Monday, October 1, 2012

Stop the Slapping! A budget-friendly DIY chain guide.

This summer I bought a new mountain bike.  Ok, so it wasn't really "new," in the strictest sense of the word, but I did buy it from an actual bike shop.  I just happened upon a left-over Trek Remedy demo bike with a small frame that only a vertically challenged rider could love.  With a little TLC, it was riding and shifting smoothly, eating up bumps my hardtail would have had me in tears over.  I soon realized that this all-mountain machine with 6" of front and rear suspension travel was a LOT more bike than I was used to, and that it quickly boosted my confidence in quickly descending very rugged terrain. 

As I barreled down yet another boulder-strewn old jeep trail, I noticed how the tranquil sound of rocks bouncing off my spokes was rudely interrupted by the annoying clatter of chain striking chainstay.  I began my investigation with the rear derailleur.  As I hadn't had this problem on my other bikes, I assumed the tensioning system must be askew.  Spinning on the work stand, there seemed to be adequate tension on the chain to keep it in line, so maybe I was mistaken in my previous supposition.  Maybe the problem resulted from my new-found love for rocky descents and technical terrain?  While the cockpit of this new burly all-mountain beast afforded a place of smooth security as the earth undulated underneath, the rear wheel and drivetrain experienced a much more violent interaction with terra-firma.  It would seem that my chain slap symphony was due to hurling my drivetrain mercilessly down the rugged fall line. 

Ok, source located - now, what do I do about it?  My good friend Google had quite a few wonderful solutions to my predicament, but the price tags ranged from a low end of $50 to a high end over $300!  When I added "DIY" to my search engine query, some interesting ideas began to emerge.  Riders were using all kinds of materials they had lying around the house to keep that chain where it was supposed to be as they danced with gravity down the slopes.  Some were using zip ties, others bushings and spacers, some using bolts and a few even fabricating with wood.  The one thing most of these ideas had in common was the basic premise that the shorter the unsupported length of exposed chain, the less deflection the chain will undergo when the driveline deflects rapidly (due to a bump, obstacle, etc).  As I set out to scrounge up some materials for my own version, I began to think about the various recumbent bikes/trikes that I'd owned and how most of them had extremely long chains run across long spans between supports.  Some of the bikes I'd owned were plagued with chain slap issues (my vintage RANS Stratus was one of the worst offenders), while others seemed to manage the chain effectively regardless of how rough the road became.  Many of the quieter drivetrains used chain tubes to support and guide the chain along its long (and sometimes circuitous) journey.  Could it be as simple as adapting a recumbent bike product for a diamond-frame implementation?

Idea hatched - time for parts gathering.  The core of this idea involved a chain tube.  These are typically made from PVC or Teflon tubing (1/2"?) and can be found either on recumbent bike supply websites or in the yard irrigation aisle at your local home improvement store.  I wanted a length short enough to minimize friction, but long enough to ensure a smooth chain entry/exit impervious to twisting and binding - in my case, I guessed about 2" long.  Now, one thing I learned riding recumbent bikes with chain tubes: they are not the easiest things to secure.  The up-side to this type of tubing is that it is quite low-friction.  The down-side is that this low-friction property makes it a pain to securely affix.  I've had more than my fair share of chain tubes sucked into my front derailleur when the zip tie I had securing it slowly slipped further and further down the tube's length.  I thought about using a hose clamp, but the thought of an unsightly silver clamp on this beautiful bike was more than I could bear.  As I rummaged through a parts drawer I came across a piece leftover from the installation of a rear cargo rack on one of my other bikes.  Some racks come with universal C-clamps to wrap around the seat stays and anchor the cargo rack in cases where the bike doesn't have rack mounting bosses brazed on.  These small metal clamps have most of their surface coated in an anti-slip rubber or vinyl material - perfect for our implementation. 

Once the chain tube was securely bolted in the C-clamp, the attention turned to securing the apparatus to the chainstay.  Since I'm not one of those single-speed-riding monsters I've been passed by on the trail, the chain guide had to be able to move and pivot to accommodate my triple front crankset and 10-speed rear cassette.  Enter my ever-present pals, the zip ties.  The simplest part of the project, one zip tie around the chainstay and another from that zip tie to the bolt securing the chain tube's C-clamp.  The tie around the chainstay should be secured VERY tightly so it doesn't allow the tube to "migrate" forward along with the chain and risk contacting the front derailleur.  The tie connecting the chainstay tie with the chaintube c-clamp bolt should be left as an open circle, in my case about 1" in diameter.  This allows the chain tube to pivot on the zip tie and to deflect left and right as the chain is shifted through the various gears. The optimal size of this connecting zip tie "O" will depend on the specific bike geometry and how far the chainstay is from the lower return path of the chain from crankset to rear derailleur.  Some contact with the chain tube in all but the smallest of gears (granny gear + smallest rear cassette cog) quiets the chain, but too much (i.e. tube pulled too far up toward the chainstay) creates additional friction and noise when in the large chainring and could limit lateral deflection, creating difficulty in shifting smoothly between the front three chainrings.

I will include a side note about guide positioning fore-n-aft.  Some DIY-ers reported issues with the guide deflecting sideways and contacting spokes or knobby tire treads, sometimes with not-so-great consequences.  I anchored my guide so as to bump into the smooth sidewall of the tire, should the chain & tube ever deflect sideways to that extent.  Chain slap most likely would be further minimized by moving the guide further aft toward the center of the chainstay, but I didn't want to risk that positioning for a prototype run.

Last step: threading the chain through the tube.  I was in luck as I had a removable master link in my chain - popped that open, threaded the chain through the tube and snapped it back together.  No master link?  Get out the chain tool, I guess.  Theoretically I suppose one could cut the chain tube down one side along its axis allowing it to be wrapped around the chain before attaching the C-clamp, but I'd worry that the tubing would spiral in on itself, coming out of the C-clamp and lodging itself in the rear derailleur - probably at the most inopportune time.  If I hadn't had a master link in my chain, I would probably have removed a link from my chain and put a master link in - but that's just me.

Maiden voyage: success.  A couple rides around the block and it was off to the trail.  The thing that struck me was how unnoticeable the chain tube apparatus was.  In many gear combinations I couldn't hear the chain passing through the tube at all, and in others it was just perceptible.  Going up the trail the bike performed as usual - shifting as smoothly as ever through the range of gears.  At the top, I opened the rear shock up to its full range and began the descent.  The difference was immediately evident.  I would estimate an 70-85% decrease in chain clatter, the rest of which can probably be attributed to drive-side chain slap (to which I have no solution thus far).  My descent was now (mostly) peaceful and serene again, the gentle ping of displaced stones rocketing off the tensioned spokes (and occasionally the aluminum frame) and the chain silently and dutifully propelling the bike downward.

A final note on durability.  In any system - especially one of inherent contact and friction between two surfaces (chain and chain tube) one would expect one or both components to exhibit wear.  The soft plastic of the chain tube will need periodic inspecting and eventual replacement.  The time between tube replacements (at a cost of about $0.05 if you bought the 20' irrigation tubing roll!) can be extended (probably up to 400%) by periodically loosening the C-clamp and rotating the chain tube 90 degrees.  Simple, basic, cheap and effective.  Probably one of my favorite "little" projects yet.

Happy riding!

*** Disclaimer: Once again, if you use this idea or a part of it in a project of your own, you do so at your own risk!  If your chain tube breaks loose, jams in your rear wheel causing you to fly off a cliff, don't blame me - I've lost plenty of perfectly good bike components (and skin) to hairbrained ideas such as these.

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