Tire Fit Gauge

How wide a tire can I fit on my bike? It’s a common question, and yet it’s difficult to figure out, especially when you plan to change wheel sizes.

Hahn Rossman has developed a simple tire fit gauge that allows checking tire clearances on your bike. Put the disc for the tire width in the slot for the wheel size. Place the gauge on a dummy axle (any old hub axle will work). Rotate the gauge around the axle to check clearances between the chainstays, at the back of the seat tube and between the seatstays.

It’s such a simple tool that you wonder why nobody else has made one before. Above, you can see that if your frame has clearance for 700C x 23 mm tires…

… you may also be able to fit 650B x 38 mm tires…

… but you need a lot of extra clearance to fit 26″ x 54 mm tires.
The outer diameter of all wheels is the same, but it’s the clearance to the chainstays that is often too tight. The gauge makes it easy to visualize where the widest portion of the tire will be. It eliminates the guesswork that can make conversions to different tire sizes a hit-or-miss.
The Tire Fit Gauge is laser-cut from plexiglass. It’s in stock now. Click here for more information.

20 Responses to Tire Fit Gauge

  1. Mark Petry December 14, 2018 at 4:38 am #

    That’s a very simple and elegant tool. Great idea.

  2. larryatcycleitalia December 14, 2018 at 5:10 am #

    “…but it’s the clearance to the chainstays that is often too tight.” I’m not so sure about that based on experience. I put a vintage bike together for Eroica CA a few years ago and had plenty of room between the stays and under the brakes. But I couldn’t put the rear wheel on unless I deflated the tire!
    A gizmo like this might have helped me know this beforehand, so BRAVO to the inventor and you for making it available.

    • Jan Heine, Editor, Bicycle Quarterly December 14, 2018 at 5:20 am #

      Yes, the horizontal dropouts of many vintage racing bikes add an extra layer of complication. These days, it’s often also the brakes that don’t open wide enough to remove the wheel…

  3. Eli December 14, 2018 at 5:35 am #

    Will you sell an appropriate dummy axle for those of us living in small apartments who don’t tend to keep old parts around? Or can you recommend a third-party source?

    • Jan Heine, Editor, Bicycle Quarterly December 14, 2018 at 1:57 pm #

      The tool originally was intended for framebuilders, who have dummy axles in their frame jigs. You can get the axles from the makers of frame jigs. We also sell the Nitto dummy axle. It’s smaller in diameter – add a few layers of tape, and the Tire Fit Gauge fits.

      • Steve Brands December 15, 2018 at 10:09 pm #

        This is completely unrelated, on page 78 of the BQ article on Ostrich bags, what is the purpose of that little pin brazed to your fork steerer? I couldn’t find another place to ask the question. I’m sure that pin caught others eye too.

        • Jan Heine, Editor, Bicycle Quarterly December 16, 2018 at 4:06 pm #

          The pin is a brass brush that transmits the power to the taillight. It runs on an insulated brass ring pressed into the head tube. The taillight wire connects to the brass ring. The brush on the fork also is insulated and connects to the wire that comes from the Edelux headlight’s taillight output.
          This not only eliminates the wire between frame and fork (which tends to get snagged and break), but also makes disassembly for Rinko easy, without needing a connector that will eventually malfunction after years of exposure to the elements.

  4. Chris Lampe December 14, 2018 at 6:31 am #

    Any plans for discs for even wider tires? Plenty of folks out there with 29’ers that are interested in going 27.5 x 3.00. I know of a bike shop that actually keeps an extra 27.5 x 3.00 rim/tire combo in the back so people can check their frames for clearance before investing in new wheels and tires.

  5. Dan Eldredge December 14, 2018 at 6:43 am #

    Jan, Wheel Fanatyk claims (in its blurb for its rim-tire meausure tool) that “the same size tire can vary over 10mm in diameter, mounted to different rims.” If this is true then I assume that Hahn’s tool design for each tire size is the widest-case diameter to ensure that the clearances will always be at least what Hahn’s tool indicates?

    • Jan Heine, Editor, Bicycle Quarterly December 14, 2018 at 2:00 pm #

      The tool is designed for the actual size. So the 38 mm tool shows a true 38 mm tire. Figuring out how wide the tires from makers really are is another matter… At Compass, we state the actual widths in the specs, if they vary from the nominal specs.

  6. Dave Walker December 14, 2018 at 6:45 am #

    Jan (and Hahn), Such a simple and practical tool; thanks for this contribution. I could add something potentially useful to this effort if you’re interested. Some time ago, I worked out the math to calculate the actual tire width and height as a function of linear bead-to-bead tire width, rim width (internal), and tread depth (as I was interested in MTB applications, but same might apply for CX tires as well). The issues are that the actual tire width and height depends on the rim width used (and this is a big deal on modern plus-sized MTBs) and the tire size as embossed on the sidewall isn’t always an accurate reflection of the actual tire width: Bead-to-bead (B2B) width is a more accurate way to do it, obviously. Once you have a database of B2B widths for various tires then it’s easy to compare them for a given application. It’d be nice to have this as an on-line tool for users, and what better place to have it than here?

  7. John B. December 14, 2018 at 9:23 am #

    “It’s such a simple tool that you wonder why nobody else has made one before.”
    My friend Niall (@pioneervalleyframeworks) made something like this, as well as crank templates, several years ago:

  8. buckyrides December 14, 2018 at 9:52 am #

    That is a very cool device. – james

  9. mackenzy December 14, 2018 at 10:48 am #

    Now that you’re on a roll making so many great new products…do you think Compass (soon to be Rene Herse) will you ever consider making a light-weight clamp-on front “suicide” shifter?

  10. David Paranka December 15, 2018 at 6:50 am #

    Hello – Thank you for all the informative posts and articles over the years … in this case, I beg to differ when you say, “The outer diameter of all wheels is the same” – that may be true for Compass brand tires, but, several years ago too-tight fender clearances on a bike led me to measure, and the radius of a Rivendell Nifty-Swifty was a good 4 mm less than the radius of a SOMA B-Line when mounted on the same wheel (650b) which made all the difference with tire clearance underneath the fork crown.

    • Jan Heine, Editor, Bicycle Quarterly December 16, 2018 at 4:14 pm #

      The gauge is a measuring tool, so it shows how much a true tire of that width and height needs. As you and others point out, many tires are not true to their stated widths, for a variety of factors. The user needs to consider these factors when making their tire choice. Making a tool that doesn’t show accurate measurements doesn’t make sense. For example, if you know that your tire is 2 mm oversize, then just use a larger disc (or check that you have an extra 2 mm all the way around the disc).
      In an ideal world, we’d have discs for each rim/tire combination in the world. If I was a framebuilder who uses an unusual rim/tire combination a lot, I’d probably make my own disc for that setup.
      Most of all, you’ll always need at least 3 mm to allow the wheel to wobble and the tire to stretch, so just because the tool barely clears doesn’t mean a tire that size fits. The tool really is intended for visualizing where the tire will be when a framebuilder has the frame in the jig and can’t install a wheel to check the clearances.

  11. Andy Stow December 15, 2018 at 7:29 am #

    Very cool, I was going to make one of these out of plywood. Now I don’t have to!

  12. Garry Armsworth December 15, 2018 at 10:55 pm #

    Great idea and discs that represent tire widths of 28 and 32 mm would be even greater!

    • Jan Heine, Editor, Bicycle Quarterly December 16, 2018 at 4:00 pm #

      For 28 and 32 mm tires, use the 34 mm disc. If it clears all around, you have save clearances for a 28 mm tire (which is just 3 mm smaller in each direction). For a 32 mm tire, you want a little clearance around the 32 mm disc.

  13. SteveP December 17, 2018 at 7:11 am #

    I just had a frame made and while the CAD said 48mm tires would fit, even Compass 38mm tires (measure true to size on my rims) were close on the fork. I had about 5mm clearance under the steerer tube at the crown. Interestingly, 44mm Compass tires (which run smaller) were only 1mm taller, so still usable.
    The issue (which I have seen before) is that builders tend to cut a unicrown steerer tube perpendicular to its axis. This means the trailing (rear) edge) is closer to the tire than the leading edge. Ideally, the steerer would be cut perpendicular to the fork rake and remain equidistant from the tire front to rear (actually closest in the center)