How do I know if a larger cassette will fit?

[screenman]

I think what some may be saying is losing a few pound in body weight may be a healthier option and make climbing far easier. The old saying far cheaper to lose a pound of the rider than a pound off the bike.

I hope your recovery goes well and that you are sticking to any physio prescribed, stay away from the big hills until your previous level of fitness returns, would be my advice.

 

[Rohloff_Brompton_Rider]

I think what some may be saying is losing a few pound in body weight may be a healthier option and make climbing far easier. The old saying far cheaper to lose a pound of the rider than a pound off the bike.

I hope your recovery goes well and that you are sticking to any physio prescribed, stay away from the big hills until your previous level of fitness returns, would be my advice.

For those of you who are offering technical advice thank you.....those of you offering unsolicited unwelcome health advice...

 

[PpPete]

Can only agree Pete, just because some can climb anything with the standard gearing doeasn't mean everyone can, especially if you are on the 'portly' side. Without hijacking brompton's thread completely, would you have any idea if a rear mech that came on a compact set up 50/34 and a 12-28 cassette would be able to take a 30t rear sprocket?
It's a 105 presumably short cage.

Same as above can you guys help on this one as well
Ultegra compact currently running a 11-25 on back can I go to a 12- 28 or 30?
It says my rear mech is
Shimano Ultegra GRAY NEW 6700A/SS 20 Speed - no idea what that means and will I need a new chain?
Sorry to jump in on thread

Sorry - no clue whether they put short cage or medium cage on a compact.
I only ever rode compact once on a rented bike and I did not like.

As per Slowmotion - look up the part number of your mech in the Shimano techdocs.
Have a look at the total capacity (the wrapping referred above) then add "a few".
My 6600GS is rated at 37 total, but I run it at 42 normally and have run out to 45 (total difference) on occasion when I had to borrow a wheel at short notice.

 

[screenman]

For those of you who are offering technical advice thank you.....those of you offering unsolicited unwelcome health advice...

Please accept my apologies.

 

[Rohloff_Brompton_Rider]

Please accept my apologies.

I was joking!

 

[ColinJ]

I was joking!

Well, in that case, do whatever you did last time and get back down to 16-stone-odd rather than 18- or 19-stone-odd!

(Cycling 12,000 miles on a Brompton in just over a year, wasn't it? )

 

[RecordAceFromNew]

Same as above can you guys help on this one as well
Ultegra compact currently running a 11-25 on back can I go to a 12- 28 or 30?
It says my rear mech is
Shimano Ultegra GRAY NEW 6700A/SS 20 Speed - no idea what that means and will I need a new chain?

Sorry to jump in on thread

There are two distinct limits imposed by rear mechs that you need to check before lowering gearing: a) max rear sprocket size the mech/hanger can handle, and b) wrap capacity which ensures the chain is still adequately tensioned on small front small back when the chain length is correct for large front large back.

According to this, for a 6700 SS (i.e. the short cage version) you have no problem going to 28T regarding a). Depending on the frame/hanger (in particular the distance between the axle and the rear mech fixing bolt) it is very likely that you can go to 30T. In any case you can check by measuring the clearance you can see between the existing 25T cog and the top jockey wheel when you are on that gear - every 2mm allows an extra tooth - if you don't get enough clearance adjusting the B screw may well give you enough.

Regarding b), based on the linked tech doc the 6700 SS has an official wrap capacity of 33T. If you put a 12-30 on that leaves 33 - (30 -12) or 15T for tooth differential between the largest and smallest rings on the chainset. So whether a 12-30 cassette breaches b) depends also on what chainset you have - if it is a standard double or compact you should be fine. If not it depends on how wide the difference is and how conservative Shimano's spec is in this instance.

 

[Sterba]

At the NEC Bike show I saw a new SRAM cassette for MTB that went to something like a 40 tooth cog, the spread of gears was so wide that you didn't need a front changer.

 

[slowmotion]

At the NEC Bike show I saw a new SRAM cassette for MTB that went to something like a 40 tooth cog, the spread of gears was so wide that you didn't need a front changer.

I need one! I'll stick with the triple on the front though...

 

[RecordAceFromNew]

At the NEC Bike show I saw a new SRAM cassette for MTB that went to something like a 40 tooth cog, the spread of gears was so wide that you didn't need a front changer.

May be you are referring to this? But they need specific freehub/wheel, for those of us still on 10, 9 and 8 (yes!) speed this has been available.

 

[Rohloff_Brompton_Rider]

Well, in that case, do whatever you did last time and get back down to 16-stone-odd rather than 18- or 19-stone-odd!

(Cycling 12,000 miles on a Brompton in just over a year, wasn't it? )

10,000 and it was 15 odd stone thank you very much, but I still used 19" bottom gear to sit and spin.

 

[youngoldbloke]

There are two distinct limits imposed by rear mechs that you need to check before lowering gearing: a) max rear sprocket size the mech/hanger can handle, and b) wrap capacity which ensures the chain is still adequately tensioned on small front small back when the chain length is correct for large front large back.

According to this, for a 6700 SS (i.e. the short cage version) you have no problem going to 28T regarding a). Depending on the frame/hanger (in particular the distance between the axle and the rear mech fixing bolt) it is very likely that you can go to 30T. In any case you can check by measuring the clearance you can see between the existing 25T cog and the top jockey wheel when you are on that gear - every 2mm allows an extra tooth - if you don't get enough clearance adjusting the B screw may well give you enough.

Regarding b), based on the linked tech doc the 6700 SS has an official wrap capacity of 33T. If you put a 12-30 on that leaves 33 - (30 -12) or 15T for tooth differential between the largest and smallest rings on the chainset. So whether a 12-30 cassette breaches b) depends also on what chainset you have - if it is a standard double or compact you should be fine. If not it depends on how wide the difference is and how conservative Shimano's spec is in this instance.

Apologies for jumping in, but can't find tech docs for the new 6800 Ultegra. I know it will accommodate a 32T rear sprocket with the GS mech - do you think this means it will probably take a 34T, given the careful nature of Shimano's recommendations? (- and assuming there is an 11 speed cassette with 34T available). As things stand, a 50/34T compact with 32T would give you a 28" lowest gear, compared with 29" from a 'standard' road triple set up - 30T x 27T.

 

[Cyclopathic]

Ok, as I was lead to understand this the rating given to a rear mech such as say 30 does not relate to the number of teeth it can handle on the rear cassette but the combined difference of teeth of the front rings and rear cassette range.

For instance if you have 50/36 on the front the difference is 14. And you have 12 to 28 on the cassette the difference is 16. Adding together gives 30 so any rear mech with a 30 rating or over will suffice. 30 seems to be about the limit for short cage mechs.

Anything with a triple will probably have a long cage even if the rear mech only has 26 teeth. To me this makes sense as the function of the cage length is to take up slack from the overall length which is a function of the differentials of both front and rear.

 

[RecordAceFromNew]

Ok, as I was lead to understand this the rating given to a rear mech such as say 30 does not relate to the number of teeth it can handle on the rear cassette but the combined difference of teeth of the front rings and rear cassette range.

For instance if you have 50/36 on the front the difference is 14. And you have 12 to 28 on the cassette the difference is 16. Adding together gives 30 so any rear mech with a 30 rating or over will suffice. 30 seems to be about the limit for short cage mechs.

Anything with a triple will probably have a long cage even if the rear mech only has 26 teeth. To me this makes sense as the function of the cage length is to take up slack from the overall length which is a function of the differentials of both front and rear.

As I mentioned above rear mechs have two parameters relevant to capacity for down gearing. The Largest Sprocket spec does relate to the number of teeth on the cassette. You can see them in e.g. the section entitled Specification in the 6700 SS tech doc I linked to above.

 

[RecordAceFromNew]

Apologies for jumping in, but can't find tech docs for the new 6800 Ultegra. I know it will accommodate a 32T rear sprocket with the GS mech - do you think this means it will probably take a 34T, given the careful nature of Shimano's recommendations? (- and assuming there is an 11 speed cassette with 34T available). As things stand, a 50/34T compact with 32T would give you a 28" lowest gear, compared with 29" from a 'standard' road triple set up - 30T x 27T.

I have never seen the relevant tech doc for 6800 either. However the exploded diagram does indicate one thing - that the cage length of the 6800 has no impact on max sprocket size, only wrap capacity, because the upper parallelogram and cage profile down to the top jockey axle are common between the SS and GS models. The dropdown spec here however does say it would only accommodate a 28T large cog. A +6T push is quite a lot which imho would be impossible to predict without trying, but certainly not unprecedented.

What I would suggest is to get a wheel with 34T cassette (irrespective of speed) on and see if you have the mech already, but be prepared to apply all the tricks available under the sun on the B screw.

Although this is purely conjecture since I have not tried it, I don't think it necessarily matters whether you can get a 34T 11 speed cassette if you have a Shimano freehub and if you are so inclined and are prepared to do some work. The reason is that the sprocket thickness of 9 and 11 speed Shimano cassette sprockets are 1.8mm and 1.6mm respectively, the difference of which has to be inconsequential in operation, and you can get a 9 speed 34T sprocket without spider easily and cheaply on e.g. a sacrificial HG50 cassette. All that remains is to sand one of the plastic spacers to around 2.1mm which should get the spacing about right for 11 speed.