Changing double to triple - BB choice/Crankset choice?

[poppet]

Hello all,

I'm changing the stock double on my CdF 20 Ltd (2015) after a beautiful but hard September cycling across France. I'm changing it to a Triple with the following components...

4703 Triple 10 Speed Crankset
4703 Triple 10 Speed Front Mech
Tiagra 4703 3x10 Speed STI
4700 10 Speed Rear Mech

Currently fitted are

Double brifters - Shimano ST-460
10 speed cassette 11-34
10 speed rear derailleur Shimano RD-350
Shimano FD-460 double front derailleur
Double crankset FC-R460 48/34T
Shimano SM-BB4600, 68mm

Happy with this choice so not looking for suggestions for alternatives (unless these are world changing!) but just some advice on Bottom Bracket choice. Will I need to change it? Is there an easy way to find out what length I will need? Any other help with the process? (maybe I'm imagining this being more complicated than it actually will be!) I'd like to keep the 11-34 cassette but some websites indicate it's a maximum 34 with a triple chainset, any truth in this or is it get awayable?

Your's semi-clueless.

[Brucey]

.....but just some advice on Bottom Bracket choice. Will I need to change it? Is there an easy way to find out what length I will need?....

this is almost a non-question, since FC-4703 is a HT-II type chainset, i.e. it comes with its own BB spindle bonded to the RH crank, so it is one length only, no choice.

Depending on the BB cups (which hold the BB bearings) that you use, you can adjust the chainline slightly but if you just use the matched parts you shouldn't need to, provided the frame was designed with a road triple in mind. Now I don't know if this is the case or not, but I'm sure that others have done this kind of build and will let you know, even if you can't get a straight answer out of the manufacturers.

cheers

[poppet]

Gotcha. Interested to hear other people's experience if they have stuck a triple on a Croix de fer?

[hondated]

poppet you have probably done this anyway but just in case you haven't check the chain ring lever to see whether you can get three clicks on them. I say that because I recently took a chance with buying some Shimano levers on Ebay at a cheap price and hey were what I needed a triple but the advertiser hadn't advertised them as such.
Brucey makes an interesting point about frame design because I recently tried to transfer my Campag triple from a 2000 Bianchi onto a later frame but the BB wouldn't
fit. So I ended up with a Compact which because it was too much like hard work, being the largest rear sprocket I could use was a 29t.

[poppet]

Yes - unfortunately they are definitely doubles.

Just had a thought, could I put one of these MTB triples on it without dire consequences? 42-32-24T, 48-36-26T or 22.30.40t

http://www.chainreactioncycles.com/shim ... prod108982
http://www.chainreactioncycles.com/shim ... prod106873

[Valbrona]

All triples are not the same.

Most road triples are based around 130 and 74mm BCD. In practice, this means the following: that you can fit any size of outer chainring; that the smallest middle you will be able to fit is a 38t; and you can fit pretty any size of inner chainring.

I prefer a triples based on 110/74 BCD, and this allows the fitting of a smaller middle ring. One such triple is made by Stronglight, and it fits onto a regular square taper bottom bracket, ie. it is not an 'integrated-axle' type crankset.

[cycleruk]

If I understand your list of current equipment:-

Double brifters - Shimano ST-460
10 speed cassette 11-34
10 speed rear derailleur Shimano RD-350
Shimano FD-460 double front derailleur
Double crankset FC-R460 48/34T
Shimano SM-BB4600, 68mm

Then I think your current BB will be suitable.
The new crankset may or not come with a new BB anyway?
You may need a new longer chain for the new 50T chainring.

[531colin]

I have been trying out a double chainset using this crankset http://www.spacycles.co.uk/products.php?plid=m2b0s109p3051
...and these chainrings.http://www.spacycles.co.uk/products.php?plid=m2b0s210p3056 as the inner chainring 74 BCD
and in the middle ring position a chainring similar to these http://www.spacycles.co.uk/products.php?plid=m2b0s210p3055 but bigger, 110 BCD
I tried 30/46 chainrings which I couldn't fault with 105 11 speed, so i see no reason they shouldn't work with 10 speed.
So you can save a lot of money and get a gear as low as a "road" triple, as long as you don't mind losing a few gears at the top end.
30/46 worked perfectly for me, I think you could easily go to 26/42 and get lower still.
Its in early development at the moment....you need to blank off the big ring position of the triple, either with a chainguard or filling pieces....to follow.
Others do similar chainsets, but more money....http://www.gravelbike.com/?p=1799

Mountain bike triples are designed for a different chain line, and may not play nicely with road front mech.

[Ray]

I replaced the outer 50t ring on my Tiagra road triple with a Stronglight chainguard from Spa Cycles. Gives me 30 and 39 rings, which I find fine with 11 or 12t top sprockets, but you could easily fit something bigger than the 39 - or, indeed, smaller than the 30.

Note that the (double) front changer could not be lowered to the recommended 1-2mm spacing from the larger ring because it would have fouled the chainstay. Works perfectly, though.

[Brucey]

FC-4703 is a queer bird; it uses shimano's unevenly spaced 4-bolt ring mounting, and whilst the inner ring is indeed a 74mm bcd, and the middle is 110BCD, I'm not sure there are any other chainring sizes that fit other than the 30T/39T that shimano offer...?

you can see the EV techdoc here

[url]si.shimano.com/php/download.php?file=pdf/ev/EV-FC-4703-3934.pdf[/url]

cheers

[poppet]

Exciting - I wouldn't mind going off brand and getting a lower geared chainset.

So to summarise the considerations when choosing a new chainset are to make sure the BCD match the old one coming off the bike and making sure it is road not MTB. Spa Cycles seem to have lots of choice. Any worries about going to low with the number of teeth?

[Brucey]

I'm not sure that you need the BCDs to match unless you are planning to swap chainrings around...?

FWIW a 110/74 five bolt BCD setup has been pretty much the default option for touring bikes for many years now.

For some years you could also get 130/74 (or in campag 135/74) five bolt chainsets; these are OK too but usually limit you to ~39T as the smallest middle ring. They make good sense on machines that have 'road bike gearing' on the larger rings, but if you have very low gearing there is often a big drop from the middle to inner ring which not everyone likes. In more recent times on their road triples shimano have gone with

a) a larger inner ring BCD, meaning that ~30T is the smallest inner ring you can fit (and campag have done likewise with their road triples too, more's the pity) and/or
b) some new and weird bolt pattern that means you can't buy different sized chainrings anyway (as per FC-4703).

It doesn't so much matter if you have MTB or Road cranks if they have a square taper BB, because in most cases (but not quite all, sadly) it is possible to use a different BB spindle length to adjust the chainline.

Smaller chainrings are a little lighter but they also mean that the whole transmission (gear ratio for gear ratio, power for power) may be a little less efficient and will wear a bit faster too. They also make the gear intervals you get slightly different, too. They do have the benefit that (for any given front/rear mech capacity) you can get a wider range of gears, though.

cheers

[poppet]

No, as little fettling as necessary...

Just after getting a lower geared bike still keeping brifters and not creating too much of a frankenbike!

[poppet]

PS Brucey - thanks for your advice on the V pipe/Tiagra hack to get the clothes lines out the way of the bar bag. Worked a treat for my trip across France.

[531colin]

Exciting - I wouldn't mind going off brand and getting a lower geared chainset.

So to summarise the considerations when choosing a new chainset are to make sure the BCD match the old one coming off the bike and making sure it is road not MTB. Spa Cycles seem to have lots of choice. Any worries about going to low with the number of teeth?

As Brucey says, BCD isn't really an issue.
The Spa stuff is square taper not external bottom bracket, so there is plenty of options to get the chainline right....Bobby at Spa built the one I rode, he can tell you which length axle you need.
"Too small" a chainring is only a problem if you run out of gears at the top end, or if you can't get the front mech. low enough because it fouls the chainstay. Brucey has a formula for "X" number of teeth makes the chainring diameter "Y"mm smaller......
and just replacing the chainset, keeping the shifters is much less money!