Author Topic: Derailleur into wheel. Why?  (Read 1222 times)

JJ

Derailleur into wheel. Why?
« on: January 06, 2019, 09:05:39 pm »
Yesterday on the Poor Student, I had my second such incident in 6 months.
I'm confident that the limit screw was properly set both times, and the second mech was fitted by a mechanic on the 3-Down, not by me.  The remainder of it was still solidly screwed into the dropout.

So what could it be?  The frame doesn't seem to flex unduly.  I thought maybe it could be something to do with the freehub, but I think I was pedalling both times.

I'd like to find out because I REALLY don't want it to happen at speed.

JJ

jiberjaber

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Re: Derailleur into wheel. Why?
« Reply #1 on: January 06, 2019, 09:09:32 pm »
Bent mech hanger? (did you get that checked when the RD was replaced?)
Regards,

Joergen

LMT

Re: Derailleur into wheel. Why?
« Reply #2 on: January 06, 2019, 09:26:55 pm »
Changing gear under load.

Re: Derailleur into wheel. Why?
« Reply #3 on: January 06, 2019, 09:30:49 pm »
Mechs get knocked all the time on everyday bikes.   I avoid using derailleur geared bikes for utility work for this reason; you can come back to the bike and find that it has been knocked whilst parked and the mech doesn't need to be bent much before it goes straight in the back wheel the first time you go for bottom gear.

Also note that the shifter will usually pull the mech hard into the stop screw when bottom gear is selected; if the screw is not secure so that it can turn unintentionally that will allow the mech to go too far. Also over time (not much time with some mechs) the stop screws wear where they touch and this allows the mech to go too far too.  If the frame is flexy, depending on the cable routing, frame flex can also tighten the cable and pull the RD hard into the stop screws.

Finally slop in the pivots allows the mech to flop about and this can make the difference between it going in the spokes and not.  SOP is to set the bike up on the workstand and to push the RD towards the spokes by hand. If it touches the spokes when correctly adjusted (for good shifting) then the RD is probably knackered and needs to be replaced. A final insult is that a lot of RDs could hardly be better designed to snag in the spokes and cause a catastrophe....

FWIW a lot of 11s 'road' setups seem to involve a normal running clearance between the mech and the spokes of about 0.5mm in bottom gear. This is OK on a well looked after race bike but IMHO it is an essentially unmaintainable proposition  on a lot of bikes that are used every day.

Recently I saw an 11s bike that made a noise in bottom gear; it turned out that one spoke had been replaced but the spoke crossing hadn't been braced; the tiny difference this made to the position of teh spokes was enough to risk a tangle.

This leads me to a final point; if you have a wheel without enough spokes in and/or that is not laced tight enough, spokes will be going completely slack in the rear wheel whenever you are pushing hard on teh pedals and/or riding out of the saddle; this can provoke a  tangle too.

cheers


Torslanda

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Re: Derailleur into wheel. Why?
« Reply #4 on: January 06, 2019, 09:32:32 pm »
Any mechanic worthy of the name would have checked it. Certainly in a workshop. If I'd had any doubts about the alignment I would replace the hanger as a matter of course.

If a hanger wasn't readily available and it was bent I'd be inclined to lock the bike out of bottom gear and tell the rider to replace it ASAP.

My inclination is to suggest you replace yours without a second thought.
VELOMANCER

Well that's the more blunt way of putting it but as usual he's dead right.

Ben T

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Re: Derailleur into wheel. Why?
« Reply #5 on: January 06, 2019, 09:32:43 pm »
I know you don't want it to happen, but take slight solace in the fact you're unlikely to change into the lowest gear at speed.
Unless you put on overalls, boots, and a helmet with a high tech pre fitted lamp - and you dig coal - nope, you don't know me.

Kim

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Re: Derailleur into wheel. Why?
« Reply #6 on: January 06, 2019, 10:11:11 pm »
Foreign object entanglement?  Plastic bags are canonical, but you'd probably have spotted one of those...
To ride the Windcheetah, first, you must embrace the cantilever...

Re: Derailleur into wheel. Why?
« Reply #7 on: January 06, 2019, 10:13:51 pm »
If you have a spare rear wheel you can use it as a DIY rear mech hanger alignment tool.

First remove the rear derailleur and then simply screw the spare wheel (removing quick release first) in to the threaded hole where the mech was removed as they are the same thread - make sure the mech hanger and where it interfaces with the wheel are clean.

You basically then have 2 wheels that should be parallel to each other. I use a piece of dowel to gauge the distance between the 2 wheels at several positions around their circumference.

If there are noticeable differences its remarkably easy to push on the spare wheel to bend the mech hanger back as it should be (you then realise how easy the mech can be bent by say dropping a bike on its side)

Someone may come along and advise why you shouldn't do this but it has worked for me in the past on a bike that I have subsequently used for many years and miles with no problem.

At the very least even if you decide to not bend the mech back you'll see whether it's bent or not.

Re: Derailleur into wheel. Why?
« Reply #8 on: January 06, 2019, 10:19:46 pm »
If the chain jams in the cage for any reason it will pull the mech into the wheel. So check the chain for bent links, especially if it hasn't been changed from the previous incident.

Also, I once had a jockey wheel bolt come loose and ended up with the mech in the spokes*. If your mech is in pieces this might be the explanation.

(* I can't quite visualize *how*, mind)

JJ

Re: Derailleur into wheel. Why?
« Reply #9 on: January 06, 2019, 10:58:23 pm »
Lots of good suggestions there!  Thanks folks.
I'll re-check the hanger.  It isn't replaceable, but knowledge is power.

The frame is fairly flexy, but not ridiculously so.  It's a Chinese Ti job, made to my drawing a few years back.  I'd be gutted to have to let it go because it's COMFY.

The first one was changing gear under power.  The second time, I think I was just pedaling along in the middle ring but I could be wrong.  Things are hazy!  Could have been debris.  It was pitch dark so I didn't see anything, and I was busy nattering with my companions.

The cage was broken first time, but intact the second.  The parallelogram was broken both times.

I built the wheel myself.  It was tight.  Needs some attention now, though!

The first time, the chain was broken.  It's hard to know whether that was cause or effect.  I assumed cause, but now I don't know.  Two different kinds of bad luck so soon after each other seems unlikely.

Sooo.  Check the alignment.  Grease the freehub.  Check the clearance on the big cog.  Fit one of those plastic spoke protectors for the time being.  Anything else?

Re: Derailleur into wheel. Why?
« Reply #10 on: January 07, 2019, 07:18:14 am »
there are some other causes of derailleur mangling too, which fall into the 'mystery manglement' category; they can seemingly occur at any time.  Debris entering the RD has been mentioned. However it is also possible  for

- the chain to unship (at the bottom) on the way into the RD.  If it then jams somehow the RD is very quickly dragged towards the sprockets and the mech is destroyed. A worn tension pulley makes this more likely, as does running cross-chained. Clues afterwards include that the side plates are badly worn where the chain enters the RD.

- the chain starts to break, i.e. one side plate cracks and is pushed to one side. It is basically 50:50 as to whether the side plate pops off in such a way as it snags and jams or will squeeze through the RD. It if snags then the RD is dragged into the sprockets pdq.

- derailleur pulley bolts backing out; older RD designs have both pulley bolts that are both accessible from the RHS. Newer ones have the lower accessible from the RHS and the upper from the LHS; this means that if the bearing/bushing starts to bind it doesn't start to undo the upper pulley bolt. However the bolts can still work themselves loose and this can cause the mech to go into the spokes and/or the chain to jam in the RD.

- derailleur misassembled; if the inner side plate is fitted upside down (which is surprisingly commonplace and easy to do), the chain can more easily derail on the way into the RD and cause a jam

- chain breaking wholesale or coming partly unshipped from the chainring; if the RD is suddenly presented with too much slack, in some installations the tail of the RD (the bit with the lower pulley mounted in it) can flick backwards so far that it tangles with the sprockets. [Normally an RD with a sprung upper pivot will swing backwards on the upper pivot and such tangles are prevented; however if the upper pivot bushing is a bit draggy and the lower one isn't, the mech may fold itself up in an unusual way. Single pivot mechs are often thrown further backwards than normal when the chain breaks, and can tangle that way.]  In particular the loop that goes over the lower pulley (which helps prevent wholesale chain derailment) can tangle with the sprockets.  If the top run of the chain is still intact, simply pedalling can wreck the RD. If you stop and wheel the bike backwards, again the RD is mangled.

- forced shift;  forced downshifts (under large pedalling load) can cause problems simply by overloading the RD. However the chain is often pushed directly by the top of the outer side plate and momentarily the load here can go up enormously once the chain hits a ramp on the new sprocket and starts to derail.  This also causes the chain to drag on the side plate in the direction of chain travel, creating an ACW torque (as viewed from the RHS of the bike)  that may become larger than the resisting force (supplied by the tension spring in the mech).  This may mean that the chain lower run becomes completely slack part way through a forced shift, in a way which doesn't happen during a normal downshift.  If slack enough, the chain can derail on the way into the RD and precipitate a jam/tangle that way.

- draggy lower pulley bushing; this creates an additional torque on the pulley cage so that RDs with a sprung upper pivot tend to ride higher than normal when pedalling forwards; this can somehow precipitate the whole RD tangling with the sprockets. A clue that there is something wrong and this might happen is if the RD assumes a very different position when pedalling backwards than forwards.

- draggy pivot in RD; if the rear derailleur can't extend quickly enough (so as to take up chain slack) then the lower chain run tends to be slacker than it should be. This can precipitate the chain coming unshipped on the way into the RD and tangling.

So there are plenty of potential mechanisms by which a tangle can be caused.  Note that some RD faults (esp with the lower pulley) tend to greatly accelerate wear in the lower pulley so that in a few hundred miles the pulley can go from being OK to seriously worn and this can soon precipitate a jam/tangle.

 cheers

Re: Derailleur into wheel. Why?
« Reply #11 on: January 07, 2019, 08:55:16 am »
All of which ^ does nothing to dispel my sense that derailer gearing is a massive bodge that somehow, inexplicably, mostly works fine...

Re: Derailleur into wheel. Why?
« Reply #12 on: January 07, 2019, 11:37:03 am »
yes, "c'est brutal, mais ça marche…" is often quoted in relation to derailleur gears, and not without good reason....

BTW the OP also mentions that he has doubts about the freehub. If this goes draggy (very likely with some eg Mavic ones) then any (even very brief) break in pedalling can cause the chain top run to go completely slack. When you start pedalling again all this slack can get into a tangle at the front and if the RD is a bit sticky too the slack can get tangled at the back.

cheers

Torslanda

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Re: Derailleur into wheel. Why?
« Reply #13 on: January 07, 2019, 11:57:33 am »
All of which ^ does nothing to dispel my sense that derailleur gearing is a massive bodge that somehow, inexplicably, mostly works fine...

Because it is . . . ?

<OT>Am currently working on a Pinnacle. 650b with 47mm slick tyres, SA 5 speed, eccentric BB, full guards & rack. It rides like proper stealth machine, doesn't feel heavy and outwardly appears bombproof. It just gathers momentum.

Such machines are almost impossible to sell - they don't have sex appeal, 'what d'ya mean it's only got 5 gears' etc. but it's so simple and a delight to ride and maintain. I'll also wager it's less likely to be stolen - for all the reasons listed above.</OT>
VELOMANCER

Well that's the more blunt way of putting it but as usual he's dead right.

JJ

Re: Derailleur into wheel. Why?
« Reply #14 on: January 07, 2019, 12:04:07 pm »
All of which ^ does nothing to dispel my sense that derailer gearing is a massive bodge that somehow, inexplicably, mostly works fine...

I'm with you.  Derailleurs are rubbish.  They just happen to be better than the alternatives!

Yes, I did mention the freehub, but only because I was scratching around to find a cause.  That hub is fairly old now.  It's outlived at least 5 rims, but I've never noticed a problem with it and I'm pretty sure I was pedalling on both occasions.  Frame flex is still a possible culprit.  I had had a few auto-changes when honking ever since that derailleur was fitted, but I was sitting down when it went, thankfully.  Actually I was quite pleased with myself, as I rode the locked wheel to a standstill and then gently toppled over, still clipped in.

The chain was intact, albeit very twisted, so that wasn't it, but the bottom of the cage was bent right out of shape so maybe the chain had caught in it somehow.  Then again it had been through the spokes so hard to tell.   I did feel something hit my legs shortly before, so perhaps litter, but no sign of it.  Anyway the mech went in someone's bin, so no forensic examinatoin now, and I doubt I'll ever really know.

I'll put it all back together, and replace the funny shaped spoke.  I'll check everything that you've all kindly proposed and I'll use a plastic guard for my peace of mind.  Shall I get one with Batman on?  Oh, and I'll get a half-link for the toolbag.

Re: Derailleur into wheel. Why?
« Reply #15 on: January 07, 2019, 01:30:40 pm »
<OT>Am currently working on a Pinnacle. 650b with 47mm slick tyres, SA 5 speed, eccentric BB, full guards & rack. It rides like proper stealth machine, doesn't feel heavy and outwardly appears bombproof. It just gathers momentum.

Such machines are almost impossible to sell - they don't have sex appeal, 'what d'ya mean it's only got 5 gears' etc. but it's so simple and a delight to ride and maintain. I'll also wager it's less likely to be stolen - for all the reasons listed above.</OT>

(Continuing OT - what are the modern SA 5-speeds like? Are they branded Suntour these days? I think I've looked at that Pinnacle as a commuter before - I rather fancy one.)

Re: Derailleur into wheel. Why?
« Reply #16 on: January 07, 2019, 01:53:37 pm »
I hated my 5 speed (SRF5W) - it has a very crude key mechanism for shifting between rations (1<>2 and 4<>5) which can’t shift under any sort of load and also has dangerous dead spots if it’s even slightly out of adjustment. The key mechanism also needs to move for the 3/4/5 shifts, so it doesn’t even work smoothly as a three speed like it theoretically could.

OTOH I love my six speed Brompton, which uses a 3 speed SA.

zigzag

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Re: Derailleur into wheel. Why?
« Reply #17 on: January 07, 2019, 01:54:44 pm »
perhaps worth mentioning that those running campag rear derailleurs are at higher risk (vs shimano) as the mech cage is about 2mm closer to the spokes than a shimano one, talking of 11s groupsets. i've only realised this when setting up the bike on a tacx neo trainer, where the campag mech cage would rub against the flywheel when shifted into bottom gear.

Re: Derailleur into wheel. Why?
« Reply #18 on: January 07, 2019, 07:22:46 pm »
I'll suggest a work-around for this problem, which is mitigation of the consequences. The traditional solution was to fit a plastic ring to protect the spokes, or more precisely to spread the load of a derailled chain over a few more spokes & IME prevent external damage to the spokes. I'll leave judgment of how well it worked to those who have more experience than I.

Wowbagger

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Re: Derailleur into wheel. Why?
« Reply #19 on: January 07, 2019, 07:27:27 pm »
I subscribe to the view that you can set a bike up perfectly on a bike stand, but that as soon as a human is on it, the frame will flex.

We had this problem with our first tandem when I sent the derailleur into the spokes and broke 6 of them. Luckily the other 42 got us home OK. But I took the drastic solution of buying a tandem with a Rohloff hub  and substituting one sort of breakage for another.
Oh, Bach without any doubt. Bach every time for me.

Re: Derailleur into wheel. Why?
« Reply #20 on: January 07, 2019, 07:29:52 pm »
the current SA hubs are still branded SA not sun tour or sun race (Sun Race bought sturmey archer).


2002-2008 they appear to have made a revised version of the ball-locking 5s 'sprinter' hub. 2009-2016 they made hubs with a (W) suffix, so X-RD5 (W) and X-RF5 (W).  In about 2016 they  launched the C50 series of 5s hubs which are larger and heavier than those that came before. 

The (W) hubs use a different shifter from the sprinters and share almost no parts with them.  They have a wider gear range but have several problem areas

1) the pretty alloy thumbshifter overhsifts and sticks on downshifts and this very commonly causes slippage in the hub. If this is not rectified the hub invariably breaks. You can cope with the usual duff shifter if you pull the lever back to the detent after every downshift.  I have a scheme for rebuilding the shifters so they work properly, which appears to be 100% successful, but it isn't really a DIY proposition.

2) the bearings for the planet pinions are pretty hopeless; the planet pinions are solid and have spigots on each end, about 2mm long. These sit in holes in the planet cage and make the crudest, draggiest plain bearings imaginable, that wear out faster than is acceptable. it is bloody amazing that they have the never to make a hub like that; it is almost as if they have learned nothing from the past.

3) like most current SA hubs (and unlike older ones) there is no tab washer on the RH cone.  This is required to prevent the RH cone from precessing inwards and destroying the hub. This commonly happens with both 3s and 5s hubs currently. You can fit a tab washer for pennies why they have omitted this vital part is a mystery to me.

4) There isn't enough grease inside the hubs

5) what grease there is, is not very good quality; I recommend something with solid lubricants and corrosion inhibitors, and more of it.

6) the (W) hub has exactly same actuator plate issues as current SA 3s hubs.

7) there are at least two springs that cause drag inside the W hub, all the time, and there is very little you can do about it.

8 ) the locking key assembly really needs threadlock on the small screw that holds the two halves together; without it the two halves can come apart and this results in a hub-smashingly terrible fault condition in which both suns try to be locked at the same time.

9) when the (W) internal breaks, I don't think there is another SA internal that goes into that hubshell, because it is a different depth to any other SA one. I have not yet found an easy way of making a 3s internal up that will go in a 5s (W) hubshell.

The shifting in the (W) hubs is pretty clunky (worse than in a ball-locking sprinter) especially 5-4 and 2-1 but improves once the hub is both run in and lubricated correctly.  Most of these hubs break (usually due to a duff shifter) before they have a chance to wear out on the planet pinions.

But having said all that a (W) hub with a hub brake is a pretty useful hub on a utility bike; efficiency is fairly good in the middle gear so if you view it as 'a singlespeed with options' it will work pretty well. I've had one on my utility bike for a few years now, mostly in gear 3 with a load on. Without a load on I use gear 4 quite a lot and the extra drag is noticeable.

Whether the (W) hubs will continue to be available is open to question; they are mostly no longer listed on the SA website.

cheers

Re: Derailleur into wheel. Why?
« Reply #21 on: January 07, 2019, 09:12:15 pm »
the current SA hubs are still branded SA not sun tour or sun race (Sun Race bought sturmey archer).

Yup, got my suns mixed up! The hub I was thinking of was the Sunrace RX-RK5; is this just a rebranded SA design?

Re: Derailleur into wheel. Why?
« Reply #22 on: January 07, 2019, 11:02:54 pm »
a lot of 11s 'road' setups seem to involve a normal running clearance between the mech and the spokes of about 0.5mm in bottom gear. This is OK on a well looked after race bike but IMHO it is an essentially unmaintainable proposition  on a lot of bikes that are used every day.
I had a 9s that was like this after I'd re-spaced the rear dropout.  When I rebuilt the wheel after an entanglement I did it "backwards"  (What Musson calls Option B lacing) so that the cage was much less likely to catch in the spoke-cross.  I've built all my rear wheels that way ever since.

Re: Derailleur into wheel. Why?
« Reply #23 on: January 08, 2019, 01:25:20 am »
the current SA hubs are still branded SA not sun tour or sun race (Sun Race bought sturmey archer).

Yup, got my suns mixed up! The hub I was thinking of was the Sunrace RX-RK5; is this just a rebranded SA design?

RX-RK5 is a newish hub design (was initially referred to as a 'C50' design) with rotary shifting. I have only seen these hubs with Sturmey Archer branding.  This hub uses a sun locking scheme that is like that used in the SA 8s hubs but the gears are still configured as per other SA 5s hubs. This means there is a sliding clutch (as per a three speed more or less) which does the 4-3-2 shifts, and the sun locking only is swapped in the 5-4 and 2-1 shifts. The idea of the 8s style sun locking is that these 5-4 and 2-1 shifts should be more or less seamless. I'd expect the baulkiest shift to be the 4-3 one.

http://www.sturmey-archer.com/files/catalog/files/85/PART%20LIST%20-%20RX-RK5.pdf

  This hub is incredibly heavy for what it is. It uses a longer cable pull and the (larger diameter) thumbshifter could offer more accuracy. However it does no such thing, because the gubbins that is inside the shifter is identical to that in the smaller shifter, so you get just the same angle error, and a proportionately greater cable pull error..... ::-)  I've only seen a few of these hubs in regular use and thus far no blow-ups. But I'm not holding my breath...

cheers

Torslanda

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Re: Derailleur into wheel. Why?
« Reply #24 on: February 11, 2019, 10:59:51 pm »
Coming back to mech into spokes, I had a similar job recently which deaded a 105 short cage mech in excellent condition and hanger. In this case it was freehub bearing failure - Fulcrum Quattro - with sufficient wobble that if I had missed it the NOS 6700 mech would have been an expensive mistake...
VELOMANCER

Well that's the more blunt way of putting it but as usual he's dead right.