Suspension Systems Explained

We’re faced with a bewildering range of suspension systems, each claiming to be the best thing since sliced bread. Steve Hinchliffe takes an independent look at the popular designs to separate the fact from fiction.

A little over a decade ago, dual suspension bikes were largely the domain of elite semi-or fully-professional riders, or those with too much disposable cash burning a hole in their pockets. These machines were relatively expensive, not always reliable and many recreational riders just didn’t bother with them, happy to have their weekend trail entertainment on a quality hardtail of some description. These days, full suspension bikes are more affordable, lighter, more efficient, and almost totally dependable; the result is that full-squish bikes now make up a huge proportion of the ‘average’ mountain biking market.

With billions of dollars at stake in the global market, bike manufacturers are constantly searching for an edge over their competitors, busting their butts to get a bigger share of the pie; marketing departments are working hard to convince you, the buyer, that their suspension design is the best pedalling, best braking, plushest, and the most awesomely rad system on the market; any others just won’t cut it. While there’s already a broad range of suspension designs on the market, it seems like every year one brand or another comes out with a new system claiming to make all the rest obsolete; something they’ve spent a bucket-load of time and dollars developing.

With so much money (and kudos) at stake, it’s not surprising that the lines have become somewhat blurred as to which of the many performance claims are legitimate, and which are little more than what you do with your cranks — spin. At the risk of offending a whole swag of journos, PR experts and marketing gurus, we thought it was about time to de-bunk some of the myths that have, for too long, been circulating around in the bike industry. This list is by no means complete, but covers some of the more prevalent, and potentially misleading, furphies surrounding the poorly understood subject of rear suspension.

Cut Throught the Hype

For the sake of this article, we have grouped the suspension designs into two main categories; single pivots, and multi pivots. A single pivot design has only the main pivot between the bottom bracket and the axle. It may be a simple single pivot, like a Santa Cruz Heckler, where the swingarm drives the shock directly, or it may have a series of linkages to drive the shock, like a Scott Spark. Either way, it is still a single pivot. Multi pivot bikes have more than one pivot between the bottom bracket and the rear axle; typical examples include the common horst-link (Specialized FSR and Ellsworth ICT) style bikes, and also the short-link four-bar variants (DW-Link, VPP, Maestro etc). There are a small number of designs outside these two main categories, such as those with ‘floating’ bottom brackets, (see more at the bottom of this page), however the vast majority of bikes on the market (probably more than 90%) fall into these two designs, and even those that are truly ‘outside the box’ share characteristics similar to the main two styles; Trek’s ABP and the Weagle designed Split Pivot for example have the axle path of a single pivot with the theoretical braking characteristics of a multi link.

Also note that there’s a whole lot more to any suspension system than pivot placement alone. Shock and linkage spring rates as well as special damper technologies (such as Brain/Terralogic and platform damping) will also contribute to make or break any bike—single pivot, multi link or otherwise.

So let's get stuck into it!

 

Myth #1: For a single pivot (with or without linkages), positioning the main pivot at the chain line produces suspension which is unaffected by pedalling forces. 

The diagram below shows a single pivot suspension swing arm turned at 90-degrees; the main pivot is on the ground with the wheel supported above it. Let’s assume this whole unit is perfectly balanced (i.e. in equilibrium). If we imagine the chain hanging down from the cassette on the right hand side, which direction would you pull the chain in order to turn the wheel without making the whole system fall over; to the left of the pivot, through the pivot, or to the right of the pivot?

The correct answer is actually straight down; if you didn’t get it right first time (I didn’t), don’t be upset; even physics lecturers can often be caught out on this one. The only force that will not have any effect on this equilibrium is one which is parallel to the line between the main pivot and the rear axle. Translating this back to your bike, you can see that when the pivot is positioned at the chain torque line (CTL), the suspension actually extends and resists compression when you put power down through the pedals; the suspension is only neutral when the pivot is somewhat below the CTL. Neutral suspension does not mean no bob under hard pedalling; it merely means that the rear wheel is completely unrestrained and free to react to any vertical forces that it encounters—think bumps/dips and rider weight shifts. 

 

Myth #2: A suspension design can use chain tension to overcome pedal-induced suspension bob, but still be fully active when pedalling over bumps.

When you put hard-earned energy into your pedals, you want as much of it as possible to generate forward momentum—any uppy-downy movement of the suspension is a waste of valuable effort. Because riders rarely pedal in perfect, completely balanced circles, modern suspension designs use ‘chain growth’ to help counter your bike’s inherent tendency to ‘bob’ under pedalling load.

Through strategic positioning of the various suspension pivots, the rear axle is made to move further away from the bottom bracket as the suspension moves through its travel. What this means is that whenever your suspension compresses, the lengthening chain pulls slightly backward on the chainrings; if you aren’t pedalling at the time you’ll rarely notice it, but if you are putting down the gas, one of two things, or a combination of both, can happen.

Firstly, if the bump is small and you are cranking hard, your pedal effort can resist the chain’s attempt at getting longer, in which case the suspension will be unable to move—that means no pedal bob, but much less than 100% activity of the rear suspension. Secondly, if force to the suspension is quite large (or your effort is feeble!), the rear wheel will move through its travel and the chain will grow. From this you’ll feel the pedals ‘kickback’ against the soles of your feet as the cranks try to turn backwards with the lengthening chain.

Lastly, and probably most common, is a combination of the two; there is some suspension movement and hence kickback, but also there is some resistance to pedal-induced bob and slightly restricted travel.

Different pivot locations will produce a different emphasis of these two traits; some will prioritise efficient pedalling at the expense of bump absorption and pedal feedback, whilst others will give you a smooth ride and unrestrained pedalling at the expense of efficiency, but there is no free lunch to be had—you simply cannot have the benefits of chain growth without some of the detrimental aspects. Anyone who tells you otherwise is not being 100% honest. 

 

Myth #3: All single pivot bikes lock up under brakes while multi pivot bikes remain fully active.

This is one of the most persistent myths in the MTB world, and as such deserves a more detailed explanation. When you apply your bike’s rear brake on a smooth surface, one of three things can happen; the suspension can either try to extend, compress, or it can remain completely neutral. Which of these it will do is determined by what’s called ‘anti-rise’ (AR) which is calculated by knowing the Centre of Mass (CM) of the bike plus rider, and the Instant Centre (IC) for the particular suspension design. These are pretty simple to figure out, so here’s how you do it.

The three images above are simplified drawings of a bike and rider; the CM is always positioned just in front of and below the riders navel. If you have a single pivot bike (the first image), the IC is situated at the main pivot point. For a multi pivot bike (the other two examples), the IC can be found by drawing a line through the two lower pivots, and also the two upper pivots; where these lines meet is the IC.

To calculate the AR as a percentage, we first draw a line from the rear tyre’s contact patch through the IC; then draw a line horizontally forward from the CM, and lastly draw a line vertically from the front tyre’s contact patch. The intersection of the line through the IC and the vertical line determines the anti-squat value, which is expressed as a percentage of the length of the vertical line up to the height of the CM.

Using the single pivot as an example (the first bike), the line through the IC intersects the vertical line 108% of the way up; hence the AR is 108%. In the second image, a short-link multi-pivot, the line through the IC intersects 100% of the way up, so the AR is 100%. The lower diagram is a traditional multi-pivot design and it has an AR figure of 62% — see, easy!

An AR value of 100% means the braking forces should equal out to allow neutral and uninhibited suspension response. A value less than 100% means the suspension extends under braking while greater than 100% should cause the rear end to squat. Bear in mind that the AR value is not fixed, and changes constantly as the bike moves through its suspension travel.

So with all this science, it should be simple to figure out which bikes are active under brakes right? Wrong!

You see, of the multi pivot bikes on the market, all of which claim (and are generally accepted) to have fully active braking, AR values can vary from below 45% to around 110%; they clearly can’t all behave the same. Additionally, virtually all single pivot bikes also have AR values that fall within this 45-110% range, so they behave under brakes exactly the same as their multi-pivot cousins. Not only that, but by moving your weight around on the bike, you can dramatically alter the CM, and hence the AR value, of any bike, regardless of linkage design. For example, even if you had a perfectly ‘neutral’ design with a particular body position, by moving your weight rearward you will make it squat (compress), and by moving forward you will make it extend. Similarly, by moving your weight up or down you change the AR value, because the IC is determined by frame geometry, not rider position.

Whether the disc calliper is mounted to the swingarm or the seat stay is largely irrelevant, as any torque forces from the calliper are not isolated by a pivot point as is often suggested. Torque applied to a frame member by braking will be transferred across a pivot point. If it didn’t, designs with a chain stay mounted pivot would suffer marked power loss, as the chain torque would be isolated and in some way unable to drive the rear wheel. As pointed out earlier, it's the AR value that does influence the suspension characteristics under braking, so that's the figure you should take note of.

The only sensible assumption from all this is that marketing claims of full activity under brakes are just that; marketing. All suspension designs have the potential to be active or otherwise under brakes, with the single most important factor being the position of the rider’s weight. Whatever bike you ride, over time you’ll automatically adjust your style to extract the best performance from its braking behaviour.

 

Myth #4: Multi pivot bikes have a more complex axle path than single pivots, and are hence better at absorbing bumps.

It is widely accepted, and almost certainly true, that the rear axle’s path is the main factor determining bump absorption. Specifically, a path which moves backwards as it moves up offers the least resistance to obstacles; hence the often-touted benefits of a ‘rearward axle path’. Here again there are compromises, because in general a rearward path introduces more chain growth, and hence the feedback and suspension locking behaviour when pedalling mentioned in Myth #2. Because of this, axle paths generally tend to be rearward initially, but then become more vertical as you go deeper into the travel; many often end up moving forward at the end of the stroke.

A single pivot design can only give a completely circular axle path, with the centre of the circle being the main pivot. This doesn’t mean, however, that they cannot have a rearward/upward path; the axle will always move backwards whilst it is below the height of the pivot, will be moving vertically at the pivot height, and then begin to move forward above this point.

Whilst it is true that multi pivot bikes have the potential to have axle paths that vary greatly from this circular arc, the reality is that they very rarely differ to any significant extent, especially for horst-link style designs. These, in particular, have paths which usually vary by less than 3mm from the completely circular path of an equivalent single pivot; in functional terms, they’re essentially the same animal. Even with some short-link four-bar designs (DW, Maestro, VPP, etc) having somewhat more varied paths, the actual differences in axle path are very small, and are certainly not enough to make or break your riding experience. Again, be sceptical of claims of a ‘perfect’ axle path by any specific design; suspension performance will always be a compromise between bump absorption, pedal feedback, stiffness, weight, and other performance. Also, be aware of qualifying words in marketing claims; given the relatively small variation in all axle paths, statements like ‘nearly vertical’ can apply to virtually any design on the market.

The Logical Conclusion

So in the end, what we’re saying to you is this; there are great single pivot designs on the market (just ask the Athertons), and there are great multi pivot designs out there too (Mr Hill can suggest at least two). Suspension performance is not based inherently on the type of design, but in its execution; any suspension type can be made to work as well, or as badly, as any other. So when you’re out looking for your next dual suspension purchase, don’t disregard a bike just because it uses a simpler design, and don’t get too caught up in the marketing hype surrounding the ‘next big thing’ in suspension. Look at the geometry, the build quality, and the intending use of your new bike, and above all else, take it for a test ride! The proof is always in the pudding, and all that sales spiel will fade to insignificance once you hit the trail. And when someone tries to tell you that ‘system X does this’, or ‘pivot point Y is the best’, just have a little chuckle to yourself, because you know better.

References & FurtherReading

For those interested in delving further into the science of suspension performance, we highly recommend a paper called ‘Path Analysis’ by Ken Sasaki, upon which this article is largely based. Although nearly a decade old, it is still probably the most detailed and objective analysis of MTB suspension behaviour available. You can find it at http://raystrax.com/PathAnalysis/index.htm. Likewise, we also recommend the suspension simulation program ‘Linkage’ by Gergely Kovacs, which calculates and shows numerous performance parameters for around 700 different suspension bikes; you can find it at www.bikechecker.com. Enjoy! 

 

 

Floating Bottom Brackets

As mentioned briefly in the intro, the other type of suspension design you’re likely to see around is a Floating Bottom Bracket (FBB) design. The most common ones are the GT I-Drive and Mongoose Freedrive, both of which are based on the same patent, but have differences in execution. The somewhat rare Maverick bikes also use a FBB, and the new 2011 La Pierre DH Team is also running a variation of a FBB.

By definition, a FBB design is one where the bottom bracket is neither on the front nor the rear triangle, and instead is mounted on a separate link which is free to move (or float) independent of the two main halves of the bike. The general aim is to have the bottom bracket move rearward as the suspension compresses; this reduces chain growth and its negative side effects somewhat, allowing for a more compliant, rearward axle path. In fact, FBB designs are capable of having axle paths that would, in practice, be impossible for any other viable suspension design. Because of this trait they tend to be exceptionally good climbers, especially over stepped, square-edged terrain, where the rearward axle path allows them (and the rider) to maintain forward momentum more easily.

As with any design, however, they are not without their drawbacks. Firstly, they tend to require a lot of pivots in a very small area around the bottom bracket, so stiffness and/or weight can be an issue. Secondly, if the bottom bracket moves a relatively large amount (the Freedrive for example) there is a somewhat strange sensation when riding over bumps—it’s something you get used to quickly, but not to everyone’s taste. Also, if the bottom bracket moves a lot it means that when standing on the pedals the rider is somewhat connected to the swingarm; this means that whilst the bike will still absorb bumps and maintain momentum well, more of each impact will need to be absorbed by the rider’s legs. In other words, they don’t feel as plush as other designs. If the amount of bottom bracket movement is kept limited in order to avoid the factors mentioned above (e.g. I-Drive), then there will be little difference between the particular FBB design and a single pivot with the same main pivot location. As with all designs, it’s a matter of execution and personal preference more than anything else.