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Annoying shifting problems, I cant figure it out.

1K views 12 replies 6 participants last post by  JB. 
#1 ·
I'm setting up my new Titus Racer X, but I'm running in to some problems with my drivetrain. I run:
SRAM 990 cassette
SRAM 991 chain
SRAM X.9 Rear and front derailleur
RaceFace Deus crankset

When the chain is running on the largest rear sprocket and largest front chainring it won't stay on the front chainring, it just skips down to the one in the middle. Now what causes this behavior? I can't figure it out. The chain also seems to be running a bit rough on the front chainrings. It's not smooth like you would expect it to be.
 
#2 ·
thats called cross chaining and is not recommended, especially big/big as your chain is at its maximum, as well as the deraillier . Ideally the best combo is middle middle ,as you get further away from that the shifting and performance gets worse

the chain may be a little short causing the downshift issue, but dont crosschain and you should be ok
 
#6 ·
JB. said:
Well, actually it's like that for the three biggest rear sprockets, not just the biggest. The chain should be long enough. Although I'm crossing, shouldn't the chain stay on?

Could it have anything to do with chain line?
It could but should be pretty standard are you running a spacer in the drive side?
 
#7 ·
It may be a chainline issue, that's easy enough to eyeball. But why are you concerned that the drivetrain doesn't perform well when cross-chaining?

Just don't cross-chain. Ever. Seriously.

Other than that, all new drivetrain components may be a little rough at first until they mesh together. Give it a little time, that's pretty normal.

Cheers, Chris
 
#8 ·
Well, I'm not concerned about me not being able to cross chain. It's more a matter of everything not working the way it should. The hole front part of the drivetrain does not appear to be smooth as silk. I'll measure up the chainline tomorrow, maybe thats the problem.
 
#9 ·
JB. said:
Well, actually it's like that for the three biggest rear sprockets, not just the biggest. The chain should be long enough. Although I'm crossing, shouldn't the chain stay on?

Could it have anything to do with chain line?
well the ideal chainline is when in middle/middle the further out from that the worse the chainline.
to check the chain length, wrap the chain around the big/big without the deraillier and add 2 links
if the length is right you could need to adjust the FD, or you could have a worn or bent tooth on the big ring
also make sure the BB is not loose and the chainrings are tight as well
 
#10 ·
You have 27 POSSIBLE gears, but many are redundant, and bad to be in. It is helpful to study a gear inch chart. It tells how many inches the bike travels with each pedal revolution. Here is one from my drivetrain. https://sheldonbrown.com/gears/index.html



As you can see from the chart, the gear ratios from being in the big ring - big cassette combos are also achieved in the middle chainring. The chain is much straighter, quieter, more efficient, and lasts longer like this. You will also notice that your rear derailleur is almost wanting to explode from the long amount of chain.

Same goes for granny gear and small cassette cogs. Your chain has almost no tension, and the derailleur has little tention. Bent and unhappy chain, and those gear ratios are also available in the middle chainring.

The problem when shifting the front derailleur is that the gear ratios jump a bunch. If you go from the middle to the granny, it is too easy to pedal, too few gear inches. The solution is to then pop off a few rear shifts to make up for this. Same goes when going to the big ring. It gets real hard to pedal, so you need to move the rear a couple cogs to make the transision smoother.

With practice, this becomes ingrained in your brain. Everybody accidentally cross chains now and again, but try not so, and your drivetrain will shift better and last longer.
 
#11 ·
laurenlex said:
... You have 27 POSSIBLE gears, but many are redundant, and bad to be in ... the gear ratios from being in the big ring - big cassette combos are also achieved in the middle chainring. The chain is much straighter, quieter, more efficient, and lasts longer like this ...
well put ... if you look at the chain line from the back of your bike when you physically put the chain in the largest chainring and as large a rear cog as possible, you will notice that the chain is not quite straight ... ;)

what is important is the ratios that the gears are in which should be accessible to you at other combinations instead of large front - large rear.
 
#12 ·
Thanks for your response guys. It may seem as you think I'm new to bike mechanics, but I'm not, I actually work part time (I'm a student) at a bike shop. I'm quite familiar with it and I never cross chain when I'm out biking. But when setting up the limit screws I always use biggest/smallest, smallest/biggest combo to get it just right. And I've never had any problems with the chain not willing to stay on the 44t chainring. That's why I'm asking.
 
#13 ·
For the last day I've tried to make sense of this. I measured the chainline to be 49.25mm, witch is good. I've tried moving over the BB shell spacer from drive side to non drove side and, but it didn't help. I also tried putting om my old LX crank set. When I did this everything worked out fine. The chainline is the same. The only difference between the to crank sets is RF has 44t as biggest, while LX has 42t.
 
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