Introducing 'Behind the Numbers' - A New Suspension Analysis Series

Geeking out is damn good fun. But more than that, we can start to objectively analyse what’s going on with our bikes and form a connection between numbers and ride feelings and vice versa.

Feelings can only tell part of a story, and by the same token, numbers can only tell a different part. It’s in the combination of the two where we attain all the information and can start to understand the whole of what’s going on.

A bike is a myriad of parts that the information at ground level has to pass through to get to the rider. Behind the Numbers is a new series that takes a deeper look at the suspension link of the chain. And hopefully with the information we can start to understand this myriad of parts a little better.

The jungle of bold marketing claims and jargon is dense and tough to wade through. Biking is all about fun, and if we can get the tools to pick the right bike for our individual needs, we can extract the most amount of fun possible out of every ride.

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After quite a while badgering everyone in the industry with an email address (sorry about that!) I eventually, and what still seems by pure luck, landed a job working for SCOTT in Switzerland as a bike engineer.

When I started there, I was wide eyed and in a new land full of breathtaking mountains, excessive amounts of cheese and some of the most passionate and fun-loving people that one could hope to share adventures with. Those adventures ensued, be it in learning the right mixture needed to bake some of the most smile-inducing bikes that I’ve ever ridden, or off up a mountain side, with no more than a half-eaten cereal bar and sheer hope to get you to the tiny hütte on the horizon for the night.

We developed all manner of bikes and parts covering hardtails, full suspension, aluminum, carbon, stems, chain guides and even stuff that never saw the light of day. The variety of skills and experience on offer to learn was vast and wealthy. Three highlights include the Scale XC hardtail, the Voltage FR freeride bike and the Voltage dirt jump hardtail.

All things, sadly, come to an end. And with a lump in my throat I left SCOTT in 2017 to set out ticking off the riding spots on a list that adorned my fridge door. After 14 months, 12 countries, god knows how many kilometres and many more sets of brake pads than I am proud to admit, I had only one place that I wanted to return to — Switzerland.

I now call Champéry home, and run a small engineering consultancy to the bike industry called Garage Bike Project.

For me there was no other choice in my head than Champéry. Besides the riding that is on the doorstep and in the adjacent valleys, the crew of like-minded passionate pinners and the ability to scare yourself and have your socks blown off by the scenery on every ride, the opportunity to fluidly mix work and riding is the cocktail that I discovered to extract my best and most efficient work, while having a blast doing it.

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Definitions

Before we delve into analysing the plethora of bikes available, we need to explain a few key terms that describe what’s going on.


Instant Center

This is the point that the rear axle is rotating around. On something like a single pivot bike, it's easy to see as the fixed main pivot on the main frame. On more complex, multi-link bikes this point can be out in space and not a physical part of the bike. As the suspension compresses and links move around, so does the instant center. This instant center is used to help calculate the leverage ratio, anti-squat and anti-rise.



Leverage Ratio

This the ratio between how much the shock moves versus how much the rear wheel moves. Most of the time it’s always looking at moving the shock 1mm (or whatever unit you would like, seeing as it’s just a ratio) and seeing how much rear wheel travel is produced via the linkage system.

1:1 would mean that the shock and rear wheel move at the same rate. 2:1 means that for every 1mm the shock moves the rear wheel moves 2mm. 3:1, the rear wheel moves 3 times the shock and so on and so forth. The higher the number, the higher the leverage ratio.

The leverage ratio can also tell us how much force the suspension system transmits to the shock from the rear wheel, and how fast or slow the shock will move.

A high leverage ratio will transmit more force to the shock, this is why you need bigger springs or more air for higher ratio bikes. A low leverage bike will transmit less force to the shock, requiring less spring force to push back.

The inverse, however, can be said of the shock speeds. At high leverage ratios the shock will be getting compressed slowly, and as damping is dependent on the shock speed, it will generate less damping force. A low ratio will be compressing the shock faster (it’s getting closer to 1:1) and will generate more damping force.

As links rotate and rear triangles move, the leverage ratio does too, and as we plot the leverage ratio for every point along the travel of the bike, we get the leverage ratio curve.



Linear

Linear describes a leverage ratio curve that doesn’t change much throughout travel, i.e. a horizontal line.

Progressive

Progressive describes a leverage ratio curve that goes from a high ratio to a low ratio, so on the graph, it goes down.

Regressive

Regressive is the opposite of progressive and has a leverage ratio curve that goes from a low ratio to a high ratio.

Leverage Ratio Percentage Change

When we look at the difference in leverage ratio between the start and end of travel, we calculate the progression of the leverage ratio. Usually it’s described as a percentage. If a leverage ratio drops from 3:1 to 2:1 it has a change of 1, so we can say it’s got a progression of 33.3% (a third of the starting leverage ratio).




Mass Transfer

When you slam on the anchors or mash the pedals your mass will move around. Physics is quite lazy, and so a mass always wants to remain where it is, be it still and not moving or just trundling along at a constant speed. If you start pedalling from a standstill, or brake while you’re moving forwards, your mass will want to stay right where it was while you’re actually starting, or stopping, moving. This lag causes our mass to transfer either forwards or backwards, and how our suspension system deals with this transfer can be described in two ways.

Anti-Squat

This is how the suspension system will deal with acceleration and the mass transferring backwards.

Imagine a big solid bar of steel propping you up, right at your center of mass; if you accelerate, you’re not going to compress this solid bar. The amount of force that the bar, or suspension system, pushes back with is exactly the same amount of force that your mass is falling backwards with. In other words, it’s 100%.

0% would mean that the suspension system doesn’t push back at all, and your mass transfer squats the bike into its travel.

200% would combat all the mass transfer forces and then still have the same amount again to push back with and so would make your mass move forwards.

Below zero figures mean that the suspension system is going the other way, and actually compressing itself further into travel while the mass transfer squats the bike too.

As you go further into your travel, the rear axle and BB get further apart from each other and stretch the chain. If you hold your feet where they are on the pedals, it needs a force to combat this chain stretch.

Anti-squat, however, is not the “chain force”. This seems to be getting used a lot amongst the industry at the moment, and it’s simply not true.

A suspension system will have a certain amount of anti-squat without the chain, and depending on what gear you’re in and how the chain lines up with the suspension system, it will either help, do nothing or have an adverse effect on the bike trying to combat the mass transfer.

Anti-Rise

When we talk about anti-rise, we talk about how the bike will react to mass transfer while braking, and so having the mass shift forwards. This pitching forwards effect would usually have the suspension system extending and having the bike rise out of its travel. Just like anti-squat it’s described as a percentage of how much the bike combats, or helps, the mass transfer.

100% would keep the mass at the same position while you brake by compressing the suspension, and 0% does nothing to combat the mass transfer and allowing it to move forwards with the braking.

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Assumptions

Riding a bike is not a static process. That little quarter horsepower motor is moving violently all over the shop. To analyse things like anti-squat and anti-rise we have to make some assumptions to do the analysis in the first place, and then make it easier for comparisons between bikes. Otherwise it would be a mess.

These things are only analysed in a 2D nature, bike bolt upright and the mass transfer and reactions acting in the same plane. We assume a fixed center of mass height from the ground and we assume a fixed fork length. And for specifically anti-rise calculation we’re only braking with the rear brake.

For the center of mass, or COM, height we take 1150mm above the ground. For anti-squat calculations, where the chain comes into play, we use the extremes of the cassette and a cog in the middle. For example, on a SRAM Eagle cassette that would be a 50T, 24T and 10T. For bikes designed a bit more for pedalling up things, we take a 30T chainring for 29er bikes and a 32T chainring for 27.5. For bikes designed for going down, we take a 36T chainring and use the SRAM 7spd DH cassette as a reference.

These assumptions of course do not represent reality entirely, but as long as you know them then you can read the curves just fine and draw all the conclusions you need to understand what’s going on.

As previously mentioned, a bike is the sum of its parts. You can have the best suspension in the world, but if you mess the information path up with something as simple as tire pressure, then the outcome is going to be a negative feeling. We’re only focussing on one part of a bike here.

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How It's Calculated

First, we need to measure the bikes, pinpointing each pivot’s location in space relative to some hard points on the bike, like the BB and rear axle. But we measure everything to know the link lengths, angles and layout of the bike.

These measurements get plugged into a 3D Computer Aided Design software package, where we create a 2D kinematic layout of the bike. There are only a certain number of dimensions that need to be inputted to have the sketch of the bike fully defined. But we measure more points on the bike to have more information to ensure the analysis is as accurate as possible.

From our software we can output all the data about the leverage ratio, anti-squat, anti-rise and axle path. But it’s not the prettiest and most intuitive to understand. So, we transfer the raw data over to a spreadsheet to form the graphs that we’ll have in the analyses.

The other tool used for the analyses is experience. I’m definitely not an expert, and am dubious of anyone who claims to be. We’re always learning. But what I’ve learnt so far - be it with development, engineering, riding or even just chin scratching with friends - provides the tools to speculate how the numbers will transfer into the real world.

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Reviews and Tech Behind The Numbers Dan Roberts