How to pick the correct springs for you!

What are the springs for?

The primary purpose of suspension springs in any vehicle is to support the sprung mass. In a motorcycle this means the springs job is to support the weight of the bike and you, and this is generally how we work out the right spring.

Springs also impact the performance of the suspension, and its not uncommon for riders to deviate from the recommended spring rate in order to fine tune the bike. With this said we generally find that the dampers have a much bigger impact on performance once the correct springs are fitted.

How do springs work?

Coil springs are made from high-tensile steel wire that undergoes a relatively complex heat treating process to ensure stability and durability.

All motorcycle springs have 4 important specifications, free length, small end diameter, large end diameter and rate.

Free Length

A springs free length is its height when completely unloaded. You need to remove it from the bike to measure its free length correctly as all springs are installed with a small amount of "preload" to ensure the spring doesn't bounce of its seat when the forks or shock fully extend.

Free length is generally measured in mm or inches depending on your region.

The appropriate free length will usually depend on the bike, and has little to do with the rider.

Small & Large Diameter

For forks springs the small and large diameter is measured as the outside diameter at each end and is usually the same (i.e. the spring is usually parallel and not tapered). The outside diameter is important because it needs to fit inside a fork tube.

For shock springs the small and large diameter is measured as the inside diameter at each end and can be the same (parallel) or different (tapered). The inside diameter is important because the spring needs to fit around the shock tube.

The appropriate small and large diameter will usually depend on the bike, and has little to do with the rider.

Rate

The spring's rate is a measure of its stiffness. A higher rate results in a stiffer spring.

There are two conventions for spring rate, kg/mm & N/mm. They both measure the exact same thing, and the only real difference is that N/mm is exactly 9.81x kg/mm. The reason they both exist is historical, force used to be measured in kilograms, where as nowadays kg is a unit of mass and N (Newtons) is a measure of force. Technically speaking kg/mm as a scientific unit doesn't make sense, but we sweep that fact under the rug and generally use both.

Both units represent the amount of force (in Newtons) or mass (in kg) required to compress the spring 1 mm. For example, a 6kg/mm spring will take 6kg to make the spring 1 mm shorter and 12kg to make the spring 2 mm shorter.

Don't confuse this number with your weight, a 10kg/mm spring is quite stiff. Considering a spring like that has approximately 120mm of travel, completely bottomed out the spring would be pushing back with 1200kg of equivalent force!

So how heavy are you?

This may seem like a simple question but it can have a very important impact on the bikes comfort and performance.

You need to consider the following:

• How heavy am I when I am wearing all of my usual riding gear?
• How much luggage do I carry?
• How often do I carry it?
• Do I take a pillion? How often?
• How heavy is my pillion?

If you don't take luggage or a pillion, pick the spring based on your weight in your gear.

If you take luggage or a pillion then how often you carry that extra weight needs to be considered. We find the following process works well:

• Consider the spring based on the usual weight (you + your gear + any luggage you always carry)
• Consider the spring based on the usual + extra weight (same the same weight as above + the extra weight you carry)
• Work out the percentage of time you carry this extra weight.

If your answer to number 3 is 50% of the time, then pick a spring half way in between the two. For 70% of the time move towards the stiffer spring.

If you are unsure of your result then we recommend you contact us for assistance.

How do I find the right springs for my bike.

There is generally two ways to do this:

• Find your bike on our website using the make/model/year boxes above. Enter your weight into the "Calculate Spring Rate" box and pick the closest available spring.
• If no spring data is available then the "Calculate Spring Rate" box will be unavailable. Refer to your owners manual or do some research to identify the rate of the standard spring, and work upwards (if you are heavier than 70-80kg) or downwards (if you are lighter than 70-80kg) from there.

What about preload and sag?

The bike's "sag" is a useful way to identify if a spring rate is too low or too high for your weight. It is part of the setup process of the bike, so we recommend you see our other articles for offroad bike setup for adventure and dirt bikes, along with our road bike setup for tarmac riders.