Recently I made the case
that more is more when it comes to suspension travel. But it may surprise you to learn that many bikes don't actually offer the travel they claim. Some have more travel than advertised, but more often they have less. Sometimes quite a lot less.
I must give credit to Alan Muldoon of MBR magazine for drawing my attention to this. Never content to take any bike brand at their word, he's been measuring the travel of test bikes for years. Because some of the bikes he measured were way off-spec, he convinced me to start doing the same in 2018. Two of the first bikes I measured were the 2019 Specialized Enduro and Scott Ransom. The Enduro had a claimed 160 mm of travel, but my test bike with its Fox DPX2 shock was delivering just 148 mm; meanwhile, the Ransom claimed 170 mm but delivered 173 mm. So, instead of a 10 mm travel difference, the bikes had a 25 mm gulf between them. I'd already noticed the Ransom far outperformed the Enduro in the rough, but was more imbalanced unsupportive in the corners. Measuring the travel provided a credible reason why. Measuring travel
I'll admit that measuring travel isn't precise. I estimate my method has an error of +/- 3 mm, but that's good enough to spot when a bike deviates significantly from the claimed travel. I put the bike upside-down on smooth, level ground, adjusting the dropper post until the axles are level, then measure from the floor to the centre of the axle. Next, I deflate the shock and thread a ratchet strap from the bottom of the rim to the seat tube. Ratcheting it tight while allowing the remaining air to escape the shock, I can be sure it's fully bottomed out. Then I measure the distance to the floor again and take the difference.
Of course, you could use linkage software to calculate the travel, but this assumes you know the real-world stroke and eye-to-eye length of the shock and a lot of the variation in travel comes down to this.Why travel can changeMarketing
Customers often shop for bikes by travel, so brands might occasionally stick a badge on a bike that fits it neatly into a particular travel segment based on intended use, rather than its actual travel. One brand told me off the record that they once had a bike that measured up at 158 mm travel but they sold it as a 150 mm bike because that's what customers wanted. More common is just to round up to the nearest 10 mm. Compromise
Frame designers have to balance many considerations when designing a suspension linkage: weight, cost, ease of manufacturing, reliability, progressiveness, standover clearance and (most importantly of all) how many water bottles
can fit around the shock. Stipulating that the bike must deliver exactly
X mm of suspension travel is an unnecessary extra constraint. If moving the shock mount a few millimetres will make it less likely to break, or better still, allow a water bottle to fit, then who cares if the travel figure changes a bit?Shock stroke
Most bikes have a leverage ratio of between 2:1 and 3:1, meaning for every millimetre the shock moves, the axle moves 2-3 mm. So if a shock measures 1 mm short on stroke length, the travel will drop by as much as 3 mm.
Fox say their shocks are built to a tolerance of +/- 1 mm on stroke length, but in the real world, it's often difficult or impossible to access every last millimetre of that stroke. Modern air shocks are designed such that the negative air spring prevents it from topping out harshly when riding and helps improve the off-the-top sensitivity. But in some cases, this can make them unable to fully extend without being pulled by an external force. If you forcefully extend the rear axle or over-inflate the positive chamber, you can usually get these shocks to deliver their full travel, but whether or not you'll ever use those last few millimetres on the trail is another question. Similarly, some modern air shocks have substantial bottom-out bumpers which make it difficult or impossible to fully compress the shock.
Don't get me wrong, shocks that don't fully top-out or fully bottom-out are undoubtedly a good thing on the trail. But depending on how you measure it, the "usable stroke" can be as much as 2-3 mm less than the fully-extended stroke length in the most extreme cases, corresponding to 5-8 mm less travel.Geometry adjustments
Some bikes have adjustable chainstay lengths or longer chainstays for larger sizes. Increasing the chainstay length usually increases the length of the effective swingarm, which means more travel. Roughly speaking, for every 10 mm increase in the swingarm length, suspension travel will increase by about 2-2.5%, so about 3-4 mm for a 150 mm bike. Not a huge amount, but for bikes where the chainstay length can vary by as much as 20 mm, it's worth considering. Note this isn't true for brands like Forbidden where the change in chainstay length is achieved by moving the bottom bracket relative to the main pivot, not by changing the swingarm length.
The leverage ratio for the 2019 (non-high-pivot) GT Force, courtesy of Dan Roberts. In the low setting, the leverage ratio is slightly lower meaning less travel, though not by much in this case.
Less obvious are flip chips at the shock eyelet, which are designed to raise or lower the bottom bracket height. Flipping these can change the average leverage ratio and therefore the amount of travel that's extracted from the shock. Ironically, the low/slack setting, generally considered better for descending, usually offers less travel. The difference is usually only a millimetre or two, but in some cases where the flip-chip is mounted on a short rocker link, it can be much more. Axle path
Does this matter?
Axle path diagrams are often shown with a compressed scale. This makes it look as if the forward or rearward component of the travel contributes significantly to the overall distance the axle moves along the perimeter of the arc, or diagonally from the start point to the endpoint, as opposed to the vertical component of the travel which is normally quoted.
But if the axle path is drawn to scale, like in the diagram opposite, it becomes clear that measuring travel vertically or diagonally doesn't make much difference. Even for a bike with an extremely rearward axle path (a Forbidden Dreadnought in this case) the difference is only about 3 mm. For bikes with conventional axle paths, the "diagonal travel" is usually around a millimetre more than the vertical travel.
Yes and no. Though important, suspension travel is far from the only thing that affects suspension performance. Ultimately, if a bike rides well it rides well, whether or not the travel matches what's written on the frame badge. Besides, I don't want you to think that most bikes are centimetres off the quoted travel figure. Most I've measured are within a few millimetres - close enough that any difference could just be a measurement error and certainly not enough to worry about.
But just as riders want to know exactly how much a bike weighs, they should know how much travel it's got. In those (rare) cases where a bike has notably less travel than claimed, it can help to explain why it rides the way it does. It's a bike review cliché to say a bike feels like it has more or less travel than it claims to. In some cases, that's because it does!