chain bending from side to side

Veggie said:

I just completed a 7 speed freewheel to 8 speed cassette conversion. I
was able to keep the vintage Shimano SIS rear derailer, as many on the
net had said.

When I first tried it out, the derailer was preventing extreme gear
combos. It couldn't do the left most chainring and rightmost cassette
cog, and vice versa. The middle chainring could use all cassette cogs.
I had since adjusted the rear derailer to allow all combos.

It made me wonder... Should extreme combos be allowed? It seems to put
side bending forces on the chain, as it is straight when on the
ring/cogs but then has to bend diagonally. How do 9 cog cassette users
deal with this?

The conventional wisdom is that severe cross-chaining (small/small and
big/big sprocket combinations) should be avoided since they place
excessive single-side strain on the chain and will increase the rate
of chain wear. It has been observed that some der setups have
problems shifting into those combinations reliably; in my experience,
this is most often observed with low-end ders.

The same would hold true for 9-speed and 10-speed cassettes, but most
such setups employ better grades of der and shifter; these tend to be
able to handle the cross-chaining more predictably in most cases, but
that still doesn't make it a good idea to use those gear pairs.

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Where's the hatrack said:

Quoted message said:

I just completed a 7 speed freewheel to 8 speed cassette
conversion. I was able to keep the vintage Shimano SIS rear
derailer, as many on the net had said.

Quoted message said:

Quoted message said:

When I first tried it out, the derailer was preventing extreme gear
combos. It couldn't do the left most chainring and rightmost
cassette cog, and vice versa. The middle chainring could use all
cassette cogs. I had since adjusted the rear derailer to allow all
combos.

Quoted message said:

Quoted message said:

It made me wonder... Should extreme combos be allowed? It seems
to put side bending forces on the chain, as it is straight when on
the ring/cogs but then has to bend diagonally. How do 9 cog
cassette users deal with this?

Quoted message said:

The conventional wisdom is that severe cross-chaining (small/small
and big/big sprocket combinations) should be avoided since they
place excessive single-side strain on the chain and will increase
the rate of chain wear. It has been observed that some der setups
have problems shifting into those combinations reliably; in my
experience, this is most often observed with low-end ders.

I wouldn't worry about that as much as the gouging it does on the
aluminum chainwheel, and these cost more than chains, or at least they
did. Chains need replacement anyway, but chainwheels should last many
chain lives.

Quoted message said:

The same would hold true for 9-speed and 10-speed cassettes, but
most such setups employ better grades of der and shifter; these tend
to be able to handle the cross-chaining more predictably in most
cases, but that still doesn't make it a good idea to use those gear
pairs.

Chains at a steep angle to the derailleur eat up idler wheels fast.
Focus on the parts you don't want to replace often.

Jobst Brandt

Veggie said:

I just completed a 7 speed freewheel to 8 speed cassette conversion. I
was able to keep the vintage Shimano SIS rear derailer, as many on the
net had said.

When I first tried it out, the derailer was preventing extreme gear
combos. It couldn't do the left most chainring and rightmost cassette
cog, and vice versa. The middle chainring could use all cassette cogs.
I had since adjusted the rear derailer to allow all combos.

It made me wonder... Should extreme combos be allowed? It seems to put
side bending forces on the chain, as it is straight when on the
ring/cogs but then has to bend diagonally. How do 9 cog cassette users
deal with this?

More recent bike designs are less tolerant of bad combos for a couple of
reasons: thinner sprockets and shorter chainstays (not to mention much
more expensive cassettes).