Obscure SPD history question

[Brucey]

PD-M525



PD-M737



the rear jaw is of course only half the story; the front part of the cleat isn't always located in the same way between pedal models, and some of the  float comes from the front jaw.   However IME all 'early' SPDs (up to and including PD-M747) are designed to work with SM-SH50 and/or SM-SH51 (first version), which has a narrow protrusion at the 'heel' of the cleat.   None of these pedals offer any appreciable float if you use the current version of SM-SH51.  I think the first pedal which used the current(ish) jaw shape was either  PD-M515 or PD-M324.

cheers

[rogerzilla]

I think there were undocumented changes:

Screenshot_20200921-192952_eBay by rogerzilla, on Flickr

[Brucey]

possibly but it is still a much narrower jaw opening than that found in  more recent pedals.

FWIW it is possible to 'convert' older pedals so that they have more float when used with current cleats, simply by grinding the rear jaw opening.

It is also possible to use cleats which are slightly shorter than normal; if the jaw is against the stop when the cleat is in the binding, this can give float which is almost entirely unrestricted over a small range. By contrast the float which is usually obtained has some resistance caused by the curvature of the jaw openings and the jaw spring pressure.

cheers

[Brucey]

I found this picture lurking on the web



which is taken from the SPD patent which resulted in the 'open binding' variant, eg PD-M520.   The overlap shows how the cleat is supported from beneath in this pedal.  This shows the cleat 'shoulders' may not provide as much support as I supposed in this pedal (they do in others BTW), but the edge of the 'racetrack' opening does provide support. As mentioned upthread this edge is flat in SM-SH51 but chamfered at the sides (almost to a knife edge) in various earlier cleat designs such as SM-SH55.

You can also see the 'kicker ramp' #67a, without which the pedal won't release cleanly once the cleats are a bit worn.

In case you have not seen it (it is on cyclingtips too) this article is interesting

https://www.pinkbike.com/news/the-20-year-history-of-the-shimano-pd-m520.html

However the 'invention of the SPD' is actually the invention of the third 'open' version of the binding.  The first (seen on all the early pedals) was superseded by the second version pedals which use the 'bignose' (and bigheel) versions of the cleats eg SM-SH51(v2) (as illustrated above) . PD-M520 etc are the third version, and are certainly the most successful, remaining virtually unchanged for about 20 years.

cheers