What am I doing wrong?
I'm on my 4th (fourth!!) attempt to run a tubeless setup on my Cervelo S5, with Conti GP5000TL on Mavic Cosmic Elite UST wheels. The tyre inflates properly with little effort, seat properly and seem to seal around the rim. I can't hear any hissing, but the tyre deflates slowly overnight. This only happens with the rear wheel - the front sealed first time and has been problem free. And other tubeless wheels I own have been easy to set up. So hopefully I'm trying to do this correctly!
I've tried the originally installed Mavic tubeless rim tape, and 3 attempts with others. I've installed the tape and then left it overnight with an inner tube inside to help the tape adhesive bond to the rim.
I'm using the Mavic shaped valves which are designed specifically to match the centre channel on the rim.
I'm using 18mm tubeless specific rim tape which fits neatly into the rim. The rims have a specified 17mm internal width, but that's between the outer edges of the braking surface, not the tyre bed, which is slightly wider.
There's plenty of sealant in the tyre (40ml), and I'm shaking/bouncing/riding the tyre in every orientation to ensure it's available at all possible leaking points.
I *think* what's happening is that imperfections in the rim surface (there are a couple) make it impossible for the rim tape to stick properly, and the air/sealant just sneaks underneath at these points, leaking into the interior of the rim - there's no leaking sealant visible externally.
Any ideas for what else I could do?
Or should I simply return the wheel to the vendor, on the basis that it doesn't perform as advertised.
I haven't used any of the recent Mavic road UST stuff, but this is my best educated guess (having used a bunch of other tubeless stuff).
1. In general, I see a higher leak-down rate of tubeless tires compared to (butyl) inner tubes. So there's a little bit of this that's going to happen... but not as fast as you're experiencing.
2. When you say it deflates overnight - are the tire beads staying seated in place? As in, they're not coming undone and unlocking past the bead shelves? The reason I ask this is that I'm wondering if they aren't getting fully seated in the first place.
3. MTB UST stuff always worked best when you used a UST tire + UST rim. Things could get wonky when you deviated. For example, I had a UST gravel tire bead catastrophically fail during inflation on a Stan's rim (which are more intended to be used with tube-type tires converted to tubeless, or "tubeless-ready" tires). And I've definitely seen reports of non-UST MTB tires not seating or sealing as well on UST rims. UST was conceived as a system, whereas a lot of the tubeless-ready stuff is a little more 'wild west'. I've heard that their road UST tires and rims work really well when used together.
4. How tightly do those tires fit on those rims? It seems possible that they're too loose, and you may need to build up additional layers of tubeless tape.
5. Did you tighten down the valve stem a lot, and use an o-ring under the locknut on the inner side of the rim? Are you sure that the hole you cut in the tape for the valve stem hasn't grown, or that the tape hasn't split around this area? I've seen this result in leaks. My tubeless installation video
shows the details of how I cut valve holes, and talks about tightening down the lock nut sufficiently.
6. What sealant are you using, and how old is it?
7. It's possible that your tape is leaking, as you mentioned. I've had the best luck with Stan's tape (and Hed, and Vision Tech, which seem to be identical). You can remove the tape, clean the rim with alcohol, let it dry, and install fresh tape (TIGHTLY), install inner tubes, and pump to max pressure overnight. This has always worked for me. Old-school tubular folks would lightly sand aluminum rim surfaces to roughen them up for better adhesion... though I'm sure this would void your warranty if you did it to your rims.
Greg Kopecky - Slowtwitch Tech Editor
Founder of Minimal Multisport Athlete website, blog, podcast, and Youtube channel: https://www.minimalmultisport.com/