Ask a Mechanic: More on Bottom Brackets, Tire Widths

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

Question: Can I use Shimano outboard bearing bottom bracket cups from a Dura-Ace road crank on my Shimano XT mountain cranks? They look like they are the same, so it should work right?  From: Carl

Answer: Looks can be deceiving Carl. Much like the lady at the local watering hole that looked great until your sobriety returned, bike parts often promise compatibility by beguiling us with their fancy labels and comely machined edges, only to disappoint us when they are fully assembled. This is one of those cases.

Shimano Dura Ace 7900 Bottom Bracket Cup Set

Looks can be deceiving – these Dura Ace BB cups won’t work with mountain cranks, even though they may look compatible.

That is why calipers can be a mechanic’s best friend. Shimano road bottom bracket cups are 1mm per side wider than mountain bottom bracket cups. This means that if you have a 73mm bottom bracket shell (road bikes have a 68mm shell) you would need to add a .5mm spacer behind the drive side cup and your chainline will move inboard by 1mm. This could negatively affect your shifting, but it might work. If your mountain bike has a 68mm shell, you would need to add a 1.5mm spacer to the non-drive side and a 4mm spacer to the drive side. Resist the temptation to do this. There will not be enough threads left on the drive side cup to safely engage the bottom bracket shell threads in your frame. Avoid the heartbreak of destroying a frame by attempting to save $30 on the correct bottom bracket.

Because some readers may be wondering if a mountain bottom bracket will work with a road crank, the answer to that question is “yes” if you add a 1mm spacer behind each cup. The only limiting factor is whether your frame has enough threads in the bottom bracket shell to accept the longer threaded portion of mountain bottom bracket cups. Of course this setup will also require you to omit the installation of the plastic sleeve that mounts between the two cups for sealing purposes, as it is too long to fit.

Question: What is the deal with tire width measurements? Many manufacturers claim to make tires that are the same width but there is a huge discrepancy in width from one brand to another. How do I know which tires are marked correctly without mounting them?  From: Brandon

Answer: Humans are prone to exaggeration. This is especially true when it comes to lengths and widths. Just ask any of our buddies at Tackle Warehouse how big the bass was they caught last weekend! Or, just ask your girlfriend what dress size she wears for a reverse of the bass example! Tires tend to follow the bass model.

However, exaggeration is only one part of the problem. Rim width, tire manufacturing variances, air pressure, tubeless vs. tubed installation, the age of the tire, and even the design of the rim’s bead hook will have an effect on tire width. For example a Continental Grand Prix 700x23c is true to size when mounted on a standard 19mm wide road rim. But put the same tire on a wide Hed Belgium rim, and the width grows to around 28! The profile increases as well, though not as dramatically. Tires that are mounted tubeless have a greater tendency to stretch after they have been installed for a month or so. Mountain tires can grow as much as 10mm due to casing stretch (tubed tires stretch too, it just takes much longer). Then there is the issue with mountain tires of whether the manufacturer is measuring the casing width or the knob width. Each manufacturer does it differently. With some tires the knobs are the widest part of the tire, and in others it is the casing.

Unfortunately, the only way to really know a tire’s width is to mount the tire on a rim and measure it. That is why we are mounting tires and taking our own measurements and posting them in the product description so that you don’t have to. We use standard width rims to best approximate what most of our customers use. Specifically, we use a WTB Laser TCS trail rim with a 21mm inner width and inflate the tires to 32 psi. If your rim is wider or narrower, take the difference between our rim width and yours and multiply it by .75 and you will get the approximate difference in tire width for your application. We also use tubes to take these measurements because it is impractical to do a tubeless mounting for every tire we carry since we would have to give away the test tire after it was coated in sealant. Plus there is the time delay issue of tire stretch that would require us to age each tire before measuring. A safe approximation is to add 5mm (~ 0.2”) to our measurement of the width of any mountain tire that you plan to mount tubeless.

Some basic generalizations that apply to tire width by manufacturer are as follows: Michelin and WTB tires run narrow. Specialized, Schwalbe, and Continental tires generally run true to size with respect to width but tend to have a tall profile. Maxxis mountain tires are now true to size after being narrow for many years. Just remember that these are all generalizations and that some tire models within these brands don’t fit the stereotype. The WTB 2.4 Mutano Raptor is a prime example, as they tend to be true to size. Oftentimes manufacturers will describe tires as being “high volume.” This tends to refer to the profile rather than the tread or casing width. High volume tires offer a cushier ride and better traction in the rough stuff. We will let you know in our description if this claim is true or not, or if the manufacturer should have claimed it but didn’t.

In the end though, it isn’t about how big your tires are; it is how you ride them that matters!

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 University. When he is not riding, wrenching, or writing he enjoys spending time with his wife and two children.