Hi, we're Pam and Gilbert of 2 Bikes 4 Adventure. We're often asked how fast we can go on a Brompton,
now this depends more on the rider's fitness than the bike's limitations. Nobody mistakes us
for fast cyclists.But we can offer some insight into how a Brompton compares with the road bike
in terms of speed. Reviewing the Brompton gearing table we showed in a previous video,
let's select a six-speed configuration with a 54-tooth chainring installed on a CHPT3
frame. This highest gear would provide 106 gear- inches, a very potent gear development on a par
with a similarly priced road bike. Assuming a rider that could maintain a 90 rpm cadence on this bike,
this would translate into a very respectable 45 kilometers per hour (or 28 miles per hour).
But, a more appropriate question would be: for the same amount of effort
what is the difference in speed between a Brompton and a similarly priced road bike?
We'll compare a 12 kilogram S6L Brompton with a 73 kilogram rider and a similar
rider on a 10 kilogram road bike, both riding on a smooth flat road.
We'll focus on three major factors that influence bicycle performance: Drivetrain efficiency,
Rolling resistance and Aerodynamics. A significant portion of the chain friction is created
as it articulates through the various chain wheels. Observe in this close-up how the inner and outer
links pivot around this bearing pin (highlighted in red) as the chain disengages from the rear sprocket.
Let's repeat that movement back and forth to clearly visualize what is happening. The deflection
or angle of friction at the disengagement point is proportional to 360 degrees divided by the
number of teeth on the sprocket. So, the smaller the sprocket, the greater the angle of friction.
This friction loss applies as well to the upper and lower jockey pulleys that are particularly
small compared to the cassette sprockets. Friction loss is energy loss, meaning less efficiency.
Finally, but to a lesser degree, the lateral chain deflection or
offset will cause additional friction as the rider selects the largest or smallest sprocket.
For example, a generic derailleur like the Shimano Dura Ace using its most efficient or biggest
sprocket, would provide 94% efficiency at 60 RPM and 100 Watts. It drops to 92% on
the middle sprocket and 91% for the smallest sprocket, so a significant difference between
the largest and smallest sprockets. As the power level supplied by the rider increases,
surprisingly, the efficiency improves as well, reaching up to 98% during hill climbing.
So, road bikes are at their most efficient during hill climbing and less efficient at high speed.
Comparing this with the Brompton 6-speed, the chain routing is surprisingly similar,
mainly due to the presence of the gear tensioner on the Brompton.
The main difference remains hidden from view, inside the internal hub.
If we could open it, we would see it consists of a planetary gear system containing a fixed
central gear, called the sun gear. Around it rotates four smaller gears called planets. The planets are
connected together by a planet cage. The outside part is called the ring gear. When 1st gear
is selected, the bicycle sprocket connects to the ring gear and the 16 inch wheel to the planet cage.
This results in a 1 to 0.64 gear ratio and a friction loss of about 3 %.
In 3rd gear, the mechanical connections are reversed. The sprocket now drives the planet cage
and the wheel is connected to the ring gear for a ratio of 1 to 1.57 and still a loss of about 3 %.
The magic occurs in 2nd gear, also known as direct drive. In this setting the sprocket and
the rear wheel are mechanically connected together via the ring gear, thus eliminating most of the
friction loss. The efficiency is about 95 % at 150 watts, mainly due to chain friction.
The lowest and highest gears are substantially less efficient as we've seen. If we compare with the
derailleur, it's clear the Brompton rider should favor the 2nd hub gear as much as possible.
Next, we'll look at the effect of wheel size on rolling resistance.
As we pressurize a tire and add weight to it, the tire deforms slightly and a contact patch
is created. The patch area roughly equals the weight carried by the tire divided by the tire
pressure, in this example 1 sq in. or slightly less than 7 cm2.
As the tire rolls forward, the front of the contact patch deforms the tire and energy is
absorbed by the tire casing. The energy is restored at the back of the contact patch.
The difference between the energy absorbed and restored is a source of rolling resistance.
We might think that if we have a larger diameter tire with the same physical
properties and therefore the same contact patch, the rolling resistance would be identical but...
The difference resides in the percentage of tire deformation for a given contact patch.
In this example, the Brompton tire would have 10 % of its circumference deformed while on the larger
road bike the circumference would only be deformed by 6 %. This would translate to an equivalent of 40%
reduction in rolling resistance as can be shown in this laboratory test done on Schwalbe tires.
Notice how the rolling resistance decreases as the wheel diameter increases. Assuming a tire pressure
of 90 PSI, our Brompton rider equipped with 16-inch Marathon Racers cycling at 20 km/h
will experience 50 watts rolling resistance while a similarly loaded road bicycle equipped with
Marathon Racers as well will experience 40% less rolling resistance or about 29W.
Let's see how that's this translates into riding speed differences. On a flat road,
as the rider rotates the crankset, the bicycle accelerates until the sum of the aerodynamic drag
and rolling resistance equals the power supplied by the rider, at which point, the speed stabilizes.
A 20 km/h Brompton has a rolling resistance about equal to the aerodynamic drag.
If however, we replace the Brompton by a road bike, the rolling resistance will drop by 40%,
leaving more power available to accelerate the rider. How much faster can the road bike rider go?
Assuming both cyclists apply the same power input and use a similar
riding position and then factoring in the drivetrain efficiency we discussed earlier,
we can use an online bicycle power calculator to evaluate the speed difference between the bikes.
At very low speed, the low efficiency gear hub is no match against the efficient large sprockets
found on the road bike. A 5 km/h Brompton would be overtaken by the road bike
50% faster at 7.5 km/h. As we accelerate, the
Brompton 2nd gear can be used and the increase in aerodynamic drag starts affecting both riders
and somewhat masks of Brompton's poor rolling resistance. At high speed, both bicycles suffer from
their poor gearing efficiency. If we average out the speed difference over the entire speed range,
we can sum it up to a 2.3 km/h difference between the Brompton and the road bike.
In percentage, this would be about a 10% increase over a casual 20 km/h ride.
A 60 km touring trip would see the road bite cyclists arriving 19 minutes or so
before the Brompton friend, more than enough time to drink a victory beer.
So far we have assumed that both riders have a similar riding position,
in other words, that the Brompton handlebar has a similar stack to the road bike.
The effective speed on aerodynamic drag is well known. As the speed doubles, the power
required is multiplied by eight. So, while it's relatively easy to accelerate from 5 to 10
or 10 to 20 km/h, it takes considerable effort to accelerate from 20
to 40 km/h. One way to reduce the level of effort required is to reduce the frontal
area. This is much easier to do on a road bike with drop handlebars than on an upright Brompton.
When the rider's hands lower from the top of the handlebar to the brake hood position,
a 20% reduction in aerodynamic drag results. Moving to the drop bars will save an additional 5%
and if the bicycle is equipped with an aero bar, a total of 30%
could be saved. None of that is available to our unfortunate Brompton rider.
So, if we revisit our speed difference chart and assume a break hood position while the Brompton
rider is going at 25 km/h, the road bike will now exceed 29 km/h.
The difference increases further as the bikes accelerate and the road cyclist assumes the
drop position. Fast cyclists should therefore expect a 4.5 km/h advantage
over a Brompton rider. In percentage, roughly 15% if cruising at 30 km/h.
Using our previous 60 km/h journey, the road bike would arrive 16 minutes before the
Brompton competitor, still enough time for a beer. What can we take away from this information?
The 2nd gear (+/-) is our friend, equivalent to 3rd and 4th gear on a
6-speed Brompton. Expect 3% more efficiency when using it compared to low and high gear settings.
For casual rider (15 to 20 km/h) expect a road bike to be about 2.3 km/h
or 10% faster for the equivalent effort. For a fast rider (25 to 30 km/h)
expect a road bike to be about 4.6 km/h or 15% faster. We hope that
when you arrive at your destination, you enjoy a celebratory beverage no matter how fast you cycle.