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I have a Norco VPS Six with the following dt components

Drivetrain
Crankset Truvativ Hussefelt, dual 32/22 with bashguard, Isis spline
Chainguide
Pedals Wellgo flat pedals with steel insert pins
Chain Sram PC-950 chain
Freewheel Sram PG-950 cassette 9sp, 11-34T
FrontChanger Shimano Deore 31.8
Rear Changer Sram X9 9-SP long cage
Front Shifter Sram X9 Trigger Shifter
Rear Shifter Sram X9 Trigger Shifter

Lately i have been having problems with my second lightest gear, in that it will periodically slip out of gear. As you can imagine this is extremly annoying while im huffing and puffing and spinning up a hill.Does anyone know the culprit? I had a local bike shop look at it and had a base level tune up but that one gear still slips. Any ideas?
 

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If this is a problem that slowly developed over time on a bike that had always been OK, the place to start is chain or cassette wear. Over time chain's stretch, not like an elastic band, but because with wear at the pins, each link is able to move farther from it's neighbor under tension. That changes the pitch or link to link distance from the original 1/2" to something larger and allows a chain to climb up and disengage from a sprocket. It also causes sprocket wear which it the other half of the problem.

Start by measuring your chain for stretch by putting a pencil across your chainstay so you can tension the chain slightly with the crank without the wheel turning. Use a 12" ruler to measure 11" of chain from the 1/2" mark to the 11-1/2" mark (don't use the ends because they're often worn and not truly 0 and 12"). Since the chain is 1/2" pitch, every pin will line up with a 1/2" mark, but when it's stretched they'll be slightly beyond the marks. Start with a pin lined up at 1/2" and if at the other end the pin is more than 1/16" beyond the mark your chain is toast. If it's far beyond the mark it's possible or likely that your cassette is also. (BTW- if you told them of your problem, the shop should have checked this as part of a tune up)

Start with chain wear as the most likely, others include minor trim errors in the RD, the RD upper pulley being too close to the sprocket, a nicked or chipped tooth on that one sprocket, and more if none of those.
 
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