Wrench Tip Wednesday: Set the Sag
WARNING, I am not a suspension expert but understanding how it works is something that interests me. Having the bike properly set up helps the overall rideability of it. I’m not saying you have to go spend a bunch of money to get your suspension tuned but suggesting to at least have your sag is set correctly. Doing so insures the correct size springs are in your bike along with allowing for better overall handling.
The sag can be decreased to make quicker handling but will reduce high speed stability, this is good for tight courses because it can help improve steering accuracy. It is also recommended to do so in muddy conditions to handle weight build up. Increasing the sag, improves stability in high speed and sandy tracks. A spring too soft will ride high in the rear because it needs preloaded more than a stiffer spring. A stiffer spring will have a lower initial ride height but will feel firmer on bumps.
Static sag is a measurement of the bikes distance that the rear end of the bike settles while on the ground, but only under the load of its own weight (no rider) and is checked before and after rider’s sag. For 125cc and above, the number should be 35mm +/- 5mm. Rider’s sag is the distance that the rear end of the bike settles when on the ground and under the load of the rider’s weight in full gear. This number should be 105 mm +/- 10mm. Linkage bikes should be 95mm – 105mm range and KTM PDS should be 105 mm – 115mm. If the measurements are above range values you need a softer spring. If it is below, then you need a stiffer spring.
The most convenient and easiest way to measure is using Motool Slacker, a digital scale. Center the tool in the middle of the axel and always place the clamp in the exact same place every time. Put the bike up on a stand with both wheels off the ground and measure the static sag. Then, have the rider hop on in full gear and in “attack position”, standing not sitting in a granny position. Bounce up and down a few times and check the number. Just like anything, there is different way to do everything. I have heard some people like to stand up and then sit down naturally to measure the riders sag. Other people check both standing and sitting and do an average of the two. Everyone is entitled to their own opinion and this is just what I have learned over the years from experience and talking to different people. Have the rider get off and check the static sag measurement.
Don’t have the digital tool? I hope you like math. A measuring tape can be used, preferably metric. Put the bike on a stand, with the wheels off the ground. Measure the distance from the rear axle to a vertical point on the rear of the bike. This is L1. Put the rider on the bike in full gear, standing, compress the rear suspension and slowly release back up. Measure between the two points rear axle to a vertical point on the rear of the bike. This is L2. Lift up on the rear slightly (about 20mm) and slowly release it downward, particularly at the end. If there was no friction it would continue to drop a bit more. Measure between the same two points. This is L3. The difference between lifting up L2 and pushing down L3 is caused by friction. If it is more than 4mm or 5mm the linkage needs attention. Sag is calculated by averaging L2 and L3 subtracting it from fully extended. Riders sag = L1 – [(L2 – L3)/2]
As I mentioned above, if your linkage bearings are seized up, this prevents the rear shock working properly. Therefore, you will not get your sag set and your shock will not be moving like it should, you will be in for a rough ride. Keep up on your maintenance and your dirt bike experience will be much better.
If further assistance is needed with setup Jim at Shock Zone Suspension would be a good place to go.
Shoutout to the customer that got lucky enough to be in the picture to make sure I did not fall! haha I wanted to give a general idea of what it is like to check it.