See more bike failure-related pictures at http://pardo.net/bike/pic.
"Road" shoes historically have stiffer soles than MTB shoes, and many riders find them more comfortable. Road shoes are also typically lighter and may have different ventilation.
Some older road shoes had a strip of rubber on either side of the cleat area, giving both stability and traction when the shoe is on the ground. However, newer models tend to elminate the rubber, so wider "road" cleats can be fitted. These shoes often have SPD bolt holes but are poorly-suited to use with SPDs: when walking, most weight is on the steel SPD cleat, making the shoe tippy, and the steel cleat gives poor traction on most surfaces.
Some cleats are available with outboard rubber blocks, referred to by Shimano as "pontoons". These typically clamp under the same bolts that afix the cleat and simulate the rubber strip of older shoes. (Beware that pontoons still do not give traction under the toe.)
"Pontoon" cleats are compatible with only some SPD pedals. The incompatability is non-obvious and can lead to the shoe jamming in the pedal unpredictably.
From http://draco.acs.uci.edu/rbfaq/FAQ/8h.4.html as of 2008/06:
Subject: 8h.4 SPD cleat compatability
From: Eric Salathe
Date: Wed, 10 Mar 1999 11:52:55 -0800 (PST)
According to the Shimano web page FAQ:
Frequently Asked Questions
19) What cleats work with which pedals?
The SM-SH70 and SM-SH71 work best with both the PD-7410 and the PD-6500. The SM-SH51 and SM-SH55 work with the PD-M747, M636, M545, M535, M515, M434, M323, A525, M737 and M525.
There are a couple usable combinations which can be substituted for the recommended cleat:
PD-M747, M636 M545, M535, M515, M434 can use all cleats (70,71,51,55).
The PD-A525 and PD-M323 work with all cleats except SM-SH70.
The new SH-90, SH-81/91 and SH-82/92 are only compatible with the PD-7700, PD-6600 and PD-5500 SPD-R type pedals.
From http://www.sheldonbrown.com/shimano.html as of 2008/06:
SPD Pedal/Cleat Interchangeability M858 M747 M636 M545 M535 M515 M434 M737 M525 A525 M323 7410 6500 SPD-R SM-SH70 (No Float) ? ok ok ok ok ok ok no no no no yes yes no SM-SH71 (With Float) ? ok ok ok ok ok ok no no ok ok yes yes no SM-SH50, 51 "Single-release" no yes yes yes yes yes yes yes yes yes yes no no no SM-SH52 "Single-release" yes yes yes yes yes yes yes yes yes yes yes no no no SM-SH55 "Multi-release" yes yes yes yes yes yes yes yes yes yes yes no no no SM-SH56 "Multi-release" For PD-M959, PD-M540 & PD-M520 pedals. SM-SH90 SPD-R (No Float) no no no no no no no no no no no no no yes SM-SH91 SPD-R (With Float) no no no no no no no no no no no no no yes Table courtesy Eric Salathé
- "Single-Release" cleats hold the shoe more firmly, only release on heel-out rotation (yaw axis.)
- "Multi-Release" cleats are easier to exit, with rotation in either the yaw or roll axis, either direction.
In the above, the SH70 and SH71 are the cleats with pontoons.
In some cases, the root cause of cleat/pedal incompataiblity seems to be a slightly taller cleat/plate/bolt assembly used to hold the pontoon. The extra height causes the bolt heads to protrude slightly, and they in turn sometimes overlap with a part of the pedal. When the shoe and cleat are twisted to release, the edge of the bolt head can catch on the edge of the pedal part, and other constraints on the cleat motion mean the cleat has nowhere to go and jams.
One workaround is to bevel or round the edge of the bolt head, making a shoulderless ramp instead of a lip where there the cleat and pedal (sometimes) contact. It would probably also work well to slightly bevel or round the pedal part, though doing so is more complicated and removes a chrome protective layer so may accellerate wear.
In this case, material was removed from the bolts using a belt sander, being careful to keep the bolt cooled to avoid changing the base material.
Removing material from the bolts may weaken them slightly, however, in this application with a sub-65kgf rider, weakening seems acceptable because this part of the bolt raarely fails: SPD failures are typically either the hex wrench hole rounds out, or breaking the two-hole oval "washer" between the bolts and the cleat.
Here is the pontoon cleat with the bolt edges beveled:
Here is a non-pontoon cleat, note the bolts sit deeper in the cleat: