Hubs shifting during race

Buy a 50mm ball hone and hone the inside of the hub, that will clean it and remove high spots in the bore. Also clean the axle with some really fine grip sandpaper. Put in new clamping bolts.

I have experienced similar running wide on a short axle with some hubs, and put it down to hub design.
(96mm hub, 50mm onto axle)

On hubs with 2 pinch bolts that allow the hub to grab all round evenly it has been fine, however, where there was a key-way at 90deg to the pinch bolt you are asking 3/4 of the hub to 'ull around the axle to clamp effectively, which is simply not viable.

Result is hub moving, leaving galling on the axle and inside of the hub.

Don’t get how a grub screw holds it in place considering the amount of pressure on the hub. If the main hub screw can’t hold it in place how does a small grub screw keep it there.

Use some chalk/crayon put it on the axle the part where the hub is mounted
and your problems are gone.

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It helps because the grub screw digs into the key slightly. Same as your axle bearings and collars.

With the pinch bolt in the hub you have a large clamping force over a large area, low pressure.

All that said, you really shouldn’t need or rely on a grub screw in your hub. But it’s an option if you need it.