Bike frame size/geometry question.

keirtristan

I've been thinking about this for some time now, how does the frame size of a biek affect it's geometry?

When the tyres are the same size and the front fork is the same size on all sized frames, how does that work? What adjustments need to be made and where to ensure they retain their geometry?

robertpb

Just think of it as a big "V" as you extend the seat and down tube the bike gets taller and the crossbar gets longer.

The head tube stays in the same place at its base but grows with the frame.

That's it in a simple way.

keirtristan

Does the down tube change as well or does that stay the same length?

GHill

keirtristan wrote:

Does the down tube change as well or does that stay the same length?

It has to change or else you'd end up with the fork pointing under the bike

Usually they try to keep the head angle and seat tube angle relatively constant, that means a longer wheelbase along with longer reach and usually a taller bike. I have no doubt that some companies do things differently.

robertpb

Either, it depends on how much longer you you want to make the frame, but the angle between it and the seat tube changes as it's extended.

You will notice that the seat tube on MTB frames may grow by 100mm but the head tube by only 30mm. The increase in top tube will be somewhere between these two figures say 70mm.

You now have a longer taller top tube but the geometry and down tube are the same.

To make the frame longer you increase the down tube but change the angle with the seat tube keeping the head tube in the same place relative to the ground.