this seems like a question I should know the answer to already, and yet I find myself second guessing myself, so where better to ask but here.
When installing tubeless tires, is there a max safe pressure to use to get the beads to pop into place?
For example, installing gravel tires with a stated max pressure of 60PSI, of course that’s max pressure for riding, but would it be an issue to put 120-140psi into my airshot to get them to seat properly? I get a bit nervous going about 120psi and thus often have 90% seated tires with a bit of bead not seated properly.
Pumping up your Airshot to 120-140PSI is a different thing than inflating your tire to 120-140PSI. It should be fine. The tire has a greater volume, so there’s a pressure differential once you release the air.
Like you I have to put at least 120psi in my Airshot to seat 28mm road tyres, anything less and they often don’t seat properly. The Airshot volume is 1.15 L, so when you seat a tyre the final tyre pressure will be inversely proportional to the combined volume of the Airshot and tyre, plus the volume of air that escaped as the beads seated.
Starting with 120 psi, assuming no air escapes, and roughly estimating the volume of your gravel tyre at 0.7 L, the final tyre pressure would be about 75 psi. But of course air does escape as the bead seats so it’s very unlikely the final seated tyre pressure would exceed the stated max tyre pressure of 60 psi. Even if it does that won’t damage the tyre or the rim, so you’re safe.
Just for interest, I have to change a 28mm tubeless tyre soon so I’ll check the final pressure in the Airshot before I disconnect it. Easy to do, put a pressure gauge on the Airshot filling valve and that will be the final tyre pressure (I’ll post the result).
You could do the same to confirm you aren’t doing damage.
Just did this with 28mm GP5000 TT TR on Zipp 808. Anything less than 120 PSI in the Airshot didn’t seat the tires, and after seating the tires had right around 65 PSI in them with no sealant.