You may be wondering whether a heavier bike will travel downhill faster than a lighter bike.
In this article we break down each of the forces that affect how quickly a bike can travel downhill and answer the question: are heavier bikes faster downhill?
Are Heavier Bikes Faster Downhill?
A heavier bike will travel downhill faster than a lighter bike when all other factors including drag, rolling resistance, and weight of the cyclist are equal. This is due to the fact that the force of gravity has a greater effect on heavier objects.
Are Heavier Bikes Faster Downhill?
In order to fully answer this question, we need to consider all of the forces acting on a bike traveling downhill.
When a bike is traveling downhill, gravity is the force acting to pull the bike down the hill, whilst drag and rolling resistance work against gravity to slow the bike and cyclist down. Or in other words:
Total Force Acting on Bike Downhill = Force of Gravity – Drag – Rolling Resistance
The force of gravity is affected by the total weight of the bike and the rider, as well as the gradient of the hill. The heavier the bike, the greater the gravitational force. The steeper the hill, the greater the gravitational force.
The main factors affecting the drag are the speed of travel and the frontal surface area of the bike and the rider.
To visualize this, think about putting your hand out of a car window with your palm facing forwards. The faster the car travels, the more force you feel from the air hitting your hand.
Now picture turning your hand 90° so your palm is facing you. When traveling at the same speed you feel less force from the air hitting your hand than when your palm is facing forwards, as you’ve reduced the surface area of your hand. (This essentially means less air is hitting your hand as you’ve made it a smaller target).
Other factors such as the clothing worn by the cyclist will have an effect on drag. Loose-fitting clothing results in more drag than tight-fitting clothing for example. However, for this analysis we can ignore this effect by assuming it is only the weight of the bicycle that differs, and that the cyclist is dressed identically.
Rolling resistance is the energy that is lost when the bike wheels are rolling. The weight of the wheels, tire width, tire diameter, tire tread, and inflation pressure all contribute to rolling resistance.
(There are also frictional forces that oppose the forward motion of a bike, however for a well-maintained bike the friction in the drivetrain is nominal compared to drag and rolling resistance forces.)
So now we’ve identified all of the forces acting on a bike going downhill, we can determine whether or not heavier bikes go faster downhill.
If all other variables are the same, including the surface area of the bike and the rider, the geometry of the bike, and the rolling resistance, the heavier bicycle will accelerate faster downhill and will have a higher top speed.
Why Don’t Pro Cyclists Ride Heavier Bikes?
Now it’s clear that a heavier bike means higher downhill acceleration and top speed if all other factors are equal, it’s worth asking why pro cyclists don’t take advantage of this.
There are a number of reasons. First of all, it’s the total mass of the bike and the cyclist that determines how much force they generate due to gravity. Differences in the weight of the cyclists themselves are far more significant than the weight of their bikes.
This means that increasing the weight of the bike results in a relatively small benefit to downhill speed.
This benefit would be more than outweighed by the negative effects of the bike being harder to pedal on the flat or going uphill, as a heavier bike requires more pedaling power to move.
Some cycling disciplines that have no real flat or uphill sections, such as downhill mountain biking, will benefit from the effects of a heavier bike.
However, as with anything, there is a sweet spot. Downhill mountain bikes are the heaviest mountain bikes and so will travel downhill faster, but they are heavyweight primarily due to having robust frames and components to deal with the demands of the sport, as opposed to being intentionally heavy for any speed benefit.
We wrote an article discussing why downhill mountain bikes are so heavy here, give it a read if you’d like to learn more.
Another reason pros don’t use heavier bikes is that for most cycling disciplines there are much more significant factors than bike weight that can be optimized for downhill speed.
The best example is drag. Minimizing drag has a far more noticeable effect on increasing downhill speed and will also increase top speed on flat sections. That’s why we see lycra, aerodynamically shaped helmets, and of course, shaved legs in pro cycling.
