Hahn Rossman’s Ingenious Tire Clearance Checker

Jim’s Tech Talk

By Jim Langley

A few Tech Talks back, I introduced my current project, updating a 1970s Lejeune in order to ride it in the upcoming Eroica California ride. If you missed it, here’s a link: https://www.roadbikerider.com/quick-tips-for-upgrading-vintage-road-bikes/.

Sadly, it turns out that I probably won’t be able to attend Eroica this year. Still, I’m going ahead with refurbishing this vintage steel rig for future editions of the ride. And, because this classic French racer rides so nicely I’m going to use it more than just once every few years.

For this, I want to go to wider tires from the 25mm ones already on the bike – and the fatter the better to smooth the ride. With wider rubber all the rage right now, there are more options than ever. I’m probably going to go with Panaracer Pasela Pro Tites, which come in 25, 28, 32, 35 and 38mm widths (my wheels are 700c).

Tire Sizing Has Been a Crap-Shoot

Before I order the tires, I need to make sure they’ll fit the Lejeune. Up until only recently, there hasn’t been an easy way to look at a bicycle and know with a high likelihood of success how wide you can go.

This is because while tires are marked with their nominal size, they may actually inflate larger or smaller. Occasionally they’re significantly over or under size. And, with old (and modern) frames and forks, you can’t be sure how much clearance you have without actually trying the tires out. Also, what fits on the front might not on the rear.

Roll the Dice and You Might Lose Your Money

If you take your best guess based on the tires on a bike and the clearance remaining, you might get lucky and pick a winning wider set of sneaks. But, you might also be wrong and discover that the new treads rub on the underside of a brake or fork, or on the chainstays, seat tube or even the down tube.

In that case, hopefully you can return the tires for narrower ones. That’s something to check before buying new tires not after, since mounting and definitely riding on tires could void a return policy. 

I’ve made educated guesses in the past, returned some tires and lost money on some I couldn’t, too. So, this time I wanted a more surefire measurement. 

Hitting the Jackpot with Tire Sizing

And that’s how I found Hahn Rossman’s super cool Tire Fit Gauge sold by Rene Herse Cycles and for only $25.

In my photos you can see the tool and how it works. It’s designed to fit onto a 13mm axle, which is what’s on framebuilder’s jigs (according to the instructions). So, to get it to fit into frames, Hahn says to use an axle set, which you might get from an old pair of hubs. I had an old hub set collecting dust, so I took the axles out (photo). 

Any rods that fit into your bicycle frame and fork would serve the purpose, such as wood dowels or perhaps pencils taped in place. With through axle frames, you might be able to use your axles. Just make sure the Tire Fit Gauge arm fits correctly, with the tool’s axle slot seated on the axle.

Standard quick-release axles like I used are 9 and 10mm diameter (front and rear respectively). That’s too small so I wrapped electrical tape around them until the Tire Fit Gauge arm fit snugly. You’ll do the same with your “axles.” Then, with the dummy axles in place in the fork and frame, you’re ready to use Hahn’s tool.

Using the Tool

In the end of the Tire Fit Gauge’s arm opposite the axle there are slots for 559mm (26-inch), 584mm (650B/27.5-inch) and 622mm (700C) diameter wheels. And included with the tool are tire discs in six widths/sizes, 23, 34, 38, 42, 48 and 54mm. 

To use the tool, you place the arm on your dummy axle with the disc you want to try in the correct wheel diameter slot. By swinging the arm through its arc you can check clearance at the key touch points.

Up front, these include, the fork blades, fork crown, brake, and don’t forget the down tube. While on retro velos there’s likely loads of clearance there, I’ve seen modern carbon rigs (like my Cervelo S5) where one 25mm tire barely clears the down tube and the next brushes it.

For the rear, check at the seat stays and chain stays, the brake and/or brake bridge and the seat tube. On my Lejeune I mounted the brakes I plan to use to ensure the disc cleared. I looked for a few mm of clearance at all points around the disc, not the largest size that fit. That way, should a tire run a little oversize, it should still fit. 

In the photos with the gauge between the frame tubes, notice that the tire size disc that fits is the 38mm. I would have guessed larger based on looking alone because it appears there’s a lot more room especially at the fork (see photo of fork and wheel only). 

Choose a conservative width

The thing is, that you can never be 100 percent certain how tires will fit or change shape on different rims. The Tire Gauge tire discs are round. It might be that on your rim, your tires are more oval than round, which can make them taller and increase the chance of rubbing on the fork or brake or the frame. 

Because of this, I plan to run 35mm tires in the Lejeune rather than 38s. And, in general, using this tool, I wouldn’t go with the maximum disc size that fits, but one that allows clearance for whatever shape your tires take on your wheels (and you might have more than one pair of hoops, I know).

If you’re running fenders or other accessories that approach the tires, you will want to test with those in place, too. I’m very happy to have Hahn’s tool on hand and think if you’re building bikes or regularly swapping out tires, you’ll really like it, too.

Ride total: 9,569