Pie plates are plastic rings placed against the spokes on the rear wheel of dérailleur equipped bicycles. They protect the wheel and derailleur against damage by preventing the chain or dérailleur from going into the spokes if the rear dérailleur is misadjusted. I once had to replace spokes, chain, derailleur and chainring for want of a spoke protector because my dérailleur hanger got bent in on the train.
If you ride a high end road bike, you're expected to fiddle with the limit screws like a tweaking meth user, so get rid of the pie plate. Ditto for fixed gears, singlespeeds and hub geared bikes, which have no need for spoke protectors. If you ride your bike for transportation, though, and your bike is flung against other bikes, crammed into closets, dropped on the sidewalk and you otherwise don't want to mess with the gear adjustments, keep that spoke protector in place.
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Snork... I think my pie plate on the fast hybrid got brittle and flaked off after 10,000 miles or so ;) we'll see how long hte one on the Dahon lasts! But hey, I'm allowed to have one since I'm on a hybrid and a proud Fredwina :)
Just to let you know, these are most commonly referred to as "Cosmic Ray Deflectors" by a lot of wrenches. Owing to their redundancy if your set screws are properly adjusted.
Also, I find that most "CRD"'s are rather flimsy, too small, or so brittle that they are practically useless on the majority of rigs that come through our shop. So if you must use one, please have it properly sized or replaced as necessary. Otherwise the "phred factor" goes through the roof with these odd pieces of cycling gear.
Yeah, that's one thing that BSNYC didn't address... was the faintest utility of them. I think they are one of the ugliest things on the bike, but if it ever saves me from a long walk home or unecessary repairs...
A while back in Mountain Bike (I believe) published a reader's question on this topic. The reader wanted to know how to get a spoke protector, and what type to buy. The writer suggested he should march into his bike shop and insist on a Big Wedgie, that the shop would know exactly what he meant, and he'd get what he deserved.
I wouldn't suggest Fritz needs his undies snugged, but I don't see too much utility to the devices. If your hanger is bent during a ride, straigten it if you can, or use the inner limit screw to keep the derailleur out of the spokes. At the very least, avoid shifting that far in.
If you're unlucky, and you don't discover the problem until the overshift occurs, a spoke protector isn't much help, especially since the lower part of the derailleur cage can still get into the spokes.
I've untangled many of these messes over the years (even on one of my own bikes a couple of decades ago), and I've never had cause to say, "Phew, good thing you had that spoke protector." Happy Trails, Ron
Wish I would have had one a couple weeks ago. Threw my chain off of the largest cog, into the spokes, pulled the derailer in and wrapped it up and around. $170 later I had to replace the hanger, derailer, 3 spokes and the chain.