Two chaps came to see me recently for Bike Sizing exercises. The first chap was considering buying two bikes, an MTB and a touring bike. He was about 175cm tall, with slightly longish arms and legs but still fell very squarely into the 54cm size category of the touring bike he was after.
The second chap came in a few hours later and was confused. He was keen on a Canyon road bike, but being an internet retailer, had no opportunity to go and sit on one somewhere. He was 178cm tall (5’10″) and had been putting his dimensions into the Canyon website size calculator. The reason he was confused was that the calculator kept telling him he was a 53, whereas his instincts (and mine) said 56.
He thought he may have been doing the measurements incorrectly, so I did them for him, and sure enough, the website suggested 53. But I could now see why, he had extremely short arms for his height and simulating the 53 geometry with my jig confirmed he needed that size.
Comparing the two chap’s dimensions afterwards, the arms of the shorter 1st customer were a full 6″ (15cm) longer than the 2nd!