BMX KNOWLEDGE FROM RIDERS
BMX Frames & Geometry Explained
Choosing the right BMX frame isn't just about picking a brand, colour or top tube size. Frame geometry can make a huge difference to how your BMX feels, from how quickly the front end responds to how easy the bike is to manual, spin or hold a line at speed.
In this guide we'll explain the main BMX frame measurements including Top Tube (TT), Head Tube Angle (HT), Seat Tube Angle (ST), Chainstay Length (CS), Bottom Bracket Height (BBH) and Standover (SO), along with frame construction and important compatibility such as BB type, headsets, dropouts, tyre clearance and brake mounts.
Once you understand what the numbers mean, you can use the Waller BMX Frame Geometry Finder to filter the BMX frames we sell by the geometry you're actually looking for.
Quick BMX Geometry Guide
TT ā Top Tube: The main measurement used to describe BMX frame size and how much room the bike gives the rider.
HT ā Head Tube Angle: Affects how responsive or stable the front end feels.
ST ā Seat Tube Angle: Affects the effective position and feel of the cockpit.
CS ā Chainstay Length: The length of the back end. A major influence on how easily the front wheel lifts and how responsive or stable the bike feels.
BBH ā Bottom Bracket Height: Affects how responsive or planted the bike feels beneath the rider.
SO ā Standover: The height of the frame around the rider and a big part of how the frame looks and feels.
BMX Frame Geometry Explained
Top Tube (TT)
Top tube length is one of the main measurements used when choosing BMX frame size. You'll normally hear riders describe a frame simply as a 20.75", 21" or 21.25" top tube, for example.
A shorter top tube generally creates a more compact-feeling bike, while a longer top tube gives the rider more room. Rider height is a useful starting point when choosing a top tube, but personal preference and riding style also play a big part.
Don't look at top tube length in isolation. Two BMX frames with the same top tube can feel noticeably different because of the rest of their geometry, particularly the head tube angle, seat tube angle, back end, BB height and standover.
Head Tube Angle (HT)
The head tube angle has a major influence on how the front end of a BMX feels. A steeper head tube angle generally gives a quicker and more responsive front end. This can make front-end tricks easier but can also make the bike feel more twitchy at speed.
A more laid-back head tube angle generally gives the bike a calmer, more stable feel. That's one reason you often see slightly more relaxed head tube angles on frames aimed towards trails, transition and riding at higher speeds.
Even small changes can be noticeable. Moving from something like 75Ā° to 75.5Ā° doesn't sound huge on paper, but geometry works as a complete package and experienced riders can definitely feel the difference between frames.
Seat Tube Angle (ST)
Seat tube angle is easy to overlook, but it contributes to the effective position and feel of the cockpit. A steeper or more laid-back seat tube angle changes the relationship between the top tube, BB and the rider's position on the bike.
This is another reason two frames with the same stated top tube won't necessarily feel identical. When comparing frames, look at the complete geometry rather than choosing purely from the top tube measurement.
Chainstay Length (CS) / Back End
Chainstay length is what BMX riders will often refer to simply as the back end. So when someone says a frame has a 13" back end or a 13" chainstay, they're talking about this measurement.
A shorter back end puts the rear wheel further underneath the rider. This generally makes the front wheel easier to pick up and can make manuals, spins and other technical riding feel more responsive.
There is a trade-off. Because the rear wheel is further underneath you, a short back end can also loop out more quickly in a manual. A longer back end generally requires more effort to lift but gives the bike a more stable, less twitchy feel.
What Does Slammed Mean?
When a BMX frame's back end is described as slammed, it means the rear wheel is positioned all the way forward in the dropout. For example, a frame listed as having a 13" chainstay slammed measures 13" with the wheel in that forward position.
The chainstay becomes longer as you move the wheel backwards in the dropout to set your chain tension.
The chainstay measurements used in the Waller BMX Frame Geometry Finder are the slammed measurements, giving you a consistent starting point when comparing different frames.
Bottom Bracket Height (BBH)
The BB height describes how high the bottom bracket and crank axle sit relative to the wheel axles and the rest of the bike.
A higher BB generally contributes to a more responsive feeling and puts the rider higher relative to the wheel axles. A lower BB generally gives the bike a more planted and stable feel.
As with the rest of BMX geometry, BB height shouldn't be judged on its own. A high BB combined with a steep head tube angle and short back end can produce a very different bike to one with a lower BB, more laid-back head tube angle and longer back end.
Standover (SO)
Standover is measured from the centre of the BB to the top of the top tube around the seat tube area. You'll normally hear this described simply as something like an 8.5" or 9" standover.
A lower standover creates a smaller, more compact-looking rear triangle and gives more clearance around the rider. A taller standover gives the frame a more traditional BMX silhouette and more frame around the rider's legs.
Standover can make a surprisingly noticeable difference to both the look and feel of a BMX, so it's worth comparing if you're trying to replicate the feel of a previous frame.
Find a BMX Frame by Geometry
KNOW THE GEOMETRY YOU WANT? LET WALLER BMX FIND THE FRAMES THAT MATCH.
Instead of opening dozens of BMX frame geometry charts and comparing them yourself, the Waller BMX Frame Geometry Finder lets you search the frames available at Waller BMX using the measurements that actually matter to you.
FILTER BY: TOP TUBE ā¢ HEAD TUBE ANGLE ā¢ SEAT TUBE ANGLE ā¢ CHAINSTAY ā¢ BB HEIGHT ā¢ STANDOVER
Replacing a frame you loved?
If your favourite BMX frame has cracked, been discontinued or simply isn't available anymore, look up its geometry and use those measurements as your starting point. You can then filter current BMX frames to find something with similar geometry instead of guessing which replacement might feel closest.
Want to change how your BMX feels?
Start with the geometry of your current frame and then change the measurements you want to experiment with. Maybe you want a shorter back end to make the front wheel easier to pick up, a steeper head tube angle for a more responsive front end, a different BB height or a taller standover.
Use those measurements in the filters and you can see suitable frames from different BMX brands rather than having to search every manufacturer's geometry chart individually.
Compare BMX frames across different brands
You don't have to choose the brand first. Tell the filters what geometry you're looking for and use the results to discover frames from across the BMX brands available at Waller BMX.
USE THE BMX FRAME GEOMETRY FINDER ā
BMX Frame Materials & Construction
Most quality aftermarket freestyle BMX frames are made from 4130 chromoly, but two 4130 frames aren't necessarily constructed in the same way. Manufacturers use different tubing profiles, heat treatments, butting and frame junctions to balance strength, stiffness and weight.
Butted Tubing
Butted tubing varies the wall thickness along the tube. This allows manufacturers to keep more material where strength is required while removing unnecessary material from lower-stress areas to save weight.
Heat-Treated 4130 Chromoly
You may also see frames described as using heat-treated 4130 chromoly. Heat treatment is used as part of the manufacturing process to achieve the material properties and strength required by the frame manufacturer.
Gussets & Invest Cast Parts
Frame manufacturers can reinforce or shape high-stress areas in different ways. Depending on the frame you may see gussets, invest cast dropouts, invest cast wishbones or yokes, seatstay bridges and other specially formed frame junctions.
These aren't automatically signs that one frame is better than another. They're different ways manufacturers design the frame around the strength, stiffness, tyre clearance, weight and riding characteristics they're trying to achieve.
Integrated Seat Clamps
Most modern aftermarket BMX frames use an integrated seat clamp, meaning the clamping system is built into the frame rather than requiring a separate seat clamp around the outside of the seat tube.
BMX Frame Compatibility
BB Types
The main BMX BB types you'll come across are Mid, Spanish, USA/American and Euro.
Mid BB is standard on the vast majority of modern aftermarket freestyle BMX frames. Spanish BB uses smaller press-fit bearings, USA/American uses a much larger BB shell, while Euro BB uses threaded cups and is particularly common on BMX race frames.
If you're transferring your existing cranks to a new frame, check the frame's BB type and make sure you have the correct BB for your crank axle.
SHOP BMX BOTTOM BRACKETS ā | READ BMX DRIVETRAIN EXPLAINED ā
Integrated Headsets
An integrated headset is standard on modern aftermarket freestyle BMX frames. The headset bearings sit directly into shaped bearing seats in the head tube rather than using separate headset cups pressed into the frame.
Older BMX bikes and some entry-level bikes can use different headset systems, so if you're moving parts from another bike it's worth checking first.
SHOP BMX HEADSETS ā | READ BMX STEERING EXPLAINED ā
10mm & 14mm Dropouts
Modern aftermarket freestyle BMX frames normally use 14mm dropouts for a 14mm rear hub axle.
10mm dropouts are commonly found on BMX race frames and some other BMX setups. If you're transferring an existing rear wheel to a new frame, check the dropout size against the axle on your rear hub.
Some frames also have integrated chain tensioners built into the dropouts to make positioning the rear wheel and setting chain tension easier.
Tyre Clearance
Modern freestyle BMX tyres commonly reach 2.4" wide, so tyre clearance is worth checking when choosing a frame ā especially if you want to run the back end slammed.
A frame may have plenty of clearance with the wheel further back in the dropout but become tighter around the chainstays, seatstays or brake area as the wheel moves forward. If you want to run a 2.4" tyre slammed, check the manufacturer's maximum tyre clearance rather than assuming every modern frame will have the same amount of room.
Removable Brake Mounts
Many modern freestyle BMX frames use removable brake mounts. This allows the frame to look clean when ridden brakeless while still allowing a rear U-brake to be fitted when required.
One important thing to know is that BMX removable brake mounts aren't universal. Different frames and manufacturers use different mounting systems, so don't buy a set of removable mounts based purely on how they look.
One common design ā seen on various frames from brands such as Cult, United, Fit, S&M and BSD ā uses a bolt-through style mount. Other frames use a threaded removable brake lug that screws directly into the frame.
Even within brands, mount designs can change between frame models or generations. Check whether the mounts are included with the frame or sold separately and which specific mount kit your frame requires.
SHOP BMX BRAKE MOUNTS ā | READ BMX BRAKES EXPLAINED ā
Gyro Tabs
If you want to run a gyro, check whether the frame's head tube has holes for removable gyro tabs. Many modern freestyle frames are designed this way, allowing the tabs to be removed completely when running a straight cable or riding brakeless.
As with brake mounts, don't assume the gyro tabs are included with every frame. Check the frame specification and what's supplied before ordering.
SHOP BMX GYROS ā | SHOP GYRO TABS & SPARES ā
Adding a Complete Brake Setup
If you're building a frame from scratch or converting a brakeless BMX back to brakes, remember that the caliper is only part of the setup. You'll also need the correct lever, cable, cable hanger, straddle cable, brake mounts and fitting hardware for your frame.
A complete brake kit can be the easiest option when you need the main brake components together.
SHOP BMX BRAKE KITS ā | LEARN ABOUT BMX BRAKE SETUPS ā
Shop BMX Frames
Ready to choose your next frame? Browse the Waller BMX frame collection and use our detailed geometry filters to narrow down the available frames by the measurements you're looking for.
Whether you're trying to replicate the geometry of an old favourite or deliberately change how your BMX feels, you can compare frames by top tube, head tube angle, seat tube angle, slammed chainstay length, BB height and standover.