BMX Frame Geometry
BMX Frame Geometry
The BMX frame
Arguably THE most important part of your bike, but also one of the most complicated. With all the various lengths and geometries out there, shopping for a new frame can be somewhat overwhelming at first glance. In this article we will be discussing everything you need to know about BMX frame geometry and how changes in geometry can affect your riding.
If you’ve browsed through our website, or any manufacturer websites, you’ve no doubt seen mention of bottom bracket heights, headtube angles and everything in-between. But what does it all mean and why does it matter? Well, depending on what it is, sometimes very little and sometimes quite a bit so let’s dive right in. Everything discussed in this article will be in relation to a 20” BMX frame, but the principles are still the same no matter what size or type of bike you have.
The toptube
Let’s start with the simple toptube length with is measured from the center of the headtube, straight along the side of the frame to the center of the seattube. This can easily be measured yourself, but make sure you’re measuring along the side and not the top as the seattube and headtube angles can throw off your measurement. The lengths listed in the illustration below are not set in stone, but they are good rule of thumb to determine what size toptube will best fit the rider. If you’re a racer you will want to err on the longer side for extra stability.
The headtube angle
Next up is headtube angle. Imagine a line running down the center of your headtube and a line running parallel with your axles. The angle at which they meet is your headtube angle. The higher the number the steeper the headtube angle. For example, most race frames will have around a 74° headtube angle. This is a bit more laid back and in turn provides more stability, especially at speed. Freestyle frames will often have a steeper headtube which shortens the bikes overall wheelbase and gives you a quicker, more responsive front-end feel. Most freestyle frame these days are around 75 to 75.5° with some ultra tech street frames coming in at a steep 76° to 76.25°. A steeper headtube is ideal for tricks like nose manuals and hang-5s, but keep in mind it will make the bike feel a bit more twitchy and less stable at speed.
The Seat tube
The Seattube angle is measured along the center of the seattube to where it meets parallel with the center of axles. The majority of freestyle frames will have a 71° seattube angle, but this is where things can get a little tricky if the manufacturer changes this angle slightly. As an example, let’s say you have a 20.75” toptube frame with a standard 71° seattube. Pretty normal, right? However, imagine we were to angle the seattube back ever so slightly to 69°, BUT we still kept the toptube at a measurable 20.75”. In effect by pushing that seattube back, you’ve also pushed your bottom bracket forward ever so slightly as well. And since the chainstays are connected to the bottom bracket, that’s also shortening your overall wheelbase.
Confused yet? Okay let’s say we have two nearly identical frames both with a 20.75” toptube. The only difference being one has a 69° seattube and the other has a more traditional 71° seattube. If you were to ride both frames back-to-back, you may notice the frame with the 69° seattube actually feels shorter overall, even though both have a 20.75” toptube. A little confusing for sure, and likely something you won’t encounter much as most companies stick to the standard 71°, but there are some advantages to this. By having a more laid back seattube angle you’re giving yourself a little extra clearance up top, but the bike will ride like a shorter frame because of it’s overall shorter wheelbase.
Standover height
On the subject of seattubes, let’s talk seattube height, also known as standover height. This measurement is taken from the center of the bottom bracket, straight up the seattube to the middle of the steattube/toptube junction. These measurements usually range from 7” up to 10” tall. Standover height is all personal preference, but typically you will find that most park oriented frames will have a lower standover. This brings the toptube down lower and can make it easier tricks like tailwhips. A taller standover is usually found on more street-oriented frames. It gives a more traditional look and is generally preferred by taller riders. Many riders also prefer a taller standover for tricks like barspins, but it certainly is not a requirement.
The bottom bracket
Now we’re on to bottom bracket height and this measurement can definitely play a role in how your bike rides. Bottom bracket height is measured from the center of the bottom bracket shell, straight down to where it meets the ground. Most BMX bottom brackets heights will range from 11.5” to 11.8” with some variances here and there. Do not try measuring this at home because it likely will not be the same as advertised. That’s because 20” BMX frame geometry is calculated off a “standard” 20” overall wheel size. However, modern BMX tires are much wider and taller than before. If you were to measure your BB height it will almost certainly be taller than advertised due to the height of your tires. Just know that all BMX companies use this same formula so it is consistent from brand to brand.
So what are the advantages and disadvantages of certain bottom bracket heights? Well, a lower bottom bracket height (say 11.5” – 11.6”) will provide a more stable ride. If you ride trails, pools, or in general just like to go fast, you might want to consider a lower bottom bracket. On the other hand, a taller bottom bracket height (11.7” – 11.8”) can make the bike feel a bit more agile and easier to spin as you’re sitting higher up on the bike. And if you’re someone who does a lot of crooked grinds, a taller BB height can also give you a bit more clearance so you’re not grinding away at your chainstays.
Chainstay length
Finally, we come to chainstay length which is the measurement used to determine the length of the frame’s back end. Chainstay length is measured from the center of the bottom bracket, straight back to the center of the rear axle. Most modern frames will range as short as 12.5” for a super tech street frame, to 14” and longer for a trail or race frame. Chainstay length is something you can measure at home if you’re precise. Again, just make sure you’re measuring center to center.
Often you will see this chainstay length listed as a range (for example 13” – 13.3”). These measurements are calculated with the rear wheel slammed completely forward in the dropouts and with it pulled out to its furthest usable length. If nothing is specified, assume it’s to the middle of the dropouts. A longer chainstay length will provide a more stable, balanced ride, especially in the air or at speed. A shorter chainstay length will make the bike more maneuverable and easier to whip around, at the expensive of some extra stability.
While there are many frames out there designed for specific riding styles, don’t ever feel like you must choose one style of riding over the other. There are plenty of frames designed to have “middle of the road” geometry that can be used to ride anything and everything. Overall frame geometry comes down to personal preference so don’t stress yourself out if you can’t decide right away.