For some stupid reason, the BMX industry has made a tradition of solely advertising the top tube length as if the top tube length is the only factor that changes the way your bike feels. The top tube alone is a vague measurement or basis to choose the most expensive component on your bike. There are so many other factors that play a huge role in how your bike will roll (see what I did there?). If you have ever picked a frame solely on the length of the top tube this article is just for you. If you believe by changing frames you’ll be able to hop higher, spin quicker, loop out less, manual better or catch tail whips easier you just may be right.
Before we dive in keep in mind the primary concern is where you meet the bike and where the bike meets the ground. Your point of contacts are relative to one another. So even though I will focus on one dimension and make it seem it only changes on point of contact it may change others as well. Also, consider that long and low objects are generally more stable than short and tall objects. To give you an idea, imagine racing a limo above 200 MPH compared to driving a smart car at the same speed. The smart car has a very short wheelbase while the limo has a longer and more stable wheelbase. Which one do you think will flip first?
Head Tube / Steer Tube
This connects your down tube, top tube, forks, stem, and handlebars. The tube will dramatically change the way your bike steers and feels due to shortening or lengthening the wheelbase. Most head tubes are the same length but he angles vary.
Generally, street riders like a steeper HT (i.e 75.5*) because it allows for riders to get into a nose manual and spin out of the manual easier. A steep head tube effectively shortens the wheel base which brings the front wheel underneath the rider and also makes the front wheel feel light while pulling up. A steep HT may also make steering more twitchy at high speeds.
A mellow head tube angle (i.e. 74.5) give riders more stability at high speeds, brings the bars closer to the rider, brings the front wheel out and thus makes the wheelbase longer. Many riders preach it’s easier to nose manual with a mellower head tube angle (75* – 74.5*). The longer front end allows for a wider range of balance but it puts the wheel forward which makes it harder to get into the nose manual. The balance point and range will primarily be affected by this angle, top tube length, and your fork reach. If the front end is short it’ll be similar to balancing a spoon on your finger. With a longer front end, you’ll have more time to adjust and more room for error such as balancing a broom on the same finger.
Seat Tube & Stand Over Height
The seat tube is measured from the center of the bottom bracket to the center of the top tube. The length of this tube is changed to make tailwhips easier, raise your seat for hang 5’s and when looking at overall geometry you will have more traditional triangles when this tube is longer.
By decreasing the angle of the seat tube it effectively lengthens the frames front end but bringing the seat and bars further apart. When increasing the angle, or making it more straight up and down, you are making the front end shorter. This brings the seat and handlebars closer to each other too. Some people like a steep seat tube angle (69*) to keep the desired top tube length while shortening the frame overall. Take this for example:
“Imagine a frame with a 71-degree seat angle 20.5inch top-tube and 75degree head angle; you might think that this would be very similar to a 20.5inch top tube frame with a 72-degree seat and 74.5-degree head angle? BUT you would really feel the difference between the two. The front end of the second frame would be longer by 0.15inches because of the seat angle and another 0.15inches longer because of the shallower head angle, giving a total change of well over a quarter inch.
So remember a degree steeper on the seat tube will mean a frame’s front end is effectively 0.15” longer and a half-degree steeper head angle will make a frame’s front end effectively 0.15” shorter.”
Top-tube length is measured from the center of the seat tube to the center of the head tube. The angle of the top tube is determined by the frames other dimensions and lengths. The length of this tube can affect the balance point on the front end, wheelbase, and how much room you have between the bars and seat. One indication of needing a bigger top tube is hitting your knees on the bars or stem. If one is seeking to gain an extra 1/8 of an inch of clearance they may want to change the head tube angle rather than getting a larger top tube. But, if the persons wants more room for their knees but wants to keep the steering the same then they may consider going with a longer top tube. While the top tube length is important remember the head tube angle, chain stay length, seat tube length or angles can affect balance points, wheelbase and how much room you have to throw barspins too.
Bottom Bracket Height
This is the distance from the ground to the center of the bottom bracket. This will greatly affect the point of contact of your feet.
The lower your bottom bracket is the more stable and heavy your bike will feel. Note, this will also make transitions feels slightly more sluggish while making it slightly more difficult to loop out and pull up.
When the bottom bracket is relatively higher to the plane of your wheels axles it will make it easier to loop out while giving riders the feeling of a “lighter” bike when hopping.
This measurement is taken from the center of your cranks to the center of your rear axle. Chain stay lengths have dramatically changed over the past few years and as with the other dimensions, will change the wheelbase. All BMX bikes had chain stays over 14 inches in the mid-2000’s and now most chain stays measure less than 14 inches.
A longer chain stay makes pulling up to manual or bunny hop harder. In return, a long chain stay allows for a wider range of balance in the manual.
If you shorten the chain stay you may get a quicker pop when bunny hopping. This will make it easier to get into a manual while making the balancing more difficult. It’s similar to the spoon analogy I provided above. You will have less time to adjust the balance with a short back end but in return, you are able to get a more responsive pop.