Mondoman

I've noticed a clear plastic disc mounted just inboard of the cassette (adjacent to the spokes) on rear wheels. Typically, its diameter is an inch or two greater than that of the largest cog. It seems to be a protector of some sort, but what is it protecting, and what is it called (so I know what to ask for at the store)?

carpediemracing 

It's a spoke protector.

It protects the spokes in case the chain goes past the large cog. Although a properly adjusted derailleur shouldn't let that happen, you never know. If a chain does get into the spokes it can cause a lot of damage to the spokes. Many riders don't use spoke protectors. To replace it you'll need to take the cogs off the rear wheel.

cdr

Torchy McFlux

It's a spoke protector or "pie plate", "dork disc", "nerd coaster", etc. It helps prevent damage to the wheel if the rear derailleur is poorly adjusted or bent thus allowing the chain to shift too far inwards.
Some people have issues with them:
https://www.bikeforums.net/classic-vintage/415249-amusing-pie-plate-rant-bikesnobnyc.html

Cadfael

In addition to the above...

Many cyclist remove this. It can look a bit tacky on a bike that is well looked after and well maintained... a cheap bit of placky just sitting there doing nothing it would seem but getting full of oil and crud, but it does serve a purpose. If you are absolutely sure your read RD is set up correctly you could take the chance and remove it, the rear cluster DOES look better for the lack of it in my opinion. It was a no brainer for my bike... I never ever go to the largest ring on the back cluster. But at the same time something could damage your RD which could send the chain over the cluster.

Personally, I like the large metal ones you see on some cheep bikes with slots cut into them.

Mondoman

Thanks, folks! Since I will be attempting my first derailer install/adjustment within the next month or so, I'll put one in.

capwater

All the above mentioned names are quite appropriate. It is a holdover from the days of friction shifting. With properly adjusted indexed shifting, especially STI type, you're never going to need it. It's rarely seen on anything other than older bikes and some low end models. Ditch it.

Mr. Underbridge

I'm personally not that confident - on the other hand, I've been living with a lightly bent hanger for about 6 months now (just lazy) that prevents me from even getting to the largest cog - it's like a built in dork disc!

Bill Kapaun

Friction shifting has nothing to do with it!
A derailleur stop screw is a derailleur stop screw.

cooker

I 'm not sure I agree. With a friction shifter, my feeling is you can "throw" the chain a little beyond the derailleur limit by shifting quiickly over a few cogs. On one of my bikes I have a hard time shifting to the big cog from the one right next to it, but if I first shift a couple of gears higher (smaller cogs) and then throw the friction lever back fast, I can get the chain to leap on the big cog. So in theory if my limit screw was set to allow easy shifting to the big cog, I might be able to throw the chain into the spokes with a fast downshift over several gears.

Camilo

Untrue. Many new bikes - some fairly high end (maybe all??) come with spoke protectors, aka DD.


A stop screw is a stop screw. Whether it be friction or indexed, the limit the derailleur can move toward the spokes is limited by the stop screw, period. An index shifter is just as capable of throwing the derailleur into the wheel as a friction shifter if the stop screw is not adjusted properly.

Your friction shifter experience is simply the result of an improperly adjusted stop screw imho, of course.

I_bRAD

Sounds like you could do with a different range for your next replacement!

Sci-Fi

As long as it isn't falling apart (cracked, chipped, flopping around, etc), most will leave it on, if only for 'possible' spoke protection in case the chain jumps in there for whatever reason. Removing it is up to you. Some remove it for aesthetics, peer pressure, weight savings, etc.

Cadfael

Absolutely, I have another bike that I inherited from my father in law that has a better range for me. But even that bike is sans dork disk... and I do use the largest sprocket on that bike...

Camilo

I wish I had your father. That tiny portion of the bike looks very nice!

airosen

The disc that came on my '07 Trek 1500 is actually rather inconspicuous, and I think I'll leave it on, at least until it becomes a fugly shade of yellow.