It sometimes seems like topics are coincidental. I had a big interaction with today's topic yesterday having planned to post this post a few days ago, but the Stan's and RaceFace carbon post took priority. Then you realize that some topics aren't really coincidental, they are just always active. Alloy nipples is one, and a post on those will follow soon. Tire and rim width is a subject that's never more than an hour away from happening, so let's discuss it.
People most often want to know if an AForce Al33 build can be used with a 28mm tire. That's the number one question on the topic and in any given week will be the question we get the most. Yes is the answer. The internal width of the AForce or Boyd, Easton, HED, FSW3 or Mavic road builds we sell would have been considered crazy wide for mountain biking not that long ago. These are all also quite appropriate for gravel tires, and for the most part they are all excellent for cross. Cross is the most demanding of a tubeless interface, by far, so we tend to get pretty specific about that but that's getting away from the topic.
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For aerodynamic optimization, you want the rim's external width to be just about the same as or very slightly wider than the tire. But that's optimization, and we're talking about differences of hundredths of a mile per hour there. When you're talking about using a 28 for rolling resistance versus a 25 for aerodynamic optimization, rolling resistance overcomes aerodynamics in almost every instance (and if "aero trumps weight" we need a new cliche about the relationship between rolling resistance and aerodynamics). Plus most people are looking at 28s for either comfort benefit or to be able to use them on "not quite road" roads, which are totally orthogonal to the "aero vs rolling resistance" discussion. And then the question of will a 28 fit your bike, and what is a "28" anyway, as most of the 25mm size of the tires that people want to use measure 28 or more when inflated on a current rim. For road, I use Vittoria 23s that measure 26 and change. So yes, you can use a 28 or 33 or 42 on any "road" build we offer and it will be just ducky.
But then yesterday's interaction was talking on a forum about hookless rims, and I advocated for them (within their domain, they are great), and a guy thought that we were doing our own product (we totally aren't - topic for another day) and he hoped that it would be very wide. And we've known for a long time that there's this belief out there that if some wide is good, more wide must be better, but we believe quite strongly that that's a fallacy. Rather, there is a range of tire width to rim width ratios that will work best.
For road, aerodynamics optimizes at about equality between tire width and rim outer width, like in this sketch (these are concept sketches, not dimensioned drawings, btw):
This is also a good handling match. With a tire that's a whole bunch wider than the inner width of a rim, you get the dreaded lightbulb. That is known to be aerodynamically lame, and more important to me it handles like crap. The tire is unsupported in hard turns and it's not good. Here's a sketch of that:
See the lightbulb shape? As a general rule, the wider tires are going to tolerate and even need a bit more overhang (lightbulb shape) than narrower tires, for several reasons. One, wider tires are generally used in "less paved" situations, and a bit of tire overhang protects the rim from damage. It's much more normal to hear about people damaging rims without having a flat tire now.
For another, the tire can actually roll if the rim's inner width is too wide relative to the tire. This should intuitively make sense, but if the tire's "pressing out against the rim" component gets too small, well, you see what happens.
For a third, again on the "less paved" deal, if the tire sidewalls are vertical, the tire's contribution to suspension recedes. Gravel and cross, you definitely want that suspension. Need it, in fact.
For a fourth, the tire's grip in turns really starts to suck. Like in this crappy sketch:
You've got the near vertical sidewalls, but note also that the turning knobs come off of the side of the tire and the knob profile gets very flat. When you lean into a turn on this, it creates a very "on, then off" grip scenario. The side knobs are unsupported and then you lean enough that they are not in ground contact. No bueno.
So a better tire/rim combo for a knobbed tire looks more like this crappy sketch:
You have a little overhang to protect the tire and maintain suspension, and the turning knobs are well supported and in a place where they'll stay in ground contact. All good. And this is why we think that a 30mm inner width rim is a bad match for a 35mm gravel tire. There really aren't any hard and fast rules, and different companies will vary in their recommendations.
Last, this GCN video is pretty good and hits some of the topics here.
13 comments
According to DT Swiss and their Road Revolution study they did in 2017, the 28mm GP4000 is faster than the 25mm GP4000 when the average rider speed is less than 35km/h. So, that’s most of us. For Elite and Pro riders averaging greater than 35km/h the narrower frontal area of the 25mm GP4000 tyre is more of an aero benefit than the higher Crr is a penalty. That test was on their new ERC 1400 27mm outer wheels.
As the Aero race bikes (Giant Propel, Scott Foil etc) move to disc brakes, they are also being fitted with 28mm tyres from the factory. There aren’t a lot of wheel options optimised for 28mm tyres, yet. 28 is the new 25, and Vittoria just released their Rubinos in a 30mm width. That tells me their 28mm sales are growing and their looking ahead to meet demand for wider options.
I get wider is better until it isn’t, but maybe at 32mm it still is?
I theorise that for the average enthusiast averaging 25 to 28km/h a 32mm tyre would be faster than a 28mm tyre, as the aero penalty at lower average speeds is even less. The weight of a well made 32mm tyre such as the Compass is still below 300g, and the ENVE 4.5AR rim at 31mm wide is quite light, so a 33mm wide version would maybe only be a 500g rim?
Maybe the next Enve rim will be 33mm or 34mm wide?
I see Zipp just released their 303 wheels in a 650B size. Maybe smaller wheels is the way to keep tyre diameter and bike handling in check as tyres go wider?
Ben – Thanks. Tom actually chimed in on FB with that saying. I’d love for it to be more annoying and dogmatic, much more absolutist, and 6 or 7 words shorter, along the lines of “rolling resistance (insert verb here) aero.” I hate shading, subtlety, and accuracy. What we need more of is sloganistic dogma, darn it!
I believe Tom Anhalt already coined the cliché about rolling resistance vs aero: “low Crr can make up for a great deal of aerodynamic ‘sins’”
bikeblather.blogspot.com/2013/04/why-tire-crr-matters.html
Terry – Good question, with three answers. One, the fit with a tire that’s comfortably more than 28mm wide gets a bit close in my bike. Two, most road bikes designed more than an hour ago (mine was designed in 2014) were designed around a 23mm tire that’s 23mm, not 26. You start doing weird stuff to trail and handling and blah blah when you add somewhere on about 3/4” to the diameter of your wheels. No factor exists in isolation. Third, rolling resistance improvements shown assume same tire pressure for the two sizes. I’ve ridden 25s that are actually 28 at 85/95 and it’s not that fun. 23s that are actually 26 at those pressures work like a champ. So equalizing comfort probably equalizes rolling resistance or somewhere close to it in this ecosystem for me.
Another Dave – Certainly that site informs a lot of our thinking on this, but I wasn’t aware of that big tire’s good rolling resistance. What pressure was that at (see response to Terry for reasoning there). But, “road plus” or even “road fat” is unlikely to ever see the big time for a few reasons. For one, aerodynamics really would start to suck. Yes you can optimize wider with wider rims etc but for outright low yaw angle aerodynamics, skinny is king. 2.35” is >2x the frontal area of a 28mm tire. Second, the 2.35” G-One is to the gram twice as heavy as a Vittoria Corsa G+ 25. That much weight at the tire is going to SUCK. But let’s say we want a 35mm wide road tire. That means somewhere on about a 32 or 33mm inner width rim with 38mm outside, and to get any kind of aero juice out of that you’d need to go about 50mm deep, but if we’re talking about TT kind of aero, you’re in the 90mm depths. Know how much that rim weighs? About 750g each, very conservatively. Then you have the bike structures to support it. Your fork is wide, giving you yet another ding on aerodynamics. If you want anything like a road rear triangle that actually handles well, your Q factor (distance between pedals) is going to be disgusting (I have snake hips and would probably need hip replacements after one year of riding that) and the drivetrain would struggle to match. So, you know, wider is better until it isn’t.
Rolling resistance… Another great November blog post, to serve as food for thought!
Jarno at bicyclerollingresistance.com posted a great article a few years ago, comparing rolling resistance for Conti GP4000S II tires having different widths (23mm vs 25mm vs 28mm) using the same wheel/etc—and for any given air pressure, the 28mm tire had less rolling resistance than the 25mm and 23mm tires, and in that order: https://www.bicyclerollingresistance.com/road-bike-reviews/continental-grand-prix-4000s-ii-2014
Perhaps even more surprising was his 2016 test of Schwalbe’s 2.35" wide “Big One” (replaced by their current 2.35" wide G-One) niche-specific beach racing tire, which at the time had lower rolling resistance than any 23mm-25mm road racing tire he had tested: https://www.bicyclerollingresistance.com/mtb-reviews/schwalbe-big-one-2016
Schwalbe’s Big One has since been dethroned as the tire with the least rolling resistance by at least 10 other tires (and all are skinny 23-25mm racing tires), but it’s rolling resistance is still impressively low, especially when accounting for it’s mountainous 2.35" girth.
Factor in what that GCN video said about how a wider tire with lower air pressure can further reduce rolling resistance in the real world by significantly damping road vibration (increasing rider comfort as well), and the argument for wider rims to support wider tires looks even more attractive.
I must be living under a rock, because I hadn’t been aware that ENVE’s SES 4.5 AR disc road rims had a 25mm internal width until reading this post, but that 25mm internal width for a road rim is mildly mind-blowing. To put that in perspective, Santa Cruz cited data from ENVE in 2015 arguing that mountain bike rims having 26mm to 31mm internal widths (ie ENVE’s M60 HV and M70 HV rims) were the optimal widths to support the side walls of mountain bike tires up to 2.5" wide. So a roadbike-specific disc rim with virtually the same internal width as ENVE’s M60 HV rim seems crazy, but in a very good way.
Anyway I’m just curious, but if a rim with ~20mm internal width (like AForce’s Al33) creates an optimal aerodynamic interface for 23-25mm wide road tires, and a ~25mm internal width rim (like ENVE’s SES 4.5 AR) creates the similar optimal aerodynamic interface for a 28mm wide road tire, and Schwalbe’s 2.35" wide beach racing Big One tire had slightly less lower rolling resistance than Conti’s GP4000S II, then where might all this be heading? Will tire manufacturers make road racing tires wider than 28mm, and will rim manufacturers keep making wider carbon road disc rims optimized for such wider tires?
My preferences probably lie outside the bell curve compared to most other roadies, but after learning of Jan Heine’s light (although reportedly fragile) Compass 35-44mm wide road tires, then seeing road bikes like Open’s U.P. that has clearance for both 700×40 and 27.5″ × 2.1″ tires, then reading 2 years ago about Schwalbe’s 2.35" beach racing tire having less rolling resistance than almost all other 23-25mm road racing tires, then seeing the emergence of 650B “road plus” tires (marketing b.s. aside ?) like WTB’s 47mm wide Horizon and Byway tires, I started daydreaming of wide U-shaped aero rims designed for wider road tires and bikes that could accommodate them.
So maybe one day soon it won’t seem extreme to have a versatile “multi-surface” road bike that has all the increased grip, comfort, decreased rolling resistance, and-no-F’s-given attitude about craptastic road surfaces (whether potholed northeastern roads, Flandrian pave, or Italian strade bianche) that wider tires provide, but mounted on rims aerodynamically optimized for their width.