Science

The Power Process:

To begin to understand the process of forces that produces power, we must consider the laws of physics and how these apply to the bicycle rider. While cadence multiplied by strength equals power, cadence and strength themselves have limits dictated by the riders cardiovascular capacity and efficiency of movement, or coordination.

Power = Work x Time

In basic terms this means a cyclist moves forward by pushing on the pedals for a given time period. (Technically if a rider pushes on the pedals but the bike is prevented from moving forward, and therefore the crankarms do not move, then the rider has produced a force but not work).

Breaking this equation down, if we look at what makes up Work we get:

Work = Force x Distance

This means as a rider applies Force (from muscle strength) times Distance (rotation of the crankarms) we get a measure of Work. More Force requires more strength. To produce Power then, a rider applies Force (the leg strength) over a Distance (the crank revolution) to create Work for a period of Time.

Force = Inertial Mass x Acceleration

(Inertial Mass is the pushing leg and Acceleration is the legs tendency to begin to rotate the crankarm)

Torque = Rotation x Force

Torque is the tendency of a Force to rotate an object about an axis. What this means is that Torque = Force (muscle strength) x Length of lever arm (crankarm in the case of a bicycle).

Power Output = Torque

(Force applied to a crankarm by strength of riders leg) x Rotational Speed (frequency of the applied Torque, high RPM/lower Torque =low RPM/higher Torque) x Time

Power = ((Mass x Acceleration) x Distance) x Time

An endurance rider must be able to maintain a low percentage of their max force to utilise slow twitch muscle fibers to minimise fatigue during the event. If a rider does not have sufficient aerobic capacity or cannot develop sufficient aerobic capacity to main a high cadence (therefore low percentage of max force) then the rider must gain more muscular strength to be able to perform at the required power output. A rider should create a fitness profile of different aspects and limits of their physiological capacity to be able to improve their performance in desired areas as effectively and efficiently as possible. Just performing a standard V02 Max test does not provide in depth enough and adequate enough information to write a training program to result in maximum power output gains.

Rotational Inertia:

Inertia is the resistance of any object to a change in its state of motion or rest. This is important because the rotational inertia of a stationary trainer resistance unit has a significant influence on a riders power application and pedaling coordination.

High inertia trainers use heavy rotating units and generally replicate riding on a flat road at low to mid power outputs. They can struggle however, to provide enough resistance at high power outputs. A high inertia trainer only requires force to be applied on the down stroke of the pedal cycle, limiting muscular training benefit. Once the heavy fan or flyweight builds up momentum, the rider has then “topped-out” the trainer, where an increase in pedal speed in the hardest gear does not have any significant effect on resistance.

Virtually any tire-roller type trainer is a low inertia/high friction device. Manufacturers attempt to overcome the friction and add resistance with flyweights. At low pedal speeds the high friction causes a stalling effect through the pedal stroke “dead-center” which forces the rider to employ a compensatory pedaling technique.

The Revbox Erg with huge diameter fan relies solely on air as resistance and will increase exponentially, making it impossible to “top-out”. The fan is very lightweight, minimising rotational inertia for maximum muscle activation while pedaling. The chain drive connection has the lowest possible friction and provides the exact conditions a rider needs to perform high intensity training at any pedal speed and any power output.

Friction Effects:  

There is a significant difference in the mechanical friction of the Revbox Erg compared to the typical tire-roller type trainers. While friction adds to resistance, it is actually an undesirable component as it interferes with a rider’s pedaling dynamics at low cadences.

The major flaw with existing stationary trainer designs is that the tire-roller friction is 2.5 – 3 times more than that of on road rolling resistance. This creates a negative affect for any low cadence strength training. This anomaly is called “stalling” and it refers to when the cadence is sufficiently low enough and the friction sufficiently high enough to cause the trainer unit to attempt to stop rotating when the rider has little leverage on the crankarms (the crank “dead-center”). Stalling is a characteristic unique to magnetic or electro-magnetic trainers as when resistance is increased the unit rotating speed decreases. This extremely low inertia combined with the high friction of the tire-roller connection, will attempt to halt or stall the riders pedal momentum at the point of least leverage (the 12/6 o’clock positions being the “dead-center”).

The Revbox Erg, with mechanical friction less than half of on road rolling resistance, and only 17% of the friction of tire-roller trainers, means that the fan unit does not attempt to stall, regardless of pedal speed. Pedal technique and power transfer are optimised as the rider is able to rotate the cranks with a natural unobstructed motion.

Resistance Limits:

Different trainer designs have varying resistance limits, some of which make many stationary trainers unsuitable for high intensity training. High inertia trainers for example use very heavy rotating weight to help provide resistance. Once up to a speed that has built momentum in the heavy resistance unit, it will experience a tailing off in resistance rate. This phenomenon of “topping out” is when a stationary trainer unit nears its mechanical limit and cannot provide resistance high enough to match the power output of the rider.

Most tire-roller trainers will exhibit both stalling and topping out. The Revbox Erg with its large air braked fan, being 580mm in diameter but only 1.1kg in weight, does not experience either effect and will continue to provide increasing resistance no matter how hard the rider pedals. To match the resistance range of the Revbox Erg, a high inertia stationary trainer would have to weigh over 30kgs (66 pounds).

Magnetic based stationary trainers create resistance by an eddy current of the magnetic field. It is possible for a powerful rider to break free of this current at very high intensities, instantly resulting in zero resistance. Magnetic or electro-magnetic units therefore, have wattage limits, and to train at certain power outputs require undesirably high pedal speeds. While an electro-magnetic trainer has the ability to act as a low or high inertia trainer, it will still always have a limit, combined with the associated issues of the tire-roller interface.

Rehabilitative Exercise:

The Revbox Erg lends itself perfectly for use as a rehabilitative tool for sports centers who may prescribe to clients recovering from injury, low intensity, low resistance exercise. While this may seem counter intuitive to the high performance use of the Revbox Erg, as although the resistance ramps up to extremely high levels, the low end resistance is actually much less than standard tire-roller trainers.

Regular stationary trainers with a high friction tire-roller interface require a high initial force to turn the pedals of the bike. In fact, the resistance of this friction interface is approximately four times that of rolling resistance on the road. The Revbox Erg in contrast requires just 17% of the amount of force of a regular trainer for the rider to start rotating the crank arms. For rehabilitative purposes a person recovering from injury is able to ride on a bicycle setup with the Revbox Erg with very low resistance, even less than what is possible to keep a bike in forward motion on the road.

Suitable for not just bike riders recovering from injury, but any person who requires low resistance, low impact, cyclic motion exercise to gradually build back range of movement, coordination and strength. Combining the Revbox Erg with an adjustable fitting bike and a crank set based power meter, a coach or sports science professional will be equipped with an extremely versatile tool that can be utilised for rehabilitation through to high performance fitness testing.

Fitness Testing:

For a rider to begin to train effectively an Athlete Fitness Profile should be developed and tracked over time. A standard VO2 Max test evaluates one aspect of a cyclist’s capacity but for accuracy relevant to the riders cycling discipline other tests should be considered that pertain to certain performance criteria or the goal event.

With its huge resistance range the Revbox Erg can be used as a tool to create an AFP (Athlete Fitness Profile) of a bike rider, which then can be performed at other stages throughout the training plan or season. This is potentially a great additional service to offer for coaches, fitness centers and bicycle stores wanting to raise their level of professionalism within the industry.

An AFP can be different for various disciplines or it may even be specific for certain riders, but it should cover as broad as possible the power, cadence, strength, and coordination aspects of the event or goal requirements. Due to rider fatigue it may be necessary to perform parts of the AFP Test over several hours or days. Training program durations and intensities should be based on specific prescribed values of Power, Heart Rate, Cadence and Incline that are relevant to the rider’s goal performance or event.

Example of General Athlete Fitness Profile Test
Maximum Anaerobic Cadence Test 1,2,3,4,5
Tests for difference in pedaling coordination at various gradients
Duration: 10 seconds
Conditions: Low gear, 1 Level ground, 2 Gradient 5%, 3 Gradient 10%, 4 Gradient 15%, 5 Gradient 20%
Recorded: 10 second average power
Maximum Aerobic Cadence Test:  1,2,3,4,5

Tests for difference between maximum and sustainable cadence
Duration: 3 minutes
Conditions: Low gear, 1 Level ground, 2 Gradient 5%, 3 Gradient 10%, 4 Gradient 15%, 5 Gradient 20%
Recorded: Average Cadence, Average Power, Average HR, Max HR
Maximum Anaerobic Power Test:  1,2,3,4,5

Tests for difference in maximum anaerobic power at various gradients
Duration: 10 seconds
Conditions: Any gear, 1 Level ground, 2 Gradient 5%, 3 Gradient 10%, 4 Gradient 15%, 5 Gradient 20%
Recorded: 10 second Average Cadence, 10 second Average Power
Maximum Aerobic Power Test: 1,2,3,4,5

Tests for difference in maximum aerobic power at various gradients as percentage of M.An.P
Duration: 3 minutes
Conditions: Any gear, 1 Level ground, 2 Gradient 5%, 3 Gradient 10%, 4 Gradient 15%, 5 Gradient 20%
Recorded: Average power, Average Cadence, Average HR, Max HR
Functional Threshold Power Test: 1,2

Tests for difference in FTP and MAP at best and worst gradient results from MAP test
Duration: 20 minutes
Conditions: Any gear, Two Gradients
Recorded: Average Power, Average Cadence, Average HR, Max HR
Functional Threshold Power Test: On-Road

Tests for difference in FTP indoors and FTP outdoors on road
Duration: 20 minutes
Conditions: Any gear
Recorded: Average Power, Average Cadence, Average HR, Max HR

The results compared between the two environments need to be analysed as to the differences (rider skill on road, inability to maintain ideal gear ratios, etc) and this worked on in the riders training program

 

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