|
|
|
|
Bike Geometry 102 This article deals with common geometry and what it changes on your bike. If you have not read Part 1 (Geometry 101), I HIGHLY recommend reading it first as this will not tell what each is referring to. This article deals with common measurements and what each one does for you. Top tube section does not go into much more detail than part one had, but rear end length does. Top tube length A person 6 foot, on a 20in frame, may not spin as fast as you would think. The rider can feel cramped, and he does not have the room to get leverage to spin. On the same note, that 20in frame may feel too long for a person who is only 5 foot. You have to judge it based on what you *should* be on. Another odd thing top tube length people do not realize, is you get leverage from it. A long top tube frame will spin slower, but it can spin easier. You have more room to pull with. It also tends to jump straighter. Rear end length has the same effect. This is why S&M Holmes and Dirt Bikes had such long back ends by today's standards. They were jumping frames. What you need to do is find what you need for your style, height, and intentions. A 6foot dirt jumper on a 19in. frame will/can have a lot of trouble jumping. the bike often refuses to jump straight. Fine if you like to spin, and can control it, imagine a Cadillac on to of a skateboard (and it did not crush), yeah, it can cut a turn fast, but can you control it? Below is a basic size guide. Until you find out how you ride, what you like doing, and understand, how it can change your riding, I suggest following it. 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. I am saving the lay back discussion for Geometry 103 Rear End Length The common miss-conception is that a short rear will manual easier. Not necessarily true. If it was, tightrope walkers would carry short poles instead of long ones. The proper length again is where the goal is. What's long for one person, is short for another. Especially when combined with top tube lengths. People are under the rear end is in control of manuals. Again, not entirely true. Same for spins. The rear is all about levers and fulcrums ( pivot point). The shorter the front end, the longer the back end will feel. Why.. Your bottom bracket. It is your pivot point. If you take a frame with an 18in top tube, and a 14.75 rear end, it will spin like crap. Take a 21.5 top tube, and that same back end will feel normal, provided these frames fit you right. Why do so many frames have a "long" (14.25-14.5) length, two reasons.. One is many frames for street are based on dirt jumping fames. Street and dirt are related in many ways. Two.. Foot clearance. A person wearing a size 11 shoe, will have trouble with hitting the peg if they have a short rear end (13in). Back to the tightrope.. A short rear will allow the front to come up faster, but that same speed will also allow it to shift your balance point faster, making you have to correct more often and faster when you do. If you doubt this, ask Rob-O why his rear is at about 14.5. You know the boy can manual. Its the right spot for that size frame. Some people may go a little either way, but its pretty close. For a frame under 20.5 (not including 20.5) rear end length should go under 14in. How far under is debatable. By the time you reach 18in. The rear end should be close to 13in. Many frames derived from longer ones will not go this short. Why? because it was not originally designed fro that length and some believe the shoe problem still exists for people who ride frames that short. Which is true. For Racers there is another issue for rear end lengths. Gate starts. A person with power will need a longer rear end to keep the front end from coming up and looping out. Too long and you cannot power the front end up at all. Spinners usually like shorter back ends. Unfortunately there are so many different levels of power Vs. Spinners, its impossible to say which will work best for you. Luckily for racers there is something that can be done. Raise your bars. That will give you extra leverage to pull up. Lower them for less leverage. This unfortunately does have other side effects, but it will change your gate start dramatically. A handlebar change and single chain link, can make the fastest person out of the gate, the last one out. My advice, get the rear as close as you can, then use the bars to fine tune. When you buy your next frame, look for a frame that closer matches what you need.
Bottom Bracket Height A high bottom bracket is like a jet fighter. Its inherently unstable. Top heavy if you will. This makes them great for tables, carving, and any other activity where you lean the bike. It also tends to make the front end feel longer, and the frame feel more stiff and tight. Odd.. It makes it feel like a short in height frame, even though your technically up higher. Some people can actually feel this in the frame. Others cannot. High BB can also make manual adjustments easier. Combine this with a short top tube, really short rear end, and it would be a nightmare to control in a manual. It would however make it easy to pull up. A low bottom bracket is like a drag racer. Long and low. Its very stable. Makes the bike feel planted. Racers usually use low BB because of this, although it has been on the rise lately. You can power the cranks, and it will just go. While they spin the same as any other BB, they do change manuals. Making it harder to pull up. Once there it helps though, by making it stable. High is 12in. A new measurement has been cropping up lately for street. 11.8. Its is a bit strange. It feel like a 12 when being ridden. Its balance is a little of both 12 and 11.75. Which is not what you would expect since its very close to 11.75 instead of 12. Most people will not be able to tell a difference between any of these sizes, which is evident by how many people ignore BB height when they buy a frame. They simply notice it feels "different" and adapt. Steering Angle
and Seat Tube Angle
|
||||||||||||||
