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Context: Newbie at cleats, and using Decathlon SPD SL 3 bolt cleats for road bikes.

Is it easier to unclip using a shoe with higher stiffness index? I currently use a shoe that is around 6-7 on the stiffness index. Is it easier to unclip if I changed to a shoe that is around 10 stiffness index (for example, using Shimano RC702)

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  • Even in the vertical direction, cycling shoes should be much stiffer than the pedal spring tension (so using multi-release cleats like SH56 won't result in a noticeable difference). In the horizontal direction, there's no contest. The second moment of area for a tall plate is huge.
    – MaplePanda
    Mar 15, 2023 at 0:09

3 Answers 3

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In my experience: No.

Cycling shoe soles are extremely rigid in the “sideways” direction. Even with soft and flexible running shoes there is very little movement in the heel if you hold them at the front and twist sideways at the heel as hard as you can (with your arms). You are basically trying to bend a flat beam in the direction where it’s strongest (highest second moment of area).

Even if you have very stiff shoes, how tight those shoes are will have a much bigger impact when it comes to turning/twisting your feet. I’ve come dangerously close to failing to unclip when I’m too lazy to properly tighten the laces on my shoes.

Apart from that, how strong the springs in the pedal are has the biggest influence.

You can buy cleats with different release angles. In my experience having a smaller release angle makes it easier to unclip, but maybe that’s just because I have bad hip flexibility. I think worn cleats and pedals have a similar effect. Dirt or anything else which increases friction also makes it harder to unclip.

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    This is my experience as well. Mar 14, 2023 at 10:21
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I'm not going to answer the question as written, Because Michael is correct that sole stiffness has no effect. I'll focus on some other aspects of the clipless pedal experience.

Shimano SPD-SL pedals all have an adjustment screw to change the release tension. More experienced riders often prefer theirs tighter. In part this is acclimatization, so you may find that easy release matters less to you with time. Shimano also makes a version of their road pedals (the RS-500) that has a generally lower spring tension (but it's still adjustable). Look's Keo pedals have a similar adjustment. Speedplay doesn't have adjustable release tension, but it has standard and easy release pedals. Personally, I would see if you get used to the release tension, and consider adjusting your tension setting.

Sole stiffness doesn't matter to clipping out. It also may not have much of an impact on performance unless the sole is very flexy, and I do mean as flexy as a sneaker (I'm not even sure if such cycling shoes are commercially available). I have owned shoes with high-grade carbon (max stiffness index), moderate-grade carbon (probably would be rated an 8 or so on a 10-point scale), and nylon (probably rated 5-6). I don't notice a big difference in performance on any of them. I'm not aware of any hard scientific testing in this regard. Bike Radar attempts to quantify the difference in this YouTube video. I've heard some people say that too stiff soles may be uncomfortable on very long rides, and I believe I've experienced this myself. However, this could easily be other aspects or the shoe, or just the fact that it was a very long ride. As an aside, this is an example of a nocebo effect, which is like a placebo effect except that you perceive harm.

Also note that stiffness indices are not objective. They're just each manufacturer's own self-report of how stiff the soles are. When I got a pair of 2010 Specialized S-Works shoes, the max stiffness was 11, as in "it goes up to 11" from Spinal Tap. The max is now 15. For all you know, if you devised a deflection test, the soles wouldn't have 26.7% less deflection for a given load (that is, 4 points divided by 15 points = 26.7%).

Why would you need stiffer soles? The overall quality of construction may be better with the higher end shoes, and the closure systems may be better and have more adjustability. For sure they look nicer. This does provide a placebo effect, which is maybe not objectively worth paying $400 for, but which you might expect to improve motivation and time to exertion. The higher end shoes may have lower stack height - that is, the height between your sole and the cleat, influenced by how thick the sole is. That enables you to lower your saddle and reduce your aerodynamic drag marginally. Really the last would only matter at the margins.

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Short answer: yes the stiffer and tighter the shoe, the easier it is to unclip.

This does not mean that you should go for stiffer and tighter. Comfort is important. But is means that too loose will make unclipping difficult.

Up to you to strike a balance.

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  • Hi, welcome to bicycles. As noted in the other answers, stiffness and tightness are not necessarily correlated; on its own, how much difference does the stiffness of the shoe make?
    – DavidW
    Mar 15, 2023 at 20:29
  • If the sole is too soft (the opposite of stiff), there is play before the cleat comes off the pedal. Like when the shoe is not tight enough.
    – thiebo
    Mar 15, 2023 at 20:56

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