Seatpost sizing

Gary Brunsdon said:

Ive just bought a secondhand Dawes Galaxy tandem 'Galaxy Twin tour
series 97' - and I need to change the seatposts that arent marked with
the size? I've measure them with callipers and they appear to be 26.2 ish?

Wondered if this sounded right? - or if anyone knew how/ where to
confirm this or not?

Try here http://www.sheldonbrown.com/seatpost_sizes.html

Regards

Alan

Gary Brunsdon said:

Ive just bought a secondhand Dawes Galaxy tandem 'Galaxy Twin tour
series 97' - and I need to change the seatposts that arent marked with
the size? I've measure them with callipers and they appear to be 26.2 ish?

Wondered if this sounded right? - or if anyone knew how/ where to
confirm this or not?

I can't confirm that that is the intended seatpost size for your tandem,
but 26.2 is a common-ish seatpost size. If you're competent enough with
callipers to measure to an accuracy of .1mm and your existing seatpost
fits then you've got it right. If you'd have said 26.1 or 26.3 then
those are not common standard seatpost sizes (they tend to come in .2mm
increments).

JimP

--
Let's think the unthinkable, let's do the undoable, let's prepare to
grapple with the ineffable itself, and see if we may not eff it after
all. - DNA

Jim Price said:

Gary Brunsdon said:

Ive just bought a secondhand Dawes Galaxy tandem 'Galaxy Twin tour
series 97' - and I need to change the seatposts that arent marked with
the size? I've measure them with callipers and they appear to be 26.2
ish?

Wondered if this sounded right? - or if anyone knew how/ where to
confirm this or not?

I can't confirm that that is the intended seatpost size for your tandem,
but 26.2 is a common-ish seatpost size. If you're competent enough with
callipers to measure to an accuracy of .1mm and your existing seatpost
fits then you've got it right. If you'd have said 26.1 or 26.3 then
those are not common standard seatpost sizes (they tend to come in .2mm
increments).

JimP

I think I'm ok with the Callipers and theyre quite good ones so I guess
accurate.

One thing that did suprise me though - the front does seem to be 26.2,
whereas the stokers tube is 26.8 - but I guees I shoulddnt be too
suprised as I suppose a tandem is built out of different size tubes to
allow for extra strenghth where its needed?

Gary Brunsdon said:

I think I'm ok with the Callipers and theyre quite good ones so I guess
accurate.

One thing that did suprise me though - the front does seem to be 26.2,
whereas the stokers tube is 26.8 - but I guees I shoulddnt be too
suprised as I suppose a tandem is built out of different size tubes to
allow for extra strenghth where its needed?

You're right, you shouldn't be too surprised. It's normal to put the
heaviest rider at the front on a tandem, but seatposts down to 23.8mm
are easily strong enough for most people so there is no need for the
front seatpost to be bigger than the rear.

JimP

--
Let's think the unthinkable, let's do the undoable, let's prepare to
grapple with the ineffable itself, and see if we may not eff it after
all. - DNA

"Jim Price" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Gary Brunsdon said:

I think I'm ok with the Callipers and theyre quite good ones so I guess
accurate.

One thing that did suprise me though - the front does seem to be 26.2,
whereas the stokers tube is 26.8 - but I guees I shoulddnt be too
suprised as I suppose a tandem is built out of different size tubes to
allow for extra strenghth where its needed?

You're right, you shouldn't be too surprised. It's normal to put the
heaviest rider at the front on a tandem, but seatposts down to 23.8mm are
easily strong enough for most people so there is no need for the front
seatpost to be bigger than the rear.

Which will be better at resisting twisting, bigger or smaller? The front
seatpost has the stoker hauling on their handlebars to contend with.

cheers,
clive

Clive George said:

Which will be better at resisting twisting, bigger or smaller? The
front seatpost has the stoker hauling on their handlebars to contend
with.

I would have thought bigger when all else is equal. It depends on wall
thickness as well as diameter, which does vary from one seatpost (and
frame tube) to another.

For instance, ITM Big One 25.0 seatpost has a thicker wall than the same
model in 27.2.

~PB

Clive George said:

"Jim Price" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Gary Brunsdon said:

I think I'm ok with the Callipers and theyre quite good ones so I
guess accurate.

One thing that did suprise me though - the front does seem to be
26.2, whereas the stokers tube is 26.8 - but I guees I shoulddnt be
too suprised as I suppose a tandem is built out of different size
tubes to allow for extra strenghth where its needed?

You're right, you shouldn't be too surprised. It's normal to put the
heaviest rider at the front on a tandem, but seatposts down to 23.8mm
are easily strong enough for most people so there is no need for the
front seatpost to be bigger than the rear.

Which will be better at resisting twisting, bigger or smaller? The front
seatpost has the stoker hauling on their handlebars to contend with.

Prolly not much of an issue given the number of years people have been
relying on 22.2mm or thereabouts tubular stems to mount handlebars onto
bicycles, plus the forces of the pilot on the seatpost are not really
lined up with the twisting forces from the stoker in the sort of way
which would lead to a beefed up part being needed. You wouldn't want to
go and put an expensive stupid-light racing seatpost on the back, though.

JimP

--
Let's think the unthinkable, let's do the undoable, let's prepare to
grapple with the ineffable itself, and see if we may not eff it after
all. - DNA

"Jim Price" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Clive George said:

"Jim Price" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Gary Brunsdon wrote:
> I think I'm ok with the Callipers and theyre quite good ones so I guess
> accurate.
>
> One thing that did suprise me though - the front does seem to be 26.2,
> whereas the stokers tube is 26.8 - but I guees I shoulddnt be too
> suprised as I suppose a tandem is built out of different size tubes to
> allow for extra strenghth where its needed?

You're right, you shouldn't be too surprised. It's normal to put the
heaviest rider at the front on a tandem, but seatposts down to 23.8mm
are easily strong enough for most people so there is no need for the
front seatpost to be bigger than the rear.

Which will be better at resisting twisting, bigger or smaller? The front
seatpost has the stoker hauling on their handlebars to contend with.

Prolly not much of an issue given the number of years people have been
relying on 22.2mm or thereabouts tubular stems to mount handlebars onto
bicycles, plus the forces of the pilot on the seatpost are not really
lined up with the twisting forces from the stoker in the sort of way which
would lead to a beefed up part being needed. You wouldn't want to go and
put an expensive stupid-light racing seatpost on the back, though.

Stoker can put far more force through their bars than a front handlebar,
simply because their bars are fixed rather than being on a moveable wheel.
Think how easy it is to move handlebars mounted with that 22.2 mm tubular
stem if you hold the wheel still.

Re strength - yes, don't use CF seatposts for captains.

cheers,
clive

Clive George said:

"Jim Price" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Clive George said:

"Jim Price" <[email hidden]> wrote in message
news:[email hidden]...
> Gary Brunsdon wrote:
>> I think I'm ok with the Callipers and theyre quite good ones so I
>> guess accurate.
>>
>> One thing that did suprise me though - the front does seem to be
>> 26.2, whereas the stokers tube is 26.8 - but I guees I shoulddnt be
>> too suprised as I suppose a tandem is built out of different size
>> tubes to allow for extra strenghth where its needed?
>
> You're right, you shouldn't be too surprised. It's normal to put the
> heaviest rider at the front on a tandem, but seatposts down to
> 23.8mm are easily strong enough for most people so there is no need
> for the front seatpost to be bigger than the rear.

Which will be better at resisting twisting, bigger or smaller? The
front seatpost has the stoker hauling on their handlebars to contend
with.

Prolly not much of an issue given the number of years people have been
relying on 22.2mm or thereabouts tubular stems to mount handlebars
onto bicycles, plus the forces of the pilot on the seatpost are not
really lined up with the twisting forces from the stoker in the sort
of way which would lead to a beefed up part being needed. You wouldn't
want to go and put an expensive stupid-light racing seatpost on the
back, though.

Stoker can put far more force through their bars than a front handlebar,
simply because their bars are fixed rather than being on a moveable
wheel. Think how easy it is to move handlebars mounted with that 22.2 mm
tubular stem if you hold the wheel still.

Indeed, I've even managed to bend them in accidents. I've not been able
to straighten them afterwards, though. However, I would have thought
that you could protect the captain's seatpost from such excessive force
by using a smaller diameter stem to connect the stoker's bars. That way
you control where any flexing occurs.

What sort of sizes get used on real world tandems?

JimP

--
Let's think the unthinkable, let's do the undoable, let's prepare to
grapple with the ineffable itself, and see if we may not eff it after
all. - DNA

"Jim Price" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Quoted message said:

Quoted message said:

Prolly not much of an issue given the number of years people have been
relying on 22.2mm or thereabouts tubular stems to mount handlebars onto
bicycles, plus the forces of the pilot on the seatpost are not really
lined up with the twisting forces from the stoker in the sort of way
which would lead to a beefed up part being needed. You wouldn't want to
go and put an expensive stupid-light racing seatpost on the back,
though.

Stoker can put far more force through their bars than a front handlebar,
simply because their bars are fixed rather than being on a moveable
wheel. Think how easy it is to move handlebars mounted with that 22.2 mm
tubular stem if you hold the wheel still.

Indeed, I've even managed to bend them in accidents. I've not been able to
straighten them afterwards, though. However, I would have thought that you
could protect the captain's seatpost from such excessive force by using a
smaller diameter stem to connect the stoker's bars. That way you control
where any flexing occurs.

Eh? That'd just be all floppy, surely? (Fortunately it isn't actually
necessary - on the whole, things seem to work as they are.)

Quoted message said:

What sort of sizes get used on real world tandems?

Same as single bikes. Ours are all 27.2mm. I believe Mr Thorn's products
tend to be too.

cheers,
clive

Clive George said:

"Jim Price" <[email hidden]> wrote in message

Quoted message said:

Indeed, I've even managed to bend them in accidents. I've not been
able to straighten them afterwards, though. However, I would have
thought that you could protect the captain's seatpost from such
excessive force by using a smaller diameter stem to connect the
stoker's bars. That way you control where any flexing occurs.

Eh? That'd just be all floppy, surely? (Fortunately it isn't actually
necessary - on the whole, things seem to work as they are.)

I'm not thinking a lot smaller, just a tiny bit so that the stem flexes
before the seatpost, so in a disaster scenario you'd hope the stem would
bend or break first, thus only taking out one of the riders. e.g. for
the OP's 26.2 seatpost, you'd use a 25.4 stem (equivalent to a threaded
1 1/8 threaded MTB stem).

JimP

--
Let's think the unthinkable, let's do the undoable, let's prepare to
grapple with the ineffable itself, and see if we may not eff it after
all. - DNA

Jim Price said:

Clive George said:

"Jim Price" <[email hidden]> wrote in message

Quoted message said:

Indeed, I've even managed to bend them in accidents. I've not been
able to straighten them afterwards, though. However, I would have
thought that you could protect the captain's seatpost from such
excessive force by using a smaller diameter stem to connect the
stoker's bars. That way you control where any flexing occurs.

Eh? That'd just be all floppy, surely? (Fortunately it isn't actually
necessary - on the whole, things seem to work as they are.)

I'm not thinking a lot smaller, just a tiny bit so that the stem flexes
before the seatpost, so in a disaster scenario you'd hope the stem would
bend or break first, thus only taking out one of the riders. e.g. for
the OP's 26.2 seatpost, you'd use a 25.4 stem (equivalent to a threaded
1 1/8 threaded MTB stem).

JimP

Thanks for the info - I have just recieved an email back from Thorn
Cycles which expresses great suprise that the tube sizes would be
different. The guy reckons that he has never ever come across a dawes
tandem that has that?

Also - he suggests that dawes tandems from that era normally have 26.8
or 27 tubes.

So - I've had another look at my setup.

The existing 26.2 seatpost in the front, although fitting in, does seem
to wobble a little from side to side - before its clamped.

The 26.8 in the rear seems to fit really well.

If you try the 26.8 in the front, it wont fit in at all..

So - I'm still confused - but now feel that maybe taking the bike to my
local bike shop would be best? (Ken Apps cycles in Hastings.)

Do you know what they'd do in order to assertain the right post? -
specialist tools? ....range of tube sizes to try?

Gary Brunsdon said:

Thanks for the info - I have just recieved an email back from Thorn
Cycles which expresses great suprise that the tube sizes would be
different. The guy reckons that he has never ever come across a dawes
tandem that has that?

Also - he suggests that dawes tandems from that era normally have 26.8
or 27 tubes.

So - I've had another look at my setup.

The existing 26.2 seatpost in the front, although fitting in, does seem
to wobble a little from side to side - before its clamped.

The 26.8 in the rear seems to fit really well.

If you try the 26.8 in the front, it wont fit in at all..

So - I'm still confused - but now feel that maybe taking the bike to my
local bike shop would be best? (Ken Apps cycles in Hastings.)

Do you know what they'd do in order to assertain the right post? -
specialist tools? ....range of tube sizes to try?

Well, I suppose if you measure the external diameters of the seat tubes
and they are the same then something might be up. Also, check that the
vertical slot which allows the seatpost collar to tighten has parallel
sides - if not it may be deformed. A visual check for roundness of the
hole would also be advisable - see if there are any gaps with a seatpost
in place. If there is a small out of roundness, that can be corrected
with a seat tube reamer, but you shouldn't get too carried away or you
will seriously weaken the seat tube. If as you say the existing seat
tube wobbles from side to side but not by the same amount in other
directions, there is probably something wrong, but it should be fixable
if the frame is steel.

Do you have the facility to take digital pictures and post them
somewhere you can link to? If so, take a closeup of the slot and one
from above each side of the seatpost entry.

JimP

--
Let's think the unthinkable, let's do the undoable, let's prepare to
grapple with the ineffable itself, and see if we may not eff it after
all. - DNA