Which new steel?
The short answer to your question is that Reynolds 725 is ‘better’ than 525. They are chemically the same steel (0.3% carbon, alloyed with chrome and molybdenum), but 725 is heat-treated, boosting its strength by about 50%. Thus a tube of 725 can be made with thinner walls, saving weight without loss of strength – provided that will not make the frame too flexible. (The stiffness of steel is an unchangeable property.)
To make a tube lighter whilst preserving not only strength but also stiffness (in bending and torsion), one must not only make it from a stronger metal but also increase external diameter simultaneously with reducing wall thickness. A fatter, thinner-walled tube is more vulnerable to denting, and making it from stronger steel only slightly improves its resistance to such damage. With a diameter-to-wall-thickness ratio in excess of the engineer’s 50:1 rule of thumb, lightweight steel bike frames are already dicing with dents!
So whilst the answer is simple, the question is much more complicated. What do you mean by better? If you mean lighter, the answer is probably not. Reynolds’ catalogue shows that for every 725 frame tube (in the main triangle at least), there’s a 525 tube of identical diameter and wall thickness or ‘gauge’ (usually 0.8/0.5/0.8mm). And as neither is available in a great variety of sizes, there’s little opportunity of making a lighter and equally stiff frame from 725 versus 525, except for a few grams here and there. (There is much more choice of tube diameter and gauge in some other new steels, such as 631 and 853.)
If by better you mean stronger, then yes, the 725 frame should be less likely to break – unless the builder makes some very strange tubing choices indeed! If a better frame for you would be just as strong but lighter and more flexible, then that is possible with 725, but only if the designer of the 525 frame has chosen to build with unusually fat tubes.
How do these compare with 531? Reynolds 525 is about the same strength, whilst the much stronger 725 is comparable to 753 (but far easier to weld). How they differ is that whereas a 531 down-tube, for example, was almost always 28.6mm diameter, but came in various wall thicknesses, modern tubes come in a variety of diameters, most of them fatter and all much the same wall thickness. Comparisons are more about dimensions than materials. A simple 531 double-butted decal gives no clue about wall thickness – apart from thicker at the ends than the middle!
In the 1980s, Reynolds sorted their tubes into sets, and ‘531 Competition’ meant a 28.6mm down-tube with 0.91/0.61/0.91mm walls. A ‘531 Professional’ down-tube was 0.71/0.55/0.71mm (–17% in weight and stiffness), and a ‘Super Tourist’ 1.02/0.71/1.02mm (+14%).
The lightest 525 or 725 tube that could possibly be used as a down-tube is also 28.6mm and 0.7/0.4/0.7mm. But at 28% lighter and more flexible, I doubt that’s stiff enough for a down-tube and think it’s intended as a top-tube. It’s 18% lighter and 3% stiffer than the 1in 531C top-tube. The usual 0.8/0.5/0.8mm walls of 525 or 725 result in a down-tube that’s still 14% lighter and floppier than 531C, but (as suggested above) modern steel frames are usually stiffened by a fatter top-tube. Many have a fatter down-tube too, 31.75mm diameter, which comes out 5% lighter and 17% stiffer than 531C – but more easily dented.
For a bit more resistance to accidental damage of all sorts, I’d choose 725. Otherwise, there’s not much in it. You can generally reckon on the modern frame being a bit lighter and stiffer than your old one.
This was first published in the February / March 2014 edition of Cycle magazine.