Cyclist
Bike frame stiffness: Why it matters and all you need to know
When it comes to cycling performance, stiffer means better. Or at least that’s what the bike industry marketeers would have us believe. They may have a point, although just what the word ‘stiffness’ means in bicycle engineering is somewhat more complex than a simple ‘more is better’.
Along with weight and aerodynamics, stiffness is one of the engines that drives modern composite bike design. But while a number on the scales and the way a frame cuts through the air are relatively easy concepts to wrap one’s head around, stiffness is much more difficult to quantify.
Can it be put into numbers? Sure. There are all sorts of complex jigs that manufacturers use to measure stiffness, but for the average cyclist it’s less about exact figures and more about feel. Broken down to its absolute fundamentals, stiffness is the opposite of compliance, which is a mark of how prone a frame is to bending in certain planes.
‘Compliance is a deflection from a defined input,’ explains Reggie Lund, mechanical engineer at Trek. ‘In other words, how much the frame bends when a force is applied to it. We are typically looking at it in terms of ride handling, so things like cornering, climbing and sprinting.’
In general, the ‘stiffer’ a frame is, the more it resists forces being applied to it through pedalling and cornering. In theory, what this means for the rider pushing the pedals is that the maximum amount of power possible is transferred from body to bike, with as little as possible lost to frame flex.
In layman’s terms the right level of stiffness keeps the frame from twisting during cornering, making one wheel follow the other through the arc of the corner.
That said, dialling the stiffness up to 11 throughout the frame isn’t necessarily going to result in a better bike. It’s a bit more nuanced than that.
‘When it comes to everything that is impacting the steering, I really think higher stiffness is better,’ says Stefan Christ, head of research and development at BMC. ‘I almost think you cannot have enough of this. But when it comes to pedalling, we know that higher is not automatically better.
‘What we want to do is tune the stiffness so that the frame is loaded like a spring with each pedal stroke. So we want it to be high, but not crazy-high, because a frame that’s too stiff isn’t going to support that pedalling motion, particularly when out of the saddle.
‘Imagine doing a big climb on a track frame. It would feel very odd, because a frame that stiff is not going to support your movement.’
Twist and shout
There are two key types of stiffness designers and engineers must consider: lateral (side-to-side) and torsional (twisting).
‘Torsional stiffness deals with the position of the two wheels relative to each other and how they get twisted or not twisted up as you’re going around a corner,’ says Graham Shrive, Factor’s director of engineering.
‘If you’re going around a corner on a flexy frame, the front and rear wheels will start to come out of plane. That’s how you get understeer or oversteer. This primarily transmits through the down tube and across the chainstays, which is why you see brands doing these great big down tubes. Because it’s really a torsion tube at that point – it really twists up.’
Then there’s lateral stiffness, which resists side-to-side motions. This needs to be high in the fork for controlled steering, and also in the rear triangle and bottom bracket to deliver efficient energy transfer through the pedals.
‘You can really feel this when you’re out of the saddle on a nice, steep hill,’ says Shrive. ‘You get that sensation of the bike surging forwards with every pedal stroke. That’s a good indicator of bottom bracket stiffness.’
The real magic, claims Shrive, is in the relationship between bottom bracket and torsional stiffness, and how the ratio of the two affects the way a bike rides. Too much or too little of one relative to the other can throw the bike’s handling out of whack.
‘I have what I call the “golden ratio” between torsional and bottom bracket stiffness,’ says Shrive. ‘On our jig it’s pretty much 50/50. We use newton metres per degree as the measurement of torsional stiffness, because it’s not just a displacement, it’s also like a twist, so there’s an angular component to it.
‘Then with bottom bracket stiffness it’s just millimetres of displacement that we’re measuring. If you have too much of one, it starts to overpower the other.’
Shrive got this idea from the old steel-framed racing bikes of yesteryear. Although much more flexible than today’s carbon-framed bikes, well-made steel race bikes often have this exact same ratio of bottom-bracket to torsional stiffness, which results in a lively, responsive ride.
‘On those bikes you can actually use the bottom bracket to drive the rear wheel in. Imagine with torsional stiffness that the rear wheel comes out of plane with the front wheel and you start to understeer the corner and drift to the outside; with a really flexy frame you can just load up the outside pedal and force the rear wheel back in.
‘And good riders do that. You don’t even think about it. It’s just like a biomechanical thing that your body does. You just twist your hips a bit, push down and scoop back into the corner.’
Stiff competition
In other words, it’s not a simple matter of ‘stiffer equals better’. Rather, it’s about making the frame and fork precisely as stiff as they need to be to enhance performance without compromising ride feel and responsiveness. Engineers and designers have two important tools at their disposal to achieve this.
Firstly, they can use tube profiles to their advantage. A wider tube is a stiffer tube, but this comes with a weight penalty and affects aerodynamics. Secondly they can play around with what’s known in the trade as the carbon layup schedule.
This refers to the type of fibres, their shape (individually called ‘plies’), number, orientation and position in a frame. This is what allows engineers to tailor stiffness and compliance to different parts of the structure, and what makes carbon fibre composites crucially anisotropic – displaying different levels of stiffness in different directions – as opposed to isotropic, like steel, where a material displays the same stiffness in all directions.
‘Imagine trying to increase the stiffness for pedalling,’ says Trek’s Lund. ‘If you double the material up in the seatmast, you’ll see zero per cent change in your pedalling stiffness because the strain in the seatmast from pedalling is basically zero.
‘However, around the bottom bracket there is typically a lot of strain based on the loading, so by placing the extra material in those high-strain areas you reduce the overall local movement, which reduces the overall displacement.’
It’s a delicately balanced recipe that bike brands all strive to cook up, but there’s another important ingredient that’s essential for a well-rounded ride.
A question of comfort
There still exists a perception among some cyclists that tyres should be thin, pressure should be high and frames should be as stiff as boards. After all, how can you claim to be going fast if your teeth aren’t chattering and your bidons aren’t rattling?
Thankfully for our bidons and our backsides, science tells us differently. It’s now widely accepted that a 28mm tyre run at lower pressure is faster on the average road than a 23mm tyre inflated to feel like granite.
And the same is true of frames. We now know that stiffness and comfort aren’t mutually exclusive, and there are other arguably more important factors at play in determining how the bike feels out on the road.
Shrive has conducted a number of blind studies in an attempt to pin down the relationship between stiffness and comfort, with some interesting results.
In one instance, at a large bike brand he worked at prior to Factor, he took three different tier frames from the same line of bikes, weighted them so they all read the same on the scales and had riders assess them in terms of comfort. The frame shape was the same across all three, with similar stiffness, the key difference being thicker tube walls in the cheaper frame.
‘The wall thickness was like a quarter of an inch at some points, but you’d ride the thing around and it’s like, “Oh, this is really comfortable, I can’t even feel the road through it.” And it was crazy stiff. None of that other stuff made any difference. It was the wall thickness that was the real factor.’
Even with all the tests and studies in the world, there’s no escaping that comfort and ride feel are personal. Some riders love a cushy ride, while others want to feel every bump in the road.
However, while ride feel is subjective, all the engineers we spoke to agreed absolute stiffness shouldn’t be the goal of a bike. Rather the key is how stiffness combines with all manner of other variables, from compliance to rider weight to terrain. It’s complex stuff.
Question time
Reggie Lund, mechanical engineer at Trek, explains the basics of stiffness and ride feel
Q: How would an overly stiff frame affect ride feel?
A: ‘It’s the cycling equivalent of running barefoot on concrete,’ says Lund. ‘We’ve tested extremely stiff frames in the past and our test riders did not like them.’
Q: How would an overly flexible frame affect ride feel?
‘A bit like running across a bouncy castle. Think of how much of the rider’s power is being lost to the muscle movement required just to stabilise the system.’
Hard financial truths
Why stiffness is often sacrificed at the altar of price
One of the issues facing carbon engineers in terms of stiffness is that they’re increasingly being backed into a corner by budget constraints and forced to work with lower-modulus fibres. A big reason for this, claims Graham Shrive, director of engineering at Factor, is that customer expectations of lavish paintjobs with fancy finishes are eating up available spend.
‘The actual cost of many of those shimmering, faded paint schemes will represent about a third of the price of the frame. That’s putting pressure on my engineering colleagues to use cheaper fibres. At many brands other than Factor, you have weight goals, you have stiffness goals, you have minimum features like internal routing or whatever, and then you also have a price goal. You go to the factory, you lay those out and it’s a negotiation.
‘Typically, what I would focus on would be the percentages of the different kinds of fibres, so you go back and forth with that. But it’s like Keith Bontrager said: light, stiff, cheap – pick two.’
• This article originally appeared in issue 134 of Cyclist magazine. Click here to subscribe
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