Swapping chainset on road bike for a MTB one

[PeteT]

I'm looking to swap the chainset on my road bike from a Truvativ 50-39-30 to a Shimano MTB triple of 42-32-22. I realise I may need a new BB. My front changer is a Tiagra triple with a 22 - tooth capacity. I won't be changing the cassette, which is a 12 -28T coupled with Shimano 105 medium cage dearilleur.

I presume that, other than a possible new BB, I won't need to change anything else such as the chain as I'm adding smaller rings at the front rather than larger ones?

Cheers for your help.

 

[mickle]

I presume that, other than a possible new BB, I won't need to change anything else such as the chain as I'm adding smaller rings at the front rather than larger ones?

You'll definitely want to shorten the chain to account for the difference of eight teeth between the outgoing and incoming largest rings, and I'd be tempted to fit a new front mech, even though it's highly likely that the mech you have will work passably well once you've dropped it down the eat tube a few mm.

 

[MacB]

I'm assuming you have STI shifters, they don't play well with MTB front mechs, you can reroute the cable but I've read mixed results with trying to combine the two. But if you install the MTB chainset as standard then the Tiagra front mech may struggle to reach the necessary limits of throw due to the chainline difference. The rings sit further away from the frame with a MTB chainset compared to a road, 5mm based on Shimano published numbers.

I did run a non standard chainset with STIs but I used a Stronglight one and could select a square taper BB to suit, dropping the Tiagra FD down the seatube worked perfectly. If it's a two piece, Hollowtech II, chainset then you are more limited on chainline adjustment.

 

[PeteT]

Thanks guys - MacB, may I ask what were the ring sizes on your Stronglight? Although I've mentioned 42-32-22, I'm not averse to going a bit bigger on all three if it makes things less complicated; having said that, I think the max. for the granny would have to be 26T to enable my old legs to cope with some serious hill-climbing I've got planned.

 

[MacB]

Thanks guys - MacB, may I ask what were the ring sizes on your Stronglight? Although I've mentioned 42-32-22, I'm not averse to going a bit bigger on all three if it makes things less complicated; having said that, I think the max. for the granny would have to be 26T to enable my old legs to cope with some serious hill-climbing I've got planned.

you could be in luck Pete, I have changed back to a hub gear and was planning on listing up my numerous parts for sale, this wasn't some cunning plan by the way. So I'll be selling:-

Stronglight chainset - silver - 175mm arms - 46/34/24 rings, Shimano UN54 BB to match and I also have spare unused 36t and 28t rings. The chainset itself did less than 200 miles, but it did function perfectly with Tiagra STIs and a Tiagra 4503 triple front derailleur.

That's about £85 worth of kit, I reckon I'll be asking for £50 - I just haven't gotten around to taking pics of all the stuff yet, I can get that done tonight if you're interested.....Al but no problem if it's not quite what you're wanting.

 

[PeteT]

ARRGH! This is yet another downside of being a shortarse! I would have had that lot off you like a shot...except I've only got a 27 inch inside leg, so my crank length is usually 165 (or 170 at a push, which I have at the moment)....damn..... but many thanks anyway

 

[MacB]

ARRGH! This is yet another downside of being a shortarse! I would have had that lot off you like a shot...except I've only got a 27 inch inside leg, so my crank length is usually 165 (or 170 at a push, which I have at the moment)....damn..... but many thanks anyway

No probs, I would have kept it but have recently decided on a 180mm crank, still Spa Cycles is where to get a similar chainset, I think they are on at £59 at present. Obviously you have to buy a BB as well, I wouldn't recommend going with the JPS one I think the UN54 is better value.

 

[RecordAceFromNew]

As Mickle and MacB said it can potentially be a little bit more involved than you mentioned.

As Mickle said you should move the front mech down, and if it is spot on at present by 16mm to be precise (2mm per tooth difference). However not all frames allow it (e.g. if seat tube flares out or if you have a brazed-on not band-on Tiagra front mech), so worth checking.

As MacB said the problem of mtb front mechs is that they require different (greater) cable pull for road sti to deliver. But you "might" be able to get away with using your existing mech. You can back your front mech's H screw all the way out and see how much further can the front mech move out - the outer ring of a triple chainset on 47.5mm mtb chainline (having consciously choosing bb avoiding 50mm mtb chainline) is around 55.5mm from the bike's centre.

 

[MacB]

triple chainset on 47.5mm mtb chainline (having consciously choosing bb avoiding 50mm mtb chainline) is around 55.5mm from the bike's centre.

is it that much? I've been looking into this a bit and numbers aren't easy to find, but I want to use an XT H2 triple for a Rohloff and require a 54mm chainline. I thought the middle was 50mm and the outer 56mm and was hoping that reversing the spacers ratio, two on non drive to one on drive, would near enough give me the 54mm. Though this would be at the expense of moving the whole chainset across.

Have you got any links to detail on this?

 

[RecordAceFromNew]

For 47.5mm this is a somewhat out of dated table.

Some current ones.

As you mentioned earlier, HT2 could be more limiting (personally I am a square tapered man, and 8 speed! ). Do you have a specific model in mind?

You are not specifying one of those heavy 14 speed things on your new foldable project per chance? (Editor: which he read about and has been eagerly and secretly awaiting details...)

 

[MacB]

For 47.5mm this is a somewhat out of dated table.

Some current ones.

As you mentioned earlier, HT2 could be more limiting (personally I am a square tapered man, and 8 speed! ). Do you have a specific model in mind?

You are not specifying one of those heavy 14 speed things on your new foldable project per chance? (Editor: which he read about and has been eagerly and secretly awaiting details...)

It was the Shimano XT M770 I was looking at, basically need an XT model to get the 180mm cranks, plus I want them in black.

Projects, ah, erm, well, the 29er is complete with Rohloff and SONDelux(both in anodised red), the road bike has shed it's derailleur wheelset and is currently sporting a front SONDelux(in black) and the SRAM I-9 rear. There is a planned replacement frame for the road bike, longer top tube and using rohloff specific rocker dropouts from Paragon Machineworks, which are on order at present. Then the new frame will get the SONDelux front wheel and a new Rohloff rear wheel. At which point the I-9 hub will be built into a dinky wheel for the BikeFriday Tikit, this is being ordered as a single speed with the quick fold and the folding rear rack.

But I do need to start offloading some gear, I've been compiling another big list just need to get round to detailed pics and descriptions. This will include wheels, cassettes, chains, chainsets, BBs, bars, seatposts, spacers, stems, thumbies, barend shifters, FDs, RDs. Some of this is totally unused as I changed my mind on colour schemes part way through When the new road frame arrives then the original Ti frame, with headset, will be up for sale. If I really like the rocker dropouts then it's feasible that I'll have another 29er frame done and so that would go for sale as well. But that may be a step too far as the only alteration would be the dropouts, whereas the road frame is being redone with a much longer ETT due to my move away from drop bars. I may even be letting go of one of the SONDelux wheels as it has a black hub and I'm rather taken with the anodised red look that I have on the 29er. It's either that or I buy a black Rohloff instead and I don't think that's what I want.

So, no, nothing much doing on the project front

 

[mickle]

It was the Shimano XT M770 I was looking at, basically need an XT model to get the 180mm cranks, plus I want them in black.

Projects, ah, erm, well, the 29er is complete with Rohloff and SONDelux(both in anodised red), the road bike has shed it's derailleur wheelset and is currently sporting a front SONDelux(in black) and the SRAM I-9 rear. There is a planned replacement frame for the road bike, longer top tube and using rohloff specific rocker dropouts from Paragon Machineworks, which are on order at present. Then the new frame will get the SONDelux front wheel and a new Rohloff rear wheel. At which point the I-9 hub will be built into a dinky wheel for the BikeFriday Tikit, this is being ordered as a single speed with the quick fold and the folding rear rack.

But I do need to start offloading some gear, I've been compiling another big list just need to get round to detailed pics and descriptions. This will include wheels, cassettes, chains, chainsets, BBs, bars, seatposts, spacers, stems, thumbies, barend shifters, FDs, RDs. Some of this is totally unused as I changed my mind on colour schemes part way through When the new road frame arrives then the original Ti frame, with headset, will be up for sale. If I really like the rocker dropouts then it's feasible that I'll have another 29er frame done and so that would go for sale as well. But that may be a step too far as the only alteration would be the dropouts, whereas the road frame is being redone with a much longer ETT due to my move away from drop bars. I may even be letting go of one of the SONDelux wheels as it has a black hub and I'm rather taken with the anodised red look that I have on the 29er. It's either that or I buy a black Rohloff instead and I don't think that's what I want.

So, no, nothing much doing on the project front

Oh eM Gee! You've got it baaad!

 

[albion]

Mine was a hybrid but this is how it went.

a) Bought a 42/34/24 chainset to repace the 48/38/28

b) Chainline out.

c) Bought bottom bracket.

d) Adjusted front mech down.




All OK.

 

[RecordAceFromNew]

Are you aware that with your 56mm chainline kludge planned for the drive side (and therefore essentially 44mm on the non-drive side) you will be riding around in a circle constantly?

Worse, it is not helped by the Coriolis force unless you up root and move to the southern hemisphere...

Is that why you need 3x14 gears?

 

[PeteT]

Thanks all. I've spoken to those helpful guys at Spa Cycles and it seems a straight bolt-on solution is the Stronglight rings (I'm going for the 48-36-26 - so no need for new cranks or BB). Just not sure whether I'll need to shorten the chain or not. John at Spa reckons the Tiagra front mech will cope.