Plastic "pie dish"

[Catrin]

My LHT has this round piece of plastic behind the rear cassette. What is the purpose of this? Does it protect the cassette?

[OakLeaf]

It's there in case the chain would come off and get stuck between the cassette and the spokes. If that would happen, the wheel would seize.

It's a very rare occurrence, since the low limit screw on the RD as well as the whole design of a derailleur system should prevent it. Still, if it does happen it could be catastrophic, which is why manufacturers are required to put the pie plate on. But they're at best messy, and at worst clattery and noisy, which is why most people take them off.

There's also a bit of peer pressure, as they're considered geeky, but obviously that's no reason to do anything; and weight weenies like to save the couple of dozen grams.

I'd ignore it unless it's making noise or making it unreasonably hard to clean your cassette.

[nscrbug]

It's there in case the chain would come off and get stuck between the cassette and the spokes. If that would happen, the wheel would seize.

It's a very rare occurrence, since the high limit screw on the RD as well as the whole design of a derailleur system should prevent it. Still, if it does happen it could be catastrophic, which is why manufacturers are required to put the pie plate on. But they're at best messy, and at worst clattery and noisy, which is why most people take them off.

There's also a bit of peer pressure, as they're considered geeky, but obviously that's no reason to do anything; and weight weenies like to save the couple of dozen grams.

I'd ignore it unless it's making noise or making it unreasonably hard to clean your cassette.

This actually happened to ME...so I speak from experience. My chain did fall into the spokes as I was trying to downshift into an easier gear while climbing a hill. The wheel did seize, but luckily I had unclipped beforehand and was able to avoid, what could have been a nasty fall. This happened on my Cervelo, which already had the "dork disk" removed. My current Cannondale, also had it removed before I took the bike home. But my "backup" Trek bike, still has it...and I have no intentions of removing it.

[Owlie]

It's never bothered me. I just left it. But then, I'm a "road geek."

[ny biker]

My new bike has one but it's small and black so it's not too noticeable. I'm not planning to remove it.

[DMC]

My LBS removed it immediately without asking once I paid for the bike. I think they are so used to roadies asking for it to be taken off, they just do it to save time.

Took the bike home, and my bike nerd friend went to work removing all the stickers, spoke reflectors and front reflector.

Went in for a mega bike re-fit a few months ago, and my bike fitter mumbled something about 'looking like a noob' as snatched the rear reflector off as well.

I asked if this was a bad idea and he noted that the reflector was blocked by my seat pack anyway and I already attach blinky LED lights to the front and back of my bike when I need them (they live in my seat pack at all times).

All these people dedicated to keeping me from looking like a bike dork, and yet they do this while wearing the tackiest racing kit designs I have ever seen.

[lunacycles]

it's true. the "pie plate" is one of many, many things bike snobs use to identify a newbie. It's totally unnecessary if your bike is set up correctly, but as a poster noted, it might save your life some day...

I routinely remove them, and never install them on any of the bikes I sell. That being said, who cares? If it doesn't bother you, it might be your friend some day. Also, if your chain does fall behind your cassette, in addition to potentially seizing the rear wheel it can also really dig into the spokes and compromise/make them weaker as a result. I have seen a lot of drive-side spokes frighteningly compromised from this occurring.

the one thing I've always disliked about them is that uv exposure turns them brittle, their little clips onto the spokes break as a result, and they become a rattling nuisance. But, otherwise, they are harmless...and weigh approximately 42 grams.

[TxDoc]

It's totally unnecessary if your bike is set up correctly

Exactly.
And, if you are going a little bit faster you will notice the horrible noise they make...

[laura*]

Also, if your chain does fall behind your cassette, in addition to potentially seizing the rear wheel it can also really dig into the spokes and compromise/make them weaker as a result. I have seen a lot of drive-side spokes frighteningly compromised from this occurring.

At the bike co-op where I volunteer, it is a regular occurrence for people to come in with a broken drive side spoke - and when we get the cassette/freewheel removed, all the outer drive side spokes are mangled.

It's totally unnecessary if your bike is set up correctly

Bikes fall over, derailleur hangers bend; limit screws might be set wrong (because the shifter is set right); twigs can snag in derailleur cables thus changing gears; twigs can snag in chains or a chain can seize, thus causing the derailleur to get pulled into the cogs; and so on. In other words, even when a bike is set up correctly, things happen. It is much better to have just a destroyed derailleur, than a destroyed derailleur and rear wheel.