Shimmy back wheel

Eye rattle on the trail ? 🙂

Looked down and the back wheel was out of line.

As in:
- sat to one side of the frame, but 'straight'.
- at an angle in the frame.
- wobbling as you ride along.
- tyre wobbling but rim not wobbling.

?

Rim wobbling side to side / and possibly up down

Rim wobbling side to side / and possibly up down

Only when riding, not when you've got home?

I assume you've:
- spun rear wheel with no weight on and it's fine
- cycled on it around some corners or off a bump etc to see if you can re-create the issue
- checked every spoke tension
- checked qr/axle

I had that once. Broken frame with the crack opening when I was sat on it but closed up and hard to spot with no weight.

I should have qualified my statement with it's probably not that, but something to check once you've ruled out all the obvious wheel stuff outlined above.

Ok we have a bunch of spokes on the NDS that are slack.
Is it a simple as nipping them up? Pick a tight one and pull to the same tension?

Yes - basically nip them up.
It *can* pull the wheel other ways, so go easy.
Syd and Macky show how to tighten with and without a truing stand:

Fab, I’ve had a quick look. I’d say 12 spokes on the NDS are slack. 2 at some sort of tension.

I guess the wheel is just a shoddy build

I find that some wheels need a 'good' true & tension. It's one of the few things I pay a good local wheel builder to do.

Bike still under warranty??

Back to the shop you go.

Low spoke tension.

If it's convenient then take the bike back to have the wheels tensioned.

If not convenient and you're comfortable doing it yourself then....

If the wheel is true then go over it with a spoke key increasing the the spoke half a turn at a time. Spokes should feel taught when squeezing two together.