Chain tightens, then loosens; What's up?



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J

Joe

Guest
Hey,

I'm new to the sport of BMX. I got a new bike (DK Dayton), but notice that the chain tightens and
loosens as the pedals spin. I called the bike shop that I got the bike from, and they suggested a
new chain. The bike's brand-new, so it's not as though I have been beating on it. Can I just
continue riding the bike and get the chain stretched out so it spins through smoothly?

Also, I would assume that I should tighten the chain while the pedals are in the 'loose stage'. The
chain will get rather tight while I spin through the tight phase though, will that damage anything?

Thanks...
 
G

Gary Smiley

Guest
Your front chainring is not round, and is probably eliptical. Tighten the chain while the pedals are
in the tight stage. If you tighten the chain while the pedals are in the loose stage, when you pedal
around to the tight stage, the chain will be too tight, and could break at that point.

joe wrote:

> Hey,
>
> I'm new to the sport of BMX. I got a new bike (DK Dayton), but notice that the chain tightens and
> loosens as the pedals spin. I called the bike shop that I got the bike from, and they suggested a
> new chain. The bike's brand-new, so it's not as though I have been beating on it. Can I just
> continue riding the bike and get the chain stretched out so it spins through smoothly?
>
> Also, I would assume that I should tighten the chain while the pedals are in the 'loose stage'.
> The chain will get rather tight while I spin through the tight phase though, will that damage
> anything?
>
> Thanks...
 
M

Michael Dart

Guest
"joe" <[email protected]> wrote in message news:[email protected]...
> Hey,
>
> I'm new to the sport of BMX. I got a new bike (DK Dayton), but notice that the chain tightens and
> loosens as the pedals spin. I called the bike shop that I got the bike from, and they suggested a
> new chain. The bike's brand-new, so it's not as though I have been beating on it. Can I just
> continue riding the bike and get the chain stretched out so it spins
through
> smoothly?
>
> Also, I would assume that I should tighten the chain while the pedals are
in
> the 'loose stage'. The chain will get rather tight while I spin through
the
> tight phase though, will that damage anything?
>
> Thanks...
>
>

Bent crank or bb axle? Hold a pencil against the frame pointing at the chainring as a reference and
see if it 'wobbles' when you turn the crank.

Mike
 
M

Matt O'Toole

Guest
"joe" <[email protected]> wrote in message news:[email protected]...

> I'm new to the sport of BMX. I got a new bike (DK Dayton),
but notice that
> the chain tightens and loosens as the pedals spin. I
called the bike shop
> that I got the bike from, and they suggested a new chain.
The bike's
> brand-new, so it's not as though I have been beating on
it. Can I just
> continue riding the bike and get the chain stretched out
so it spins through
> smoothly?
>
> Also, I would assume that I should tighten the chain while
the pedals are in
> the 'loose stage'. The chain will get rather tight while I
spin through the
> tight phase though, will that damage anything?

I'm not exactly sure what you're talking about, but it sounds like your chainring might not be
perfectly centered. This problem comes up sometimes with fixed gear bikes. The solution is to loosen
the chainring bolts until they're just tight enough to hold, but loose enough that the chainring can
be tapped back and forth with a hammer for adjustment. Then turn the cranks slowly, and as the chain
gets tighter, tap the chainring back a little. By doing this several times, you can eventually get
the chainring perfectly centered. Not all chainrings can be adjusted this way, though. In fact, this
procedure can be a real pain. You might go back to the shop, suggesting this is the problem, and see
what they have to say. Maybe some others here will have some other ideas too.

Matt O.
 
S

Sheldon Brown

Guest
joe wrote:
> Hey,
>
> I'm new to the sport of BMX. I got a new bike (DK Dayton), but notice that the chain tightens and
> loosens as the pedals spin. I called the bike shop that I got the bike from, and they suggested a
> new chain. The bike's brand-new, so it's not as though I have been beating on it.

A certain amount of out-of-roundness of the chainring is not unusual, especially on less expensive
cranksets. Sometimes you can adjust it a bit, as described at:

http://sheldonbrown.com/fixed-conversion.html#tension

> Can I just continue riding the bike and get the chain stretched out so it spins through smoothly?

It's unlikely that this has anything to do with the chain, rather that the chainring is not
perfectly centered.

> Also, I would assume that I should tighten the chain while the pedals are in the 'loose stage'.
> The chain will get rather tight while I spin through the tight phase though, will that damage
> anything?

Yes, it will. Set it so that even at the tightest position, it shouldn't bind.

Sheldon "Sometimes Eccentricity Is Bad" Brown +----------------------------------------------+
| My mind is aglow with whirling, transient | nodes of thought careening through a cosmic | vapor
| of invention! --Mel Brooks |
+----------------------------------------------+ Harris Cyclery, West Newton, Massachusetts Phone
617-244-9772 FAX 617-244-1041 http://harriscyclery.com Hard-to-find parts shipped Worldwide
http://captainbike.com http://sheldonbrown.com
 
C

Cy Galley

Guest
Is the pedal chain ring marked Bio-Pace?

"Sheldon Brown" <[email protected]> wrote in message
news:[email protected]...
>
>
> joe wrote:
> > Hey,
> >
> > I'm new to the sport of BMX. I got a new bike (DK Dayton), but notice
that
> > the chain tightens and loosens as the pedals spin. I called the bike
shop
> > that I got the bike from, and they suggested a new chain. The bike's brand-new, so it's not as
> > though I have been beating on it.
>
> A certain amount of out-of-roundness of the chainring is not unusual, especially on less expensive
> cranksets. Sometimes you can adjust it a bit, as described at:
>
> http://sheldonbrown.com/fixed-conversion.html#tension
>
> > Can I just continue riding the bike and get the chain stretched out so it spins
through
> > smoothly?
>
> It's unlikely that this has anything to do with the chain, rather that the chainring is not
> perfectly centered.
>
> > Also, I would assume that I should tighten the chain while the pedals
are in
> > the 'loose stage'. The chain will get rather tight while I spin through
the
> > tight phase though, will that damage anything?
>
> Yes, it will. Set it so that even at the tightest position, it shouldn't bind.
>
> Sheldon "Sometimes Eccentricity Is Bad" Brown +----------------------------------------------+
> | My mind is aglow with whirling, transient | nodes of thought careening through a cosmic | vapor
> | of invention! --Mel Brooks |
> +----------------------------------------------+ Harris Cyclery, West Newton, Massachusetts Phone
> 617-244-9772 FAX 617-244-1041 http://harriscyclery.com Hard-to-find parts shipped Worldwide
> http://captainbike.com http://sheldonbrown.com
 
J

Joe

Guest
I took a few photos of the chain ring area to let you guys know what exactly I'm talking about.

http://www.geocities.com/boardmethod/photos.html

I stuck a small gum eraser on the chainstay and pressed it into shape next to the chain ring. I then
spun the chain ring around to see if it was more oval the circular. The chain ring rubbed the eraser
quite smoothly the whole way through. It appears to be quite circular. I then pressed the eraser
closer to the chain ring, so when spun, the chain ring left a mark on the eraser. The indent made on
the eraser was quite narrow. This would seem to mean the that chain ring is quite solid and true.
While the chain was off, I spun the freewheel. It didn't appear to wobble around at all.

I am starting to wonder if indeed the chain is bad. It is a KMC brand. The tension of the chain does
vary quite dramatically. When I have the time, I could take a shot of the chain in the tight stage,
and then in the loose stage.

What do you think? Replace the chain?
 
A

A Muzi

Guest
"joe" <[email protected]> wrote in message news:[email protected]...
> Hey,
>
> I'm new to the sport of BMX. I got a new bike (DK Dayton), but notice that the chain tightens and
> loosens as the pedals spin. I called the bike shop that I got the bike from, and they suggested a
> new chain. The bike's brand-new, so it's not as though I have been beating on it. Can I just
> continue riding the bike and get the chain stretched out so it spins
through
> smoothly?
>
> Also, I would assume that I should tighten the chain while the pedals are
in
> the 'loose stage'. The chain will get rather tight while I spin through
the
> tight phase though, will that damage anything?
>
> Thanks...
>
>

All the components of the system, while relatively round, are imperfect. When you spin the crank
around you show the errors of roundness in all the various parts of the crank bearing assembly and
the rear hub/freewheel. This is normal although the amount of variance is less with better-quality
equipment.

Set your chain so the tight spot is such that you can still deflect the chain a half inch with
a light finger pressure. Overtightening the chain as you suggest will very quickly eat up all
the various bearing assemblies. Err slightly to the loose side, not the tight side, of perfect
chain tension.

Since the bicycle is new, perhaps you might ask the dealer to revisit that adjustment?

--
Andrew Muzi http://www.yellowjersey.org Open every day since 1 April 1971
 
S

Sheldon Brown

Guest
joe wrote:
> I took a few photos of the chain ring area to let you guys know what ex=
actly
> I'm talking about.
>=20
> http://www.geocities.com/boardmethod/photos.html
>=20
> I stuck a small gum eraser on the chainstay and pressed it into shape n=
ext
> to the chain ring. I then spun the chain ring around to see if it was m=
ore
> oval the circular. The chain ring rubbed the eraser quite smoothly the =
whole
> way through. It appears to be quite circular. I then pressed the eraser=

> closer to the chain ring, so when spun, the chain ring left a mark on t=
he
> eraser. The indent made on the eraser was quite narrow. This would seem=
to
> mean the that chain ring is quite solid and true. While the chain was o=
ff, I
> spun the freewheel. It didn't appear to wobble around at all.
>=20
> I am starting to wonder if indeed the chain is bad. It is a KMC brand. =
The
> tension of the chain does vary quite dramatically. When I have the time=
, I
> could take a shot of the chain in the tight stage, and then in the loos=
e
> stage.
>=20
> What do you think? Replace the chain?

As I said before, it's unlikely it's the chain. Probably the chainwheel =

or rear sprocket is not perfectly centered.

Make a note of where the cranks are pointing when the chain is at its=20 tightest. The give the
cranks a turn and see if it gets tight again=20 with the cranks in the same orientation. If it is
consistently tight at =

a particular angle of the cranks, the problem is in the crank, chainring =

or possibly bottom bracket.

If the tightness occurs at different crank orientations, it's probably=20 the rear cog or hub. Cheap
track sprockets (and, occasionally, even=20 fairly expensive ones are sometimes out of round.

It's also possible that _both_ sprockets are out of round by small=20 amounts, and that when they
_both_ line up facing in opposite=20 directions, that's when the chain gets tight.

Sheldon "Not So Tight It Binds, Not So Loose It Can Fall Off" Brown
+----------------------------------------+
| I never did a day=92s work in my life; | it was all fun. --Thomas Edison |
+----------------------------------------+ Harris Cyclery, West Newton, Massachusetts Phone
617-244-9772 FAX 617-244-1041 http://harriscyclery.com Hard-to-find parts shipped Worldwide
http://captainbike.com http://sheldonbrown.com
 
A

Ant

Guest
"joe" <[email protected]> wrote in message

> I am starting to wonder if indeed the chain is bad. It is a KMC brand. The tension of the chain
> does vary quite dramatically. When I have the time, I could take a shot of the chain in the tight
> stage, and then in the loose stage.

Dude, ill bet money it ain't the chain. have you checked the archives? peruse
http://tinyurl.com/9a6o. many people have asked this before. there are many answers.

cheers
 
B

Bikefixr

Guest
REAL common on singles like BMX. The front chainring hole is probabally a bit off-center. Find the
medium between the loose and tight, adjust to that. The chain will stratch a bit and it'll run
smooth. The problem is NOT the chain. Amazes me to this day how stupid some shops have become.
 
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