The importance of frame compliance over rigidity

[531colin]

IMG_5884 by 531colin, on Flickr

This is the easiest comparison I could make between a 631 fork blade and a Reynolds "R" fork blade, without doing lots of spannering.
Bearing in mind CJ's comments above about the different steels being comparable in stiffness, I really don't see how a 631 fork is stiffer than an "R" fork.

IMG_5886 by 531colin, on Flickr

Goodness only knows how old that is or where I got it! There were no 631 blades for disc brakes then.

CORRECTION that is in fact a 631 blade for disc brakes, with the larger diameter tip. The taper still starts high up.

[531colin]

I am going to say where I think the "comfort" comes from in a traditional steel frame; this is a frame with a one inch top tube and steerer, and inch and eighth seat and down tubes. (the steerer is thicker wall than the top tube)

When the front wheel goes over a bump, the fork, the steerer, and the front of the top and down tubes flex, so that the frame "nods its head".

A lot of people observe that an old-fashioned curved fork in an old-fashioned frame flexes more than a straight fork.
Theres nothing wrong with the observation, but theres everything wrong with attributing the flexing to what you can see, the curve in the fork. Look at my picture in a previous post. If the fork is to flex at its curve, the lever is only the fork offset, so between 40 and 60 mm. In fact, the fork flexes fore and aft at the tip, like a pendulum, and the head tube nods, because the lever is somewhere around 350mm.

If the front wheel, acting through the lever of the fork, can flex the frame's main triangle, then the back wheel can flex the rear triangle, and possibly the frame as well.

[531colin]

Long ago, Spa were looking for a different factory (in the far East) to produce steel frames.
We sent out drawings of a couple of sizes of steel (rim brake) Audax bikes, and the prototypes duly came back.
One numpty had made theirs up in really big tubing, instead of the "just slightly oversize" tubing we used (and is still used)

A couple of us rode them, which was an interesting experience. With the same carbon fork, same wheels, same 25mm tyres at the same pressure, riding the "new factory" bikes (on a tar/granite chip road) was like having the soles of your feet beaten with a stick.

I don't know if the seat/chain stays were also oversize, its a long time ago.

[Carlton green]

..................
My daily ride is built from 18-23 which I had thought was likely not of any worthwhile quality. However research elsewhere says it’s perfectly fine (fit for purpose) and your post suggests to me that that should be the case too (if designed and built well). It glides along and seems to get me up the hills too … the fitter I get the better it climbs.

"18-23" is a Raleigh trade mark for tube; theres a (2007) post here from CJ viewtopic.php?p=30668&hilit=18+23#p30668 saying what it is and how its made; but to me more importantly CJ also says all varieties of steel have the same density and the same Young's modulus (stiffness); where fancy steels differ is in their strength, so you can have less of the fancy steels, this usually means thinner walled tube. The backside of this is that some frames made of cheap oversize tube (which is also relatively thick-walled) are really stiff; good for powerful riders' flat-out efforts, or a camping load; less good for light riders all-day comfort on an unloaded bike.

Thank you for that link and the other insights too; funnily enough I was reading that thread the other day. That thread, and another on a different site, prompted my earlier comment. Now here’s a comment from a well known Yorkshire man that I cheekily lift from the CUK thread:

Minor differences in the tubing will be overwhelmed by differences in frame design and standards of construction.

It’s really interesting following the thread and hearing the voices of experience - certainly challenging preconceived ideas.

And from Hubgearfreak:

… and if it's anything like my 18-23 raleigh, a joy to ride

but not the lightest. by the time you add mudgaurds, rack, lock, tools, water, yourself. the difference between it & 531 say will be around the 0.02% mark

Being myself several kilos overweight a slightly heavier frame is of little concern, but how it rides is.

Raleigh's 18-23

Cheaper and heavier isn't necessarily inferior.

they are also virtually invisible to theives

Yes, different isn’t always inferior, and the less attractive something is to thieves the better.

Looking through the material specifications I believe that 18-23 is a low carbon steel, as such - unless I’m mistaken - it can be welded and its characteristics don’t change with (high temperature) heat. So 18-23 is much more readily repaired than ‘alloy’ steels, more easily worked in manufacture and doesn’t soften after being (high temperature) heated.

18/23 steel refers to the Carbon content of the steel.

I used to work for British Steel and we supplied 18/23 steel to Tube Products in the Midlands.

I spent quite a few days at their factory watching them make it into tubes.

It is good strong steel - at the time they were making prop shafts for Bedford trucks and Jaguar cars.

For interest, the chemical specification of the steel was:

Carbon 0.18-0.23%
Silicon 0.035% max
Manganese 0.4-0.6%
Phosporus 0.025%max
Sulphur 0.01%max
Nitrogen 0.008%max
Aluminium 0.025-0.06%max

We supplied it as hot rolled pickled strip which they then cold rolled and roll formed into tube.

[Brucey]

......If the front wheel, acting through the lever of the fork, can flex the frame's main triangle, then the back wheel can flex the rear triangle, and possibly the frame as well.

sorry, I don't quite agree with the logic even if I do agree with the sentiment. FWIW I think that straight seat stays may see a form of Euler buckling. The chainstays are often a lot like a set of straight fork blades but they don't get to flex in the same way (which might be just as well, given how corroded some of them get) except perhaps in soft-tail and curly stay designs

[jimlews]

In CTC magazine of August/September 1994 Mr Chris Juden wrote:

"I don't think variations in frame stiffness made any significant difference to comfort scores, except for a more or less springy fork".

[Brucey]

I more or less agree if we are comparing strictly conventional frames (edit; designed to carry a load). However, if we include soft-tails and frames with curly/superthin/superflexy seat stays, it is far less clear-cut.

[pwa]

As a big bloke (tall, and not slender) I think if I were going out buying a bike for long day rides today, I'd separate the contradictory requirements of stiffness (which I need) and a cushioned ride (which I want) by combining a relatively stiff frame with tyres that are big enough and supple enough to do the suspension job. And that would mean brakes that are not the restrictive calliper brakes I used to go for.

[slowster]

Bearing in mind CJ's comments above about the different steels being comparable in stiffness, I really don't see how a 631 fork is stiffer than an "R" fork.

Bear in mind that I did not claim that it was. I was repeating Dave Yates' comment stating that 853 was too stiff for an audax fork, and that 'R' blades were a better choice. I don't know if he has made any comment comparing them with 631 forks, although I think that 631 was often his favoured/recommended choice for most customers for a frame.

Although I linked to Dave Yates' posts about this on YACF, it is something which he has commented on before in a magazine article which I once read (possibly in Arrivee). The background was that having been a racer/time trialist, he had started to do a lot of audax rides, and he completed a Super Randonneur series (200km, 300km, 400km and 600km) and Paris-Brest-Paris. He made a frame/fork in 853, but found the forks uncomfortably harsh during the SR series rides, and presumably switched them for something else before PBP.

I don't understand how 853 would be stiffer than 'R' blades, or 631, or any other steel alloy, if all other things are equal, given as you say the Young's Modulus is the same for all. And yet if Dave Yates says so based not only on his experience and knowledge as a framebuilder, but also as a rider doing PBP etc., I am not going to suggest he is wrong.

Maybe there is more to it than Young's Modulus. I think Brucey touched on this in this post - viewtopic.php?p=944790#p944790.

[slowster]

......If the front wheel, acting through the lever of the fork, can flex the frame's main triangle, then the back wheel can flex the rear triangle, and possibly the frame as well.

sorry, I don't quite agree with the logic even if I do agree with the sentiment. FWIW I think that straight seat stays may see a form of Euler buckling. The chainstays are often a lot like a set of straight fork blades but they don't get to flex in the same way (which might be just as well, given how corroded some of them get) except perhaps in soft-tail and curly stay designs

A few years ago I bookmarked several posts about frames and framebuilding. One was 531colin's frame alignment guide, another was the post I linked to in my post immediately above. The other two were this one where you went into some detail about seat stay flex - viewtopic.php?t=105498, and this one about the different contributions/relationships between the blades, steerer and top/down tubes to stiffness - viewtopic.php?p=1157626#p1157626.

[drossall]

I think my response to this kind of issue will always be affected by the apocryphal story of a pro team, back in the days of steel frames. It's said that their well-worn old frames from last season were finally taken away and replaced by shiny new ones. They all agreed that these rode far better than their tired old frames. Only then was it revealed that the old frames had simply been taken away and resprayed.

I don't believe that paint is normally claimed to affect either compliance or rigidity, but clearly it can

[531colin]

As a big bloke (tall, and not slender) I think if I were going out buying a bike for long day rides today, I'd separate the contradictory requirements of stiffness (which I need) and a cushioned ride (which I want) by combining a relatively stiff frame with tyres that are big enough and supple enough to do the suspension job. And that would mean brakes that are not the restrictive calliper brakes I used to go for.

I think thats the view of the vast majority.

[531colin]

I have a problem with that "Fairlight" design; I don't see how the oval top tube can flex to provide comfort unless the very oversize down tube flexes at the same time.

[rareposter]

I have a problem with that "Fairlight" design; I don't see how the oval top tube can flex to provide comfort unless the very oversize down tube flexes at the same time.

Surely it's not flexing? Flex is bad - and in a traditional double diamond frame shape, pretty much impossible anyway unless you're engineering in leaf spring type designs at which point it's not a traditional double diamond frame anyway.

What it should be doing is dispersing forces from the ground via the tyres and along the tubes to be compliant. But compliance is not the same as flex.

At least, that would be my interpretation but I suspect the terms are being used interchangeably within some parts of this thread so I guess I'm asking a question as to which term is more accurate?

[Brucey]

...... I don't see how the oval top tube can flex to provide comfort unless the very oversize down tube flexes at the same time.

unless the head tube is mysteriously flexible too...