Jim’s Tech Talk
by Jim Langley
Before we get started with this week’s topic, which is on how to determine how wide a tire your fork (and frame) can accept, I want to point out that we received great comments on last week’s story on shifting 2X drivetrains for beginners (these drivetrains have two chainrings in front).
The whole goal of the story was to attempt to keep it simple, so that even complete rookies might have enough confidence to hit the road on a 2X. But, as often happens when it comes to bicycle shifting and gearing, the topic became more complicated as the feedback rolled in.
Which is actually great because it added value by providing something that should interest even expert shifters. We heard from some of you with helpful tips and RBR’s own, coach Rick Shultz, who also posed a gearing puzzle for us to try to solve.
I pitched Rick’s pop quiz to John Schubert, who passed it along to Crispin Miller, both former editors of Bike Tech, a famous cycling science journal published in the 1980’s. Miller is also an engineer who wrote his PhD thesis about bicycle drivetrains as a student at MIT.
Note: For a peek back, in this Bike Tech from February, 1984, Miller looks at Chain Behavior in Front Derailleurs – a great subject even today.
Suffice to say, that if you’re enjoying the shifting discussion, be sure to return to last week’s TT and follow the comments thread top to bottom. You’ll enjoy it and might even learn something. Thanks for the awesome comments everyone. Here’s the link: https://www.roadbikerider.com/how-to-shift-2x-bicycle/.
Q & A: Determining The Widest Tires That’ll Fit In Your Fork/Frame
Houston roadie and RBR Premium Member, Phil Lehmberg sent in the question we’re tackling this week. Phil asks,
“Other than buying a dozen different tires to try on my HED Belgian + rims, is there any way to tell which 28mm-width road tires will fit as FRONT wheels? I have found, the hard way, that several brands of 28c (tubeless and tubed alike) tires mount “too tall” and rub the inside crown of my fork although they easily clear the brake pads with some small adjustments.
I have two Lynskey frames and the 28’s work on the REAR wheels on the newer frame but not on the older frame so I just run 25’s on the older frame. I would certainly like to run 28’s on both front and back of the newer frame.”
Not So Easy to Know
Determining tire compatibility on a given frame and fork may seem simple, but it’s not. Because tires can vary even between two of the same make and model. That shouldn’t be the case, but I’ve seen it and know it happens.
And, Phil’s trouble is the tire is touching where it passes beneath the fork crown, not on the sides, which is the more common issue in my experience. That’s a challenge because while tire companies give the nominal width of their tires (which often is only an estimate), they don’t list how tall tires are.
Complicating fitting further, how tall the tire is when mounted can vary with different width rims and different tire pressures, too. If you’ve ever mounted a 23c tire on one of the new wide carbon rims, you’ve seen how the tire that stood tall on the narrow rim now is so low it barely provides protection for the rim.
Unfortunately, I’ve never seen a comprehensive tire chart showing the maximum mounted height of tires or even specs inside a specific tire box providing this information.
But, I have a few thoughts that might help Phil.
The first is to call Lynskey. Tire clearance is something I would expect to find on design drawings for a frame. Even if Lynskey spec’d someone else’s fork for their frame, since they designed the bike for it, they should at least have an idea what size tires fit – or have a number for the fork’s tire clearance.
Now, if Phil bought only the Lynskey frame and then purchased a fork, I would try reaching out to the company that made the fork. Just like the Lynskey engineers, there should be a fork engineer who knows what tires fit that fork.
With the fork clearance number in hand, Phil would have one piece of the puzzle. Next he would need to find the largest tire available that fits inside his fork without rubbing. It’s possible that there’s a 28c tire that would fit since tires vary so much. But, finding it might require a lot of googling and reading road bike forums. For example, Weight Weenies has an active forum that’s free to join. For that matter, a company like Lynskey has lots of riders using its products and might have learned from customers what wider tires fit.
Replace the Fork
Speaking of forks, one way to solve the problem would be to throw money at the bike in the form of a new fork that’s guaranteed built to accept the tire sizes he wants to run. Phil could likely sell the old fork to recoup some of the cost of the new.
Become Your Own Expert
It sounds like Phil has already tested multiple tires, so he might be able to do his own test fitting and measurements so that he knows what clearance is needed and which tires and sizes work.
There’s actually a new tool that might help with this testing and measurement. It’s from Rene Herse Cycles (formerly known as Compass Cycles). It’s called the Tire Fit Gauge and sells for only $25.
This Tire Fit Gauge is essentially a tire-shaped feeler gauge that swings on an axle you place in your frame and fork (remove the wheels first). The tool comes with different size tire circles so that you can pass them through the stays and fork to see what size allows sufficient clearance (photos).
I hope one of these ideas helps Phil get on a wider front tire he likes. If you have any suggestions for him on 28c road tires that aren’t too tall, please post a comment below. Thanks!
Ride total: 9,410