Can you remember when you last set your rear-shock sag? If you answered yes, then this article may just be confirmation of what you already know. If you answered no, and the concept of sag-setting sounds a bit foreign, then this article will provide the exact knowledge you need in order to get the most from that rear shock on your full-suspension bike.

In almost 30 years of working on mountain bike suspension, Robbie Powell has lived (and worked) through the evolution of rear suspension on bicycles. His business, Robbie’s Bicycle Concept, specialises in servicing and repair of mountain bike suspension so he’s more qualified than most to answer our burning rear-shock questions.

A trunnion-mount shock on a Trek Top Fuel (vertically orientated next to the seattube)

From the outside, most rear shocks look similar. Is this the case or are there different types of rear shock designs? If so, what are they?

From a spring point of view there are air-spring and coil-spring shocks, damping can be either oil or air and there are two main designs, trunnion, and standard mount. In the past the industry flirted with pull shocks, but these have fortunately not been continued.

What is the most common problem you see with rear shocks that come in?

The most common problem would be basic wear and tear from lack of service, but we also see some scratching and damage on the oil body and damper shaft.

What is the best way to prevent this problem?

Stick to the manufacturer’s recommended service schedules and if you ride your bike hard, have it serviced more often. Most of the time the riders don’t know what the service intervals are with the rear shocks, so they only bring them in when the problems become evident.

[As a guide, the two most popular rear suspension brands are Fox and RockShox. Fox recommends a minor service on your rear shock every 30 hours of riding and a major service ever 120 hours or a year, depends which comes first. RockShox recommends a minor service after 50 hours of riding with a major service every 100 hours. Other brands are similar]

All the pieces of a dismantled rear shock being serviced at RBC

How often should mountain bikers check the efficiency and sag of their rear shock?

I check my rear shock sag and feel every weekend before I do a long ride. I always check the sag before I ride to make sure the shock has not lost any air. I hardly ever check the pressure and always use the sag O-ring as the guide to the shock’s efficiency. Most times I see riders becoming more confused by trying to work with shock pressures rather than sag. Depending on the shock size and air pressure, some air will go into the shock pump and will drop the air pressure so it seems like the shock is leaking air but its just used the shock pump as an additional air chamber, so the pressure is lowered.

 

Take for example a 165/38mm air shock, these are typically fitted to bikes with high leverage rates and have high-pressure requirements to overcome the higher leverage so a small air can will hold a lot of pressure. Any added space for the air to expand into will make a significant impact on the overall pressure.

For example, pumping the shock to 250psi then take the pump off and add it again and the pressure will be 235psi because some of the air has gone into the pump. Every shock we service comes with a card showing the length and stroke of the shock and the sag requirements from 10% to 30% in millimetres to make it easier to set up the shocks without worrying about the pressure.

To find out how to set your own suspension, check out Robbie’s video here.

Lee at RBC tests a rear shock during it’s service.

What is the normal duration of riding required before a rear shock needs to be serviced?

It all depends on the type of shock and the type of riding. Bikes with higher leverage ratios and shorter-travel shocks have more heat build-up so require more service. Bikes with longer-travel shocks and lower leverage rations have less of these issues. Air shocks tend to suffer from heat issues so these will require more servicing than coil shocks. Typical issues with shocks will be stiction or changes in the damping characteristics like bouncing or topping out, but can include leaking oil, adjusters not working correctly, squelching noise or leaking air.

What do you do to service a mountain bike shock – what is the process?

The process is dependent on the type of shock, but we do the following:

  • Check the shock for any external damage
  • Note the air pressure and shock settings, release air the pressure, open the aircan
  • Test the shock on the hand dyno to confirm damper issues, move the settings to open
  • Open the damper and check for damage or wear on the piston and shims
  • Confirm the type of service and parts required, quote the customer
  • Strip the shock completely, cleaning all the parts with isopropyl alcohol
  • Rebuild the damper with the seals and parts from the manufacturer
  • Set the IFP height and fill with oil
  • Inflate the IFP chamber with nitrogen to the manufacturers spec
  • Check the shock on our hand dyno to determine that all the functions work correctly
  • Dyno the shock if the customer requested a specific tune.
  • Assemble the aircan seals and fit the aircan.
  • Inflate the shock to the maximin pressure permitted by the manufacturer.
  • Equalise the aircan pressure on the hand dyno
  • Check the shock for air leaks in a water bath then set the pressure back to the pressure that the shock came in at
  • Clean and reset to the customer settings
  • Determine the shock length, stroke and sag settings, fill out the shock card and send to customer

A standard-mount shock on a Norco Revolver (beneath the toptube).

If a rear shock has been damaged, is it repairable?

All depending on the shock type and age as well as the manufacturer, some will support full part replacements and keep redundant stock and others don’t, however most shock parts are available and can be replaced by a suspension service centre. There are a number of aftermarket manufactures that offer support for parts not made available by the manufacturer.

Can local bike shops do a rear shock service?

Local bike stores are the front lines of suspension care and should recommend regular servicing for their customers. Most rear shock aircan services can be done in store with the right equipment, service kits and skills. However damper services and more advanced tuning or changes to the stroke should be done by a qualified suspension service centre.

More suspension-related content here:

EIGHT THINGS SA MOUNTAIN BIKERS SHOULD KNOW ABOUT SUSPENSION FORKS

WHY RBC IS SOUTH AFRICA’S PREMIERE MTB SUSPENSION SPECIALIST

 

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