Bike Geometry 103
Bike Geometry By Sheepdog
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  In the last episodes we dove into how common geometry, and more detailed look at top tube and chain stays. If you have not read them I highly suggest reading them. If you already have a good basic understanding, skip to the second one. You really should understand those before reading this one. What this article will deal with is top tube slope or slant, and seat tube angle and head tube angles. 

Top tube slant/slope
  Most people take it for granted that top tubes are measured and angled relatively the same. Unfortunately, that's not true. There are as many way to setup a jig, as you can imagine. In this industry, riders often ride whatever they think feels fine, and looks good. When it comes to designing their own frame, they are at the manufacturers whim, which may not always be what a rider would want. Which explains why some frames are terrible. Same goes for the actual design. Some companies have no clue about designing for strength, common sense, or an idea for geometry. One company had a guy design a frame from a desk. He never rode. Another company simply threw another companies frame into their jigs and started welding duplicates.  It happens.. A lot. You have non-riders and non-engineers using  people as guinea pigs. See the Taiwan/Clone parts article for that.

  Slant effects top tube length, which is why some manufacturers will not tell you how they measured. I base it off going along the top tube, but what if it slants, do they measure where a normal top tube would be? Where would a "regular" top tube be for that matter? Unfortunately, I cannot answer some of that. What I can tell you is what to look for.

  For the most part, for each drop of 3/4in the top tube will change approx 1/16in.. The further it changes, the faster that increases. Not that any manufacturer will go much lower than an inch or 2. Though I would not put it past them. As it droops it will effectively lengthen the frame. So if they say it is 20.5in top tube with a sloped top tube, it will likely feel closer to 20.75in. On the other hand if a frames top tube is almost horizontal, and labeled a 20.5in, It will fell more like a 20.25in frame. 

  As has been said, without knowing how they measured though, this may be irrelevant. Because cause it depends on how they measured it. If you expected them to measure along the top tube, and they didn't, then the frame will not feel how you thought it would. Again, many will not tell you how they did, claiming it is a "company secret", "industry secret", or some other B.S. Like I said, many, cannot tell you, because they do not know.

Seat tube angles in relation to top tube length
   Ahh the seat tube angle. If there is one angle people take for granted, its this one, and they should. Kinda. Most frames use the same  we have since the early 80's. It works well and helps keep frame geometry pretty simple. Which is a good thing

   Years ago, Redline made a frame with a  69 degree seat angle (Redline RL-20 II). I loved it, especially with a quick release. When the seat was low, or when standing, it felt like a short frame. Put the seat up, and it rode more like a race cruiser, more stretched out. It was great. Except that while it was considered a 20.5in frame, it was closer to a 20. 

   Think of a pendulum. With the hinge at the top tube and the BB at the bottom of it. As it swings, and changes angle, the BB moves. At a 69 degree slant the bb moves approximately 1/2in forward compared to a 20.5in frame. So a 20.5in top tube frame with a 69 degree head angle now has the bb positioning of a 20in frame, and the seat position of a 21! As the seat rises, it goes further back. 

   A steeper seat angle has the opposite effect. A frame with a steeper seat tube, will feel shorter than normal while sitting, and longer when riding, because the BB has moved back, and the seat forward. 

   Very few companies mess with it anymore. It used to be common to move it. Sticking with 71 has been good, it allows us to adjust the rest of the frame around it, besides, 71, seems to work well for people and will not likely change.

Lay back seat posts and top tube length
   Remember these? In the early days of flatland, they were used for a couple reason. The frames were shorter then. A 21 was monstrous (many think it still is). When the Free Agent limo and S&M Holmes came out, people were surprised. No one would ever ride a 21, or 21.5in frame they thought. Its only because no one knew any better. 

   Back in the days when flat was king, short frames were good anyhow. The longer riders though, needed more room, but a shorter frame. A lay back gave them the room while sitting, but still was a short frame. It was also used to clear the wide bars we used back then. You needed to be able to grab the seat, and bar end at the same time for tricks. Most riders just thought they looked cool. Shops sold them based on the looks, bar positioning, and rider height, whatever was needed at the time. 

   At the time, it did not matter, you did not have longer frames to really choose from. You took what was offered, and most were about 20-20.5in. Nowadays, you are much better buying a frame that fits you properly and running a stronger, straight post. This is one place where having more riders controlling companies has paid off.  The lay back, was a band-aid. Pro's were selling bikes to kids, who did not know any better. Had they designed the frames with kids sizes in mind, it would have almost sure never seen production, and even if it still had, it would never have been as popular.

Head tube angles versus top tube length
   Another angle taken for granted. Although it has changed over the years, it remains relatively constant throughout the industry. In the early 80's the angle was 71, and grew to 73 by the end of it. One after another just slowly increased it, until we use what we do today. I remember when 71was considered to be the best, then 72 and then 73. people thought for sure 73 would remain the preferred angle. Just as I am sure people think 74.5 will. The rate of change has  slowed so we are nearing the optimal angle. however riding style dictates it. So it can still change. Only time may tell.

  Head tube angles can be tricky. Changing it does more than people realize, and most do not realize it changes often because of you. A different fork, tire, tube, liner, lower headset cup.. All effect your head tube angle. Unfortunately, it does when a manufacturer makes a frame as well. White Bear made a frame once with a 78 degree angle, the fork also was super tall., Almost large enough for a 24in wheel. Supposedly it was a frame built for one of their team. I find that hard to believe. The whole story behind it was very strange so I wont go into here, but it is relative. had they changed the fork to a  normal one, the head tube would have increased to almost 80 degrees! 

   The same happens but on a minor scale if you put a smaller height lower cup, and the opposite if you use a larger cup. What does this have to do with new frame geometry. Well, what fork and headset did the manufacturer base the jig from?  I would say its not a big deal, but there has been a manufacturer who had a batch of frames slip through with 5 degrees shallow angles. How it happened, who knows. Supposedly a mix-up in Taiwan. 

   So how does it effect you, more than you think. An S&M Holmes Ng has a head tube angle of 73 degrees. A Fit Series One has a head tube of 74.5 degrees. What is interesting, is that both frames have the same top tube length, BB height and seat angle (the rear end length, head tube angle, and tube thickness is the biggest differences). Yet a Fit Series one feels almost 1/4in. longer on the top tube. Why is this?

   Going back to the pendulum, placing the hinge at the top tube and head tube junction, if you move the fork back, simulating a steeper angle, you can also see that the forks top, will move slightly forward. If you extend that up higher, as in having your bars connected to it, you will also notice them moving forward. The higher you go, the more pronounced it becomes. 

   While it may not be as much problem as it used to be, it is something to pay attention to if you are changing from an older frame to a newer one, or just changing angles. It will effect the frames feel, in more ways than just faster handling. When switching from a  race or trails frame to a street or flatland frame, this could also be a consideration.

 Conclusion
   While I hope you got something out of this 3 part series, I hope I did not confuse people too much. I know there is a lot info here. Some you can do something with, some you cannot. I hope that it helps people understand geometry is a whole concept not just a couple angles lazily thrown together. This guide will hopefully help some of you find what you really want in a frame and weed out those that would be inappropriate. - Sheepdog

 Supplement..
   I was not going to go into stems and bars but felt it necessary. 

 Stem length and handling
   Some riders often use stem length to compensate for top tube length. That is wrong for several reasons. Short stems do serve a purpose but its not really intended for this. The only real way to compensate for this is to get a different frame.

A short stem is meant to customize handling. A shorter one will increase the steering angle (only in feel). Head angle not steep enough? A shorter stem will help. Too steep, a longer one will make it feel more like a chopper. Mini frames use short stem because its appropriate size for them. They are designed to use that size. Flatlanders use short stems to decrease the change of how the front end feels when the bars are backwards and to make the steering faster.

While it can be used to fine tune top tube length, its similar to the lay back seat post. Its a band-aid. The true answer is a longer frame. A better option is to lower your bars.

 Handlebars, all you want to know, and less
   While I would love to tell you all I can about this, frankly there are few rules about it to my knowledge. I can tell you some general thoughts on it, but that is about it, its all preference.

   Terms:
Sweep, or back sweep, this is the angle backwards. This is a measurement most companies tell you.
Upsweep. Rarely given number, this is how much angle up the bars have.
Height. height from middle of the stem clamp, to middle of the grip height.
Width. I am sure you know this.

Going in order gives you the least info first so I will go reverse of that list.

   Width, generally I have noticed its based on shoulder width. A wider shoulder guy, will usually use wider bars. There is no real rule as to how wide though. Its still personal preference.

   Height. Bar eight effects how long your top tube feels. See earlier parts of this guide for more on this.

   Upsweep. Rarely given or known. S&M is one of the few companies who give you all the specs you really should have, then again, they may be one of the few actually using any upsweep, though that's doubtful. While does effect the power of your pull, we have no real rules, to base it on, and few numbers to pick from. Luckily it varies very little. Too little or too much can hurt your wrists over time. While a bar may feel great to you, over a few weeks you may notice your wrists getting sore. This could be the cause, but I would look more into sweep as being the culprit.

   Back sweep. A very tricky number. Many people buy whichever bar they think looks cool or a friend recommends. Unfortunately, its MUCH more complex than that. Once again there is no complete rule, there are some guidelines.

   The generic rule is that 8-10 degrees is best for your wrist. Its not always right.  The real rule is  too much or too little can damage your wrists. Supposedly you should not go over 13. Though some bars do, and people like them. 

   My rule. Do not change more than 5 degrees unless you are having problems. If your wrists are used to 3 degrees sweep, and you go to a 13 degree, your wrists will not be accustomed to it and you may have problems. While a bar may feel great to you, over a few weeks you may notice your wrists getting sore. This could be the cause. If you are having problems, go to the other extreme. There is no rule as to where to start unfortunately other than the generic. 

   One thing I have been trying to find a connection between wide shoulders and narrow bars liking less, and narrow shoulders  on wide bars liking more, or similar, however, there is none. While this theory could work, it doesn't. It just goes to show how people are different.

 Handlebar angle
   Most people take it for granted that you run with the same angle as the fork. This is one angle where upsweep can help counter for that some. Upsweep can correct the relation to your body if they are not aligned properly. but, what is properly? For flatland, the angle is different. Jumpers like them inline, street rider as well. Some prefer back more and others more forward. 

   Again, there really is no rule, run what's comfortable. While it can be used to adjust top tube length, its not a good idea as leaning the bars can really effect your handling and mess up the sweep angles on your bars. If your bars are giving your wrists trouble, you may want to start here, as this does not require changing bars.

 2 and 4 piece handlebars
   Just a different design for the most part. Each has advantages and disadvantages.

   4 piece  are usually stiffer than a 2 piece. They also have the option of using different size tubing. Such as 1in. instead of 7/8in. While they are more out of the way for your knees, when you do make contact with them, it will usually hurt more, as they do have a sharp corner at the bottom instead of the nice rounded tube of a 2 piece. Also, die to more heat from welding, and the added stiffness, when they decide to end their existence, they will be more likely to break compared to a 2 piece. Though I am not saying they are any weaker or stronger than 2 piece, they tend to be about 1/2 pound heavier.

   2 piece tend to flex a bit more, and be easier on the knees if you do hit them. They are also more likely to bend, than snap, when they do decide their life is over. - Sheepdog