Tuesday, March 4, 2008

How a Clipless Pedal Works

Hey! Want to say screw off to your enemy at the races?

Yes, just do it. Screw off.

No no no hold it! Hold it ... I didn't mean THAT!!

I meant : Take a screw off his pedal cleat when he isn't looking. Literally.

Well, I'll come back to this in a sec. First lets see how a clipless bicycle pedal works. All the nerdy wordy descriptions aside, I'll let you look more at pictures.

This is the Shimano PD-520 Pedal, a set of 3 year old beaters I have on my fixed gear. This version is made in Malaysia.

1 and 2 are the clipping sites for the cleat, something you attach to the back of your shoe. 3 is the body, 4 the spindle, and 5 is the threading that fastens into a crank arm clearance.

Member 2 is able to move back and forth (black arrows below), whose resistance to do so is set by tensioning a torsional spring 6.

As one rotates an adjustment screw clockwise (purple arrow), the black spring 6 moves outward shown by the red arrows, clamping member 2 at its outer arms more tightly. The result is that it becomes harder to clip in and clip out. This is handy if you want a more secure attachment. Some people's feet accidently clip out while climbing hills or sprinting.
Now to the cleat that attaches to the bottom of your cycling show. It looks like this, without screws :

Oooh , look at that beaten up sole, will you? The carbon footprint (no pun intended) explains everything about the bicycle racer - how his foot reacted when he clipped in for the first time, how he fell, this rebellious war between untrained feet and machine element.

By radio isotope dating and measuring the amount of C-14, cycling experts can take the sole and test its age to know when the racer actually began pursuing this crazy interest.

Naaa, joking.

Anyway, the assembly taken apart looks like this :

Observe 3 members. The first one in order at the top is what clips into the member 2 on the pedal.

To show you a cutaway of how this happens, I had to use bare arm strength to clip the right cleat into the right pedal. The turning motion (purple arrow) is actually provided by your feet. You clip one portion of the cleat into the immovable member 1 of the pedal and rotate the ankle of your feet so that the other part of the cleat displaces the moving member 2 (red arrows) to clip in. The difficulty of clipping in is again dictated by the spring tension 6 as mentioned above.

To clip out of the pedal, your ankle does the exact opposite!

Now observe that two screws are used to fasten the cleat onto the SPD compatible holes in your shoe. Say that one of these screws went missing. This could happen due to insufficient tightening and consequent loss while on a ride, or deliberate horseplay.

Then there only remains one screw. One screw WILL NOT be able to ensure proper fastening to the holes in the shoe sole. Say also that the spring tension is set to HIGH making it even harder to unclip.

A clipped-in rider, unaware of this situation and then attempting to unclip by twisting his foot will not unclip after repeated attempts. This is because the cleat turns with the pedal. The relative motion between the two is almost zero!

He may be able to use some superhuman strength to twist his feet so much that the remaining screw unfastens but this is unlikely while on the bike. I mean, its difficult to do. Even if its possible, chances are you could damage the sole of the shoe.

This gentlemen, is screwing-off.

P.s : Don't try to harm anyone with this ploy. :) The post is meant to show why its important to ensure your cleat screws are sufficiently tight before riding.

To handle this situation, its advisable to unclip other feet, get off the bike, remove your feet from the shoe in question, while having the bike supported (maybe by a friend). Once you have your feet off, use an Allen key from your multi-tool to set the tension on the spring low (if possible) and free member 2 of pedal using the Allen key as a lever. This will get the shoe off from the pedal. Its easier to do all this with the pedal removed from the crank arm using a wrench.

Speaking of which, SPD pedals aren't really known for their float (degree to which you can rotate feet while clipped in for natural movement). Removing a screw from the cleat may be a way to "engineer" float. Funny...

1 comment:

Chris said...

Good post Ron. I have been wondering how much pedal stack height really affects pedaling efficiency. Any advice on that?