Steel Frame Tubing - Fact and Mythology Posted on 19 May 19:26 , 1 comment

Is it 531?

We get a lot of questions about old steel frames people have found and are wondering "If they are any good", by which they mean "What is the quality of the tubing?". There are several tubing manufacturers Reynold, Columbus, Tange and Dedaccio being the most famous. Major manufacturers often had their own tubing brands: Tru-Wel was Raleigh's. Manufacturers' own tubing is not always inferior. Perhaps they just didn't want to pay a premium for a well known brand.

The myths inevitably start with Reynolds. Like "531 is the best!" 531 is their brand name for a steel alloy: Chromium-Manganese in fact. The generic "Chromoly" is another alloy. So why does the alloy matter?

There are 3 aspects of tubing to take into account: Composition, Construction, and Heat Treatment.

Composition

This is the composition of the steel alloy used to make the tubing. The primary one is carbon content, but minute quantities of other metals are alloyed with the steel to enhance its properties to make it better suited for bicycle frames. The best steel frames are "springy" and give a livelier feel when riding. The alloy makes no direct difference to the weight, but a better alloy may enable the tubing to by made with thinner walls and therefore result in a lighter frame. Reynolds 531 is just a brand name for their own chrome-manganese alloy. As we know in the modern era, brand names are all powerful and consumers (that's you and me!) will pay more for a recognised brand. It is a guarantee of confidence in the quality. Generic chrome-moly frames are not necessarily "gas pipe". This is just a derogatory term to make you want to buy a "brand".

4130 steel - an alloy steel - which is commonly known in the bike industry as chrome-moly, contains the following alloying agents: 0.28- to 0.33-percent carbon, 0.4- to 0.6-percent manganese, 0.8- to 1.1-percent chromium, 0.15- to 0.25-percent molybdenum, 0.04-percent phosphorous, 0.04-percent sulphur, and 0.2- to 0.35-percent silicon. The other 95 percent is iron. Reynolds 520 and 525 are 4130. It has the big advantage to volume frame builders that it can be welded, whereas the old 531 ca not be welded. Instead it has to be brazed or silver-soldered.

1020 steel, which is called plain carbon steel, and has significantly lower strength than the chromium-molybdenum steels, so the tubing needs to be thicker.

Construction

How the tubing is made is also important. The cheapest tubing is made from flat steel rolled into a tube and welded along the seam. The weld weakens the tubing so it needs to be of a thicker gauge and therefore heavier to be strong enough. The best tubing is die-drawn and seamless. This can be made thinner. Once the tube is made and painted, you can't tell how it has been made, but put your finger inside the tube and you will feel the join. You may have heard the terms "butted" or "double butted". This means the tubing is thicker at the ends, where it is brazed or welded to another tube and thinner in the middle. This makes for an even lighter tube. You may be surprised to learn how thin the tubing is in the middle and how easily it dents, which is why butted tubing is usually reserved for lightweight racing bikes. "Double butted" just means it is thicker at both ends, tapering to the middle and "Single Butted" means it is thicker at one end. Single butted tubes are usually used for stays and forks. Some modern tubes are "triple butted" meaning that there are 3 different wall thicknesses - the thinnest being in the middle.

Heat Treatment

Forming and welding the tubing affects the properties of the steel, so tubing is often "heat treated" i.e. heated to a specific temperature and cooled at a controlled rate, to minimise these effects. Brazing or welding the tubing together to make a frame can degrade or destroy the treatment. This where the skill of the frame builder comes in: under-heating can result in a poor joint and overheating can damage the tubing, possibly resulting in the frame cracking and breaking at some time in the future. This why some tubing manufacturers require frame builders to be licensed to build with certain tubes (Reynolds 753 is an example). Steel tubing was always brazed for this reason as welding creates higher temperatures. Some modern steel tubing can be robot-welded without damage. ~Hence cheap steel frames of good quality emerging from factories in Taiwan nowadays.

 So it seems that the simple steel tubing used to make bike frames is actually a high tech product! It's a complicated business, which is why we tend to rely on brand names and why Reynolds rules! 

Size Does Matter!

Perhaps you have found an old frame, that has been re-sprayed and you have no idea of its quality? Or perhaps it has been resprayed and someone has affixed a sticker, but you are not sure its is pukka? Well it's kind of obvious when you think about it, but seat-post size is an indicator. Nearly all steel frames have a seat tube of 28.6mm outside diameter. The seat post is of course, the inside diameter. Every schoolboy (used to) know that Reynolds 531 butted has a seat post diameter of 27.2mm. So the wall of the tubing is 0.7mm thick at the ends and even thinner in the middle (I said you may be surprised). At the other end of the spectrum some heavyweight bikes take a seat post of 25.0mm which implies a wall thickness of 1.8mm - more than twice the thickness and hence more than twice the weight, especially as it is not butted ! This doesn't mean the frame will be twice as heavy, as there are bottom brackets, lugs, stays and forks to take into account, but it will certainly be a lot heavier relatively. So as a rule of thumb, if you are looking for a lightweight frame, the thicker the seat post the better. This does not of course take into account the composition or construction of the tube and the skill of the frame builder.  So the perennial question is 531 better? The answer is nor necessarily.

What tubing does my frame have? If it is 531 butted  the seat post size will be 27.2mm. If it 531 plain gauge it will be 26.8mm. Anything smaller implies a thicker tubing and a heavier frame.