Rear Mech Cage Length Question

[EasyPeez]

I ride a 11-28t Shimano 105 cassette, and that does me just fine all around the hills of northern England, but this summer I'm heading to Flanders to take on some of the classic cobbled routes and climbs, including the Koppenberg etc.

I'm worried that the combination of steep and bumpy might be a bit much, so am considering sticking a bigger cassette on for the trip - ideally an 11-32t.

Would I be ok with my current medium cage 105 derailleur? This suggests yes -

https://accidentalrandonneur.wordpr...-derailleur-cage-length-short-medium-or-long/

but my mate is telling me no. Who is right?

Either way, I think I'd need a longer chain - can someone remind me how to calculate how many links to cut it at?

Thanks.

 

[TheDoctor]

Personally I'd stick the cassette on and try it. If the current rear mech won't do it, then replace it, but I've normally found that the spec can be pushed a little, if you don't mind a bit of noise occasionally. As for length, I always go long enough for the big-big combo.

 

[greenmark]

Your mate is wrong.


First, what year 105 do you have? The most recent ones (late 2018/2019) or something a bit earlier. The latest ones can cope with 34T largest sprocket.

If it is a couple of years' old then it will be a 105 5800-series. Medium cage is called a "GS" (as opposed to their SS short cage derailleurs).
Shimano states that the maximum capacity for the 5800 series GS rear derailleur is 32T at the back.

As well as largest sprocket, the other thing you have to look at as the derailleur capacity, which Shimano says is 37 teeth. This is the difference in teeth between the front chainrings and the difference in teeth between the smallest and largest sprocket. So if you are using 50/34 and 11-32, then just check that 50-34+32-11 = 37, which is within the derailleur capacity.

Note however, that Shimano is being safe on this. People have successfully used 5800 GS with 11-36 cassettes and 50/34 chainrings.

 

[EasyPeez]

Thanks @greenmark - all useful advice.

It's an older mech so I'll stick to a 32t max - ergo 50-34 (16) + 32-11 (21) = 37

I won't consciously ever choose to ride 'big/big' anyway, so the derailleur shouldn't be taken to capacity in that sense, but I would want to know that the set up had the right capacity, just in case of any misshifts etc.

Do you just wrap the chain around the 2 biggest cogs in order to work out how long to cut it? I thought I once saw someone post a way of calculating it on here but can't seem to find it now.

Thanks for your help.

 

[dickyknees]

Any help?

http://productinfo.shimano.com/#/com/3.8?cid=C-453&acid=C-454

 

[Cycleops]

One of the ways is large to large (omitting the Derailleur) plus two links, there are other methods but all will come back to the same result.

 

[raleighnut]

I'd think if you avoid large-large and small-small you'll be fine and like @Cycleops says wrap the chain over the large sprockets but not through the mech and add a pair of links for chain length.

EDIT you may need to twiddle with the 'B' screw to move the rear mech away from the largest sprocket too.

 

[pawl]

I changed my Ultegra short cage for a 105 maedium so i could run a 32 large cog With hindsight I think I could have fitted a 30 cog with the original cage. Withh a medium cage 34 is a maximum.105 v Ultegra changes are as crisp as the Ultegra ,the only difference is that105 is slightly heavier.

 

[slowmotion]

To find out what the chain length should be, use this...

http://www.machinehead-software.co.uk/bike/chain_length/chainlengthcalc.html

It works every time.

 

[TheDoctor]

What I'd do would be use the current length, which is suitable for 28T, and add 4 to make it suitable for 32T. No measuring required. It assumes that your current length is right.

Someone will probably be along in a minute to tell me I'm an idiot.

*obliges*


That would work perfectly well.

 

[Jody]

I run Sora on the road bike and swapped 28 for a 34 on a medium cage mech with no issues.

 

[EasyPeez]

Thanks everyone, for your helpful replies.

I had no idea that this method was sufficient -

One of the ways is large to large (omitting the Derailleur) plus two links

I also can't envisage how it would be as, if and when you are running big-big or a combo close to it, it surely takes more than 2 links to account for the amount of chain running through the mech and around the 2 jockey wheels? Clearly it's me that's the idiot here as I must be overlooking some simple physical factor.
Anyway, I trust the wisdom of the CC Community far more than my own intuition, so I'll give it a go once the new bits arrive.

I'll also compare it to this, though, just out of curiosity to see if they yield the same result -
http://www.machinehead-software.co.uk/bike/chain_length/chainlengthcalc.html

Thanks again.

 

[EasyPeez]

I also had no idea that adding a link for every extra tooth on the biggest rear sprocket was a thing.

What I'd do would be use the current length, which is suitable for 28T, and add 4 to make it suitable for 32T. No measuring required. It assumes that your current length is right.

Though interestingly/worryingly the calculator that @slowmotion linked to tells me I should only add one extra link if going from 28 to 32.....

 

[slowmotion]

Who you going to believe ... Some software of doubtful reliability? Or a guaranteed idiot?

When I don't have the software available, I predict the chain length in a complicated voodoo ceremony involving animal sacrifices.

 

[roubaixtuesday]

I also had no idea that adding a link for every extra tooth on the biggest rear sprocket was a thing.



Though interestingly/worryingly the calculator that @slowmotion linked to tells me I should only add one extra link if going from 28 to 32.....

I think that's correct. The chain only contacts half the sprocket, so it only needs to cover two extra teeth, not four. And one chain link does, indeed, cover two teeth. I would guess the current chain is very likely long enough in reality, though worth checking carefully on the stand.

You can apparently get half links to extend a chain by less, though I've never used one myself.