What exactly is chromoly?

Could anyone please tell exactly what chromoly is and what it’s properties are? Is it just strengthened steel?

Ta, James

good question OWS!

I have just had a chromolly hub put in my MUni (thanks Mike!!)
Seems strong… Roger says it is stronger than a Suzue.

Someone please tell us…


Chromoly is a high carbon steel. Meaning yes it is stronger and you can design stronger/lighter components with it. It is brazable and weldable commonly used in aircraft and solar cars.


Check out the construction of the solar car in action! Yeah, I got to hold the torch half of the time.


From the name, I believe it is an alloy of steel or a coating containing both chromium and molybednium (sp?) for extra protection against the elements.

It’s an steel alloy containing chromium and molybdenum, hence the name.
Check out this website for more info Metallurgy for Cyclists


Cool, thanks. So can it be welded to regular steel? Or can only regular steel be welded to regular steel etc? How expensive and hard to get is it? Also how strong/stiff etc is it in comparison to other unicycle related metals/materials (ie carbon fibre)?

OK, you asked for it…

Yes, basically, chromoly is strengthen steel.

At first, steel is a blend of iron with other elements. The composition of different steel and aluminium can be very complicated, like those NASA-areospatial-secret-alloy…

There is some basic standard classification for steels. The AISI-SAE standard is the most used in the USA. Those are 4 numbers designations, that tell the composition of the alloy. (f.e. the number you see on KH frames: 4130)

There is a serie for each type of steel or alloy:
-10xx, 11xx, 12xx, 15xx = Carbon steels
-13xx = Manganese steels
-41xx = Chromium-Molybdenum steels

The most common steels named ‘‘chromoly’’ are the 4130 and the 1045. Note - their properties change if a heat treatment is done on the part.

Some mechanical properties:

Tensile Strenght (the point where it breaks):
-low carbon steel, as rolled = 61 000 lb/in2
-1045, as rolled = 90 000 lb/in2
-4130, normalized = 97 000 lb/in2

Composition of the 4130:
-0.28-0.33% Carbon
-0.40-0.60% Manganese
-0.035%max Phosphorus
-0.040%max Sulfure
-0.15-0.35% Silicon
-0.80-1.10% Chromium
-0.15-0.25% Molybdenum
-small quantities of certain elements are also present.

My answer is too much resumed to be clear, so feel free to ask if you want more informations.:stuck_out_tongue:


oh what really sucks is that the army has taken preatty much all the chromoly steel, so the guy who is building my trials uni cant finish it because of the stupid army.

You can’t be serious…

Chromoly is readily available in the U.S from a number of suppliers. I’d even bet that there’s enough chromoly scraps laying around some machine shop in Denver to make a trials frame.

I don’t know who fed you the excuse of chromoly not being available because of the U.S. Army, but I’d get a second opinion. Further, you probably wouldn’t be enjoying the lifestyle you do now if it wasn’t for our armed forces.

Wow that was a lot of detail! Thanks! So… can it be welded to regular steel? Or can only regular steel be welded to regular steel etc? How expensive and hard to get is it? Also how strong/stiff etc is it in comparison to other unicycle related metals/materials (ie carbon fibre)?

Chromolly is steel with chromium and molybdenum in the mixture. (“Steel” is any of a number of alloys of iron and other metals.)

When I was a bicyclist in the 1970s/80s, the big thing was Reynolds tubing. Reynolds 531 was the thing to have. Then they brought out Reynolds 753, 453, 501 and 500 (amongst others).

Very roughly, the 3 digits referred to the proportions of three of the various elements included in the alloy (e.g. Reynolds 531 included those three elements in a ratio of 5:3:1 to each other.)

Also very roughly, it happened that the higher the number, the “better” the alloy. So 753 was better than 531, and so on.

Some time in the mid - late 80s, I noticed the term “Chromolly” being used more often. I got the impression that this was a blurring of the distinction between the various grades, so that a basic chromolly alloy could be sold as “the same as” a better one - in much the same way as they used to sell mediocre bikes on the basis of the number of gears (rather than the range of those gears), and they now sell mediocre bikes on the basis of front and rear suspension (rather than the quality of the suspension).

The Pashley MUni has Reynolds tubing, but I note that the stickers only say “Reynolds Tubing” rather than 531, etc.

Thing is, chromolly is stronger than normal steel, weight for weight, and is typically used to make bikes lighter rather than stronger. That means they use less of the steel so the strength is the same but the weight is reduced. However, the stiffness of the steel is approximately the same, so, with less of it to be stiff, the chromolly bike frame ends up more flexible than the standard steel one.

Hmm. They probably need it to protect themselves against stupid bullets. Which is where I’d rather see it go until those boys can get home. But like someone else already mentioned, any shortage of chromoly is most likely a local or temporary one, specific to the supplier your builder works with.

Thanks for the great info on chromoly!

In the US 4130 tubing seems to be easy to buy in multiples of 1 foot lengths. I easily found a few websites which stocked suitably dimensioned material.

In the UK however it seems harder to buy, you can easily get ~5m lengths but nobody seems to sell short bits. It is cold drawn from a large lump, I believe each drawn bit comes out around 5-7m when the required wall thickness is achieved.

T45 is another suitable tube, which is nearly identical to 4130 but is based on a British standard. Race cars, Dragsters and proper go karts are all made of 4130, as you live in Oxford there my be a suitable local fabricator to buy off-cuts from (although a lot of their tubing would be too thick).

You shouldn’t overheat the tubes so, apparently, brazing is best. However race cars are TIG welded (very expensive equipment) rather than the cheaper MIG.

Personally I want to learn how to make my own frames so am planning to take an evening class this September on Oxy-Accetelene welding/brazing.

WOW! Thanks very much everyone!

All you need to know about Chromoly in ONE thread - cool! :sunglasses:

Keg, I also want to learn to weld. Not sure which type (not even sure if there is types, just you said you were going to learn oxy- Accetelene welding/brazing.) I dont know anything about welding at all yet.

I know this is slightly off topic to ‘What is chromoly’, but what is the different to welding and brazing, and what different types is there, like Keg said.



We had a little chat about it before



Wow again! Thanks a lot for all the info! Could you weld the standard bearing housings available on unicycle.com to 4130 then? I’ve been considering a welding course for a while… not entirely sure which types of welding it entails, so will have to look into it!

Thanks to stupid army stuff, your state of Colorado is not part of Germany, etc. Also, your uni guy is giving you a line of BS as the reason for not finishing your uni if he is blaming it on a chromoly shortage.

You can weld all sorts of disimilar metals to each other. The weld becomes an alloy of all the metals involved. If it will be strong is a whole other matter. If the metals are pretty similar you should be fine.

While this thread is wandering off I’ll vote for more unicycles and less guns and bombs. Is anyone making a "make chromoly unicycles, not war’ T-shirt? :slight_smile:

Alot of Molybdenum,(Moly) was taken out of Climax CO. A huge mine complex sits ontop (and under) Fremont pass between Copper Mountain and Leadville. (given all those names, the area is obviously mineral rich). Nearly all the “Moly” that went into warships/planes during WWII was pulled from that mine. The mine is now dorment due to the low need for Moly. If the “Stupid Army” was in such dire need of Moly, the mine would have roared back into operation. It’s not a dead mine; there is still lots of mineral there. Just no market for it right now.

Basically, blaming the Army is misguided. Your frame builder is feeding you a load a crap. Time to pack up your stuff and go next door to the next frame builder.

ps… I spent 9 years active duty with the U.S. Army so yeah, I’ll defend them here. Without armed defenses, we’d all be screwed.
This is a internationally used forum so I won’t go any farther, but we can all agree that every country benifits from their armed services.