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Technical FAQ: Tires widths, cassettes, and coasting

Tire widths vs. rim widths

Dear Lennard,
I have read about how fast 25mm tires are, but I was wondering what actually is meant by a 25mm road tire. When I put a 23mm on a wide clincher rim they measure 25mm; does that make the 23mm the fastest? When I put a 25mm on a wide rim it measures up to 27mm, and a 28mm tire on a wide rim ends up being 29mm. Does the research involved with tubular tires and the speed test only work for 25mm tubulars?

I was also wondering if you lose the lower pressure advantage on wide clincher rims if you put, let’s say, a 25mm tire on a 23mm wide rim. I am figuring that with a 28mm tire on a 23mm rim, you should not go lower than the recommended tire pressure labeled on the tire.
— Jonathan

Dear Jonathan,
This is a subject for a whole lot of testing and more articles. It gives me the willies the way people put tires of different widths on rims of different widths without clarity about what combinations should not be considered, much less ridden.

The European Tire and Rim Technical Organization (ETRTO) is supposed to protect us from dangerous combinations. It creates tire and rim standards for the EU, and the numbers imprinted on tires are the result. Since the United States has no equivalent organization, we depend on ETRTO’s standards. ETRTO sizes also apply to rims and tires for other types of vehicles besides bicycles.

There is an ETRTO bicycle technical committee that meets periodically. It has representatives of tire companies and of rim companies. The thing is, tire engineers know an enormous amount about tires, while rim engineers know a lot about rims. But a harmonious combination of those things is rarely embodied in a single person. Often, in fact, manufacturers send technically unqualified people to the technical committee meetings who don’t contribute but rather were sent there to take notes so the company is forewarned about what is coming down the pike.

It should be obvious that at a certain point, the rim becomes so wide that the tire is no longer mounted safely. Obviously, if the rim’s inner width exceeds the tire’s rated width, the rim can no longer securely hold the tire onto the rim when inflated.

We also know that a rim can be too narrow for a tire. A very wide tire on a very narrow rim would result in a tire so round that its walls meet the rim at too low of an angle to secure the beads; it could not result in the lightbulb-shaped cross section of the tire on the rim that is required for safe use.

So what about the gray area in between? Do you know how wide of a rim you can safely use with each of the three tire widths you mentioned in your letter? Mavic has such a chart, but it does not pretend to be a universal one; it only applies to its own rims and tires.

I also don’t understand something about your question. Are you saying that you believe that 25mm is the fastest size? Because I’m quite sure nobody can say that as a general statement. There are tires wider than 25c with lower rolling resistance than a 700x25c, just as there are tires narrower than 25c that are lighter than a 700x25c; depending on the riding conditions, either one of those could make the bike’s speed faster.
― Lennard

Wide-range cassettes

Dear Lennard,
I read this Tech FAQ and have a simple question: what 11-36t cassette were you referring to? A SRAM 1170?

I’m thinking of getting an 11-speed Ultegra mechanical group and running an 11-36 with a 48-32 crankset (I run a similar 10-speed setup now).
— Steve

Dear Steve,
Yes; I have a SRAM 11-speed PG-1170 (SRAM Force) 11-36 cassette on two of my road disc gravel bikes. Shimano offers a road 11-speed 11-34 cassette in Ultegra as well as in 105.
― Lennard

Dear Lennard,
I am currently running an 11-32 SRAM cassette on my gravel rig with a Force WiFli rear derailleur and full Force drivetrain. I recently became aware of a Shimano Ultegra 11-34 cassette and would like to add a bit more to the gear range. What, if any, changes would I have to make for this transition?
— Frank

Dear Frank,
You may be able to just slap it on with no adjustments, and it may work just fine, especially if your current chain is too long.

It is likely that you will need to replace your chain with one that is longer by a link or perhaps two. And you may need to tighten the b-screw a bit so that that the chain doesn’t get pinched between the upper jockey wheel and the 34-tooth cog.

I doubt this would be required, but the worst-case scenario would be that you would need to add one of these derailleur-hanger extenders.
― Lennard

Campy-Shimano compatibility

Dear Lennard,
I am currently running Campy 11-speed Athena, and I was considering switching to Dura-Ace 9100. Can I still use the current rear wheel (11-speed Chorus cassette) or do I need to switch to a Shimano compatible wheel?
— Mark

Dear Mark,
Yes, you can use that wheel with that Chorus cassette on Dura-Ace 9100, and it will work great. See this piece I wrote a few years ago.
― Lennard

More on descending speed vs. rider weight

Dear Lennard,
I enjoyed your recent column on scaling laws and descending. I have good real-world example (and a spreadsheet model) for you. I’m Michael, 6-foot-1.5 and 175 pounds. My main bike bud, Karen, is 5-foot-1 and 100 pounds. I’m assuming my bike and gear weights 20 pounds, while Karen’s is 16 pounds. We’re both in our early 60s but still fit. I’m a pretty good athlete, but Karen was a semi-pro runner with an old cat-3 bike racing license.

Our weight ratio is 1.75. If you take the cube root (1.205), that is almost exactly our height ratio. I’ve estimated my Cda at .38, so I divide that 1.45 (1.205 squared) to estimate Karen’s Cda.

I assume we’re coasting down a seven percent grade and have reached our terminal velocity. Each of the grey outline boxes are completely separate, and you can copy them as many times as you like.

I have double checked this a bunch, and I’m confident that these results are good. The interesting thing is that even though I am much bigger than Karen, I only coast down a seven percent grade 2.5 mph faster than her (this seems consistent with my personal experience — I don’t drop Karen very fast on descents). If our bikes+gear were proportional to our weights, the ratio of our speed would be the square root of the difference in our height. That also applies to cannonballs at their terminal velocity.
— Michael
Retired science editor and writer, UC Berkeley College of Chemistry

More on front derailleur mount too high

Dear Lennard,
I had a similar problem as Geoffrey, who needed to mount a front derailleur but the braze-on was too high. I had a mechanical groupset, so I used a SRAM Force derailleur with two mounting bolt positions. But I later discovered this adapter.
— Richard

Dear Richard,
That’s a perfect solution! Thanks!
― Lennard

Quick correction to last week’s column

Dear Lennard,
As I read my note I found an error! The formula should be: area/mass

h^2/h^3 = 1/h

I apologize.
— Pierre

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