Large tandem chainrings?

[Monkey Boy]

What is the largest combination of chainrings you can put on a
CoMotion Speedster tandem? My wife & I just purchased a tandem & we
want the largest chainrings possible.

[LittleWheelsandBig]

Do they need to clear the chainstay? Greenspeed do chainrings to 80
teeth and companies like Highpath and TA do custom chainrings but
there aren't many front derailleurs that work well with significantly
oversize chainrings.

[russell...@yahoo.com]

As already mentioned, front derailleur shifting on really large
chainrings isn't great. The outer cage of the front derailleur needs
to conform to the curve of the chainring to get good shifting.
Derailleurs are generally curved to match 53 tooth chainrings.
Another problem is the difference between the inner chainring and the
outer chainring. Assuming you are using a triple crankset. More than
30 teeth is bad. The larger the outer ring, the higher you have to
mount the front derailleur to clear the outer ring. Thus the longer
drop to the small ring. You would likely only be able to use the
largest cogs with the inner ring if the difference between inner ring
and outer ring is more than 30. This may not be too big of a problem
since you really only need the lowest gears when using the inner
ring. But with the huge drop from outer to middle to inner rings,
shifting will be bad. The chian will drop for a second or two or
three and you will be in freefall for awhile. Then hopefully the
chain will land on the ring and you will have gears again. Assuming a
triple crank with 130/74 or 110/74 bcd. And no need for really low
gears, then you can run a 32 inner ring, TA makes one, and say a 60
outer ring. This is 28 tooth difference and a front derailleur should
handle it OK.

[Dark Horse]

What's on there now? (ring size, crankset, etc)
What's the wheel size on that tandem?
Why?


Dark Horse

[LittleWheelsandBig]

On Jul 4, 2:27 am, Jan Heine <hein...@earthlink.net> wrote:

> When we rode PBP in 2003 on a tandem, our largest gear was a 48-14,
> with 650B wheels. We got to Brest in 23 hours. We rode with lots of
> other tandems at first, and nobody ever passed us on the downhills.
> We didn't spin tremendously fast - never more than 120 rpm. (Our
> maximum recorded speed was a 64 km/h or 40 mph, coasting with the
> captain in the aero tuck.)
>
> For those not interested in a fast time, large gears can provide some
> fun, but at the expense of wasting precious energy. Even then, one
> has to ask how large is large enough. If you equip a 700C tandem with
> a 53-12, you get a gear that is 33% larger than the largest we used
> in PBP. We generally coasted above 44 km/h (27.5 mph). A 53-12 would
> allow you to pedal up to 58 km/h (37 mph).

Interesting, you must have spent a lot of time either in top gear or
coasting.

A couple of friends rode the 1999 PBP on a 26" wheeled tandem and
after the first two days changed the cassette from a 13t top cog to
one with an 11t top cog because they were sick and tired of 'having to
coast every downhill'. One was part of the team that held the world-
record fleche distance (770 km, now 778 km by the French), so quite
experienced. I guess they preferred to roll the pedals over
sometimes, rather than spin quickly all the time, particularly with
the strong tailwinds that year.

[Cupcake/The Sock Monkey]

The cranks are FSA with a 53-39-30. The cassette is an 11-28. The
front derailleur is an Ultegra & the rear derailleur is an XTR. The
wheels are 700. The low gear seems pretty low and I had a tandem a
few years back and would occasionally spin out a 53x12. My wife & I
both are gear mashers and I think we could use more high and less low
gears. I would like higher gears but I don't want to make shifting
sloppy. I appreciate everyone's input.
Val

[Lisa Susan McPhate]

We have 54-42-30 FSA Gossamer cranks on our tandem with a 29-13
cassette, campy front and rear derailleur, and campy shifters. It
shifts fine. If you switch to a 54 chain ring, you would probably
want the middle ring to be a 42. We are fine with a high gear of
54/13, but we could easily change it to a 11 if desired. I have heard
of people having 55-42-30, but have no personal experience with how
those shift. The switch from 12 to 11 on its own is significant.
With the XTR derailleur, you should easily have quite a wide range of
gears and have them shift well. Tandems are inefficient climbing; I
wouldn't lose that low gear, until you prove you don't need it. We
spin out around 35 mph or so. Neither of us are mashers and I don't
like descending fast.

lisa-susan mcphate
oakland, ca

[Lisa Susan McPhate]

ps we have 700 wheels also

[Bill Gobie]

Re avoiding sloppy shifting, I recently ran across an interesting
discussion about orienting pinned & ramped chainrings for good
shifting. The gist is that you want the pickup pins on the middle and
large rings to engage the centers of the chain's outer plates. When
this happens the chain is synchronized with the larger rings' teeth
such that the first link to engage will drop smoothly into the valley
between teeth. This is why matched chainring sets have index marks
for correct installation.

If a pickup pin lifts the chain at some point other than the center of
a side plate, the chain will not be synched with the larger ring
resulting in difficult engagement on the larger ring.

The point at which a pin lifts the chain depends on the how the teeth
on the smaller ring line up with the pins on the bigger ring.
Rotating one of the rings on the spider changes that relationship.
When installing unmatched chainrings the goal is to find the
orientation that results in the greatest number of good shifts. On a
five-bolt crank there are five orientations of each ring with respect
to the next larger ring. For a given pair of rings, there might be an
orientation that gives a good shift for every pin, and conversely
there might be an orientation with no good shifts. Hopefully you will
find an orientation that gives at least one or two good shifts.
Ideally one of the good shifts should occur when the pedals are
vertical so the chain is under the least tension.

The middle of this thread is a flame war; read the first few posts
then skip to the end:

http://catrike.yuku.com/topic/3743

Bill Gobie

On Jul 4, 2009, at 5:55 PM, Cupcake/The Sock Monkey wrote: