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Analysing Front & Rear Axle G-Force Data

Comparing data between an ideal setup to my setup

It's important to note that although this is an insight in to suspension behaviours, it's not the only way we can look in to this, and collective G-Forces measured at the axles aren't only about suspension. They also give an insight in to other contributing factors that results in forces being transmitted to the suspension, bike and rider. For this post I'll focus on how we can use the G-Force data to determine how the suspension is dealing with these forces and how the rider weight distribution can also affect this.

The graph below shows G-forces registered over a three second window of time at the front and rear axles on a 203mm travel DH bike with 30% sag front and rear and a 62° head angle on a track at Antur Stiniog.

Front & rear axle G-forces from Antur run

The graph below shows the same information over the same period of time for my bike (203mm of travel front and rear, 62.4° head angle and 30% sag front and rear) on a track at Rogate Downhill.

Front & rear axle G-forces from Rogate run

The G-force scale (vertical) show the same units and variance (+40 G through to -20 G) with both forks maxing out around 35 G’s and shocks at around 30 G’s so it’s a good comparison between the two runs. The biggest difference between the two runs are the types of terrain, which is highlighted by the larger range of forces registered in the top graph over the three seconds.

The top graph shows a rear shock which is more active than mine, with similar amounts of forces being registered and handled to the fork. The rear axle forces on my bike are smaller and fewer than the front, meaning the fork is more active than the shock which would mean there are compromises to the grip at the rear wheel compared to the front.

When you compare the areas in the red squares you can clearly see that the front and rear axles in the Antur run are almost identical in comparison to the Rogate run, where the rear axle trace is not only different in shape but also in frequency.

The graphs below show the peaks of G-forces in more detail, which give you an idea of how your suspension is handling the forces.

The details in G-force traces

The Antur run is on the left and Rogate on the right.

You can see the peaks for both axles are almost the same shape in terms of angles on the Antur run, compared to the slightly wider rear axle peaks on the Rogate run. This would show that the rear shock rebound is slower than the fork rebound, unlike in the Antur run where both suspension units are handling the forces in the same way.

There are also smaller and fewer G-forces being registered at the rear wheel in the Rogate run, with the Antur run showing both axle traces with remarkably similar amounts and forces being registered. This would suggest that the rear shock could do with less compression damping on my bike which would result in the rear wheel tracking the ground more consistently and providing more grip.

When you overlay the rider input trace over the front wheel G-force trace you can see the rider weight distribution is very similar. The Antur run is the top graph.

Rider Input trace overlaid front axle G-force from Antur
Rider Input trace overlaid front axle G-force from Rogate

This shows that the rider weight is having a similar effect of the suspension units on both bikes, despite there being more rider input in the Antur run due to more G-forces being registered. Proportionally, they are very similar. This is when compared with each other but also when you compare the rider input trace to the relating front G-force trace in each run.

If there was more rider input on the Rogate run in comparison to the Antur run, that would show that my weight is more over the front of the bike which would result in potentially higher/more G-forces being registered at the front and lower/less being registered at the rear, which could result in adjustments being made to the shock when it could be an issue with weight distribution rather than shock settings. This also relates to the fork settings and rider inputs as well.

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