A recent article on fitting double rim tapes has provoked some interesting discussion on the RCUK forum concerning the way clincher tyres work, including the following:

John Hardy: “As R Hallett has said, the idea is that one side of the tyre is supposed to have room to contract, leaving the other room to expand a bit, just enough to go over the rim edge.
Others think this is arguable, as both tyre edges have a fixed diameter and cannot expand.”

Steve Middleton:”Just for the record, fitting an extra rim tape won’t make the tyre less likely to blow off the rim. It will only make it harder to fit, hence the perception is that the tyre is a tighter fit on the rim. If you have a blow out, the other bead won’t be sitting in the rim well so the rim tape makes no difference.”

First of all, the article was not talking about “blowing off the rim” as in the tyre being forced over the edge by excess inflation pressure. Assuming the rim does not split, this will eventually happen to any clincher tyre however carefully fitted since the bead, whether wire or folding, is elastic. Ie. it will stretch and, as it is pulled by inflation pressure, will ultimately stretch enough to pop over the rim. At the Continental tyre factory (and presumably others) there is a test rig, used to check tyre fit, which is pressurised with water to obviate risk of explosion and on which tyres can be “inflated” until they pop off, which invariably occurs once 170-180psi is reached.

Incidentally, the hook on a hook bead rim is more important in retaining the tyre than the unbroken loop of the bead itself, which is why high-pressure clinchers must be used on a hook-bead type rim. If you don’t believe this, try cutting the bead of a folding tyre in a few places. Fit it and inflate it carefully so the inner tube keeps the bead tucked under the rim hook; you should be able to get to 100psi… This is not possible with an older non-hook clincher rim such as Westwood and indeed maximum inflation pressure on non-hook bead rims is limited by the propensity of the bead to stretch and pop over the rim.

So, in this respect, Steve Middleton is correct: an extra rim tape will have no effect on blow-off due to excess inflation pressure (or overheating from braking).

The original article’s purpose was to propose a way to combat a potential problem alluded to by John Hardy’s comment. The important point to bear in mind is that the principle of the detachable clincher tyre used today relies on the well of the rim. The idea was patented by Charles K Welch in 1890 and the patent, bought by John Boyd Dunlop, was the basis on which the Dunlop fortune was built, since it quickly proved by far the best means of making a tyre readily detachable. 

The concept relies on concentricity: the rim well, bead and rim flange are concentric when the tyre is inflated, with the flange circumference greater than the bead circumference. Ordinarily, there would be no way to get the bead over the flange without stretching it by force. Welch realised that, by making a well or trough in the centre of the rim, he could drop the bead into it on one side and thereby on the side diametrically opposite find enough slack in the bead to pass over the opposite side flange. Therefore, those who “think this is arguable, as both tyre edges have a fixed diameter and cannot expand” misunderstand the clincher concept.

So, the deeper the well or trough, the more easily the bead will pass over the flange and the easier the tyre is to fit and remove. Obviously flange height will affect the process, since a small dimensional difference between the height of the flange and the bead seat will require a less deep trough and vice versa. In any case, reducing the depth of the trough also reduces the amount of slack available to pass over the opposite rim flange and therefore makes fitment slightly harder.

The purpose of fitting a second or even third rim tape is to fill the trough and make it harder for the tyre to come off the rim once it is deflated, which may save a fall in the event of a puncture.