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Friday, May 29, 2015

Make it Single Speed

So, you're just riding along.  All of a sudden, you're pedaling, but the bike's not moving.  For some reason, your rear derailer no longer works and you're out on some gravel patch miles from any cell coverage, let alone a coffee shop.  What do you do?

Riding remote on back roads is rough on your drivetrain.  Among other things, muddy, small pieces of gravel can easily wedge between chain links and jam inside rear derailers, resulting in broken chains, derailers or derailer hangers.  This is not an uncommon end to many attempting gravel races.  Equipment choices and rider attentiveness reduce the likelihood of such a catastrophic failure, but ride long enough in bad conditions and stuff happens.  See prior post: Mechanicals

But a broken derailer does not have to end your ride, if you can convert your drivetrain to a single speed.  It's not hard.  And the resulting self-sufficiency and peace of mind is priceless.  Here's how.

Geared Black Mountain Cross bike quickly converted to a workable single speed. 
Start with a thorough reading of the writings of Mr. Sheldon Brown, the Pied Piper of single speed and fixed gear conversions.  See: Sheldon Brown Single Speed Conversion  His articles provide the conceptual framework and detailed procedures to handle most any technical issue.  Look there for step-by-step instructions.  This post is just an introduction and some gravel related fixes.

Generally, one must remove the chain, align the chain in a relatively straight line from a single chain ring to a single gear, shorten the chain to an appropriate length and tighten the chain to an appropriate tension.  That's it.  It really is that simple, if you have done it a few times and have the right equipment.  Off you go.

To start, remove the chain and clean it as best you can, even if you have to use some precious water.  Examine it closely.  You will be shortening the chain considerably and may as well remove links that are clogged or twisted or otherwise questionable.  Even in nasty conditions, you should have more than enough good links remaining.

Align the chain in a relatively straight line from a chain ring to a single gear.  For a given chain ring, you can usually choose one gear of two, maybe even three, on the rear cassette to get a sufficiently straight chain line to work well enough.  Remember that you're looking for a short term solution to finish the ride, not necessarily an optimal single speed set up.  With two front chain rings, I plan to use the smaller one.  If the conditions are bad enough to cause this problem, I'll need the lower gear.

To keep the chain on track, shoot for a relatively straight chain line, parallel to your line of travel.  Here, this chain line is slightly off center, although I think it would probably hold up.  The front derailer will help, so you have a little leeway, if your chain is in good shape.  A better chain line here would be at least one cog in, and probably two.
Wrap the chain around the chain ring and gear, draw the chain together to determine the appropriate length, remove unwanted links and close the chain.  All done, right?  Not so fast.

The final step is a deal breaker for many:  tighten the chain to an appropriate tension.  That means the chain must be loose enough to operate, but not so loose as to slip off.  The operational tolerance here is not high.

On a typical modern bike, the rear derailer provides this chain tension and the rear wheel simply slides into a vertical slot on the frame.  However, without a working derailer, or similar chain tensioner, such a frame has no capacity to adjust the chain tension because it cannot change the fore/aft distance between the chain ring and the rear cassette.  The likely result is a chain tensioned too loosely and/or a chain running at a poorly angled chain line.  Maybe you can limp home that way.  Maybe not.

Enter the archaic, old school semi-horizontal dropout, yet another thoughtful design feature of the Black Mountain Cycles monster cross frame that played a significant part in my purchase decision.  See prior post: An All Road Bike  With such a dropout, the frame itself allows enough fore/aft adjustment to tension the chain for a single speed conversion.  A sturdy quick release skewer will firmly hold the rear wheel in place in a steel frame, even under hard pedaling.  Issue solved.  Ride saved.

Old school semi-horizontal dropout, yet another thoughtful design feature by Mike Varley at Black Mountain Cycles.
Addendum 1.  The Black Mountain Cycles monster cross frame comes with a pair of very nice limiter screws so that you can pre-determine a precise position on each side of the frame to more easily position the rear wheel.  That's great for an operational derailer drivetrain and a necessary feature back when frames were built to less stringent tolerances.  For a single speed conversion, those limiter screws limit the amount of fore/aft adjustment and their removal in the field is a pain.

Black Mountain Cycles Monster Cross bike dropout with limiter screws.
(photo by Black Mountain Cycles)
So, I simply removed the limiter screws and slid the rear wheel all the way back into the dropouts.  Maybe that means less than optimal shifting or other negatives, but it opens the entire length of the dropout for fore/aft adjustment to tension the chain.  In addition to simplifying an emergency single speed conversion, sliding the wheel to the back of the dropouts creates more mud clearance by moving the rear wheel a little further from the bottom bracket area.  It also effectively increases the bottom bracket drop from a published 70 mm to my bike shop measured 75 mm, which provides a bit more stability at higher speeds.

Back to geared within a few minutes, with the wheel all the way back into the dropout.
Addendum 2.  If a single speed conversion during an event is more than a remote possibility, I pre-determine the single speed gearing to use, install a cassette with the best gears lining up with the chain rings, and even carry a spare chain, at the appropriate length, with Power Links.  If conditions make such a conversion highly likely, I may just start the ride as single speed.

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