MTB tyre pressure

13/06/23 13:51 Filed in: Riding

I was reading an article about CushCore the other day and it suggested that a number of pro enduro riders actually use the XC version of the insert over the more aggressive Pro version because they primarily use it to prevent burping.
Interesting idea. I have 2 bikes with the XC inserts and 2 with the Pro inserts. Since going CushCore I have not burped a tyre once. I used to do it quite regularly back in the days of skinny rims not necessarily designed to be tubeless, with moderately fat tyres and the lowest air pressure I could get away with (mid-20s). Land a drop a little bit sideways - burp. Slide into a corner and the tyre suddenly finds traction - burp. There were numerous initiating factors, but always the result was losing 10+ psi and having to stop and pump it up. Sometimes much of the sealant came out with the rush of air too.

Taking a step backwards, there are three roles an insert can play.
Many, but not all, sit between the tyre beads and prevent them moving inwards. Thus no burping.
All of them occupy some volume of air in the tyre and act like a fork air chamber spacer (eg, a RockShox token) to reduce the volume of air in the tyre and "ramp up" the pressure rise more quickly on hitting a bump. This is the pathway that led to the development of both CushCore and the original tyre insert, the Schwalbe/Syntace ProCore. I've read an interview with the Syntace guy who wanted to better couple the movement of the tyres with the movement of the suspension. That it also helped the other two traits I'm discussing here is just a bonus. Regardless of the sophistication of the suspension mechanisms on a bike, the tyre is a large and uncontrolled suspension element (as in Formula 1 cars). In the Syntace experiments, they found that the tyre moved completely before the fork moved at all - but ProCore coupled the two together more closely. Which was better for control and therefore traction.
The third is the primary reason many people go to inserts, and that is for rim protection when you hit something really hard. ProCore is essentially running a road tyre inside your MTB tyre. The MTB tyre is tubeless, but the road tyre uses a tube at around 80 psi. That much pressure pushing inwards on the upper rim floor became much more pressure in a bottom-out situation. Enough pressure to crack a carbon rim not designed to have such loads on the normally unloaded upper floor surface. Hence the recommendation not to use ProCore with carbon rims. Any insert that sits down in the rim well can transfer too much load to that upper rim floor and crack a carbon rim - even if the foam ones don't do it very often. My DH bike runs DT Swiss aluminium rims, so no issue there. And my enduro bike runs Zipp 3Zero Moto rims, so no upper floor to worry about (they are built like cheap metal rims without a hollow structure in the middle - part of the reason they are so different to other rims).

On my 26" wheeled DH bike I use to run DH tyres and pretty high air pressure and still put big dents in the rim every season. The Sender has had CushCore Pro in the 29" DH tyres since day 1, and there are zero marks on the rims from impacts, despite running very low pressure. The benefit of the low air pressure is amazing grip (the tyre can conform to everything so it hangs on much better) and lower rolling resistance (on rough terrain, high air pressure uses energy that lower air pressure does not - by lifting the bike up and over more obstacles that would not happen if the tyre deformed over the object instead).

I titled this article "tyre pressure" because I actually wanted to talk about the effects of tyre pressures on MTB riding. But I had to discuss the impact (pun intended) of inserts first.
On my old 26" wheeled DH bike, the Santa Cruz V10.4, I tried running some non-DH tyres for a while. I had to run about 36 psi in the rear tyre to even hope to make it down a single run without a pinch flat. Even at that high pressure, I would get a flat every third day. This was with tubes. Once I dinged up the rims a lot, I replaced them with Stan's Flow rims and ran tubeless. I still had to run around 34 psi in the rear to protect the rim, which got dents in it regularly. Mid-30s tyres do not grip optimally.

I remember a group skills class I was running about 10 years ago. One student liked to run the max sidewall pressure on his tyres. That was 65 psi. He literally had zero grip. His plan of action was to aim the front tyre at ridges and rocks that the tyre could bounce off. I like to get students to experiment with tyre pressure so they can feel what different pressures do for them. Then - pre inserts - I was running mid or high 20s psi in my XC bike tyres. He was incredulous that this could work. (How would he have felt about my 16 psi front tyre pressure with insert?) In the end he relented, did some experimenting, and settled on 45 psi. If he's out there, and reads this, I'd hope he'd do the experiment again and get down into the 20s where traction starts to get really good.

So, as I wrote in the prior paragraph, I can go down as low as 16 psi in the front tyre (I've even run this on the DH bike but usually use a touch more there - 19-20 psi). Lower and it moves around laterally too much for good "feel". On the rear I can get away with about 21 psi without bottoming out the tyre and feeling it regularly. But in higher speed corners, this low pressure squirms too much. It feels imprecise and also disconcerting (like something is broken or loose). I run 25 psi out back to avoid that feeling.

While running less pressure on rough terrain lowers rolling resistance, it does not turn a Maxxis Assegai into a Maxxis Ikon. Reinforced, chunky and heavy tyres still roll with greater effort than delicate, small knob and light tyres. XC racers run at the edge of puncturing (and frequently do) to maximise performance. I feel like I could do anything with a DH double-wall tyre and not damage it, but fast racers are proof that this isn't true. Regardless of the tyre's construction, minimising its rolling resistance requires minimising its pressure.
Incidentally, this applies to road tyres too. Hence the recent move towards wide rims and even wider tyres on race bikes from club level to World Tour level.

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MTB tyre pressure