Excellent question. I'll be following……

If not having float hurts your knees, you want float.

and/or - having float gives you some leeway in not having to set your cleats 100% perfect since your foot can twist a bit.

That is about all there is to it.

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
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Freelance sports & outdoors writer Kathryn Hunter

I don't currently have knee problems, but can a little bit of float prevent developing knee soreness or injury?

Float helps give a little bit of wiggle room when setting up your cleat fit. Zero float (and stiff shoes) mean you need to have your clear position dialed in exactly. Some float let's you get your cleats close but not exact and still be comfortable. OTOH, float makes power transfer feel less direct.

All pedal systems have float, you only have zero float if you specify it. I assume you are referring to speedplays, which actually have a considerable amount of float, unless you tuned it out.

Also, i can't imagine racing tris with zero float. A slight amount of soreness from no float may be totally acceptable after a ride, but could screw up a run.