Help With Crank Arm/Pin Problem...

Quoted message said:

The left crank has extra play in it, so when it reaches the tip of its
arc, it loses friction and drops a bit for the length of its play.
This leads to a jerky motion on each revolution on the left side. I
looked at it, and the bolt that connects the crank arm to the frame is
the culprit, however I have no idea how to fix it. There's one
component that almost looks like a very large hex bolt, but it's thin,
and there's another one that looks like a rough ring.

The crank is pressed onto a square taper. If the crank gets loose, it will
be destroyed in short order. That sounds like your problem. To remove the
crank, you need a crank puller tool. First remove the crank bolt and the
washer underneath it. Then thread the crank puller into the crank.

See:
http://draco.acs.uci.edu/rbfaq/FAQ/8f.11.html

and

http://www.parktool.com/tools/CWP_6.shtml

Art Harris

xmp333 writes anonymously:

Quoted message said:

I recently purchased a road bike from a flea market. This is a Puch
Prima frame, but with assorted components -- basically a
FrankenBike. I put some work into it -- removed broken fenders,
trued wheels, etc..., but it's got a problem that I don't know how
to solve. I couldn't find any info in my bike maintenance book
either.

Quoted message said:

The left crank has extra play in it, so when it reaches the tip of
its arc, it loses friction and drops a bit for the length of its
play. This leads to a jerky motion on each revolution on the left
side. I looked at it, and the bolt that connects the crank arm to
the frame is the culprit, however I have no idea how to fix it.
There's one component that almost looks like a very large hex bolt,
but it's thin, and there's another one that looks like a rough ring.

Quoted message said:

I may have a bike store look at it, but since the bike only cost me
$20, I'm not keen on spending too much on fixing it.

Quoted message said:

Any advice would be greatly appreciated.

From your title, it seems you have steel cranks with cotters, round
tapered pins that engage flats on the crank spindle. These cannot be
tightened by the retaining nut on the cotter, and the cotter cannot be
secured anymore because it has a dimple in it that will also make
their removal difficult.

Cotters are notoriously too soft for the job and in olden times when
high quality racing bicycles still used steel cranks, only a few
brands of cotters would stand up to hard pedaling. Of these "Acier
Dupratnik" was the top of the line. Remember that torque from the
left crank goes through both left and right cotters and that they are
both shot. I don't know whether you can find reasonable replacements
but the whole bicycle is probably not worth the bother if it is so
equipped. New cranks and BB will cost more than the bicycle is worth.

New cotters are usually not ready to install but need to be filed to
fit properly. Since this is a hand operation, cranks were often not
exactly 180 degrees apart. The flat on the cotter should be at as low
an angle as practical. I always cut them on a milling machine to fit
my Cinelli (Magistroni) Cranks and a Campagnolo spindle.

Jobst Brandt
[email hidden]

may have said:

Hi All,

I recently purchased a road bike from a flea market. This is a Puch
Prima frame, but with assorted components -- basically a FrankenBike.

The left crank has extra play in it, so when it reaches the tip of its
arc, it loses friction and drops a bit for the length of its play.
This leads to a jerky motion on each revolution on the left side. I
looked at it, and the bolt that connects the crank arm to the frame is
the culprit, however I have no idea how to fix it. There's one
component that almost looks like a very large hex bolt, but it's thin,
and there's another one that looks like a rough ring.

If the hex bolt goes through the crank at a point immediately next to
the shaft on which the crank is mounted, you have what is called
"cottered cranks". In that case, the cotter is most likely worn out.
It will probably be difficult to remove, and obtaining a useful
replacement may be a challenge. You can buy a pair of cotters (it may
be that the *right* cotter is actually the one that's loose) and
replace them in the hope that the problem will be alleviated enough to
make the bike pleasantly operable, but be aware that these cotters wer
supplied in a number of sizes, and you'll need to match what's there
now. If there are no cotters, the crank's probably been loose for
long enough that the square taper hole is wallowed; if that's the
case, it's almost certainly junk.

Quoted message said:

I may have a bike store look at it, but since the bike only cost me
$20, I'm not keen on spending too much on fixing it.

Personally, I would look at the possibility of acquiring a more modern
set of used but serviceable cranks (cotterless) and a matching bottom
bracket. (That's what the shaft-and-bearing assembly is called, or
just BB in many cases.)

Otherwise, the bike's probably little more than a potential parts
donor.

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