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What is this? Some kind of chain guide?

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What is this? Some kind of chain guide?

Old 09-13-16, 04:25 AM
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Stokestack
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What is this? Some kind of chain guide?

Hi all.

The chain jammed around the chainring on my Trek 920 Singletrack a few weeks ago. So I'm replacing a bunch of worn stuff. I also noticed that the jam apparently broke this thing.



Anybody know what this is?

Thanks!

Gavin
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Old 09-13-16, 05:56 AM
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I'd hazard a guess at a chainline guide for the size crank (42, 34, 24) in use.
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Old 09-13-16, 06:54 AM
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It's to stop chain suck - chain suck plate.
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Old 09-13-16, 08:25 AM
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It was, OP ^, but now it's torn up. the above was for adding to frames that lacked the fitting bosses on your found bike.
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Old 09-13-16, 08:45 AM
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Agreed with anti chain suck device. And I'll add that it did it's job so many times it finally broke. Like a good helmet does. Andy.
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Old 09-13-16, 01:37 PM
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Thanks, guys. Is this something I should try to replace?

Worst case I guess I can just bash this one back into shape, and hopefully with the new drivetrain parts it won't really come into play.
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Old 09-13-16, 02:07 PM
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First- this guard can be removed and with chain and rings in good shape that are not subject to bad shifting technique (we call that "jam shifting") chain suck is not usually a problem for most.
Second- This guard is broken. So merely hammering it back into shape will not really fix it fully. And AL doesn't like to be hammered anyway.
Third- The best thing to do is to fab up a new one. Assuming the rings will be the same sizes as before a simple stencil of the old guard placed on a 3/16" (or 1'4") AL plate and a drill, hacksaw and some filing will create a new one in short order. Remember to compensate for the distortion due to the crack/breakage spot. Andy.
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Old 09-13-16, 06:51 PM
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If you're replacing worn stuff with a modern-ish clutch rear derailleur, I'd just take it off.
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Old 09-13-16, 08:31 PM
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I'm not sure a clutched rear der will solve chain suck. Andy
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Old 09-13-16, 08:35 PM
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Originally Posted by Jiggle View Post
If you're replacing worn stuff with a modern-ish clutch rear derailleur, I'd just take it off.
What is a clutch rear derailleur?
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Old 09-13-16, 10:01 PM
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Those more up on current stuff might say otherwise but... To reduce chain slap and the resulting upper chain run wave when coasting over severe terrain many modern rear ders now have a clutch pack built into the cage pivot. Thus cage rotation in one direction is still fairly free but in the other direction the clutch will engage and reduce the cage's ability to rotate freely. Some ders have this feature able to be turned off and on Andy.
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Old 09-14-16, 08:44 AM
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Originally Posted by cyclist2000 View Post
What is a clutch rear derailleur?
It keeps the chain tight during bumps so it's far less likely to fall off the ring.
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Old 09-19-16, 01:19 AM
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Thanks for the info, guys. I'm not replacing any derailleurs, but I am replacing the chain rings and rear gears and chain.

I'll correct the distortion of the plate if I make a new one. Coincidentally, I needed to bend some aluminum bar stock today and it broke a couple of times. So I started annealing the bend areas with a propane torch, and hey... it worked.

Gavin
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Old 09-19-16, 08:42 AM
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Originally Posted by Andrew R Stewart View Post
This guard is broken. So merely hammering it back into shape will not really fix it fully. And AL doesn't like to be hammered anyway.
I agree with all you say, Andrew, but is the plate shown actually aluminum? Looks more like stamped steel to me. Could be wrong, though.
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Old 09-19-16, 08:49 AM
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Also, the original plate looks like a poor design - there is a very thin web between the actual chain stop and the oval bolt holes. The thing broke near one of those holes. If you remake a new one, you might wish to make the outside outline first, then see where it fits, then drill two single holes to accomodate the bolts to get that fit. Since the mounting bolts were not butted up against one end of the original oval hole, this should give you more "meat" where the thing broke.
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Old 09-19-16, 09:10 PM
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IME these anti suck devices were made of AL (or plastic like Shark fins and Dog Tooths). Like helmets they were meant to be sacrificial and consumables. So as a minor feature they were made as low cost as possible.

I will say that while I have seen dozens of these devices (mostly in the early to mid 1990s, aprox.) I don't remember seeing and plate ones with fractured structure. Lots of heavily gauged ones but no broken/cracked that I remember. Not that this hasn't happened before but I can only talk to my experiences. Andy.
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Old 09-26-16, 03:45 AM
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Thanks, guys.

It's aluminum. If necessary I'll make a new one; I spend a lot of time cutting aluminum bar stock for custom camera rigs, so I should be able to handle it.
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Old 09-26-16, 04:54 AM
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It looks like Tektro makes a similar plate. Maybe it could be adapted.

Tektro Chain Aligner (Anti-Chain Suck Device): RPB-102A ? FreeLander Bicycles
Tektro Chain Aligner (Anti-Chain Suck Device): RPB-100A ? FreeLander Bicycles

I wonder if modernizing the chainring set would help eliminate the need for the plate.
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Old 09-26-16, 06:59 AM
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Originally Posted by Andrew R Stewart View Post
First- this guard can be removed and with chain and rings in good shape that are not subject to bad shifting technique (we call that "jam shifting") chain suck is not usually a problem for most.
For road biking, perhaps. But mountain biking has a whole host of different things that can happen to cause chain suck. Bikes are shifted under more tension so chainrings are more likely to be damaged. Thankfully, chain suck doesn't happen that often on the outer and middle ring...the derailer has more tension and does a better job of pulling the chain off a tooth with a burr. That's not the case with smaller inner rings. The low tension on the rear derailer spring and the smaller diameter of the ring lead to the chain catching on any imperfections more often.

One way to ameliorate the problem is to use a steel ring on the inner ring. It's less susceptible to damage than aluminum rings.

Originally Posted by Andrew R Stewart View Post
Second- This guard is broken. So merely hammering it back into shape will not really fix it fully. And AL doesn't like to be hammered anyway.
Yes, the guard is broken. But aluminum doesn't like bending. But aluminum doesn't have a problem with "hammering"...aka ductility...it's actually one of the most ductile metals.

But this isn't a support member and even bending it back into shape shouldn't be much of a problem. The issue might be keeping that partially broken bit in place. Replacing the small washer on the bolt with a larger fender washer would help keep it in place.

Originally Posted by Andrew R Stewart View Post
Third- The best thing to do is to fab up a new one. Assuming the rings will be the same sizes as before a simple stencil of the old guard placed on a 3/16" (or 1'4") AL plate and a drill, hacksaw and some filing will create a new one in short order. Remember to compensate for the distortion due to the crack/breakage spot. Andy.
Or go to the manufacturer and see if they have a replacement.
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Old 09-26-16, 08:15 AM
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Originally Posted by cyccommute View Post
For road biking, perhaps. But mountain biking has a whole host of different things that can happen to cause chain suck. Bikes are shifted under more tension so chainrings are more likely to be damaged. Thankfully, chain suck doesn't happen that often on the outer and middle ring...the derailer has more tension and does a better job of pulling the chain off a tooth with a burr. That's not the case with smaller inner rings. The low tension on the rear derailer spring and the smaller diameter of the ring lead to the chain catching on any imperfections more often.

One way to ameliorate the problem is to use a steel ring on the inner ring. It's less susceptible to damage than aluminum rings.


Note I said "rings in good shape". I totally agree that Mt Biking is far harder on equipment in general and the drivetrain specifically. Also agree that steel inner rings make good sense.



Yes, the guard is broken. But aluminum doesn't like bending. But aluminum doesn't have a problem with "hammering"...aka ductility...it's actually one of the most ductile metals.


And AL will much more easily work harden (if think that's the metallurgical path) then show cracks. Or at least when I have shaped AL it happens very easily. I believe this is why melt forging is a common way to form AL as the metal's properties when near it's melt temp are far more forgiving of cracking and during it's cooling the metallurgical conditions go through changes which reduce future cracking doe to the forming. But I'm no expert here.

But this isn't a support member and even bending it back into shape shouldn't be much of a problem. The issue might be keeping that partially broken bit in place. Replacing the small washer on the bolt with a larger fender washer would help keep it in place.



Or go to the manufacturer and see if they have a replacement.
Of course this is the most obvious choice. But a 20+ year old product which has been off the market for nearly that long might be also out of production. Andy.
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Old 09-26-16, 08:25 AM
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Originally Posted by Andrew R Stewart View Post
And AL will much more easily work harden (if think that's the metallurgical path) then show cracks. Or at least when I have shaped AL it happens very easily. I believe this is why melt forging is a common way to form AL as the metal's properties when near it's melt temp are far more forgiving of cracking and during it's cooling the metallurgical conditions go through changes which reduce future cracking doe to the forming. But I'm no expert here.
It depends on the alloy. I doubt that the alloy used for this piece is too exotic. I am also talking about only a small amount of working the material. Just enough to bend it back into the same shape and then using a fender washer to hold it in place. If Stokestack has access to a welder, it wouldn't take much to just weld the broken bit back together.

Originally Posted by Andrew R Stewart View Post
Of course this is the most obvious choice. But a 20+ year old product which has been off the market for nearly that long might be also out of production. Andy.
I'm thinking more along the lines of a shop having some NOS parts. Or even Fleabay having one around.
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Old 09-27-16, 02:54 AM
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My thinking was the same as cyccommute's: This thing isn't critical, so if I bend it back into shape it may be strong enough to do its job. After I replace most of the drivetrain, it shouldn't come into play that often (I hope). I don't have any welder (let alone a TIG welder), so unfortunately that's not an option.

Thanks for those links, Clifford. If necessary, I'll try one of those.

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