One of my favorite debates to sit back and listen to in crowds of bike people is the effect of rim width on tires.  There isn’t exactly a consensus of opinion on that topic, but there ARE some passionately held beliefs.  Today I had a chance to get some ACTUAL TEST DATA on this subject while shuffling wheels and tires around at work, so I did, and thought I’d share it with anyone who’s interested...

The tires used were both WTB Vigilante 2.3 x 29”.  One is a little older and has more tread wear, and they don’t have exactly the same graphics, but they are exactly the same tire otherwise.  One tire is mounted on a Gravity Worx 29” x 40mm wheel, and one is mounted on an Easton Haven Carbon wheel with an external rim width that measures 28.45mm with my calipers.  The side by side photo of the same tire on two different width rims doesn’t do justice to the differences.  In person it looks a lot more dramatic.  At the end of the day though, it’s the actual measurements that really matter, so measure I did:  28.45mm rims = a max carcass width of 56.42mm.  Same tire on a 40mm rim = 64.08mm. So… an 11.65mm wider rim resulted in an increase in overall tire carcass width of 7.66mm.  Interestingly enough, the difference in measurements of the actual tread width was a lot less.  About 60mm wide on the 40mm rim vs. about 58mm on the 28.45mm rim when measuring across the outside of the tread lugs.  So going 11.65mm wider on the rim actually only increases the width across the tread by about 2mm.  Draw whatever conclusions you want.  I’m just presenting the data…

While I was doing all this tire mounting and measuring I decided to also attempt to settle once and for all a question that I’ve heard hotly debated more than once in a bike shop:  Does increasing or decreasing rim width have any effect on overall wheel/tire diameter?  I have always maintained that rim width makes no difference in overall wheel and tire diameter because the amount of material in the tire at the center of the tread doesn’t change when you change rim width.  More than one individual has patiently explained to me that when the beads move further apart the tread is somehow “sucked in” at the center as a result.  After rolling both of the above mentioned wheels through one rotation and marking the developed length on the ground I’m officially calling “bullshit” on the idea that your tire diameter changes when changing rim width.  According to this theory (that I DON’T subscribe to), the 40mm wheel should have had a shorter developed length because the tire beads were spread further apart than on the 28.45mm wheel.  In fact, the 40mm wheel wound up having a developed length 2.5 cm GREATER than the narrower wheel, which is exactly the OPPOSITE of what you would expect if tire diameter DID change with rim width.  So why the difference?  Upon closer inspection, the tread on the 40mm wheel was noticeably less worn, which made the knobs taller, and also increased the diameter relative to the more worn tire on the narrower rim.  If one tire’s tread is worn say, 4mm more than the other, that would be a difference of 8mm in overall wheel diameter, which when multiplied by 3.14 = 25.12mm or in other words, just about the difference between the two wheels’ ACTUAL MEASURED CIRCUMFERENCE.  If there’s anyone out there who has read this, and still thinks that rim width effects tire diameter and can offer a plausible explanation of how I’m wrong, I’d love to hear it.