MOUNTAIN BIKE SUSPENSION 101: Springs (Part 2 of 6)

What is a spring?

The most common springs are exactly what you imagine them to be - coiled up bits of metal that bounce - often referred to as coil springs. By definition, a spring is defined as an elastic object that stores mechanical energy. There are many spring designs (such as leaf springs on certain cars), however in the mountain biking world, forks and shocks are typically air sprung or coil sprung.  

Spring rate

The spring rate is the term used to refer to the amount of force that is needed to compress a spring a given distance.

Coil springs are a great visual example of this - they come in different spring rates, which are measured in the pounds of force to compress the spring one inch (pounds per inch).

If you isolated the spring from a coil shock and noted the number on it, for example 400 lbs, it would mean that if 400 lbs of force was applied to the spring, it would compress one inch, 800 lbs will compress it a total of two inches, and 1200 lbs will compress it a total of three inches. Each additional inch requires an additional 400lbs of force.

An important distinction for coil springs is that they compress linearly, meaning the increment of force required to compress the spring will stay the same as the spring compresses through its stroke, in this example, it will always take 400 additional lbs to compress each additional inch. An exception to this would be progressive springs. 

Air springs however, don’t work that way, they are considered naturally progressive. Progressive means that the spring rate is higher towards end of the travel than it is earlier in the travel, in other words, it's generally softer at the start and firmer at the end.

As air shocks compress, pressure builds up naturally as space begins to run out, that is, the volume in the air spring will decrease, subsequently increasing the shock’s spring rate. So in an air shock, it might take 400 lbs to compress the first inch, 425 more lbs to compress it from one to two inches, and 475 more lbs to compress it from two to three inches. These numbers are made up, and depend on the size of the air chamber (which can be reduced by adding volume spacers), and how much air you pump into it. In other words, an air shock will be typically harder to compress as it gets deeper into its travel (ie progressive). To what extent it is progressive depends on its specific configuration.

What is Sag?

Sag is the amount your suspension compresses under your own weight (plus your gear) when you’re just standing on the bike, and having it properly set up is the first step to ensure that your suspension is neither too stiff nor too soft, putting you in the ideal riding position to engage the trail.

Sag is an indicator of a ballpark appropriate spring rate because it tells you how stiff the spring is relative to your weight. It’s the preferred method of getting started with shock spring rates or air pressures. It is hard to measure consistently with forks though, because friction and body position can cause it to vary a lot even if you haven’t changed anything.

Setting up your suspension with an appropriate amount of sag will ensure that the bike is balanced front to rear. Too much sag may cause you to bottom out constantly, and throw off the bike's balance / geometry, making it more difficult to corner, jump etc. Too little will feel extremely stiff and uncomfortable, and you won’t get the most out of your suspension performance. There isn't a "correct" amount of sag, there's an appropriate range for each individual case. 
 

Many frames will have a recommended amount of sag, as ultimately the manufacturer will have reasons to why they have designed with a particular ride quality in mind. We won’t go into the specifics of these terms, but some bikes are designed to be softer and more forgiving and others are designed to feel firmer and more responsive to rider input.

If there is no recommendation, there is no hard and fast rule for how much sag you should run, as each rider has their own preferences, expectations, riding style, terrain, etc. However here are good ranges to stay within:

• For the rear shock - 20% - 35% sag is typically recommended. The recommendations are usually provided by the frame manufacturer for that particular frame.
• For the fork- 15% - 25% sag is typically recommended. Be aware that fork sag is hard to measure - usually the manufacturers of the forks have good starting points for air pressures, and these are usually printed on the back of the fork leg for Fox and Rockshox forks.

For the most part, this can be used across the board for trail bikes and downhill bikes. Again, keep in mind you may want more or less sag depending on your bike, weight, and riding preferences. For example, if you’re into cross country riding, some may want to compromise on suspension performance for increased pedaling efficiency - but we won’t go into that. It is important to remember that recommended starting points are just that - starting points - so experiment with it yourself and find what works for you. 

Setting Sag - Air

Measuring sag on an air fork or shock is relatively simple as they usually have a small rubber o-ring known as a ‘sag indicator’:

1. Gear up as you would for a ride, find a friend to help you
2. Set your bike up somewhere level where you can put your full weight on the bike while balanced / get a friend to sit on the front wheel to hold the bike upright for you
3. Step onto the bike and ensure the shock or fork is in the wide open position / not locked.
4. Give it a bounce or two to overcome some of the suspension stiffness & ensure equalization. Don’t touch the brakes while doing any of this!
5. Move the o-ring to the bottom of the stanchion or air shaft as a placeholder. 
6. Slowly step off the bike - ensuring not to compress anything further, and measure the displacement of the o-ring
7. Do the maths (maths is hard, but makes for great suspension)
   a) [Sag amount / total travel of fork] * 100
   Eg. [25mm / 160mm] * 100 = 15.625%
   b) [Sag amount / total stroke of shock] * 100
   Eg. [15mm / 60mm] * 100 = 25%
8. Adjust your shock or fork's air pressure if you want to run more or less sag by inflating / deflating. Repeat the steps above to get a new measurement - ensuring that you are bouncing on it a few times after adjusting air pressure in order to ‘equalize’ the air in the positive and negative spring chambers.

Setting Sag - Coil

Measuring sag on a coil fork follows a similar procedure to an air fork, however accessing the shock stroke measurements can be slightly more difficult.

1. Gear up as you would for a ride, find a friend to help you
2. Set your bike up somewhere level where you can put your full weight on the bike while balanced / get a friend to sit on the front wheel to hold the bike upright for you. Make sure not to touch the brakes!
3. Step onto the bike and ensure the shock or fork is in the wide open position / not locked.
4. For a fork:
   a) Move the o-ring to the bottom of the stanchion as a placeholder. If you do not have an o-ring, a zip tie will also do the trick.
   b) Slowly step off the bike - ensuring not to compress anything further, and measure the displacement of the o-ring
   c) Do the math (math is hard, but makes for great suspension)
        [Sag amount / total travel of fork] * 100 = Sag %
        Eg. [25mm / 160mm] * 100 = 15.625% 
   
   
5. For a shock:
   a) Measure the eye-to-eye distance whilst you are on the bike
   b) Slowly step off the bike - ensuring not to compress anything further
   c) Do the math
   [Regular (fully extended) eye-to-eye measurement] - [Sagged eye-to-eye measurement] = Sag amount
   Eg. [230mm] - [215mm] = 15mm
   [Sag amount / total stroke of shock] * 100 = Sag %
   Eg. [15mm / 60mm] * 100 = 25%

6. Adjusting the spring rate to get a specific sag measurement means changing the physical spring. Many manufacturers have different spring rates available in either 25 lbs or 50 lbs increments. Though this may be one downside to running a coil, there are also many benefits also. It is also a common misconception that tightening the preload on the spring will make it stiffer. It does affect sag, but it does not affect spring rate. While preload is critical on motorized vehicles (because 2mm of ride height change on a race car is a very big deal), on mountain bikes it should be kept to a minimum.

While sag is an important first step to setting up your suspension, tire pressure and your compression / rebound damping adjustments can also make or break your setup. We’ll be covering those in the coming articles.

Pressure vs volume spacers

Volume spacers or tokens are a useful tool that allows you to customize the spring rate of your air suspension. They are designed to reduce the amount of air volume in an air chamber, and can be added to most air forks and air shocks. 

By adding tokens and reducing the amount of space available, the air pressure will ramp up more quickly, making it less likely to bottom out. If you’re a heavy rider and are constantly bottoming out, a volume spacer or two adds the support you need. This should only be considered once you have set up your sag correctly, and should not be used in the instance of bottoming out constantly due to incorrect initial pressures.

By removing tokens and increasing the amount of space available, the spring rate is softened and the amount of force needed to achieve a bottom out is reduced. For a lighter rider that struggles to use all of their suspension, removing a token may help you take advantage of that bottom end of the travel.

Air vs Coil 

There are many factors to consider when choosing to go air or coil for your fork or shock. 

Forks

While mountain bike forks in recent years have typically been air sprung due to their light weight and adjustability, coil springs offer a more linear spring rate and lower friction. Whatever your choice may be, if you’re looking to turn your air fork into a coil, we make a conversion kit here.

Shocks

Choosing an air shock vs coil shock for your bike becomes a little more complicated as you’ll need to factor in your frame’s design. You can read more about that here.  

We’ve summarised the pros and cons of each below:

Next:

Part 3/6 - Dampers

Part 4/6 - Understanding Rebound Damping

Part 5/6 - Understanding Compression Damping

Part 6/6 - Servicing & Maintenance