It really depends on the surface. Crushed limestone, smooth ‘grade’ 1 or 2 you’re best bet is super shoes. Honestly it’s probably the fastest on even the roughest gravel as well, but people tend to go lower stack on rough gravel and trails because it’s very easy to turn an ankle on high stack, low width shoes like the next%.
I’m with you on the instability. No way I would ever try these on trails. Even on the portion of IMWI that goes on the nearby football field the Next% felt a little weird. Do you have experience with that type of shoe on gravel? I wasn’t really sure from your reply.
I’ve used them on all sorts of surfaces, and even on the roughest of gravel they are still the IMO fastest, just not stable at all. The only surface I’ve anecdotally felt them to be slower is on softer dirt trails. Shoes just kinda sink into the softer dirt and the bounce is non-existent.
That’s where I start wondering about things I don’t have expertise in. To me, the foam is primarily responsible for the energy return. That being said, the foam is acting on the outsole, not the ground (with the outsole acting on the ground), right? Thus I believe the outsole to be more important in loose surfaces than on paved surfaces. The shoes with so much exposed foam or lots of separations between the outsole pieces seem to need something solid to be landing on and pushing off from. So maybe a solid outsole helps the foam do its job on loose surfaces? Maybe the surfaces aren’t loose enough for it to actually matter.
I’m a research analyst, not any sort of engineer though. So it could be crazy talk.
And my primary goal is reduce soreness the day after. I have done enough long distance triathlon to know the ‘super shoes’ make a difference for me the next day.
The outsole on non-trail shoes is really just for durability. You could make a case that 5k/10k shoes would be faster with no outsole at all, if they would last the race.
The real place where an outsole design is important is on slippery surfaces. Slippery surfaces (like finely crushed limestone) have a primary loss in slipping of the foot, so mechanical contact would be best (i.e. spikes), but spikes would produce a point load on soft foam unless the outsole was made very stiff. Loose surfaces like rocky gravel and mud have such a deleterious effect on running form that it’s hard to isolate the effect of supershoes. BUT if you were to put a solid plate outsole on supershoes to help the foam on rocky surfaces the shoe would just slide off of any non-level rocks.