Welcome to our Ask a Mechanic column where our expert mechanic Daniel Slusser answers your bike maintenance questions. If you have a question for Daniel, please post it on our Facebook Wall or e-mail Daniel directly at firstname.lastname@example.org.
Question: I just bought a Schwalbe Big Apple tire for my 29er and the tire size printed on the side says that the tire is for a 28″ wheel. I’ve never seen a 28″ wheel before. Is this a new wheel size or what? From: Bill
Answer: Tire sizing is kind of a mess at the moment. The current sizing is based on the outer diameter of the tire. This means that a small 29er tire is actually a 28, hence the unusual sizing printed on the Big Apple. Unfortunately, or fortunately the rest of the industry ignores that tire size denotes outer tire diameter and labels tires 28″ tires as 29×1.5″. With the renaissance of 650B and the debate over whether this wheel size should be called 27″ or 27.5″, only add to the confusion. And why is 650B a different diameter than 650C? To have the patience to read the answer to that question requires a level of interest in the subject that no sane person can muster, but I digress.
Claimed tire width measurements are usually way off too, especially with mountain bike tires. That’s why we mount and measure every mountain bike tire we carry and denote the inner rim width of the wheel we mounted the tire on.
I know what you are thinking, “Why can’t the industry just get it together and come up with a consistent sizing nomenclature?” The answer to that question is, ‘They already have.’ In fact this style of sizing has been around for a very long time but most people either don’t see it or don’t understand it. ETRTO is the measurement we are all looking for.
This measurement is expressed as a width followed by a bead seat diameter in millimeters. For example, your Schwalbe Big Apple is a 55-622. At 55mm, this tire works out to 2.165 inches wide. Of course this doesn’t exactly match the printed size of 2.15″ wide. Why is this? It’s a mystery to me. However, I’ve found that the metric width is typically more accurate compared to the width in inches found on the side of the tire (no matter which company is making the tire). That second number, 622, is the diameter of the tire bead. 622mm is the bead seat diameter for all “29er” and “700c” wheels. If you are interested, check out the charts created by the late, but definitely great, Sheldon Brown that gives a full rundown on what each size roughly converts to.
Daniel Slusser is a professional bicycle mechanic with over ten years of experience. He holds a bachelor’s degree in business administration from HSU and a master’s degree in history from Cal Poly. When he is not riding, wrenching, or writing he enjoys spending time with his wife and two children.