Stiffness in bicycle frames
Holy Grail or Fool's Gold?
About 30 years ago as we set up our cycling holidays company in France I was looking to have my first custom touring frame built. Of course I wanted the best and was taken by an article in one of the Cycling Magazines about a new Steel frame tubing made by 'Reynolds'. The article was headlined - “What's Stiff and has Purple stickers on it?'” Apart from the rather pathetic phnar, phnar attempt at humour the article persuaded me to have my new frame built of Reynolds 653 tubing – the subject of the magazine article.
Sadly the article was totally inaccurate and the frame produced performed nothing like I expected. Why? Because the subject of frame stiffness is probably the most widely misunderstood aspect of bicycle frames and some of the true reasons for the fashion for 'stiffness' being the be-all-and-end-all of frame design well hidden in recent times...
So I though I'd write a short and simple article on the subject to try to make sense of one of the most important aspects of bicycle design and perhaps help you not to make the mistake I did...
A very short primer on metal properties...
If we are looking at making a bicycle from metal tubes – like 99% of bikes – then the most surprising fact is that one steel is the same stiffness as the next – likewise one Aluminium is the same stiffness as any other and ditto for Titanium. This is so counter-intuitive and against so much of manufacturers 'hype' that it need a little explanation. For more a detailed summary of how alloys vary see my article article on bicycle frame materials but here I'll keep it simple.
All steels (for example) are equally stiff – they share the same Young's Modulous. So it you take a bit of mild steel and apply a force to bend it it will deflect exactly the same distance as if you'd done the same test with an identically dimensioned piece of the highest technology steel alloy. Of course the big difference is that the better the alloy the more force you can apply and the steel will still 'spring' back where the mild steel will bend permanently – that is effectively what you get for your money... It means you can use less steel and the frame will not bend permanently – i.e. you get a lighter, stronger frame BUT all things being equal it will be less stiff. This applies to all metals.
So how do you make a frame stiffer?
There are two ways of making a tube stiffer – either increase the wall thickness of a tube or increase the diameter of the tube, so your tubes look 'fatter'. The first method just means using more material and thus more weight so a bit basic and not really what a cyclist wants. The second allows the same amount of material – and so weight of metal – to give more resistance to bending but to do this the tube walls must be thinner. There comes a point where these thin walls become very easy to dent, but as our expensive steels are more resistant to permanent bending you can go a lot thinner with say Chro-Mo than mild-steel.
You can actually do some clever things with this – for example 'butted' tubes have greater wall thickness at the ends (where they are under most strain) than the middle. Also by making the tube oval in cross-section you can make the tube stiffer in one direction and less stiff in another, something that may be highly desirable as we will see later.
Why do we want a stiff frame?
So now we know how to make a stiff frame comes the $64,000 question – is a stiff frame a good idea?
Well first let's have a look as to why a stiff frame may be good and in some circumstances essential – prepare to learn a couple of things the manufacturers don't tell you!
The most fundamental and unarguable reason is that a bike has two wheels and a rider and you need to keep them all in the same place relative to each other;-) Imagine if the front and rear wheel could twist independently – the bike would steer all over the place, be lethal on fast descents and a touch on the brakes or 'honking' on the pedals would have you out of control. So as a minimum the frame must be stiff enough to maintain the integrity of the bike, something you'd think would be obvious, but is far from always the case...
Power transfer... This is the biggie – the one those flashy adverts sell to you... Why waste all your energy bending a frame back and forth when you stand on those pedals? And it's a crook...
Remember that fine quality steel springs back rather than bending permanently? Well a spring is an energy store. So when you stand on the pedals and some of your energy goes into bending the frame – well the vast, vast majority of it comes right back to you as the frame springs back;-) So far from losing the energy it is merely the delivery that is different and that can be felt in the stiff frame reacting to inputs much more sharply – it'll feel more responsive and 'quicker'. MUCH more important is that in a sprint finish the frame will retain its integrity better – watch a slo-mo of a sprint finish and you'll see the amount frames and wheels flex – the stiffer frame will be much more stable at that time of maximum stress and that is just as important for a touring cyclist carrying 50 kg in front and rear panniers as it is for a TdF cyclists cresting the Ventoux.
So stiffness is good and the main advantage is keeping the frame 'straight'...
But there are other important reasons why frames have to be stiff, and they are the ones manufacturers generally keep quiet about.
'Fatigue' – Frame manufacturers don't like talking about this but the fact is that all metals show 'fatigue' – that is that if you flex a piece of metal back and forth it will eventually break. We've all done it – bent a bit of wire back and forth until it breaks. But what most people don't know is that this happens before metal is bent permanently (unlike that wire). The metal will keep springing back but 'remembers' each bend and eventually cracks and fails. That's why all Aluminium aircraft components have a 'life' and are discarded after a certain number of hours or landings – because they can fail catastrophically without any visual signs of weakening.
This 'fatigue life' applies to Steel, Aluminium and Titanium. BUT both Steel and Titanium display a 'fatigue limit', a limit of deflection below which they will NOT fatigue. Just to clarify that – if you bent a Steel or Titanium tube back-and-forth 'just a bit' they would never, ever fail from fatigue. Aluminium on the other hand remembers every flex no matter how small.
So if you design a Steel or Titanium frame so that it is never flexed beyond its fatigue limit then it will last forever – with an Aluminium frame the designer MUST design the frame to flex as little as possible so that the frame life is well beyond the normal life-expectancy of a bicycle. Here it's very important to note that of course the higher the quality of metal, the more resistant the resultant tube is to fatigue.
If you look at the cycle market, especially racing bikes, one big change has been the replacing of high quality steel with ever cheaper Aluminium frames which by nature have to be stiff and so have fat (large diameter) tubes (Aluminium is 1/3 as stiff as steel). So an industry has given us thousands of very lightweight, fat-tubed frames that are very stiff and sold us on the idea that that is a 'really good thing' and so even Steel frames have tended to follow the same goals.
The other reason 'stiff' is 'good' has been largely ignored. With the increasing number of rear cogs (11 or even 12 now) the amount of cable pull between gears has become ever smaller and ever more precise. Those cables run under the bottom bracket on most bikes*. If a rider stands or even pushes hard on their pedals the bottom bracket area is the most stressed part of the bike and will flex one way and the next – pulling on the cables. This causes 'ghost-changes' – imagine a sprint finish, you leap onto the pedals and with each turn the rear derailleur changes up and down two gears! So modern bikes MUST be very stiff around the bottom bracket – it's got nothing to do with energy transfer and everything to do with gear changes. If you took say a 531c frame from 1970, fitted 12 gears (or even 8 in my experience) and 'honked' hard the bike would be changing gear by itself – so the evolution of more and more gears has caused a fundamental redesign of the bottom bracket area of the bike...
So – the pursuit of ever stiffer frames has been part dictated by stability in sprints especially as riders have become more powerful and heavier, but equally by changes in materials and technology other than frames – and of course that curse of the cycling world... Fashion...
So where is stiffness a bad thing?
Here we can be far more simple. There is only one reason why you'd want a frame to be less stiff – comfort...
I remember reading many years ago that when old professional cyclists retired the first thing they would do would be to throw away their harsh race frames and go and buy a classic hand-built steel frame to ease their way comfortably into retirement;-)
Up until now we've been sort of concentrating on racing cyclists, but even for them comfort is a major problem. On the Tour de France riders will be in the saddle for upwards of 6 hours and a harsh ride can make their lives misery. A bike needs to absorb road shocks and more specifically the 'buzz' that comes through from the tarmac. With 18/22mm tyres the racing cyclist can't even rely on tyres to take the sting out. Forks likewise need to be able to flex to avoid 'riveter's syndrome'. Because a quality Steel or Titanium is quite happy being 'sprung' these can offer a relaxing ride whilst feeling very 'alive'. In this article this 'disadvantage' of stiffness has taken up 1/10th of the space of the advantages, but its importance cannot be underestimated.
Compromises.
By now I hope you have grasped the concept of 'stiffness' and so will appreciate that it is not one of those things that is automatically more-is-better. Every bike is a compromise and that compromise will be dictated by what the bike is used for and by the rider. The idea that a 80kg world-class sprinter will require the same stiffness in the frame as the bike for an 50kg girl riding 200 km in a day is ludicrous and yet sadly that's exactly what many manufacturers will give you. The perfect bike should be stiff enough for the purpose it is to be used for and to allow longevity and yet springy enough to be comfortable. But pity the manufacturer – when they produce their next wonder bike they know that it will be ridden by some 120 kg bloke pottering to the shops and just as likely an 18 year-old whippet of 50 kg attacking the pack up the Tormalet. That bike has to cover all those types.
And cheats;-)
Think that that world-class sprinter is riding the same bike as the one you ride? Nope – his team will have specced that bike for his weight and power, likewise that 50kg climber will be riding something the sprinter could rip apart from pure power. In the days of Steel at the top level the bikes ridden up the Pyrenees were so light, with such thin tubes that most were discarded for fear of failure after a few climbs. A long road stage would demand a comfy bike, the next day a bike looking identical to the spectator (and potential purchaser) would be twice as stiff for the Criterium round the cobbles... But even now specific bikes will be produced for specific riders.
But there are two other 'cheats' the first of which I've already hinted at. By using oval tubes a bike can be made stiff in one direction whilst springy in another. As an example having a horizontal oval in the tubes going to the bottom bracket this area can be stiffened to lateral load whilst being more springy vertically.
The big advance in modern times though is the use of composites. Because they can be made in wildly different shapes, with varying wall thickness and even alignment of fibres the designer has complete control in 3d of a frames stiffness and 'spring' – fatigue isn't an issue and the result can be a lovely ride and immense strength and stiffness where needed. Sadly there are two caveats. Because moulds and construction techniques are fixed each frame size has to be a compromise between the potential customers physical forms and abilities – and secondly for a touring cyclist once damaged (and damage is easy on such frames) the frame is generally scrap.
Which rather neatly brings us back to the very start of this article;-)
A Tale of Two Frames
Remember that Reynolds 653 touring frame I had built (beautifully – by Bob Jackson)? Well in my ignorance (and the ignorance of the writer of the original review) I didn't realise that Reynolds 653 was the lightest frameset Reynolds made – in fact the LEAST stiff frameset in their catalogue! Drawn to '531 Professional' gauges but made with better steel it was as stiff as wet spaghetti. Luckily Bob Jackson knew what they were doing and it actually produced the most gorgeous frame to ride – deliciously comfortable with enough fork-rake to be stable and with the short rear-end I'd specced so that it was in fact quite stiff in the rear triangle and climbed brilliantly. It also coped with rear panniers and the tent when touring and was plenty stable enough even coming off French Pyrenean mountains.
But in Autumn and Spring tours where I had front panniers too it would shake her head when I started off – then child No1 arrived along with a lot more weight and provoked a speed wobble that though never dangerous was something you had to watch out for.
No 2 and shortly after No 3 arrived and a baby trailer and front and rear panniers? No chance, pretty wobbly and climbing in low gear gave masses of chain rub and ghost changes as the bottom bracket flexed. So a wonderful bike, but no longer suited for the role it was called on to perform.
Back to Bob Jackson and an identical frame built but this time with tandem-weight 531. To ride it was heavy, stiff as a gate and nothing like as comfortable or lively. But you could load anything onto it and it would just shrug it off, and loaded up it became comfortable again. Not something to lightweight tour on, but if you were a family man or riding around the world a wholly appropriate compromise re weight and stiffness.
And in this age of mass-produced bikes of excellent quality that remains the one big advantage of a custom builder. Give them your measurements, what the bike is going to be used for and a good one will choose wholly appropriate tubing in a way that no production bike could, no matter how expensive.
*Sadly running cables along the top tube and down the stay just means that the need for absolute stiffness is transferred to these parts of the frame.
A Small Favour
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