help with gears please

theraggyone

I built my latest road bike last October with full Sram rival 22 (GXP compact chainset,11-32 cassette , force 22 front mech and rival 22 mid cage rear mech) and i had no issues at all with set up etc it all worked flawlessly .

For my 40th birthday in march i got a Sram red 22 GXP compact chainset , force 11-26 cassette, new GXP BB and force 22 short cage rear mech with a new 11 speed Sram chain.

i fitted all of this with no problems at all and it shifts brilliantly .Overall im really pleased but there is one thing that bugs me. in the big - big combination and second biggest - big ( i know cross chaining is bad) if i back pedal half a turn my chain comes off of the inside of the big chainring .This isnt usually a problem except when stationary and moving the pedals to position to set off but it is starting to annoy me as it never happened with the rival set up and i dont seem to be able to stop it, does any one have any suggestions?

is the sram red 22 chainline differant to the rival 22 one?

step83

From what I know the chainline is the same. When you setup the front mech did you take into account the Yaw on the mech? Be worth checking when you shift it to that position where the chain is sitting.

Also dropping rear cassette size did you reduce the chain length as well for the new chain?

svetty

Step83 wrote:

Also dropping rear cassette size did you reduce the chain length as well for the new chain?

^ this

theraggyone

yes ive made sure chain length is as short as it can be and i have spent ages dialling in the front mech so that it doesnt rub in any combination and shifts sweetly . never had this issue with any of the previous bikes ive built ,im a bit stumped by it.

step83

Its possible then the chain may be a little too short so the tension is lifting it off the front ring when you back pedal. You able to get a photo of it at all on big to big so we can see the tension its set with?

theraggyone

that sounds possible , i kept the links i removed from the chain so i will add an extra link tonight with a quicklink and see if that helps at all its certainly worth a try although it has no shifting problems and runs in big - big fine if i want. im at work at the moment so cant supply a photo unfortunatly. thanks for the help its apprecieted. the more ideas the better

theraggyone

Ive just been and had another good look. Its definatly the chainline, there is enough links in the chain if you push the lower jockey wheel you can create more slack in the chain and it shifts flawlessly. In big-big if you turn the cranks backwards you can see the chain struggling to cope with the chainline at the bottom of the chainring then it doesnt catch and comes off.... chainline looks good in centre of cassette it doesnt seem to be offset at all very strange. Looks like I will have to live with it..