Physiology of Fixed



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Mike Latondresse <mikelat@no_spamshaw.ca> wrote in message news:<[email protected]>...
> [email protected] (Carl Fogel) wrote in news:[email protected]:
>
> > dvt <[email protected]> wrote in message news:<[email protected]>...
> >> Benjamin Weiner wrote:
> >> > The fact, which many of us have noticed, that high chain tension often occurs at a particular
> >> > crank position points to the chainring (or less likely, the crank) as a source of error.
> >>
> >> Carl, read this previous paragraph carefully. Benjamin is not the first person to posit this
> >> theory, but he says it quite clearly.
> >>
> >> Dave dvt at psu dot edu
> >
> > Dear Dave,
> >
> > Sounds as if the chainring would then be out of round in a significantly measurable way,
> > doesn't it?
> >
> > Of course, others have been saying that it's a trickier combination of two significantly
> > out-of-round sprockets, front and rear, which occasionally reach their worst positions at
> > the same time.
> >
> > But that also sounds as if they should be measurable, too.
> >
> > It's an interesting situation, with so many people insisting that there's a clearly observable
> > phenomenon (varying chain tension) that either cannot or should not be measured at what's
> > supposed to be the source (gear teeth running out of true).
> >
> > Carl Fogel
> >
> Carl, haven't you dragged this dead end discussion along enough. Many of us who ride fixies have
> watched the chain tension alter and most of us who have read Sheldon's stuff have used his
> suggestions to adjust tension so that it is a non-issue. Please do not give me one of your long
> explanations, just give it up.

Dear Mike,

Short explanation. I measured 18-inch sections of the same worn chain. They varied about 1.7mm. So I
think that the chain is likely involved.

Carl Fogel
 
> > Carl, haven't you dragged this dead end discussion along enough. Many of us who ride fixies have
> > watched the chain tension alter and most of us who have read Sheldon's stuff have used his
> > suggestions to adjust tension so that it is a non-issue. Please do not give me one of your long
> > explanations, just give it up.
>
> Dear Mike,
>
> Short explanation. I measured 18-inch sections of the same worn chain. They varied about 1.7mm. So
> I think that the chain is likely involved.
>
> Carl Fogel

I own a fixed gear and Carl's questions got me thinking. So I replaced my old worn chain with a
brand new one and I would estimate that I decreased chain sag at the loose point by 20 - 25 %

I think Carl is right that chain wear is part of the problem. If chain wear is caused by road grit
wearing out links in the chain, why would chain wear be uniform unless the pieces of road grit are
nice enough to organize themselves by size and jump into each link of the chain out of a sense of
fair play ?

ps I loved the link to the Mark Twain story that you sent out. Now I want to try a high wheeler.
That would get rid of chain sag !
 
[email protected] (Matt Cahill) wrote in message news:<[email protected]>...
> > > Carl, haven't you dragged this dead end discussion along enough. Many of us who ride fixies
> > > have watched the chain tension alter and most of us who have read Sheldon's stuff have used
> > > his suggestions to adjust tension so that it is a non-issue. Please do not give me one of your
> > > long explanations, just give it up.
> >
> > Dear Mike,
> >
> > Short explanation. I measured 18-inch sections of the same worn chain. They varied about 1.7mm.
> > So I think that the chain is likely involved.
> >
> > Carl Fogel
>
> I own a fixed gear and Carl's questions got me thinking. So I replaced my old worn chain with a
> brand new one and I would estimate that I decreased chain sag at the loose point by 20 - 25 %
>
> I think Carl is right that chain wear is part of the problem. If chain wear is caused by road grit
> wearing out links in the chain, why would chain wear be uniform unless the pieces of road grit are
> nice enough to organize themselves by size and jump into each link of the chain out of a sense of
> fair play ?
>
> ps I loved the link to the Mark Twain story that you sent out. Now I want to try a high wheeler.
> That would get rid of chain sag !

Dear Matt,

Glad that you liked Twain's story. It helps explain why full-size tricycles were so popular
back then.

As for chain-wear, what struck me was that a chain has more moving parts than the rest of the
bicycle put together, but is a beast to measure for evenness all along its length--typically, we
just slap a ruler over the handiest twelve links, see that there's a convenient sixteenth of an inch
of wear--well, closer to an eighth--and replace it. If the new chain skips, we mutter and replace
the gears, too, vowing not to let things wear so far next time.

(Only an idiot would clean and stretch a worn chain out taut on an aluminum frame and then spend a
happy hour measuring the bejeebers out of 53 eighteen-link sections with dial calipers and feeding
the results into a spreadsheet.)

I like your point about the grit being unlikely to organize itself evenly. Once a pin-roller
interface wears a little, it can admit bigger chunks of grit that serve to enlarge it more rapidly,
which in turn admits larger chunks, and so on--it would be remarkable if this process proceeded
uniformly over a hundred pins and rollers.

Of course, the individual differences average out somewhat over the eighteen or so links that
are under load, which is why I used that actual length instead of the more dramatic single link
variation. The 18-link sections still showed significant variation, about 1.7mm from shortest
to longest.

To be fair, the chain that I measured was badly worn. It was roughly 1.3% when replacement is
suggested at .5% to 1.0%. yet it was still running fine without any skipping on 53 x 11 at what
another poster was kind enough to call a brisk pace. But the gears had worn so badly that replacing
the rear cogs wasn't enough--the new chain stripped off the worn front chain-ring the instant that I
tried to pedal off and had to be replaced.

Interestingly, fixed-gear bikes probably wear their gears and chains out more rapidly and in
different ways than multi-gear bikes for several reasons.

First, the wear is concentrated on a single rear gear and a single front gear, instead of being
spread out over two or three front gears and six to ten rear cogs.

Second, there's no derailleur chain-tensioning to help smooth things out. (On the other hand, the
fixed-gear chain doesn't run at an angle or rattle as it climbs from gear to gear.)

Third, the fixed-gear transmission load is abruptly reversed whenever a fixed-gear bicyclist brakes.
While braking is probably only a tiny part of riding compared to normal pedalling, it's also
probably much stronger than normal pedalling--we can decelerate much more violently than we can
accelerate, and both are accelerations as far as physics is concerned.

From a practical point of view (hardly my interest), any uneven chain wear might explain some
difficulties in adjusting a fixed-gear to avoid binding.

That is, while a few cranks of the pedal will serve to expose the tight spot in a perfect chain
between two rarely-in-phase lumpy gears (4 & 53/55ths turns for a 53 x 11), far more cranking is
needed to find the tightest (and loosest) configuration with two lumpy gears and an unevenly
worn chain.

And although you can do some centering on a lumpy cog or chain-ring, there's no adjustment for an
unevenly worn chain--it can only be replaced. If this trick reduced the magnitude of your chain sag
with the same gears some 20-25%, then your chain may well have been worn unevenly.

In the unlikely event that you still have the old chain, feel free to mail it to me and I'll stretch
it out, measure 18-link sections, and post the results.

Thanks,

Carl Fogel 309 Grace Pueblo, CO 81004
 
> In the unlikely event that you still have the old chain, feel free to mail it to me and I'll
> stretch it out, measure 18-link sections, and post the results.
>
> Thanks,
>
> Carl Fogel 309 Grace Pueblo, CO 81004

Carl,

Unfortunately I threw the chain out. It was quite worn. It was visably stretched when compared to
the new chain side by side. Interestingly it worked fine on the derailleur drive train which was on
before the conversion to fixed gear. The rear cog and chainring where probably worn in a mutually
compatible way with the chain.

At this point I have a virtually new rear cog (maybe a few hundred miles) and a new chain. My next
step is to replace the worn chain ring. I wouldn't be surprised if that once I have all the new
parts chain sag will be fairly minimal, as Sheldon described on the new bike he was putting
together. I suspect that new parts will probably mesh better and be more consistant at all points.

I'll let you know when I've completed the last part of my "research".

Thanks, Matt
 
On 26 Dec 2003 11:29:27 -0800, [email protected] (Carl Fogel)
wrote:

>To be fair, the chain that I measured was badly worn. It was roughly 1.3% when replacement is
>suggested at .5% to 1.0%. yet it was still running fine without any skipping on 53 x 11 at what
>another poster was kind enough to call a brisk pace.

I just replaced my chain and rear cassette (8-speed) for the first time. Mileage approx.
7000km/4350miles. I replaced it because the chain started to skip in the rear. Measured the stretch
at 2.6%, a 12-inch section was 12 5/16 long. It worked fine up to that point...

>But the gears had worn so badly that replacing the rear cogs wasn't enough--the new chain stripped
>off the worn front chain-ring the instant that I tried to pedal off and had to be replaced.

I had just replaced the crankset so I didn't have that problem, luckily.
 
On 27 Dec 2003 10:05:15 -0800, [email protected] (Carl Fogel)
wrote:

>Dear Jens,
>
>Dear Lord!
>
>I'm not surprised that your chain began to skip on the rear cogs. In fact, a chain worn to 5/16
>inches in twelve links may be causing hearts to skip beats throughout rec.bicycles.tech.
>
>So much wear seems unusual in only 5,000 miles, but perhaps this chain was on a mountain bike or
>used on horrible muddy roads?

Both. Also used through two winters. I just got the tools for changing chains myself so I'll keep an
eye on the new one and see how fast it wears.
 
Jens Kr. Kirkebø <[email protected]> wrote in message news:<[email protected]>...
> On 26 Dec 2003 11:29:27 -0800, [email protected] (Carl Fogel) wrote:
>
> >To be fair, the chain that I measured was badly worn. It was roughly 1.3% when replacement is
> >suggested at .5% to 1.0%. yet it was still running fine without any skipping on 53 x 11 at what
> >another poster was kind enough to call a brisk pace.
>
> I just replaced my chain and rear cassette (8-speed) for the first time. Mileage approx.
> 7000km/4350miles. I replaced it because the chain started to skip in the rear. Measured the
> stretch at 2.6%, a 12-inch section was 12 5/16 long. It worked fine up to that point...
>
> >But the gears had worn so badly that replacing the rear cogs wasn't enough--the new chain
> >stripped off the worn front chain-ring the instant that I tried to pedal off and had to be
> >replaced.
>
> I had just replaced the crankset so I didn't have that problem, luckily.

Dear Jens,

Dear Lord!

I'm not surprised that your chain began to skip on the rear cogs. In fact, a chain worn to 5/16
inches in twelve links may be causing hearts to skip beats throughout rec.bicycles.tech.

So much wear seems unusual in only 5,000 miles, but perhaps this chain was on a mountain bike or
used on horrible muddy roads?

Carl Fogel
 
Carl Fogel wrote:
> I measured 18-inch sections of the same worn chain. They varied about 1.7mm. So I think that the
> chain is likely involved.

How, then, do you explain that the tension variation is often periodic with one rotation of
the crank?

Dave dvt at psu dot edu
 
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