Road bike had a worn chain so have replaced today and whilst at it I changed the cassette, cheekily adding a larger sprocket for a bit more climbing assistance (I don't go uphill well :()
Did the chain at the same length as the one I took of, so went to check it whilst on the stand and the rear mech looked very strange in a stretched flat sort of way?
Thought I will need to add a link, so checked on small chainring and smallest sprocket first and the chain was hanging not under tension from the rear mech 😯
Its a 6600 Ultegra SS mech, spec shows I may be stretching the limits?
Max Cassette 11-27 (now have a 11-28 on)
Front Chainrings Max difference 15 Teeth (currently 16, 34/50)
Total Capacity 29T or less ???? not sure what mine is.
Anyone got any ideas to get round this, got a 100k Audax to do tomorrow!
no real way round it if the teeth difference is more than the mech can take
Fix it so you can get the big cogs at the rear [ for the hills] then adjust the rear mech so it cannot change to the smallest cassette cog and you therefore dont run the risk of dropping the chain?
Not perfect but better than jamming a chain on a long ride
don't use 50/28 or 34/11 ?
The capacity of your current drivechain is (50t-34t)+(28t-11t)=33t which exceeds the capacity of your mech (assuming it's 29t) by 4t. Assuming the cassette follows the sequence 11, 12, 13, 14, 16, 18, 21, 24 & 28t you can set it up so you don't use the 50t/28t combination, or the 34t with any of the 11, 12, 13 & 14t gears or buy a mech with a bigger cage.
alternatively, put a mtb 9sp mech on and use all the "wrong" combinations you want
Edit. Ignore that.
But it seems obvious to not use the big/big or small/small combos.
Dérailleurs usually are rated in terms of "tooth-difference capacity," a number that represents the dérailleur's capacity to take up slack in the chain. To find the minimum tooth-difference capacity needed for the gearing you have selected, subtract the number of teeth on your smallest rear sprocket from the number of teeth on the largest. Do the same for the front sprockets, and add the two numbers. For example, if your rear sprockets run from 14 to 26 teeth, the rear difference is 12. If your chainwheels are 36-52, the front difference is 16. Adding the figures for front and rear, we get a total difference of 28 teeth. If you get a dérailleur with a 28-tooth capacity or more, it should handle the range. If you use a dérailleur with insufficient tooth difference capacity, the chain will be too slack in some gears or too tight in others and you probably will not be able to get it to shift properly.
http://sheldonbrown.com/derailleur.html
Thanks Sum and qwerty, that was the calculation I was struggling with 😳
On a closer look it would appear the cassette I took off was a 12-25 so the 11-28 was stretching capacity all ends up, I'm blaming having too many bikes and not enough brain capacity 😆
I've taken a 12-27 cassette which is almost new off of a set of cross wheels and this although still slightly over capacity appears to be working much better and should get me round tomorrow.
And as has already been mentioned will have to steer clear of 50/27 and 34/12 😯
You are not meant to use big on big or small on small anyway
Just get a new rear mech in GS (medium) size rather than SS (short) (and add a couple of links to the chain so that it's long enough.
SGS is 'long' or what MTB'ers think of as normal, short for MTB mechs is GS, and SS is only available in Zee and Saint group sets. Note that 10s MTB isn't compatible with 10s road, but 9s MTB mechs are compatible with 9s and 10s road group sets (and 8s if anyone still runs it).
105
http://www.spacycles.co.uk/products.php?plid=m2b32s107p1444&rs=gb&vid=4119
I wouldn't set a bike up so that you can't use big/big. Too short a chain can cause quite a lot of damage when you select it by accident...
As above. You definitely need to have enough chain to get big/big if you want to avoid jamming chains, destroying mechs etc. You will have slack in small/small but that doesn't really do much harm.
The OP has already said that he could select big/big with the 11-28t cassette without destroying the mech, jamming the chain, dying etc. so I'm sure he will be fine if he accidentally selected big/big with the 12-27t cassette on his Audax tommorrow. I guess there was enough slack in the chain with the original 12-25t cassette for him to get away with it.