geometry and sizing

[bikenut2010]

I should know this stuff but...


In these days of buying used bikes online, I'd like a little guidance on this:

I have a couple of nice steelies, both columbus tubing, one is 53 seat ct-ct and 54 top, the other is 54 seat and 54.5 top ( Colnago super ).
They are both sport frames with close clearance geometries ( i.e. no room for mudguards. )

would a similar size frame with mudguard clearance typically have a greater standover height because of the increased wheel to frame clearance , or would, as one would imagine, a mudguard friendly bike have a more relaxed geometry returning the standover height to about the same as a race frame?

perhaps there's no hard and fast rule due to the huge diversity of designs ( i'm talking about trad. horizontal top tube frames ).


Any insight would be valued.

 

[fiks]

Top tube height from the ground wouldn't change if the fork blades were longer and the position of the brake bridge higher, would it?

If the top tube and seat tube lengths and the frame angles stayed the same, then the main difference would be a shorter head tube.


Anyway IMHO top tube height from the ground, aka standover height, should not be used to size a frame. It should be the height and position of the bars relative to the bottom bracket.

 

[bikenut2010]

sizing???

Anyway IMHO top tube height from the ground, aka standover height, should not be used to size a frame. It should be the height and position of the bars relative to the bottom bracket.

That's most useful, care to expand a little? I'd like to learn more...

 

[fiks]

As long as the bars, saddle are in the right position relative to the bb, then the position and the lengths of the frame tubes (ie frame size) are largely arbitrary and is mainly down to fashion. Of course that's not to say one particular geometry will give a "better ride" or handling, but when it's a matter of 1 or 2 cm, it's it's not make a huge difference.

For example, take two bikes with the same riding position. The first frame is much smaller than the second one, but it has a much longer exposed seatpin and a longer and upward pointing stem. That would be the current fashion.

 

[bm0p700f]

I tend to do the same but think of it differently. What is important to me is the bb centre to saddle top height, the top tube + stem length and the saddle to handle bar vertical drop.

When these are to your liking you have a correctly fitted bike. Getting all this info from the internet is sometimes difficult. If the forks have been cut it is a little difficult to add length to the steerer tube without an extension bar.