Seat Post Diameters
 

I'm trying to help out a friend's child, age 13, riding an almost
grown-up sized mountain bike with a sloping top tube and not enough seat
post for his size. I measure the post, with a digital caliper, at about
31.5 mm. I didn't want to take it out since it had been slipping and I
just yesterday managed to get it to stay put, but I think it's roughly 9
or 10 inches in length.

Is 31.5 an 'official' seatpost diameter? This one slips a bit, in part
because it's a slightly sloppy fit, so I'm thinking maybe 32 mm is the
right size.

If anyone wants to provide a link to what we're looking for (right
diameter, longer than what we've now got), that'd be great.

Thanks in advance.

-S-

Seat Post Diameters
 

Guy wrote:
Steve Freides wrote:

31.5 mm



31.55mm == 31.6mm

31.6 x 410mm post should yield about 12in usable length.

Very good, and thanks.

-S-

Seat Post Diameters
 

Steve Freides wrote:

I'm trying to help out a friend's child, age 13, riding an almost
grown-up sized mountain bike with a sloping top tube and not enough seat
post for his size. I measure the post, with a digital caliper, at about
31.5 mm. I didn't want to take it out since it had been slipping and I
just yesterday managed to get it to stay put, but I think it's roughly 9
or 10 inches in length.

Is 31.5 an 'official' seatpost diameter? This one slips a bit, in part
because it's a slightly sloppy fit, so I'm thinking maybe 32 mm is the
right size.

31.6 mm is the largest of the common diameters. Next size down
is 30.9 mm.

If anyone wants to provide a link to what we're looking for (right
diameter, longer than what we've now got), that'd be great.

To cure the slippage, you can't beat a double clamp. But you
need to know the outer diameter of the seat tube:

http://www.chainreactioncycles.com/M...x?ModelID=8213

You'll also need to know what length of seatpost you want.
Thudbusters are long, comfortable, and expensive:

http://www.thudbuster.com/products.html

John

Seat Post Diameters
 

On 9/6/2010 4:40 PM, Steve Freides wrote:
I'm trying to help out a friend's child, age 13, riding an almost
grown-up sized mountain bike with a sloping top tube and not enough seat
post for his size. I measure the post, with a digital caliper, at about
31.5 mm. I didn't want to take it out since it had been slipping and I
just yesterday managed to get it to stay put, but I think it's roughly 9
or 10 inches in length.

Is 31.5 an 'official' seatpost diameter? This one slips a bit, in part
because it's a slightly sloppy fit, so I'm thinking maybe 32 mm is the
right size.

If anyone wants to provide a link to what we're looking for (right
diameter, longer than what we've now got), that'd be great.

Thanks in advance.

-S-



http://www.jensonusa.com/store/produ...+Seatpost.aspx

31.6x350, $15.99

Seat posts can make you crazy, with all the slight size differences.
Also look at the clamp. Some clamps have welded on "ears" on either side
of the seat tube slot, some are collars, which can be removed from the
seat tube.

Sometimes the ear type can get bent and wind up touching before full
clamping force (or they may just be badly made). As A Muzi recommends,
lubricate (threads, cams, pivots, etc.). I lubricate between the clamp
and tube of the collar types, too.

If you have a good LBS available, that might be the easiest way to get a
good fitting seat post, a little (0.1's of a mm) too big or too small
makes for problems.

Seat Post Diameters
 

On Sep 6, 3:40*pm, "Steve Freides" wrote:
I'm trying to help out a friend's child, age 13, riding an almost
grown-up sized mountain bike with a sloping top tube and not enough seat
post for his size. *I measure the post, with a digital caliper, at about
31.5 mm. *I didn't want to take it out since it had been slipping and I
just yesterday managed to get it to stay put, but I think it's roughly 9
or 10 inches in length.

Is 31.5 an 'official' seatpost diameter? *This one slips a bit, in part
because it's a slightly sloppy fit, so I'm thinking maybe 32 mm is the
right size.

If anyone wants to provide a link to what we're looking for (right
diameter, longer than what we've now got), that'd be great.

Thanks in advance.

-S-

Probably 31.8.

Seat Post Diameters
 

Guy wrote:
Steve Freides wrote:

It was variously 31.5x, so I'm pretty sure
it's slightly-too-small 31.6.



31.6mm is to be the ID of the tube, not the OD of the post.

Sounds like ordering a post that says it's 31.6 mm is what's called for
here, so I should be good. Thanks for clarifying what's being measured.

-S-

Seat Post Diameters
 

On Sep 6, 10:19*pm, John Henderson wrote:
landotter wrote:
On Sep 6, 3:40*pm, "Steve Freides" wrote:
I'm trying to help out a friend's child, age 13, riding an almost
grown-up sized mountain bike with a sloping top tube and not enough seat
post for his size. *I measure the post, with a digital caliper, at about
31.5 mm. *I didn't want to take it out since it had been slipping and I
just yesterday managed to get it to stay put, but I think it's roughly 9
or 10 inches in length.

Is 31.5 an 'official' seatpost diameter? *This one slips a bit, in part
because it's a slightly sloppy fit, so I'm thinking maybe 32 mm is the
right size.

If anyone wants to provide a link to what we're looking for (right
diameter, longer than what we've now got), that'd be great.

Thanks in advance.

-S-

Probably 31.8.

That's the typical seat tube outer diameter for a 27.2 mm post.
31.6 mm is a post size.

31.8 is a common post size as well as bar and clamp.

Seat Post Diameters
 

Steve Freides wrote:
Guy wrote:
Steve Freides wrote:

It was variously 31.5x, so I'm pretty sure
it's slightly-too-small 31.6.



31.6mm is to be the ID of the tube, not the OD of the post.

Sounds like ordering a post that says it's 31.6 mm is what's called
for here, so I should be good. Thanks for clarifying what's being
measured.

You said the post wouldn't hold its position. Shouldn't you be measuring the
seat tube ID instead?

Seat Post Diameters
 

MikeWhy wrote:
Steve Freides wrote:
Guy wrote:
Steve Freides wrote:

It was variously 31.5x, so I'm pretty sure
it's slightly-too-small 31.6.



31.6mm is to be the ID of the tube, not the OD of the post.

Sounds like ordering a post that says it's 31.6 mm is what's called
for here, so I should be good. Thanks for clarifying what's being
measured.

You said the post wouldn't hold its position. Shouldn't you be
measuring the seat tube ID instead?

Yes, that would be a good idea, too, although I found I was able to get
it to stay put just by adjusting the QR - the young man's parents hadn't
done that, or hadn't done it quite tightly enough.

But let's ask - if the post measures 31.55 mm, is there any standard
seat tube size (the bike is a Trek low-end MTB) anywhere near that,
i.e., do they make bikes with a 33 mm ID and not 32?

-S-