To conclude our first series of Behind the Numbers, and before we move onto anything new, we're putting all the bikes we've analysed so far onto the same graph, with the same scale, to see how certain layouts and bikes give different curve characteristics.
You may be wondering why the Grim Donut is not adorning these graphs, and it's simply because that bike sits aloof from all other bikes, quietly in its own category, incomparable to other such "traditional" bikes.
For the analysis we looked at leverage ratio, anti-squat, anti-rise and axle path. But for this comparison article we went back and added in an analysis for the pedal kickback, due to popular demand.
Keep your eyes peeled in the coming weeks for our next series of Behind the Numbers, this time focussing on five trail bikes.
We kicked off the series with the Specialized Stumpjumper EVO 29. As the name suggests, it's a 29er and has the ability to adjust the geometry and suspension via a chip in the shock extender. It uses a Horst pivot design with a small link compressing the shock.
Next up was the unconventional Marin Mount Vision. This had 27.5 wheels and no adjustability but a suspension system using sliding elements rather than just rotating pivots.
After that we looked at the GT Force. Another 27.5 bike but with a flip chip located at the lower shock mount. Another Horst pivot design but uses a rocker link to compress the vertical shock.
Then it was the turn of the Orange Stage 6. A 29er with no adjustability and a single pivot suspension system.
And finally, we looked at the Santa Cruz Megatower. A 29er with chips at the shock and the rear axle, giving four possibilities for the bike to be configured. A VPP, or Virtual Pivot Point, suspension system is used, but you can also call it a short, counter-rotating link system.
Leverage ratio is the ratio between how much the rear wheel moves versus how much the shock moves. If we move the shock 1mm, how many mm is that going to move the rear wheel?
A higher leverage ratio means that for 1mm of shock movement the rear wheel moves more. Or looking from the rear wheel's perspective (the inverse of the leverage ratio), it would move the shock less for a given amount of rear wheel travel. A lower leverage ratio means the rear wheel moves less for a given shock movement, say that 1mm. The inverse means that for a given rear wheel movement it would move the shock more.
Higher leverage ratios multiply the input force at the rear wheel more but would generally generate less damping force, seeing as they are moving the shock slower. Lower leverage ratios multiply the force going into the shock less but generally generate more damping force from the faster shaft speeds.
When we talk about leverage ratios, we use the terms progressive, linear and regressive. These are less a systematic description of just a curve on a graph and more a description of how the bike is going to feel when you ride it. Once a shock is bolted in all bikes will exhibit an increasing wheel force as you go through the travel, so it's not a description of the wheel rate curve. A progressive leverage ratio curve is one that starts high and gradually reduces in leverage ratio. A linear leverage ratio is one that has little to no change from start to finish and looks more horizontal. A regressive leverage ratio is the opposite of a progressive one and starts at a low leverage ratio and gradually increases.
Looking at the bikes we have a pretty good mix of suspension systems and as a result, a good mix of curve characteristics.
The Megatower is by far the most progressive, or has the most change from start to end, ranging between 27% to 32%.
The Stage 6, however, is the polar opposite and is the most linear, or has almost no change between the start and end.
In-between these extremes sit the Force and Stumpjumper EVO, both with some progressivity but also exhibiting more the traits of a linear leverage ratio.
Then we have the Mount Vision, which shows us what regression is with its increasing leverage ratios at the start of travel, before rounding off in a hump and giving a progressive leverage ratio.
As a rule of thumb, regression in a leverage ratio curve is something to avoid. Some argue that at the end of travel, leverage ratio regression will counteract an air spring's natural ramp up in force, but having straighter lines avoiding regression translates to a more predictable suspension feel and also addresses the damping's relation to leverage ratio as well as the spring forces.
The shape of the curve and its overall change are two things to consider. But you can have two identical curves that could be at different leverage ratios. So, we need to look at the actual ratio numbers to understand more about what's going on. The Mount Vision has low leverage ratios, even dipping under 2:1 at the end of travel. Whereas the Megatower has much higher leverage ratios all the way through travel. This is going to affect how much spring is needed to achieve a desired shock stroke sag while also affecting where the rear axle sits at this sag point.
Anti-squat is describing how the bike is going to react to acceleration and the load transfer that comes with it. We've taken a really close look at anti-squat in our Enginerding
article, which goes into the details of how we analyse it, what assumptions we need to make and what happens when we come back to real life with our anti-squat percentages and curves.
Anti-squat requires a chain in the mix for the analysis, and given we have multiple gears, that anti-squat figure changes depending what gear you are in. For our analysis we've chosen a light climbing gear, 50-tooth, a mid-pack gear, 24-tooth and the bottom of the cassette with the 10-tooth. All together we have thirty different curves, which put all onto the same graph becomes a good catalyst for making you go cross-eyed. So instead we've broken the anti-squat down into those three gears making it easier to digest. For the 29" bikes we took a chainring size of 30 teeth and for the 27.5" bikes it was 32 teeth.
The characteristics of the different suspension systems can also be seen here. The two Horst pivot bikes, the GT and the Specialized, show similar characteristics and the levels of anti-squat decrease as you go through the travel. The Orange keeps the horizontal theme going and this is where we start to see the recognizable curves of the Santa Cruz's short counter-rotating links which we'll see later on too. As the bottom link of the VPP suspension system rotates towards the end of travel it shoots the instant centre of the system forwards and down and so creates this exponential-like drop off in the curves.
With the Marin we actually see a relatively uncommon characteristic in having the anti-squat percentage increase as you go through the travel. This is down to the suspension design that Marin have employed on the Mount Vision with its use of normal rotating pivots as well as a large sliding cylinder.
You ask and you shall receive.
Pedal kickback is looking at the resulting amount of crank rotation from a fixed length of chain as you go from zero to full travel. The distance between your bottom bracket and rear axle will usually change, but if the chain is a fixed length then something needs to give. Also consider that this chain extension can be a rotation at the rear wheel or simply a rotation of the cassette that will never be felt due to you travelling faster than the resulting tug on the cassette. But when you've got the cassette engaged, like when you're climbing or locking the rear wheel when braking, it can be something to consider.
We have the same deal for the pedal kickback as we did for the anti-squat, with the analysis broken down into the same three gear combinations. As there's some link between anti-squat and pedal kickback, it then makes it easy to correspond a certain amount of pedal kickback to the appropriate level of anti-squat for that bike.
A simple rule for all bikes is that as you increase the size of your cassette cog you will have more pedal kickback degrees. So, climbing gears will have more than your gears generally used for going downhill. What is also interesting to consider is that it's not just the amount of pedal kickback degrees, but how fast they are inputted to you as a rider. A bike that delivers you more pedal kickback but in a slower manner may be easier to deal with than a lesser amount of degrees dealt to you in a fast manner.
The Stage 6 shows the most pedal kickback, due to its high main pivot. The Mount Vision is not far behind in terms of degrees whereas the two Horst link bikes with generally low anti-squat percentages show lower levels of pedal kickback. All these bikes have fairly straight anti-squat curves and so have pretty damn straight pedal kickback curves. But when we look at the Megatower we again see a different shape of curve. With that extreme drop off in anti-squat towards the end of travel we also see a slight reduction in the amount of pedal kickback. It's not a complete reduction in the amount, as we've already generated quite a bit of pedal kickback before we get to the portion of the curve that flattens or drops, but compared to the Mount Vision, which the Megatower follows quite early on it would be between 2 to 3 degrees less.
Anti-rise is describing how the bike will react to the load transfer from applying the rear brake. It's got the same measurement in terms of a percentage but to calculate it we don't use the chain line and instead look at just our instant centre, front and rear contact patches and our center of gravity height. A bike with 100% anti-rise will theoretically neither extend or compress the suspension when you grab a load of rear brake. Below 100% the bike would extend, or rise. Above 100% the bike would compress the suspension.
All bikes in the enduro category exhibit less than 100% anti-rise, meaning that they should all rise somewhat due to the load transfer induced by braking. The Orange exhibits the highest anti-rise, thanks again to its relatively high pivot when compared to the rest of the bikes. The Marin and GT have the least deviation in the anti-rise curve, meaning that their response to load transfer is going to be pretty similar if you're at sag or at the end of your travel. The difference being the amount of rise being less with the Marin and more with the GT due to its lower anti-rise percentage amounts. A more consistent response from your bike could translate to more predictability.
The Specialized's anti-rise actually increases with travel, which could be perceived as a handy trait. At instances where you're deeper in your travel, and possibly have some violent terrain or actions causing this, the bikes suspension would react less to the load transfer from braking, reducing the number of things trying to unsettle your bike's geometry. It doesn't increase violently, meaning that it could still be seen as in a predictable amount of change, giving you some secret support while still not leaving you to second guess what the bike will do at any given moment.
With the Santa Cruz we see the characteristic curve shape that we came to recognise in the anti-squat graph. For the most part, the curves follow a very similar path and it's only towards the end of travel that we see the biggest change between the high and low chip positions. For the Santa Cruz the values either touch or go below zero. At zero the bike will not provide any countering force and the suspension will rise solely due to the load transfer. Below zero the bike will enter a pro-rise zone and exert an additional force that will help the bike rise even further. But due to the shape of these curves these tendencies would only be in the very final stages of travel.
All the curves on one graph with the same scale.
What is straight away clear is that all the bikes spend most of their travel with the axle moving forwards. Only in a full-on high pivot bikes, like the Forbidden Druid and Deviate Highlander, would we see that reversed and the axle moving rearward more than forwards.
That said, there are two clear groups of bikes here, with the Mount Vision and the Stage 6 not only having the most rearward travel but also the least forwards movement. Something that is reflected in the pedal kickback amounts, but something that will no doubt help their performance of the rear wheel and impacts in all travel positions.
On the other hand, the Megatower, Force and Stumpjumper EVO all have less rearwards movement of the axle coupled with far more forwards movement. The Megatower actually has the most forwards movement of the axle and is combined with the most pedal kickback of these three remaining bikes, due in part to its increased travel over the rest of the bikes.
The Stumpjumper EVO has almost no rearward movement in its axle path and despite being around 25mm less travel than the Megatower finished with the axle further forwards. In the way that one of the downsides of true high pivot bikes is their rapid change of geometry as you go through travel, this can also be true of the rear axle moving so far forwards.
Centre of Gravity (CoG) height is 1,150mm above the ground.
Anti-squat and anti-rise always assume a static center of gravity. In the real world this is rarely the case, but this needs to be done for analysis’ sake to allow it to be calculated and compared to other designs and bikes.
Fork is at full travel. There’s no industry standard for the fork in anti-squat and anti-rise analysis. Changing the fork travel to be at sag throughout the analysis does result in some change to the curves. Having the fork compress at the same rate as the rear suspension could be a more realistic way of analysing, but adds complication to the analysis and makes comparisons harder. As long as these assumptions about AS and AR are known and understood, it’s easier to analyse and compare bikes.
Final Thoughts Previous Behind the Numbers Articles:Santa Cruz Megatower Suspension AnalysisGT Force Suspension AnalysisOrange Stage Six Suspension AnalysisMarin Mount Vision Suspension AnalysisSpecialized Stumpjumper EVO Suspension AnalysisIntroducing Behind the Numbers - A New Suspension Analysis Series