I'll name that thingumajig in...

[SoreTween]

What us the correct name for the plastic disk that sits between cassette & spokes?  Mines bust and I can't find the right search terms to order a new one.

Ta.

[rafletcher]

Ummm, spoke protector?

http://www.wiggle.co.uk/shimano-spoke-protector/?lang=en&curr=GBP&dest=1&sku=5300030826&kpid=5300030826&utm_source=google&utm_term&utm_campaign=UK_PLA_Components&utm_medium=base&utm_content=mckv%7CsXM85yeR1_dt%7Cmcrid%7C67090789382%7Cmkw%7C%7Cmmt%7C%7Cmrd%7C5300030826uk%7Cmslid%7C%7C&mkwid=sXM85yeR1_dt&pcrid=67090789382&prd=5300030826uk&pgrid=17507338382&ptaid=pla-121252369982

[Kim]

But everyone calls it a dork disc.  Which is silly, as it's really a "look, I've got a new bike" disc.

[SoreTween]

Thanks.  Spoke guard I think was the closest I tried.

[Torslanda]

Choose carefully. There are models for freewheels and freehubs.

The latter come in different sizes because they clip onto the hub flange, further complicated by spoke symmetry. Different units for 32 and 36 hole wheels.

[phantasmagoriana]

But everyone calls it a dork disc.  Which is silly, as it's really a "look, I've got a new bike" disc.

Except when they're riding a 20-year-old bike, and the disc is cracked and yellowing.

[robgul]

.... but back to the OP - why do you want one?

- if the indexing is correct the chain will only go as far as the last sprocket.  Most of the bikes I see in the shop that have them either have cracks or the nibs that grip the spokes have bust and the plastic disc rotates with the wheel and makes a scratchy sort of noise.

Rob

[Kim]

Protection against low-flying Tesco bag attack, or similar unusual circumstances?

(Not that I think a dork disc would have saved the rear mech in that instance, but it might have reduced the spoke damage.)

[Brucey]

FWIW I have seen some spoke protectors (for cassette hubs) that are intended to rotate with the sprockets rather than the wheel. In some cases they rub on the spokes in a worrying fashion.

My view on them is that they may reduce the amount of damage should the rear mech go into the wheel, but they are no substitute for keeping the rear mech adjusted so that it won't do that.

cheers

[benborp]

Low flying generic blue (not Tesco) plastic bag/rear wheel interface in my case resulted in no damaged spokes even without a spoke protector.

However the ripped off rear mech did manage to take out the seat stay.

Is there ever an instance when a mechanical failure can cause a mech to massively over-shift up the block? Spoke protectors in my experience only seem to ever protect against chronic maladjustment.

[Kim]

IME the chain escaping off the top of the block with a correctly adjusted mech tends to be the result of back-pedalling or operating the drivetrain in reverse (ie. the wheel driving the pedals as the cycle rolls backwards).  Usually in combination with  a) bouncing around on horrid surfaces   b) manhandling cycles in and out of cars  or  c) negotiating Silly Sustrans Gates with a recumbent.

The trick of course is to spot that it's happened and rectify the situation *before* attempting to apply force to the pedals in the usual manner.

[Brucey]

....Is there ever an instance when a mechanical failure can cause a mech to massively over-shift up the block?....

1) when the rear mech takes a knock (eg when parked or when the bike falls over) it will overshift subsequently

2) the low gear stop screws wear where they bear against the mech, so the derailleur slowly goes out of adjustment.

Sneakily, the last cable pull (gear 2 to gear 1) on most indexed shifters is longer than the others, i.e.  long enough that it always pulls the mech into the stop screw, even if it is adjusted badly. 

When servicing the bike (on a workstand), it is a good policy to pedal forwards whilst  grasping the rear mech and to push it in the way which would occur if you pulled really hard on the cable and the mech is nudged sideways. If the chain unships inwards there is almost certainly a better setting for the low gear stop screw that allows low gear to be selected whilst not overshifting.

cheers

[SoreTween]

.... but back to the OP - why do you want one?

I'm wondering that too, I think perhaps I don't.  Automatic reaction to finding a broken part.
The inside of my cassette was absolutely foul.  I suspected it was bad having spent a good long time cleaning it on the wheel a while back but by 'eck, you could grow potatoes inside still. With a disc on inspection was impossible.

(another) slow brane day.

[citoyen]

Is there ever an instance when a mechanical failure can cause a mech to massively over-shift up the block?

I’ve wrecked a Di2 rear mech by overshifting it into the wheel, which in theory shouldn’t actually be possible.

I suspect the cause was a bent hanger. Impossible to know for sure.

[Cudzoziemiec]

But everyone calls it a dork disc.  Which is silly, as it's really a "look, I've got a new bike" disc.

Except when they're riding a 20-year-old bike, and the disc is cracked and yellowing.

Then it's a "Look how well I care for my sprockets" disc.

[Cudzoziemiec]

Is there ever an instance when a mechanical failure can cause a mech to massively over-shift up the block?

I’ve wrecked a Di2 rear mech by overshifting it into the wheel, which in theory shouldn’t actually be possible.

I suspect the cause was a bent hanger. Impossible to know for sure.

The future is RFID (or summat) on sprockets, with mech locating each sprocket individually regardless of what the shifter does.

[Kim]

The future is RFID (or summat) on sprockets, with mech locating each sprocket individually regardless of what the shifter does.

It sounds daft, but it's the sort of thing Shimano would manage to make work, if only because it's a useful source of incompatibility.

[arabella]

When I cba to adjust the wheel in the dropouts of my (sob sniff now stolen) fixie my saggy chain would jump off the sprocket and start sawing through the spokes.  Until I had to abandon a 600 mid-way and thus started paying more attention and also added a metal spoke protector in case.

[MartinC]

The future is RFID (or summat) on sprockets, with mech locating each sprocket individually regardless of what the shifter does.

With the improvements dictated by indexing (profiled sprockets, jockey wheel float, profiled chain sideplates, etc.) you've virtually got this now if you use friction shifters.

[quixoticgeek]

It sounds daft, but it's the sort of thing Shimano would manage to make work, if only because it's a useful source of incompatibility.

Ye gods. Please don't give them ideas like this! Shimano are bad enough at being incompatible even with themselves, without giving them DRM! Stick an RFID chip in each sprocket and before long you'll be getting a 2000km license for your groupset...

J