from Rookie’s keyboard
Hello, friends
Many rookies wrongfully conclude that smaller wheels always equal a slow bicycle. And while there’s some logic behind this statement, every bike theorist has to know that ultimately bicycle speed is a function of GEARING rather than wheels size.
What does this mean?
It’s simple.
If largers wheels equaled more speed, then tractors should be faster than cars, right? And F1 models should have the largest wheels on the planet? As big a T-rex?
The top speed of a vehicle is determined by its gearing (a.k.a. the mechanism through which the rider/engine transmits power to the wheels) and the power output of the engine/rider.
The drivetrain consists of chainrings (fron) and small cog(s) at the back.
When you spin the cranks, the chain transfers power to the rear cog(s) and respectively rear wheel.
A larger chainring and/or a smaller rear cog, equal more power transfer to the rear.
The term gear ratio refers precisely to the relation between the chainring and the cog.
For instance, if the chainring has 44 teeth and the rear cog has 11, the gear ratio is 4:1.
In that case, each full revolution of the front chainring, results in 4 spins of the rear wheel.
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 per minute and thus a greater traveled distance.
The formula for speed is Speed = Distance/Time.
A bike that covers more distance in the same amount of time than another bike is moving faster regardless of wheel size.
Example:
| Wheel Size | Gear Ratio | Wheel Circumference | Tire Width | Time | RPM | |
|---|---|---|---|---|---|---|
| Bike A | 26″ | 44:11 | 2075mm/207.50 cm | 2.00 inch | 60 seconds | 80 |
| Bike B | 27.5″ | 34:11 | 2153mm/215.30 cm | 2.00 inch | 60 seconds | 80 |
The table above contains data for two hypothetical set-ups. Both bikes have identical tire width but different wheel sizes.
Bike A:
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/60 seconds.
The traveled distance can be calculated by multiplying the wheel’s circumference by the number of wheel turns.
In this case, the distance is 320 x 207.50 cm = 66400cm = 664.00 m = 0.66km
Bike B:
In 34/11, the rear wheel spins 3.09 times per 1 crank revolution.
At 80 rpm, the rear wheel makes 3.09 x 80 = 247.2 revolutions.
Thus, the traveled distance is 247.2 x 215.3cm (wheel circumference) = 53222cm = 532.22m = 0.53km.
The formula for speed is: Speed = Distance/Time.
In the example case above, we have the following speeds:
Bike A’s speed = 664m / 60s = 11.0667m/s = 24.75 mi/h = 39.84 km/h
Bike B’s speed = 532.22m / 60s = 8.87 m/s = 19.84 mi/h = 31.93 km/h
Conclusions:
Bike A has the potential to be 24.7% faster than Bike B despite having smaller 26″ wheels. The potential for extra speed comes from the higher gearing.
But, if both bikes have the same gearing, then Bike B will be faster than Bike A.
Acceleration
Don’t forget that smaller wheels are easier to spin from a dead stop and therefore accelerate faster.
A bike with 26″ wheels is easier to get up to speed than a bike with 29″ wheels, for example.
Speed Maintenance
Smaller wheels may be easier to accelerate, but they have smaller inertia and require more effort to keep spinning.
Meanwhile, larger wheels are more difficult to get up to speed, but once there, they maintain the speed with less effort. Hence larger wheels are considered better for covering long distances.
Rolling Resistance
Another parameter that directly impacts speed is rolling resistance. The lower the rolling resistance of a tire is, the less effort is required to maintain a higher speed.
Higher rolling resistance, on the other hand, is detrimental.
If a 26″ wheel is equipped with slick tires, it will have a lower rolling resistance on paved roads but reduced grip on off-road terrain.
Кnobby tires offer more grip when riding off-road by digging into the ground. On paved roads, however, they are are slower and noisier.
A Perceived Feeling of “Slowness”
Many people who go from 26″ wheels to 700c/29″, for example, immediately feel faster.
Why? First, 26″ bikes are either retro MTBs, large BMXs, or old commuters.
Meanwhile, 700c wheels are found on road bikes, touring bikes, commuters, and modern MTBs (700c = 29″).
Road bikes and even touring bikes tend to have much higher gearing than retro MTBs for example.
A road bike offers a much higher speed potential not only thanks to its larger wheels but thanks to its high gearing.
When switching to larger tires, you will have to produce less power to keep them rolling on flat roads. The reduced energy requirement will make you feel faster and lighter even when you’re not technically moving at a higher speed than before.
Road Bikes Always Win
A 26-er with a high-gearing and slick tires can be plenty fast. But when speed is the main goal – road bikes are always the winner.
The aggressive geometry in conjunction with high gears, light weight and large tires with low rolling resistance facilitate the maintenance of decent speed levels.
Until next time,
Rookie
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