You've finally reached the top of the hill and look forward to an easy descent.
A road sign indicates the downslope angle ahead. Hmm, maybe this won't be so relaxing. How fast will
you end up going? Should you brake on the way. Do you have to worry about your wheels overheating
or a tire blowing out? Hi, we're Pam and Gilbert. In this video, we'll investigate the maximum speed
you can expect to reach while going downhill and the most effective braking strategy to survive it.
We'll start with a quick explanation of Terminal Velocity. This concept is most often applied to
falling objects. As the earth's gravity pulls downward and accelerates the descent of the object,
the aerodynamic drag generated increases until both forces equalize and the object
stops accelerating, reaching what's called Terminal Velocity. This speed is a function
of the shape of the object, its weight and the air density surrounding it. How does this apply
to your Brompton? Well, as a bicycle coasts downhill, three major factors come into play.
The gravity pulling toward the bottom of the hill. This force will increase depending on
the total weight of the bicycle and the slope gradient. The tire rolling resistance which is
also dependent on the bicycle weight and the slope. In addition, it is affected by the type of tire and
the pavement quality. And finally, the aerodynamic drag, which is a function of the bicycle speed
and the aerodynamic properties of the bicycle and its rider. Like the falling objects we saw
earlier, the bicycle will accelerate until the sum of all forces equalizes, at which point the speed
will stabilize. Adding more weight to the bicycle should accelerate it. However, the increased drag
caused by the frontal area of a T-Bag or a Large Borough bag will negate some of the speed gain.
Expect a Brompton with such a setup to go faster than a unloaded Brompton
only if it carries more than 20 kilograms of luggage. In all cases, a higher terminal
velocity can be reached if the rider assumes a low head and shoulder position.
Finally, the maximum speed reached will be dependent on air density. Everything else being
equal, a downhill rider in Denver would go about 10% faster than a similar rider in Singapore due to
the lower air density found at higher altitude. The expected terminal velocity as a function of slope
gradient is as shown in this graph. For example a 4.5% slope should provide around 40 km/h [25mph]
terminal velocity as shown in this video from Victor of Everyday Cycling filmed in Atlanta.
More significant slopes are often found at higher altitude. For example, Russ and Laura
from the Path Less Pedaled channel cycle down the Big Hole Pass in Montana. Located above 2000 meters,
it has an average gradient of 6.5% at the location where the video was filmed.
Laura reached a speed of 55 km/h [35mph] while Russ, assuming a more
aerodynamic position on his bicycle, went 20% faster reaching 68 km/h [42mph].
Now, let's look at braking while going downhill. Assuming a 10% slope at low
altitude, the predicted terminal velocity should be around 63 km/h [40mph].
If you apply full brake force, the bicycle will be stationary at the top of the hill. Definitely
a safe and secure position, but not much of a solution if you want to reach your destination.
As you release the brakes by 5%, the speed will increase to 15 km/h [9 mph]
and the brake pad will generate 400 watts of heat. Releasing the brakes by 30% will increase
the speed to 35 km/h [22 mph] and the heat generated will peak over 600 watts,
the equivalent of a small microwave or toaster. As the brakes are released further, the bicycle will
eventually reach terminal velocity and no brake heat will be generated. As can be seen, maximum heat
generation occurs between 20 and 50 km/h [12-30 mph] To estimate how this affects rim temperature,
we need to look closer at the difference between a Brompton and a full-size bike. This
cross-section of a Brompton wheel shows the tire, inner tube, rim tape and aluminum double wall rim.
Now, let's compare it to a similarly priced full-size bike equipped with aluminum wheels.
Not only is its diameter larger providing more thermal absorption capability, but the rim cross
section is deeper and thus more aerodynamic. As the brakes are applied, the energy is mainly
absorbed by the rim, the brake pads having much lower thermal conductivity than aluminum.
The Brompton wheel, with its lower aluminum mass and thus smaller thermal capacity
will see its temperature increase faster than the larger wheel of its bigger cousin.
Furthermore, convection cooling due to airflow around the wheel
will be better on a full-size wheel due to its deeper rim and the lack of fenders.
Using a mathematical model developed by David Wilson (reference in the description), it's possible
to roughly estimate the rim temperature during a steady state descent, assuming both wheels are
used for braking. With no brake applied, the rim will be roughly at ambient temperature or 20c [68F]
and the bicycle will be going at 63 km/h [40 mph]. If we apply brakes
to reduce the speed to 50 km/h [31 mph] the rim temperature will increase to 55c [131F].
Further deceleration of 35 km/h [22 mph] will increase rim temperature to 85c [185F]
At 20 km/h [12 mph], the rim will will reach 100c [212F]. For comparison purposes,
the mathematical model predicts approximately 75c [167F] for a 20-inch wheel bicycle and less than
50c [122F] for a full-size road bike. Rim overheat during descent can therefore be a serious issue on
a smaller wheel bicycle equipped with rim brakes like the Brompton. Ideally, and if road conditions
permit, riders should avoid constant braking, instead braking intermittently in order to give
the wheels a chance to cool off. But what exactly occurs if the rim reaches higher temperature?
Unfortunately, there's no definitive temperature threshold at which we can predict failure.
One thing that is easy to evaluate however is the expected tire pressure. Using the Ideal Gas Law,
assuming a 100 psi tire [6.9bar] at 20c [68F], the pressure will be 127 psi [8.8bar] at 100c [212F].
As a rule of thumb expect a change of 3 psi per 10c. The combination of high
temperature and high pressure increases the probability of tire failure. For example, tire
patch adhesive could start melting, creating an air leak. Rim tape can melt, allowing a spoke to
perforate the tire. In a more serious scenario, a poorly installed tire could eventually blow out
as it is ejected from the rim. So, to sum up, a rider of average weight on a C-line bike should reach
45 km/h [28 mph] on a 5% slope and in excess of 60 km/h [40 mph] on a 10% slope. A touring
setup with a large front bag will increase this speed only if more than 20 kilograms is carried.
And remember your smaller wheels are prone to overheating during descent, thus our advice is
avoid constant braking. Use your riding position as a speed brake, unzip your jacket for example.
When you really need to brake, use both front and rear wheel braking to equally distribute the
thermal effect. And stop by the side of the road to cool off your rims if you suspect overheating.
We'd like to hear your experience with maximum downhill speed.
Let us know in the comments and include where you were and your bike set up.