Just installed a used SLX (M665) and (***warning complete newbie re hydraulic brakes***) when braking the front wheel visibly warps/moves to the left. I tried reseating the wheel to make sure it’s straight. The rotor runs true, the wheel spins freely between the pads – but when braking this bending is alarming. Anyone got an idea?

Yes, you’ve got the caliper mounted too far to one side. The pads retract enough to prevent rubbing thanks to the servo wave thing, but it means you need to mount the caliper by eye to make sure it’s aligned properly.
Get the bike in a stand with a light so you can see the clearance between rotor and pad. Wheel out, and use a flat screw driver or tyre lever to push both pads all the way into the sides of the calipers so the pistons are fully retracted. Be careful not to knacker the pad surface too much. Wheel in, and undo the two caliper mounting bolts. Now align the caliper so there is the same amount of clearance both sides, and fore and aft. Once you are sure it’s even, dothe mounting bolts up half a turn at a time watching the pad clearance as you do so.

Wheel done up tight? Spoke tensions good? If yes and yes you may have noodely forks and you may or may not die.

What forks?
What wheel + hub, and are the spokes tight?

Check spoke tension

I think scapegoat thinks you mean the rotor is warping. Do you mean the rotor or the wheel rim. I am guessing wheel rim, and can only think some kind of some tension issue.

If it is the wheel rim my money’s on noodley forks.

It’s deffo the wheel not the rotor, the rotor is straight as a die. Forks are Rock Shox Tora TK. What 😯 that noodly fork biniss?

OK so I checked the caliper/pad alignment again, bang on with rotor slap bang in centre. Then I activated the brake and rocked the forks back and forth, and wheel skewed to left again side while static.

Then I checked to see how tight my QR skewer (Superstar steel) was…and retightened it…

I’ll get me coat…

😆
Better to find that out now than on a ride when your wheel falls off.

Chortle!

I’m not sure what you’re describing. The whole rim moves left, or parts of the rim bend out of plane to the left, or the wheel turns to the left?

If you apply the brake and pull the bike backwards, does it do exactly the same as forwards?

As above, I’m suspecting spoke tension. It wouldn’t be obvious with rim brakes, but with disc, you’re holding the hold and trying to move the rim, that pulls on the spokes, and if they are tighter on one side it may move the rim. If you “ping” the spokes going to each side of the hub, do they all sound the same?

In addition to skewering myself with shabby QR tightening habits, further to yr ample (thnks!) suggestions I checked the spokes and this is a pretty bad wheelbuild, the back one has already seen two broken spokes from light riding in as many weeks – time to get them off to my mate the (proper)wheelbuilder…

(Alex DP17’s/Shimano – stock on a Genesis Core 20)