Road/MTB compatibility

[neilob]

Will a 10speed Dura Ace shifter (7800) work well with a good quality 10 speed MTB rear mech like the XT M781 Shadow? My rear at the mo is short cage only and I want a bigger cog than 27 at the back but med/long cage Dura Ace is pretty much unavailable. I could of course use 105 I know.

[hamster]

Road 10 uses the same cable pull ratio as 9 speed, so you need to use a 9 speed MTB rear mech. MTB 10 speed is different.

[Brucey]

yup. The only road 10s mech that is different is RD-4700; this has the same pull ratio as road 11s mechs. Otherwise any 9s mech (road or MTB) or any road 10s mech (4700 excluded) will work with your RH shifter.

BTW there is a DA 10s mech with a long arm (and an ultegra version) meant for running a triple as you say; this comes up from time to time NOS/used on ebay. An alternative is to fit the longer plates (that are available as spares) to a short arm mech, and make your own.

cheers

[neilob]

Aaah...you can adapt a short cage mech? Brilliant! Any idea where to get the bits I need plus approx cost? My legs will be grateful.

[peetee]

Its worth experimenting with the short cage mech if that's what you have. If you are careful about not cross-chaining (eg big cog to big ring) the short mech can cope. I have run a short XT on my MTB for decades. It has a 24-36-46 on the front and 12-28 on the rear.

[Brucey]

I have had a similar setup, but I have arranged it so that the chain is long enough to use big-big, and I advise anyone to always set up their transmission likewise.

It is just asking for (big) trouble if the chain is not long enough for this....

cheers

[Cugel]

I have had a similar setup, but I have arranged it so that the chain is long enough to use big-big, and I advise anyone to always set up their transmission likewise.

It is just asking for (big) trouble if the chain is not long enough for this....

cheers

Agreed - the strain of the chain will be a pain if it's not long enough for big-big, even if used only briefly (and assuming it will go in at all without a horrible graunching event). Having a longer chain may mean that the chain rubs on the mech body when in small-small but this is just noisy. And the chain may sag a little bit. One doesn't run small-small anyway, except accidentally.

In addition, I found that a short arm Ultegra 10-speed mech would cope with a 30t sprocket with no issues, as long as the b-screw is properly adjusted. This doesn't accommodate a wide gear range, though - no triple chainset with the 12-30 cassette, for example.

All in all, though, better to have a long arm rear mech designed for a wider range of rings & sprockets, if the wide range is what one desires. I reckon a good quality long-arm rear mech can be had for the price of 3-4 café stops, which are nice but the beans on toast & cake is gone after 3 minutes, unlike a nice rear mech. (I wolf my grub; a bad habit, I know).

Cugel

[neilob]

My ideal requirement is for a 11-32 cassette and a 50/34 front. Would one of those hanger extenders make it work? Apart from chain length of course. I am much more likely to use 50 32 than 34 11.

[peetee]

I have had a similar setup, but I have arranged it so that the chain is long enough to use big-big, and I advise anyone to always set up their transmission likewise.

It is just asking for (big) trouble if the chain is not long enough for this....

I can see your point but perhaps I am unique in always being aware what gear I am in and what gears are available from there. I have never used extremes of gears accidentally any more than I put the car in 1st when I should have selected 5th.
Oh dear, Perhaps that wasn't a good analogy on a bike forum.

[Gattonero]

I have had a similar setup, but I have arranged it so that the chain is long enough to use big-big, and I advise anyone to always set up their transmission likewise.

It is just asking for (big) trouble if the chain is not long enough for this....

cheers

That is not a good idea especially with Shimano Rd's. If the cage can't cope, to leave the chain longer than usual it still makes the cage moving forward and keeping the second pivot of the mech pulled, this makes the top jockey wheel too close to the large(st) sprocket(s) with possible jamming. It's easy to spot this by a knocking sound of the jockey wheel, even with the "B-tension" screw set to give the maximum distance, the 2nd pivot gets pulled too much and wins the rate of the spring in the 1st pivot (the one that pivots in the frame Rh dropout).
Rather than taking the chance and self-destruct the parts (and possibly the wheel, it becomes a very expensive mistake!) is better to source a long cage to be fitted on the existing mech. I've used for a while a D.A. 7800 modified with a long cage, it worked very well with an 11-32 9sp cassette and double 42-28 chainrings.

[Brucey]

I didn't say it was a good idea to routinely use the big-big combination, only that it was a very bad idea to have a setup where this was impossible. Not the same thing at all!

You know when you are pushing it because the 'not for regular use' gears would run OK when pedalling forwards, but exhibit the 'grrr' sound of the guide pulley hitting the sprocket when you backpedalled.

BTW the short arm rear mechs on MTBs BITD may have been an affectation for some, but on rigid MTBs especially, it dramatically reduced the chances of the chain flapping about too much or unshipping; (I note that trials riders do the same thing to this day, for similar reasons). At one time the long cage RDs mostly had steel side plates which weighed an extra ~50g or so; with the extra chain involved and the (effectively) weaker springs, this was enough to make the RD articulate very heavily around the B pivot on bumpy ground; they just wore out very quickly because of this. A shorter RD also meant that it was less likely that the RD would pick up twigs, grass, nettles, mud and brambles, all of which are regular features of some parts of the British countryside.

I first did this mod back in the 6s uniglide days; my new 'Deore' RD (with a long cage) had a messy collision in my workshop with a short cage 105 RD (the first indexed model I think) and the result was that I ended up with a short cage Deore mech on my MTB and my chum ended up with a long-cage 105 RD on his touring bike; win-win!

cheers

[Gattonero]

The XT rear mech with SS cage was popular in the early 90's, but it's worth to note that the maximum rear sprocket used was a 28t, also it was forbidden to "cross" the chain. May people have to abandon a race because of this

Btw, the discipline of trials is almost inevitably done on single-speeds, and the ratchet is in the cranks

[peetee]

Here is 'Dennis' sporting his short cage XT mech which he asked me to fit in 1991. Dennis has used it to get me up and down Helvelyn, Hardnott, and Wrynose, most of the North Cornwall coast path (when it wasn't frowned upon) and mine trails, hundreds of miles of Hampshire forest trails and bridleways and fast descents (40mph +) in the Wye Valley, and a few races to boot.
It has never caused him any pain and still works like a dream.