I recently learned that I’ve been setting my race sag wrong all these years, which explains why I’m always having to stiffen up my rear shock.  So I decided to snap some pics and blog it the way it’s supposed to be done.  Maybe someone will learn something.  In the article I’m using my 1999 Honda CR250 as the example but the steps and methods will be the same for almost all newer bikes.  Once you know what you’re doing the race sag can be adjusted in well under 30 minutes but if you’re a first-timer give yourself an hour or so…..

Let’s start off by explaining what "race sag" is and why you should care about where it’s set.  Race sag is a measurement of how much the rear suspension compresses with the rider on it compared to no weight at all on the bike (no weight meaning the bike has to be on a stand as explained later).  Race sag is the MOST important suspension adjustment on your bike.  It can be the difference between getting bucked off the bike through some whoops or being able to fly through them like Stewart.  The reason the rear shock spring is adjustable is so you can adjust the rear shock to suit your weight.  Motocross springs are designed to fit a range of riders weight and will need to be adjusted to be right for you.  Everyone should set their race sag and check it frequently.  4 inches of race sag is the starting point for all 125cc and up dirt bikes.  I’m going to assume everyone is starting with a set of springs designed for their weight and riding ability.  Stiffer and softer springs are available if you need them.  I’ll explain more as we go….
 
Section 1 – Measure your race sag

So the first thing to do is measure where your race sag is set now.  To do this the bike must be placed on a stand so the rear wheel is off the ground as shown in the first picture.  This was the mistake I had always made, I wasn’t putting the bike on the stand.  It makes a big difference if you don’t put it on the stand so always put it on the stand when measuring your race sag!  Go from somewhere near the axle (I use the centerline of my chain adjuster bolt  where it goes into the swingarm) up to the seat bolt or another fixed location in that area.  Write down this number.  Now set the bike on the ground and get on it.  Work the suspension up and down a few times to ensure a good result.  Now if you can, have someone balance the bike while you pick up your feet and have someone else measure it.  If you don’t have a 3rd person try to barely rest the handlebars against something to balance you but the bike must remain as upright as possible, so don’t go leaning it way over onto something.  If you have to you can tip-toe it, just remember we want all of our weight on the bike, not the ground.  As shown in the second picture, take the measurement from the same spots as before and write this number down.  Subtract the 2nd measurement from the 1st measurement and that is what race sag your bike currently has.  Again, the race sag starting point for all 125cc and up dirt bikes is about 4".  If you want, you can check your owners manual to see what race sag starting point your bike manufacturer recommends.  For my 1999 Honda CR250 they recommend 3.9" as a starting point so that’s what I used.

Section 2 – The tear-down

Now that you’ve got your measurement and you’ve decided to adjust your race sag lets get the bike back on the stand and start tearing it down.  You can’t get to the rear spring adjustment on most bikes so you usually have remove the rear subframe.  If you can easily get to the spring adjusters you can skip ahead to the section dealing with spring adjustment.  Don’t let removing the rear subframe scare you, it’s actually quite easy.  The first thing to do is get the seat and rear plastic off as outlined in the above images.

As long as your fender is only attached to the subframe (most bikes are) you won’t need to remove it.  But you will most likely have to remove the exhaust.  Just un-bolt it from the subframe and slide it out as shown above.

Okay, now that the plastic and exhaust is off you can un-bolt the subframe and disconnect the airbox from the carb (if the airbox is attached to the subframe it will come off with it).  Simply remove the bolts as shown in the first 2 images and then loosen the clamp on the airbox tube on the back of the carburetor as shown in the 3rd image.  Most subframes are going to have 3 or 4 bolts attaching it to the main frame so make sure you get them all.  Once all of the bolts are removed and the carb inlet clamp are loosened simply give a gentle tug on the subframe and it should pull right off.

Section 3 – Adjusting the rear spring

Okay, so now you should have something that looks like this and you’re thinking it looks kind of cool and wondering whether or not you could land a triple without losing your nads.  I’ll let you decide.  I want you to take a look at the second image which shows the top of the shock and the upper and lower adjusters.  The upper adjuster is just used to "lock down" the lower adjuster and keep it from moving.  The yellow lines are from the factory and they show you how tight to make the upper adjuster.  If your adjusters don’t have those lines on them put them on there now before you loosen the upper adjuster.

Before you go any further make sure you clean and lubricate the threads on the shock.  It’s pretty common for them to get dirt and stones in them and if you don’t clean them out good you’ll mess up the threads which could result in having to buy a whole new shock.  Once the threads are clean go ahead and loosen the top adjuster with either a long punch or a screwdriver.  Just hit it with a hammer to loosen it up.  BE CAREFUL NOT TO HIT THE THREADS ON THE SHOCK!  Once the top adjuster is loose go ahead and loosen it way up to get it out of your way.  Now you can adjust the spring with the lower adjuster whichever direction you need to go.
Tightening the adjuster will stiffen the spring which will reduce your race sag.
Loosening the adjuster will soften the spring which will increase your race sag.

How much you have to adjust the spring will be different for everyone.  You’ll just have to adjust it, put the subframe and seat back on (see the next section), measure it, and adjust again until it’s right.  If you find you are having to put your adjusters all the way to the end of the threads either on the top or bottom, your spring is not right for your weight.  If you’re tightening down your adjuster all the way and still have too much sag, you need a heavier spring.  If you’re loosening it all the way and still can’t get enough sag, you need a lighter spring.  Once you have it set you can lock the top adjuster back down to the lower adjuster as shown in the first 2 images.  Make sure the lines we spoke of earlier end up lined up again as shown in the 2nd image.
As long as you’ve got it all apart now is a good time to do some cleaning as well.  These areas are usually pretty hard to reach with the subframe on.  Use whatever cleaner you prefer but I’ve found bug and tar remover works pretty darn good and is available at most automotive parts stores.

Section 4 – Putting it back together

Okay, so whether you’re just putting it back together to check your adjustment or if you’re all done adjusting here are your next steps.  Basically just reverse your tear-down procedure.  Slide the subframe back in while paying close attention to the air box tube which needs to be lined up on the carburetor inlet.  Depending on how tight your fit is, this can be a real pain in the ass so just take it slow and try not to get frustrated getting the airbox tube on properly as shown in images 1-3.  Once that tube is on go ahead and tighten up the clamp as shown in image 4 (don’t need to bother tightening until you know you’re done adjusting the spring).  Replace and tighten all the bolts holding the subframe onto the bike and put the seat back on.  Now you’re ready to verify your spring adjustment by re-measuring your race sag.  If you want to you can replace your exhaust and side panels (next image set) before re-measuring but they shouldn’t have much affect, if any, on the measurement.

Okay..now that your race sag is where you want it to be go ahead and button it up.  Slide the exhaust back on and tighten it’s fasteners, then put your side panels back on.  Once it’s back together go ahead and verify your measurement again and hope that it’s right so you don’t have to tear it all back apart again!

Adjustments to the spring WILL affect your other suspension adjustments.  You’ll probably have to tweak your rebound and compression adjusters once you get out and see how the bike is acting.

Now that you know what you’re doing go ahead an adjust the rest of the bikes in your garage!
Note: No matter how many suspension adjustments you make to your Harley and chopper they’ll never land as good as your MX’er.

This entry was posted on Monday, October 16th, 2006 at 2:23 pm and is filed under Gearhead. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.