bottom bracket chainline front derailleur

[stephenjubb]

I bought a Dawes 2003 Super Galaxy last March.

The cranks were too far in for my knees so I changed my bottom bracket from Shimano UN54 Bottom Bracket Square Taper
size 68-110mm to 68-115mm thus moving the cranks out 2.5mm. This did help my knees.

The bike mechanic from the LBS did mention I may have problems using the large 48 tooth outer crank and he was correct.

This did not bother me on tour as I rarely used it.

However I would like to fix this as changing from small to middle to outer was never exactly smooth.

Am I correct in assuming that with a bottom bracket of size 68-110mm should give a chainline (measured as per Sheldon Browns instructions) should be 47.5mm.

With the new bottom bracker of 68-115mm the chainline is now 50mm, makes sense given the extra 2.5mm on each side from the increase in bottom bracket.

What confuses me is before the change in bottom bracket when it was 68-110mm the shimano specs for the Tiagra front derailleur mention a chainline of 45mm so how did it work in the past?

However this does not matter so much, merily curiosity if anyone can answer, I am focused on a solution.

I have found the Shimano Deore front derailleur FD-M511 at

http://techdocs.shimano.com/media/t...eSystem/SI-F740I-En_v1_m56577569830612824.pdf

has a chainline of 47.5mm-50mm, chainstay angles match those of the Tiagra FD, however the technical docs for the Deore FD
state it can only be used with a crankset of 44-32-22T and mine is 48/36/22 tooth chainrings, so may not work with a 48 teeth outer ring.

The specs also state it will only work with a maximum bottom bracket of 68-113, I suspect 1mm in the new bottom bracket of 68-115 shouln't make a difference.

The specs also state it can only be used with Rapidfire M9 (Shifting lever) ST-M510 / SL-M510 shifters. Will they work with Tiagra 9 Speed STIs?

It also seems the diameter of the band on is L (34,9 mm) with shimmies to fit it on my seat tube of 28.6mm. This may cause problems with the FD being further out.

It seems to me based on the evidence above it will not work?

Can anyone please give their opinions?

 

[RecordAceFromNew]

Hi Stephen since Dawes tend to push component limits/mix to achieve touring gearing, and since there isn't sufficient info on your chainset, old chainline, pedal set up or your specific requirements, my comments below should really only be taken with a big lump of salt.

To answer some of your questions, 1) afaik Shimano's 115mm and 113mm UNxx bb's should deliver the same chainline for the same chainset (because the 115mm is 2mm longer only on the non-drive side), and 2) using the shims of an FD should have no impact on its chainline compatibility (because it is designed with the L and the shims just turn the S/M downtubes to an L).

Unfortunately the other news are not so good.

Nearly all current Shimano road triple front mechs (such as the Tiagra) are designed for 45mm chainline. If you are now having a 50mm chainline with your new bb, then you have moved even further out from its designed scope.

As you have noticed, Shimano mountain mechs are designed to work with 50mm chainline, however if you have Shimano road sti's then the problem is that they don't pull enough cable for mtb front mechs. There is a chance that fiddling with cable attachment could make one work but it can not be taken for granted.

Finally since your chainset is a 48/36/22, its range of 26T is well beyond any Shimano front mech's design spec, and while depending on chainstay angle etc. it can work, set up is likely to be critical to smooth shifting.

To better appreciate the gearing challenges unleashed by your longer bb and potential solutions, you might want to read this.

What I wonder, however, is whether you have considered options other than changing the bb to move the pedals out. It seems to me you could have achieved same e.g. by choosing a chainset-bb combination with the same chainline but a larger q factor, or simply changing your pedals.

Hope it helps a little.

 

[youngoldbloke]

What I wonder, however, is whether you have considered options other than changing the bb to move the pedals out. It seems to me you could have achieved same e.g. by choosing a chainset-bb combination with the same chainline but a larger q factor, or simply changing your pedals.

Hope it helps a little.

- such as pedal washers, eg. 1.5 & 2.5mm washers or even 20mm spacers

 

[Davidc]

- such as pedal washers, eg. 1.5 & 2.5mm washers or even 20mm spacers

While I agree in principle, having used them in the past, be aware of the slight problems.

They're expensive and the ones I had (different and now defunct manufacturer) cracked after a few years and 20k miles.

The spacers increase the chance of grounding the end of the pedal when cornering so you need to corner correctly, with the pedal on the inside of the corner at the top of its rotation.

I'm noy certain of the reason, but if you thought pedals' threads go tight with time just wait until you want to get the spacers unscrewed from the cranks! A little grease applied when fitting them helps.

I haven't used spacers on either of my present bikes, but have considered it on the Dawes Horizon where I do sometimes find the spacing too tight.