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I have a new road bike with thru axle and disc brakes (Ultegra). I noticed that depending on the how tight I screw the axle in the frame, the brake disc will be closer to either side of the brake pads. I didn't pay attention at first, so now the rear left pad is much more worn that the right one.

Is there a way to find the perfect torque to make sure that the disc is perfectly centered?

You should screw your through axle according to the manufacturer's specified torque value range. The torque applied to the axle is not meant to be used for adjusting relative brake/disc position, even if you observe such effect. Keeping the axle under-torqued is dangerous, and over-torquing it might damage the fork/frame dropouts.

A proper solution would be to adjust the position of the brake caliper itself relative to the disc. One possible way to achieve that:

1. Screw the wheel's axle to manufacturer's recommended torque.


2. Loosen both bolts that attach the caliper to the fork/frame. Do not unscrew them too much; you should just be able to wobble the caliper by hand.


3. Squeeze the brake lever so that both pads hug the disc. If the pads were symmetric, then the caliper would become centered relative to the disc. In your case, you should probably do this with a new pair of pads.


4. Keeping the lever pressed (by using a rubber band, rope etc.), tighten the caliper bolts so that the caliper will stay in the new position. Make sure to tighten these bolts to a proper torque as well (typically in a range 7-9 N×m, but it may be different)

Now, when you release the brake lever, the distance between pads and disc surfaces will be equal (given that both pads move at the same rate, of course).

I suspect a little that it was loose at one point and there won't be much movement of the brake within the thru-axle's window of "good" torque values. In other words, the range given for allen-headed "stealth" type thru axles, the only type that readily take a torque wrench, is usually about 9-13Nm, and I doubt there would be movement of the brake without about that range.

Torque it so that it's firmly bottomed out and you can feel there's heavy resistance to further preloading of the threads. Make sure there's some grease on the threads to make it easier to sense what's going on. The nature of big coarse aluminum threads actually makes it pretty clear to tell through wrench feel when you've reached the "bottomed out" point.

If it's still vague and you're having trouble with the brake wanting to move, that's unfortunate. Make sure the hub isn't missing any hardware and that any removable bits of the dropouts are tight. Finally, you can use a marker to make an index mark to set the rotational position of the axle to every time. For example, on the drive side where the end of the axle is visible when installed, you could draw a line across its face and onto the dropouts. You still have to make sure it's tight when you reinstall it in case something shifts/wears, but for the most part that should make it easy to reproduce the "adjustment."