giskard

I'm in the process of choosing a new bike that I want to last for years of daily use commuting to work about 15 miles per day, and with some weekend riding with my cycling buddies. I've narrowed down my choice to a couple of bikes, one with a Reynolds 520 steel frame and the other with a Reynolds 725 steel frame. I've read the specs on both frame materials, basically the 725 is a heat-treated version of the 520, so it's stronger than the 520 (as borne out by the specs I've linked to). But I'm guessing with the added strength of the 725, the bike manufacturer will have made the frame from thinner tubing to reduce the weight so the overall strength of the frame might not be much better than the 520 frame, and will in fact be a lot stiffer than than the 520 frame, and therefore the 725 frame could possibly be less durable and/or more prone to failure?

I'm wondering what any frame experts make of what I've said, or can offer any further advice or info?

Thanks in advance.

Andrew R Stewart 

The 725 might be made from larger diameter tubing. This is what would make the frame stiffer. The use of stronger material allows the thinner walls that allow the larger diameters without a weight gain. But for your use range (especially commuting IMO) the frame's design and braze ons are a greater issue.

I build my own frames and chose basic tubing for my commuter (which has gone on a number of credit card tours as well) in traditional diameters for a few reasons. First is that it works very well. Second is that with the banging around that can happen on both a commuting bike (locking up, car toting, carrying gear bags) and on a supported tour (being handled by staff, less then ideal transporting, being leaned up against walls and fences often) the thicker walled top tube helps prevent dings. Third is the lesser cost. And did I say that it works really well, the slightly more flexie nature of the traditional diameter walls is nice on the long tour days too.

But I also attached all the braze ons that I needed for the fenders, racks, pumps, cages, chain (peg), slotted cable stops and guides with threaded adjusters that I deemed right for a bike that will need routine servicing for consistent performance. In fact in daily use it's these add ons that make the greater difference then any amount of flex or stiffness. One bolt does only one attachment job on my bikes.

Lastly I designed the frame to be able to use tires up to a 28mm width with fenders, have stable handling and proper stand over height without a reach compromise.

So I'd look at the two possibilities with an eye for the details that affect your use of the bike. IMO the current mania about stiffness is a red hearing and totally misplaced media manipulation. Andy.

fietsbob

Past what the tube set is , Look into a custom hand made frame for the more meticulous building care,

if you wish for a super light steel frame that lasts.. the who as well as the what.

and don't crash it.

HillRider

If the tubing outside diameters are the same, the frame stiffness will be identical or perhaps very slightly lower if the 725 has thinner walls. Otherwise compare weights and, if the 725 is lighter, the walls are thinner. As to overall strength, both will be more than adequate.

Wilfred Laurier

I would go for the heavier (probably the 520) frame as it likely has thicker walls and will be more resistant to denting, which is a real (although generally not to important) risk with a commuting bike. The heavier one will also likely be a bit stiffer, which might or might not be somewhat advantageous, depending on your weight and the amount of stuff you want to carry.

These are all generalizations, though, and may or may not be true for specific bikes. Other concerns (such as bike fit) are probably more important. I would avoid getting the lightest steel frames for a commuter bike as my experience has been that they possibly do not have the durability for day-in-day-out use for decades.

Also, what bikes are you considering? Are they from the same manufacturer?

giskard

Thanks Andy, that's all really good information and advice. I believe the frames of the two bikes are double-butted, so thicker at the weld ends of each tube.

So if the tube diameters of both frames are similar then the 725 will be much stronger than than 520? The practicalities you mention are worth me bearing in mind when choosing which frame/bike to go for, assuming they both test-ride well. Regarding stiffness, yeah, the media seem to imply that unless the frame is really stiff then some of your cycling energy is dissipated by the frame flex (especially on hill climbs), but I'm guessing the reality is that such losses are negligible?

I'm pretty sure both frames are rack and mudguard ready, with plenty of clearance for wide tyres if needed.

giskard

I'm ideally after the best compromise between frame lightness and durability, if that's possible?

giskard

Thanks, so you mean that a thinner-walled 725 will be less stiff than the 520 if same dia. tube? I guess that makes sense and from what you and others are saying, greater stiffness will come from a large dia. tube?

giskard

I'm pretty heavy so a stiffer frame will be welcome and I suspect the 520 frame will be heavier, though given both bike are from different manufacturers and with differing componentry, weight comparisons of the frame may be difficult to assess. I wouldn't necessarily be carrying much, just a fully-laden backpack on my back and some wet weather gear/shoes on the rear rack.

My conundrum is that I want a bike that will last, that will be a reliable commuter but also allow me to keep up with my friends on their weekend road-bikes, so I guess I'll be compromising more towards durability than lightness and out-and-out speed, just a shame I can't have both but then that's life I suppose.

The bikes I'm considering are the Norco Search S1 and Genesis Croix de Fer 20, the latter is the 725 frame, the former the 520. I'm trying out both bikes next week but on paper, the Norco appeals to me more because of the 105 group set and the carbon fork, both of which my (frame-cracked) Kona Jake the Snake has, though the Tiagra components on the Genesis are also very good.

fietsbob

A professional Frame building specialist will select a tube set Mix in consideration of your Weight.

Maybe you would be better served with a mass produced frame as the Commuter and the Hand made piece of Metal Arts And science custom one for the weekend?

In the late 80's and early 90's There were a Lot more Regional frame builders and store fronts in Britain that there are remaining.

FW Evans used to make their own Frames , and now sell Taiwan made brands, as an example.

giskard

That's a sensible and good suggestion but I'd need to store 2 bikes rather than just one and then there'd be the increased cost

It might be worth my while talking to some frame builders in London about what I'm after.

davidad

My understanding is that the 725 tubes were designed to be silver soldered in a lugged frame to keep the heat below the transition temp. If they gas tungsten arc welded it should be heat treated to bring it back to spec.
The 520 will be ok as welded.

The most important component on the bike is the engine. If you have a big enough engine you can keep up with your group no matter the horse.

Andrew R Stewart 

Ah, more data. The more a tube is tweaked (heat treated, thinner walls, manipulated) the more likely the cracks will be. There's a reason that heavy thick walled frames of relatively low strength steel are both common and long lasting. Yes, the problems with marketing sourced data is that it's hard to make direct and unfiltered comparisons. So your riding the two bikes is the right next step. Which feels right is important, but try to look past the moment for the long term if you can. Andy.

rpenmanparker 

There is a lot of talk here about what could be the case and happen, but not enough emphasis on the most likely situations and outcomes. The 725 tubing signifies a higher performance bike. It is plenty durable enough. There is no need to think of it as fragile. OP needs to find out how the tube sets compare in gauge and diameter to know how they will compare in weight and stiffness. Thicker walls make a steel bike heavier and stiffer. Larger diameter does more to increase stiffness than does thickening the tube walls in the usual scenario. So the common compromise is for the diameter to be increased and the wall thickness decreased. That gives very good stiffness at a reasonably light weight. The thinner walls benefit strength wise from stronger tubing like 725 to make up for the thinning. Yes thinner walls and larger diameter tubes are more susceptible to denting than otherwise, but not drastically so. Designers generally stop the thinning and increasing the diameter before they get to that critical situation. If you can afford the 725 bike, that is the one to have. If you can't the 520 bike is a workman-like compromise. Just don't let misplaced concerns turn you off the better product for no good reason.

Trakhak

Steel frames of moderate weight are almost certainly still the best choice for durability in a commuter bike, but you did ask about lightness as well. It's always worth looking at the article at the link below (an English translation of a frame fatigue test performed by some engineers for the German magazine Tour in 1997).

Short version: of the high-end, racing frames tested, the aluminum and carbon fiber frames survived the testing best, and the steel frames did considerably less well. Of course, they tested racing frames, not touring frames; heavier steel frames might have done much better in the test than the steel frames they tested. Still, the results seem counter-intuitive if you believe that steel frames are always the durability champions.

EFBe Biketest: TOUR-Frametest

giskard

Thanks for posting that very informative link, from that my understanding is that the lighter alu frames did well because of their design compared with the steel frames, and as you say, they didn't test steel touring frames. From what I've read, alu is easier to work with (compared to steel) to design very strong frames.

As a larger rider, a frame's strength and durability is more important to me, hence my consideration of steel frames in favour of aluminium, but the particular characteristic of steel over alu frames - that appeals to me - is that aluminium will always fail due to fatigue, whereas my understanding is that steel has a fatigue limit, meaning that steel will only fail due to fatigue if forces exceeding the fatigue limit are cyclically applied, and that limit is higher for high-strength steel.

Of course, the other factor in all this is that generally the frame designers make allowances for the characteristics of the frame material in the design of their frames.