Nothing from the list for some time - have I been de-subscribed? Going through my huge pile of old Classis Bike magazines - often read and re-read - I found some tyre size conversions that puzzle me. Old Size Modern Metric 2.50/2.75 3.10 80/90 3.00/3.25 3.60 90/90 3.50 4.10 100/90 4.00 4.25/85 110/90 4.70 none 120/90 4.50/5.00 5.10 130/90
A bit of arithmetic shows that all is not equal. There are 25.4mm to one inch. Old sizes and modern are expressed in inches whilst metric is, well, metric. Thus 4.00 (as in 4.00X18) is 101.6mm. 110/90 is 110mm width and 90% aspect ratio equals 99mm height a smidgeon less than 101.6mm. Modern 4.25/80 is size, 4.25" and 85% aspect ratio that calculates to 3.61" so it seems a little small although the width is larger. The 3.25X18 is 3.25" equals 82.55mm. The said equivalent is 90/90 that to me works out at 90mm X 90% equals 81mm equals 3.18". Working this way and assuming a 90% aspect ratio, I get a theoretical 113/90 to replace a 4.00 and a 92/90 for a 3.25. OK so now that all is clear,I'll just go into my local tyre shop and demand a 113/90X18. Regards Paul (non metricated and still thinks in thou) H
