With Rims, Inches rules!
After flirting with different systems, tyre size codes have gone almost completely metric. There are still a few relics around in R.V. and aircraft tyres, based on inches, but passenger tyres are coded in metric. All except the rim size, so really it becomes a hybrid of both. Why didn’t they get rid of the inches then.
The answer, is that there are just too many inch rims to change. For example, U.S.A. makes over 60 million rims a year, and they last forever. Rims are used all over the world, from Afghanistan to Zagreb, ending up on all sorts of vehicles. They all rely on the fit between tyre and rim, which is designed to tight tolerances to hold it on. It just got too hard to effect the change. Conservatively, it would take 50 years or more of confusion and danger to change the world over to all metric. It was easier to call it a “nominal rim size” in inches.
There are in fact, metric rim sizes, which are designed to be used with only two types of tyres- Michelin TRX and its derivatives, and Dunlop Denloc run flat tyres. These are specially matched tyres and rims, of different diameters to the inch norm. They must NEVER be fitted to inch coded rims.
The rim diameter is known as the “nominal rim diameter”. A fourteen inch rim doesn’t measure fourteen inches anyway, because of the designed interference fit between the tyre bead and the rim ledge. It is measured only with a specially designed ball tape, because the rim ledge is a taper, and dependent on just where the measurement is made on the taper, you yet a different result, so a tape measurement is useless. For example, a 14 inch rim (nominally 14) has a specified diameter of 13.968 inches, with a plus and minus tolerance from 13.9838 to 13.9523 inches. The rim taper leads the tyre bead up to a radiussed flange which stops the bead going any further.
There is a further complication. So that the bead will not unseat readily if deflated accidentally or purposely, a “safety hump” is placed inboard of the flange, of diameter just below the final specified ledge diameter. During fitting, the tyre is inflated until the pressure is sufficient to slide the lubricated beads over the hump. Placed far enough down the taper to allow the beads of the tyre to seat on the ledge between flange and hump, it holds the deflated tyre in position hopefully long enough for the motorist to slow to a halt.
The logic is that you can still steer the car until the beads become dislodged, after which it becomes uncontrollable.
While it is possible to design a tyre/rim combination to pass this bead unseating test (see Dunlop Denloc above), the easiest way is to incorporate these safety humps. The bead unseating test is a requirement of the Australian Design Rules, and the rims used in the test procedure do not have a safety hump- the tyre is a tight fit anyway.
As with all things engineering, an unfavourable combination of tolerances can result in a very tight, or very loose fit between tyre and rim. Some tyre makes have the reputation amongst fitters of being difficult to break the seal. The design manual for the engineers from that particular tyre maker may specify a tight squeeze of the rubber and fabric between bead hoop built into the tyre, and the rim. Others are a little more relaxed about it. Different styles, or applications even within the one make, may be tight or loose, dependent on how the tyre is to be used.