The front and rear spoilers of F1 cars are mounted very close to the two sets of gear. I suspect that the downforce vectors are located between the axles, in which case there is no unloading of an axle. In the case that spoiler(s) are located beyond an axle:
- the moment arm is enormously less that that from a tailplane so that the result is very little unloading of the opposite axle -- or the car goes off the road
- Any unloading from one spoiler is more than counterbalanced by the action of its opposite number.
In the case of a/c we are dealing with moment arms of a hundred feet or so as opposed to much less in F1.