"When Climbing Hills, Should You Prioritize Weight Reduction or Aerodynamics?"

(This article was originally published in Japanese in July 2022, and later translated into English with the help of ChatGPT.You can read the original Japanese version here.)

As someone passionate about doing well in amateur hill climb races, this is one of the most important questions for me.

In recent years, it's become common to hear things like:

“Even on climbs, if the gradient is below X%, it's better to prioritize aerodynamics over weight reduction.”
“By redesigning the frame to reduce drag, I managed to shave XX seconds off my time on the ○○ Pass.”

Every time a new road bike is announced, we see these kinds of claims. It's fascinating that improvements in aerodynamics can reduce your time on a climb by tens of seconds.

However, in parallel with this aero trend, disc brakes have become the standard on road bikes (perhaps aero came slightly earlier?). Disc brakes, by their design, tend to add weight to the frame, wheels, and components compared to rim brake setups. In bikes of the same grade, the total weight difference is roughly 500 to 700 grams. As someone serious about climbing, I don't obsess over every gram, but 700 grams is hard to ignore.

(Just to add my own situation: I built my Specialissima with a 12-speed Ultegra, adding another 299g. Due to rising costs in the bike industry, Dura-Ace was out of my reach.)

So the question becomes:

“Sure, modern bikes have gotten faster thanks to aero frame shapes and integrated cabling. But with the extra weight from disc brakes, aren't we back to square one? When it comes to climbing, wouldn’t a lightweight rim brake bike still be faster?”

I suspect this is a common question for many cyclists—not just me.

This post is my attempt to investigate that question for myself.

Title: Emonda vs. Specialissima: Hill Climb Showdown Vol. 1

(Yes, I tend to slap “Vol. 1” on these kinds of posts… whether there’s a Vol. 2 is yet to be seen.)

To start with the conclusion:

On a 4.42 km hill with an average gradient of 5.4%, the lightweight rim-brake Emonda was about 4.5 seconds faster per climb.

Based on this, we can estimate a time savings of around 22 seconds on a course like Mt.Fuji Hill Climb race.("The Mt. Fuji Hill Climb is the most famous uphill-only cycling race in Japan.")

However, this data assumes riding at 4.41 w/kg. Interestingly, when I increased the output to 5 w/kg, the 900g-heavier disc brake Specialissima actually pulled ahead by a small margin.

Test Conditions

Bike Specs

TREK Emonda SLR 2018 (rim brake)

• • Weight: 6.65 kg (with meter, pedal weight added manually)

  • Wheels: Roval CLX50

  • Groupset: SRAM eTap 11s

 Bianchi Specialissima Disc 2021

• • Weight: 7.55 kg (including meter and pedals)

  • Wheels: Mavic Cosmic SLR45

  • Groupset: Shimano Ultegra 12s

Test Course & Reason for Selection

I conducted the experiment on Katsuoji Temple Hill in northern Osaka, a well-known local climbing route. The profile: 4.42 km long with a 5.4% average gradient.

The Strava KOM is 9:43 (average speed: 27.3 km/h), while my PR is 10:46 on the Emonda (24.6 km/h). That’s a gap of over a minute on a short 4.5 km hill—pretty significant.

Why I chose Katsuoji:

• No traffic lights in the segment

• Realistic gradient for actual hill climb races

• No downhill sections

• Nearby coin parking for logistics

• Familiar landmark for cyclists, making time comparisons easy

Incidentally, the Fuji Hill Climb course is 24 km long with an average gradient of 5.2%. Multiply the Katsuoji time by 5.5 to 5.7 for a rough estimate of your Fuji time (taking wind and fatigue into account). Drafting in a race group could reduce this further.

Personally, my goal is to earn the Gold Ring at Fuji Hill, so I also wanted to investigate which bike would serve me better for that event.

[2022.7.26 Update]

My Fuji Hill time was 65:16. If I ran Katsuoji in 12 minutes, multiplying by 5.7 gives ~68:30. Subtract ~3 minutes for draft effect, and the numbers line up well.

In short, being able to ride Katsuoji three times in a row in 11:30 might be a good benchmark for hitting gold at Fuji.

Additional Test Conditions

• 4 total climbs (2 on each bike) at a consistent 265w (4.41 w/kg)

• Bonus climb at 300w (5 w/kg)

• Consistent riding position: seated, on the hoods

• Ignored variables like wind, cars, or temperature differences

• Same kit for all runs; carried a wind vest in the rear pocket each time

• Same power meter (Assioma Duo) for all climbs; calibrated when switching bikes

• Water bottles filled with 200ml of water, no drinking during the climb

• All tires were the same brand (new), weights balanced across bikes

• Tire pressure set to 5.0 bar for all tires

• Emonda used butyl tubes, Specialissima used latex tubes (based on real-use conditions)

• Drivetrain was cleaned and lubed identically for both bikes

• Emonda had a newer chain; Specialissima had ~1,500 km of wear

Test Results

At 265w, the Emonda was slightly faster. At 300w, the Specialissima pulled ahead.

Bonus Test Data

Measured separately on a 4.67 km / 9.7% average gradient segment on Mt. Rokko:

• Specialissima: 20:45 @ 282w

• Emonda: 19:41 @ 287w

Over the same distance, the steeper grade resulted in nearly a 1-minute difference, which could be huge in a race.

Summary

So what’s the conclusion? “Lightweight bikes are faster for climbing! Aero benefits are just marketing!” — I’d love to make such a bold claim, but that’s not how I see it.

It really depends on the course and the rider’s power and weight. In fact, when I increased the power, the heavier aero bike actually won.

Generally, the more power you can sustain, the more effective aero bikes become. On the other hand, for lighter riders or steeper courses, a lightweight bike has the edge.

This test wasn’t meant to favor either bike. I love both my Emonda and my Specialissima, and there’s no bias here. Please keep that in mind.

Some may think:

• “Given such a small difference, I’ll go with the disc brake bike for better control.”

• “No need to buy a new aero bike — rim brakes are still good enough.”

• “I can’t hold 4.4 w/kg. For me, a lightweight bike makes more sense.”

• “Maybe it’s not the disc brakes — maybe I need a 7.0 kg aero disc setup.”

• “Emonda at 6.6 kg still isn’t light enough — maybe even lighter is better.”

• “Who cares about the weight? Just buy an Aethos and be done with it.”

• “The differences are too small to draw any conclusion.”

As for me, I was honestly surprised. I thought the Emonda would be about 10 seconds faster in both runs.

After this test, I feel far less negative about the weight of disc bikes. During the rides—especially the bonus climb—the Specialissima actually felt lighter, probably thanks to the latex tubes.

In the end, this took a lot of effort and prep, but I’m glad I did it. I hope someone finds these results useful.