New 105 to MTB rear - Possible?

[Albert]

I have been told that it would be OK and would have no issues if I were to switch my rear (10 Speed Compact Double Road New 105) deraileur running a 12 - 27 cassette to an SLX running an 11 - 34 cassette, provided I change the chain.
Is this correct? I am adapting a road bike into a "climb anything" long distance Audax/light tourer and any comments would be very helpful and much appreciated.

 

[RecordAceFromNew]

Is this correct?

As long as the SLX is not the 10 speed version, since all Shimano 10 speed mtb rear mechs are DynaSys and have different cable pull requirement to your current road mech. You can use 9 speed SLX, e.g.

 

[boydj]

If your current mech is a short cage mech, then you might be better changing the derailleur to a compatible long or medium cage mech to cope with the wider range of gears.

 

[Albert]

Thanks both of you. Using a 9 speed MTB rear derailleur seems to be the universal view.
But - am I being thick here and missing something? - does this not mean that the result is 9 speeds from a 10speed cassette?

 

[Alun]

No, you'll still have 10 speeds, the term "9 speed derailleur" is a bit misleading. I have used a deore rear derailleur to get the result you're after and it works. The deore derailleur also has a cable adjuster which can be useful, many MTB derailleurs don't as they have the adjuster on the lever which you don't get with road levers

 

[Tomba]

I've got a SLX rear mech for sale if you want a cheapish experiment. Used for about 300 miles.

 

[Albert]

Thanks everyone for all the info and the offer Tomba. I have already tentatively arranged for my LBS to do the work, but was unclear as to whether it would definitely work as the mechanic has not done a 10 speed conversion and I wanted confirmation that he was right in what he said. I have very little knowledge as you have probably gathered I will (subtly) make sure that he knows to use a 9 speed rear derailleur.
Thanks again.

 

[Albert]

All done! Deore rear mech, SRAM chain and 11 - 34 cassette. All 10 gears working very nicely and no issues at all.
I now have my bike geared to go up 20%ers with a bit of baggage without my legs breaking.
Thanks again.