BMX Part
Chromoly vs Hi-Ten Steel
Chromoly (4130) and hi-ten are the two steels used for BMX frames and parts: chromoly is stronger and lighter for the job, while hi-ten is cheaper and heavier.
Updated 4 min read940 words
Chromoly and hi-ten are the two kinds of steel used for most BMX frames and parts. Chromoly, usually 4130 steel, is an alloy with chromium and molybdenum that is stronger, so parts can be made lighter for the same strength. Hi-ten, short for high-tensile steel, is a plain carbon steel that is cheaper but heavier and less tough.
What the difference means
Both are steel, and both are welded into frames, forks and handlebars in much the same way. The difference is strength. Chromoly can take more stress before it bends or breaks, and it handles repeated loads better. That lets makers use thinner tube walls, which saves weight while keeping the part strong.
Hi-ten needs thicker walls to be strong enough, so a hi-ten part is heavier. Even then, it tends to bend, dent or crack sooner under hard riding.
Types and standards
4130 chromoly
4130 is a standard steel designation. The "41" marks it as a chromium-molybdenum steel, and the "30" means about 0.30 percent carbon. When a BMX brand says chromoly, it almost always means 4130 or a close relative.
Within chromoly, there are further differences:
- Heat treated. Many frames and parts are heat treated, sometimes under a named process such as Odyssey's 41 Thermal treatment, which Sunday uses on its frames. Makers say it raises strength, and heat treating after welding can also relieve stress around the welds. Heat treated chromoly is common on aftermarket frames, forks, bars and axles.
- Butted tubing. The tube walls are thicker at the ends, where the welds and loads are, and thinner in the middle. This saves weight. It is used mostly on chromoly, since hi-ten is not strong enough to make thin walls worthwhile.
- Branded tubing. Some makers use their own named or proprietary chromoly tubing. These are usually 4130 based with particular heat treatment or wall profiles.
Hi-ten
Hi-ten, or high-tensile steel, is a general term rather than one exact grade. It is plain carbon steel without the alloying elements of chromoly. It is cheap, easy to work and weld, and strong enough for light use.
Mixed frames
Many entry level bikes use both. Common labels include:
| Label | What it usually means |
|---|---|
| Full chromoly or 100% chromoly | Every tube in the frame is chromoly |
| Chromoly front triangle | Top tube, down tube and seat tube are chromoly. The rear end is hi-ten. Common on entry level completes. |
| Chromoly down tube | Only the down tube is chromoly |
| Hi-ten | The whole frame is hi-ten |
The fork and bars may be listed separately. A frame can be chromoly while the fork or bars are hi-ten.
Comparison
| Property | 4130 chromoly | Hi-ten |
|---|---|---|
| Strength | Higher | Lower |
| Stiffness of the metal | About the same | About the same |
| Density of the metal | About the same | About the same |
| Weight of a finished part | Lighter, thanks to thinner walls | Heavier |
| Fatigue resistance | Better | Worse |
| Cost | Higher | Lower |
| Rust | Rusts if bare | Rusts if bare |
| Common on | Aftermarket frames and parts, mid to high-end completes | Budget and department store bikes, some entry level completes |
Where it matters on a BMX
| Part | Chromoly | Hi-ten |
|---|---|---|
| Frame | Standard on aftermarket frames | Common on cheap completes |
| Forks | Standard aftermarket | Budget bikes |
| Handlebars | Standard aftermarket | Budget bikes, can bend in crashes |
| Cranks | Three-piece chromoly cranks are the norm | One-piece or two-piece steel cranks on cheap bikes |
| Axles and pegs | Heat treated chromoly is common | Rare on quality parts |
| Dropouts | Usually chromoly plate | On hi-ten frames |
Aluminium is also used in BMX, mainly for race frames, rims, hub shells and sprockets. Titanium appears on a few high-end parts. Neither is common for freestyle frames.
Sizing and compatibility
The steel type does not change part sizes. A chromoly frame and a hi-ten frame of the same design take the same headset, bottom bracket, seatpost and wheels. You can fit chromoly bars and forks to a hi-ten frame, which is a common way to upgrade a cheap bike.
Weight is the practical difference. A chromoly frame is noticeably lighter than a hi-ten frame of similar strength. Aftermarket chromoly freestyle frames commonly weigh somewhere around 2 to 2.6 kg (about 4.5 to 5.7 lb), depending on size, tubing and features. Many current 20 inch frames are listed at around 2.1 to 2.4 kg. On a complete bike, the gap grows when the forks, bars and cranks are chromoly too. A lighter bike is easier to bunny hop and throw around, which matters most for children and smaller riders.
How to choose
- Light riding, small budget. A hi-ten or part chromoly bike is fine for learning basics on flat ground.
- Jumps, drops, street and park. Choose a full chromoly frame, fork and bars. They last longer and are lighter.
- Children. Weight matters a lot. A full chromoly bike can make learning easier.
- Adults and heavier riders. Chromoly is the safer choice. See BMX for adults.
- Upgrading. If you have a hi-ten bike, chromoly bars and forks are good first upgrades, followed by three-piece cranks.
In Australian bike shops, the jump from hi-ten to full chromoly usually happens somewhere between entry level and mid range completes. See how to choose your first BMX for indicative price tiers, and cheap vs expensive BMX bikes for what the extra money buys.
Maintenance
- Touch up chipped paint to stop rust on both steels.
- Store the bike somewhere dry. Moisture inside the tubes can rust them from the inside.
- Check welds for cracks regularly, especially around the head tube, the bottom bracket and the dropouts.
- Retire bent bars or forks. Straightening steel weakens it, and hi-ten in particular may crack later.
- See BMX maintenance basics.
Related parts
Frame geometry, forks, handlebars, cranks and dropouts.
Frequently asked questions about Chromoly vs Hi-Ten Steel
Is hi-ten steel bad for BMX?
Not for light riding. Hi-ten is fine for a beginner learning to cruise, manual and bunny hop. It is heavier and bends or cracks sooner under hard jumping, drops and street riding, which is why most riders move to chromoly.
Is a chromoly frame stiffer than a hi-ten frame?
Not because of the steel itself. All steels have nearly the same stiffness and density. Chromoly is stronger, so makers can use thinner tube walls. That makes the part lighter for the same strength. Stiffness then depends on tube size and wall thickness.
Does chromoly rust?
Yes. Chromoly is not stainless. Both steels rust if the paint or plating is chipped and the bare metal stays wet, so touch up chips and store the bike dry.
How can I tell if my bike is chromoly?
Look for a 4130 or chromoly sticker on the frame, fork and bars, and check the maker's spec sheet. A magnet will not help, since both are steel. Watch for labels such as chromoly front triangle, which means only part of the frame is chromoly.
Related articles
Frame Geometry
Frame geometry is the set of measurements, such as top tube length, head tube angle and chainstay length, that decides how a BMX frame fits and rides.
4 min readForks
A BMX fork holds the front wheel and steers the bike, with offset, steerer size and dropout width setting how it fits and handles.
3 min readHandlebars
BMX handlebars are the tall, swept chromoly bars that steer the bike and take the rider's weight, defined by their rise, width, sweep and clamp diameter.
4 min readCranks
Cranks are the two arms that connect the pedals to the bottom bracket spindle, turning a rider's leg power into rotation of the sprocket and chain.
3 min readDropouts
Dropouts are the slotted plates at the back of a BMX frame and the tips of the fork that hold the wheel axles, and their slot size sets which hubs will fit.
4 min readCheap vs Expensive BMX Bikes
Expensive BMX bikes buy stronger chromoly steel, sealed bearings, better heat treatment, lower weight and resale value, while cheap bikes suit casual use.
3 min read