PoorBob

Trying to figure out what the best rim tape size would be.

My rims are 21mm internal width and 25mm external.

Thanks,

Bob

ThermionicScott 

Tubeless tape should span the whole width from sidewall to sidewall on the inside. Does your 21mm mean between the beads or the sidewalls?

masi61

21 mm bead to bead might require 23 or even 24mm to account for the curvature of the well.

PoorBob

21 is bead to bead.

So the tape should go bead to bead? No gap or 1mm- gap on each side?

I suspect I could take some painters tape to get the width including the curvature or valley in the rim the pull it off to measure the true distance of surface.

ThermionicScott 

Adding a millimeter per side -- perhaps a smidge more -- seems like a good rule of thumb. Pacenti recommended 21mm tape for my SL23 and Brevet rims, both with a bead-to-bead width of about 19mm.

well biked

Something like 2-5 mm wider for the tape than the internal width of the rim, depending on the rim. Example (and this is obviously a very wide rim example): For WTB i45 (45mm internal width) rims, WTB recommends their 50mm tape. The extra 5mm of tape width is used for the contour of the rim channel.

Pop N Wood

Trying to learn here. I see lots of recommendations saying it only needs to cover the spoke holes. Putting tape on the channel edges cuts into possibly already tight clearance for the tire.

Does everyone do it this way?

Crankycrank

What make and model are your rims? The mfr. may have a recommended width on their website.

ThermionicScott 

Covering the spoke holes is important, but you generally want the tape to extend under the tire beads to ensure everything seals well. It's also a part of the tire fit, increasing the rim diameter slightly so that the tire is less likely to come unseated if you have a flat.

The tight fit of the tires is a feature, not a bug.

Cooperator

Yes. It is best for the bead of the tire to compress the tape against the rim. That will create the best seal.
Fwiw I've found that cheap 1" Gorilla Tape from the hardware store to be an excellent width for 19-21mm internal width rims.

Pop N Wood

In my grand experience of setting up one set of wheels tubeless I had trouble with the tire sitting on the tape causing the tape to unseat when the tire was being seated or removed.

The wheels came with I think Stans rim tape, which I tore every time I tried to seat it into the spoke channel. Went to gorilla tape, which like you note was a perfect bead to bead width and super easy for a noob to work with. This spring I pulled the gorilla tape and went with a tape cycocommute recommended. The gorilla tape pretty much came off when I pulled the tire. The new tape goes the full width of the channel but is much thinner and a slicker surface than the gorilla tape, so hopefully no more worries about losing seal on the tape with trailside repairs.

Kapusta

I used to be one of those folks who thought it only needed to cover the spike holes.

I was wrong.