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The Cyclist guide to frame stiffness

Jonathan Manning
10 Jun 2016

Stiffness in a road bike is essential, but it must also be balanced against comfort, weight and cost. We investigate how.

Search the internet using the term ‘stiffness’ and you won’t get far before seeing adverts peddling pharmaceutical solutions for gentlemen’s issues. Bicycles and bedrooms, it seems, share an unswerving paranoia about limp performance, which explains why the marketing bumf that accompanies every new bike is invariably peppered with claims about how the frame is now super-stiff, as well as being lighter and more comfortable. But, in theory at least, the three qualities are in conflict, and bike designers are continually trying to find the sweetspot between them by tinkering with tube dimensions and material science.

Pedal to the carbon

When engineers talk about frame stiffness, they’re really addressing two different areas of a bike’s performance. The first relates to having sufficient lateral stiffness to allow a rider’s pedalling input to transfer as efficiently as possible to the road. The second concerns the predictability and stability of a bike’s handling.

In terms of lateral stiffness, each time your foot stomps on the pedal you create substantial lateral (side-to-side) stresses along with torsional (twisting) forces, which combine to lever the lower portion of the frame out of alignment. Every millimetre of frame movement absorbs precious energy that could be channelled to the road, so minimising this flex effectively maximises pedalling efficiency, hence the relentless focus on the stiffness of frames. 

‘How you get the energy you are putting through the cranks to the rear wheel is really about the bottom bracket, chainstays, dropouts and wheel stiffness,’ says Gerard Vroomen, co-founder of Open bicycles and previously co-owner of Cervélo. This challenge is complicated by a bike’s single-sided drivetrain, which creates an uneven load on the rear end of the bike. The need to resist the greater forces on the right side of the frame is the reason why many bikes opt for an asymmetric design of chainstays and seat tubes.

But as Ben Coates, Trek’s road product director, reminds us, what you do to one area of a bike directly impacts another: ‘You can increase the stiffness at the bottom bracket, for example, by adding laminate to the lower side of the head tube, but every piece of laminate affects the entire bike. You risk adverse effects if you don’t understand how adding or honing materials affects the other parts of the bicycle.

‘We can make bikes stiffer than even the biggest and most demanding riders in the world want, but just making it stiffer, or lighter for that matter, isn’t necessarily a recipe for a better bike. The conversation must begin with how you want the bike to ride, not where you want it to be stiff.’

Engineering stiffness

How do designers stiffen a frame in just the right places? The answer lies in the diameter of the cross-section of tubes, as well as their length and, in the case of carbon bikes, the multiple layers of carbon fibre
used in their construction. 

‘The greater a tube’s diameter, the stiffer it’s going to be,’ says Adam Wais, CEO and founder of handmade carbon bike manufacturer Rolo. ‘And that’s before you even start looking at materials.’ 

This explains the tendency in bike design towards oversized down tubes, bottom bracket junctions and chainstays. Advances in carbon fibre have allowed manufacturers to reduce the thickness of tube walls, giving them the freedom to create gargantuan-looking tubes without adding weight. 

So if huge down tubes and bottom brackets are used to channel every watt of rider power to the road, why not follow the same philosophy at the top tube and head tube to resist cornering forces and ensure precision steering?

As you lean a bike into a corner, three large forces converge: gravity, which pulls vertically downwards; kinetic energy, which keeps you moving forward, and centripetal force, which pushes you outwards – to the left when turning right and vice versa. If the frame is too flexible, these forces can push the wheels and head tube out of alignment, leading to imprecise steering. 

‘You need the wheels to track, and the better they track front to back, the more predictable they are through a corner,’ says Thomas McDaniel, product manager at BMC. ‘Say you’re going through a corner you’ve ridden a thousand times, so that you’re confident and carrying a lot of speed, but one day you find a massive rock right in the middle of the line you always take. How well can the bike accept your need to make a change in the middle of a corner? That’s where front-end stiffness comes into play.’

It turns out that if a bike is too stiff at the front end it becomes difficult to lean, which creates a different type of handling problem. Chris D’Aluisio, creative director at Specialized, takes up the story by recalling the moment when the company’s Tarmac SL4 replaced the SL3. Previously, when Specialized developed a new bike it would use the 56cm frame size as the benchmark for a new set of targets, including stiffness. Once the targets were hit, the frame would then be scaled, with slightly smaller tubes for the smaller frames and bigger tubes for the larger frames.

‘With the SL4 we made the 56 stiffer and lighter, and taller riders – size 56 and above – said, “Wow! It’s so much better.” But at my size, 52, it rode worse,’ says D’Aluisio with refreshing candour. ‘It was too harsh, not only vertically but also as you leaned into a corner. It had too much front-end stiffness, and it didn’t allow the bike at smaller sizes to be compliant over the road mid-corner, so it would cause the front to chatter, especially over a bumpy road surface, which could be unnerving.’

It turned out that the adjustments made in scaling the bike had not gone nearly far enough, and that smaller frames, with tubes of a similar cross-section and carbon lay-up to the 56cm, were incredibly stiff because those tubes were shorter.

‘The small bikes were proportionally much stiffer than the larger bikes, which is completely opposite to what the rider needs,’ says D’Aluisio. ‘The taller rider, with a longer seatpost and higher centre of gravity, requires the bike to do a lot more work. When you ask the bike to move from right to left in a manoeuvre, that bike is doing all the work to move that rider’s weight from one side to the other and catch that rider from falling. We have to regain traction as the rider turns.’

As a result, Specialized decided it needed to effectively treat each different frame size as if it was its own separate custom project, a process it calls Rider First Engineering.

This helps explain why engineers aren’t simply making the whole bike as stiff as possible, but there’s another reason too: comfort, also known as compliance, which is the ability of the frame to contend with imperfections in the road surface and absorb vibrations from the tarmac.

Having created the Cervélo R3, a bike that enjoyed podium positions over the brutal cobbles of Paris-Roubaix for seven consecutive years, Vroomen knows what it takes for a bike that can deliver power to the rear wheel while protecting the rider from the very worst of surfaces.

‘Ideally you want as little vertical stiffness as possible so you get some comfort and compliance,’ he says. ‘But the tube you enlarge to get stiffness in torsion also becomes bigger vertically and stiffer vertically, and it’s not easy to uncouple those two factors. In that sense it will always be a compromise – the most comfortable bike will be unrideable because it’s so flexible in all directions, and the stiffest possible bike will also be unrideable because it’s bone-jarringly stiff, which is not only uncomfortable but slower too. You need some sort of compliance to take out the roughness of the road.’

Tyres do much of this work, but designers also introduce a degree of flex into the frame, particularly in the seat tube and via very thin or flattened seatstays, as a means to dissipate the road shocks travelling up through the rear of the bike to the rider. 

Arguably the best example of how vertical compliance can be divorced from bottom bracket and head tube stiffness is the Trek Domane, the race-tuned bike that carried Fabian Cancellara to the top step of the podium at the Tour of Flanders and Paris-Roubaix. The Domane’s seat tube is ‘decoupled’ from the top tube, allowing the seat tube to flex almost independently of the frame without sacrificing stiffness. The new IsoSpeed decoupler in the latest Domane SLR even has ‘tuneable compliance’, so that the rider can personalise the level of stiffness.

‘The Domane tuning range starts at roughly the same level of vertical compliance as the Madone [Trek’s aero race rig] and goes up to 35% more compliant without affecting pedalling stiffness,’ says Coates.

Frames of the future

The relentless pursuit of stiffer, lighter, more comfortable frames shows no signs of abating, with manufacturers on a continual quest to explore new materials and technology. British bike brand Dassi, for example, is investigating the possibilities of graphene, the wonder substance that holds extraordinary potential if manufacturers can find ways to harness its capabilities.

‘Graphene exhibits properties that far exceed traditional carbons that are laid up in a bike frame,’ says Stuart Abbott, a former aerospace design engineer who set up Dassi six years ago. ‘There is a mystical opportunity to create a frame that could weigh as little as 300g, because graphene is so much lighter and so much stronger that you have the opportunity to replace areas of carbon that might be 2mm or 3mm thick in certain places on the frame – particularly around the bottom bracket – with something that is 100 times lighter and one-thousandth of the thickness.’

For now, however, a quick reality check is in order for anyone on the brink of buying a new bike. Cycling is not like Formula One, where the best car always wins, all but irrespective of the driver. Lots of this year’s major races – including Paris-Roubaix, the Tour of Flanders, Milan-San Remo, Paris-Nice and the Tour of Romandie – have been won by riders on different bikes. In other words, ultimate performance is about the rider, not the bike, and there’s no objective ‘right’ level of stiffness in a frame, only the level that’s right for you.

Top 10 stiffest bikes

How do you measure a bike’s stiffness? Sadly we don’t have a lab to do the flex testing, but we know a place that does. Our friends at German cycling magazine Tour are renowned for their scientific approach to bike testing, which includes subjecting each bare frame to a rigorous battery of bench tests to ascertain stiffness values. 

An equation based on the stiffness score that frame achieves at the head tube (measured in Nm/°), divided by the frame’s overall mass (in kg), provides a stiffness-to-weight index that gives Tour its leaderboard for the stiffest frames…

Bike Date Stiffness-to-weight
1. Cervelo Rca Jan 2015 142Nm/°/kg
2. Specialized Tarmac SL4 Dec 2011 141.2Nm/°/kg
3. Cannondale SuperSix Evo Ultimate Sep 2011 139.2Nm/°/kg
4. Canyon Ultimate CF SLX  Jan 2016 131.5Nm/°/kg
5. Trek Emonda Dec 2014 131.3Nm/°/kg
6. Focus Izalco Max Jul 2013 127.1Nm/°/kg
7. Felt F1  Dec 2011 125.3Nm/°/kg
8. AX Lightness Vial Evo Jan 2015 125.1Nm/°/kg
9. Storck Aernario Oct 2015 123.9Nm/°/kg
10. Rose X-Lite Team 8000 Dec 2014 123.7Nm/°/kg

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