Suspension Terminology - Stop Making Nonsense
With the burgeoning number of full suspension bikes on the market these days there are a huge variety of terms that get thrown around by marketing heads, reviewers, forum-dwellers and riders, allegedly with the intention of describing and analysing the way a particular bike behaves or feels to ride. For the uninitiated, some of these terms might as well be in a foreign language for all the sense they make.
To make matters even worse there’s an overabundance of words that are either misused, misunderstood, or too vague to actually convey anything tangible anyway. In order to help you see through the often nonsensical jargon and extract meaningful information from someone talking about sagging from eye-to-eye whilst post mounting their damper to a full floater, we’ve compiled this not quite comprehensive list of common terms, as well as a not always serious description of what they do, could, or should mean.
Air Can or Air Chamber: The part of an air-sprung suspension unit into which the air is compressed by the piston. The higher the air volume, the more linear the suspension unit will feel; the smaller the volume, the more progressive it will be.
Air Spring: The spring formed when an airtight piston compresses air in a sealed chamber or air can. For everything but dedicated DH bikes air springs have all-but replaced coil springs as they are lighter and the spring rate can be altered by simply adding or releasing air. Air springs are inherently progressive, however by altering the size of the air chamber and/or how far the piston pushes into it, they can be made to feel more or less linear.
Anti-Squat: Not leg extensions at the gym, but rather the ability of a suspension system to resist the tendency to compress when you accelerate forward. Suspension ‘squat’ is caused by the rider’s weight moving rearward as the bike accelerates forward underneath them (because you are not a rigid, fixed part of your bike); this rearward weight shift is generally proportional to the degree of acceleration, so the faster you accelerate, the more your weight shifts rearward, and the more your suspension squats under power. This is one of the main contributing factors to energy wasting, rider-induced ‘pedal bob’, or the tendency for dual suspension bikes to move up and down as you pedal.
Some dual suspension designs overcome this phenomenon by using heavily a damped rear shock that resists compression; however this adversely affects bump absorption, even when the rider isn’t pedalling. There are also shocks that overcome this tendency by using an inertia valve design which has its own pros and cons. Many manufacturers design their bikes so that chain tension from pedalling tries to extend the suspension, and this force resists the squatting effect (hence the term ‘anti-squat’). The biggest benefit of using chain tension to control squatting is that the two opposing forces are proportional, so when you accelerate faster the anti-squat force is also higher. Additionally, this means that the shock doesn’t have to control pedalling forces, and can have a softer compression tune and hence be more supple and absorb impacts better.
The downside is that the anti-squat effect provided by chain tension will vary at different points in the travel, and will also vary with each different chainring/cassette cog combination. Because of this, dual suspension bikes will be optimised for a particular gear combination and percentage sag; the further you go from this ‘balanced’ situation, the more either suspension squat or extension will overpower its opposing force. As chain-induced anti-squat inherently relies on a degree of chain growth, at its most unbalanced this method will result in high levels of pedal kickback with a noticeable stiffening or lockout of the suspension.
Axle Path: Exactly as it sounds, this is the line or path that the axle follows as it moves through its travel. All axle paths are, overall, simple C-shaped curves, with the short-link four bar designs potentially showing the most variation from completely circular, and single pivot designs (linkage driven or otherwise) being absolutely circular.
All rear suspension designs can deliver paths which are either predominantly rearward, vertical, or forward; which of these occurs depends entirely on the precise location of the pivot points. Marketing departments often stretch the truth here;Santa Cruzonce claimed that their VPP axle paths were S-shaped but have since admitted that this was exaggerated, and others use qualifying terms like ‘nearly’ vertical, which could be applied to virtually any bike’s axle path due to the limited variation across all designs.
It is widely accepted that predominantly rearward axle paths offer the best bump absorption, however they also lead to increased chain growth and hence pedal kickback. Forward axle paths limit these two negative effects, but tend to produce less anti-squat when pedalling, and suspension performance tends to suffer, especially on mid-to-large square-edged impacts. Because of the need to balance these two conflicting characteristics, most real-world axle paths tend to fall into a fairly narrow design window, with different manufacturers (and sometimes different bikes within a brand) prioritising one or the other of these performance parameters.
Bearing: The majority of suspension pivots use sealed, radial cartridge ball bearings to allow movement between the links; these are just like the bearings in a skateboard wheel, only (usually) bigger. Some high-end bike designers use double row (i.e. two sets of balls in one casing) bearings, and/or angular contact bearings (where the inner and outer races are at an angle to the plane of rotation) to further increase durability and resistance to lateral forces. Another excellent variation for suspension use is full complement bearings (sometimes calledMAX bearings), where the internal spacing cage is removed to allow more balls to be added, hence decreasing the load on each individual ball. Although not technically ideal for oscillation applications, ball bearings have proven over time to be an effective solution and they usually keep the suspension components moving freely for extended periods. And that’s enough talk about balls…
Bottomless: Whilst nothing at all to do with how you feel after riding a hardtail for three hours on a rutted fire road, this is one of the most overused and potentially meaningless terms in suspension jargon today. Does it mean that the suspension is too firm and therefore all available travel was never used, or full travel was used but without any obvious sense of this occurring? Without a qualifying statement, treat this one with the contempt it deserves.
Bottom Out: Possibly what happens when you ‘step-off’ your bike and rip your knicks or shorts by sliding along the gravel, but more commonly used to describe using all of the available suspension travel. This one also needs some qualification; whilst you don’t want harsh bottom out (in either context), gently bottoming your suspension (feel free to snigger here) is something you should do fairly regularly, otherwise you’re not using all the suspension that your bike has to offer, and which you’ve paid for.
Brake Jack: a) What you shout to your friend Jack when he’s obviously going too fast for the next corner, and will probably take a late exit off the cliff edge; b) A term originally coined to describe the tendency of some early dual suspension designs to extend violently under braking, all too often resulting in a highly unpleasant OTB incident. In modern usage, it usually refers to any highly aberrant suspension behaviour when the brakes are applied, whether extending, compressing, or simply become stiff and ‘locked’.