Everything You Need To Know About Rear Shocks

HOW DO I MEASURE A REAR MOUNTAIN BIKE SHOCK?

It is important to understand that your rear shock size on your full suspension mountain bike is not measured by the amount of rear wheel travel, but by the eye-to-eye length, and the stroke length

Eye-to-eye length is the distance between the two mounting points on the shock, or the rear shock mounts on the frame.

How to measure eye to eye length

Stroke length refers to the total distance the shock can compress. You can measure the stroke length by subtracting the eye to eye length when the shock is fully compressed, from the eye to eye length when the shock is fully extended – you should get relatively close to the shock’s stroke length.  In most cases, a bottom out bumper is incorporated at the end of the shock’s stroke, which helps deter harsh “bottom-out’s” when the shock compresses all the way very hard. That bottom out bumper can affect the measurement you find by a few millimeters. 

eye to eye length
Fully extended
eye to eye length at bottom out
Fully compressed (we recommend slowly letting the air out of your shock with a shock pump)

WHAT IS REAR SHOCK MOUNTING HARDWARE?

The type of hardware we use and install into our Double Barrel shocks is a traditional axle and spacer assembly.  There is an axle which is pressed into the shock mounting eyelet, and spacers that slide over the axle and make up the gap between the frame’s mounting tabs and the shock mounting eyelet.

Rear Shock hardware
Hardware axle and spacers
rear shock hardware
Complete rear shock mounting hardware assembly

A mounting bolt (that comes with your bicycle’s frame) will pass through the hardware axle and thread into the frame or the frame’s linkage, which secures the hardware and shock in place.

rear shock mounting bolt
Mounting bolt holding rear shock mount in place

In most cases, there is a bushing that is pressed into the shock’s eyelet, which the hardware axle is then pressed into. The bushing we use and currently press into our shocks are called Igus bushings, they are on off-white cream color. 

Igus bushing
15mm bushing is pressed into the standard 15mm diameter shock mounting eyelet

 A bushing is needed to allow for some rotational movement of the hardware axle when the rear linkage pivots – If there were no rotation allowed at all between the shock eyelet and the hardware axle, the rear linkage would not move freely and the rear shock would absorb all of the extra leverage, which is not ideal.  Bushings are meant to wear, rather than the alloy material that your shocks mounting eyelet is made of. They are easily replaceable, and we replace them during every in house factory direct service.

HOW DO I MEASURE REAR SHOCK HARDWARE?

Rear shock hardware is labeled first by the width of the hardware axle, and then by the diameter of the mounting bolt. You must confirm the width of the hardware axle in millimeters, (or the distance between the shock mount tabs) and the diameter of the eyelet that passes through the hardware axle (or the diameter of the mounting bolt).

measuring shock hardware width
Width of the hardware axle
bolt hole diameter
Bolt hole diameter

The two images above would represent 24mm X 8mm hardware. 

WHAT IS A 15MM OPEN END EYE?

A 15mm open end eye simply means there is no bushing that is installed into that shock’s mounting eyelet. Industry standard shock mounting eyelet diameter is 15mm

rear shock eyelet
15mm open end eye eyelet vs eyelet with a bushing pressed in

15mm open end eye

 You will use the proprietary mounting hardware that came with your bike’s frame on a 15mm open end eye mount.  When upgrading or swapping your shock, you do not need any additional mounting hardware for the 15mm open end eye side, simply swap the proprietary hardware over to the new shock.

WHAT IS A TRUNNION MOUNT?

A Trunnion mount shock mounts the shock body directly to the linkage – there are two threaded holes on the shock body and linkage bolts thread directly into them.  Trunnion mounts have started to become more popular within the last few years with frame manufacturers. A Trunnion mount shortens the overall eye-to-eye length of the shock while allowing it to have the same amount or more, of shock stroke.  This can be advantageous to frame designers.  

Trunnion mount Air CS
Trunnion Mount shock

No traditional axle and spacer hardware assembly is needed for the Trunnion mount side – you will use the mounting bolts that come with your bike’s frame. However the opposite shock mount may require a traditional mounting hardware assembly, or it may be a 15mm open end eye.

WHAT IS SAG?

“Sag” refers to the amount of suspension travel that is compressed when the bike is on flat ground under rider weight and riding gear.  Setting the correct amount of sag on your suspension plays a crucial role in the overall performance of your full suspension mountain bike. Setting sag should always be the first step in setting up your suspension.

You might be thinking, why is setting sag so important? Sag affects the geometry of the bicycle, and frame designers design frames with geometry and sag in mind – ideally you would want to experience the bike in the way it was intended and designed to be ridden. When you have too much sag, you do not have a high enough spring rate to support your rider weight.  This can result in pedal strikes and the suspension going through it’s travel too easily, and “bottoming out” often, and with rear shocks, this can result in pedal strikes and less pedaling efficiency. 

When you do not have enough sag, or the spring rate is set too high, you risk not utilizing all of your suspension travel, and again affecting the intended geometry and overall performance of the bicycle.  

We recommend first checking with your bicycle manufacturer on the suggested amount of sag they intended for your bike.  If that information is not available to you, we recommend 25-35% sag depending on your shock size and intended use.

HOW DO I MEASURE SAG?

With air shocks, the first step is to charge the air spring (pump air into the air valve) to around 100 psi.  Take your shock pump off and find a place to cycle the shock through it’s travel 15-20 times. This is to ensure that the positive and negative air chambers are equalized.  Put your shock pump back on, then pump the shock up to around your rider weight. Take the pump off and cycle the air through the shock again. Find a flat surface and have your normal riding gear on and get into a ready position, and try not to move.   If you ride on trails that require you to be out of the saddle and in an attack position frequently, set sag when you are in your attack position.  If you are riding on paved trails where you are mostly seated while pedaling, set sag when you are in the seated position.

attack position
Attack Position
Seated Position
Seated Position

Slide the travel indicator o-ring all the way down to the wiper seal:

sag indicator

 Being careful not to bounce the suspension, dismount the bike.  Measure the distance between the wiper seal and the o-ring

measuring sag

Take that measurement and divide it by the shock’s stroke, then multiply that number by 100, and you should have the percentage of sag that is currently set.  For example, on a 210X55mm shock, if you were to measure 13mm of sag, that would be 23.63% sag: 13 / 55 = 0.2363 X 100 = 23.63. In this case, you would want to drop some pressure out of the shock so you could get closer to 25-35% sag. 

HOW DO I MEASURE SAG ON A COIL SHOCK?

With coil sprung rear shocks, adjusting sag and finding the measurement is a little different than air shocks. There is no air valve that you can increase or decrease air pressure into, instead there is a preload collar that you thread up and down the damper body to compress or release tension on the external spring, which affects the amount of sag. It’s important to note: the preload adjustment on coil sprung shocks only affects sag.

cane creek oil IL
Technician threading preload collar

The first step is to confirm the eye-to-eye length and stroke length of the shock.  Then find a flat surface, and hop on the bike with all of your normal riding gear on.  Get into your ready position, and measure the distance between the eye-to-eye length – you will need someone to assist you with this step.

Subtract the eye-to-eye when in the ready position, from the shock’s fully extended eye-to-eye length, and that is your sag measurement.  Just like with air shocks, calculating the percentage of sag is the same – take the sag measurement, and divide it by the shock’s stroke, then multiply that number by 100 and you’ll have your percentage of sag.

Another method: with all Cane Creek Double Barrel coil shocks, you can dislodge the bottom out bumper from the spring clip and use it as a travel indicator o-ring like on an air shock.  Be careful not to scratch the damper shaft during this step.  Slide it up to the base of the damper body, hop on the bike, and measure the amount of distance between the top of the bottom out bumper and the base of the damper body.

Cane Creek Coil IL
Technician sliding bottom out bumper up the damper shaft
Cane Creek Coil IL
Measuring distance between bottom out bumper and damper body

Coil springs are limited to the amount of support or range they have.  For example, if you have a 250 lb rider that is on a spring rate that was intended for a 150- 175 lb rider, there simply is not enough preload adjustment available to be able to get close to an appropriate sag setting. If less than 1 turn of preload is needed to achieve proper sag, you should change to a lower spring rate. If more than 6 turns preload are needed to achieve proper sag, you should change to a higher spring rate.

 

This article might not cover every single rear shock question you might have, but it should hopefully provide you with some answers. All Cane Creek shocks are backed by our commitment to customer service. If you have any further questions about one of our shocks, feel free to contact us.