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This post examines the seat tube angle’s effect on a BMX frame.

Seat tube angle

The seat tube angle (STA) is formed by a horizontal line passing through the middle of the bottom bracket and another line passing through the middle of the seat tube (image below).

Effective seat tube angle?

The effective seat tube angle (ESTA) is formed by a horizontal line passing through the bottom bracket and another line passing through the bottom bracket and the middle of the junction where the seat post meets the saddle (image below)

Since most BMX bikes have a straight seat tube, the effective seat tube angle, and the actual seat tube coincide unless the rider moves the saddle too much to the front or back.

The Seat Tube Angle’s Effect On a BMX Frame

• Effective Top Tube

The effective top tube (ETT) is the horizontal distance between the point where the top tube and headtube meet and an extended line from the seat tube.

The seat tube angle directly affects the effective top tube. A steeper seat tube angle makes the effective top tube shorter. A slacker seat tube angle makes the effective top tube longer.

• Reach

The term reach refers to the horizontal distance from the center of the bottom bracket to the middle of the head tube. (image below).

The reach and the effective top tube may seem like identical measurements but they are not.

One of the differences is that the reach applies only when the rider is pedaling out of the saddle which is the case for BMX bikes.

Technically, the seat tube angle has no direct effect on reach if the length of the top tube changes along with the seat tube angle.

For example, if you make the angle steeper but keep the length of the downtube the same and shorten the top tube, the reach will remain the same.

If you slacken the seat tube angle while making the top tube longer and keeping the downtube length untouched, the reach will once again remain the same.

However, if you keep the top tube the same length, then changes to the seat tube angle will affect the downtube and with it the reach.

In that situation, a slacker STA will decrease the reach whereas a steeper STA will increase it.

I know that the above seems a bit complicated, but I have a long dedicated article on the same topic explaining everything in greater detail with images.

• Wheelbase

The wheelbase is the distance between the front and rear axle. Consequently, the seat tube angle can affect it too.

How?

As mentioned in the article on reach, we observe the following:

If we take a frame and make the seat tube angle steeper without making the top tube shorter, the downtube will have to increase in length. A long downtube pushes the front wheel forward and thus increases the wheelbase.

If we slacken the seat tube angle in the same conditions, the downtube will have to get shorter. Consequently, the wheelbase will decrease.

In short, a steeper STA= longer wheelbase; a slacker STA = shorter wheelbase.

A shorter wheelbase is less stable but makes the bike more agile and responsive.

What is the most common seat tube angle of BMX Frames?

Freestyle BMX

Conclusion: The vast majority of freestyle BMX bikes have a 71° seat tube angle.

Race BMX

Conclusion: Many race BMX models have a slightly steeper (72°) seat tube angle. Thus, the difference between freestyle and race seat tube angles isn’t substantial.

What Are My Options?

If you want a freestyle BMX, the steepest seat tube angle that you can have is about 71°. If you want a slacker seat tube angle, you will have fewer options, but technically, you can find models with a 69° angle.

The advantage of 71°, besides the higher number of options, is that the top tube is true to its length. (e.g., a 21″ top tube is 21″) and the bike is more “predictable”.

A 69° STA angle would require a slightly longer top tube when all other parameters are the same because the distance between the head tube and the seat tube is greater. Thus, a bike with a 69° seat tube angle could feel “too long”.

If you want a race BMX bike, the same applies, except that you have more models with a 72° seat tube angle.

Can I Steepen The Seat Tube Angle of My BMX Bike?

The actual seat tube angle cannot be changed once the frame is built. You can steepen or slacken the effective seat tube angle by moving the saddle forward or backward.

However, the effective seat tube angle of a BMX bike is close to irrelevant because the rider is not meant to pedal in a seated position.

Thus, it makes little sense to play with the saddle’s position from that perspective. Truth be told, the saddles of BMX bikes are there primarily to rest between trick attempts.

The Seat Tube Angle Is Not Crucial

Truth be told, the seat tube angle is not the most important property when it comes to BMX frames. The following parameters should be higher priorities:

• Frame quality (In general, Cr-Mo frames are the best.)
• Frame size (Taller riders need a longer top tube.)
• Head tube angle
• Chainstay length

Summary: What You Need To Know

• The seat tube angle of a BMX is not the most crucial measurement, but it has its importance.

• The most common BMX seat tube angle is 71°, but there are also models with 69° angles.

• If the angle of a frame is slackened from 71° to 69°, the top tube will have to get longer if the downtube is to be preserved the same. In this case, the longer top tube makes the bike feel…longer.

• If the angle of a frame is slackened from 71° to 69° without elongating the top tube, the downtube will have to be shorter. Consequently, the head tube will get closer to the bottom bracket and the bike will actually feel shorter.

• The safest way to acquire predictable handling is to go for the classic 71° angle.

• There are other more important parameters such as frame quality, head tube angle, and chainstay length.