Are there any studies comparing drag of wider rims vs narrower ones?
I get that wider = more comfy and has lower loss due to impedance on rough roads, but i am curious as to what the aero tradeoffs with rim width are.
I’ve been searching but i cannot find any information specifically on this area.
Thx!
I think the concept is, at 0 yaw, narrow = fast.
Any thing after that, due to tire width interactions the stall angle increases with the wider wheel. So they get faster as the yaw angle increases. Plus bigger tire = less pressure = lower rolling resistance.
I think the concept is, at 0 yaw, narrow = fast.
The space shuttle is massive and traveled super fast and had a Cd approaching .1 depending on the phase of flight.
Are there any studies comparing drag of wider rims vs narrower ones?
I get that wider = more comfy and has lower loss due to impedance on rough roads, but i am curious as to what the aero tradeoffs with rim width are.
I’ve been searching but i cannot find any information specifically on this area.
Thx!
The often quoted rule of thumb is 105% of measured tyre width for the rim is said to be optimum. So if you are using a 25mm tyre you should have a 26.25mm rim ETC, but there must be a optimum width to depth ratio, also aero efficiency is measured as CDA with the A part standing for area if you double the area or width of the rim you double the drag.
-The most aerodynamic rim for a given tire is the one that’s properly sized for the tire (~105% as mentioned). Putting a big tire on a narrow rim creates a “lollipop” profile that creates a lot of flow separation and high pressure drag.
-Given a properly sized rim/tire combo, narrow is more aerodynamic than wide.
Where a lot of the confusion comes in is mfgs saying “our wide rim is more aerodynamic”…which is only true when they start with the assumption that you will be using a wide tire. What they are not comparing is a wide tire with a wide rim to a narrow tire with a narrow rim.
I think the concept is, at 0 yaw, narrow = fast.
The space shuttle is massive and traveled super fast and had a Cd approaching .1 depending on the phase of flight.
Re-entry aerodynamics are not about reducing drag so much as evading heat.
Re-entry vehicles are blunt to push out the shock wave, they’re not optimised f o r drag reduction… although I’m not sure what your point was, so…
I am a mere mortal. My TT speed is about 25 mph. Obviously faster downhill and slower up.
I made a spreadsheet tab once to totalize and graph my yaw for a ride against assuming the local weather station was usable.
Even I spent 60 to 75% at yaws 7 deg and less. Over 50% at 5 deg or less.
I find the move to 25mm optimized fine. 28 is getting a little stupid. Lose weight and don’t be fatties and you won’t need 28’s to ride tarmac on a TT bike.
I have ridden 28’s on gravel plenty of times.
Not having bought race wheels for a long time, I am still sporting deep wheels with a 14 internal 20mm external rim. When my tire of choice (Bontrager Aerowing 19-20mm) began to be outdated, I looked to see what newer tires might work with that wheel. I had a GP4000S that measure pretty close to the claimed 23mm so I tested it against the older tire and found that the improved rolling resistance was a wash with the narrower, more aero tire. Eventually I found a GP Supersonic in 20mm that I am going to race in the future, but it is so tiny and I am going to have to pump the thing up to 100 or 105 to give me any peace of mind from pinch flats.
(BTW, I was using Tom A. rolling resistance spreadsheet to compare the tires)
Well drag is CdA so a 5-10% wider tire would add roughly that much additional drag for the front wheel when you go from a 20 to 23. The problem with going back to narrower is both the reduced tire volume and the lack of decent tires narrower than 23 mm. Even the Conti SS 20 with no flat protection rolls at least 3 To 4 watts slower than a Corsa Speed.
I still think the AeroCoach testing indicates the 23 mm Corsa Speed is Still the fastest choice combining both CdA and Crr. That is a pretty narrow tire by current standards.
Still technically the lowest, but I’ve personally seen more races ruined by those tires than any others, so I ended up purchasing a set of 25/23mm of the Michelins awhile ago to race on this season. Of course I haven’t even mounted them yet but the 23 should work well on my 25mm external front wheel. 23mm GP 5000s measure ~24.5 on them right now.
I have a brand new 20 mm supersonic of the new construction. DM me if you want it, not going to use it
-The most aerodynamic rim for a given tire is the one that’s properly sized for the tire (~105% as mentioned). Putting a big tire on a narrow rim creates a “lollipop” profile that creates a lot of flow separation and high pressure drag.
-Given a properly sized rim/tire combo, narrow is more aerodynamic than wide.
Where a lot of the confusion comes in is mfgs saying “our wide rim is more aerodynamic”…which is only true when they start with the assumption that you will be using a wide tire. What they are not comparing is a wide tire with a wide rim to a narrow tire with a narrow rim.
We have to differentiate between the rear and front wheel: the above pictures should be applicable to the front wheel alone. The rear wheel sits mostly hidden behind the frame. I would say you can go beyond the 105%-rule on the rear because of that.
Also: the above pictures show only the front side of the front wheel, on the back side the air first hits the rim, then the tire.
I think the concept is, at 0 yaw, narrow = fast.
The space shuttle is massive and traveled super fast and had a Cd approaching .1 depending on the phase of flight.
This is a really bad analogy.
A 2.5" is never going to be more aerodynamic than a 20 c. I would love to be proven wrong, are we going to see monster trucks beating f1 cars? 
In the (20c?) 23/25/28 c debate the other variable becomes rolling resistance. What is the right answer? Like most complex engineering problems, the answer is probably a combination we don’t really know, but it likely depends.
As another poster mentioned, the aerodynamics you wrongly quoted are not relevant to this discussion.
I still think the AeroCoach testing indicates the 23 mm Corsa Speed is Still the fastest choice combining both CdA and Crr. That is a pretty narrow tire by current standards.
My v1 CS 23s measure up at 24.4 on both my rims (Bonty D3 and Spec 321), I wouldn’t consider that “pretty” narrow. I’d say that’s the perfect trade off size (Actually I’d rather they run true to size). But you’re right with what seems to be popular. For some reason people want the ability to run 30mm tires even if they’re slower and no more comfortable than a comparable 25. I guess I’m just lucky being light(er) that I can “get away” with running narrower tires.
…We have to differentiate between the rear and front wheel: the above pictures should be applicable to the front wheel alone. The rear wheel sits mostly hidden behind the frame. I would say you can go beyond the 105%-rule on the rear because of that.
Also: the above pictures show only the front side of the front wheel, on the back side the air first hits the rim, then the tire.
I agree, there’s an awful habit in cycling of cherry picking the easy part of the answer and pretending the rest doesn’t exist! But then in this case the truth is rather hard to deal with usefully on a discussion forum like this.
Illustrations almost always shows the leading edge of the forward side of the rim at right angles to the incident airflow as in the post above. In reality, even in still air (zero yaw), that only occurs at one point on the wheels circumference. Every distance above/below the centreline has a different chord length. So even already massively simplified to ignore yaw, rotation, flow interference and wind gradient, it’s not simple. Add in any one of those 4 factors and it gets much more complex. Include all 4 and it’s just not something I fancy tackling. I’m much too lazy these days. Basically airflow around a wheel is very three dimensional no matter what.
Where a lot of the confusion comes in is mfgs saying “our wide rim is more aerodynamic”…which is only true when they start with the assumption that you will be using a wide tire. What they are not comparing is a wide tire with a wide rim to a narrow tire with a narrow rim.
^^ Exactly this.
I get that for a given rim size, the rim should be 105% of the tire width, and that overall, a wider rim + wider tires may have better rolling resistance / reduced impedance on rough roads.
But what I have not seen is how a matched WIDE rim+tire combo pairs up against a matched NARROW rim+tire combo, where both tires are suitably chosen to match the appropriate rim. IOW, what is the drag curve of an narrow combo, with a modern rim profile, compared to the wider combos that are in vogue today?
At 0 yaw, the narrower combo will be faster. Is there something about a wider rim/tire combo that makes it more aerodynamic at other yaw angles? Because if not, then we are basically sacrificing a little bit of aero for improved gains in rolling resistance. That makes logical sense, conceptually, but it leads me to my other query:
Why are we assuming that wider/softer is always faster?
The data indicates that as you increase tire pressure, rolling resistance improves up to a point; after that, it gets a lot worse due to impedance. For a given tire size, there is an optimal inflection point at which you have the best overall riding efficiency (or total rolling loss) for that tire.
Eg, as per the Silca blog and Tom A’s tests here, that would be 110psi on smooth asphalt and 100psi on rougher asphalt with 25mm tires:
https://blog.silca.cc/...stance-and-impedance
Let’s assume a wider tire. Starting at the left of the scale, at a given pressure, a wider tire will have a lower Crr and that number will continue to drop as tire pressure increases - until you reach the inflection point. Now, depending on where this inflection point is reached, the optimal watts could be higher or lower than the 25mm tire.
It would have to be lower, in order to cancel out the aero cost of being wider. Is that so? Does anyone have a sense of these numbers?
Because as it stands, something seems to be missing in this whole “these 30mm OD wheels are going to make you faster” mantra.
Why are we assuming that wider/softer is always faster?
Who is assuming that?
The typical assumption is that optimal tire width is contextual, hence why plenty of people have 50mm tires on their gravel bikes and 23mm tires on their smooth-road TT bikes.
The reason that designs have been optimizing around higher tire widths is that the current thinking is that we’ve been erring lower than appropriate to cover a reasonable sprawl of typical use cases. I mean, I don’t think that there are a lot of people out there who will tell you that a 30mm tire on an appropriate rim is faster than a similarly-built 22mm tire on an appropriate rim, on the polished wooden floor of an indoor velodrome.
As far as what the reality is, I have no idea. Whatever it is, the differences across traditional road tire sizes are small enough that it’s a challenge to to measure and characterize them. That’s part of what’s driving the trend: erring wide seems to be a generally a safer bet than erring narrow.
Why are we assuming that wider/softer is always faster?
Who is assuming that?
The typical assumption is that optimal tire width is contextual, hence why plenty of people have 50mm tires on their gravel bikes and 23mm tires on their smooth-road TT bikes.
The reason that designs have been optimizing around higher tire widths is that the current thinking is that we’ve been erring lower than appropriate to cover a reasonable sprawl of typical use cases. I mean, I don’t think that there are a lot of people out there who will tell you that a 30mm tire on an appropriate rim is faster than a similarly-built 22mm tire on an appropriate rim, on the polished wooden floor of an indoor velodrome.
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The velodrome comparison is obvious to the point of being facile, honestly. I am referring to the general and increasingly wide-spread acceptance of wider = better for all tarmac use cases.
Look around on the Interwebz and everyone is talking about riding 28-32mm tires at 55-65psi as being the best option for speed. If it were just that, no big deal - that’s a personal preference.
Instead, let’s look at all the new wheels coming out - my Roval CLX64s are 30mm in width. The Zipps are even wider and dont let you go higher than 75psi or something like that. And as far as i can see, for tarmac, the data doesnt even support rolling efficiency being optimized at 70-75psi, let alone aero + rolling efficiency.
Without making this an argument of semantics, you dont think that the market is nudging everyone towards wider rims+tires for all road use, without factoring in that road conditions play a role?
It seems to me that there is a very clear missing element of data here. Put quite simply, what are the aero vs rolling tradeoffs, on good asphalt, of:
So far, I have yet to see any data. Hence the question.
I think the concept is, at 0 yaw, narrow = fast.
The space shuttle is massive and traveled super fast and had a Cd approaching .1 depending on the phase of flight.
The space shuttle was a lot smaller, in comparison, when attached to the fuel tank and SRBs. An A380 flies. A 747 flies. Would a longer and thinner aircraft with the same capacity as either fly faster? Yes, not considering other factors, but you’d probably need additional weight to construct such a long aircraft…
The space shuttle also operated on its own in low density environments, or had the advantage of falling through the atmosphere where drag helps slow down and go down. I’m guessing what you meant to say was that everything is a compromise…
Why are we assuming that wider/softer is always faster?
I guess I could go find it, but I recall reading in a recent post that one of the FLO brothers posted that their new design–optimized for “wider” tires–is faster than their old design which is optimized around “narrow” tires.
As far as softer, it’s a comfort thing. CRR is likely to be equal if you reduce the pressure in the more voluminous tire. It’s a win/win.
Me, I stick to narrower is faster and I am happy to see people running fat 5000s in my races. 😉