1/2 sprockets at the back in one go, and I do not like double shifts.

The 1991 Mercian I am currently building will replace the Roberts and use some components transferred from it. I wanted to use non-STI shifters, square taper BB and stick with 105mm chainline. Gearing-wise I wanted to insert a middle chainring between the 40t and 24t. Without regard to constraints I would have chosen 32t, but the Spa TD-2 seemed the obvious choice and the smallest ring that fits on 110mm BCD has 33t - good enough. I look forward to finding out how I get on with it.

Chris

[Chris Jeggo]

I just looked on ebay and the going rate for 'deer head' FD-M700 appears to be about £35 (*cough*....) used or about double that NOS.

There are other (less expensive) shimano FDs which might do though...

cheers

Thanks, Brucey, for that really useful suggestion. £35 does seem OTT, but if it's the only good tool for the job I'll buy it.

Chris

[nsew]

I am building a touring bike with a 3x9 transmission. Hubs are Shimano 105 and chainset is Spa TD-2 on its recommended 113mm BB so chainline is 45mm. Downtube is 28.6mm. Chainrings are 40-33-24t to get the gear ratios I want. MTB FDs are designed for smaller chainrings than road FDs, but I think their 50mm chainline would be likely to make my inner ring unreachable. Also, my chainstay angle is about 62deg while MTB FDs are spec'd for larger angles, but this is probably not so critical. So what would the shifting be like using a road mech with its larger-radius outer cage plate? And with the mech mounted high because of outer-middle minimum tooth difference requirements? For example, Shimano Sora FD-R3030-B needs an 11t difference so I would have to mount the mech as if I had a 44t outer, or thereabouts; total capacity is 20t so that's OK. That calculation is much the same for a triple MTB mech such as Shimano Deore FD-M591 with 12t outer-middle difference and 22t total capacity. I am using a down-tube friction LH lever so indexing is not an issue; I am happy to overshift and then trim if necessary. So what FD should I buy? I am trying to do something Mr Shimano doesn't want me to do, so what is the least worse compromise? Would it help to consider 8-sp or 10-sp mechs too? Or obsolete FDs readily obtainable on eBay?

This Ultegra level FD will shift your combination with ease. I’m currently shifting 9sp 44-32-22 with a 50mm chainline and previously a 7sp 42-39-24 with a 47.5mm chainline. This appears to be the 28.6-31.8mm version but is missing the 28.6 shim in the photo. You would obviously need to confirm that with the seller.

https://si.shimano.com/pdfs/si/SI-5MW0A-002-ENG.pdf

https://www.ebay.co.uk/itm/Shimano-CX70 ... SwqA1e08kk

[nsew]

Scratch that. That’s the top pull version as is the other on eBay. The German online bike shops carry them but be aware there’s a few versions.

[gaz]

I've been using road doubles to friction shift small outer triples since the late 80's.

Height relative to the outer chainring is governed by clearing the chainstay, I can't say I've never overshifted but it's been extremely rare.

Pictured is a Suntour Superbe Pro, BITD I also used Shimano Dura-Ace 7401/7402 (IIRC) and 1050, typically running a 38T outer. No experience of trying the same with current mechs.