Shifting problems on Stronglight 86mm BCD cranks

David Damerell said:

I recently acquired an '89 Super Galaxy tandem. This has
Stronglight cranks on, with a single 86mm BCD for a triple.
The bicycle's original gearing was 48/42/28 chainrings with
a 13-28 six speed rear freewheel.

Of course, tandems being pretty quick, we found ourselves
spinning out the
48/13 almost immediately. Hence I ordered a 52t 86mm
chainring and fitted
it.

However, the chain falls down the gap between the 52t and
42t chainrings, which is a nuisance. Furthermore, I
intended to replace the rear hub with a 7-speed cassette
hub; we plan to do the End to End on this tandem and I
don't expect to be able to find 6s freewheels in the back
of beyond, I have a bunch of existing 7s gear, and an 11t
top sprocket would also alleviate the gear range problems.
However, I'm concerned that a narrower chain will only
exacerbate this problem.

I'm not sure what the answer is. "New cranks" is obvious,
but expensive. Returning to 48/42/28 might sort it out, but
then the gearing's less useful. Another idea that springs
to mind is drilling into the 52 and 42 just below the teeth
on the 42, and bolting them together - both to make them
slightly closer, and to make it impossible for the chain to
descend below that level. Grinding down the flats that
engage with the spider would also decrease the spacing, but
seems pretty drastic.

I'd be grateful for any advice people can offer.

Dear David,

Could you be using an unusually thin chain?

Last August, Pete Biggs asked about fatter chains for 5 and
6 speed bikes, and Dianne_1234 shamelessly published widths
from 6.60mm to 8.61mm:

groups.google.comgroupsOpen ↗
rec.bicycles.tech%26start%3D20%26hl%3Den%26lr%3D%26ie%3DUTF-
8%26group%3Drec.bicycles.tech%26scoring%3Dd%26selm%3Dad63da-
ab.0308251725.3f05c39%2540posting.google.com%26rnum%3D29

or

tinyurl.com27po5Open ↗

Carl Fogel

David Damerell said:

However, the chain falls down the gap between the 52t and
42t chainrings, which is a nuisance. Furthermore, I
intended to replace the rear hub with a 7-speed cassette
hub; we plan to do the End to End on this tandem and I
don't expect to be able to find 6s freewheels in the back
of beyond, I have a bunch of existing 7s gear, and an 11t
top sprocket would also alleviate the gear range problems.
However, I'm concerned that a narrower chain will only
exacerbate this problem.

Quoted message said:

I'm not sure what the answer is. "New cranks" is obvious,
but expensive. Returning to 48/42/28 might sort it out,
but then the gearing's less useful. Another idea that
springs to mind is drilling into the 52 and 42 just below
the teeth on the 42, and bolting them together - both to
make them slightly closer, and to make it impossible for
the chain to descend below that level. Grinding down the
flats that engage with the spider would also decrease the
spacing, but seems pretty drastic.

The crankset may have been designed with wider, 5/6 speed
chain in mind, where the rivets stick out more. Many
chainrings are asymmetric - perhaps the 42 can be turned
around, However, there is a better solution.

I "pinned" a 52 in an attempt to mimic the Shimano pins that
assist 42->52 upshifts. I tapped small holes in the
chainring a bit below the teeth, and inserted short M5
setscrews that poke inside by 1-2mm, just enough so the
chain doesn't hit them when it's on the 42. If you do this I
think the setscrews will push the chain inside enough to
land on the 42 when downshifting.

I used a tapered tap and didn't run it quite all the way
through, so the screws are a tight fit. This keeps them from
vibrating out.

Benjamin Weiner said:

David Damerell said:

However, the chain falls down the gap between the 52t and
42t chainrings, which is a nuisance.

The crankset may have been designed with wider, 5/6
speed chain in mind, where the rivets stick out more.
Many chainrings are asymmetric - perhaps the 42 can be
turned around

Curiously, I noticed this after posting; for now I've
flipped the 42.

Quoted message said:

I "pinned" a 52 in an attempt to mimic the Shimano pins
that assist 42->52 upshifts. I tapped small holes in the
chainring a bit below the teeth, and inserted short M5
setscrews that poke inside by 1-2mm, just enough so the
chain doesn't hit them when it's on the 42. If you do this
I think the setscrews will push the chain inside enough to
land on the 42 when downshifting.

Ingenious - but do they help with upshifts? The derailleur
cage doesn't provide much upwards hoik, because it has to do
most of its work for the 28->42 shifts.
--
David Damerell <[email hidden]> Kill
the tomato!

David Damerell said:

Benjamin Weiner <[email hidden]> wrote:

Quoted message said:

Quoted message said:

I "pinned" a 52 in an attempt to mimic the Shimano pins
that assist 42->52 upshifts. I tapped small holes in the
chainring a bit below the teeth, and inserted short M5
setscrews that poke inside by 1-2mm, just enough so the
chain doesn't hit them when it's on the 42. If you do
this I think the setscrews will push the chain inside
enough to land on the 42 when downshifting.

Quoted message said:

Ingenious - but do they help with upshifts? The derailleur
cage doesn't provide much upwards hoik, because it has to
do most of its work for the 28->42 shifts.

Yes, slightly. This is on a 52-42-30 with plain rings and an
old RSX STI triple front lever with no trim adjustment. If
it was a friction front shifter, the plain rings would work
ok. With the STI, there is a limited range of adjustment
where upshifts work, the der can push the chain down to the
granny, and chain rub is tolerable. The pins improve front
upshifting enough to widen this workable range. Late model
Shimano rings also have some shorter teeth to help the
downshifting - I've been tempted to file down a couple of
teeth to imitate that.

Front STI is hardly ideal, but front shifting can be clumsy
even with a friction shifter, especially under load. I know,
it's "better" not to shift the front under load but if you
have hills it's sometimes unavoidable. The mutated
chainrings seem to help with that.

WRT the derailleur cage and upshifts, sometimes rotating the
FD about the seattube slightly helps with that. Triple
fronts can be a battle between quicker shifting and
tolerable chain rub. I'd like to see the manufacturers
improve front shifting before adding more rear cogs, but
given the degrees of freedom in any crankset/FD combination,
that may be quite hard.

Benjamin Weiner <[email hidden]> wrote: [discussion
of shifting problems on 52/42/28 Stronglight cranks]

Quoted message said:

WRT the derailleur cage and upshifts, sometimes rotating
the FD about the seattube slightly helps with that. Triple
fronts can be a battle between quicker shifting and
tolerable chain rub.

That (rotation) might be an idea. Chain rub is not so great
an issue, because the chainstays are quite long, presumably
in order to bash the stoker about a bit less over the bumps.
--
David Damerell <[email hidden]>
Distortion Field!