The Biking Hub: Mountain

MTB 101 - Gear » Cory

An introduction to mountain bike rear shocks

So you’ve made the plunge and purchased a full suspension mountain bike. You’re excited to get out on the trails and go faster than ever before but, on your first ride, the bike doesn’t feel the way you think it should. At this point it’s important to realize that dialling in your rear shock isn’t an exact science and you should play around with some of your shock’s settings and make notes about what you like and what you don’t. If you’re not happy with the feel of your shock after you’ve done this, then it’s time to take a few additional steps.

If you purchased your new bike from a local shop, you should take it back and explain what you like and what you don’t about the overall ‘feel’ of the suspension. Was it too mushy? Or did you feel like you were riding a pogo stick? An informed shop employee can use this information to help you get your rear shock ‘dialled in’ so you can get the most out of your new ride. If you purchased a used bike, you can still go to a local shop for some well-informed advice, but expect to pay a little for the worker’s expertise. Remember that they’re taking valuable time out of their day to help you get going on a bike that they didn’t sell to you.

Regardless of where you purchased your bike from, it never hurts to go into a shop armed with the basic information I mentioned above and to know a little bit about some of the key terms that will be used when discussing your suspension setup. This article is meant to serve as a primer that brings you up to speed on the terminology of mountain bike rear shocks and, while it’s not a definitive guide, it’ll help you on your way and get you closer to dialling in your bike’s rear suspension in a way that you’re happy with.

Types of rear shocks

There are two primary types of rear shocks that are used on modern mountain bikes: Air shocks and coil-over shocks. Typically, you’ll find air shocks spec’d on cross country, all mountain, and some freeride bikes, with coil-over shocks being spec’d on some all mountain, many freeride bikes, and almost all downhill bikes.

Air shocks. Some of the more popular types of air shocks include the Fifth Element CV/T, the Fox Float RP3, the Fox DHX Air and the Manitou Swinger 3-Way Air, and the Rock Shox Pearl. These shocks are lighter than their coil-spring counterparts and extremely ‘tunable’ but, in general, are less able to deal with the extreme forces generated by freeriding and downhill racing, which is why they’re not usually found on this type of bike.

Each of the shocks mentioned above uses air to simulate the springs that are used on coil-over rear shocks. While each shock use air different volumes, the air settings, typically measured in PSI, are used to set the sag of your shock, something that’s important to achieve the type of ride you’re looking for (see below).

Coil-over shocks. Some of the more popular names for this type of rear shock include the Fox Vanilla, the Fox DHX, the Progressive Fifth Element, the Manitou Swinger coil-based shocks, the Manitou Revox, and the Marzocchi Roco. Other popular brand names include Romic, Avalanche, and Cane Creek.

Typically, coil-over rear shocks are found on bikes that are intended for heavier duty use and, as such, are spec’d primarily on freeride and downhill bikes. They use specially-designed springs that are help set your shock’s sag. Increasingly, coil-over rear shocks are using air to fine tune the ride characteristics of the shock.

Shock stroke

Shock stroke is a measurement of the total travel that the shock moves through when going from full extension to complete compression. Typical shock strokes range from 1.5″ to 3″, although you can find the odd exception out there. Why is your rear shock’s stroke important? In addition to being related to things like shock leverage ratios and rear-wheel travel, shock stroke plays a key role in determining your shock’s sag.

Sag

Sag is a measurement of the amount that your shock compresses when you’re sitting in your saddle in a normal riding position. You can figure out the sag of your rear shock by dividing the amount that your shock compresses when you’re sitting on the bike by the shock’s stroke and multiplying the result by 100. For example, a shock with a 2″ stroke that compresses by .5″ when you sit on the bike has a sag measurement of 25% (.5″ / 2″ x 100%).

While the ‘ideal’ amount of sag for your shock can usually be found in your shock’s user manual, keep in mind that sag settings vary, depending on the type of riding you do and your personal preference.

Spring rate

On coil-over rear shocks, spring rates influence your bike’s sag measurement. These rates are usually found stamped on the actual spring of your coil-over rear shock and are in a format that goes something like 550 x 2.25. The first number indicates the number of pounds of force required to compress the spring one inch; the second figure indicates the stroke for which the coil is intended (ie. 2.25″).

It’s important to select a coil that allows you to achieve the desired amount of sag so you can get the most amount of travel out of your rear shock. A spring rate that is too heavy will prevent you from using all of your bike’s rear wheel travel while one that is too light will result in you blowing through the travel and bottoming out your bike’s suspension more frequently than you’d like.

Be absolutely sure that you’ve got the correct coil on your shock — a spring that is shorter than your shock’s maximum stroke can bind, something that is dangerous and can damage your shock and even your frame.

Rebound damping

Rebound damping controls the rate at which your shock returns to its extended position after full or partial compression. Things like riding style and personal preference influence the way you set up your shock’s rebound setting. Fortunately, most popular rear shocks feature external rebound dials — something that makes it easy to make tool-free adjustments on the trail.

Remember how I mentioned that you should make notes about the things you didn’t like about the ‘feel’ of your bike’s suspension and I made some silly comment about a pogo stick? Well, a rear end that ‘pogos’ usually does so because the rear shock’s rebound setting is too fast — if you slow it down a bit, your bike will ride a whole lot better.

Compression damping

Compression damping controls the rate at which your shock, well, compresses. If you’re into big hit trails that have lots of rocks and plenty of square-edged hits, you’ll probably find yourself running a relatively high compression setting so you don’t blow through your travel prematurely. Similarly, if you ride smooth singletrack you’ll likely run a relatively low compression setting so your shock can help with small bump compliance.

A number of the popular rear shocks out there have both low and high speed compression settings while others have their own names for what, essentially, is compression damping. Compression damping is one of those things that is difficult to explain in words, so if you have any doubts about what it does, don’t hesitate to ask for advice at your local bike shop.

Summary

As I mentioned in the introduction, this article won’t help you become an expert when it comes to rear suspension but it does cover the basics so you can walk into your local shop and talk about your shock without a bewildered look on your face. Remember that setting up your bike’s rear suspension isn’t an exact science so you’ll probably need to be patient and spend some time fiddling around with your rear shock’s settings to achieve the type of ride that you imagined when you made your original purchase.

Also keep in mind that there are a number of proprietary damping and valving systems out there — ProPedal, TPC, SPV — that allow you to fine tune your shock even more minutely. For more information on these systems, refer to your user manual or ask the people who are familiar with the products.

This entry was posted on Wednesday, February 22nd, 2006 at 12:27 am and is filed under MTB 101 - Gear. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.