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Building a singlespeed- need chainlength advice(6 posts)

Building a singlespeed- need chainlength adviceAuriaprottu
May 28, 2003 5:23 PM
I've got an old Miyata touring/cross frame I'm building into a budget SS. A trip to my LBS found me pretty comfy with a 40/18 MTB. Since I have lots of parts around, I plan on using a 46t chainring with a 21t cog from a cassette for the hilly areas around my home. I've got cassette spacers for the rear and stack spacers for the crank to get the chainline right. I'd buy a Shimano BMX cog, but they only go up to 18t. I'd like to use a large chainring for aesthetics, and find a cog that closely matches the 40/18 gear I rode at the LBS. I'll err on the side of comfort if necessary.

Here's the rub. my frame has horizontal dropouts, but no adjusters, and no holes for them. I suspect part of the answer to this question may depend on the length of my chainstays (they're about 18"), but should I encounter any problems with the length of the chain? In other words, is this one of those "in-between" lengths that won't allow me to put the axle all the way back in the dropouts and still have a tight chain? I'd like to have all the info before I go back to the LBS to have the work done. Thanks in advance.
We don't use no steenking adjusters ...Humma Hah
May 28, 2003 5:51 PM
I've got adjusters on the Paramount, but I'll probably pull them off and store them as original parts.

The usual technique is to set the axel nuts a little loose (notice I didn't say QR skewers), pull the wheel back, eyeball that it is centered between the chainstays, and snug the nuts. I like to adjust the chain so it has maybe 1/4" of slack at the tightest point when the crank is rotated ... hopefully the chainwheel is running centered so there's little difference in slack as you rotate it.

When you get the tension you want, torque them nuts down until you grunt. QR skewers may work, but I've had them slip on me ... they're better left to ders and vertical dropouts.

This is a technique every kid with a coasterbrake and an adjustable wrench used to learn, and its been the standard technique on everything but track bikes since the first chain was put on a fixie about 120 years ago.
QRs work great, HH.Steve_0
May 29, 2003 10:37 AM
The beauty of the QR is the quick-flip for instant tight. No slippin-o-the-axle while spinning nuts.

I've never had a QR slip in over a decade. Really no diff in force than the same QR on the same HD, but using a freewheel and deraileur.
They're not always that quick...OverStuffed
May 29, 2003 4:30 PM
I'm sure they do work fine. I ran a QR on my horizontal dropouts for a few years (before I was fixed). When I first put it on, though, I wasn't tightening it enough. It takes a little practice to get a better feel for how tight it needs to be. I generally tightened it all the way with my fingers, then closed the lever, backing off only as much as I needed to force the lever closed. After switching back to a solid axle, I think the QR was a bit too much work to change a flat.
sounds like too much work alright;Steve_0
May 30, 2003 12:02 PM
perhaps your QR was defective in some way? Once properly adjusted, you should never need to loosen or tighten the 'nut' again. Just flip the lever.
Cogs, and two resources for chain length...p chop
Jun 1, 2003 7:53 AM
One is FixMeUp, a calculator on the Fixed Innovations web site:
http://www.peak.org/~fixin/

Another is a SS gear calculator that you can download, found here:
http://www.teambigtime.com/software/ssConvert.htm

For cogs, I went to a shop and paid almost nothing for some old Shimano twisty-tooth cassette cogs (pre-ramp era), which come in 21 and many other sizes. Just inspect each cog because some of them have a couple of shorter teeth, presumably to help shed the chain for shifting (exactly what you don't want); make sure your cogs have all tall teeth.

I like the FixUp calculator, because it shows you all the possible gears for a given range of chainstay length on a graph. It tells how long of a chain you'll need for each, also. I've used it to set up both SS and fixie projects.

Two pieces of info to have on hand are your chainstay length, measured from the center of the BB spindle to the center of the rear axle (which in your case will be a range of lengths, since you're horizontal), and the actual circumference of the tires you plan to use (best to do the roll-out test: mark a spot on the floor where the tire valve is, roll in a straight line until the valve comes to the floor again, measure.)

The chainstay length if far more crucial, since it will give you the chain length and possible gear combos. The tire circ will contribute to the calculation of how big of a gear you get for a given combo. The downloadable SS calculator just estimates wheel circ from a selection of sizes, so it doesn't require actual wheel circ. On FixUp, you could also estimate wheel circ and the chainlength/chainstay info won't be affected.

Good luck and good riding! A little patience will pay off during the build.