Geometry and all that.

percusski

Wondering how the geometry of a frame actually affects performance, does anyone have any links to reliable info on this?
Surely with the adjustabilty of all bikes it seems the rider can be set in the optimum position on pretty much anything so why all the variation in frame shape/design and why are some designs preferable to others?

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John.T

The geometry mainly affects how a bike handles. You can usually get a good position an any frame that more or less fits you by using a seat pin with or without set back and different stem lengths. Generally the longer the wheel base the more stable the bike will be. A steeper head angle or more fork rake will make the steering more twitchy. I all depends on how you want your bike to feel. This is why you should always test ride if you can.

meagain

"The geometry mainly affects how a bike handles"....and rides I think.

Geometry is a pretty all-embracing term. Wheelbase, chainstay length, front-centre distance, head tube length etc are all linked to the angles of the tube intersects - in particular seat and head - and the fork characteristics.
The "engine" (rider) could have exactly the same position between bars and saddle and pedals on bikes with enormously different "geometries"!

percusski

So it's primarily a 'feel' thing then, can the geometry of a frame actually affect performance i.e. would a horizontal top tube make the bike quicker?

meagain

percusski wrote:

So it's primarily a 'feel' thing then, can the geometry of a frame actually affect performance i.e. would a horizontal top tube make the bike quicker?

Not sure that that is what "we" meant - feel, yes, but an extra long wheel base for example will affect "performance" (especially as related to handling - how quickly responds to steering input, say).

I can't think of any reason why a horizontal top tube (everything else being equal!) would or could make a bike quicker. On the contrary, a sloper being lighter and stiffer (everything else being equal again!) should transfer power more effectively and more effecirntly and thus be "quicker" for any given power input. I'm leaving aside the question of exposed seat post length for the more technically minded!

John.T

Every thing else being equal a sloping TT will not make much if any difference. It may make the frame a little stiffer but if the horizontal one is designed correctly there will be no difference. Things like chain stay length, front centre length (from BB to front hub), TT length and frame angles will all make more difference. Tube profile and material will also have more affect. The main thing that will make a bike quicker is the rider and their position on it with regard power output and aerodynamics.