Aerodynamics of gravel set ups? Swissside test

Swiss side published some wind tunnel testing for gravel tires and wheels.
They measured some huge differences between a ‘standard’ aero road bike and gravel bike, and between various widths and tread designs of gravel tires.

Several things jump out at me:
They claim a 40 Watt difference at 0 yaw for an aero road bike vs gravel bike, at 30 km/h.
Has anyone here compared their power numbers on those 2 bike types? Since 30 km/h is usually under 200W, that difference seems huge to me.
I would be interested if anyone had some rough numbers to compare.

The second part that I find strange are the large differences they measure with wider tires or knobbier tires.

For example, they measure ~1.5W more going from a 35mm > 40mm tire, and ~2W going from the smoothest to the knobbiest tire in the same width, at 30 km/h.
If I understand correctly, this is at 0 degrees yaw, so at more realistic yaw angles, you would expect it to be even a bit more.

Hunt on the other hand, measured in the same wind tunnel, only ~0.5W difference, going from a 38mm, low tread, tire to a 42mm, knobby. And this was consistent on several rims.
They even measured at 32km/h, so slightly faster, and listed the numbers for a weighted average of yaw angles, so again, if anything, you’d expect their differences to be larger.

Obviously you will get different results with diffrent wheels and tires, and when measuring just the wheel vs the whole bike, but the fact that Swissside are seeing about 4 times as much differnce between tires widths and knobby-ness, as Hunt, makes me really wonder what is going on.

They’re comparing a fully aero S5 with integrated cockpit, 65mm deep wheels, and 25mm smooth road tires to an Aspero with 25mm wheels and 40mm gravel tires and using rolling resistance data from a road scenario… a 39w difference at 30kph, if anything, seems low to me.

I don’t run a power meter on my gravel or mountain bikes but, riding each on the road, my ‘seat of the pants’ feeling would fall somewhere in the same neighborhood between road and gravel and a even a bit more of a gap between the gravel bike and my 5010 with 2.5" tires.

Hunt is comparing roughly similar tires so them seeing only a tiny gap isn’t a surprise.

Hunt saw 0.5 W difference between tires that were 4mm different in width, at 32km/h.
Swissside saw 1.5W difference between tires that were 5mm differnt in width, at 30km/h. (So 3 times as big a difference).

So if you find the Hunt results unsurprising, don’t you find the Swiss Side results surprising then?

Did you see where they pulled the rolling resistance from? I didn’t see it, but maybe I just missed it.

Unless they were testing the same tires (and arguably wheels), trying to extrapolate results for gravel setups is challenging at best.

Tire / knob designs will play a huge role in the aerodynamics.

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Both brands tested each tire with the same wheels.
In Hunt’s case, they tested with 3 different wheels, and the differences between the 2 tires were fairly similar with each wheel set, so wheel selection(at least in this category of wider aero wheels) does not seem to make a huge difference.

Swissside did use the same tire design, and only changed tire size. With both Conti and Schwalbe they found a fairly consistant ~1.5W for each 5mm jump in size.

Hunt did change the tire model, but if anything, based on Swiss side’s other testing, the knobbier Terrravail Cannonball would incur even more aero resistance than the smoother G-One , even if they were the same size.

So that can not explain the difference, there must be something else at play.

I just reread, and both of them tested a complete bike, no rider, so no difference there either.

Are you sure? That is not how i read it…

Aero Road Bike Setup:
Cervelo S5 with Swiss Side HADRON2 Utimate 625, Continental GP5000
25mm tyre, 7.0bar pressure.

Gravel Bike Setup:
Cervelo Aspero with Swiss Side GRAVON 250, Continental Terra Speed TR
40mm tyre, 3.0bar pressure

Compared to

Conduct the test using the Schwalbe G-One 38mm and the Teravail Cannonball 42mm tyres

So the test with the smaller difference is starting from a wider baseline tire (38) vs. a narrower tire (25mm)…and definitely not the same wheels. Also different frames.

I think there is so much noise between the two tests that trying to draw correlation is almost impossible.

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Incorrect… Hunt saw a 0.5w difference between highly similar tires on identical wheels that were only 4mm different in width.

Swiss Side tested highly dissimilar tires that were 5mm apart on vastly different wheels.

Given the absolute lack of consistency in testing protocols between the two and the highly different test subjects, the large gap in difference is exactly what one should expect.

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Blockquote > *Aero Road Bike Setup:

Cervelo S5 with Swiss Side HADRON2 Utimate 625, Continental GP5000
25mm tyre, 7.0bar pressure.

*Gravel Bike Setup:

Cervelo Aspero with Swiss Side GRAVON 250, Continental Terra Speed TR
40mm tyre, 3.0bar pressure

Compared to

Conduct the test using the Schwalbe G-One 38mm and the Teravail Cannonball 42mm tyres

“So the test with the smaller difference is starting from a wider baseline tire (38) vs. a narrower tire (25mm)…and definitely not the same wheels. Also different frames.”

Sorry, the Swiss side article has two parts. One is a comparison of overal resistance of an aero road bike vs gravel bike set up.
That is the equipment you quoted.

The 1.5W difference for wider tires was the second part of the test, only gravel tires, and all on the same rim:
It was run on the same rim, using 3 different widths of the same Schawalbe tire model, and 2 different widths of the same Conti Tire model.
In each case, they saw about 1.5W more air resistance when moving formthe 35mm tire to the 40mm tire. On the Schwalbe they saw an even bigger (2.2W) increase when they went from 40mm to 45mm wide.
All on the same rim.

Hunt only compared 2 differrent tire models of 38 and 42mm size, so indeed, the differtn tread design could explain why there was such a small increase in drag. However, that would mean that that Terravail is essentially a more aerodynamic design, which seems a bit unlikely when you look at the final part of the Swiss side test, where they are seeing smoother treaded tires have lower drag than ones with bigger knobs.

Huh? Either you or I are reading somehting wrong, or we are talking about two completely differnt things.
The way I understood it
Swiss side tested identical tire models, only the width was different:

Blockquote Tyres: Schwalbe G-One Allround in 35mm, 40mm and 45mm widths.
Continental Terra Speed in 35mm and 40mm widths
And the wheel was the same:

Blockquote Cervelo Aspero with Swiss Side GRAVON 420 and various tyres

And I guess it is pretty subjective whether we call the old G-One 38mm that Hunt test very similar or very different from the Terravail Cannonball 42mm. I was calling them pretty diffenrt, because they are a differnt tread design, with a bit ore agressive side knobs, vs the Swiss Side tests, where the trread design was almost identical (since they were the same model tire, just wider)

I think you just kinda proved my point….there is a lot of noise between those two tests. Frame here, different wheels there, tire width differences, etc.

I did not bother to check, but were the tests run in the same wind tunnel? If not, there is another variation. But even if they were, most aero experts are somewhat hesitant to compare results from different time periods.

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Wait a minute!
As I read both your responses above, I sounds like perhaps you are talking about Swiss Side comparing the gravel set up to the aero road set vs the Hunt tests of various wheel and two gravel tires, and that my remark was about that.

Sorry for the confusion.

Both companies measured the extra drag a wider tire crreates on the same wheel and the same bike, and for that Hunt saw a very small increase, ~0.5W, and Swiss Side about 3 times as much.

So that is the two tests I am talking about, not the road vs gravel set up.

Edit: I think i found part of it: the absolute numbers are much higher for Swiss side, about double the Hunt numbers, so maybe Hunt is listing one wheel and Siwss Side two wheels, even though both tested a pair of wheels, and did so installed in a bike.

If that is correct, Hunt sees about 0.8W increase going up 4mm, while Swiss Side sees ~1.5W. still a big difference, especially considering how consistant Swiss Sides results were with their two tires and even 3 sizes of one.

Swissside also does not list what yaw averaging they are using, so perhaps they used one that weighs higher yaw angles stronger, which both brands show to have a negative effect on wider tires.

I didn’t actually read far enough into the Swiss Side page to see that but it still isn’t an unexpected result.

Ultimately, what we have is Swiss Side doing a test comparing the same tire in a width that’s roughly 14.3% greater seeing an increase of a bit over 4% in power compared to Hunt doing a test that shows a roughly 10.5% wider tire seeing an increase of a little over 2.5% power. So, a difference in how much width is increased of about 35% or so and an power disparity of about 55-60%… given the non-linear nature of aerodynamics impact on power and the differences in testing, that doesn’t seem at all out of the realm of expected results.

In any case, as Henri has mentioned a few times, these aren’t remotely comparable setups and the variability between them means trying to pit the results against one another is pointless.

Well, I read two things:
1st: A bike specialising in aero will be more aero than a bike for other use cases - no surprises there.

2nd: tyre width obviously gives a penalty aerodynamically, albeit small - but the gains side and use case is not only aerodynamics - not in every case aero is everything.

But what about the elefant in the room: would an aero bike with the ability to accommodate wider tyres still be faster than with smaller tyres? If you can’t put tyres >28mm into an aero bike no one will be able to find out… But I guess the move to 26/28 does tell part of the story.
My gut still tells me that there will be some kind of cut-off where wider gives diminishing returns. But have we already reached that?