In this post, we’re going to go over all the different specifications & features of modern freestyle frames.
These are all important specs to consider when purchasing a new BMX frame. The specs, or specifications, are the geometric measurements of the frame that give it it’s own unique feel.
Top Tube (TT) Length
This is the first and most important spec to take in to consideration when it comes time to pick your new frame. It is the length between the frame’s Seat Tube and its Head Tube, and it is used generally to describe the size of a frame. Most modern 20″ frames come in actual sizes between 20.25″ & 21.75″. Typically you will choose a size based on your own height, but personal preference also comes into play. A slightly longer top tube will be more stable & comfortable, but harder to manipulate for some tricks. A slightly shorter top tube will be less roomy & stable, but can make a lot of tricks simpler. Here is a list of estimated frame sizes based off of height. Using the this plus the information above will help you decide on an appropriate frame size for yourself.
5’1″ – 5’3″ / 20.25″
5’4″ – 5’6″ / 20.5″
5’7″ – 5’9″ / 20.75″
5’10” – 6’0″ / 21″
6’1″-6’3″ / 21.25″
6’4″ – 6’6″ / 21.5
6’7″+ / 21.6 – 22″
Chainstay (CS) Length
This is the length between the Bottom Bracket and the frame’s Drop Outs. They generally span from 12.7″ to 14.5″. This spec is solely based on personal preference.
The shorter the Chain Stays, the easier it is to manipulate the bike for tricks. They also make it easier to get up on the rear wheel for manuals and such, but at the same time makes it harder to “lock in” and balance it. Shorter chain stays also make for a twitchier bike.
The longer the chain stays, the harder it is to initiate manuals and rear wheel tricks, but the easier it is to lock in and balance them. Longer chainstays will make for a stable tail end of the bike.
Head Tube Angle (HTA)
This is the angle of the Head Tube, which is relevant to steering, stability, and front wheel tricks. The Head Tube Angle will usually range from 74 degrees (mellow) to 76 degrees (steep.)
The steeper the head tube, the more responsive your steering will be, and the easier it will be to initiate front wheel tricks like nose manuals.
The mellower the head tube, the more stable your front end will be, aswell as it will be easier to lock in and balance front wheel tricks.
Bottom Bracket Height (BBH)
This is the height of the Bottom Bracket. It will usually range from 11.5″ to 11.8″.
Lower bottom brackets are renound for bringing a super stable feel to the bike, aswell as making it easier to balance manuals and other rear wheel tricks.
Higher bottom brackets make for a more “poppy” feel in the bike, especially when hopping and doing tricks. They make it easier to initiate rear end tricks, but harder to lock into them.
Also keep in mind that when you change your BBH, it changes the distance between your feet and your handlebars. Lowering your Bottom Bracket by .25″ will make your bars feel .25″ taller.
Stand Over Height (SOH)
This is the length of the Seat Tube. This will usually range anywhere from 7″ to 10″. The taller you go, the harder it becomes to do tricks such as can-cans or tailwhips, as the Top Tube is now that much higher and more in the way. Taller Stand Overs are also said to make for a stiffer, stronger frame. This spec is pretty dependant on personal preference and riding style.
Seat Tube Angle (STA)
This would be the angle of the Seat Tube. This alters the effective feel of the Top Tube, in correspondence to the Stand Over Height. The industry standard for STA’s is 71-degrees, but can range from 69-degrees to 72-degrees.
Basically, when you change the STA, you have to add or remove some Top Tube material, without altering the position of the Bottom Bracket. And the taller the SOH, the larger the difference.
Example: A 21″ TT frame with a 9″ SOH and 71-degree STA will ride identical to a 21.3″ TT frame with a 9″ SOH and 69-degree STA (assuming all other specs are the same aswell.)
This is because the difference between the 71-deg and 69-deg on a 9″ SOH is 0.31″, meaning you have an extra .31″ of top tube but your Bottom Bracket is still the same distance from the Head Tube.
Now that you’ve figured out what specs should suit your body and riding style, you are ready to choose a frame based off of its features. The features of the frame are the technical specifications that decide its structural integrity and its weight.
Most aftermarket frames are Heat Treated, which means the tubes have been heated and cooled in a way that strengthens them. There are two different ways a frame will be treated.
The first and cheaper method is to use heat treated tubes to build the frame. However, the heat of welding can alter the heat treated properties of the tube near the weld, which brings me to method #2.
Post-weld Heat Treatment is a process that consists of heat treating the entire frame post-production so that the properties of the metal are the same through out the weld. This is the most efficient way to strengthen both the tubes and the welds.
Investment Casting in BMX is essentially a way to move a weld away from a joint for a stronger or larger connection. It is done by casting pieces of the frame, such as a Head Tube or Drop Outs, with segments of the corresponding tube built in to it. This usually, but not always, makes for a more durable section of the frame.
Some frames will use Ovalized Tubing for the Down Tube and the Chain Stays. Due to the shape of the tube, they are much less likely to dent or bend on the bottom of the tube. This is preferable for riders who hit their their frame in those spots on grinds and such.
This option is only available from Sunday Bikes. Similar to Ovalized Tubing, they make the tube dent proof by adding “waves” to the bottom of the tube. This feature is limited to a few of their frames.
Some companies will use Tapered Tubing to save weight without a significant difference in strength. A tapered tube is one that is slightly narrower in diameter on one end than it is on the other.
Butted Tubes are tubes that are thicker on the ends than in the center. This can be done for two very different reasons.
The first being that you can essentially beef up the end of the tube for a stronger weld with very little difference in weight compared to a straight gauge tube.
The second method is to remove material from the center of ideally a straight gauge tube, making a lighter tube without sacrificing strength from the weld.
Info taken from: https://www.vitalbmx.com/forums/General-BMX-Talk,2/BMX-Frame-Buying-Guide,1317923#edit_post_2273371