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Gear issues
I have an issue with my gears. They regularly slip into higher gears, especially if they are under load. When I ride on a nice, flat area and change gears, I can change up and down nicely as expected but when under a bit of load, nasty slippage which also sounds terrible.
Obviously something needs adjusting and I would appreciate some tips about what I need to look at. Do I need to adjust the cable tension or something else? It is a reasonably new bike so I am thinking that it is unlikely to need anything replaced although I am open to being told otherwise!
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When you say, "reasonably new" does that mean you've done nothing to it since leaving the store? Shifter cables usually stretch and bed in after a few rides so give the barrel adjuster a couple of turns and see how you go. Just go the right way.
Or... if you're like me and can never remember which is the 'right' way, just turn it one way and if that makes it worse, go back twice the amount in the other direction
The way I remember which way it goes on Shimano shifters at least (not sure on Sram as I've never used one) rotating the adjuster anti-clockwise (undoing the adjuster out of the shifter) effectively lengthens the outer cable, meaning the inner cable has to 'take up the slack' (for lack of a better description) and tighten the inner cable.
1. Undo the bolt on your rear derailleur that clamps the cable.
2. Shift onto the smallest gear (unless you have a reverse swing Shimano system)
3. On the barrel adjuster of the shifter, turn it until it is flush against the body of the shifter (or derailleur depending on system) and cannot be turned any further.
4. Now let the barrel adjuster out two full turns
5. Tension the cable at the rear derailleur and tighten the bolt (some pliers are handy here)
6. Shift up one gear and turn the pedals at normal speed. If you hear the chain wanting to jump up or down, or it does actually jump, turn the barrel adjuster a quarter turn at time until the noise or jumping stops. You should find a sweet spot where the chain runs quieter when its not trying to shift up or down. This is usually just trial and error but if you watch the rear derailleur jockey wheel a you move the adjuster you'll see which way the derailleur moves to, so you know that if jumps up and your adjustment moves the jockey wheel in, just turn the barrel the opposite way.
After writing all that, I also remembered this useful video too:
http://youtu.be/SjJfKO_tAo0
If you have a normal rear dérailleur then it will want to shift towards the smallest cog if there is no tension in the cable. It is the cable tension that pulls it up to the larger cogs. If you notice it changes to the smaller cogs properly but going back up to the larger (easier) cogs is where you have a problem then you need more tension. If the problem is in the other direction you need less tension.
Once you work out the more or less thing then simply turn the barrel adjuster 2 clicks in the appropriate direction. As Rob said, if you don't know which direction to turn the adjuster then 2 clicks and check, 2 more clicks if it doesn't work then if it still doesn't work or gets worse assume you have gone the wrong way so turn back the 4 clicks then go 2 in the other direction.
If a small number of clicks isn't a fixer then you have something more than cable stretch to deal with such as a bent dérailleur or hanger so best then to go through the process as someone else described above where you undo the cable and start from scratch.
My bike used to ghost shift up bumpy hills and when going quick in bumpy sections - was fine elsewhere. When the suspension was working/flexing it turned out it was pulling the cable due to how my rear suspension and cable routing work.
Some tape around the cable & frame down near the derailleur and some zip ties and it doesn't move now and is finally fixed.
Check that your derailleur is done up tight on your frame, as its a new bike it may have come loose. I recently installed a new one and it came undone and I had the same problem as you, it jumped gear when under tension.
Had a problem where my lockring would come loose, all would work fine until put under load then the cassette would shift slightly causing a gear change.