May the Force be With You

My approach to bike fitting is based around three main principles. Firstly, I want to identify all the variables that could impact the issue my client is experiencing. Secondly, I want to understand the mechanisms behind any intervention that I might want to make. Combining a good knowledge of the variables with a thorough understanding of the mechanisms behind each one is a great starting point for identifying which intervention is most likely to help your client.

But there’s a third part to my approach which is more broad reaching and applies even when clients aren’t experiencing discomfort or overuse issues. That third part is forces – how do they act on the body, and how are they produced by the body. Understanding these forces is the most important element of bike fitting. If you only understand forces and don’t bother with the first two pieces, you’ll still help a lot of people; maybe not all of your clients, but still a decent number.

And that’s because the forces explain so much of what happens to a rider on a bike.

It’s difficult to dig down into forces in a blog article because I think it’s much easier to explain visually. I might do a little video at some point to help but in the meantime, I’ll try to keep it simple.

Every force has an equal and opposite. If you want to get fancy then this is Newton’s third law of motion, but you don’t need to know that to understand it. Basically, if you’re sat on a bike and your upper body is relatively stationary (i.e. you’re not falling forwards) then the forces acting on you are balanced. As you lean forwards to reach the bars, gravity pulls you down towards the ground. Without opposing forces overcoming gravity, you would fall forwards. You can overcome those forces in a couple of ways – supporting your weight on the bars, engaging your core, or using the help of pedal reaction forces. And it’s balancing these three that will result in the most comfortable position for your client.

Pedal reaction forces are the opposite force that pushes back when you push down on the pedal. You can feel them every time you pedal, but probably more so when you’re out of the saddle climbing a steep hill. Those pedal reaction forces are much bigger, so they take a lot of effort to overcome them and still propel the bike forwards.

But they aren’t just a bad thing that needs to be overcome. They also help keep your upper body in a sustainable position and help with saddle stability, too. That’s because as you push down and away from yourself, the reaction forces push back and normally a little rearward. This keeps you in your seat and helps reduce the amount of core activation or arm support required to sustain your position.

If you’re not convinced, here’s a little exercise to show what I mean. Stand against a wall with your bum and heels in contact with the wall. Keep your upper body straight and extended, fold your arms across your chest and lean forwards by hinging at the hips. You’ll quickly lose contact between your bum and the wall and begin to topple forwards. Now repeat the same thing but this time move your feet 5cm forwards. Keep you bum in contact with the wall just like before, and hinge forwards from the hips. What you should see is that you can hinge forwards further and sustain it for longer with your feet slightly in front of the wall. This is because the ground reaction forces are vertical when your heels are in contact with the wall, so there’s nothing pushing your bum back into the wall. But when you move your feet forwards, and keep your bum on the wall, the ground reaction forces are rearward and help give you a better result in the test.

This is what we’re doing during a bike fit, and it’s one of the main reasons that aggressive forward seat positions aren’t sustainable for the average rider. A pro produces around 400W, which creates a lot of force at the pedal and therefore experiences a lot bigger pedal reaction force than the rest of us. Combined with a very low body fat percentage and not much upper body mass, those large pedal reaction forces don’t have much of a toppling effect to overcome to keep the pro’s upper body in position. So, the pro can go further forward and lower, and still have it be relatively comfortable and sustainable over long rides.

By comparison, a recreational rider that’s carrying a few extra kilos (no judgement, I’m not at my race weight either!) and only putting out 150W creates a much smaller pedal reaction force and has more upper body mass to resist.

In all cases, bike fitting is about balancing the forces acting on the rider with the forces created by the rider. And that’s the main reason I had to flip my stem when I put on weight and stopped training as much – gravity was having a bigger impact and my reaction forces couldn’t overcome it.

Hopefully this is a good example of how important forces are in cycling, but this all gets a lot more complicated when you start peeling back the layers. It’s one of the topics we’ll definitely cover in my mentoring, so if you’d like to nerd out on forces, give me a shout and we can set up a call.