Bike Geometry 101
Bike Geometry By Sheepdog

Ever wonder what the measurements are on your frame, and how they effect you. Then this is for you. I will outline what each angle, and measurement is, how to measure it, and how it effects you and your riding. A second section will be added going into even more detail about geometry and will make your head spin, unless you have the basics.

   This article only addresses how to measure, and what the measurements are. Part 2 will detail what are common measurements and what each one does for you.

Frame size
  The most common thing people start looking for in a frame is Top Tube Length. Top Tube length is used as a reference for a frame size. A 21in top tube, is considered a 21in. frame. This is the measurement along the top tube from the middle of the headset, to the middle of the seat tube. This measurement accommodates rider size. A longer top tube, is better for a taller person, while a shorter top tube will work better for a shorter person. Everyone is a little different in size.

Red lines represent the center of each tube Measure the green line from red to red.

   A person's height is used as a general reference. If you have shorter legs and or arms, you may feel better on a shorter frame. Someone with a short body but gorilla length arms may feel more comfortable on a longer frame. However some people cannot tell any difference, and can ride any size, and any bike.

Some people are under the impression that all arms are the same size for your height. Not true. Find a couple buddies the same height as you and ask what their inseam is, or compare arms. There will be differences.

   If you are unsure of what to get a trip to the local bike shop is in order. Or try some friends bikes and see what size they are to determine what works best for you. However there is a general sizing many people follow. Its not an exact science, and some people use the exact opposite of what is recommended, and love them. Until you know what you like, its best to start with a recommended size.

Remember if you are in or about to start puberty, buy a bit bigger.

Remember, this is only a starting point. Fine tuning comes from experience.

   A final note about top tube length. it used to be common to use a layback seat post to compensate for a short top tube, it really does not change the actual feel of the bike while you are standing. This will be explained more in 102 or 103 of this article.

Rear End Length
   This is where personal preference really kicks in for some people. Rear end length is measure from the center of the cranks (bottom bracket) to the center of the axle in its forward -most position. Some manufacturers will tell you the maximum length as well. Some will tell you center of the cranks to the center of available axle positions. 

Red X's indicates center of B.B. and center of axle in forward most position.
Again, measure green from center of red to center of red.

   The reason this one is important to people, is manuals. Many people feel a short rear end make manuals easier. They also make the back end spin faster. Too short can be a problem for larger riders, so be aware. When its really short, your heel can catch on your axle pegs. Which is why you rarely see a really short rear end, on a longer frame. Taller people tend to have larger feet.

Bottom Bracket Height
   This one is a mystery to many. However its a very important measurement that changes the way a bike feels greatly. Bottom bracket is the bearings for your cranks. Some people prefer high, some prefer low. Its height is measured in relation to the ground. While it sounds simple enough to measure, it is actually the most difficult as tire sizes, inflation and more, can all effect the measurement. What's worse, is manufacturers will often not tell you how they measured it on their frames. Many companies will not even tell you the measurement. Fortunately I have a simple way of measuring it Eliminating much of the things that will effect your measurement.

   All you need is a ruler, and some string or a piece of flexible wire. Flip your bike over and make sure the handlebars are straight. On the none peg side of the bike, loop the string over the front and rear axles. and bring the loose ends together so you can grip both ends.

X's show axle centers and B,B, spindle center. Blue is the string. Put the string over the axles like so. Leaving the ends like is shown.  You do need the wheels on, though it is not shown.

   Hold the string tight, not too tight or the handlebars will move (another person can hold it of you cannot do this one handed).

Hold where shown.

With your other hand measure from the string, to the middle of your crank arm bolt. 

Measure green. 

Note, you should go at a 90 degree angle to the string.

   Take the measurement and if you run 14mm axles subtract 5/16in., if you run 3/8 axles subtract 3/16in. This is to compensate for being over the top of the axle. Now add 10 inches (for your wheels, you ride a 20in).

Measurement - half axle thickness + 10 inches = bottom bracket height

   If you did it correct it should be close or the same as what the manufacturer claims. On my Fit Series One it was exactly 12inches. As was expected.

Steering Angle or Head Tube Angle
   This measurement is fairly common. It tends to become more steep by about 1 degree every 5 years, though I expect it to taper off soon, as we are quickly closing in on perfecting frame geometry in my opinion. It has already shown signs of slowing as many still run the same angles from 10 years ago.

Red is level, measure green angle. Not easy. Tires and other factors make this tough. Bike lean being one, as well as flat ground. Too many variables even if you eliminate the tires.

   Though we tend to view it as the fork angle, its actually the frame we measure as forks sometimes angle slightly from the frame. Basically this measurement is very tough to do accurately on your own. Luckily frame manufacturers are more than happy to show off these numbers. 

Seat tube angle
   Another common measurement. Obviously the angle of the seat post compared to the ground. Again, tough to measure on your own, but manufacturers are more than willing to give these out.

Red is level, measure green angle. Not easy. Tires and other factors make this tough. Bike lean being one, as well as flat ground. Too many variables even if you eliminate the tires.

Watch for the next chapter of this describing common measurements and what they will do for you. The third chapter will dive further into differences in measuring techniques and how they will effect you. 

Part 2 is here.