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Correct Top Tube vs. Correct Frame Size



 
 
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  #1  
Old July 28th 05, 01:52 AM
RP10128
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Default Correct Top Tube vs. Correct Frame Size

I need a bike with a top tube of about 50.5 cm. To get a top tube of
that length, I need to get a frame that is one or two sizes too small.
That is, I would have to get a 49 cm. frame, whereas my inseam would
indicate a 52 cm. frame. My LBS says that it doesn't matter that the
frame is so small, but this strikes me as being counter-intuitive. Is
the LBS right? Should I get the correct size frame and install a very
short stem?
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  #2  
Old July 28th 05, 02:03 AM
tntcoach
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Default Correct Top Tube vs. Correct Frame Size

I would say the LBS is mostly correct. The most important part of the
fit is your leg extension and knee position. As long as your seatpost
extends high enough for proper leg extension and your saddle position
and arm reach places your knee plumb with the pedal axle at 3 & 9
position you should be alright. If your stretched out reaching for the
bars your going to have back problems. If you are sitting too far
foward and your knee is forward of your pedal then your going to have
serious knee problems. My suggestion is to fit the bike to your torso
and legs first, then adjust for reach. If you don't trust your LBS go
to a different shop or google "bike fit".

Good luck.

-Michael

  #3  
Old July 28th 05, 02:06 AM
Dave Thompson
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Default Correct Top Tube vs. Correct Frame Size

RP10128 wrote:
I need a bike with a top tube of about 50.5 cm. To get a top tube of
that length, I need to get a frame that is one or two sizes too small.
That is, I would have to get a 49 cm. frame, whereas my inseam would
indicate a 52 cm. frame. My LBS says that it doesn't matter that the
frame is so small, but this strikes me as being counter-intuitive. Is
the LBS right? Should I get the correct size frame and install a very
short stem?

No, a short stem makes for a bike that doesn't handle as well as it should.
The primary measurement for a bike *is* the top tube. A taller seat post can
make up for the frame size otherwise. Also, as you get more fit and in
better condition, you will have a tendency to want to get *longer*, that is
be more stretched out.

How did you arrive at the 50.5 dimension for the top tube?


  #4  
Old July 28th 05, 02:19 AM
RP10128
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Posts: n/a
Default Correct Top Tube vs. Correct Frame Size

"Dave Thompson" wrote in
:

RP10128 wrote:
I need a bike with a top tube of about 50.5 cm. To get a top tube

of
that length, I need to get a frame that is one or two sizes too
small. That is, I would have to get a 49 cm. frame, whereas my

inseam
would indicate a 52 cm. frame. My LBS says that it doesn't matter
that the frame is so small, but this strikes me as being
counter-intuitive. Is the LBS right? Should I get the correct

size
frame and install a very short stem?

No, a short stem makes for a bike that doesn't handle as well as it
should. The primary measurement for a bike *is* the top tube. A

taller
seat post can make up for the frame size otherwise. Also, as you

get
more fit and in better condition, you will have a tendency to want

to
get *longer*, that is be more stretched out.

How did you arrive at the 50.5 dimension for the top tube?




In two ways. First, I have been riding my current bike (a 52 cm.
Trek) with the saddle slid all the way forward, and with the nut that
secures the seat post clamp slid all the way forward. With the
saddle in that position, the distance from the mid point of the
saddle to the center of the head tube is about 50 cm. Second, I feel
that the top tube of my current bike (which has a top tube of 52.3
cm.) is about 2 cm. too long.

  #5  
Old July 28th 05, 02:26 AM
RP10128
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Posts: n/a
Default Correct Top Tube vs. Correct Frame Size

"tntcoach" wrote in
oups.com:

I would say the LBS is mostly correct. The most important part of

the
fit is your leg extension and knee position. As long as your

seatpost
extends high enough for proper leg extension and your saddle

position
and arm reach places your knee plumb with the pedal axle at 3 & 9
position you should be alright. If your stretched out reaching for
the bars your going to have back problems. If you are sitting too

far
foward and your knee is forward of your pedal then your going to

have
serious knee problems. My suggestion is to fit the bike to your

torso
and legs first, then adjust for reach. If you don't trust your LBS

go
to a different shop or google "bike fit".

Good luck.

-Michael


You suggest that I fit the bike to my torso and legs first, and then
adjust for reach. But do you mean that I should get the smaller
frame with the shorter top tube, or buy the larger frame and then
install a very short stem?
  #6  
Old July 28th 05, 02:44 AM
Mark Hickey
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Posts: n/a
Default Correct Top Tube vs. Correct Frame Size

RP10128 wrote:

I need a bike with a top tube of about 50.5 cm. To get a top tube of
that length, I need to get a frame that is one or two sizes too small.
That is, I would have to get a 49 cm. frame, whereas my inseam would
indicate a 52 cm. frame. My LBS says that it doesn't matter that the
frame is so small, but this strikes me as being counter-intuitive. Is
the LBS right? Should I get the correct size frame and install a very
short stem?


Where it comes to fit, the only thing that matters is the relative
position of three points...

1) Saddle
2) Crank
3) Bars

Now obviously, it's possible to get the above items right without
having a bike that's a joy to ride (think "really, really long stem").

In your case, if you can get the three "fit points" right with a stem
that's no longer than 10-11cm at the most, INCLUDING getting the bar
HEIGHT right, then you should be pretty good to go. The only real
down side to riding a "shorter" (vertically) frame is that the bar
height RANGE is lower. If you don't need the bars up around the level
of the seat and/or can live with lots o' spacers and/or a stem with
considerable rise, the "shorter" frame should work.

A tool to help you work out the stem issue:

http://www.habcycles.com/fitting.html

Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $795 ti frame
  #7  
Old July 28th 05, 07:20 AM
Ken
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Posts: n/a
Default Correct Top Tube vs. Correct Frame Size

RP10128 wrote in news:Xns96A0D45BAB0C5richardpu2aolcom@
216.196.97.131:
I need a bike with a top tube of about 50.5 cm. To get a top tube of
that length, I need to get a frame that is one or two sizes too small.
That is, I would have to get a 49 cm. frame, whereas my inseam would
indicate a 52 cm. frame. My LBS says that it doesn't matter that the
frame is so small, but this strikes me as being counter-intuitive.


Top tube is more important, but if your seattube is too small then (depending
on frame geometry), your head tube may be very low. If you want your
handlebars several inches below saddle height, that may not be a problem. If
you want a more upright riding position, then you may have fit or handling
issues.
  #8  
Old July 28th 05, 09:58 AM
Ron Ruff
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Posts: n/a
Default Correct Top Tube vs. Correct Frame Size

RP10128 wrote:
I need a bike with a top tube of about 50.5 cm. To get a top tube of
that length, I need to get a frame that is one or two sizes too small.
That is, I would have to get a 49 cm. frame, whereas my inseam would
indicate a 52 cm. frame. My LBS says that it doesn't matter that the
frame is so small, but this strikes me as being counter-intuitive. Is
the LBS right? Should I get the correct size frame and install a very
short stem?


Mark Hickey had some good advice above, but I just wanted to add that
seat tube angle can also vary between frames, and it's good to consider
this in addition to the top tube length. A 50.5cm top tube on a frame
with a 74 degree ST angle is not quite the same as one with a 73 degree
ST angle. If you put your seat in the same position relative to the
bottom bracket on both, then the reach to the bars will be about 1 cm
longer on the 74 degree frame (since you will need to slide the seat
farther back on the rails to get it in the same position).

Anyway, it's a very good idea to get properly fitted... there are lots
of online calculators and articles, and/or you could get it done at a
shop.

I just noticed that the price of a Habanero went up $100! Hasn't it
been $695 since the beginning? Maybe the recent currency value changes
in China had something to do with this...

-Ron

  #9  
Old July 28th 05, 01:16 PM
Pete
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Default Correct Top Tube vs. Correct Frame Size


"RP10128" wrote in message
31...
I need a bike with a top tube of about 50.5 cm. To get a top tube of
that length, I need to get a frame that is one or two sizes too small.
That is, I would have to get a 49 cm. frame, whereas my inseam would
indicate a 52 cm. frame. My LBS says that it doesn't matter that the
frame is so small, but this strikes me as being counter-intuitive. Is
the LBS right? Should I get the correct size frame and install a very
short stem?


Keep in mind that the smaller the frame, the lower the handlebars will be.
For example, you probably wouldn't be able to get them anywhere near saddle
level, because the saddle will be that much higher in relation to the frame.
Pete


  #10  
Old July 28th 05, 01:47 PM
Qui si parla Campagnolo
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Posts: n/a
Default Correct Top Tube vs. Correct Frame Size



RP10128 wrote:
I need a bike with a top tube of about 50.5 cm. To get a top tube of
that length, I need to get a frame that is one or two sizes too small.
That is, I would have to get a 49 cm. frame, whereas my inseam would
indicate a 52 cm. frame. My LBS says that it doesn't matter that the
frame is so small, but this strikes me as being counter-intuitive. Is
the LBS right? Should I get the correct size frame and install a very
short stem?


Problem with a too small frame is the head tube is short and then the
hbars will be very low. To raise to a ridable height, need a lots of
rise stem or lots of spacers under the stem or something.

Frames from different makers are all different. perhaps a custom?

http://www.waterfordbikes.com

RS-14 series is only custom, made to order.

 




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