# How do I measure rear shock sag?

Hi Lee,

I’m a beginner mountain biker from Peru and want to thank you for a great book, it’s helping me a lot!!

I have a question: how do i calculate the recommended 25% sag?. Do i base it on the stroke of my rear shox or the eye to eye distance? Thanks in advance for your help.

Regards, César (in Peru)

Hey César,

Your sag is a percentage of your shock stroke. Check this out:

On an air shock, you can measure sag with an O-ring on the shock shaft. Under your weight, the O-ring should move one-half inch.

On a coil shock, measure sag from eye to eye. Let’s say your shock is 200mm (8 inches) long. Under your weight, it should be about 188mm (7.5 inches) long. To make this measurement, you need a friend with a tape measure.

To measure your sag, get onto your bike in your normal riding position. I really hope that’s with all your weight on your feet, butt off the seat and weightless hands. Bounce up and down and let the bike settle. Reach down and slide the O-ring against the body of your air shock, or have your buddy measure the length of your coil shock.

I hope that makes sense. Have fun!

— Lee

2 replies
1. Cesar says:

Thanks Lee sure helps!!

2. SeanH says:

I really like that you mentioned bouncing on the bike and letting it settle before setting the O-Ring. I figured that out by trial and error. This is the first place I have seen it brought up.