Something that rarely anybody talks about is the seat tube angle.
Do you care about it? Do you ever think about it? Does it even matter?
Honestly, NO, it does not.
For instance, the head tube angle is way more important and will make a MUCH more significant difference to your riding style.
And the other very important thing is the chainstay length.
But let’s get down to the nitty-gritty of the mysterious seat tube (ST).
In general, MOST of the BMX frames will have a 71-degree head tube angle.
That’s just the standard that works WELL regardless of what riding style you’re into. Street, park, dirt, flow, speed, tech – that 71-degree is universal.
You will rarely see a frame (Cult Dehart and 2 Short, for example) with a seat tube at a different angle, say 70 or even 71.5.
The Seat Tube Angle Explained
So, what’s up with the angle no one really talks about and no one really cares about?
Is it really as important as some other frame specifications?
Everyone is all about the top tube length, the head tube angle, the standover height, the bottom bracket height and the chainstay length.
Even pros HARDLY mention the ST angle unless they have a signature frame and they want to do something untraditional.
As an example, Chase Dehart, instead of going with a 71-degree one, made his frame with a 70-degree seat tube angle.
The ST angles affect the center of gravity. Moreover, the steeper the angle, the shorter your bike will feel, and vice versa.
In other words, if you have a 21″ top tube (TT) frame, it will:
- Feel shorter if the ST angle is 72-degree
- Feel longer if the ST angle is 69-degree
So that’s that.
The angle at which the seat tube is will “tweak” the length of the frame while the TT stays the same.
A frame with a 71-degree seat tube makes a 21″ TT frame true to its length.
If you’d change the angle of this same frame to 69-degree, the top tube length stays at 21″, but the angle pushes the bottom bracket more forward (toward the head tube).
This change then makes the bike feel as if the top tube would be around 21.2″ (+/- 0.2″).
Please note that the standover height also impacts how “long/short” the frame will feel based on the ST angle.
One more thing!
Because the change in angle “pushes” the bottom bracket forward or backward, it alters the wheelbase slightly.
Should You Worry About The Seat Tube Angle?
No, because almost all modern frames come with a 71-degree one.
However, here’s where it can get tricky:
- If the change (based on the traditional 71-degree) in the seat tube angle is 0.5 degrees, the longer/shorter frame feel won’t be as noticeable.
- But if you get a frame with a 1+ degree difference in the angle, the frame can feel completely different. As mentioned earlier, a 21″ frame with a 71-degree seat tube will respond differently than the same frame with a 69-degree seat tube.
Can I Do Something About The Seat Tube Angle?
You can only artificially change it if you have a railed seat, moving the seat forward or backward.
But this won’t really do much on a BMX bike as opposed to a big-wheel bike because it affects how you seat – and you don’t seat much on a BMX bike.
You may also be interested in my article on the different BMX seat types.
Conclusion: Does Seat Tube Angle Matter?
I wouldn’t stress about it because most of the frames come with the classic 71-degree seat tube angle that fits any riding style.
I’ll only say this:
If you’re a technical street rider and you come across a frame you really like and see it has a 69-degree seat tube angle, I don’t recommend you opt for it.
Stick to 71 degrees.
- A 69-degree ST angle will make your frame feel longer.
- A 71-degree ST angle would make it feel shorter.
A laid-back, “slack” ST angle (68-, 69-degree) gives you more room so you don’t feel so “squashed.”
But I stick to the 71-degree and focus on getting a frame with the right TT length instead.
The less you complicate, the better.
Remember, the angle of the seat tube won’t make as much of a difference as the head tube angle or chainstay will – so concentrate on getting those right.