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.
Three Ideas
But, I have a few thoughts that might help Phil.
Contact Lynskey
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
I (Lars) have had a very similar issue with my ENVE SES 4.5 AR Disc wheels and my Felt road bike.
The ENVE wheels are wide by design and hold tires wider than their stated width, which I expected. But what I didn’t expect is exactly the same situation as Phil, where the tires ended up rubbing against the TOP of my fork instead of the sides. I would have imagined that stretching them out wider would make them shorter instead of taller.
I can ride 30mm Compass / Rene Herse tires on that Felt bike using the standard DT Swiss wheels that came with the bike. But even some 28mm models rub the top of the fork when I ride the ENVE wheels. Bike tires are so expensive that it’s really frustrating when you don’t know for sure if a particular set of 28mm tires will actually fit.
I got double burned in my case, because I found a set of tires that fit, only to learn later that these specific Specialized 2Bliss Ready 28mm tires are not compatible with those ENVE wheels!
https://www.roadbikerider.com/enve-wheel-tire-combination/
At the risk of sound picky:
Crispns Miller’s seminal PhD thesis — which changed the way engineering is taught at MIT — was not about bicycles.
It was about how we understand, or don’t understand, simple machines. These concepts are foundational to an understanding of bicycles, which makes the thesis relevant here.
If you’re like me, you can’t understand most PhD theses. Even the titles are incomprehensible. But this one is simply titled “So can you build one? Learning through designing. . . ” And the thesis is just as readable as the title.
Here’s the plot: While working as a teaching assistant at MIT, Crispin had been observing that MIT undergraduate students didn’t understand basic leverage and basic structures. They had poor intuition for topics such as how a truss supports a load, or how a wheel and pulley can make a small motor able to live a heavy load. (There’s your indirect reference to bicycle gearing.)
Crispin was noticing that the students who had better command of these topics weren’t the top scholars from great suburban high schools. No, they were the kids who grew up on a farm, or worked in an auto repair shop.
So he performed a series of experiments. A group of students would get a pile of materials and an assignment to make something from them: a winch that could lift X pounds, or a truss that could support Y pounds at a distance of Z. That sort of thing.
Overall, the students performed not very impressively. And the longer they’d been students at MIT, the worse they performed. And the higher their grades at MIT, the worse they performed.
Read it at this link. It will help you understand how you, and those around you, may have gaps in your intuition of basic mechanical topics.
https://www.dropbox.com/sh/c6wju19b0ch1myp/AAAtmsrXcPMFNvu7Gym9Uwdka?dl=0
Thanks for telling us more and sharing the link! Great stuff.
Jim
Although 28″s clear my (Reynolds) fork just fine, on my older wheels and older (Campy Record “pre-Skeleton”) required me to pound on the stem to force them through the brake pads.
I am in the process of upgrading to 12-speed. The specs on the new brakes claim to be compatible with 28’s. NOPE! Even better braking than before, but I still have to pound on the stem to get them through the brake pads…
You can sometimes get them in and out with low or no pressure if you are home, and simply pump them up after you put the wheel in.
I have a 2007 Cervelo R3 and wanted to use HED Jet 6 wheels (25mm rim width) and wanted to know how wide a tire I could use and whether I could use these rims at all. I spoke to HED and visited on-line forums, to no avail. When I called Cervelo, the only suggestion I got was, “maybe it’s time to upgrade your bike”.
My R3 is either a 2009 or 2010 (the all white version) with the 3T fork. 3T’s website shows that the fork should clear a 28mm. Side space is ok but I have yet to find even a 25mm that leaves more than 1mm of clearance above. With only that much space,you can hear the debris scraping the fork if you ride in the wet.
Wish someone would sell a spacer, machined to 2 – 3mm. that can fit in the front dropouts – assuming that leaves enough space for the skewer to bite..
Interesting comments that people expect side rubbing rather than top rubbing due to too-large tires. I remember needing to use one of our tandem wheels on the front of my road bike, and there was lots of side room in the Look fork, but the larger tandem tire rubbed the fork crown. In the back, it is easier to see side rubbing on the chain stays or even the seat tube if you have short chain stays, but again it seems much more likely that a too-large tire would be rubbing on the brake or brake bridge than on the seat stays.