The Benefits Of Hard-tail Mountain Bikes

It’s important to note that I’m not suggesting that hardtails are better than dual suspension bikes, or that everyone should ride hardtails. The factors that influence the suitability of a bike for a given rider are complex, and everyone’s individual requirements will be different. That said, I’m convinced that a hardtail offers performance advantages over full suspension bikes for the large majority of off-road riders and almost all cross country and marathon riders.

Full suspension cross country:

  • Pros: Nice and plush: ideal for the beginner rider.
  • Cons: Heavier than hardtail.

Hardtails:

  • Pros: Comfortable and fast, much lighter than full suspension bikes.
  • Cons: Less plush than full suspension bikes.

Because hardtails are lighter than full suspension bikes, but hardtails and full suspension bikes have identical geometry and bottom bracket heights, hardtails must be more efficient than full suspension bikes. So how much efficiency do we gain?

The efficiency of two-wheel drive, and in particular the drivetrain and the rider , compared to four-wheel drive, and in particular the tyre and the ground , is extremely important in off-road riding.

How do we quantify this efficiency?

In order to compare hardtails and full suspension bikes from a performance perspective, we need to consider the specific benefits that the suspension adds to a bike. The benefits of bike suspension can be broadly classified into 3 categories:

The suspension system does not provide any direct benefit to the bike, rather it assists the rider in riding faster and/or further by absorbing some of the bumps and vibrations from the ground. Any time the suspended bike is going faster or further than a comparable hardtail bike, a significant part of the performance enhancement is due to the impacts and vibrations being absorbed by the bike.

The suspension system absorbs some of the bumps from the ground, which then reduces the vibrations that the rider feels from his or her contact with the bike, and therefore to a lesser extent the vibrations that the rider transmits to the handlebars. The suspension system largely prevents the ground from forcing the tyres into the ground, which means that the ground cannot provide the driving force to the bike. This forces the rider to expend more energy to overcome the rolling resistance.

The relationship between a ride’s speed and the centrifugal force acting on a body moving in a circle is a fundamental physical property. It is a fundamental physical property of a bicycle that it is a two-wheel drive system, and it is a property of two-wheel drive systems that they accelerate more slowly than four-wheel drive systems. The centrifugal force acting on the bicycle is the force that causes the bicycle to accelerate more slowly than the rider. The forces on the rider are transmitted to the handlebars.

The two most significant forces on the bicycle are the centrifugal force and the weight of the cyclist. The centrifugal force can be largely eliminated by removing the rear wheel, and the weight is almost entirely eliminated by removing the front wheel. Therefore, removing both wheels reduces the forces acting on the bicycle by more than 110% with respect to two-wheel drive, with respect to four-wheel drive the bike is 97% efficient.

Any time the bike is accelerating through a turn with respect to the direction of travel, i.e. when accelerating off the bottom of the pedal stroke in turns, when accelerating out of turns, when accelerating with respect to the direction of travel in technical terrain, when accelerating in sprinting with respect to the direction of travel, when accelerating through the drive train, then it is accelerating more slowly than if it were in four-wheel drive, in other words it is less efficient than if it were in four-wheel drive.

If the rider is accelerating through the turn, then the rider is also pushing his or her weight against the centrifugal force, which is pushing against the rider. Therefore, more of the rider’s weight is pushing more of the rider’s weight against the centrifugal force. This extra force acts across a shorter lever, that is, the handlebars are closer to the rider’s centre of gravity, so the force is amplified.

The difference between two-wheel drive and four-wheel drive is the ratio of the centrifugal force to the rider’s weight, i.e. g/W. This dimensionless ratio is maximized when the centrifugal force is half of the rider’s weight, or g = 0.5W, so the ratio is maximized when the rider is maximally supported by the bike, i.e. when the body is aligned with the weight.

Thus, the rider will be accelerating more slowly when he or she is maximally supported by the bike, which is when his or her centre of gravity is aligned with the bike’s centre of gravity. This is the same action that caused the centrifugal force to increase above, i.e. the centrifugal force increases when the rider’s centre of gravity is not aligned with the bike’s centre of gravity.

The centre of gravity of a rider and a bike is typically located at the intersection of the bottom bracket and the top tube. When the bottom bracket is not aligned with the top tube, then part of the rider’s weight is not supported by the bike and therefore the acceleration is slower.

Therefore, when the bottom bracket is aligned with the top tube, the centrifugal force acting on the cyclist is the same as the centrifugal force acting on a cyclist who is not supported by a bike, therefore the cyclist is maximally supported by the bike, and the cyclist will be accelerating more slowly than if the entire weight of the bike and rider were pushing against the centrifugal force, i.e. the centrifugal force will be less than half the weight of the rider. The cyclist’s weight will be affected by the equipment and by the terrain, and therefore the centrifugal force and the weight of the cyclist will not be exactly equal to each other, but the suspension forks will still be reducing the centrifugal force to a greater extent than if the bottom bracket were aligned with the top tube. The suspension forks will be aligned with the bottom bracket and the top tube, and therefore the suspension forks will be reducing the centrifugal force to a greater extent. The benefit to the performance of a bike from a suspension fork is irrelevant to a hardtail, because the hardtail will have the same geometry as a bike with a suspension fork, and therefore a suspension fork will give no performance advantage a a hardtail.

By far the most significant contributor to the centrifugal force acting on a bike is the rider’s weight, so by far the most significant portion of the centrifugal force that a suspension fork can eliminate is the centrifugal force due to the rider’s weight. When a suspension fork is aligned with the bottom bracket and the top tube, i.e. the bike forks are aligned, then virtually all of the centrifugal force due to the weight of a rider will be eliminated. Therefore, a suspension fork aligned with the bottom bracket and the top tube will be minimally effective, and therefore a suspension fork significantly increases a bike’s efficiency for a given rider, who is maximally supported by the bike, to 74% from 97% for a comparable hardtail.

In summary, a suspension fork is aligned with the bottom bracket and the top tube, which increases the centrifugal force acting on the cyclist’s weight by 24%, but a suspension fork can eliminate as much as 74% of this increased centrifugal force, so the suspension fork is increasing the bike’s efficiency for this given rider by 12.5 percentage points.

It follows that for this given rider, if the suspension fork is aligned with the bottom bracket and the top tube, then the suspension fork is increasing the bike’s efficiency for this given rider by 12.5 percentage points. It is possible that the suspension fork is aligned with the bottom bracket and the top tube for most of the rider’s weight, i.e. for most of the time that the rider is maximally supported by the bike, but this is not necessarily the case. Indeed, it is likely that the suspension fork is increasing the bike’s efficiency for this given rider by less than 12.5 percentage points, because the bike will often be decelerating during turns, and the acceleration of the bike will be accelerating more slowly than if it were maximally supported by the bike.

It follows that for this given rider, if the suspension fork is aligned with the bottom bracket and the top tube, then the suspension fork is reducing the bike’s efficiency for this given rider by less than 24% during acceleration. Certainly, it is reducing the bike’s efficiency by significantly less than 24% during acceleration, because the bike is decelerating through turns, so the suspension fork is reducing the bike’s efficiency below the value obtained by multiplying 97% by 0.74 above.

It follows that for this given rider, if the suspension fork is aligned with the bottom bracket and the top tube, then the hardtail is more efficient than the full suspension bike. However, since the suspension fork is only aligned with the bottom bracket and the top tube for part of the time that the rider is maximally supported by the bike, then the efficiency of the suspension fork is being exaggerated.

This consideration also applies to the efficiency of the rear suspension. Certainly, the rear suspension of a full suspension bike is aligned with the bottom bracket and the top tube more of the time than the front suspension.

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