Re: Screamer timing chain idler bracket

[bruce]

Can you describe proper tension for the idler. I adjusted mine so that the
chain didn't touch the return links when they were slack. Now I am noticing
the bend in the idler and the chains are touching again when there with out
tension? Any help here?

"J D Cooper" <[email protected]> wrote in message
news:[email protected]...
> We have had our Screamer for about 5 years now, and haven't had that
> problem. I would back off the tension of the timing chain. When I
> tried to change the timing chain, the adjuster didn't have enough
> travel to produce the proper tension. When I took one link out, it was
> too short and the tension was WAY too high. I had to change the size
> of the timing gears to find a happy medium where the adjuster worked
> within the range of the turnbuckle.
>
> ANYHOW>>>> When the chain was too tight it tended to put a bending
> moment into the bracket. I suspect it would've bent in time.
> Loosening it up fixed my problem.
>
> Let us know how it works out. Screamers DO have timimg chain tension
> issues.
>
> Coop
> Cleveland

 

[Brent Hugh]

"bruce" <[email protected]> wrote in message news:<[email protected]>...
> Can you describe proper tension for the idler. I adjusted mine so that the
> chain didn't touch the return links when they were slack. Now I am noticing
> the bend in the idler and the chains are touching again when there with out
> tension? Any help here?

I can't give a definitive answer but I can tell you my experience.

First I tried making the timing chain essentially as tight as possible
(read something to that effect somewhere). What happens is, there is
so much tension on the upper idler wheel that it just chews through
the plastic, quickly releasing the tension until it reaches a point of
only moderate tightness.

After I realized what was going on I thought, "Well, what's the point
of tightening, since it's just going to loosen itself up again anyway.
Too tight has caused problems and I've never had a problem with too
loose!" Well, that was until a recent 80-miler when TWICE the timing
chain jammed. After the 2nd time we diagnosed the problem--the chain
was loose enough that (when made even looser via frame flex when
attacking a hill) the chain flopped horizontally until the point when
it could catch on the stoker's left crankarm. At that point
everything in the chainline came to a dramatic halt, bending nicely
the bolts holding the front idler assembly to the frame.

So I guess IMHO Screamer timing chain tightness is the classic
Goldilocks situation. Not too TIGHT, not too LOOSE--it's got to be
JUST RIGHT.

Sort of medium tight, or somewhat tight or taut but not too tight is
how I would describe it.

Also, make a point to tighten it up just a little every, say, 500
miles (or at a minimum, every 1000 miles).

Besides the possibility of a dramatic lock-up like we experienced, a
loose chain will be more likely to come off. This seems to be caused,
typically, by a combination of frame flex, which at certain points of
the pedal stroke causes the timing chain to become noticeably looser,
and some kind of jolt, as from shifting or hitting a bump, that
travels up & down the chainline and, if it catches it right, just
flips the chain right off the chainrings.

BTW, our mechanic suggested trying the pedals offset by 90 degrees,
partly to reduce the frame flex problem. We're experimenting with
that right now--haven't tried it long enough to give a real report.

BTW, as others have mentioned the idler wheel modifications suggested
at www.rans.com are worth looking into. In that same vein, I replaced
both timing chainrings on the Screamer with chainrings just ONE tooth
larger than the standard. The result of this is to almost eliminate
the deflection necessary in the upper run of the timing chain and also
straighten out the lower run to a degree. Right now I still have both
idler wheels on the timing chain. With the larger timing chainrings,
though, the top run of the chain actually goes UNDER the top idler
wheel--and is barely deflected at all. So I think that will
completely solve the "chews up the idler wheel" problem. (The Rans
site suggests entirely removing one idler wheel, so that the upper run
has no idler wheel at all, and I might try that also.)

--Brent

bhugh [at] mwsc.edu
Rans Rocket
Rans Screamer
Trek R200
Raytown, MO

See www.MoBikeFed.org