Compatible chainset

[quixoticgeek]

I bought a tiagra 4700 groupset to install on my genesis vagabond frame. I'm in the middle of assembling the bike and have just realised that the 50/34 chainset won't fit. It hits the frame.

Being an idiot I hadn't taken into account the chainstays limiting the Max diameter of the inner ring of the chainset.

The stock bike comes with a 40/28. Will a shimano deore 40/28 double chainset fit here? Will it work with the tiagra front derailure? 

Thanks

J

No, edited to fix phones autocorrect of chainstays to chainsaws...

[Torslanda]

The Vagabond comes as standard with a 2x10 DynaSys transmission and indeed the chainset uses just that ratio. You will also need the matching BB.

The derailleur is suck-it-and-see. Tiermat (IIRC) got a 6700 Ultegra front mech to shift on a triple 22/32/42 on an OnOne InBred, so anything is possible. As long as the cage will run low enough to clear the chainstay and give the correct spacing over the big ring I'd say yes. If not then a Deore 2x10 front mech (Genesis spec a Deore FD-M618) will handle the shifting duties but it may need a bit of trickery with cable routing at the derailleur if the cable pull ratio is different between DynaSys and Tiagra 4700.

Best of British!

[markfh]

Looking at the spec for the 2017 Vagabond, the factory build comes with Shimano Deore front and rear derailleurs which suggests that the frame is designed for a MTB chainline rather than a road chainline.  If I have read the Shimano specs correctly a Shimnao Deore double uses a nominal 48.8 mm chainline rather than the 43.5mm for a Taigra 4700 road double.  This difference in chainline may also have contributed to the chainset fouling the frame.

The difference in chainline between road and mtb doubles may also make it a challenge to use a mtb chainset (deore) road front derailleur.

[Brucey]

I agree with the above post.  If you are using dropped bar STI shifters (with indexed front shifting) then for many years there would have been trouble with a 'road' FD on a MTB chainset, trouble of several different flavours;

1. incompatible cable pull between shifter and MTB mech
2. chainline  is wider so a road FD won't reach
3. Shift ratio of the FD varies with reach so that even if the mech reaches far enough the shifting doesn't work properly.

However the issues are not as they once were if you intend to fit 10s 4700 or any 'road' 11s shifters onto an MTB double chainset, because the cable pull is different (longer than it used to be) in current road double shifters, and the demands of an MTB double chainset are different to a triple (by and large less demanding). 

So provided the FD reaches far enough and the cable pull of the shifter is slightly more than is required then you should be able to make it work at least well enough so that the trim clicks work on the big ring and the FD has one position (set by the limit screw) on the small ring that allows you to use the largest sprockets at the back. Provided you don't want to use the small-small combinations then this is enough.

  If you can match the cable pull of the mech and the shifter perfectly (eg by varying the effective length of the FD arm that the cable attaches to), you ought to be able to make the  trim clicks to work on the inner chainring too.

BTW the latest road FDs don't have a long arm (as per the first 11s FDs and 4700 10s double) but have a rotary cable mount on the mech. There is a separate adjustment for the mech reach, so that (in principle) the shift ratio of the mech can stay constant regardless of exact chainline, within limits. 

Unfortunately I have not spent long enough playing with them to work out what will and what will not work, so unless someone else knows for sure then there is a bit of 'suck it and see' ahead of you.

Worst case (I think) is that you have to make a special BO mount adaptor that brings a road FD further out than normal, so that it works properly at an MTB double chainline. But it might work OK without that level of faff.

cheers

[quixoticgeek]

Thanks all for the replies so far.

I've found a couple of mtb cranksets that look plausible, and consulting a few people, plus the replies here suggest that even if the tiagra shifter doesn't work with an mtb derailure, then a shiftmate will make it work.

So the question I now have is what bottom bracket would I need to make it fit? It's got a 68mm bottom bracket currently (BB-RS500).

What are the options?

J

[fruitcake]

A longer axle will allow the rings to clear the frame. 68mm will be the width of the bottom bracket shell, the metal cylinder at the base of the frame that the axle assembly fastens to. The axle assembly itself is known as the bottom bracket and its width is measured from one end of the axle to the other. Chainsets are generally designed to be compatible with a certain length of bottom bracket (BB). To use the chainset you've got, you'd need a longer BB axle. You've then got the issue of front derailleur compatibility, which Brucey discusses upthread. There. compatibility is determined by ring sizes and by the number of rings.

One way to check what length bottom bracket the chainset is made for is to get the model number of the chainset and make enquiries from the manufacturer (or look it up in a reliable database). If you get a chainset and you're not sure what BB axle length it needs, you can check through trial and error, fitting it to the bike and going from there. If the BB axle is too short, the chainset won't go on fully; if it's too long there'll be a big gap between the chainset and the frame and your pedals will be a long way away from each other (which might feel weird on long rides and could cause other problems).

If you then need to replace the BB for one that's compatible with the chainset, you'll need to remove the current BB using the correct tool and a long spanner. If the BB has been installed a long time it may take some work to remove it, as it could be tight. Then it may need lots of leverage and, if it's corroded, it may need treating with penetrating oil.

It's all in a day' s work for a decent bike shop. That's always an option.

----
Edit: Just realised you're using Hollowtech II, so the normal advice about bottom bracket axle length doesn't apply. 

[quixoticgeek]

A longer bottom bracket will allow the rings to clear the frame. 68mm will be the width of the bottom bracket shell, the metal cylinder at the base of the frame that the axle assembly fastens to. The axle assembly itself is known as the bottom bracket and its width is measured from one end of the axle to the other. Chainset are generally designed to be compatible with a certain length of bottom bracket (BB). To use the chainset you've got, you'd need a longer one. You've then got the issue of front derailleur compatibility, which Brucey discusses upthread. There. compatibility is determined by ring sizes and by the number of rings.

One way to check what length bottom bracket the chainset is made for is to get the model number of the chainset and make enquiries from the manufacturer (or look it up in a reliable database). The way to confirm that is to fit it to your bike. (If the BB is too short, the chainset woun't go on fully, if it's too long there'll be a big gap between the chainset and the frame and your pedals will be a long way away from each other, which might be weird on long rides).

If you then need to replace the BB for one that's compatible with the chainset, you'll need to remove the current BB using the correct tool and a long spanner. If the BB has been installed a long time it may take some work to remove it, as it could be tight. Then it may need lots of leverage and, if it's corroded, it may need treating with penetrating oil.

It's all in a day' s work for a decent bike shop. That's always an option.

Gotcha.

Shimano BB52 seems like the intended pairing for the crankset I have in mind. Shall add one to the order.

J

[fruitcake]

I wonder if the one installed is compatible. Axle length end to end is the thing.

[quixoticgeek]

I wonder if the one installed is compatible. Length end to end is the thing.

At 12 euros. I'm willing to take the punt. I can at least compare them side by side. Worst case I end up with a spare bottom bracket in stock...

J

[fruitcake]

I've just seen that you're using Hollowtech II, which has the axle integrated with the crank, so there should be no compatibility issue with other Hollowtech II BBs. (According to Sheldon Brown.) Just ignore what I've written above!

[Brucey]

In HTII bottom brackets the MTB ones come with a load of spacers and they often have a slightly longer thread on the RH cup.  The road and MTB versions have a different flange width on the cups too.

You can make a setup work with almost any BB (as SB suggests) but IMHO the least satisfactory arrangement is an MTB chainset on a 68mm BB shell using road cups; there will be too many spacers and perhaps not quite enough threads engaged with the RH cup.

cheers

[quixoticgeek]

I ordered a shimano Deore 40/28 chainset, and a BB52 bottom bracket to go with it, and to edge my bets, a FD-M786 derailleur.

It arrived in the post today. Opening the box it seems that the chainset came with a bottom bracket, which wasn't mentioned on the site I bought it from, so I have a spare bottom bracket. At this rate I'm going to end up with enough extra parts to build a whole 2nd bike...

There are 3 spacers with the bottom bracket. Any ideas how these should be distributed?

J

[Chris N]

68mm BB: 2x drive side, 1x non-drive side
73mm BB: 1x drive side

[quixoticgeek]

I've not tested it on the road, but on the workstand at least a deore 40/28, with a FD-M786 derailleur, and BB52 bottom bracket, seems to shift ok with a tiagra 4700 shifter. Will see if it works as well on the road, but at this stage, it's certainly promising.

Now I just need to get the rear to index properly...

J