Most BMX forks are the same size or extremely close.
Brands follow an industry standard for maximum compatibility and to ensure that their forks do not cause major changes to the geometry of the bike as those can notably affect handling.
Despite the similarities, it’s still necessary to take into consideration the specifications of a fork before installing it on a bike.
Axle To Crown Length
The first important measurement of a fork is the so-called Axle to Crown Length (ACL).
The ACL is the distance between the middle of the axle (the part of the hub around which the wheel rotates) and the top of the fork’s crown.
In most cases, the ACL of BMX forks is 315mm/12.4in (the industry standard). That said, in some cases, the ACL could be a bit shorter (310mm) or longer (320mm).
The ACL of a fork has a notable effect on the overall bike geometry as it affects the so-called head tube angle (HTA).
The HTA is the angle formed by the head tube and the ground. A long ACL results in a slacker HTA and a taller bottom bracket whereas a shorter one steepens the HTA and lowers the bottom bracket.
A slack HTA makes it easier to lift the front wheel but hurts the bike’s maneuverability at slow speeds and increases body drag (hardly an important characteristic in street BMX). Conversely, a steep head tube angle makes it more difficult to lift the front wheel because more of the rider’s weight is on the front end.
The next measurement that has to be analyzed is the steerer’s (the tube to which the fork’s legs attach) length.
The steerer of most BMX forks is about 160mm/6.3in. That said, bikes with extra long head tube will require a longer steerer. Since those frames are fairly rare, only a handful of manufacturers offer forks with a steerer that’s significantly longer than 160mm.
Note: If you have an MTB background, you may expect that the steerer has to be cut to accommodate each individual frame. However, this isn’t the case with BMX bikes.
First, the average fork does not have to be cut to accommodate the average frame.
Second, newer BMX forks operate with a compression bolt rather than a star-fangled nut. (The purpose of both is to “pack”/tighten the headset). Star-fangled nuts are wedged into the steerer and do not require threads. Meanwhile, compression bolts are threaded into the steerer. Thus, the steerer of forks designed for compression bolts has threads on the inside. If the steerer is cut, the threading will be cut too.
FAQ: What is the diameter of a BMX fork’s steerer?
The standard diameter of BMX steerers is 1 ⅛ inches (29mm). The headsets reflect that. That said, some BMX bikes designed for children could have a 1-inch steerer.
The next measurement that one has to take into consideration is the so-called fork rake or offset.
The term fork rake refers to the hub’s offset from the steering axis.
Or in simpler words, the offset indicates how far away the fork dropouts are from the fork’s legs.
A longer offset increases the stability of the front wheel and thus facilitates the balancing part of nose manuals. The downside of a longer offset is that the handling isn’t as snappy.
A shorter offset, on the other hand, makes it easier to initiate the nose manual but harder to balance it because it’s very easy to loop out. Also, a shorter offset increases steering sensitivity (responsiveness).
If you’re replacing a fork and want the new one to handle like the old one, you have to get a model with the same or a very close offset.
The offset of BMX forks ranges between 26mm and 33mm.