Checking your sag and spring rates is easy. There are a lot of different springs you can use on your bike depending on your weight and riding preferences. They reckon a bike straight out of the crate has a spring rate suitable for your Joe Average 75 – 80kg rider. So, if you are weighing in at over 100kg and you have a standard spring fitted, chances are your donkey won’t be riding as well as it should be.
Here’s a guide to help you find out if the sag and spring rates on your bike are right for you.
- Place your bike on a stand with the wheels off the ground. Take a measurement from the rear axle bolt vertically to a point on your bike. This will probably be on the side plate, muffler or rear mudguard. Use a marker pen to mark the spot. Record this measurement.
- Next, take your bike off the stand. It is best to set the clickers to ‘soft’ for more accurate results. While holding it upright, record the distance from the axle bolt to the mark you just made on the bike.
- Subtract the second measurement from the original one you made when the bike was on the stand and note the figure. This is the static sag. It should be around 25 – 30mm for 125cc bikes and over. And around 8 – 10mm for smaller bikes.These are guides only – for a more detailed setup of your suspension, refer to your owners manual.
- Now, to get the laden sag or rear ride height, sit on your bike in a central riding position and have someone take a measurement again from the axle bolt. Subtract this from the original measurement (when the bike was on the stand) and hopefully, you are left with around 95 – 105mm of laden sag for a 125cc or over. And 65 – 85mm for smaller bikes.
If the laden sag does not meet these requirements, you will need to adjust the spring preload. Do this by using a hammer and punch to release the top collar on the shock spring. Then gently tap the lower collar to compress the spring (more preload), or to release the spring (less preload). Make sure you tighten the top collar once you have finished to lock it in place.
So, after a bit of fiddling around, you have the correct laden sag. But if the static sag is way out, e.g. not enough sag, then the rear spring is too soft for you. And of course, if there is too much static sag, the spring is too hard.
The reason this is so, is that if the spring is too soft you have to apply too much preload to get the desired laden sag, which will offset the static sag. If you put a harder spring in, you wouldn’t have to apply so much preload. And then both the static sag and laden sag would be balanced.