A good fixed gear ratio?

wrote:


This sounds like a great low budget way to get the project done,
assuming horizontal dropouts. If the bike has vertical dropouts, you
still need a chain tensioner of some sort. I suppose a budget version
could be the existing derailer, with a very short length of cable
fastened to the frame at a given tension.

Right on the first part, but I presume that it does have horizontal
dropouts, since it's a 12-speed. That would be a pretty old bike.

On the other hand, you cannot use a chain tensioner with a fixed gear.
You have to either use an eccentric hub, or be very lucky with the
choice of gearing so that the chain tension is right. With that you
also have to worry about chain wear.



--

David L. Johnson

"Business!" cried the Ghost. "Mankind was my business. The common
welfare was my business; charity, mercy, forbearance, and
benevolence, were, all, my business. The dealings of my trade were but
a drop of water in the comprehensive ocean of my business!" --Dickens,

On Mar 9, 10:21 am, "David L. Johnson"
wrote:
Right on the first part, but I presume that it does have horizontal
dropouts, since it's a 12-speed. That would be a pretty old bike.

On the other hand, you cannot use a chain tensioner with a fixed gear.
You have to either use an eccentric hub, or be very lucky with the
choice of gearing so that the chain tension is right. With that you
also have to worry about chain wear.

Right you are Sir. I somehow mentally transposed my singlespeed,
which has a single speed freewheel, into this instance. Fixed with
vertical dropouts would require lots of luck or extremely careful
gearing choice, and still may not be possible. Singlespeed, on the
other hand, could be accomplished easily enough even with vertical
dropouts through a tensioner (or the old derailer rigged as such).
Hopefully for the OP, you're right and the dropouts are horizontal.

Dan

On 2007-03-09, David L. Johnson wrote:

wightstraker wrote:

I have two questions for you all: First, how hard would it be to
convert a 12 speed road bike to a fixed gear?

Trivial.

I'd do the work myself,
but would the parts be expensive?

It can be done on the cheap, for the price of a sprocket ($15) and a
chain ($10). That requires you to:

1) remove dérailleurs, cable, shifters.

2) remove chainrings, and put the bigger one where the smaller one was,
on the inside of the spider.

Unless you're a real gear-masher, you may want to leave the inner ring
where it is and just remove the outer ring.

You may want to get shorter chainring bolts, or jerry-rig a few
washers to space it out.

BMX single chainring bolts are cheap, effective and widely available.

3) Remove the freewheel (as it's a 6-speed rear, I presume it's a
freewheel), and put the sprocket on. It will tighten when you start to
ride, but be careful not to apply back-pressure on the pedals until you
have some miles on it, preferably including some hills.

For added security, put a bottom bracket lockring on to help hold the
cog in place. Another option would be to use a single cog BMX freewheel
intead of a fixed cog. You can pick one up at the same place you get the
chainring bolts... :-)

You may have trouble with the chainline. Ideally, the plane of the
chainring should be the same as that determined by the sprocket. The
sprocket may be offset by a couple millimeters without trouble, but more
than that could make it prone to dropping a chain, which is bad. If the
sprocket is very much too far in (this is likely), you can try

a) putting a spacer in between the sprocket and the hub. Some cassette
spacers work well, but get a metal one if you can.

Sturmey-Archer cog spacers are just the ticket for this job. They can
also be used under the bottom bracket fixed cup if you need to move the
ring out a little.

As long as your chainring and cog are designed for 3/32" width chain,
you can just shorten your current chain to make it work with the new
setup.

--

John )

wightstraker wrote:
but would the parts be expensive? Second, what would be the best gear
ratio for my bike? I mostly use it to get to around town - nothing too
strenuous; the terrain is relatively flat, but I like to move
quickly.

it is a lot of fun riding a fixie as such. The fun doubles/triples on
hills. mashing you way up the hill and spinning like crazy on the downhill.

big gear:
pro: spin slower on the downhills
con: you need bigger muscles to climb the hill

low gear:
pro: climbing uphill is a bit easier
con: you spin even more on the downhills

the best gear is one that balances your leg strength with leg speed. the
magic number varies from person to person. try different numbers to find
your number. even on flats, a lower gear will develop your spin -
because if you want to get somewhere quick, you would be spinning really
fast or even on your short sprints to get thru the traffic light will be
a spin-a-rama.

I am not sure whether pushing a big gear on flats develop power in your
legs ?

+ravi