Episode #117 – Pedal Forces, Vibration and Performance

Episode #117 – Pedal Forces, Vibration and Performance

This episode looks at optimising the mechanical forces on the pedals in different situations. Understanding these forces better will help you make adjustments on the fly, and gain more power.

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“Riders can’t piss for hours after they finish Paris-Roubaix.”CLICK TO TWEET


This week’s episode comes from two presentations at the World Cycling Congress. The first one, Muscle activity and pedal forces: do they matter in cycling performance? is by Dr Fred Grappe, and the second one, Effects of Vibration on Cycling Biomechanics is by Dr William Bertucci.

So you can get a bit of an idea of where I’m going with this show. The main area I want to talk about is how different environmental factors affect performance. Focusing on vibrations – those from cobbles, gravel or riding off-road. It was a fascinating discovery to me when I found how detailed the research in this area is.

But let’s start with you and your bike. As Dr Grappe says:

“Cyclists maintain a special relationship with the bike – the cyclist is both the master and the motor.”

The freedom that a bike gives its rider is one of the magical elements of cycling. Having control over a machine that can propel you in any direction and one which you master with strategy is only part of the equation. You would get nowhere without your motor. It produces the necessary power output to follow the strategy and direction you set.

Ok so where am I going with this – well in order for you to get the most out of yourself – and your bike you must be part of the bike. You must act as one. Especially when it comes to producing power. The pedalling movement is important.

If you break down a full revolution you will see how the heal is dynamic and follows a natural gesture. Also in this circular movement – the level of force must adapt to the pedalling cycle. The force on the pedals and changes in muscle activity is due to the regulation of power output. Intensity of the effort and cadence are important here, but there are other important parameters.

The position on the bike is important – when you sit on the bike is important to have a comfortable position. When you ride in a more aerodynamic position – the goal is not comfort but reduce drag and increase speed. Both positions apply different force on the pedal.

There are more examples though. Standing on a climb means there is a lot of rotational movement and this changes the forces on the pedal. Also, on the cobbles the vibrations play a role is changing the force on the pedals.

In every cycling environment there are opposing forces that change the force on the pedals compared to the input.

These include:

  • Wind
  • Rolling resistance
  • Weight of bike and rider
  • Mechanical Friction

In a performance sense we are looking at trying to optimise mechanical forces on the pedals. Specifically in relation to the Power Output. Which relies on minimising the opposing forces just discussed. But is also related to effective force (turning of the crank) and angular velocity.

The problem with pedalling is that it is impossible to have an effective force throughout the entire cycle. This puts the force and pedal analysis metrics from Garmin, Pioneer and others into context. Plotting the level of force according to the level of angle during the rotation becomes the best way to measure these forces.

Optimising your technique is knowing how your pedalling technique changes according to your situation. Climbing, time trialling etc.

To take this further Dr Grappe breaks down the different positions and how to optimise for Power Output:

Starting with climbing. When standing on a climb – you will experience movement in your shoulders and pelvis. Which changes the potential output.

As an example: If you produce 300W on a 7% slope the total resistance is ~60 Newtons – for an effective force of 200 Newtons. Dr Grappe believes that the most important system to overcome here is the weight of the rider on the bike. Because for each 1 kg on a 7% slope it takes ~4-5W of output. I agree with Dr Grappe.

Climbing is a great place to use Garmin’s new Seated/Standing Position metric.

It detects and flags riding position as seated or standing during a ride by comparing forces applied to the pedals. It also provides summaries of how often and how long riders have been in the position. This will be great for making adjustments to your fore and aft positions. As well as fine tuning the amount you pull on the bars while standing up on climbs.

Another area with strong opposing forces is on the flat. It’s not hills this time but the wind is a problem here – to overcome the resistance the opposing forces are 26 N for an output of 200 N.

This is in a standard position though. Where in a time trial position the total resistance reduces by 4 N to 22 N for an output of 200 N.

The most important resistance to overcome on the flat is aerodynamic drag.

When you ride on the cobbles you have vibrations between you and the cobbles including the saddle and legs and arms. The opposing force is 35 N for again, a 200 N output. Which is higher than riding in the wind on the flat. Here it’s not aero drag but rolling resistance which is the most important factor to overcome.

Dr William Bertucci has taken the effects of vibrations on cycling biomechanics to another level. We are going to look at the main effects of vibration, which are for health, comfort and cycling performance.

Back to riding on cobblestones – cobblestones are the extreme of cycling vibrations. Let’s expand it to cycling on rough roads in MTB or gravel grinding, or off-road riding on rough ground.

Every time you ride on these surfaces, compression exposure is the largest threat to your health. Especially on the upper limbs. Compression on the median and ulnar nerves in your hands. This syndrome affects nerves. blood vessels, joints of the hand, wrist and arms. It’s more than a bad bike that can damage these areas.

Also, the fingers can experience pain, decrease grip strength, and lose of finger dexterity. This is super common one which I’m sure you’ve experienced. With a special pair of gloves, like how you measure saddle pressure. They were able to measure where the pressure is localised. Which it’s half surprising to me, but the fingers cop most of the pressure.

It makes sense though. If you are trying to let the bike move underneath you while on rough ground then your fingers will have to do a lot of work to hold on. Poor little pinkies.

It doesn’t stop there though. Other than the hands and wrists, there are also possible chronic effects on the neck, lower back and damage to the spine.

This isn’t just all woo woo stuff though – science has been able to step in, and here’s the fascinating part. The researchers have an equation for recommended daily exposure to vibrations. This scale moves from below the caution zone, to the caution zone and finally the risk zone. They were then able to take this equation and work out the exposure risk for a specific amount of vibration on the pedals and saddle.

Once they had these measures they worked out a way to measure the vibrations on the bike and body while riding cobble stones. The results showed that the exposure dose is high. According to their results the maximum acceptable exposure to cobbles any rider should experience on any one day is only 7 minutes at 35 km/h. Where a race like Paris-Roubaix the time on cobbles can be up to 1 hour and 30 minutes.

No wonder riders can’t piss for hours after they finish Paris-Roubaix.

That’s health smashed but how about performance? Vibrations increase oxygen uptake. Two studies have confirmed this – the second being the more interesting of the two. They were able to link mechanical waves transmitted from the the pedals to the lower body. This then activates muscle receptors and a greater number of motor units. Leading to greater activation and oxygen uptake.

This research also suggests that vibration exposure can also decrease gross efficiency.

There is also a study on minimisation of the vibration dose and optimisation of comfort. Specifically handlebar position on compression. Three positions were tested, drops, hoods, and tops. Not surprisingly the drop position produced the highest compression exposure compared to the 2 other positions. The top position enables the highest output – explains a lot when you watch the Paris-Roubaix and riders are on the tops.

Finally some actionable advice. There has been work done on different bike parts and their influence on vibration absorption. Where they were trying to figure out how each component contributes to vibration transmission. They found when it comes to your hands, the components that have the highest influence are forks which transmit 24% of the vibrations, and the wheels which transmit 36% of the vibrations. The vibrations experienced on your saddle are the highest from your wheels, at 42% and your frame at a whopping 70%.

So this gives support to all the measures you see at Paris-Roubaix. Which you can use to reduce the vibrations in your next ruff ride…

  • Old school wheels with a wee bit of flex, but strong as an ox fresh out the box.
  • Huge tubulars up to 30 millimetres
  • Lower pressure – down to 78 pounds per square inch
  • Frames by a certain French company with Shock Absorption Technology
  • Forks – I think about Rockshox with Paris-Roubaix SL
  • Or how bout vibration absorbing seatposts
  • Extra layers of bar tape and gel pads under bar tape.

Also, it seems to have started a couple of new categories of bikes that some bike companies introduced over the past couple of years. Including endurance bikes and gravel grinders.

So next time you’re out riding on some rough road, think about the forces you are experiencing and how you can minimise them. Whether it be an aero tuck or your standing position there’s always room for improvement.


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