Out of curiosity cyclists want to compare mountain bikes and road bikes and want to know all the differences and matches between them.

The interesting side of this comparison is knowing the speed deficit, road bikes are theoretically faster than mountain bikes, but by how much?

In order to answer this question at least theoretically.

We have to calculate the difference in all points related to speed between which are aerodynamic, weight, and tire rolling resistance.

In this article, we will talk about the difference in speed in detail and try to answer the question of how slow are mountain bikes compared to road bikes?

Quick Answer

Mountain bikes can be slower by 15% than road bikes on super flat terrain when on a 5% grade climb mountain can be slower by 12.7% the main factor is aerodynamic.

The Comparison

In order to know the speed difference between a mountain and a road bike, we must know the points of difference related to speed.

Depending on this article, It is clear that the speed resisting forces are aerodynamics, the total weight and tire rolling resistance.

The speed difference is in the sum of the resistance forces divided by power given to the power output of pedals.

Speed = (Resisting Forces) / Power

It can be said that the bike that generates fewer resistance forces while cycling is the fastest, and vice versa.

This comparison is based on calculating the resistance forces of each bike and thus knowing the speed difference between them.

Note: for the comparison to be more accurate, two bikes of the same level should be compared.

Weight Affect

The forces of resistance associated with weight are gravity and tire resistance rolling resistance. 

There is a direct relationship between weight and resistance force as the higher the weight, the greater the resistance.

To calculate the resistance values, we must know the average weight of both road and mountain bike adding Coefficient of rolling resistance (crr) of each bike tire.

As we mentioned in the introduction to the comparison to be accurate and applicable, the two bikes have to be from the same level.

The average weight of a high-end road bike is 7.6kg while the weight of a mountain bike is 12kg.

Tire rolling resistance coefficient for road bike tires is 0.002 when for mountain bike tires is 0.007. 

Assuming that the rider weight is 70kg, the resistance forces of both the mountain and the road are as follows:

Bike TypeGravity (5%)Tire Rolling ResistanceSum
Road Bike38 N1.52 N39.52 N
Mountain Bike40.16 N5.62 N45.78 N

Note: in the grade of 0% (super flat road) gravity resisting force is zero for that we used 5% grade to come up with measurable results.

Aerodynamics

Or drag in another term, is related with bike aerodynamic and cyclist normal position on the bike.

Bike and rider position in aerodynamics is called frontal area and it is measured by (), when the frontal area is larger the aerodynamic resistance force gets bigger.

Bike TypeFrontal Aera (m²)
Road Bike0.423
Mountain Bike0.565
Source

Also aerodynamic is related to speed too which makes it hard to be calculated as an isolated force.

For this we will rely on gribble.com calculator for that.

Results

Here are 70kg rider pushing 200 watts on both mountain and road bike:

Bike TypeSpeed (Kmh)Speed (Mph)
Road Bike37.4723.28
Mountain Bike31.7919.75

From the above results, mountain bikes can be slower by 15% than road bikes on super flat roads.

On 5% grade:

Bike TypeSpeed (Kmh)Speed (Mph)
Road Bike16.7310.39
Mountain Bike14.599.065

From the above results, mountain bikes can be slower by 12.7% than road bikes on 5% grade climb.

Conclusion

How much slower are mountain bikes? This question comes just out of curiosity, simply the comparison is far from beginning real since both bikes are too completely different.

From the start, it’s obvious that road bikes are faster than mountain bikes on asphalt roads.
Those speed comparisons should be done on bikes from the same discipline like road and endurance bikes at least to make more sense for the comparison.

Categories: General