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For the same reason the CRF250X and many other bikes come standard with air valves in their fork caps... to release accumulated pressure inside the forks. Plus, fork cap air valves provide a quick and easy way to make temporary spring rate changes while out on the trail or track.

Note: Never, ever, EVER use an air pump to ADD pressure to your forks. You could very
easily damage the seals and/or internal parts
by doing so.

All popular motorcycle forks contain liquid and air. The liquid is in the form of fork oil (aka, fork fluid or suspension fluid), and is used for two reasons:

First, as the forks compress and rebound while riding, the fork oil is forced through tiny holes called “orifices”. Although the actual process is rather complicated, let’s just say that we can control the rate of fork compression and rebound by controlling how quickly the oil can be forced through these tiny holes. Using larger holes or thinner oil allows the oil to flow faster through the holes. Smaller holes or thicker oil slows the flow through the holes. And since oil, like all liquids, cannot be compressed, the forks will not collapse (compress) or extend (rebound) faster than the oil will flow through the holes. It’s actually a lot more complicated than this, but let’s just say that most forks have one set of holes for the compression stroke, and a different set of holes for the rebound stroke, so that the two different actions can be controlled separately (i.e., different rates).

Secondly, the amount of non-compressible fork oil inside a sealed fork tube also determines the amount of compressible air inside the tube. Since fork tubes are not vented to allow air to move in and out as the forks compress and rebound, air just naturally fills whatever space is not occupied by the oil. For example, if the fork tube volume is 1000 units (it could be fluid ounces, cubic inches, cubic centimeters, or whatever), and we pour in 500 units of oil, then air would naturally occupy the remaining 500 units of volume. Now, what happens when the unvented fork tube is compressed? Oil is a non-compressible liquid, but air is an easily compressible gas, so the air trapped inside is compressed, and it builds up pressure much like the fork spring builds pressure when it’s collapsed.

Unlike a fork spring, however, air pressure doesn’t build in a linear fashion. A 50 lb. rate spring will basically collapse 1" for every 50 lbs. applied, so its response is called “linear”. Compressed air, however, acts differently. Compress 12 cubic inches of air into 6 cubic inches of space, and you’ve doubled its pressure. Compress it again into half the remaining space, and now it’s four times the original pressure. Our 50 lb. spring example has linear response, collapsing 1" for every 50 lbs. of pressure. Compressed air, however, builds up greater and greater resistance as it’s compressed, and the smaller its space, the faster it builds up in pressure.

Fork designers know all about this, and they use know compressed air technology as a secondary spring inside the fork tube. Since air compression characteristics are known and predictable, engineers know how much air pressure accumulates as the fork is compressed. The amount of compression is thus tunable (adjustable) by varying the quantity of air inside the fork tube. How do we vary the amount of air inside a sealed fork tube?

One way is to change the amount of oil in the fork tube, which leaves more or less room for air. If we poured 700 units of oil into our 1000 unit fork tube, that would leave room for only 300 units of air, so it would build up in pressure more quickly when compressed.

Another way is to change the volume of the tube. If the fork tube contained only 800 units, and we added 500 units of oil, that too would leave room for only 300 units of air, so it would build up in pressure more rapidly when compressed.
The bottom line, and the whole point of all this talk, is that the amount of air inside the fork tube plays an important role in determining the total resistence (linear spring pressure plus non-linear air pressure) of the fork to being compressed. By adding more oil to each fork leg, the amount of remaining air is reduced, so as the fork is compressed, the air pressure builds more quickly, and will hopefully build up high enough and quickly enough to prevent the fork from harshly bottoming. By reducing the amount of oil in each leg, we allow more room for air to exist in the tube, which slows the compression buildup, making the fork action softer, but more liable to bottom out.

Our goal, of course, is to find that elusive ideal combination of oil and air amounts that allow full use of the total fork travel, but without allowing harsh bottoming. Luckily, experimentation in this area is relatively easy. Just reduce the amount of oil in each fork leg (but keep both legs the same) until bottoming becomes troublesome. Then go back to the least amount of oil that did not allow troublesome bottoming.

So how do fork cap air valves fit it with all this?

With air valves in our fork caps, we can quickly and easily adjust the amount of air inside the fork legs by changing the volume of the fork tube before it’s sealed. Here’s how.

If the air valves are opened while the fork tubes are fully extended, we admit the maximum amount of air. This means the total pressure inside each fork leg will build more quickly during compression, resulting in somewhat harsher fork action, but more resistance to bottoming (similar to stronger fork springs, but without affecting damping rates).

If the air valves are opened while the fork tubes are partially collapsed, we reduce the amount of air inside. This means the total pressure inside each fork leg will build more slowly during compression, resulting in somewhat softer and more supple fork action, but less resistance to bottoming (similar to softer fork springs).

Air valve fork caps are also useful to eliminate normal pressure build-up inside the forks, which eventually leads to harsh response and weakened fork seals. Fork seals have oil on only one side (inside the fork tube), so they are more effective at keeping the pressure in than in keeping it out. Over time, forks gradually build up pressure inside. It doesn’t build up to the point of being harmful, but it can affect tuning and overall fork action. This is why the owners’ manuals for bikes with vented fork caps usually advise venting the forks regularly.


Although we have a somewhat limited range of effectiveness, the adjustments are so easy to make, it invites experimentation. If you can extend and compress the forks by yourself, fine. If you need a helper, that’s okay too.
Begin by pulling up on the bars or leaning the bike over on its side stand until the forks are fully extended. Hold the bike there while depressing the air valve to release any air pressure inside (positive or negative). Then release the valve, replace the valve cap, and go riding. Determine for yourself how you like the fork response and current resistance to bottoming.

Then stop, hold the bike straight up and down with your weight on the seat, and release the valves again. This time you should hear a little air escape, since the forks are no longer fully extended. Replace the valve caps and go riding again. Now your fork action is softer and more supple, but less resistant to bottoming. Determine for yourself how you like the new fork action.

If you’re still not having any problems with bottoming, then try yet another step toward being even softer. Stop, hold the bike erect, remove the valve caps, and while depressing the air valves to open them, press down hard on the front of the bike to collapse the forks as far as you can. Do the best you can to equalize the pressure in both forks, then replace the valve caps, and go riding again to determine how you like the new fork action.

One extreme is to fully extend the forks while the valves are open. This allows the maximum amount of air, which makes the fork action harsher but more resistant to bottoming.

The other extreme is to compress the forks as far as possible, using several people to help, or tie-downs on a trailer, to have as little air as possible inside the forks. This makes the fork action soft and supple, but less resistant to bottoming.
Adjusting the air pressure this way is, of course, secondary to having the proper amounts of oil and air in the forks to begin with. Adjusting fork action this way should be considered a temporary adjustment only, for a particular day or ride, and not a permanent adjustment. This is simple to do, of course, since the air adjustments are so easy to make.


There are, I’m sure, nearly as many different ways to do this as there are people who want to do it. I chose an easy and simple way that costs very little (about $2.00 each for two SHORT (don’t get the long ones), automotive tubeless tire valve stems), but does require an electric drill, some spray solvent (carburetor cleaner or brake cleaner), and some drill bits ranging in size from 1/8" up to the same size as the outside diameter of the valve stem you bought. The final hole should be just a hair smaller than the diameter of the air valve stem.

Note that the tubeless tire valve stems have tapered shafts. This is important to our installation. When you measure the outside diameter, measure the small end, near the threaded portion. If you don’t have calipers, use an adjustable wrench to match the size of the valve stem (at its narrow end) to the size of your final drill bit. The drill bit should be the same size or slightly smaller than the diameter of the valve stem. As you can see in the photo, I chose to cut off the fat end of the valve stem rubber, so as not to further reduce the air volume of the fork tube, but it’s not really necessary.

1. Place the bike on a box or stand that allows the front wheel to hang free, off the ground.

2. Using a 17mm deep-well socket or a box end wrench, remove the fork caps. The fork caps are very soft aluminum, so don’t use an open end wrench or you may round off the corners of the fork caps, making them ugly and difficult to remove and tighten.

3. Determine the outside diameter of the tapered valve stem near the cap. Mine measured 13/32", so my final hole was made using a 25/64" drill bit (slightly smaller than the valve stem).

4. Starting with a 1/8" drill bit, drill a hole “dead center” (do your very best to get it dead-center!) through the top of the fork cap. Then use drill bits 1/16" larger at a time, to enlarge the hole to your final size. Never attempt to drill or enlarge a hole in soft aluminum more than 1/16" to 1/8" at a time. Again, my final hole was made using a 25/64" bit, but yours may differ, based upon the size of your valve stem..

5. As you near the size of your final hole, try pushing the valve stem up through the hole (remove the valve stem cap first, of course). Once you reach a hole size that will allow the threaded portion of the valve cap though, but not the rubber portion, your hole may be just right.

Again, your final hole size should allow the threaded portion of the valve stem to protrude through the hole, but not the rubber part. If more than 1/16" to 1/8" of the rubber part will go through the hole easily, you may have drilled your hole too large. If that happens, you’ll need to fill the hole with aluminum epoxy, let it thoroughly harden, and then start all over.

6. Once you have the right size hole, spray some solvent (carburetor cleaner or brake cleaner) inside the fork cap and all over the valve stem. (The solvent not only lubricates the rubber to make it easier to squeeze it, but the softened rubber will actually be glued to the aluminum cap when the solvent dries, helping to make an air-tight seal.) Then insert the valve stem into the fork cap, and FORCE it in as deep as it will go. Push it deep into the fork cap using a Phillips screwdriver, or tap it lightly with a hammer if necessary. Just get it in there tightly!

Once the solvent dries, you’re done. Replace the fork caps on the forks, and you now have air valve fork caps. Go riding and try ‘em out!



CRF's Only "How-To" By Gordon Banks, February 2005