Soft tail bikes are nothing new. I remember lusting over several of them in the 1990s.
But do they actually improve your comfort, speed and rear-wheel traction,
or are they just a gimmick? Today we'll be using frame deflection and vibration data to assess the
effectiveness of a handful of gravel bike rear suspension designs. I'll be estimating the spring
rates of these frames, and we'll later compare the comfort of these frames against regular
diamond ones. But first, let's discuss when suspension is advantageous and when it's not.
Here's why you should use suspension.#1 To increase traction and bike control on rough
surfaces. Suspension provides a noticeable gain in traction on rough surfaces as even a minor amount
of vertical deflection at the rear axle allows the tire to maintain contact with the ground for
longer. In addition, you get more predictable bike handling as the suspension keeps your bike
more composed. #2 To maintain forward momentum. Bumps rob energy from your forward momentum
to instead bounce your body up and down. With a suitable tyre width and pressure for the terrain,
your tyres can deform well over small road irregularities, but there is a limit to what
a tyre can do. When it comes to larger step changes, suspension systems allow you to better
maintain your forward momentum. #3 To improve comfort. Suspension insulates your body from both
vibrations and harder jolts coming up from the road, resulting in more rider comfort. And #4.
To use faster rolling tyres. Given the improved traction and bike control, you can use narrower,
lighter, faster rolling tires and achieve the equivalent grip of a more aggressive tyre
fitted to a non-suspended frame. This results in a bike that's suited to a broader range of surfaces.
Here's why you shouldn't use suspension. #1 You lose some of your pedal power on smooth surfaces.
Although suspension can improve your comfort, traction, and even speed,
a suspension damper is literally designed to remove energy from the system. This can be
a hindrance on smoother surfaces when it bobs up and down, but in the context of a gravel bike with
just 10 millimetres of travel, the difference in average speed is unlikely to go noticed.
And #2 Maintenance. A bigger downside to suspension on a gravel bike is the maintenance.
That said, the designs we will be examining today are incredibly simple compared to a full
suspension gravel bike like the Niner MCR9, which uses multiple pivot bearings and an air shock
that calls for 50 to 100-hour service intervals. The Niner's suspension performance is on another
level, however, thanks to all that complexity. Okay, let's take a look at the frame deflection
numbers of some soft tail bikes. Tour Magazin has created a standardized frame deflection test,
and have over 1,000 road and gravel bikes measured (of roughly the same size).
For the rear deflection test, the frame is secured in a jig and weight is attached to the seatpost.
The amount of vertical flex is then measured. The "N per millimetre" values that we'll be using are
the amount of force (in newtons) required to move the frame and seatpost a vertical millimetre.
Bike #1 is the BMC URS. The BMC Unrestricted is a carbon gravel bike that's using an elastomer
based system that features two pins that slide on self-lubricated bushings. The system provides
10 millimetres of vertical compliance and there are three elastomer spring rates to choose from.
Two BMC URS bikes were tested by Tour Magazine with BMC D-shaped carbon seatposts. On a number of
other 56cm BMC bikes, the same seatpost required an average of 123 newtons to flex a millimetre.
In comparison, the two BMC URS bikes measured in at 86 and 90 newtons per millimetre.
If we take the average of these spring rates, we can say that 28% less force is required to flex
the frame module of vertical millimetre than other 56cm BMC bikes with the same seat post.
Bike #2 is the Wilier Cento 10 NDR. The Cento is a carbon endurance road bike with clearance for
32-millimetre tyres. I wanted to include this bike as it has a neat linkage built into the
seat stays, which helps to dampen vibrations with its fitted elastomer. There are three elastomer
spring rates to choose between. Two Wilier Cento bikes were tested with Ritchey Link Flexlogic
seatposts. On other 56cm bike examples, these seatposts deflected at an average of 128 newtons
per millimetre. In comparison, the Centos were measured at 80 and 90 newtons per millimetre.
If we take the average of these spring rates, we can say that 33% less force is required to
flex the frame module a vertical millimetre than other 56cm bikes with the same seatpost.
Bike #3 is the Cannondale Topstone Carbon. Two years ago, Cannondale unveiled this carbon gravel
bike with both a suspension fork and a carbon leaf spring for the rear triangle. This rear
suspension system, known as Cannondale Kingpin, is said to offer between 10 and 12mm of movement
at the rear axle. This design is very lightweight, but unfortunately, there is no way to adjust the
spring rate. Two Cannondale Topstones were tested with Hollowgram SAVE 27.2mm carbon seatposts. This
seatpost on other bikes of the same size measured at 110 to 113 newtons per millimetre. Although,
it's worth noting that was with 25.4mm seatposts. This 27.2mm diameter post likely has a slightly
higher spring rate. In any case, the Topstone samples were measured at 78 and 87 newtons per
millimetre. If we take the average of these spring rates, we can say that 27% less force is required
to flex the frame module a vertical millimetre than other 56cm bikes with a similar seatpost.
And bike #4 is the Basso Tera. The Basso Tera is a lowish cost aluminium gravel bike with
a carbon leaf-sprung rear triangle similar to what we've just seen on the Topstone. The frame
is said to offer 8mm of movement at the axle, and just like the Cannondale, there is no way
to adjust the spring rate of this bike. The Tera was measured with a Microtech aluminium seatpost.
While we don't have the values for this specific post, I have found aluminium seat posts deflected
an average of 175 newtons per millimetre in 56cm bikes. The Tera achieved 110 newtons per
millimetre, which is 37% less force per millimetre than other 56cm bikes with aluminium seatposts.
If you like the technical side of gravel bikes, why not grab one of my books? The Touring and
Bikepacking Bike Buyer's Guides are updated yearly - for free - and will teach you everything you
need to know about the bikes, before allowing you to compare over 220 different bikes
at the back of the book. Now that we have the numbers, are these frames effective at providing
traction? As we would have hoped, these soft tail frames have lower spring rates than diamond frames
with the same seatpost. The cool thing is that by knowing the spring rate of various seatposts, we
can use the rate of springs in a series equation to approximate how much force might be required
to flex these frames a vertical millimetre at the rear axle. When i put the seatpost
deflection numbers into the equation, each of these frames required about 300 newtons of force
to flex one vertical millimetre. We can contrast this with the 8,500 newtons per millimetre
of 56cm steel frames that were calculated by students at the University of Brighton using
a finite element method. Or the 7,000 to 14,000 newtons per millimetre measured in a handful of
steel frames in the 1990s. When you simply apply your body weight to one of these soft tail bikes,
the rear triangles are likely dipping two to three millimetres into their travel. And when
you're on the road or trail, the ground forces will be deflecting the frame even more. With a
frame spring rate this low, the suspension is no gimmick. It will undoubtedly take the sting out of
those medium-sized bumps, allowing you to have more traction and carry more forward momentum.
But what about comfort? In the last decade, component manufacturers have put a lot of
research and design into creating sea posts that maximize your ride comfort by offering
high levels of vibration damping and vertical deflection. A question you might be wondering:
do the best carbon flex seatposts require less force to deflect a vertical millimetre than a
soft tail frame? Across multiple bikes in the Tour Magazin testing, the canyon S15 VCLS 2.0 seat post
required 72 newtons per millimetre on average. As these deflection numbers are lower than what
has been measured in our soft tail bikes, you can unlock the same or even more vertical deflection
just by selecting the right seatpost for your body weight. We have some real-world comfort testing
to check out too. Krzysztof at Gravelbikes.cc has conducted outdoor vibration tests on a Cannondale
Topstone on both a bumpy forest trail and a fast gravel road. He then compared the levels
of vibrations of these surfaces to his titanium bike fitted with a Canyon S15 VCLS 2.0 seatpost.
Unfortunately, his titanium bike was set up using a different tyre and wheelset combination,
so the results are not definitive by any measure. But using his titanium bike,
he actually found a 9% reduction in vibrations on the bumpy trail, and 4% less on the gravel road.
These are the kind of results we can expect, as the spring rate of the softest spring in
the series is always the one that dominates, and the canyon S15 seatpost is as soft as it gets.
In summary, rear suspension systems on gravel bikes are not a gimmick. My estimations suggest
that they flex vertically with 24 to 48X less force than a typical diamond frame,
which results in more traction, more comfort, more control and more forward momentum on rougher
terrain. That said, if your priority is seated ride comfort, the data suggests that fitting a
carbon flex seat post or suspension seatpost to your diamond frame can yield the same, or possibly
even less transmission of vibrations to your body. Other than for particularly rough terrain,
I think a great application for a soft tail frame design is if you use a dropper seatpost. Dropper
seatposts are very stiff vertically so a soft tail frame will play a key role in maintaining your
comfort. If you're interested in learning about suspension seatposts, check out my video on the
topic here. And if you'd like to see my favourite bikepacking bikes, click on this video here.