Words: Dan Roberts
Illustrations: Taj Mihelich
Anti-squat has become common term in discussions about bike design. But what does it mean? As simply as possible, it’s how your bike’s suspension reacts when you accelerate. Although there are other factors at play as well, anti-squat most often gets oversimplified to pedalling performance. While it's a big factor there, we need to start off with some other terms and build our understanding to properly understand its effects on bikes.
Riding bikes is a hugely dynamic process, so to analyse what’s going on we need to strip real life back to two dimensions. That way we can understand what is happening and make some calculations before we start to, bit by bit, add in real world elements and scenarios to arrive back at real life with an analysis that matches it as closely as possible.
First, we start with some definitions of mass
. As a rider we have a certain mass, measured in kg. As most of us live on Earth, we are subject to gravity accelerating our mass towards the center of the Earth. This accelerating of our mass exerts a force on the ground where we contact it. This is our weight, measured in Newtons.
The mass of the rider and bike is accelerating towards the center of the Earth creating the weight force. This is then split between the two contact patches creating a load at each. There is an equal and opposite reaction force pushing back up against this downwards load.
For the analysis of anti-squat, we need to use our center of gravity
. Every particle of the bike and rider is being attracted to the center of the Earth due to gravity, but to simplify it for calculation we concentrate all these forces to one singular point; the center of gravity or CoG
. As a bike can’t ride itself and it is similarly not as much fun to run down a trail the center of gravity needs to be that of the bike and rider combined
This combined weight of the rider and bike creates a load acting at the ground that is split between the two contact patches. Neither acceleration nor braking can cause our actual weight to transfer. During acceleration or deceleration, it’s a load transfer
that happens. Our weight can remain in a constant position while the load is transferring underneath us.
The CoG of a giraffe demands a completely different suspension design to allow it to have an efficient pedalling bike, amongst other considerations.
When you accelerate the amount of load transferred to the rear contact patch must be balanced with a reduction in load at the front. The weight always remains the same, just that if you somehow manage to wheelie your bike, all the weight is being supported by the rear contact patch.
In mountain biking the magnitude of this load transfer isn’t to the same degree as in cars or motorbikes. We simply can’t achieve the same acceleration forces, no matter how big your thighs are. However, it’s still hugely important in the system.
This load transfer happens in all bikes, no matter if you have suspension or not. But it’s with the inclusion of suspension, especially rear suspension, that we encounter its tangible effects.
With constant acceleration of the bike and rider forwards the CoG of the system is accelerated in the opposite direction and creates a moment due to the CoG height above the ground. This moment causes an increase in load at the rear accompanied by a decrease in load at the front. The weight has remained in the same place the whole time, but the load has transferred.
Under constant acceleration, the load is transferred to the rear contact patch, and in turn load is reduced at the front contact patch. This acceleration causes a rearward rotation of the bikes sprung mass and un-attended to will cause the front suspension to rise and the rear to squat.Anti-squat
therefore seeks to counter this suspension compression from load transfer. It is simply an expression of how the suspension responds to driving forces, and is expressed as a percentage.
To illustrate anti-squat and make sense of these percentages we can draw the graphical method for calculating anti-squat.
Step 1: Draw in the ground between the two contact patches.
Step 2: Draw a line up from the front contact patch through the front axle, perpendicular to the ground.
Step 3: Determine the CoG position and get a measurement for its height above the ground.
Step 4: Find the instant center of your bike.
Step 5: Draw a line from your rear axle to your IC.
Step 6: Where this line and the line of your chain cross is called the instantaneous force center or IFC.
Step 7: Draw a line from your rear contact patch through this IFC and cross over the front contact patch line. This is called the anti-squat force line.
Step 8: Measure the height, from the ground, that this line crosses the front contact patch line.
Step 9: Compare this measurement to the CoG height measurement in terms of a percentage.
Seeing as the load transfer is trying to rotate the sprung mass of the bike and rider, if your anti-squat percentage is 100% then the anti-squat forces exactly balance out the tendency of the suspension to squat under the load transfer from acceleration and the net result is that there is no suspension compression or extension.
If the anti-squat force line crosses the front axle line at the ground, we can call this 0%, as none of the tendency to squat is counteracted and the suspension compresses solely due to the load transfer.
Crossing the front contact patch line half way between 100% and 0% results in 50% anti-squat and half of the force needed to combat the squat tendencies is combatted. The net result being some suspension compression, but not as much as if we had 0%.
Going under 0% the anti-squat forces actually work with the suspension compression caused by load transfer and put us in a pro-squat zone. In this region the suspension will compress further than at 0% with the help of the pro-squat forces.
Above our 100% mark, where we’ve effectively separated the drive forces from the suspension response, the anti-squat forces will actually be more than enough to combat the squat tendencies, and the rear suspension will extend with that excess force.
The percentage of anti-squat is all in relation to the CoG. If the anti-squat force line and the CoG cross the front axle at the same height it's 100%. If the anti-squat force line crosses at the ground, it's 0%. You can then go below or above these limits with negative percentage values or above 100% values respectively.
The percentage of anti-squat tells us how much of the squat force, caused by the load transfer from acceleration, is counteracted. 100% means the two forces are equal and the net result is no suspension compression. 0% on the other hand, means none of the squat force is counteracted and the net result is the suspension compresses.
Anti-squat isn’t a fixed figure for a bike, it changes as the bike goes through its suspension travel. What we can do is draw out the graphical calculation at incremental steps in the travel until we have a curve that shows us what’s going on from zero to full travel.
Calculating the anti-squat percentage for multiple steps in the travel of the bike gives us an anti-squat curve. You can then see how the anti-squat changes as a function of rear wheel travel. Some anti-squat curves are a straight line, others have far more curvature to them.
When you pedal a bike, you’re investing energy in the system to move you forwards. Some of that energy will always be transferred to somewhere that isn’t going to help the forwards movement, like friction in your drivetrain. But ideally, you’d want as much of that effort you put in to be getting you up the hill. A system that doesn’t combat any of the squat from the load transfer is going to result in unwanted suspension compression and you bobbing your way up a climb.
Anti-squat is a good thing then, to combat the natural want of the bike to squat and compress the suspension. While the acceleration of your bike might not change with or without anti-squat, if you have some combating then you’re more likely to keep your CoG in a constant position, maintain a more favourable bike geometry and keep the load at your rear contact patch at a more constant level.
With anti-squat being good, too much of a good thing can be bad. The anti-squat needs to be within a good range and applicable to the bike’s intentions. Designers can adjust the anti-squat with pivot locations, changes in the geometry, wheel size and chain line.
The chain plays a big role in anti-squat, beginning with its duty to transfer the driving forces from the mainframe to the rear wheel. It’s from these driving forces that the whole load transfer story starts. However, a common misconception is that anti-squat is the chain force and nothing else. It’s in the combination of the chain line with the suspension pivot layout that we get our anti-squat force line. While the suspension layout can remain static, we can actually move from an anti-squat case to a pro-squat case just by changing the gears and so changing the chain line. So, the chain’s effect in the anti-squat calculation is in addition to the inherent anti-squat properties of the linkage system. The chain can then either add, do nothing or work against the anti-squat of the linkage system. There's also more ways than one to accelerate a bike.
Looking at high pivot bikes with idler pulleys we can see that they have anti-squat percentages often over 100%. This is despite there being only a small proportion of the anti-squat forces coming from the chain line, which is often positioned close to or directly through the main pivot. That means it’s then the pivot location which is providing the high value of anti-squat, the location of the pivot dragging up the anti-squat force line.
Pedal kick back is caused from the growth of the chain as you go through your travel. Some bikes have more and some less, but generally as a bike has more anti-squat it will exhibit more pedal kick back. Pedal kick back might actually never be felt at the pedals, as this chain extension can result in a rotation at the rear wheel.
It’s also with the chain that we find one of the potential downsides of anti-squat. The amount of anti-squat is followed closely by the amount of pedal kickback
for most suspension designs. The aforementioned high pivot idler pulley designs being an exception. As the suspension moves through its travel the distance between the rear axle and the chainring increases. The top portion of the chain then needs to compensate for this change in distance and causes a rotation at the cranks or a rotation at the rear wheel. It may even do nothing at all if the angular velocity of the rear wheel is higher than the angular velocity of the cassette and freehub when the chain extension gives it a good tug. Pedal kickback gets measured as an amount in degrees and its magnitude increases or decreases as the amount of anti-squat increases or decreases.
Pedal kickback is a whole other topic, with research already done into its relevance, how much is acceptable and in what scenarios the theoretical degree of motion of the cranks will actually happen. But as mentioned, the anti-squat works best in a reasonable window when it doesn’t stray into extremes and if that is the case then the amount of pedal kickback will also not enter into an extreme amount.
Now that we understand what anti-squat is, how we calculate it and what its percentages mean, we can add in more and more pieces to bring the analysis back to real life and start to consider how we can really use anti-squat.
With the anti-squat calculation only considering forces and moments from the load transfer due to acceleration we could perceive that 100% is the amount to aim for to combat all the squat tendencies. However, in real life we have more loads trying to compress the suspension than just that from the load transfer.
The stripped back nature of the anti-squat calculation only looks at the effects of load transfer. In reality there are many more forces acting to compress the suspension when we pedal or accelerate. When you push on the pedals some of that force is going to want to directly compress the suspension, so maybe it's good to counteract that too, as well as the squat from load transfer.
Even if you remain rigidly still while pedalling your CoG will want to rise up as you push down on the pedal. This acceleration of the CoG upwards is balanced by a downwards force on the bike, which will want to compress the suspension. So, pedalling a bike with that ideal 100% anti-squat figure will still result in the suspension compressing. Raising the anti-squat percentage above 100% means that we extend the suspension slightly with the extra anti-squat force. This extra extension can be used to counteract that extra vertical load put on the bike while pedalling. There is again a limit to how much extra anti-squat force we generate.
With pedalling being a cyclical motion, and the resulting power delivery following that cyclical behaviour, it can mean that we momentarily add or reduce the load on the tyres which in turn affects the amount of grip that we have. If we have a system with very high anti-squat percentages the extension of the bike will cause our CoG to rise with each pedal stroke. This rising CoG is joined by an increase in load at the contact patches, in addition to the already increased load at the rear contact patch due to the load transfer. In between pedal strokes the load transfer is reduced, so too is the load at the rear contact patch. In addition, the suspension will attempt to return to its equilibrium position and further reduce the load at the rear contact patch. This cyclical increase and decrease in load at the rear tyre can cause issues with traction.
Coming back to the amount of acceleration that a human can achieve on a bike, ebikes have upped that acceleration amount. For anyone that has ridden one they will know the hugely increased sense of gathering speed. This up in the amount of acceleration also ups the amount of load transfer and can make it happen in a shorter time. While the power delivery might be smoother on an ebike compared to a non ebike, the detrimental effects of the load transfer are more profound and tyre loading and unloading can be more severe. Added to this the system’s CoG is in a different place with the addition of the battery and motor. So perhaps on ebikes more than any the design of the anti-squat is of high importance to ensure an efficiently powered bike with good traction.
The CoG position of a rider can change horizontally, meaning the 100% anti-squat limit would remain the same. But the changes in load at the front and rear tyres from this shift in CoG could result in the effects of the load transfer and anti-squat forces being more or less prevalent.
Once we add more and more real-world factors into the anti-squat equation, we can start to see why it becomes difficult to design bikes. Once we start to see also the influence in CoG position that rider size, riding position (pedalling seated or pedalling stood up), terrain (riding on flat ground or riding up a steep hill) and the fact that a mountain bike rider is a wildly moving quarter horse power engine during even the least rowdiest rides we can start to see why we needed to strip the analysis back to a more simple beginning and add in some assumptions.
With current analyses made visible to the public, and without a lack of a standard set of rules for each manufacturer to abide by when analysing anti-squat, there are quite a few assumptions that need to be stated and understood.
Most of the time the CoG height is fixed, and no consideration taken into its varying position with rider height and seated or stood pedalling. The analysis is also conducted on flat ground, where the distribution in load between the two contact patches is closer to equal. If we were to tilt the bike uphill we would naturally start the analysis with a higher proportion of the load on the rear contact patch and any bikes that would squat with the load transfer would come dangerously close or completely remove any load on the front and cause the bike to start to loop out. For our analysis in 2D we also consider the bike to be perfectly vertical.
When climbing a steep hill our weight is acting in a different direction to when we are on the flat. Firstly, there is a component of our weight pulling us back down the hill and as the total needs to remain the same it means we have reduced overall load at the tyres. Our load distribution between the tyres is different too, with more load on the rear and much less on the front. If in this situation your bike has a tendency to squat then it won't take much acceleration for the effects of the squat to remove all the load at the front contact patch and you're left with zero traction.
Riders of different sizes will have different CoG heights and so the 100% anti-squat line will be different. It then makes sense, in an ideal world, to adapt the suspension of each size of bike to the size of rider that will be on it. A larger rider would need the anti-squat force line to cross the front axle higher up to achieve the same amount of anti-squat. It does however increase the number of variables in manufacturing a bike, but some brands do it.
As we compress the suspension to analyse the anti-squat throughout the full range of travel, we must also consider what the front suspension is doing. To keep things simpler, we can say that the fork stays extended. This will generate an anti-squat graph using lines to describe what’s going on. We could add in a degree more complexity and compress the front suspension in time with the rear, allowing the front and rear to arrive at full travel at the same time. This too would generate an anti-squat line for us to interpret. What we can also do is to analyse the anti-squat at the two extremes of zero front travel and full travel and plot two lines on the graph which would define a window of anti-squat that we would be working in. All three methods can be done, but the assumption of which one should be stated in the analysis.
We can further increase either the number of curves or windows on our graph by accounting for multiple chain lines. On most mountain bikes we have a varying chain line even with a fixed chain ring size.
A single anti-squat curve also depends on the position of the front wheel. If we draw anti-squat curves, one for zero front wheel travel and one for full travel then we generate an anti-squat window that our bike will operate in. But it can't just be taken at face value. The real world situations need to be considered again, as you'd rarely be pedalling your bike with the fork bottom out.
Analysing anti-squat needs us to simplify reality to understand how we’re combating the tendency of our suspension to squat with load transfer. Hopefully the article has gone some way to explaining how we do that. But hopefully more, our re-insertion of that analysis back into the real world and the additions of other loads, situations and considerations helps give some further understanding and food for thought into analysing our bikes.