Steel tubing for Cycle-tourists, Reynolds 531, 520, 853, True Temper – confused?
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An idiots guide to steel cycle-tubing and how not to get fixated on numbers.
This is the latest in a series of articles on Reynolds frame tubing. Many years ago (25!) I wrote a short article in the very first on-line cycling magazine – cybercyclist.com– now long gone (but it still lives on our cycling holiday website) about Reynolds 531. It came at an important time as it was then that 531 was finally being phased out in favour of more TIG-welding-friendly tubing (more on this later). You will also find a basic article on bicycle frame stiffness here.
Since then the market has changed from one where perhaps 80% of all quality touring bikes (maybe 95%+ in the UK) were made in Reynolds 531, to one where where none are. So time for an overview to help all those confused by the plethora of tubing types now available.
Steel -”The Real Deal”
I've already written an article on why steel is still the best material for a touring bike frame, but here it's important to understand what is going on with steel.
The first and perhaps most common misconception is that better steel is stiffer than something made out of steel gas-pipe. No! Stiffness is measured by a material's 'Young's Modulus' and all steels have a YM that falls within very tight limits, in fact Reynolds* themselves measure a figure of 207 GPa** for ALL their tubing regardless of quality.
So, and I'll put this in bold because it's an absolutely fundamental point –
“If you build two identically dimensioned frames, they will feel identical to ride regardless of whether made from gas-pipe tubing or Reynolds latest 953 Maraging steel.”
But only briefly – a mild steel frame built with the gauges of a top tubing would fail completely, or bend in normal use – sorry but it doesn't work;-)
Strength (UTS) and Yield Strength.
So what do you get when you pay for expensive steel? Well you get higher UTS (Ultimate Tensile Strength) and Yield Strength. These two are obviously important - the most important is not the UTS (which manufacturers tend to concentrate on) but the Yield Strength.
What's the difference? - Well in both cases the material is put under load until it fails. UTS is the point where the material actually snaps or fails completely – Yield Strength is the point at which the material goes beyond its Elastic Limit (from which it can spring back) and deforms permanently (or plastically), on a bike that would be the point where you'd permanently bend or dent a frame-tube. As a bike with bent tubes is difficult to ride it's this figure which is more important to us riders. The odd thing is that manufacturers generally ignore this figure in preference to the UTS – Reynolds for example only quote UTS on their website.*
Here the wonders of metallurgy come to our rescue as the differences between our mild-steel frame and top tubing are absolutely massive!
Mild Steel is what you'd expect your 'supermarket' bike to be made of and has a pretty pathetic Yield of 247 MPa**.
Now go to the most common, quality cycletube steel which is 4130 Cr-Mo*** – this has almost identical properties as the old Reynolds 531 and is what Reynolds 525 and 520 is made of, it's also almost certainly what any quality bike manufacturer uses for their own-brand or generic Cr-Mo tubing - This gives a typical UTS of 1110 MPa and a Yield of 951 MPa – a big improvement...
Now let's get silly and go for the very strongest steels on the Planet – called Maraging steels (like Reynolds 953) the example having a UTS of 2693 and Yield of 2683. Compare those with the two cycle tubes above and you'll see the huge potential of those 'super' steels.
The striking thing about the above figures is firstly how easy it is to bend mild steel – you'd need nearly 4x as much steel for the same resistance to failure (permanent bending) as a 4130 tube. That is why cheap bikes are so heavy**** Also you see why some steels are seen as 'brittle' - that Miraging steel has almost identical UTS and Yield so that the chances of permenently bending the steel without breaking it is practically zero.
On the other hand, if you used Maraging steel your frame would be less than ½ the weight of a 4130 frame for the same strength! Amazing! Except is doesn't work quite like that as we'll see later.
Fatigue is where a material may fail at well below its UTS because repeated flexions have weakened it. This is why every Aluminium component in an aircraft has a fixed life measured in hours before it is at risk of failure. Steel on the other hand has a Fatigue Limit. Any flexing below that Fatigue Limit will NEVER fail – this is why car springs are made of steel not Aluminium. Every time a steel component is flexed beyond its Fatigue Limit it will weaken and the count-down to failure begin. Better steels have a higher Fatigue Limit (generally around half their UTS) and so again can be thinner and flex more before failure – hence 'spring steel' - but generally speaking a frame in quality steel like 4130 should be designed to be well below its Fatigue Limit in normal use with the gauges used for cycletouring and so last indefinitely.
Seamed, Seamless, Butted and Oval tubing
The first two are simple. You can make a tube by taking a sheet of metal and then rolling it into a tube (like a rolled-up newspaper) and welding the joint. The joint is then rolled and finished to be flat. A Seamless tube is drawn from a solid billet (lump), and so has no join, no welding impurities and will be totally uniform. Seamed tubing is cheaper to make and can be made in much longer lengths, but though used by many manufacturers. it is generally considered to be inferior. If you look at a piece of cheap steel tubing you will see the join as a different coloured line running down the tube. For example the old Reynolds 531 and all better tubes from Reynolds are seamless as is their Cr-Mo 520 and 525 (identical except 520 is license built in the Far East) so not all 4130 Cr-Mo is equal.
Another construction technique is 'butting'. A cycle tube carries more stress at its ends, partly because of the join, and so by making the tube thicker-walled at the ends than the middle you can lose a little weight, or gain strength where it's needed. Perhaps more importantly, by giving a little less stiffness you can allow the frame to 'spring' a little which gives a more lively feel and gentler ride. This is why all better frames are built with butted tubing – again the Cr-Mo label doesn't guarantee this so read the small-print when comparing bike specs – if it doesn't specifically say 'butted' then it's plain-gauge and will be heavier and harsher.
Lastly we are seeing an increasing number of frames made with oval rather than round tube. By ovalising a tube you effectively make the tube act like a larger diameter tube in one direction (so stiffer) and a smaller diameter in the other (more springy). Done well this can make quite a difference to the feel and performance of a frame regardless of material.
Nowadays with almost all frames coming from the Far-East you can recon that any frame stickered with 'Thunderguts Cr-Mo' or such like will be a generic, seamed Taiwanese or Chinese Cr-Mo and watch the label to see if it is butted. I may be being unfair, but personally I like the reassurance of a big name like Reynolds (UK), True Temper (USA), Columbus (Italy), Tange (Japan) and so-on – after all it's not an exaggeration to say that your life may depend on the quality of your frame material.
But here I have skirted around many factors affecting different steels. Many Cr-Mo tubes will have different heat-treatment which alters the limits considerably – likewise for frames it is often the characteristics after joining which are the critical ones – 531's relatively poor reaction to TIG welding is why it's obsolete even though it's figures still look good against Cr-Mo.
The problem with joints.
As hinted above the one 'Elephant in the room' is what happens when you join tubes using brazing or TIG welding. The whole process of heating tubes to very high temperatires when joined has massive effects on all the steel's qualities. As one correspondent wrote to me - Comparing pre-and post joining properties is a bit like comparing a cake mix with a cake; it is 'completely different once cooked'. And of course the heating effects of TIG are much greater but concetrated over a small area than traditional brazing. My own Bob Jackson frames are joined using lugs and low-temperature 'silver' solder and so their 531 frames don't suffer much from this, but a general rule would be that the hotter the joining method the greater the change in the material and generally (not always) for the worse. Fatigue values for example can plummet in the HAZ (Heat Effected Zone). Touring framesets 'should' be thick enough to cope with this on a well-designed frame, but the details are beyond the scope of this article and will have to be covered later.
Building a Frame – bigger numbers are best?
This article is primarily concerned with cycle touring framesets so forgive me if I don't dive into the esoteric world of racing cyclists where super fit riders will stress super-thin tubes to their limits and beyond.
Looking at what I've written above it would appear that the obvious thing would be to simply buy the best tubing (the biggest number in Reynolds-speak) for your frame, but that would be to oversimplify the case. Obviously the idea of riding a mild-steel bike round the world doesn't appeal, but if the tubes were thick enough, and your legs strong enough to pedal it, the bike would actually do the job very well. However it would feel heavy, slow and very, very stiff – why? Because the tubes would have to be so thick to give the required strength and given that all steels are equally stiff it would be like a garden gate to ride.
And that is also the reason that your 953 touring frame really can't be ½ the weight of a 520 frame;-) You see what's coming? If you take a 520 frame that rides well and is stiff enough to cope with heavy touring loads you could build the same in ½ thickness 953 and have the same strength but the frame would be ½ as stiff – i.e. hopeless!
If you were doing a complete, clean-sheet sheet design you should decide on the stiffness you required in a frame and specify wall thicknesses/diameters etc to give you that and THEN calculate which frame material (with suitable headroom) would be strong enough not to fail in use. To then use a steel of higher spec would give you no advantage – just bragging rights and a posh label on the bike. And of course that frame design also has to take into account the fact that a large frame will not only generally carry more weight and need to cope with more power, but also have greater unsupported lengths of tubing and so will need either thicker, or larger diameter tubes for the larger frame sizes for a specific model. One of the reasons for shelling out on a custom built frame is that a good frame builder will ask you not only your size, but also weight, power and what you intend to carry on the bikes and size the tubing appropriately whereas a manufacturer simply has to guess an average – this was one reason 531 was so popular with framebuilders, because of the almost infinite variety of tubes available.
So expensive tubing is a waste of money?
I'm being a little disingenuous here, because you can do some clever stuff with the better tubing. Because of the much greater Yield Strength you could up the diameter of tubes and make the walls much thinner – this would get back the lost stiffness and the high Yield would still resist denting (one reason why 4130 can't be made too thin-walled) which is always a risk on a touring bike whereas it's not on a racing bike. Or in a situation where the bike is stressed well beyond the needs of normal cycle touring - e.g. after being hit by a car – the frame might survive where the lesser frame would bend... However the advantages of the former approach would be relatively small in terms of weight on a loaded tourer and the latter – a crash that would bend a 520 frame but not a 953 – so rare it's probably of minor consideration unless money really is no-object. The other argument is that most top touring frames are also Lugged rather than TIG welded but I don't intend to open that can-of-worms here...
Are there any advantages in the cheaper tubing?
Actually yes... Apart from the cost, the big advantage for cycle tourists, who tend to keep their bikes for a long time, is that they can be modified, bent about and repaired in-the-field in a way that better tubing cannot. Top tubing has a problem in that the Yield Point is often very close to the UTS (see example above) so that if you deliberately try to bend it (or 'cold-set'), for example to fit a wider rear axle or straighten a bend, the steel may well break - it's 'brittle'. If the bike needs a repair then the thicker, cheaper tubes can be welded with the sort of equipment you find away from a framebuilder + a bodge repair to get-you-home isn't going to scrap a £1000 frameset;-) Lastly, if you ever want to drill or cut the frame to add braze-ons or a dodgy repair it's very difficult with a top tubing because it is as hard as the tools trying to cut it. For these reasons an Expedition bike is probably best in something like Reynolds 525/520 if you plan to go away from 'civilisation'.
For 70 years Reynolds 531 was the faithful, go-to tubing of choice for almost all quality touring bikes. Because it is less suitable than others for TIG welding its been totally replaced in a market dominated by TIG welded frames from the Far East. My contention is that if 531 did just fine for all those years its replacements from Reynolds (520 and 525 Cr-Mo) and others will do the job. At the gauges needed for loaded cycletouring the lesser tubes are strong enough not to fail and yet can be thin enough for a good ride. Yes, if you can afford it the top-tubing will do no harm, especially if combined with the skills of a custom builder, and of course there is something very nice about riding a 'Rolls-Royce' of the cycling world, but in the end a well designed, quality Cr-Mo frame should be good enough... With this in mind we designed out BB Special touring bikes (which are used for cycling holiday hire and cycle up mountains, endure crashes, scrapes and general abuse whilst remaining light and comfortable) specifically along these lines with a Reynolds 520 frame. In over 25 years and well over 100,000 miles we've never had a frame failure and only one non-critical dent in a top-tube - I think that needs no further comment.
**MegaPascal - 1 MPa also = 1N/mm^2 also ~= 10kg/cm^2
***https://en.wikipedia.org/wiki/Ultimate_tensile_strength - I did a fair bit of internet research on various UTS and Yield figures and was amazed at how much they varied between sources to the point where I was confused and disheartened as I like to be definitive in these articles. Hot or cold-rolled, heat treatment, the effect of joining and many other parameters seemed impossible to nail down so please excuse the generalisations. For the moment (and if I find better I will edit) I have used Wikipedia figures because they come from one source and so should be relatively if not absolutely accurate but readers should be aware that there is far more to this subject than covered in this inevitably general article. – however this does not affect the general points made in this article.
**** Ever wondered why your kids 16” bike weighs more than your touring bike – well now you have the answer – most are made of mild steel...