Frame Geometry

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gilespargiter

Veteran
Location
N Wales
Far as I can see the only thing a sloping top tube does is increase the standover height. Although their are claims that it makes the drive to the chainwheel more rigid, thus improving efficiency. It certainly does not lower the centre of gravity. The usual BB height is 10 1/2" - so if you have the correct saddle height for you - no difference. It is regarded as to dangerous to lower the BB any further because of cornering. It does not shorten the wheelbase by shortening the rear triangle - this is limited by the rear wheel to seat tube clearance, or if running wide wheels, the inner chainwheel to rear wheel clearance - hence the "repopularity" of 26" wheels I guess.
The wheelbase is altered by a balance between the seat tube angle, the headset angle and the length of the top tube and to a small extent by fork rake. Short wheelbase= (at the extremes) responsive, lively - twitchy, hard ride, and often no toe clip to wheel clearance. long wheelbase= (at the extremes)slower to respond,(easier straight line) smoother ride, - mushy handling.
So, in short it is the present fashion.
 

screenman

Legendary Member
Or economic sense. 3 sizes fits all with a touch of tweaking of components.
 
The only difference between a 'regular'/'square'/horizontal-top-tube frame and a compact frame is a shorter seat tube. Angles, wheelbase and top-tube length (which now becomes 'effective top tube', ie the distance between the seat and head tubes measured horizontally) all remain the same. A compact frame requires a longer seat post.

The benefits are:
A reduction in wind resistance. Introducing an aero profile in an area of high pressure - between the thighs - has a measureable effect on aerodynamic drag. A compact frame maximises the length of the aero section. This was the primary goal behind the design of compact frames. Everything after this is a bonus.

An increase in frame stiffness. A happy side effect of the compact frame is that it drops the top tube down to bring it more in line with the major tortional loads through the frame from head to rear drop-outs.
Slightly less weight. Shorter seat, top and seat stays reduce weight (a longer seat post consumes some of this loss but it's lighter overall).
Greater comfort. The longer seat post can be tuned to deliver a degree of flex.
Greater stand-over clearance. It's a happy side benefit.
In Giant's model, fewer frame sizes can be offered, reducing costs to the consumer.

All very straightforward, but not helped by the fact that, in the beginning at least, many manufacturers didn't understand what it was about.

It's not a new idea:

Bicycles of the 1890\'s01.jpg
 
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