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Monday, 27 June 2016

How Stiff is "Stiff"?

Much of the bicycle industry has done a good job of creating the impression that different materials offer different ride characteristics. Aluminum is supposed to be stiff and light, but is also known for diminished durability and harsh ride quality; Titanium is supposed to be light, durable, comfortable and compliant, but a little flexible; Carbon fiber is supposed to be light and comfortable while simultaneously enhancing drive train stiffness; Steel (Chromoly) is supposed to be “real” and provide a comfortable and snappy ride, but is known to be a bit heavier and more flexible than other options. Right?

Not necessarily.


All manufacturers are trying to build that perfect combination of ride characteristics where stiffness and responsiveness are maximized, while the ride is still kept silky smooth and comfortable. It is not too hard to find claims of a frame being stiff, yet compliant and comfortable, with fantastic vibration damping characteristics. However, the bicycle industry has never had a good baseline testing protocol to quantify how various materials and designs actually perform in regards to specifics like stiffness and comfort. Everything has pretty much been based on “feel”, which is not a very scientific or reliable way to test a piece of machinery. 

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 CervĂ©lo. 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.

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. 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. 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.

‘Ideally you want as little vertical stiffness as possible so you get some comfort and compliance, 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.’

There is more to bicycle performance than overall frame stiffness. Frames can be too flexible for a given rider and application. Some riders may even prefer very stiff frames. However, it is clear that the old mantra of stiffer = more performance is not true for most riders.

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.  When choosing a frame or new bike, do not spend time making judgments about ride quality based upon the materials used to build a frame. Instead, approach your frame decision as an individual. 

Only consider frame options that fit you well, and then look at the design details and tubing to find the ride characteristics that will best match your needs, body and riding style. Finally, don’t forget that a bicycle is a sum of its parts. The other components (especially the wheels and the fork) that you use effect the way it will ride as much as the frame does and should be chosen based upon how they relate to the other parts around them. 


When looking at designs, keep in mind that the ride quality a frame is known for is usually based upon the experience of an “average sized” (usually a male who fits on a 55cm frame and weighs around 160 lbs) rider. What can ride great under a 160 lbs rider, might be too mushy for a heavier rider or might be too stiff and uncomfortable for a smaller rider. If you are bigger or smaller than average cyclist, it is even more important to approach frame and component decisions based upon your individual needs so that you don’t end up with a bicycle that is too stiff or too soft for your size and power.

Just remember the 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.

Source: 
Cyclist 7/2016 "Stiff Competition"
https://janheine.wordpress.com/2011/10/03/science-and-bicycles-frame-stiffness/
http://fitwerx.com/stiffness-compliance/

Ride On!

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