Bushmaster, there are 3 things one should consider before doing your conversions. ETRTO, EPI and pressure. ETRTO - (European Tire and Rim Technical Organization): Standards organization for tire and rim sizes. The ETRTO sizing (e.g. 37-622) means that the tire is 37 mm wide and 622 mm diameter bead. Basically, a well manufactured tyre (e.g. Schwalbe) will fit best on a well manufactured rim (e.g DT Swiss) EPI - Ends Per Inch (Threads per Inch): Unit for the density of the carcass fabric. the higher the EPI, the denser ther carcass and so the higher the quality of the tire. A coarser thread is used in less dense carcass manufature. These tyres often battle to seal. Schwalbe's minimum standard is 50 EPI (wire bead), other manufacturers go as low as 24 EPI Pressure - the minimum infation pressure as indicated by the manufacturer. a minimum (and maximum) pressure is usually indicated on the sidewall of the tyre. For safety reasons, this should be maintained. Tyres that roll off rims either do not conform to ETRTO or are under inflated Why is it sometimes so difficult to fit a tire? Fitting difficulties often arise when the diameters of the rim and the tire do not match perfectly. Rims may have a tolerance in diameter of +/- 0,5 mm. In addition, the height of the rim flank also may have a tolerance of +/- 0,5 mm. This adds up to a complete tolerance of +/- 1,55 in the outside diameter, or of +/- 4,7 mm in the outer circumference. This corresponds to a maximum possible circumferential difference of 9.4 mm between the largest and the smallest rim. A tire must fit on both extremes. Because a safe fit must be ensured even on the smallest permissible rim diameter, the proper centering of the tire on the largest permissible rim can prove quite difficult. The circumferential tolerance of Schwalbe tires is ? 1 mm. The Marathon Plus has proven to be extremely difficult to fit, particularly in the very narrow version. Through the rigidity of the tire, the tire slips repeatedly from the drop-center and it is extremely hard to pull the last piece of the tire over the rim flange. A third hand, holding the tire tight in the drop-center on the opposite side, is very helpful here. Instead of a ?third hand?, a zip tie or an old pedal strap can help keep the tire firmly in the drop-center during fitting. D1 Bead Seat diameter ? 0,5 mm G Rim flank height ? 0,5 mm D2 Rim outer diameter, D1 + 2 x G = ? 1,5 mm U Rim Circumference, D2 ? π = ? 4,71 mm Shamus2009-11-18 22:04:06
