13 Oct, 2016

Suspension Diaries: The Sag Test


Suspension on bikes always seems to be an area clouded in mystery, the one thing most bike shops tell customers not to play around with – if something goes wrong, it’s usually a costly, frustrating exercise to fix. In the next three articles I hope to clear up the mysteries of suspension and at the same time give you some handy tips on how best to look after your shocks. – BY THE GENTLEMAN RACER

One of the typical questions I get asked all the time is, “What pressure do I put in my fork/rear shock if I weigh 78kg?” There are many formulas and equations available, but I find they only cause confusion. Instead, there is a brutally-simple method to ensure correct air pressures. I’m sure you’ve noticed the rubber O-rings on one of your fork legs (a red one on the right-hand side of a Rockshox, a black one on the left-hand side of a Fox, Suntour or Magura) as well as on the shaft of your rear shock. These rubber bands are what you’ll use to set up your shock to the correct air pressure. Welcome to the ‘sag set-up’ method.

When correctly set-up, a bike will ‘sag’ under the rider’s weight, and how much a bike sags tells us whether there is too much or too little air pressure. The correct range that a bike sags by depends on the bike’s travel, how much the shock moves, and what the bike is being used for, i.e. the style of riding. Most cross-country bikes have 100mm of travel and a sag range of between 10% and 20%. This range of the suspension’s travel is a good guide to work out what the correct pressure of your fork should be.

Front First

Let’s start with your front fork: As most forks are “telescopic,” you can simply measure sag directly by how much the O-ring moves. To do this, make sure that the fork is not locked out and then make sure that the O-ring is pushed right down, flush against the dustwiper seals, the rubber seals that seal off the lower legs of the fork from the upper tubes (stanchions). Next you climb onto the bike, preferably in the kit that you’ll ride in, and it’s a good idea to have someone holding the bike so you can get onto it and put your full weight on.

Then it’s a case of simply climbing off the bike, as lightly as possible, and having a look at how much the fork ‘rises.’ You’ll see a measure of exposed fork stanchion between the O-ring and dustwiper, and that is your sag measurement. You’ll know you’re in the right range, air pressure-wise, if you can see between a 10mm and 20mm gap between the O-ring and the dustwiper. Less of a gap means there is too much air pressure, while too much of a gap means too little.

Back Next

The exact same method can be used for a rear shock. Just bear in mind that when they sold you your bike, they inevitably told you that you have 100mm of rear suspension, but this actually has nothing to do with your rear shock, it’s actually how much your rear wheel travels. Nevertheless, fear not, because I am going to explain how to measure your pressure correctly, and the good news is that it’s just as easy as your front.

In this case you need to know the “stroke” of the shock – thankfully pretty much all 100mm-travel cross-country bikes nowadays use a 138mm eye-to-eye length shock with a 38mm stroke. So to be able to measure what the correct pressure should be, you need to ascertain what 10-20% of 38mm of stroke will be. Quick answer is 4-8mm. However, I personally don’t always agree with this measurement, because sometimes the bike will ride harshly over stutter bumps. From my experience, I generally recommend a sag measurement of slightly more, working between 8-12mm, and I find that the shock then feels more ‘in the spring.’

This is further complicated by variances like how ‘hard’ the shock is driven by the linkage – engineers play around with leverage ratios to give the desired effect – and riding style, so if in doubt, look for the ideal, a bike that uses its full amount of travel without ever ‘bottoming out,’ i.e. blowing through its travel and becoming a rigid bike! I try to run the minimum possible air pressure in order to achieve this. More pressure than what is needed gives a bike little ‘small-bump sensitivity’, making you feel like you are playing a game of ping-pong with every little stone or root.

Most manufacturers have set-up guides and troubleshooting advice available on their sites and I’ll elaborate further in the next article, but hopefully this is the start of you learning more about your suspension.Until next time, don’t be afraid to experiment, because there is no ‘correct’ pressure, only a range that’s right for you and your bike. Shiny side up, rubber side down.