Hi, this is Pam and Gilbert of 2Bikes4Adventure. In a recent video, we explained how
the Brompton compares with a road bike on a flat road. Shortly afterward, the Global Cycling Network
published a video of an actual hill climbing speed test based on a similar premise. In their scenario,
hillclimbing champion Andrew Feather rode a two-speed Brompton in a race against GCN presenter
Ollie on a high-end road bike, an Orbea Orca. The competition took place along a road winding
through Cheddar Gorge, in Somerset England. Spoiler alert, the Brompton won by a fraction of a second.
It's difficult to compare the bike's performance as the riders were clearly not well matched.
Instead, we can use Andrew's previous record on the same climb on his road bike, a Cannondale SuperSix.
We will assume that record was set under similar conditions.
When we looked at the race data recorded by Ollie's bicycle computer,
we noticed what seems to be two distinct races as well as multiple finishes.
The end of the second climb was used in the final video as we see the racers exiting the main road
at the intersection to discuss what just happened. They seemed to ride the end of the race a second
time, probably to ensure they had enough video footage or maybe to stage a more dramatic finish.
We'll use the results of the second race in our calculations. The Brompton completed the
4.55 km route in 10 minutes 31 seconds, for an average speed of 26 km/h.
Andrew's record on his SuperSix bike was 8 minutes 34 seconds, for an average of 32 km/h.
His superbike was 6 km/h faster, about a 23% difference.
The SuperSix weighs 5.4 kg and is equipped with Schwalbe One (TT) Time Trial tires.
The 44T chainring and DuraAce 11T-28T cassette provide 11 speeds, 41 to 105 gear inches range.
The Brompton was a standard S2L, weighing 10.7 kg,
with Marathon Racer tires, a 54T front ring, 13T and 16T cassette, providing 56 and 74 gear inches.
Both bicycles were equipped with the same Shimano pedals.
Using the Cannondale performance specifications as a baseline, we extrapolate that the cyclist
generated an average of 380 watts during the race, we assume that he applied similar power on
the Brompton. We'll break down the factors that could explain the 6 km/h gap,
starting with the most significant. Rolling resistance was the biggest factor in this race.
The Brompton Schwalbe Racer would dissipate a total of 60 watts while the Schwalbe One (TT)
Time Trial dissipates about 13 watts, for a difference of 47 watts. This translates
to a 2 km/h difference between the bikes or 1/3 of the gap between them.
The second most important component during this climb was aerodynamic drag. On serious climbs,
drag can be less of a factor, but the 3.8% overall gradient of this climb resulted in average speeds
that are relatively high for climbing competition, exceeding 40 km/h during flatter
sections. While the Brompton frontal area is slightly smaller than the Cannondale, the rider's
body accounts for most of the drag. The rider assumed various positions during the race, standing
on pedals during the steeper sections, more so on the Brompton due to his lack of low gear.
Comparing the relatively poor riding position offered by the Brompton
which we estimate at .401 CdA versus the aerodynamically friendly Cannondale with
an estimated average of 0.308 CdA, the faster bike has a 1.5 km/h advantage.
Third, bicycle weight. The Cannondale SuperSix is configured for hill climbing
and is very light at 5.4 kilograms. If we could lighten the Brompton
to an equivalent weight, it would climb 1 km/h faster.
Gear efficiency: As we showed in a previous video, the smaller the sprocket the more chain friction
loss. The superbike's larger 28T sprocket creates less friction than the 16T sprocket.
On the two-speed Brompton, it's also fair to assume the jockey pulleys and bottom bracket are less
efficient than those on the Cannondale. We estimate 98.5% powertrain efficiency for the Cannondale
compared to 95.5% for the Brompton, translating into about 10 watts of additional loss
or 0.5 km/h speed difference. These factors can explain 5 km/h
of the 6 km/h difference. We'll address two more aspects.
One point we didn't discuss in our Brompton speed video is the effect of frame flexing and
rear suspension power loss. While these losses may be minor at low power settings, they are
more significant at high power, such as during a climbing race. Frame flexing is a contentious
subject, particularly in premium road bike circles. For many, lateral stiffness is synonymous
with better performance. The Cannondale SuperSix has one of the best stiffness to weight ratios
while the Brompton geometry represents a nightmare for those who worship lateral stiffness.
Brompton has used special simulation software during the design phase to minimize flexing
but the long stem, diminutive front triangle and multiple hinge points are serious limiting factors.
The presence of a rear suspension only compounds the effect of frame flexing power loss.
As we see here, the force applied by the rider varies depending on the angle of
the crank. Maximum force is applied when the pedal is at the 3 o'clock position.
Depending on upstroke technique, more or less negative force is applied as the pedal returns
to the up position. This cycle is repeated with the opposite pedal and for many cyclists, the left
leg is less powerful than the right. Adding both pedal forces results in this total crank torque.
This fluctuating force is transmitted to the sprocket via the chain.
Since the chain line is located above the pivot point of the rear triangle, it makes the triangle
move upward, compressing the suspension block each time maximum torque is applied. The amount
of movement will be proportional to the force applied on the crank and the elastic properties
of the suspension block. The up down motion of the rear triangle is known as suspension bobbing.
How much power is lost in the frame flexing and suspension?
Available test results for the Brompton are scarce, but research done on mountain bikes evaluates
losses due to rear suspension bobbing to be around 1.3%. To err on the safe side, we
will assume 2% or about 8 watts loss, which translates to 0.5 km/h.
Finally, we have to address the effect of cadence and gearing on the rider's performance.
The cadence that maximizes power output at the crank varies from one cyclist to another.
It could be as low as 70 rpm for a long distance cyclist and up to a 120 rpm for a sprinter.
In all cases, the power curve has a somewhat similar shape. It's centered at the cyclist's
optimum cadence and decreases as it deviates faster or slower from that ideal. Research
shows a deviation of 20 rpm on either side of that point results in a 6% reduction in power.
With only two speeds to select from, optimum cadence would not always be possible on the
Brompton, having to stand on the pedals while climbing and overspinning at the end of the race.
We'll attribute a 2% loss or 0.5 km/h to the poor gearing setup.
So, we see that a poorly geared Brompton is about 23% slower than the top of the line road bike when
climbing a 4% hill. What if the new Brompton 4-speed Titanium T-Line had been used instead?
With its Schwalbe One tires, rolling resistance would be reduced by 2/3,
thus gaining 1.3 km/h. Aerodynamic performance wouldn't change significantly.
While the T-Line is still heavier than the Cannondale, its 30% weight reduction due to the titanium and
carbon construction would provide an additional 0.5 km/h over the regular Brompton.
The larger sprockets and superior gear train would save about 5 watts during climb,
to gain an additional 0.25 km/h.
The more rigid suspension block and redesigned frame of the T-Line should reduce flexing
power loss. The same logic would apply to the slightly wider range provided by the four speeds
compared to the old two speed, thus reducing the effect associated with cadence inefficiency.
Overall, we would expect the T-Line to be 10% faster than the conventional (S2L) C-Line
over this race course, reducing the gap with a high-end road bike by half to about 12%. Short of
buying a T-Line, one of the most cost effective modifications to improve your Brompton speed,
uphill or not, is to upgrade the tires. But as we saw in the GCN video, the results remain highly
dependent on rider fitness, no matter what bicycle you're using. For most of us, it's not how fast we
climb or who arrives first, but if we get to the top without having to dismount and push the bike.