Which derrailleur, shifter for this chainset?

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mnichols
Posts: 1426
Joined: 22 Apr 2013, 4:29pm

Which derrailleur, shifter for this chainset?

Postby mnichols » 29 Sep 2018, 8:31pm

Could I use this chainset, which is designed for a MTB on a road bike with drop handlebars? Which front derrailleur, and shifter would I need? And then which rear derailleur and cassette would those shifters allow me to use at the back? i'd like 34 or more at the back

I don't want to be pushing the technical boundaries or mix and match more than i need to, so if I can stick with Shimano that would be great, or if the whole thing is available from another manufacturer then that's also fine, but i don't want to mix and match between manufacturers or even groupsets if I can avoid it - and yes i know this is an MTB Chainset and I need road shifters, but if i could get that to work on 105 or Ultegra that would be perfect

I should mention that the bike frame takes rim brakes, previously had ultegra but it donated all it's parts to another build and is now just a frame and fork

http://www.chainreactioncycles.com/shimano-xt-m8000-double-11-speed-chainset/rp-prod135837?gs=1&sku=sku492870&pgrid=60711843258&ptaid=pla-424556592662&utm_source=google&utm_term=&utm_campaign=PLA+All+Products&utm_medium=base&utm_content=mkwid|sgrI1Up6g_dt|pcrid|294922111661|pkw||pmt||prd|492870UK

Brucey
Posts: 35897
Joined: 4 Jan 2012, 6:25pm

Re: Which derrailleur, shifter for this chainset?

Postby Brucey » 29 Sep 2018, 9:21pm

FC-M8000 is available in three types (24-34, 26-36, 28-38); all with small chainrings. The chainline is 48.8mm.

These two things mean that no 'road' FD (i.e. that works from 'road' STIs) is specifically designed to work with that chainset. The mechs that are designed for that chainset use a completely different cable pull so won't work with road STIs, not without adaptation anyway.

In terms of the chainline, a 'road triple' mech will probably reach far enough since the double chainrings are positioned similarly to the two larger rings on a road triple. A 'get out of jail free card' chainline wise is that most dropped bar bikes have a 68mm BB shell and the chainset will do 73mm or 68mm shells, the latter with spacers. This means that if needs be you can shuffle the whole crankset to one side a mm or two if needs be on a 68mm shell. [Don't worry about the uneven tread; most bikes were like this for years anyway.]

In terms of chainring size the available 'road' FDs will all have entirely the wrong curvature to match the small chainrings and will also have a 'long tail' too, The latter means that the FD may clash with the chainstays if it is set to the correct height for the smaller chainrings. If it is left high on the seat tube, the chain is at risk of overshifting off the big ring. You can address the curvature issue by using a mech with a braze-on type fixing/adaptor and either machining the parts or including a wedge-shaped piece between the FD and the braze-on adaptor. However this also increases the risks of a tail/chainstay clash.

So there are a few options;

1) Use the matching MTB FD and control it using bar-end shifters or a friction shifter of some kind

2) Use the MTB FD and MTB pods mounted under the tops of your dropped bars (eg on a robust accessory bracket with a 22.2mm dia)

3) Use the MTB FD but use a revised cable mount (not possible with all FDs) at the mech (or a travel agent type device) to give a revised shift ratio that matches the road cable pull rather better (it doesn't have to be perfect with a double, provided you don't need to trim the FD (which indeed you are less likely to need to do with an MTB chainset because the small chainring make it less flexy).

4) Use the MTB FD but control it using a newer (most shimano 11s or tiagra 4700 10s) double road STI which has a much longer cable pull than most other road doubles; it may work OK without a travel agent being required, since the 'new' cable pull is close to that of a MTB FD. [BTW the triple versions of the shifters eg 4700 series triple, use the 'old' road FD shift ratio; a MTB mech with these shifters might move far enough to shift two rings but all the clicks would have to be used, i.e. there would be an unused click position in between the two chainrings, which would be v annoying.]

5) Use a FD that matches the road shifters you intend to use and revise the mountings so that it better matches the chainset, e.g. as per the preamble above.

At the rear things are rather simpler; current 11s 'road' mechs such as 105 and ultegra are available to accept 34T, no problem.

FWIW I'd go for option 4 and if it didn't work perfectly I'd look to revise the shift ratio slightly (via cable mount adaptation or travel agent). However one of the dumb things that might catch you out is that not all MTB mechs are convertible for bottom pull, which is how most 'road' frames come.

BTW the FC-M8000 chainset has a tread (Q, sort of) of 168mm. I find this a bit wide and (for pedalling) I much prefer one bike I have with a road triple and a sub-140mm tread value. I don't think you can easily buy a chainset with as low a tread as that quite but you can do better than 168mm without a lot of effort I think.

hth

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

mnichols
Posts: 1426
Joined: 22 Apr 2013, 4:29pm

Re: Which derrailleur, shifter for this chainset?

Postby mnichols » 29 Sep 2018, 11:43pm

Wow, thanks for the very comprehensive response. It's going to take me a while to think that one through.....one to mull over on tomorrow's ride
Last edited by mnichols on 29 Sep 2018, 11:52pm, edited 1 time in total.

mnichols
Posts: 1426
Joined: 22 Apr 2013, 4:29pm

Re: Which derrailleur, shifter for this chainset?

Postby mnichols » 29 Sep 2018, 11:51pm

Option 4 seems the best bet, but can I check, are you saying that the cable pull on a new 105/ultegra may/should work with the MTB mech, provided it can be pulled from the bottom?

Brucey
Posts: 35897
Joined: 4 Jan 2012, 6:25pm

Re: Which derrailleur, shifter for this chainset?

Postby Brucey » 30 Sep 2018, 12:18am

mnichols wrote:Option 4 seems the best bet, but can I check, are you saying that the cable pull on a new 105/ultegra may/should work with the MTB mech, provided it can be pulled from the bottom?


I have not BTDT myself but IIRC others have reported that this works OK for a double setup (but folk are happy with different things, so what suits them might not suit you...). Certainly worth a go; you can always fallback to adapting the pull ratio if it doesn't work well enough for you.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

tempsperdu
Posts: 109
Joined: 9 Jan 2014, 8:31pm

Re: Which derrailleur, shifter for this chainset?

Postby tempsperdu » 30 Sep 2018, 1:03pm

Or go Di2.
The shifter is transparent to the system so you as long as it appears on the comparability chart it should work.
Plus you can get away with only one shifter for front and rear derailleur using synchro shift.

Your first reaction to this will be 'in your dreams' etc and 'have you seen the price' etc but I put together a system by buying individual parts off 'the bay' and in sales in the UK and Germany. Ended up with an XT 2x11 Gravel bike that can run with drops or flat bars.

mnichols
Posts: 1426
Joined: 22 Apr 2013, 4:29pm

Re: Which derrailleur, shifter for this chainset?

Postby mnichols » 30 Sep 2018, 6:34pm

tempsperdu wrote:Or go Di2.
The shifter is transparent to the system so you as long as it appears on the comparability chart it should work.
Plus you can get away with only one shifter for front and rear derailleur using synchro shift.

Your first reaction to this will be 'in your dreams' etc and 'have you seen the price' etc but I put together a system by buying individual parts off 'the bay' and in sales in the UK and Germany. Ended up with an XT 2x11 Gravel bike that can run with drops or flat bars.


Thanks, but the issue is more that i wouldn't want to use electronic shifting when touring. I fully acknowledge that this may be a trust issue from my perspective, and it may not be fully thought through and therefore a bit irrational but i trust cables more if I'm in far flung, or out of the way places

I'd be interested to know about your setup though, maybe I can be convinced

mnichols
Posts: 1426
Joined: 22 Apr 2013, 4:29pm

Re: Which derrailleur, shifter for this chainset?

Postby mnichols » 30 Sep 2018, 6:39pm

tempsperdu wrote:Your first reaction to this will be 'in your dreams'


The whole thing maybe a bit 'in your dreams' at the moment. I've got four functioning bikes and I've just moved house, so got a lot of stuff further up the list

But nice to do a bit of window shopping for a bit of light relief

tempsperdu
Posts: 109
Joined: 9 Jan 2014, 8:31pm

Re: Which derrailleur, shifter for this chainset?

Postby tempsperdu » 1 Oct 2018, 9:56pm

One further thought was that bar end shifters in friction mode might work.

This write up got me looking at Di2. Substitute XT for XTR although oddly some XTR parts are cheaper than XT on ebay.
https://www.toecutters.com/single-post/ ... ikepacking
(Yes that is a pistol on the bag.)

This is the Compatibility Chart.
http://productinfo.shimano.com/#/com/3. ... acid=C-435
Looks a bit daunting to start with but once you get to understand it just print off the page you need and highlite the Part Nos then you know what you are looking for.
I did buy the wrong battery at one stage but managed to sell it for more than I got it for off ebay.
You dont need a wifi link or a computer connector as the battery charger also does that. I used the Display as Junction Box A and the distribution block on the bottom of the External Battery Holder as JB B.