A little advice please as my tubeless setup is driving me nuts.
My rear tyre leaks, today it was really bad and needed topping up every 15 minutes or so, which really spoilt my ride. It’s been doing this for several weeks now, but it holds air perfectly when the bike is in the garage, so every time I try and fix it, it seems I have been successful.
Tyre has no thorns or similar in it , checked last week. Plenty of fresh sealant last week. Tyre currently holding air fine, in the garage, but has been losing it throughout my ride earlier. I think that the leak is from the valve, but don’t understand why it only leaks while being ridden.
Any ideas?

Whenever this happens to me it is because I didn’t put the tape on properly.

Remove the tape, clean the rim thoroughly and refit some new tape, taking care that it is fully pressed down with no loose bits.

It’s quite annoying when it happens, I have a tyre right now which drops from 80psi to 40psi over a few days.

Yep, getting the rim tape right is crucial to tubeless. It “leaks” at the valve as that’s where the air works its way out from under the rim tape, the actual problem could be anywhere on the rim.

My process is:

Clean rim.
Now clean it again.
Get tape that’s the outside width of the rim – when you press the tape into the rim well it will “pull in” so that each edge sits next to the inner edges.
Mount the tyre making sure you don’t nick the edge of the tape.
Inflate and check that the tyre is mounted evenly.
Add sealant
reinflate.

The tape was shop fitted about 6 months ago, but suppose it could be iffy. Would this give the odd, only leaks while being ridden, symptoms?

I had one that wouldn’t stay up (or reinflate after I’d taken it off). New double wrap of tape fixed both issues.

Had this on one of mine.Turned out that I had one of those tubeless valves with a circular base and every time I pumped up the tyre I was turning the valve as I screwed the pump head on and off and loosening the lock ring,which caused a slow loss of air.

Can also be caused if you don’t give the back of the valve a tap with a mallet as you set up.This beds it into the tape and into the valve hole in the rim.

Rare for a tape to shift in my experience once you have set up the tyre,unless you take the tyre off and move the tape with the tyre lever.

could be sidewall flexing causing leaks to open up when riding that then seal again when not? Thats my guess

Currently inflated and holding air well. Might try tapping the valve in a bit tighter as it does seem to rotate a bit

I had a tyre with a really tiny prick hole near the bead — not idea how it even got there, as not an obvious gorse puncture – right by the rim. It was pretty OK in the shed, but once riding along, became a problem.

Was only obvious when pumped tyre right up, then rolled the wheel around in a wheelbarrow of water – really get to see the tiny bubbles at that rate.

Currently going through a world of tubeless pain with a set of Stan’s Arch rims. There are holes in the rim bed shoulder, why they’re there I’ve no idea but whatever tubeless tape I use winds up runching up as the tyre deflates, exposing the spoke holes.

I’ve even cleaned the rim with surgical spirit, nothing seems to help. Am thinking of ditching the rims entirely. It may sound drastic, but bike no 2 has Stan’s Flow rims that never seem to lose pressure. I’ve been tubeless for over ten years, but this is really pissing me off.

It’ll be the tape as others have said, this can happen in many ways, i found putting a new tyre on a rim caused it as the removal of the old tyre lifted the tape slightly, as the sealant had some sealing around the bead, meaning it moved the tape on removing the old tyre. Also check around the valve hole in the rim, sometimes the tape gets damaged there or has extra tape, causing a bad seal between the valve and rim.

As others have said, remove tape, clean it as good as you possibly can, leave it to dry out properly, then be generous with the tape (as well as getting decent tape!), i then stick a tube in and pump it to max PSI to get some pressure on the tape, leave it a day or two then remove the tube and inflate dry.

As for ‘it’s holding air well just now’, remember that’s without any forces being applied to try and force air out, stick it up to max PSI and see if it holds that, i wouldn’t tighten the valve up too much either, as you can cause damage it, it really only needs to be finger tight if the seal is good.

could be sidewall flexing causing leaks to open up when riding that then seal again when not?

This is my guess too.

Two weeks on and several more ruined rides. Decided to strip and clean everything and start again, this time, while everything was clean I spotted pink sealant coming through the sidewall. Guess this was the issue and sidewall flex causing to fail while riding!

Are they tubeless tyres? Seen so many people try and use non tubeless tyres, run into huge avoidable problems then declare tubeless is the worst thing in the world without doing it properly.

You didn’t say what make of tyre you were using. I had a Rocket Ron or Racing Ralph (can’t remember) which only lost air when riding. Looking closely the tyre was made by joining two halves down the middle. These would separate by less than 1 cm only when at the bottom and touching the ground. The wet dot at the point of escape was just one of many so I ignored it at first. Doh.