Are you ready for a new frame?
The Geometry of Choices
By Ed Leason
When we all started BMX we got what we got; whether it was a second hand bike, the Mal-Wart special, or in my case the bike I rode 10 years ago to get back and forth to school. If we were lucky we got a bike that fit right or was close enough to grow into. After we gain experience and skills, we start to notice that things could better or easier one way or another. Or maybe you had a growth spurt and growing 6 inches in a month means that the Junior frame is just too small now.
So what do all these measurements mean and how do they affect the ride of a bike. A majority of the BMX frame manufacturers will use the Top Tube (TT) as the main size indicator. However this is really a poor way to size a frame. For discussion I will be referring to a 20” class bike and not a 24” cruiser frame, although there will be lots of similarities.
The terminology used for frame geometry is:
D: Bottom Bracket Height (BB): This is the measurement from the ground to the center of the bottom bracket.
C: Chain Stay Length (CS): The distance between the center of the bottom bracket and the center of the drop outs. It is usually the average of the Min and the Max length, though some manufacturers will claim a Min CS length. This is how far the rear wheel will be from the bottom bracket.
A: Top Tube Length (TT): The distance between the center of the seat tube to the center of the head tube along the length of the top tube.
F: Seat Tube Angle (SA): This is the angle between the ground and the seat tube. With the ground as the reference plane, the angle is measured from the back of the bike. This means a larger SA will be steeper and the seat more forward while a smaller SA will mean a more slack/relaxed with the seat further back.
E: Head Tube Angle (HA): This is a measurement of how steep the forks are. Again larger HA will be steeper and the steering being quicker while a smaller HA will mean a more slack/relaxed with the steering slower.
B: Rider Area (RA): This is very seldom used measurement, which is a shame. It measures the distance between the center of the bottom bracket to the top center of the head tube. This is a good indicator of how much room a rider has between the pedals/cranks to the handle bars.
The Bottom Bracket Height is really where everything else is built around; it is the foundation of a bicycle frame. If you were to make a change to the BB everything else is going to change as well. A tall or high BB will provide the rider with increased pedal clearance. This will make it easier to pedal over rollers and coming out of corners. It will also make the front end come up easier making it easier to pick up over rollers or loop out of the gate. This is because the center of gravity has been raised higher. This will also make the bike a little more “tipsy” and cornering will be worse and you will be more likely to slide out. So the opposite would be true with a lower BB, decreased pedal clearance, better gates, better cornering, and harder to lift over rollers.
The Chain Stay Length has most to do with how “loopy” a frame is. A shorter CS will make a bike easier to lift up over rollers, manual through rhythm sections, and carve in corners. It will also be easier to loop out and fall on your tail bone. The same is true coming out of the gate making it easier to loop out. A longer CS will feel more stable and less likely to loop out but will also be slower to react and harder to manual and corner.
The Head Tube Angle is the measurement as to how “fast” the steering will be. – Hey, what happened to the other ones; Top Tube and Seat Tube?
Relax I’ll get to them in a bit. – A steeper front end or a larger HA will react much quicker. A little turn of the bars and the front end will start moving and if you’re not leaned over or prepared you’ll get bucked off the bike real quick. A more relaxed (less steeper) front end will be a little slower to turn and react, which also means it is more forgiving if you start to “get squirrelly” in the rhythm section. ALSO a steeper HA can make a bike feel a little longer. If you were to change frames and the only measurement that changed was the HA, the bars would now feel a little further away from you. This is because the stem is attached to the forks which is in line with the HA. If the HA is steeper the stem is going to be pushed a little more forward.
The HA has a slight geometry reaction. As stated if the HA where to change, the front half of the bike would feel shorter (smaller HA) or longer (larger HA). But it is a simpler and very slight reaction. However the Seat Tube Angle and Top Tube Length – see, I told you I’d get back to it – have a very strong reaction with each other with a larger effect.
The Top Tube Length is a quick and dirty method to measure how much room you have in the front of the bike. Typically you want to have enough room so that you don’t bang you knees into the bars and can feel comfortable. A longer front end will make for a more stable feeling bike assuming it’s not too long and you are now stretched out over your center of gravity. It will also make the front end harder to pick up over rollers or loop out coming out of the gate. It will also give you more room for your hips to thrust during your snap. A shorter front end will be easier to maneuver and manual as well as loop out.
The Seat Tube Angle helps determine where the seat will be. A steeper SA means that the seat will be more forward and a less steep SA means the seat will be further back. It affects the distance to the handle bars while you are seated, which isn’t very often in BMX. It also affects how far back you have to go to “tuck in” behind the seat.
Now there is also a very strong reaction between the SA and TT. If you leave the TT the same but decrease the SA, you will shorten the front end of the bike. Also a steeper SA will make the front end longer, even though the TT is the same. Also the Seat Tube length can have an effect. If the seat tube is longer (say around 10”) it will make the front end shorter keeping the same TT.
If you where to keep the seat tube the same length and keep the same front end length but change the SA from 73° to 70° the TT would have to increase approximately 0.5” to have the same front end.
Now the Rider Area will bypass all of the interactions and reactions between the SA and TT. This is why it is a shame that it is not a more commonly used measurement. However there are other interactions with the bottom bracket. If the BB is higher the RA will be shorter and if the BB is lower the RA will be longer.
Also BB and CS can have similar effects on a bike. By raising the BB or by shortening the CS the bike will manual easier or loop out easier. Basically one would want to run the BB as low as possible and adjust the CS to provide the right balance point.
But what is the right BB and CS measurements? It all depends on the personal preferences of the rider. Some riders like to have a “quick” responsive bike; some would prefer a more stable forgiving bike.
What it all comes down to is the rider’s skills, abilities and preferences. If you are looking for a new frame ask yourself what do you want to be different. If you just want a new paint job, that’s a little different, but if you want a bike that’s easier to manual, or more stable you’re going to have to do some homework. What is the geometry of your current frame? What do you want to change? What do you NOT want to change? Experience is the best way to determine what changes you desire. As you change frames take note as to what’s different between them. How do they feel, is it more maneuverable? Is it too twitchy and squirrelly? Does it feel too tight and cramped? Or is it just impossible to manual?
Since money doesn’t grow on trees and we just can’t go through frames and try them all, try your friend’s ride out. Find out what is different in the geometry between yours and your friend’s bike. Feel the difference and note whether it is an improvement or not. Also take note the way the bike is setup. Different components like a longer or shorter stem or a heavier fork will have an affect on the way a bike feels as well. But that’s another story……….
