Shimano 3 (42Ax32Ax24A) x 10 (11-36) groupset upgrade to 11-40 or 11-42

[Charl - Ride for fun :-)]

I have a Scott Spark 920 2013 with the following chain rings and group set on.

 

Crankset:

Shimano XT FC-M780 hollowtech 2 42Ax32Ax24A

 

Casette:

Shimano CS-HG81-10 11-36

 

Rear Derailleur:

Shimano XT RD-M786 SGS

 

Front Derailleur:

Shimano SLX FD-M670-E / DM

 

I would like to modify my cassette to include a 40 or 42 teeth granny gear.

 

I'm told that this cannot be done, however there are forums indicating that this is possible.

Purchasing a 2x11 or 1x11 New groupset both cost in excess of R9000 and I'm looking for a more cost effective fix.

Could you advice me if this is possible, and if it is how do I do the modifications i.e. which components do I remove and replace with what.

 

[BigTom]

Are you still wanting to keep it a triple ring on the crankset, or are you wanting to convert to 2x10 or 1x10?

[Charl - Ride for fun :-)]

Are you still wanting to keep it a triple ring on the crankset, or are you wanting to convert to 2x10 or 1x10?

I was hoping to keep the triple ring, but if that becomes a constraint then the 2x10 would become the option.

[NickGM]

I have a Scott Spark 920 2013 with the following chain rings and group set on.

 

Crankset:

Shimano XT FC-M780 hollowtech 2 42Ax32Ax24A

 

Casette:

Shimano CS-HG81-10 11-36

 

Rear Derailleur:

Shimano XT RD-M786 SGS

 

Front Derailleur:

Shimano SLX FD-M670-E / DM

 

I would like to modify my cassette to include a 40 or 42 teeth granny gear.

 

I'm told that this cannot be done, however there are forums indicating that this is possible.

Purchasing a 2x11 or 1x11 New groupset both cost in excess of R9000 and I'm looking for a more cost effective fix.

Could you advice me if this is possible, and if it is how do I do the modifications i.e. which components do I remove and replace with what.

There are others on here who can (and probably will soon) give you more comprehensive advice. But for starters, if you want to do a 1x11 conversion, I'd say you can do a good spec conversion for half of that price. The whole groupset would include brakes and crank, which I assume you don't need to replace. 

 

What you would need is a new narrow-wide chainring and 1x11 conversion bundle. 

 

Narrow-wide chain ring:

http://www.rapide.co.za/product/rapide-nw-ultralight-ring/

 

1x11 bundle:

http://www.rapide.co.za/product/rapide-1-x-11-bundle/

 

So depending on what you choose from the links above, you could come in under R4000. CWCycles also had a good special on an XT 1x11 groupset, not sure if it's still available. 

 

I suspect this is not exactly what you were asking, but hopefully helpful anyway. 

[JacBrand]

That is one serious granny.

Have you considered a smaller small chain ring in front?

[arendoog]

I am sure you can use a 11 x 40 cassette on there .Rapide supplies an extra long B screw with the cassette if you need it .I have a 11 x 42 on mine using a sram xx 10 speed shifter and derailer .I did not even use the Long B screw. I would also take the granny off and use it as a 2 x 10 .A granny gear with 40 at the back will be so slow you will not keep your balance anyway .I also have a 1 x 11 with 11 x 46 setup .there is not such a big difference between 1 x 10 and 1 x 11 .A  2 x 10 setup with a 11 x 40 at the back is PLENTY 

Edited March 10, 2017 by Blitzer

[BigTom]

Where did you get the information that it can't be done?  I would think that it could be done, but is considered impractical.  And why they might be saying it can't be done is that you would need to long a chain to fit a 42 front to 42 back cog (although no one should ride in the gear combination).

 

However, I'm guessing that the reason you may want to go to the bigger cog in the rear is that you are struggling to climb in the 24-36 combination?  If so, then I think going 2x10 might be a good option.  Your front shifter might be able to switch from 3x to 2x.  You would probably need to change the crankset to a double for proper chain alignment.  And then purchase the 40 or 42 expander sprocket (and possibly the 16t gap fill too which makes for smoother gear changes, but not absolutely necessary).

 

If you choose to go 1x10, then the front shifter would be irrelevant and you could still use your same crankset - just buy the front chainring.

[Captain Fastbastard Mayhem]

Yes, it can be done. The SGS (Long cage) RD can accommodate the 42t at the back and the spread between granny and middle across the gear range, as long as your chain is the right length. 

 

I'd suggest, though, that you get the 11-40 Sunrace cassette. That's a climbing gear that's 10% easier than your current granny (quite a bit) and relatively small outlay. 

 

You MAY need to get a slightly longer b-screw, but you can pick those up at most bolt supply stores and sometimes even a builders whorehouse. 

[shaper]

Is it physically possible to spin up a hill 24 / 42??  Forward momentum must be next to nothing and am sure it would be quicker walking.......

[DC Carbon]

There's no reason why you couldn't put the 11/42 cassette other than the chain length/practicality and derailer would possibly require changing. However you would be moving really slowly in the 24/42 gear ratio. With a 24/36 you are already close to walking pace it wouldn't make sense to go to a 42. 40 would be fine. 2x10 would be a more sensible upgrade but requires changing the crank and 1x11 is more sensible however if you battling with 24/36 granny then you would never cope with 32/42 granny ratio. You would have to go 32/50 or even 30/50

[Charl - Ride for fun :-)]

Great advice from everyone!!

Will have to consider all of the options, but it seems more practical to go to a 2 x 10 conversion and remove the 3x10 complexity.

 

The conversion kits being suggested are a lot cheaper than what I anticipated.

 

With a 2x10 at the back what would the best chainring combo on the front be?

[Captain Fastbastard Mayhem]

Won't need to change the RD, the XT 786 SGS has a capacity of 43t total. 

 

If he changes to an 11-40 and keeps the 3x, that's a 47t spread. MAY be able to get away with it, if he gets a slightly longer B screw and is super careful about shifting and gets his chain length PERFECT. 

 

Upgrading to a 1x system will not remedy the problem. 

 

2x will remedy the problem, but then the 42t will be lost at the front. 

 

2x 24/36 & 11-42 would be the logical choice. Can use the existing crank if you get short chainring bolts. 

 

Would just need a new cassette (11-42 from Sunrace - 10sp) and a new 36t chainring. Maybe also a new FD if the existing one doesn't shift lekker with the bigger 'ring, but may not need to.

 

So - all you need is a bigger 10sp cassette and possibly a B screw. 

Edited March 10, 2017 by Myles Mayhew

[BigTom]

Is it physically possible to spin up a hill 24 / 42??  Forward momentum must be next to nothing and am sure it would be quicker walking.......

Yes it is.  I can speak from experience. 

 

EDIT:  , my bad.  Got confused between my 1x setup and my 2x setup.  I can do a 24/40 on my 2x and yes, you have to spin pretty fast to keep the momentum going.  And yes, probably faster walking, but then again, it allows me to get up some hills without walking.  

Edited March 10, 2017 by BigTom

[NickGM]

 If so, then I think going 2x10 might be a good option.  Your front shifter might be able to switch from 3x to 2x.  You would probably need to change the crankset to a double for proper chain alignment.  And then purchase the 40 or 42 expander sprocket (and possibly the 16t gap fill too which makes for smoother gear changes, but not absolutely necessary).

 

If you choose to go 1x10, then the front shifter would be irrelevant and you could still use your same crankset - just buy the front chainring.

 

Actually, I'm pretty sure i have the same shifter as the OP, and when I took mine off for a recent 1x11 conversion, I noticed a handy little "3x/2x" toggle switch. Didn't even know it existed. 

[Quagga]

Did not check if someone answered this in detail yet so sorry if it has been answered.....

 

Your Rear derailleur specification can be found here

http://bike.shimano.com/content/sac-bike/en/home/mtb1/shifting---braking/rear-derailleur/rd-m780-sgs-l.html

 

You will see that they recommend a Front difference of 18T and mention a Total capacity of 43 T

 

Whata does that mean?

According to United Bicycle Institute:

1.    Determine Maximum Chainring Difference by subracting the number of teeth in the smallest chainring from the number of teeth in the largest chainring

2.    Determine Maximum Cassette Cog Difference by subtracting the number of teeth on the smallest cassette cog from the number of teeth on the largest cassette cog

3.    Determine Total Drivetrain Capacity by adding Maximum Chainring Difference to the Maximum Cassette Cog Difference

4.    Record the Maximum Cassette Cog (the number of teeth on the largest Cassette Cog)

 

So if you take your crank

Shimano XT FC-M780 hollowtech 2 42Ax32Ax24A

42-24 = 18T

36-11 = 25T

18+25 = 43T (so you are on the max that Shimano Recommends)

But some people say that you can take to a 45T total as below:

That means that you should be able to go to a 38T(not available at the back)

 

For Shimano:

SS - Short Cage Road Double - Maximum Cassette Cog is 27 and Total Capacity is 29

GS - Medium Cage MTB/Road Triple - Maximum Cassette Cog is 34(MTB)/27(Road) and Total Capacity is 33(MTB)/37(Road)

SGS - Long MTB - Maximum Cassette Cog is 34 and Total Capacity is 45

 

#NB What does this Capacity  really mean, and what will / could happen if you go outside these parameters

1. When you are in middle ring front and smallest at back the chain rubs on itself

2. when you are in the big ring front and change to big at the back (not something you should do, but everyone makes a mistake forgetting that you are in big ring) you run a HUGE risk of the RD breaking the reataining spring.

3. You have a full sus bike and you are in big ring and big or 2/3 at the back and you hit a rock / ditch etc and the suspension activates, you have the prefect senario to break the hanger and then the RD will have a nice trip around the cassette.

 

With a longer B-Screw and increasing the size of the smallest ring to 26/28T  (dont think they make them) or just going 2 x in the front (you can do this with your current shiffter even if it does not have the 2x 3x switch) you should be able to fit a bigger cassette, but then you gearing 

 

You can use this site ( http://www.bikecalc.com/gear_ratios )to look at the ratios but here is a short list

 

Current 24 x 36 = 4.7kp/h

New 32 x 40 = 5.6km/p

or 32 x 42  = 5.4km/h

 

If you change your front chainring (hope you can get one in 30 that can shift id not a narrow wide)

Current 24 x 36 = 4.7kp/h

New 30 x 40 = 5.3km/p

or 30 x 42  = 5.0km/h

 

So bottom line, is that you current setup does not really want to work with 3x and going 2x is going to be harder.

[Quagga]

If you want at least the same or easier granny gear in 2x or 1x

you will need this

 

2x you will need a 28 x 42 front with 

11-42 back

 

Or 26 x 42 to make it easier than 3x

 

or 

1 x you need 

30T front

11-42 back to be same 

28T to make it from .07 = 06 km/h easier

 

Hope it helps