Bicycle Mechanics - frame geometry basics

hi to all,

I don't get to ride other people's bikes much, but a few days ago I had the opportunity to ride a bike fitted with exactly the same wheels,components as my own - basically the same bike with a different frame and forks (same steel, both were fully rigid) but the ride felt sooooo much different that it set me thinking.

I think one very often overlooked factor for us commoners when buying a bike is the actual frame geometry - we first look at the componentry (which is now the same for most bikes in a price range anyway), then we look at frame material and size... but I don't think most of us consider the frame geometry enough to influence our decisions....

can anybody out there share some info on frame geometry basics in terms of strength and ride quality ? For example, what would the bend of the forks do to your ride ? What is the difference between having a bend in the forks or straight forks mounted on an angled head tube resulting in the same wheel position ? What about the seat tube angle ? How would a very short stem effect steering ?

cheers,

claude

Claude,
Ok here goes frame geometry has nothing to do with strength. Strength of a frame is in the tubing material selected and its assembly. Example a tig welded high tensile steel frame is not going to be as strong as lets say a tig welded Tru-Temper OX III frame. Diferent frame tubing gives you different ride qualities. Example Cro-Mo steel comes in many different alloys from many manufacturers. Each frame from various makers will feel different depending on tube selection, angles, and assembly process. The "bend" in a fork...well from here on we will call it rake. The rake on a fork affects steering. It also affects how the front wheel distributes shock. The straight rake disapates it right to the headtube and bars. The more sloped rake will use more of the wheel to disapate shock. Seat tube angle will affect your body's position on the rear end, and also it has an affect on weight distribution on the rear end. The stem length, the shorter the stem the faster the steering, however it may have a profound impact on the way your body is positioned on the bike. Ideally on an adult diamond frame you want you elbows to be plumb with your knee at the bottom of the pedal stroke, with a slight bend in your knee. You also want to have a slight bend in your elbows and back upon reaching this position. For more detail see my website. The link is at the bottom of this bar. Go to frames there is alot of links to help you.

Tubeing diffrences aside, a laid back angle of say 71/72 degrees will produce a softer although less agile ride than say a 72/73 degree angle, also wheel base has a lot to do with ride. The longer the smoother the shorter the bouncyer and twitcher it is. I dont know the age of the bike you were rideing, but a lot of the bikes from a few years ago were pretty laid back. In other words frame angle and wheel base have a lot to do with ride and handeling caracteristics.!!!