because "welding" the centre changes the metallurgial properties of the centre.

we're talking about hi-tensile material here, so to make the weld a "proper" weld and completely failsafe, after welding you need to heat the whole centre up to a dull orange and then let it cool slowly. then heat it up to an almost yellow colour, then let it cool slowly.

then the weld will be of the same quality as the surrounding material.

welded centres in frequent drag racing use WILL fail - which is why all the top guys use full spools. you don't want a weld to bust under full power and transerring 90% of the load to the unbusted weld, which makes the car lurch sideways, then makes that weld bust and then the car swings the OTHER way and then you are back to a single spinner with bits of weld floating around inside the housing running the risk of them jaming the gears and porviding full rear lock up - - - all at 100+mph.