Historically, mountain bike design has made uses of coil springs to provide bikes with suspension, however, today, more modern mountain bikes use air forks and shocks, where pressurized air acts as a spring to absorb impacts on the trail. But like mountain bike tires, air sprung suspension needs to be maintained at a certain pressure or PSI level to work optimally. And how do you adjust the PSI of your air fork or shock? A shock pump is the answer.
Understanding your suspension setup
The suspension on mountain bikes, including the front fork and rear shock, use one of two mechanisms to allow them to absorb impacts out on the trail. The two different mechanisms are coil springs and air springs.
Coil spring
Coil spring suspension makes use of a steel coil that provides even compression throughout, giving it a consistent feel and allowing it to absorb impacts consistently over the full range of travel of the suspension.
If suspension is found to be too soft or firm for a particular rider, it can be adjusted by replacing the steel coil spring with either a softer or firmer one.
Air spring
Air springs work differently than coil springs because rather than using a steel coil spring to absorb impacts, they make use of pressurized air in chamber to achieve this.
Where coil springs have an even, consistent compression rate (meaning it doesn’t get any harder or easier to compress the suspension the further into the travel you go), air springs have a variable or progressive compression rate.
What this means in layman’s terms? That air spring suspension gets stiffer the more compression that is applied to it.
Air springs are lighter weight than coil springs, which is a benefit in making your ride more efficient and less of a burden to pedal up hills.
There is something to consider on air springs though that is not a concern with coil springs: the amount of air pressure they have. Less air pressure in your air sprung fork or shock means it will be easier to compress and make the suspension more plush.
Higher air pressure will make your suspension harder to compress and give it a stiffer feel. Higher air pressure will also make it less likely you bottom your suspension out on big impacts.
The air pressure in your suspension, both front and rear, can change over time, similar to how your tires need air added to them every so often. This is where a shock pump comes in.
What is a shock pump?
A shock pump, as the name suggests, is an air pump specifically designed to add air pressure to your suspension when setting it up for the first time or adjusting it.
If you want your suspension setup to be stiffer, you’ll want to add more air to increase the air pressure, and softer suspension will require less air.
Shock pumps come with a built in gauge to allow you to measure the air pressure reading to more accurately set your suspension up.
An important question though is how much air pressure is right for you? That’s where the amount of sag your suspension has comes into play.
Setting your sag
Sag when referring to a mountain bike’s suspension basically means the amount the fork or shock compresses with the rider’s weight on the bike.
Sag is an important concept to understand if you want to make sure your mountain bike’s suspension is set up right for you.
As a rough guideline, when set up correctly, your suspension should sag about 20 to 25 percent, meaning with you on the bike (but not pushing down or purposely compressing the suspension), both the front fork and rear shock should compress about 1/5 to 1/4 of the way down.
To be clear this is simply a guideline, and to be more precise its recommended you find out the manufacturer’s suggested settings for your weight as a starting point.
Beyond the guideline we give here and the manufacturer’s recommendations, how much sag and consequently how much pressure your air sprung suspension should have will also depend on your own preferences.
Do you like a soft, buttery feel that soaks up anything and everything you hit on the trail? Or do you prefer a firmer feeling suspension setup that isn’t the most plush but results in less pedal bob and holds up better on hard impacts?
Your riding style and the types of trails you like to ride should dictate which way you lean in your suspension setup, whether that be erring on the stiffer side, or keeping the pressure lower for a softer feel.
How to use a shock pump
To use your shock pump to set up or adjust your air sprung suspension, there are a few things you need to keep in mind.
First, the shock pump itself. Most shock pumps are fairly similar, they consist of a handheld pump, an attachable hose, a gauge to tell you the PSI and a button that controls the bleed valve to let air out if the pressure is too high.
Shock pumps and suspension use a schrader valve, which is the same type of valve classically found on bike pumps (although most newer mountain bikes have tires that use a presta valve).
Air fork
For your front suspension, most forks have a label on the inside of them towards the tire that give a range of pressures that work for different rider weights. If not, you can always check the manufacturer’s website for the information.
Once you know the correct pressure, you will need to find the valve where you can attach the shock pump to add air to the fork. Most will have some sort of indicator on them where the valve is, and once you locate it, simply thread the shock pump hose into the valve stem until you see the PSI reading on the shock pump gauge rise, which lets you know that the pump is properly attached.
From there, just pump until you’ve reached the desired pressure reading on the shock pump’s gauge.
What happens if you over inflate your fork? No worries! Simply press the “bleed button” on your shock pump and slowly let air out until your fork is at the correct PSI.
Rear shock
Adjusting the rear shock pressure and adding air to the rear shock is a very similar process to what we just went over with the fork.
You will need to figure out the correct pressure for your rear shock, and then locate the shock valve to attach the pump to. Once you thread the pump into the shock and see that the pressure gauge is registering a reading, you’ll know that the pump is properly attached and its time to start pumping.
Pump until the PSI gauge shows that you’re at the correct PSI for you. Again, if you add too much air to the shock, simply press the button on the shock pump to allow air out of the bleed valve.
As a side note, be careful to not pull on the valve stem while adding air to your shock because they can sustain damage from being pulled on. Just make sure you keep your hand steady and close to the rear shock while pumping.
Difference between a shock pump and regular bike pump
One for high pressure, one for high volume
The biggest difference between a shock pump and a regular bike pump? A regular pump (tire pump) is for high volume, whereas a shock pump is designed for high pressure. Mountain bike tires have a lot of air in them, but at a relatively low PSI, and air suspension has a much lower volume of air compared to tires, but at much higher pressures.
The typical PSI for mountain bike suspension will often be over 100 PSI, and sometimes much higher than this depending on a rider’s weight. By contrast, mountain bike tire pressure will typically range from 15 to 30 PSI.
Valve stem seal
The other key difference between a shock pump and a tire pump is the valve stem seal. A shock pump creates a seal with the valve stem before it opens the valve, which prevents air loss when you remove it.
Because of the small volume of the air chamber in air sprung suspension compared to the volume of air in a mountain bike tire, the amount of air loss that a tire pump would allow if used on suspension would be more than what’s acceptable.
Accuracy of shock pump gauges
The accuracy of shock pump gauges can vary, with some being more accurate than others. Generally the more accurate gauge you have, the more expensive your shock pump will be. But most shock pumps are relatively accurate within probably 3 to 5 percent.
Having that extra level of accuracy is nice, but using the same shock pump each time you adjust your suspension will allow you to have it set up consistently. There are also other ways to gauge if your suspension is set up properly for you, like the sag we discussed earlier.
How to adjust your suspension’s air pressure if you don’t own a shock pump
If you don’t own a shock pump but have a mountain bike with air suspension, you have a couple of options. Shock pumps are relatively affordable if you look around your local bike shop or online.
As an alternative, your local shop will be able to adjust your suspension for you, and they should also be able to give you good advice as to what PSI will work best for you personally.
Frequently asked questions
No a tire pump does not work as a shock pump. Air forks and shocks require high pressure that a tire pump doesn’t deliver. Beyond this, a tire pump does not seal the valve stem before it opens and as a result allows an unacceptable amount of air to escape from the low volume cylinders of air suspension.
Yes, a shock pump is needed to add pressure to your air suspension, whether that be an air fork or shock.
The gauges of most shock pumps are relatively accurate. There can be a three to five percent variance on lower end shock pump gauges, but they still get your suspension’s pressure approximately where it needs to be.
More expensive shock pumps usually come with more accurate pressure gauges.
No, if your mountain bike uses air sprung suspension, you absolutely need to have air in it. Without air in the suspension, you will not have any support or “spring” in the shocks, and are likely to damage your fork and/or rear shock.
A small amount of air loss does occur when attaching a shock pump, and you’ll hear a hiss when attaching it. This “charges” the shock pump and allows you to see the air pressure of the shock.
When detaching a shock pump, the hiss you hear is air leaving the hose of the pump, not the suspension. Which means whatever reading your gauge says prior to removing the shock pump will be the PSI of your suspension after removing it. Therefore there is no need to “overfill” your suspension with air.
Conclusion
Shock pumps are an important tool for any serious mountain biker that has air sprung suspension on their ride. Understanding how to use a shock pump and set up your suspension properly goes a long way to making your ride fun, smooth and safe.