Tuning the Air Fork

marcparnes

Member
I ride my Superdelite primarily on the pavement with a little dirt road thrown in. The rear suspension has always felt plush to me but the front felt a bit too progressive and non-compliant over small bumps. I keep the sag at about 25-30% of total travel which translates to 85 lbs with me on it. One characteristic of air vs a spring is that air is progressive while springs are linear by nature. Progressive means that the amount of force required to compress air ramps up as the fork is compressed. This is helpful when encountering big hits like jumping over things or when hitting the front brake in a panic stop in order to keep the fork from bottoming. Springs on the other hand are linear as mentioned which means if you have a 100 lb spring it will take 100 lbs to compress it 1" and 200" lbs to compress it 2" and so forth. There are also progressively wound springs that are designed to give a soft ride on small bumps and then ramp up as the fork compresses, the best of both worlds some think.

Since our fork uses air the only adjustments we have to customize it for our needs is by either changing the amount of pressure in the fork which changes the ride height and sag or by changing the space the air has available to expand into. By decreasing or increasing the available space we can get the fork to be more progressive or more linear. If we make the space smaller pressure will increase faster making the fork more progressive resulting in a harsher ride but one that can handle big hits without bottoming. Conversely, by increasing the space pressure will increase at a slower rate resulting in a softer ride using more of the available stroke. Assuming the goal is to make the fork more compliant we will want to use as much of the stroke as we can without bottoming the fork on bigger hits or when using the front brake aggressively. To accomplish this we are going to increase the amount of available space inside the fork. This particular fork has two "air volume spacers" installed in the left fork leg. They are simply rubber spacers each about 10 mm long that do nothing except take up space. I don't know if R&M specified these two spacers or if they just come standard from the manufacturer based on their published intended Cross Country/Rally use. In any case, I removed both of them and set the air pressure at the same 85 lbs I had been using. Sag is the same but the ride is much more compliant particularly over small stutter bumps encountered on the street. Hitting the front brake to the point of skidding the tire uses no more than 75% of travel. I also tried riding over a curb with similar results. I haven't tried it yet on my dirt road but I will later today.

If you want to experiment with removing one or both of the spacers here is how you do it:

You will need a 27 mm socket or box end wrench and your suspension pump that came with the bike.

1. Remove the cap that has "AIR" on it.
2. Let all the air out of the fork by pushing on the air valve's pin with an appropriate tool like a small screwdriver.
3. Unscrew the 27 mm nut the cap was screwed into. Be careful your tool doesn't slip off and mar the surface. Its only aluminum.
4. One of the spacers is attached to the bottom of the big nut. Remove it. It takes a little effort to get it off.
5. The second spacer in my fork was just loose near the top of the fork tube. I grabbed it with a screwdriver and pulled it out.
6. Reinstall the big nut and tighten firmly.
7. Re-inflate the fork with your air pump. A good starting point is half your weight in psi so if you weigh 180 lbs start out with 90 psi.
8. Set sag. A good rule of thumb is 25-30% of total travel. The Superdelite with the Suntour fork has 100 mm of total travel so sag should be 25-30 mm. Extend the fork completely by lifting on the handlebars and then slide the O-ring all the way down till its touching the top tube. Either having someone hold the bike for you or using a wall as a support, sit on the bike as you would while riding without bouncing. Get off the bike and lift on the handlebars to extend the fork. Measure between the top of the fork tube and the O-ring. This measurement is your sag. More air will reduce sag and bleeding some air off will increase sag.

This fork is also equipped with a rebound adjuster located at the bottom of the right fork. Rebound relates to how fast the fork returns after hitting a bump. Insufficient rebound will allow the fork to return too fast which effects control and traction. Too much rebound will not let the fork return fast enough which makes for a harsh ride and also has the potential to use up travel when encountering multiple bumps in quick succession. It doesn't have as much effect on pavement as it does off-road. I always start out with rebound backed all the way out and then adjust it in one click at a time until I get the controlled and comfortable ride I'm looking for.

Go give it a try :)

Marc
 

Theguvna

Member
I ride my Superdelite primarily on the pavement with a little dirt road thrown in. The rear suspension has always felt plush to me but the front felt a bit too progressive and non-compliant over small bumps. I keep the sag at about 25-30% of total travel which translates to 85 lbs with me on it. One characteristic of air vs a spring is that air is progressive while springs are linear by nature. Progressive means that the amount of force required to compress air ramps up as the fork is compressed. This is helpful when encountering big hits like jumping over things or when hitting the front brake in a panic stop in order to keep the fork from bottoming. Springs on the other hand are linear as mentioned which means if you have a 100 lb spring it will take 100 lbs to compress it 1" and 200" lbs to compress it 2" and so forth. There are also progressively wound springs that are designed to give a soft ride on small bumps and then ramp up as the fork compresses, the best of both worlds some think.

Since our fork uses air the only adjustments we have to customize it for our needs is by either changing the amount of pressure in the fork which changes the ride height and sag or by changing the space the air has available to expand into. By decreasing or increasing the available space we can get the fork to be more progressive or more linear. If we make the space smaller pressure will increase faster making the fork more progressive resulting in a harsher ride but one that can handle big hits without bottoming. Conversely, by increasing the space pressure will increase at a slower rate resulting in a softer ride using more of the available stroke. Assuming the goal is to make the fork more compliant we will want to use as much of the stroke as we can without bottoming the fork on bigger hits or when using the front brake aggressively. To accomplish this we are going to increase the amount of available space inside the fork. This particular fork has two "air volume spacers" installed in the left fork leg. They are simply rubber spacers each about 10 mm long that do nothing except take up space. I don't know if R&M specified these two spacers or if they just come standard from the manufacturer based on their published intended Cross Country/Rally use. In any case, I removed both of them and set the air pressure at the same 85 lbs I had been using. Sag is the same but the ride is much more compliant particularly over small stutter bumps encountered on the street. Hitting the front brake to the point of skidding the tire uses no more than 75% of travel. I also tried riding over a curb with similar results. I haven't tried it yet on my dirt road but I will later today.

If you want to experiment with removing one or both of the spacers here is how you do it:

You will need a 27 mm socket or box end wrench and your suspension pump that came with the bike.

1. Remove the cap that has "AIR" on it.
2. Let all the air out of the fork by pushing on the air valve's pin with an appropriate tool like a small screwdriver.
3. Unscrew the 27 mm nut the cap was screwed into. Be careful your tool doesn't slip off and mar the surface. Its only aluminum.
4. One of the spacers is attached to the bottom of the big nut. Remove it. It takes a little effort to get it off.
5. The second spacer in my fork was just loose near the top of the fork tube. I grabbed it with a screwdriver and pulled it out.
6. Reinstall the big nut and tighten firmly.
7. Re-inflate the fork with your air pump. A good starting point is half your weight in psi so if you weigh 180 lbs start out with 90 psi.
8. Set sag. A good rule of thumb is 25-30% of total travel. The Superdelite with the Suntour fork has 100 mm of total travel so sag should be 25-30 mm. Extend the fork completely by lifting on the handlebars and then slide the O-ring all the way down till its touching the top tube. Either having someone hold the bike for you or using a wall as a support, sit on the bike as you would while riding without bouncing. Get off the bike and lift on the handlebars to extend the fork. Measure between the top of the fork tube and the O-ring. This measurement is your sag. More air will reduce sag and bleeding some air off will increase sag.

This fork is also equipped with a rebound adjuster located at the bottom of the right fork. Rebound relates to how fast the fork returns after hitting a bump. Insufficient rebound will allow the fork to return too fast which effects control and traction. Too much rebound will not let the fork return fast enough which makes for a harsh ride and also has the potential to use up travel when encountering multiple bumps in quick succession. It doesn't have as much effect on pavement as it does off-road. I always start out with rebound backed all the way out and then adjust it in one click at a time until I get the controlled and comfortable ride I'm looking for.

Go give it a try :)

Marc
What a great post, very informative for me, thank you for taking the time to write this.
 

Alaskan

Well-Known Member
@marcparnes thanks so much for this very interesting post. I am going to try it. Do you know if the same applies to the Fox 34 fork that comes with the Fox upgrade?
 

marcparnes

Member
@marcparnes thanks so much for this very interesting post. I am going to try it. Do you know if the same applies to the Fox 34 fork that comes with the Fox upgrade?
I don't know if the fork they used in your bike actually has them installed but they do equip them if specified. Easy enough to take a look. Your fork is way more sophisticated than the one on my bike. I would have upgraded but was worried about the extra height. Fox's website has all kinds of interesting info regarding their forks and shocks ie: part breakdowns, torque specs, etc. Here is a YouTube video that show's how to access the spacers in a Fox fork.

BTW, that was a great promo you and your wife made for Fly Rides which led me to your pipes which I enjoyed even more 👍

Marc
 

Alaskan

Well-Known Member
I don't know if the fork they used in your bike actually has them installed but they do equip them if specified. Easy enough to take a look. Your fork is way more sophisticated than the one on my bike. I would have upgraded but was worried about the extra height. Fox's website has all kinds of interesting info regarding their forks and shocks ie: part breakdowns, torque specs, etc. Here is a YouTube video that show's how to access the spacers in a Fox fork.

BTW, that was a great promo you and your wife made for Fly Rides which led me to your pipes which I enjoyed even more 👍

Marc
Thanks Marc. As I quit smoking after the MI, my passion for pipes transferred over to a passion for bikes.