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Ski boots are notoriously difficult to fit. The fitters will advise to go one and often two sizes down from what would at first seem like a comfortable fit.

Do you find any performance advantages from wearing tighter clipless cycling shoes? The sizes on Shimano shoes seem to be one to two sizes too small from what one would expect. Is the reason for that that they are meant to fit (very) tight?

Update: socks

Companion, not a separate, question: What is the thickness of the socks you wear with cycling shoes? I imagine cotton is a bad idea in an intense (i.e. sweaty) workout. Polyester or wool are the only two options (moisture wicking, less hospitable to bacteria growth, etc). Wool would not help much with guarding against cooler temperatures, 1- because wool socks are usually so thick they'd require a different size of shoes, 2- because the shoes are ventilated and so the inflow of cool air will anyway negate the advantage of wool. Hence polyester is the only option.

Can you confirm in your answer that the thinking above is accurate, that cycling shoes are worn with polyester socks. These, I assume, would not modulate temperature. The socks one would wear would remain on the thin side. One would modulate the temperature by full leg covers (and shoe coats if/when necessary).

Briefly: Can you comment on the thickness of socks that one would typically wear during summer? Does it boil down to preference? Some people wear soccer shoes with extra thick socks and others with thinner socks. Does the same apply to clipless cycling shoes, or does everyone eventually find that thin or thick socks are the way to go?

  • The size discrepancy you're feeling isn't my experience with Shimano, but sure, maybe I'm the odd one out :). For a little more anecdata know that about 1.5 years ago bought Giro SPD-SL road shoes online, when I got them felt like my heel was going to pop out of the shoe when walking, rode with them anyway, got faster, started racing, placed in some crits. They're still just as 'loose' off the bike, but completely fine cranking out 1000W+ on the bike ¯\_(ツ)_/¯ – Lamar Latrell Apr 9 at 6:36
  • My Shimano MTB shoes are of the exact same numeric size as most of my other shoes. – Vladimir F Apr 9 at 7:46
  • I think ski boots are actually often made a bit larger then then numerical size would suggest, to leave room for thick socks. (That's why, in the absence of such socks, it may be a good idea to go down a little.) The reason they're anyway hard to get into is simply that they're so clunky hard-shelled. – leftaroundabout Apr 9 at 10:02
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    Which number system are you using? I have found that US/UK numbers are more consistent than the European system(s), but some manufacturers and distributors have problem telling whether they use US or UK. – ojs Apr 9 at 10:45
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    Buying cycling shoes on-line (if that's what you intend to do) is not a good idea with an unknown brand and first time buy. A bike store is the better option. At least an on-line shop should give you the option to return the shoes. – Carel Apr 9 at 11:43
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In my experience you mainly want them to be comfortable.

But there are several considerations:

If the shoes are too short your toes will bump the ends, and this can become torturous 75 mile into a centurion.

On the other hand, if the shoes are loose your feet will slide back and forth. Not only can this be painful, but it means that you will not have a steady foot on the pedal, but rather the relationship between the ball of your foot and the pedal's center will be constantly changing, resulting in poor pedaling (and some additional pain in the foot).

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    Also, if your foot is sliding around, that would seem like it could cause blisters, correct? It's definitely the case in running. In cycling, we may not be hammering our feet as hard, but we can be in motion for longer than runners. – Weiwen Ng Apr 9 at 12:48
  • @WeiwenNg - Yeah, likely. – Daniel R Hicks Apr 9 at 13:05
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    There is also quite a bit to be said for manufacturers and their widths. A shoe that is too narrow (like many of the Italian styles) kills me when "krunk" down for snugness. I have a wider forefoot and have a particular brand I trust because I know that when the proper length is fitted, it won't smash my foot from the other direction. – Deleted User Apr 9 at 14:33
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    @WeiwenNg It's much less of an issue in my experience. I have a lot of miles ridden with "sloppy" foot wear that had wet, or unnecessary insulation (socks) taken out and have never had a problem. On the other hand I have boots that didn't fit right that gave me blisters just walking around in them for a few hours. – Deleted User Apr 9 at 14:37
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IMO you want your shoes to be a "generously" large fit when at rest. Not so large they slide around on your feet but still biggish while remaining comfortable.

Why? Your feet will subtly swell during exercise, making a good fit into a tight fit. That's why your slightly large fit becomes good after some time.

Naturally this assumes your socks are the same thickness as you'd wear when riding. Avoid test fitting shoes with big woolly socks, unless that's what you ride in too. Upshot here is that your winter and summer feet may vary in width by a noticeable amount.

Length of foot doesn't change as much, because the only flesh is around the heel, in the toes and between the joints. Instead its more about width of foot, across the top more where the skin is more flexible/elastic.

This is one of the reasons why velcro closers and boa dials and other tension-adjustments have grown in favour - noone's going to retie their shoelaces while riding, but you can reach one hand down and tweak a strap/knob without stopping.

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    I do not experience any significant swelling. Rather, after the initial tightening I normally have to add a click or two to tighten them more after an hour of riding (MTB Shimano shoes). I recommend finding shoes that can be tightened in the shop in such a way that they do not significantly move when you triy to move it on the foot and do not cause any hard pressing. The foot must be stable in the shoe, that is paramount. The foot must not move. – Vladimir F Apr 9 at 7:44
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    You might also consider the width of the shoe. If it is too wide it will produce folds and bulges when you tighten it which will quickly cause discomfort. – Carel Apr 9 at 8:40
  • @VladimirF Can you clarify if your question is about road or mtb shoes? I suspect the continuous nature of road cycling might work out opposite to MTB where theres a lot more standing around/waiting/not-riding. For me, the swelling starts slowly in the second hour and goes for about an hour - so after 2 or 3 I need to relax my shoe straps. – Criggie Apr 9 at 10:18
  • @Criggie I do everything in one pair of shoes (two identical pairs in fact) mostly on a gravel bike but also on a mountain bike and a road bike. I did not notice that much difference. When I d MTB, it is cross-country, so mostly continuous riding. – Vladimir F Apr 9 at 11:30
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    I like that this includes a sock bit. I own 3 pairs of the same pairs of Lakes for different thickness socks in different temperatures. In my experience, well fitted shoes are good for a narrow(ish) range of temperatures. Don't expect one pair of shoes to fit well and perform well year round unless you live in the tropics. – Deleted User Apr 9 at 14:26
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You control the angle of your skis through your ski boots. Any slack between your foot and the boots compromises your directional control. Therefore the boots need to be really close fitting. My toes are right at the end of my ski boots, and it hurts when I put them on. However, you control the direction of your bike through the handlebars, and it is possible to ride a bike fairly fast wearing flip flops. Therefore your cycling shoes just need to be a comfortable fit.

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    Power transmission efficiency will suffer and on a long ride, your feet, especially your toes, will suffer, if the feet are not firmly held in your shoes. – Carel Apr 9 at 11:39
  • @Carel: Sure, cycling shoes certainly shouldn’t be loose, just as ordinary walking or running shoes shouldn’t. But I think this accurately explains why ski boots need to be extra tight, where cycling, walking, and running shoes don’t need. (I don’t ski but I do climb, which is another setting where you rely on very precise control via your feet+shoes, and so you again want shoes which by everyday standards feel much too tight.) – PLL Apr 9 at 18:42
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    @PLL: I also want my cycling shoes to be tight because sliding around, especially fore and aft produces blooded toes and blistered heels and a kind of disconnection with the bike. Also after the first half hour I have to close the boa one or two clicks on both shoes similarly to what Vladimir F said. – Carel Apr 9 at 18:57
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Scenario: you are buying a pair of shoes, and want to determine whether it's the right size.

First of all, your feet are almost surely not identical. Pay more attention to the larger of the two. There are two useful tidbits of information to know about your feet: 1- which foot has bigger bones, and 2- which foot has bigger bulk (muscle mass, or "meat"). The answer to both questions may not lie in the same foot.

Now, if your toes touch the front of the shoes, tighten the straps. Are you able to tighten the straps so that you have adequate blood supply and get your toes off the front? If yes, you've found your right size.

Perhaps the most important part of the answer is this: You do want to tighten the straps as much as you can (while maintaining blood flow). If you don't, you will lose a small bit of efficiency. Your foot will wobble up and down during your stroke. Ideally, you do want to finish your down-stroke and immediately after find yourself able to start the up-stroke (and vice-versa). A "lag" is not just irritating if you want to go fast. It's also disturbing because, for just an instant, you will not have power, when you were hoping that clipless pedals & shoes provide power throughout the 360 degrees.

The straps themselves are likely quite adaptable. If your feet do swell during your workout, as Criggie suggests, you'll have to readjust in the middle of your ride. Whether it's safe enough to do so (for you) without stopping is an entirely different question.

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