Reynolds steel – 725 v 520

Not quite settled on which steel Genesis cross bike, Croix de Fer (725) or Day 01 (520)…….whats the difference in the frame material please?

There must be more than an lx to xt type of reason for speccing the different tubes – give them an email, I’m sure they can reveal all!

From what I understand, 725 is generally regarded as the next step down from 853 in their range, but like I said there are different reasons for selecting certain tubes types. Butting profiles, shape, curves, strength, weight etc.

I think 725 is heat treated, so its stronger for the same area of metal

that means thinner tubes can be used to save weight

if the external tube diamters are the same then the 725 will be less stiff and may have more magical steel feel….

I have a 2011 Equilibrium, which is 725 as opposed to the 520 of last year’s model. Apparently it is lighter. But sadly that is all I know!

from the reynolds website:

…[520 has] The same chemistry as our 725 range but without the heat-treatment process…

from the same source, and for the materials geeks:

520: UTS: 700-900 MPa

725: UTS: 1080-1280 MPa

so, the 725 frame will probably be a little lighter, but i doubt there’s much in it (the 520 equilibrium frame wasn’t exactly a porker).

“I think 725 is heat treated, so its stronger for the same area of metal

that means thinner tubes can be used to save weight”

correct.

“I think 725 is heat treated, so its stronger for the same area of metal

that means thinner tubes can be used to save weight”

correct.

Hmm not quite, if everything else remained the same and the tubes were made thinner then the frame would be more flexible.

thinning a tube wall has far less effect on stiffness compared to reducing diameter, the usual reductions in wall between these grades of steel have a very subtle effect on overall frame stiffness.

jameso don’t say that, you’ll undermine the whole steel/titanium/aluminium debate..

jameso don’t say that, you’ll undermine the whole steel/titanium/aluminium debate..

😆

Just did some sums on this. Reduce wall thickness by 10% whilst maintaining outside diameter and stiffness drops by 10%. However increase outside diameter by 10% whilst maintaining wall thickness and you gain 33% stiffness but only 10% more weight. So when going to a stronger steel you can drop wall thickness proportionally with the strength increase whilst increasing outside diameter by the cube root of the reduction in wall thickness and end up with a lighter frame of equal compliance.

but chiefgrooveguru, if the walls get very thin local buckling becomes an issue with frame strength, not to mention impact resistance.

CGG, thats exactly what is needed as a start for understanding this. The reason why I said

if everything else remained the same and the tubes were made thinner then the frame would be more flexible

was to assist the simplistic statement

so its stronger for the same area of metal that means thinner tubes can be used to save weight

which is a dumbed down version of saying “it is all about understanding how the increase in yield strength can allow you to vary wall thickness and tube diameter to take advantage of the increased stress that the material can take before yield. ”

It’s only because of the misconception that stronger means stiffer which the first statement can imply….

but chiefgrooveguru, if the walls get very thin local buckling becomes an issue with frame strength, not to mention impact resistance.

and this in theory is mega important because resistance to buckling (critical load) is proportional to elastic modulus (E) and not yield stress, and seeing as E is the same for 725 and 520 then the increase in yield strength has no effect on the critical buckling load.

Although it may be that the critical load is much higher than the loads in a bike frame so it might not make any appreciable difference. We shall have to defer to jameso to pass an opinion on that (or any of the other frame dudes).

Exactly Mr Blonde! That’s why you can’t take proportional advantage of the strength increase with fancy steels when it comes to reducing weight and matching stiffness, so you end up gaining some additional strength and not going quite as light as simplistic theory would suggest you can.

This is all riveting stuff fellas, really.

Which is better? Going to be a commuter, maybe a bit of CX racing. Maybe but prob not.

I’ve got a 725 Croix De Fer frame. Technically it’s ****ing heavy.

innit CGG!

OP, if it’s for commuting, the frame won’t make any difference to your journey times and I’m sure both will withstand potholes but equally fail if they get run over by a double decker.

mcboo – as others have said in the past it’s probably better to take the manufacturers judgment on what is better, if they designed to be a cx bike and you are using it as DH bike then it wont be good. It’s probably a waste of time trying to second guess bike designers as jameso has proved. So don’t worry about the material and pick a bike you like that seems fit for purpose..