Bike Frame Size

[Dave]

Hi,
How can I tell what size frame I need?. Also exactly what dimension on
the bike is the frame size? I thought that it was the distance between
the crank centre and the top of the seat post, but presumably the
height of the seat plus the protruding seat post are also added so I'm
wondering how critical it is as this can be adjsuted to a large
extent.

Ta in advance for any comments/suggestions,
Dave

[Clive George]

"Dave" > wrote in message
ups.com...

> How can I tell what size frame I need?. Also exactly what dimension on
> the bike is the frame size? I thought that it was the distance between
> the crank centre and the top of the seat post, but presumably the
> height of the seat plus the protruding seat post are also added so I'm
> wondering how critical it is as this can be adjsuted to a large
> extent.

Bad news : It's all a bit random these days! Best way is to try one and see
if it fits.

However you can get into the right sort of area by looking at what a
manufacturer makes, considering how large you are compared to average, and
choosing something which seems appropriate.

What you want also depends on what sort of bike you're after - eg I'd have a
larger framed road bike than MTB.

The frame size is crank centre to top of seat tube, not seat post. But
obviously with a sloping top tube this measurement becomes less useful. The
important one is actually the bike length - saddle to handlebars - and
height at the front (gives you a range of handlebar height - again, what
this is depends on you, with a TT racer having very low bars, and a dutch
rider (and Nick Mclaren :-) ) having very high ones.).

We can probably give more advice with a bit more information : how big are
you, how are your legs proportioned compared to your body, how much
experience do you have, what sort of riding do you want to to.

cheers,
clive

[Paul Boyd[_2_]]

On 10/10/2007 19:09, Dave said,

> How can I tell what size frame I need?

Clive has summed it up quite neatly so I'll just second it - you really
need to try the bike out as there is more to frame size than just the
distance from BB axle to nominal top tube centre. Reach is the most
significant, and simply changing the stem is not always the best answer
as it can adversely affect the handling - best to get the top tube the
right length to start with.

What sort of bike are you looking at? MTB? Tourer? Commuter?

--
Paul Boyd
http://www.paul-boyd.co.uk/

[Pete Biggs]

Dave wrote:
> Hi,
> How can I tell what size frame I need?. Also exactly what dimension on
> the bike is the frame size? I thought that it was the distance between
> the crank centre and the top of the seat post, but presumably the
> height of the seat plus the protruding seat post are also added so I'm
> wondering how critical it is as this can be adjsuted to a large
> extent.

It's sometimes bottom bracket centre* to top of seat tube (known as "centre
to top"); sometimes bottom bracket centre to centre of junction with top
tube (known as "centre to centre"). Sloping top tubes complicate things
further.

Although you get to know what bikes are like as a whole from this (more or
less), you are right to think it's not the most crucial measurement. So
also check the top tube length as this mainly governs how far forwards you
will have to lean (although this can be tuned to some extent by changing the
handlebar stem).

Try the bike in a shop if you can, otherwise be prepared for the expense of
changing or selling the bike if it doesn't fit.

* probably what you call "crank centre"

~PB

[spokes[_2_]]

How can I tell what size frame I need?.

The traditional answer to this question would probably be to take your
inside leg measurement - bare foot to crotch bone, then take off 9", or
simply divide your height by three. (Or do both and split the difference.)
Fairly crude rules of thumb, and they apply only to traditional road bikes -
not MTBs, hybrids or whatever - but if a road bike is what you're after,
they give you a reasonable starting point.
>

[Rob Morley]

In article >, spokes
says...
> How can I tell what size frame I need?.
>
> The traditional answer to this question would probably be to take your
> inside leg measurement - bare foot to crotch bone, then take off 9", or
> simply divide your height by three. (Or do both and split the difference.)
> Fairly crude rules of thumb, and they apply only to traditional road bikes -
> not MTBs, hybrids or whatever - but if a road bike is what you're after,
> they give you a reasonable starting point.
> >
>
According to that I'd want a 24" frame while actually I wouldn't want to
be on anything larger than 23"
A quick Google finds this http://www.bikebuster.com/frame-guide.html
which gives a better indication of where to start, but still doesn't
account for reach or variations in frame styles.

Of course Sheldon has something to say about it
http://www.sheldonbrown.com/frame-sizing.html

[spokes[_4_]]

"Rob Morley" > wrote in message
t...
> In article >, spokes
> says...
>> How can I tell what size frame I need?.
>>
>> The traditional answer to this question would probably be to take your
>> inside leg measurement - bare foot to crotch bone, then take off 9", or
>> simply divide your height by three. (Or do both and split the
>> difference.)
>> Fairly crude rules of thumb, and they apply only to traditional road
>> bikes -
>> not MTBs, hybrids or whatever - but if a road bike is what you're after,
>> they give you a reasonable starting point.
>> >
>>
> According to that I'd want a 24" frame while actually I wouldn't want to
> be on anything larger than 23"
> A quick Google finds this http://www.bikebuster.com/frame-guide.html
> which gives a better indication of where to start, but still doesn't
> account for reach or variations in frame styles.
>
> Of course Sheldon has something to say about it
> http://www.sheldonbrown.com/frame-sizing.html

Well, like I say, it's a fairly crude guide, but I found it on a site
onetime and it worked for me (34" I.L., 25" frame - perfick!)

[Rob Morley]

In article >, spokes
says...
>
> Well, like I say, it's a fairly crude guide, but I found it on a site
> onetime and it worked for me (34" I.L., 25" frame - perfick!)
>
I had a 25" frame once, and while I could ride it comfortably and safely
it was a revelation when I switched to a 23" frame with a longer top
tube - that was on a hack bike that I built up while the 'good' one was
being refurbished, and almost instantly I preferred it to the larger
one.

[[email protected]]

On Oct 11, 10:30 am, Rob Morley > wrote:
> In article >, spokes
> says...
>
> > Well, like I say, it's a fairly crude guide, but I found it on a site
> > onetime and it worked for me (34" I.L., 25" frame - perfick!)
>
> I had a 25" frame once, and while I could ride it comfortably and safely
> it was a revelation when I switched to a 23" frame with a longer top
> tube - that was on a hack bike that I built up while the 'good' one was
> being refurbished, and almost instantly I preferred it to the larger
> one.

I have 31" IL, which would give a 22" frame, but as I'm 6' 2", I think
this would be a little small. One web site said I should get an L
frame based on leg length but an XXL based on height. Finding the
right top tube length is a bit of a mystery to me, specific advice for
saddle height is easy to find, I've never found anything very specific
on top tube. My current bike seems OK, but I can't help wondering if I
would be more comfortable on something with different proportions.

Rob

[Nigel Cliffe]

Rob Morley wrote:
> In article >, spokes
> says...
>> How can I tell what size frame I need?.
>>
>> The traditional answer to this question would probably be to take
>> your inside leg measurement - bare foot to crotch bone, then take
>> off 9", or simply divide your height by three. (Or do both and split
>> the difference.) Fairly crude rules of thumb, and they apply only to
>> traditional road bikes - not MTBs, hybrids or whatever - but if a
>> road bike is what you're after, they give you a reasonable starting
>> point.
>>>
>>
> According to that I'd want a 24" frame while actually I wouldn't want
> to be on anything larger than 23"
> A quick Google finds this http://www.bikebuster.com/frame-guide.html
> which gives a better indication of where to start, but still doesn't
> account for reach or variations in frame styles.

The old methods (which give you 24in) were for racing/touring frames of
decades ago. More modern frames adopt a different approach; they tend to
have taller stems and much taller seatposts, and dropped/sloping top-tubes
make things much more complicated. And it totally ignores frame length.

To be honest, though it may cost, I think its worth getting someone to
measure you and work out an ideal relationship of pedal, bars and saddle for
your body. Use that to dictate the frame size within the adjustment
possible by swapping stems/seatposts/etc..


I've found all my bikes to be more comfortable since taking delivery of a
custom made bike. I was measured carefully for that bike and found it
instantly very comfortable.
So, I adjusted my other bike to have the same dimensions, starting at saddle
height and lay-back from centreline of pedal crank, then handlebars w.r.t.
saddle. End result was improved comfort.


- Nigel

--
Nigel Cliffe,
Webmaster at http://www.2mm.org.uk/

[spokes[_4_]]

Just found this - might be of use...

http://www.downlandcycles.co.uk/bespokeFitting.htm

>

[Rob Morley]

In article m>,
says...
> On Oct 11, 10:30 am, Rob Morley > wrote:

> > I had a 25" frame once, and while I could ride it comfortably and safely
> > it was a revelation when I switched to a 23" frame with a longer top
> > tube - that was on a hack bike that I built up while the 'good' one was
> > being refurbished, and almost instantly I preferred it to the larger
> > one.
>
> I have 31" IL, which would give a 22" frame, but as I'm 6' 2", I think
> this would be a little small. One web site said I should get an L
> frame based on leg length but an XXL based on height. Finding the
> right top tube length is a bit of a mystery to me, specific advice for
> saddle height is easy to find, I've never found anything very specific
> on top tube. My current bike seems OK, but I can't help wondering if I
> would be more comfortable on something with different proportions.
>
A basic top tube rule of thumb for road bikes: when the saddle is in the
right position, place your elbow against the nose - your outstretched
fingers should just touch the bars. As with all these oversimplified
'rules' it ignores most of the variables - upper/lower arm ratio,
arm/trunk ratio, saddle length, and I find the suggested length too
small - my ATB and tandem are about 2.5" longer, and my road bike about
3.5".

Frames are now generally built so that length varies with height[1], but
they still only cater for average proportions. You have a smaller
leg/trunk ratio than average, so you'll have to compromise when choosing
a stock frame size. Assuming it's a road bike go for the right length
as long as you have standover clearance. If the cranks are
proportionately sized (e.g. 170mm for L, 175mm for XXL) you might also
want to fit shorter ones, although that depends on your thigh/shin ratio
- larger and you'll sit further back/lower and longish cranks will be
OK, smaller you'll sit further forward/higher and shorter cranks will be
better. Fore/aft saddle position is determined by seat tube angle and
the particular seatpost and saddle - make sure there is sufficient
adjustment available. Generally the bony bit just inside/behind your
kneecap shouldn't be forward of the pedal spindle when the crank is
fully forward.


[1] Back in the good ol' days a 25" frame would have the same top tube
length as a 21" frame, and women wanting diamond frames were SOL because
they generally have longer legs and shorter trunks than men of the same
height.

[Rob Morley]

In article >, Nigel Cliffe
says...

> The old methods (which give you 24in) were for racing/touring frames of
> decades ago. More modern frames adopt a different approach; they tend to
> have taller stems and much taller seatposts, and dropped/sloping top-tubes
> make things much more complicated. And it totally ignores frame length.

I know.
>
> To be honest, though it may cost, I think its worth getting someone to
> measure you and work out an ideal relationship of pedal, bars and saddle for
> your body. Use that to dictate the frame size within the adjustment
> possible by swapping stems/seatposts/etc..
>
Changing the stem length can have unwanted results in handling.
>
> I've found all my bikes to be more comfortable since taking delivery of a
> custom made bike. I was measured carefully for that bike and found it
> instantly very comfortable.
> So, I adjusted my other bike to have the same dimensions, starting at saddle
> height and lay-back from centreline of pedal crank, then handlebars w.r.t.
> saddle. End result was improved comfort.
>
It's not difficult to work it out yourself with a bit of research - you
just have to use more elaborate rules than inseam - 9" or height / 3

[Simon Brooke]

in message om>, Dave
') wrote:

> Hi,
> How can I tell what size frame I need?. Also exactly what dimension on
> the bike is the frame size? I thought that it was the distance between
> the crank centre and the top of the seat post, but presumably the
> height of the seat plus the protruding seat post are also added so I'm
> wondering how critical it is as this can be adjsuted to a large
> extent.

Sit on the bike and try it.

Twenty years ago all bike builders used more or less the same geometry, so
a 20" bike from one maker was the same size as a 20" bike from another.
That simply isn't true any more. There are no simple universal
measurements. The only way to know whether it's the right size is to try
it.

--
(Simon Brooke) http://www.jasmine.org.uk/~simon/

;; Madness takes its toll. Please have exact change.