MORE THAN A FOAM INSERT
Volume spacers for your tires
When it comes to tires and tire pressures, a lot has changed in a very short period of time. This evolution is hard for many to completely get on board with, but the biggest factor is volume. Air volume is the main player when it comes to tire pressure and drastically alters settings, while rider weight is a small variable. This seems counter to what we have been taught over many generations, but that is in many cases due to the more or less static size of tires offered for bikes over that time period.
Modern mountain bikes have moved away from the 1.5-inch and even 2.0-inch tires to go larger. Then there is the adaptation of wider rims that modify the volume of air in the system. In short, what seems like small changes in tire size/rim width results in large-volume changes. This can be seen even in the road and gravel world with much larger tires and volumes that are more in relation to mountain bikes.
We are seeing technologies in the industry push the research and development of the wheel-and-tire system to evolve. We are now learning that like suspension on bikes, the volume in that system can be manipulated to offer more precise tire responses. This is where tire inserts come into play and add many layers of advantages for those willing to experiment.
Tire inserts are nothing new to the cycling industry and were first introduced as a way to minimize damage to rims as cyclists experimented with tire pressure. They are normally comprised of some sort of foam to minimize weight and are installed inside a tubeless tire, tight against the rim. This minimizes movement and ensures correct positioning that acts as a buffer between the rim and ground in more extreme cases.
A side effect of adding this foam insert is that it acts as a volume spacer. When maintaining the same pressure in the system while using a volume spacer, the result is a more progressive ramp up in pressure when a tire is compressed during riding. It is the same thing a rider will experience if he adds tokens (volume spacers) to his suspension. What does that mean for the rider? The tire is still responsive to the smaller bumps and demands of cornering with very little difference, but in the case of a larger impact, the tire will not deform as much. Not only does the tire not compress as far, it also now has this insert to act as a physical barrier, too.
On a mountain bike, this is extremely useful and could allow riders to have more tire size options. In some cases, the tire can be large but run at a lower pressure to maximize comfort and efficiency over very rough terrain. In other cases, the rider might be able to choose a smaller tire to increase clearance for mud and debris around the frame externally. The smaller tire would also benefit from lower pressures and more progressive compression. In either case, the risk of the tire pressure being too low is minimized since the foam insert helps protect the rim.
For our test, we chose two of the most popular and refined systems on the market: CushCore and Vittoria. CushCore has a full line of mountain bike offerings in the different wheel and tire sizes. Vittoria takes a slightly different approach and offers a system that is more universal and will fit a wide range of tire and wheel sizes.
CushCore offers many different sizes and kits for mountain bike needs. The kit consists of two foam inserts that are made to fit the rim/tire size you will be running. There are also two 44mm Presta valves that are specifically designed for tire inserts. The valves are specific on the rim side with holes on the sides in addition to the one at the end (like a normal valve). This is to allow better airflow in case the insert covers the hole on the end.
The continuous foam insert comes in three mountain bike options: Pro for 2.1–2.6-inch tires that fit on a 22–35mm inner rim width. XC for 1.8–2.4-inch tires that fit on a 22–32mm rim width. And Plus for 2.6–3.0-inch tires that fit on a 32–45mm rim width. Each insert weighs between 140 and 330 grams, depending on the tire size and level of protection. The fit is fairly tight on all the wheels, and it definitely requires a learning curve to install properly.
Setup can be tricky, but we found a solution that, in our opinion, was pretty easy. First, we installed our tubeless tire on the wheel dry (no sealant) and beaded up the tire. Of course, make sure you install the supplied valve that is specific to CushCore. Then we pulled one side of the tire off while the other remained beaded. With the one side open, it is easy to fit the foam insert in.
The hardest part is reinstalling the tire under the CushCore. As you work your way around, you need to tuck the tire under the foam insert to get it into the drop center of the rim. Once it is all on, then it’s like any other tubeless tire. Air it up to seat the bead. We prefer to then deflate the tire, remove the valve core and add the sealant through the valve. This normally results in less mess.
Vittoria, like CushCore, also offers a system for mountain bike tires. The Air-Liner Gravel is the kit we opted for, and the kits are offered individually for each tire, so you will need two kits. The kit includes a zip-tie used to connect each end of the foam insert, as well as a special Presta valve. Similar to the CushCore kit, the Vittoria valve also has additional holes on the sides that they call a three-way tubeless valve.
The foam insert itself is very light, and the full length supplied hit the scales at 90 grams in the size that fits 1.9–2.25-inch tires. The sizing is pretty simple: small (1.9–2.25 inches), medium (2.3–2.5 inches) large (2.6–2.7 inches) and XL (2.8–4.0 inches). The supplied zip-tie is a bit long. For the install, cut more than half of it off. Vittoria recommends its system for 29-inch wheels and below because it is a cut-to-fit system that can be trimmed for a smaller wheel size. They don’t specify inner rim widths, but we would say 24–35mm would probably be best.
The install of the Vittoria Air-Liner has a few more steps, as you have to measure and cut the foam to fit, but it is very easy. Start by installing the three-way valve, then wrap the foam insert over the rim you are going to use with no tire installed. Make sure it is tight but not being stretched, and mark the insert with a marker a few millimeters short of where it overlaps. Then, simply cut at your marked line, removing the extra length of foam. Then, using a small screwdriver or punch, poke a hole in the foam about 5cm in from each end. This will be for the supplied zip-tie to run through and connect the ends. The tie doesn’t need to be super tight, but you do want it to be tight enough to hold the two ends together.
Next, we used the same process as we did with the CushCore by installing the tire on the wheel dry (no sealant) and with no insert. We then removed one side of the tire while leaving the other bead mounted in place. We then installed the Air-Liner and the other side of the tire. It is a bit easier than the CushCore, since the overall width and volume of the Air-Liner is less. After setting the bead and having the tire mounted on both sides without sealant, we added it through the valve for another mess-free installation.
The volume and size of the CushCore is significant; the foam contacts the side of our tire. Adding air and adjusting pressure is easy thanks to the specific valves. Just make sure you have the valve up and away from the sealant that will pool at the bottom when adjusting pressure. This should always be the case with tubeless so you don’t get sealant coming back out the valve, but with the added holes in the valve, it is even more important.
To start, we ran our normal pressure of 25 psi for the same wheel and tire combo in the front and in the rear. It was obvious immediately that with the reduced air volume the tire was much stiffer when compressed. The added progression was so drastic that after only a few miles of rough terrain we started to try lower pressures.
First, we went to 23 psi, then 21 psi, then 19 psi. Yes, we went to 19 psi with surprisingly good results all around. We did have a few impacts where we bottomed out onto the foam, but due to its width and shape, the rim itself never contacted the ground. The lower pressure provided much better traction in corners and braking, while the shape of the insert almost supported the sidewall.
The Air-Liner is a smaller, less-overwhelming insert that really sits in the bed of the rim and protrudes a few millimeters past the bead wall of the rim. It takes up less space in the tire, reducing its effectiveness as a volume spacer. It isn’t better or worse, but offers a different rate of progression. Running our normal pressure of around 25 psi, the response of the tire was faster on impacts but felt more traditional. If we didn’t know they were in there, it would be hard to tell straightaway.
As we started to lower the pressure, the effect became more noticeable, since traction and compression increased but the tires didn’t feel squishy like they normally would with the lower pressure. Cornering and small-bump compliance were increased, but there was still a good level of support for big bumps and impacts. We definitely bottomed the rim out on the tire a few times when around 18 psi, but between the foam buffer and added progression rate, it was a soft touch rather than a day-ending strike that would break a rim or cut a tire.
Cornering and braking were improved, but there was less sidewall support at the lowest pressures. This led us to run more pressure in the Vittoria system to feel confident, but it was still lower than what we would opt for without the insert. We didn’t really notice any added weight from the Air-Liner.
Both systems are very interesting and, to be honest, affected our riding a lot more than we expected. The increased protection and ability to choose lower pressures added confidence to all of our rides, technical or packed flow trail. Traction was improved without the worry of damage to a wheel or tire if things got out of hand and rowdy.
Both systems add weight to your wheel system, but, in all honesty, we didn’t feel it at all. If weight is a concern, there are a few things to consider. First, the Air-Liner system by Vittoria for both tires weighs less than one CushCore tire insert. On the flip side, if you are the type who gets a lot of flat tires or really gets rowdy, the added volume and protection CushCore offers is unmatched thanks to its width. You also wouldn’t need to bring any spare tubes, since both claim you can ride them with a flat tire for over 100 miles.
We tested the run-flat ability of both systems, and while it is true the inserts definitely help, the CushCore seems to offer more rim protection. Both inserts seem to have the same durometer foam, but the CushCore has a smooth finish, while the Vittoria is a bit rougher. When we did this test, we rode 20 miles at a less aggressive pace than we normally would ride, but not nearly like we would if we had no inserts and double-flat tires. Overall, we would confidently head out for a ride without a spare tube if we had either of these inserts installed.
Running the same pressure as we would without the inserts offered a feel that was better on packed smooth trails, but when the terrain turned rough, we immediately felt like we should drop pressure. With the CushCore, it felt like we could drop more pressure than with the Vittoria, but both felt best around 19–20 psi when, without inserts, we normally opt for 25–27 psi. Neither seems to absorb sealant, meaning they should last a very long time without acting as a sponge. Either way, one of the two systems will likely be installed on our bikes from now on when we take on rougher terrain.
• More foam for more protection
• Heavier and more expensive
• Get-rowdy protection
• Universal fit
• Need to buy two kits—one for each wheel
• Light and effective
Price: $149 (complete set)
Size: 27.5 inches and 29 inches
Price: $34.99 each ($69.98 total)
Sizes: Cut to size; 29 inches and below