Are you eyeing up a shiny new cycling upgrade? 30 tabs open, checking every review and Reddit thread, looking for a little bit of certainty. “Yes, these eye-wateringly expensive wheels/helmets/socks will make you faster. Go for it.”

But is such certainty available online? Will parting with your hard-earned cash save you time and effort on the bike? Can you be sure that @cyclingGOD404 really knows what he’s talking about?  

Two Belgians, a cycling-obsessed engineer and a former Lotto Dstny pro, have built a free online tool to help you find out.

The tool is Watts the Difference, a free, physics-based simulator that tells you, in plain numbers, exactly how much speed, power, or time an upgrade will (or won’t) gain you on a ride. Before you spend big on the wrong upgrade, check out the tool below.

TL;DR: Find out what cycling upgrades really make you faster:

— Quickly compare different bike set ups to see what upgrades will make you faster
— Change your rider data and riding position to see where you can make gains
— Riding in a group, aero equipment and proper bike maintenance offer the biggest savings

What is What's the Difference?

Watts the Difference is a web-based tool that lets you model how changes to your bike, body or kit affect your overall speed and performance. Users can easily adjust various parameters to calculate how different variables will affect their speed.

You can use a preset segment, like Alpe d’Huez, or upload your local loop route GPX file, or download a previous ride from Strava and upload that as a Fit file. I prefer the latter option as it uses your actual ride data, which feels more relatable.    

How does What's the Difference work?

The simulator works by modelling the four main forces that act on a cyclist: aerodynamic drag (the wind pushing against you), rolling resistance (the friction between your tyres and the road), gravity (how much your weight and the hill work against you), and drivetrain losses (how much energy is lost between your legs and your rear wheel). 

Here's the official explanation:

Watts the Difference is a cycling simulator designed to quantify the influence of rider characteristics, bicycle configuration, and pacing plan on overall performance. The physics engine models aerodynamic drag, rolling resistance, gravity, drivetrain losses, and inertial forces using dedicated submodels for CdA, Crr, and mechanical efficiency. These submodels combine established literature with our own proprietary research, including measurements and advanced numerical simulations.

The good news is you don’t need to understand all of that to play around with the tool. You can see what upgrades will or won’t help you without a physics degree.

How do you use What's the Difference?

The easiest option is to change one variable, like swapping your standard helmet for an aero one or upgrading to 60mm deep wheels. The tool will then show you the difference in watts required, or time saved, over your chosen ride. Or you can adjust multiple variables (deep wheels, aero frame, full aero kit) to see how much time your dream set-up could save you. 

Step-by-step instructions:

• Select a course or upload a GPX/FIT file
• Select ‘compare’ under input parameters
• Adjust your current data (reference) vs your test data (test)
• Hit calculate to see the results
• To test just one variable (aero helmet vs normal helmet), make sure all other parameters are the same. 
• Change the test parameters as you like to calculate more results

How does Watts the Difference help cyclists?

In a murky landscape awash with claims about "X watts saved" or "Y seconds faster”, Watts the Difference gives you clarity over where to spend your time and money.

Made by cyclists for cyclists, it lets you cut through the marketing fluff and see what benefit a given upgrade might actually give you on your weekend ride.

Who are the people behind Watts the Difference?

The tool was built by Bart Blockmans and Viktor Verschaeve — two Belgian enthusiasts who currently work at Classified Cycling. Their shared passion for cycling and optimisation led them to create the tool, mainly because it didn’t already exist.

Bart is an R&D engineer with over a decade of experience in computational mechanics. He specialises in areas like aerodynamics, drivetrain dynamics, and multi-physical simulation. 

• He has spent his career developing numerical tools for complex engineering problems
• Currently works as an R&D engineer for international cycling & motor sports teams
• Completed senior research roles at both KU Leuven and Flanders Make
• Precisely the kind of person you want building a physics simulator for cyclists

Viktor brings a completely different perspective. He is a former pro cyclist who rode for Lotto-Soudal/Lotto-Dstny from 2021 to May 2023.

• Won a stage at the Tour de Savoie Mont Blanc and finished second at Liège-Bastogne-Liège Under-23
• Spent a career exploring how marginal gains can significantly enhance performance
• Retired early due to injury. Now the R&D Test Engineer at Classified Cycling
• Pro cycling’s loss is our gain

Why did they build a free cycling comparison tool?

I caught up with Bart and Viktor to find out why they gave up their free time to develop the tool.

"Our motivation to build it is plural," explains Bart. "In the first place, to give cyclists a means to cut through some of the marketing noise and decide what's worth doing or buying and what is not. But we also couldn’t believe that such a simple tool didn't really exist before as a freely accessible service."

That it is free for all to use is an important point. 

There are expensive consultancy services and professional wind-tunnel tests available to elite teams, and a handful of power-based calculators that have existed in various forms for years. 

Thanks to Bart and Viktor, this data is now available to you and me, too.

"Watts the Difference translates everyday cycling decisions into measurable impact — time, watts, speed”, adds Viktor. “The goal is to make the marginal gains I obsessed over as a pro measurable for all levels of cyclists. So you can decide what’s actually worth it.”

What can a cycling simulator tool tell you?

The tool is free to use and requires no sign-up. Simply head to wattsthedifference.app and start exploring. For a beginner or newer cyclist, the most valuable use cases are probably the simplest ones. You might want to know:

• Free watts from a clean chain: The hard truth no one wants to acknowledge - stop spending money, start cleaning and maintaining your bike properly (sigh)
• Tyre width: Will switching to a wider tyre make a meaningful difference on my commute or club ride?
• Tubes or tubeless? Will making the somewhat daunting switch to tubeless actually make a difference, or are good old-fashioned tubes still ok?
• Weight: How much would a new lighter bike actually save me, and can I just make cheaper/free upgrades to my existing bike for the same gains?
• Aero vs regular: How much do I gain by wearing aero kit? Should I invest in a new helmet? Do aero socks make any difference, or just look cool? Important questions.

Can Watts the Difference teach you about cycling physics?

As a bonus, by using the tool, you’ll pick up some knowledge about cycling physics. And knowledge about how your bike and body work together will vastly increase your performance more than an oversized jockey wheel! 

Many riders assume that lighter always means faster. Or that aerodynamics only matter if you're doing 40kph on a flat road. Watts the Difference makes it easy to see that the relationship between weight, speed, and gradient is more nuanced than that. 

On a steep climb, dropping weight matters enormously. On a flat fast sportive, aerodynamics dominate. Play with the numbers for a few minutes, and you get a sense of what will help you on different types of rides. 

All of this learning helps to give you more focus. Where and how to optimise your setup becomes easier with usable data. 

How accurate is Watts the Difference

To answer this question, Viktor and Bart ran a special simulation, using none other than Tadej Pogačar. 

Bart tells me of the test, “after Strade Bianche, we put the speed profile of Pogačar's 79 km attack into the app and filled in Pogačar's parameters and the wind speed & direction. We estimated his power evolution over the course, showing an average of 378W. A day later, Velon produced a graph showing that Pogacar averaged 380W over the last 79 km. Quite a nice validation.”

If it’s good enough for Pog, it’s good enough for me. 

Dirty Wknd top tips: Hard work vs easy upgrades

Hard work and discipline will always perform better than expensive upgrades. Unlike Barhatch Lane, this is a hill I will gladly die on.

Fancy upgrades will certainly increase your speed. Not least because they make you feel good about your setup - ‘feel good ride fast’ is a real thing. 

But a well-maintained and clean bike—clean drivetrain, working brakes, good tyre pressure— gives you more performance benefits than fancy noisy wheels or a pointy helmet. 

And that’s not even touching on training. Train to the best of your ability; eat well, more importantly, sleep well. Don’t drink too much, rest when you can, do some stretching, lift a few heavy things every once in a while. 

These are all hard things to do, but they should be viewed as a baseline to start from. 
‘Free’ cycling upgrades you can access today:

After many hours playing around with Watts the Difference, here are my favourite free upgrades* that you can easily action at home:

• Clean your damn chain: No joke! A dirty drivetrain is stealing your watts. Clean your chain and cassette, wax it if you can (if not, use dry lube). Make it a habit.
  • Clean/waxed vs dirty chain = 47 seconds saved over 40km
• Pump your damn tyres up: This one really is free (is air still free?). Work out your optimal tyre pressure, then make sure you're pumped before each ride. Free speed.  
  • 5.3 bar (76psi) vs 2 bar (30psi) = 5 seconds over 40km
• Fit the widest tyres you can: It’s now proven that wider tyres are better. Next time you upgrade your tyres, go as wide as you can fit on your frame.
  • 30mm tyres vs 25mm tyres = 1.37 minutes over 40km
• Inner tube material matters: Standard butyl inner tubes are slower than latex tubes. Next time you buy some new tubes, switch to latex to gain some more speed.
  • Latex vs butyl inner tubes = 55 seconds over 40km
• Aero kit isn’t a drag: Wear whatever makes you comfortable, but if you want to go faster, aero kit does make a difference. To consider when next shopping for jerseys.
  • Aero kit  (helmet, kit and socks) vs non-aero kit = 55 seconds over 40km

If you’re after more speed, these are the first things you should look at. Interestingly, some things that didn’t give a big benefit were:

• 50mm wheels vs 30mm wheels (12 seconds gained),
• Tubeless vs latex tubes (4 seconds gained)
• 50 tooth chainring vs 52 tooth chainring (1.2 seconds gained)

The beauty of Whats the Difference is that you can spend hours playing around with all the variables that matter to you. For free. Much better than doom scrolling LinkedIn next time work is slow.

What is the best cycling upgrade you can make?

Riding in a group. 

That’s it. If you want to smash your PB on your local loop, ride it with mates. Heck, even one mate will make a huge difference. 

Riding in a small group (around 10 riders) will save you over 11 minutes over 40km. Riding behind one friend will save you over 8 minutes. 

So not only is riding in a group safer and more fun, it’ll get you to that post-ride coffee or beer much quicker. Joining a cycling club or group ride is the very best way to increase your performance.

Try a free cycling simulator for yourself

You can try Watts the Difference for free at wattsthedifference.app. There's no login, no subscription, and no catch. Just a tool built by two people who wanted to make cycling a little less confusing.

Whether you're debating your first upgrade or trying to make sense of what your next ride setup should look like, it's worth 10 minutes of your time.

And if you want to join a club or group, check out the Dirty Wknd Community to find the right group in your area.