I know this was discussed a few months ago but I’d like some opinion. I train in both Pearl Izumi and Mizuno. The first is 14.2 oz the second is 12.6 oz. per shoe according to my postage meter scale. I’m a sub 3:30 IM runner and I’ve always raced in Mizuno but I find the Pearl Izumi a little bit more comfortable. Does this difference in weight matter?
I don’t think the difference between the two matters too much, but I think you could train into some much lighter shoes (in the range of 7-9oz each) and quicken your miles splits a little. There is a substantial energy savings at the foot when it is lightened, especially over the marathon distance, and especially more yet that you are running it “tired”.
Interesting as I’m in this same situation. My trainers have been giving me blisters and I’m thinking of shelving them for the “racing” shoes (lighter, but not racing flats by any means) that I use for up to 1/2 IMs. They are super comfortable but lighter than what I would normally wear for that distance.
How much does body weight factor into this? What about time on your feet (slower runners)? At what point are the shoes too light for IM running?
Bodyweight does not factor into the concept of wearing lighter shoes to enable the lower leg to swing faster with less energy expediture, per se. What I think you are getting at is how does bodyweight affect the experience of wearing lighter, less substantial shoes over your beefier trainers. I cannot answer that question for you as that would be “your” experiment. Factors such as running form, body size, distance, training loads, previous base, intelligent rest…etc all factor in how successful your experiment will be.
As far as slow runners…the energy savings may be even more substantial (on paper), but it may not be realized because energy supply is not the limiter, slow running is (for whatever reason).
My opinion is there is not a shoe that is too light for IM running if you have good form,trained in it, can wear it comfortably and it works for you. My personal experience: I go as light as I can get (7-8 oz each for size 13) with no problems (even when I cool them down with my onboard personal lower leg cooling system…if you know what I mean)
The Mizuno is the Elixr which is advertized at 10 oz. but size 12 is 12.6 oz. on my scale. I’ll work on using something lighter next year. Maybe Pearl Izumi will introduce a racing flat.
rroof recommended the Saucony Type A to me. It’s VERY LIGHT (<9oz in my size 12 feet), very breathable and quite comfy. It also has a very ‘bouncy’ forefoot.
I actually found the insole a little too squishy and put in a stiffer (EVA I think) insole. It’s a little wide in the toe box, if that’s an issue.
I really liked the Saucony Fasttwitch shoes too. I went through a few rounds of them during training but went back to Asics Hyperspeed for some reason. I really love to race in Brooks T3 (I think there is a T4 out now, both versions are under 8oz each for size 13 ). I put some elastic ez laces in them and they slip on and feel as comfortable as your favorite Saturday slippers!!
I wish my feet fit the Brooks better.
what about the 4 lbs of sweat soaking my shoes after 2m?
The type A has vent/drain holes in the soles 
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You bet weight matters for a marathon, especially an IM marathon! If you are planning a 3:30 IM marathon (top 5% split), you are likely “efficient” enough biomechanically to run in about anything. As docfuel mentioned, I really like the Saucony type-A since it is ridiculously (8.4 oz in my size) light, yet with plenty of cushioning to help out the knees/quads after some pounding. The drain holes in the sole are nice if you dump on the water like I do during the run. If you try them on and feel that you want some “more” shoe, I like the new Saucony Fastwitch 2s as well (but they got a bit heavier in their redesign) and the Brooks T4s (they fit a lot different than the Saucony’s, so one will likely feel better than the other)