Author Topic: Rear disc contamination  (Read 1242 times)

Rear disc contamination
« on: July 11, 2012, 07:50:13 pm »
Hi all

i have a bike that came with one of those plastic plate between cassette and spokes. I’m not sure what these are intended for - to stop the derailleur going into the spokes during an off? Anyway, while I was removing some other unwanted items, it occurred to me that, on a bike with a rear disc brake, the plastic plate might actually serve to help keep gear train contaminants off the rotor. Does it have any value in this respect?

PaulF

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Re: Rear disc contamination
« Reply #1 on: July 11, 2012, 08:27:53 pm »
None of my disc braked bikes have one of those discs and I haven't suffered disc contamination so I'd say that the benefit would be marginal.

Kim

  • Timelord
Re: Rear disc contamination
« Reply #2 on: July 11, 2012, 10:45:03 pm »
i have a bike that came with one of those plastic plate between cassette and spokes. I’m not sure what these are intended for - to stop the derailleur going into the spokes during an off?

More like a badly adjusted derailleur going into the spokes during a gear change.  Common causes (other than incompetent adjustment) being the spokes going slack under extreme load, or the result of an earlier mech hanger bending incident.  Probably won't stop the chain going off the top of the big sprocket, but may well reduce the damage done to the spokes in the process.

Standard fare on kids' bikes and low-end MTBs where such things are relatively likely to happen.  Though I reckon mech protectors and properly built wheels are a better approach.


+1 for what PaulF said.  Never had disc contamination caused by anything other than careless greasy fingers. 
Careful, Kim. Your sarcasm's showing...

Re: Rear disc contamination
« Reply #3 on: July 11, 2012, 10:49:31 pm »
In that case, its days are numbered.
 
Standard fare on kids' bikes and low-end MTBs where such things are relatively likely to happen.

They seem to be on all sorts of bikes in the bike shops - even moderately expensive roadie machines.

Re: Rear disc contamination
« Reply #4 on: July 12, 2012, 12:45:57 am »
Hi all

i have a bike that came with one of those plastic plate between cassette and spokes. I’m not sure what these are intended for - to stop the derailleur going into the spokes during an off? Anyway, while I was removing some other unwanted items, it occurred to me that, on a bike with a rear disc brake, the plastic plate might actually serve to help keep gear train contaminants off the rotor. Does it have any value in this respect?

I very much doubt it.

They are a requirement on new bikes, in theory at least:

EN 14764:2005
4.17 Spoke protector
A bicycle with rear gear-change sprockets shall be fitted with a spoke-protector guard to prevent the chain
interfering with or stopping rotation of the wheel through improper adjustment or damage.


But for decent bikes, with owners who keep things adjusted properly, they are junky plastic pieces of shit which should be instantly binned.

Re: Rear disc contamination
« Reply #5 on: July 12, 2012, 01:04:21 am »
... they are junky plastic pieces of shit which should be instantly binned.

I seem to recall that Pluck and I had a conversation echoing just that during our ride to Widders Belle, last weekend  :D

frankly frankie

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Re: Rear disc contamination
« Reply #6 on: July 12, 2012, 09:04:38 am »
They're a bit like insurance. 
Most insurance is not worth paying for.  ==  throw that junky disc away.
Occasionally, even when the risk is very unlikely, the consequences are so extreme, that you pay out the premium anyway.
== the chain goes into the spokes of your £200 wheel and slices clean through them before you know what's happening.  50 miles from home.
So, the one bike in our stable that has really expensive wheels - also has a spoke protector.
It's not dark yet but it's getting there.

zigzag

  • unfuckwithable
Re: Rear disc contamination
« Reply #7 on: July 12, 2012, 03:48:15 pm »
my "best" bike has 7speed cassette on 10speed freehub, so there is ample space between the biggest sprocket and spokes in case of a disaster (never happened, but there's always a hope ;D)

Re: Rear disc contamination
« Reply #8 on: July 12, 2012, 08:46:19 pm »
On the only occasion when my rear derailleur went where it shouldn’t have, wrecked spokes were the least of my worries. I’ve never, touch wood, had a random unprovoked gear hanger failure etc so I’ll take my chances with that. No, I was interested only in this notion that it might help keep the disc clean, but it seems not. Cassette tool out tomorrow. I need to change a tube with a too-short valve anyway. And clean my whole bike...

GraemeMcC

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Re: Rear disc contamination
« Reply #9 on: July 13, 2012, 01:10:30 pm »
... there is ample space between the biggest sprocket and spokes in case of a disaster (never happened, but there's always a hope ;D)

That's done it!  :o

I once had a cassette with a few spacers on the flanged side, for 6/7 speed use. Then, the chain did come off (on a hill, of course) and got well and truly wedged between the inner sprocket, the flange and the spoke bends. So wedged, that wheel was not desiring to be released. Had to resort to splitting the chain at roadside to release jockey-wheels, then losing paint (and skin off knuckles) to get wheel out, then body-weight on chain ends to release trapped chain (coz I don't carry a cassette breaker). Didn't break any spokes but did partly shave a few thou's off the bends. So than had a partial wheel re-build to replace shaven spokes when I got home...

I don't use spaced-out cassettes any more...
PBP 2011: 1234 km by Nr. 5678 in < 90 hrs. Most auspicious...

Kim

  • Timelord
Re: Rear disc contamination
« Reply #10 on: July 13, 2012, 02:07:47 pm »
I do carry a lockring tool, at least when touring and on group rides (my bike has a Centerlock disc brake as well as the usual cassette, so it seems prudent), and it's saved the day a couple of times for people in that situation.

Only had it happen to my bike the once, after it fell over and bent the mech hanger.  This was on a standard 9-speed cassette.  I expect the wheel build (spoke angles and tension) has as much to do with the wedgability as cassette spacers.
Careful, Kim. Your sarcasm's showing...

zigzag

  • unfuckwithable
Re: Rear disc contamination
« Reply #11 on: July 13, 2012, 07:42:43 pm »
... there is ample space between the biggest sprocket and spokes in case of a disaster (never happened, but there's always a hope ;D)

That's done it!  :o

:) hmm, in your case there was not enough space for the chain between the spoke bends and the largest sprocket. just checked mine, and i can fit and drag 1/8" bmx chain without jamming. but yes, it's an oddball set up - in fact all my bikes are on the odd side (or should i say "unique" 8))