Condensed answer: The potential top speed of a bicycle depends on many factors such as gearing, wheel size, tire tread and frame geometry.
All things being equal, a geared bike would outperform a single-speed model thanks to the efficiency of gears which allow the rider to pedal at optimal cadence, spare energy and climb monstrous hills.
A single-speed bike encourages aggressive sprint riding, and in some situations (e.g., short trails), a rider may have better times on it. Nonetheless, the same style of cycling can be achieved with a geared bike if the cyclist stays in a high gear and doesn’t shift.
Technical Factors That Determine a Bike’s Speed
The drivetrain of a bicycle constitutes of chainrings (front) and rear cogs (back).
Each chainring + cog combination, gives the rider certain maximum speed.
The greater the gear ratio, the faster the rider can potentially be.
The term gear ratio refers to the relation between the chainring and the rear cog.
For instance, if the rider is pedaling in a big chainring with 44 teeth combined with an 11 teeth rear cog, the gear ratio is 44:11 = 4:1
This means that in that gear, each full revolution of the cranks causes the back wheel to spin 4 times (thus you have a 4:1 ratio).
The larger the gear ratio is, the faster the bike can be because each spin of the cranks equals more revolutions of the rear wheel and thus a greater traveled distance.
The formula for speed is: Speed = Distance/Time.
Therefore, if a bike covers more distance in the same amount of time, it’s moving faster.
The next scenarios illustrates how higher gearing creates the potential for more speed.
The only dissimilarity in both situations are the gear ratios. The rest of the variables (time, crank RPM, wheel circumference…etc.) remain a constant to illustrate the difference.
Case A: Gear ratio – 44:11=4:1
Case B: Gear ratio – 50:11= 4.5:1
Time of travel = 1 minute
Rotations of the cranks (rpm) per 1 minute = 80
Wheel circumference = 220cm
In 44/11, the rear wheel spins 4 times per 1 crank revolution.
Since the rider is pedaling at 80rpm, the rear wheel makes 4×80=320 turns per 1 minute/60seconds.
If the wheel circumference is 220cm, the bike will move 220cm x 320 = 70400cm = 704m = 0.704km
The speed of the bike will be:
704m/60s = 11.74m/s = 42.24km/h = 26.24mph
In 50/11, the rear wheels spins 4.5 times per 1 crank revolution.
At 80 rpm, the rear wheel makes 4.5×80=360 turns.
If the wheel circumference is 220cm, the bike moves 220cm x 360 = 79200cm = 792m = 0.792km
The speed of the bike is:
792m/60s =13.2m/s = 47.52km/h = 29.5mph
Conclusion: In the higher gear, the bike is about 5km/h faster at 80rpm.
Therefore, if all other parameters of a single-speed and geared bike are equal or close to it, the one with the higher gearing will offer more potential top speed.
For example, if the single speed bike has a 44/11 top gear whereas the highest of the geared bike is 36:11, the single speed model provides an opportunity to reach greater speed via pedaling.
2. Wheel Size
A bike with bigger wheels is not necessarily faster, but it has the potential to be.
If two bikes have identical gearing but different size wheels, the bike with the larger wheels will offer more top speed.
Thanks to the extra circumference, each rotation of the rear wheel propels the bike further, and subsequently, the bike covers more distance in the same amount of time. Thus, the speed of the bike is also greater.
However, if the gearing of a bike with small tires is notably higher than that of a bike with larger wheels, the model with smaller tires could offer greater speed.
The tires on the bicycle are also a crucial factor when it comes to speed.
If two bikes have the same gearing and wheel size, the one with tires better suited for the terrain will offer greater performance.
A smooth, narrow tire is faster on asphalt. Hence why road bikes use slick tires of about 23-28mm.
Slick tires offer lower rolling resistance on flat roads whereas knobby off-road tires underperform in such circumstances.
If you’re riding on pavement with MTB tires, you’re at a disadvantage due to the greater rolling resistance. For that reason, people who turn their MTBs into commuters often put slicks on.
Conversely, if you want to be fast on off-road terrain, slick narrow tires won’t cut it because they have poor grip on muddy surfaces.
A single speed MTB will be faster on off-road terrain than a geared bike with inappropriate tires.
However, a single speed MTB with wide knobby tires will be outperformed on pavement by any road bike.
Slick tires + high air pressure = good performance on the road
Wide knobby tires + low air pressure = good off-road performance
If the tires of a bike are not suited for the terrain, it can be outperformed by another model regardless of gearing and wheel size.
The geometry of a bicycle influences speed too. A road bike puts the rider in a more aero position and reduces the effort needed to move forward. Meanwhile, an MTB keeps the rider in a more upright posture resulting in more drag.
Thus, an MTB cyclist is at an aerodynamic disadvantage on the road. Off-road, however, the story changes because the upright posture allows the rider to move faster and safer around obstacles because there’s less weight on the front wheel, and the chance of getting over the handlebars is smaller. Hence why MTBs have a slack headtube angle.
5. Bike Weight
When speed is the goal, light bikes have an edge because it takes less energy to accelerate and maintain speed.
For example, a fully loaded touring bike with 50kg of cargo is expected to be slower than a single speed bike equipped with a water bottle and a saddle bag.
It may sound a little counter-intuitive, but the brakes on a bike influence its speed too.
If a bike is equipped with solid brakes working in all conditions, the rider will have more confidence when riding fast.
For that reason, some people say that fixed gear bikes with brakes are actually faster than fixed geared bikes without. The safety offered by the brakes encourages the rider to be more aggressive and thus take more risks and maintain a greater average speed.
The Advantages of Single-speed Bikes
The list below contains the advantages of single-speed bikes and other factors that could positively influence speed.
Since single-speed bikes do not have derailleurs and extra gears, they could be very light.
A light bike is a fast bike.
- High gearing
Single-speed bikes have a single “compromise” gear which is substantially higher than the lower end range found a geared bike. Subsequently, the rider is forced to always ride in a relatively high gear.
If the distance is fairly short, and the terrain isn’t hilly, the rider may reach a high average speed due to constantly pedaling in a high gear.
- Sprint Through The Hills
Climbing is the weakest side of single-speed bikes. One of the ways to minimize the negative effect is to sprint (increase speed) before a climb and enter the hill with greater inertia reducing the effort needed to pedal up.
Obviously, this strategy works only for fairly short hills and isn’t effective when climbing a mountain.
This technique is also the reason why some people are faster on a single-speed bike when riding on certain trails.
- No overthinking
The absence of gears removes the need to think about shifting. The freed mental energy and focus allow the rider to concentrate on other aspects of riding.
The Disadvantages of Single-speed Bikes
The list below contains the cons of single-speed bikes when it comes to speed:
The lack of gears often forces the rider to pedal in a gear that’s too high for the terrain. The result is pre-mature fatigue that could be minimized if the gear ratio was lower.
Consequently, single-speed bikes are always inferior when it comes to average speed maintained over a prolonged period.
On a long tour, every cyclist will be able to cover more ground on a geared bike. Not only that, but the destination will be reached quicker and with less effort.
- Sub-optimal Cadence
The cadence of a rider (the number of crank spins per minute) has a great impact on average speed.
A higher cadence (e.g., 90 RPM) produces a higher average speed while minimizing fatigue.
Single-speed bikes prevent you from consistently maintaining high cadence because there’s only one gear that’s often too low or too high for the terrain ahead.
- Climbing = Page 404
Any way you look at it, single-speed bikes are poor climbers. They can do just fine if you’re occasionally meeting a hill on an 8km commute, but outside of the city, their climbing performance drops to non-practical levels. Hence why very few people use fixies and single-speed bikes for touring.
If your goal is faster climbing, gears are your best friend.
- Not Suitable for Loaded Bikes
If you want to tour or transport heavy cargo on your bike, single-speed bikes will make it a lot harder to maintain decent average speed.
- Greater Joint Stress
Pedaling in a high gear can take a toll on one’s knees. For that reason, single-speed bikes are not a good choice for long rides in the presence of injuries.
The Advantages Of Geared Bikes
Geared bikes offer the following advantages when it comes to speed:
- Fatigue Management
Gears make it easier to conquer diverse terrain by reducing the effort needed to spin the rear wheel.
For example, a 24:36 low gear has a 0.7:1 ratio. This means that the rear wheel spins only 0.7 times per 1 crank revolution. Since the rear wheel doesn’t have to spin as many times, it becomes easier to pedal and conquer hills.
- Good climbers
If the bike is equipped with low gears, it will be able to climb some serious hills.
- Optimal cadence
The diversity of gears makes it possible to maintain optimal cadence which in return results in faster times on average.
- Reduced joints stress
Gears allow you to calibrate the effort required to keep moving forward. The reduced energy output spares the joints.
The Disadvantages of Geared Bikes
When it comes to speed, geared bikes have only minor disadvantages in comparison to single-speed bikes.
The cons are:
- Additional weight
Geared bikes are heavier because they use derailleurs as well as extra cogs and chainrings.
Nonetheless, this isn’t a major downside because the extra weight isn’t substantial enough to make a difference. Also, the benefits of the additional gears make up for the extra weight many times.
Situations When a Geared Bike Is Faster
1. Long distance – nothing beats a geared bike when the objective is to cover as much ground as possible.
2. Prolonged climbing – if the bike has to climb a long hill, gears will allow you to do that faster.
3. Higher gearing – if the top gear is higher than the one on a single-speed model, then the geared bike will have the potential to reach greater speed.
4. Better tires for the terrain – if the tires of the bike are optimized for the terrain, the bike will be faster than a poorly equipped rig.
Situations When a Single-speed Bike Is Faster
1. Higher gearing
2. Better tires for the terrain
3. Sprinting – A single-speed bike encourages you to sprint and do your best to preserve momentum. Thus, if the distance is short, one may actually be faster on a single-speed bike.
Having said that, the same strategy (sprinting), can be used with a geared bike too. All you have to do is set the bike in a higher gear and stop shifting.
A geared bike can be ridden as a single-speed, but a single-speed can never be a geared bike.
The Human Factor
At the end of the day, the most important speed factor isn’t the bike, but the engine a.k.a. the rider.
If a cyclist is overweight and in poor shape, they will be a lot slower than their maximum potential regardless of the bike under them.