The stiffness of your frame plays a very large role in how your bike feels underneath
you. People often talk about bikes having a 'dead' feeling when they're too stiff,
a 'noodly' feeling when they're too flexy, and a lively feeling when they're just stiff
enough. After owning bikes that are stiff, flexy and everything in between, I believe there is a
'Goldilocks' zone for frame stiffness, where there is noticeable but minor amounts of frame flex,
contributing to that buttery-smooth ride that we all desire. We are going super-deep down the frame
stiffness rabbit hole today. We'll be discussing what frame stiffness is, when it's important,
the relationship between stiffness and frame materials, whether frame flex slows you down,
and how you might go about finding the perfect stiffness on your personal bikes.
If you value my work, I'd love to see your support over on Patreon. This platform allows me to invest
more time into researching, crunching numbers, and describing the intricacies of bikes to you.
I'll be breaking down forks, seatposts, saddles and more in the coming months. So, what is frame
stiffness? We can assess the stiffness of a frame from a number of different locations. There's the
steering stiffness, which is the amount of movement the frame twists at the head tube.
This type of flex is most noticeable when you push down on the pedals and pull up on the handlebar.
A bike that has a high steering stiffness will feel especially snappy and more reactive to your
steering inputs. Keep in mind that the overall steering stiffness is also largely dependent
on the stiffness of your fork, front wheel and handlebar. Then there's the pedalling stiffness,
which is the amount of movement your frame deflects near the bottom bracket shell when you
pedal. And finally, we can look at the vertical stiffness of a frame, which I've previously
discussed in a video about frame comfort up here. To give you a sense of the range of stiffness in
a bike frame, the most rigid, mass-produced frames are approximately twice as stiff
as the least rigid frames. I like to describe frame stiffness on a spectrum between two points.
A stiff or responsive bike has a snappy and direct feel to it under acceleration. It will have that
up-and-go bike feel, but will also feel harsher over road imperfections. Additionally, there is
a case to be made that the braking performance and traction is not as good when cornering a stiffer
bike, so stiffness is only good to a point. A flexy or forgiving bike is less communicative
of the road or trail below, and will feel laggier under acceleration. It will move around more with
your rider inputs, which feels nice again, to a point. Ok, so what factors affect frame stiffness?
A bike frame that feels stiff to one rider, may feel flexy to another. This is because there are
multiple factors determining how much your bike will move underneath you. First, we have the rider
factors. These include your power output, how much you weigh, and your riding style. For example,
whether you're riding out-of-the-saddle and accelerating quickly, or whether you're just
cruising along. Then there's the bike factor. The intended use of the bike will require varying
degrees of stiffness too. For example, a hardcore hardtail will have higher stiffness requirements
than an ultra-light cross-country hardtail, given the larger forces at play. And lastly, there's the
luggage factor. Bikes intended to carry luggage require additional frame stiffness, or the frame
might become too noodly to ride. Touring and bikepacking bikes support front and rear luggage,
and your frame is the medium that resists the twisting forces between these two points. When it
comes to the handling, stability, and general feel of a bike laden with luggage, touring frames need
to be built extra stiff. The downside to a bike that resists twisting forces is that it cannot be
optimally tuned for riding without luggage, that is, there will not be the minimal amounts of flex
that make a bike feel lively to ride - unless it's all loaded up. The pedalling stiffness also needs
to be particularly stiff with a belt drive touring bike, as the beltline has a low tolerance for
side-to-side flex. Belt drive bikes are stiffer at the bottom bracket than any other bike which
ensures that the belts cannot skip. In addition, wider handlebars have more steering leverage,
which makes it easier to flex a frame torsionally. To achieve the equivalent ride feel,
a well-designed frame with a wide handlebar will have a higher steering stiffness.
So, are stiffer bikes faster? It is generally assumed that a high pedalling stiffness is faster
because there is less energy lost to the frame. But outside of someone who is using their bike for
sprint finishes, as long as your brakes don't rub or your gear shift under a load, a bike with half
the stiffness at the bottom bracket is unlikely to make a meaningful difference to your cycling
speed, simply because the range of deflection is so small. The aerodynamics of a bike and rider,
as well as the rolling resistance from your tyres, are orders of magnitude more important.
Some have hypothesized and even field-tested that most of the deflection force is returned
back to the drivetrain on a flexy bike. But I find this scenario quite unlikely, here's why:
the frame deflection from your pedal stroke slowly builds from one o'clock to three o'clock,
then slowly releases by five or six o'clock. As some experiments have attempted to show,
the energy isn't released in one go with most of the variables locked in place.
In the noisy and dynamic environment of pedalling a bike on the open road, frame deflection is one
of the dozens of variables that store and release energy in the system. It's unlikely that just one
variable - frame flex - is responsible for the majority of the energy storage and release. A
more likely scenario is that the energy returns in part to your drivetrain, but it is also lost
as heat to your frame, wheels, tires, crankset, pedals, shoes, feet, ankles, legs and more.
Let's talk frame materials. Are steel frames more flexy than titanium, carbon and aluminium? The
data we will be using today comes from the legends at Tour Magazin in Germany. They have created a
standardized frame deflection test and have over 1,000 road and gravel bikes measured, for us to
compare. I've kept a record of almost every bike ever tested, which will allow us to understand the
deflection values of different frame materials. The n/mm values are the amount of force in newtons
required to move the head tube and bottom bracket shell a millimetre. As this is a static test,
the information is not 100% definitive about how a bike will flex under a rider. Although,
when you look at the entire data set, the force per millimetre values correlate around the
intended use of different bikes. For example, an aero race bike used in the Tour de France usually
has minimal amounts of frame deflection, and a dedicated touring bike is often stiffer again.
The head tube data shows that steel or titanium frames have less steering stiffness. On average,
carbon bikes work out to be approximately 10% stiffer than both titanium and steel,
while aluminium bikes are closer to 20% stiffer. At the bottom bracket shell, the carbon and
aluminium frames are again stiffer, with 20% more force required to move the cranks a millimetre
than either titanium or steel. These lower frame deflection values at both the bottom bracket and
head tube could explain why steel and titanium bikes are often considered to have a nicer ride
feel. But this isn't the end of the story. Let's look at the total range of deflection on all bikes
tested. Here is the lowest and highest deflecting bike of each frame material. When we look at these
ranges, it is clear that a bicycle engineer can design a very responsive or forgiving bike using
any frame material. How does a small frame compare to a big frame? Tour Magazine has the data to
help us out there too. The good news is that the average small bike frame is not obnoxiously stiff,
which could easily be the case as smaller frame triangles are inherently stiffer than bigger ones.
The fact that the stiffness values are similar means that bike engineers are doing a somewhat
good job at optimizing the ride quality of their bikes. But there is still room for improvement.
Smaller riders are often lighter and with a lower power output. This would mean that many smaller
riders would achieve a similar ride feel with more frame flex when compared to taller riders.
But here's the issue: bikes need to be designed around the heaviest, strongest rider. An example,
sprinter Caleb Ewan rides an extra small bike, and it's safe to say his power output would be
2-3X higher than the average rider of his height. Which brings me to the custom bike advantage.
While many people get custom bikes so that they can perfect their bike fit, a better reason to
get a custom frame might be to optimize the ride 'feel'. As bikes are designed around the heaviest
and strongest riders, the cyclists who have the most ride 'feel' to gain from a custom bike are
likely those who are lighter, and with a lower power output than typical. Bastion Cycles deserve
a shout out here, for allowing their customers to specify their preferred level of frame stiffness,
both torsionally and vertically. As part of the ordering process, Bastion can offer
this level of customization because they print their own titanium 3D lugs and construct their
own filament-wound carbon tubes, which is pretty damn cool. In addition, the Bastion order form
has the stiffness data of a handful of different popular bikes baked into it, which gives their
customers a sense of how their new frame will feel underneath them. Ok, so we now know that
steel and titanium bikes are, on average, less stiff than carbon or aluminium bikes. But given
there's a large range of deflection values across all frame materials, let's say you wanted to match
the flex characteristics of the average titanium or steel bike. Or perhaps you're a bit lighter
and you're looking for something with a bit more give. Almost all Trek and Look road bikes seem to
have more torsional flex baked in. Specialized is currently trending towards less frame stiffness
with their latest model Tarmac SL7 and Aethos road bikes. Giant endurance and gravel bikes
are also tested to be more forgiving. Aero race bikes and low-cost aluminium bikes are usually
the stiffest of all. The data also suggests that Cube bikes are built particularly stiff.
And for bikepacking and touring, the Specialized Sequoia, Merida Silex and Felt Broam will ride
their best with luggage attached. If I haven't mentioned your bike, there are ways to conduct
DIY stiffness tests to benchmark bikes. Here are two static tests I do before riding a new bike.
Number one: a front end wiggle test. This involves gripping the seats between your legs and pushing
and pulling on the handlebars. You'll see and feel the frame twist underneath you. Number two:
a bottom bracket deflection test. This involves locking both brakes and applying pressure
to your forward pedal. You will see the frame deflect to the side. As these tests are static,
they will only provide a snapshot of how stiff your bike will feel on the road, so you've got to
go for a test ride too. I recommend taking notes on how stiff a bike feels in the static tests,
as well as out on the road. Testing multiple bikes statically and dynamically will give you a sense
of the ride characteristics that you prefer, and if you're testing a touring or bikepacking bike,
try and do so with luggage attached. If you don't have access to any bikes, it's much harder to
know how much a bike will move underneath you. The intended use of the bike is the best clue for how
a bike will ride. A road-race bike will usually be stiffer than a road-endurance bike, and a touring
bike will usually be stiffer than a bikepacking bike. With titanium and steel, it's much easier
to predict ride characteristics because larger diameter tubes are almost always stiffer. Carbon
and aluminium are much harder to predict as there are more variables associated with the
frame design. In this case, it will pay to read some reviews from journalists that you trust,
who are ideally a similar height and weight to you. In summary, finding the 'Goldilocks' zone
for frame stiffness requires the careful consideration of your physical attributes,
as well as your riding style, bike setup, and intended use of your bike. Interestingly,
the data suggests that bikes of any frame material can be engineered to ride in a forgiving or
responsive way. However, a titanium or steel bike will have a lower frame stiffness on average,
which could help lighter riders to achieve a better ride 'feel'. But this isn't a given:
the stiffest titanium and steel bikes are much stiffer than the average aluminium or carbon
bikes. At the end of the day, I'd recommend testing lots of bikes to get an idea of the
ride characteristics that you prefer and go from there. You can support this work over on Patreon,
you can grab a copy of the Touring and Bikepacking Bike Buyer's Guides,
or you can tell a friend about these videos. All of these things bring more bike analysis to life!