MTB wheel aerodynamics for ultra races

Discuss light weight issues concerning mountain bikes & parts.

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by Weltverloren


I've been struggling to find any data (like wind tunnel test results) regarding the use of aero wheels with MTB tires.

SInce a couple years, there has been a new generation of deep and wide rims (along the lines of 3T's Discus 45), some of which are 40mm wide and deep. I'm aware that these wheels were designed with gravel tires in mind, with the 40mm outer rim width sitting flush with 40mm tires. But what about MTB tires? In most MTB long distance races (like Tour Divide), fast rolling 2.1-2.2" MTB tires are the most common choice. Even with ultra wide 40mm aero wheels, these of course won't sit flush with the rim. That of course doesn't mean that such rims won't provide an aero advantage at all.

I've been trying to find any data on whether these deep and wide rims provide an aero advantage that would outweigh the rim weight penalty of around 400g (when compared to shallow carbon rims) for MTB ultras, which given the distance typically also feature plenty of flat bits. Does anyone have any leads?

Thanks a bunch! : )

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by Hexsense

Rule of 105% aside,
If you can't even reach 90%, let alone 100% then just don't bother with aero wheels.
XC MTB tires are 56-63mm. And they are knobby. That makes it really hard to make the combo aero.
Not only you need wide wheels but you have to make it really deep too for depth to width ratio. 1.5:1 would be a minimum that make sense for aero wheels, preferably 2:1. If wheel is 56mm wide, then you'd want depth to be 84-108mm deep.

All that would make the wheels excessively heavy which negatively effect unsprung mass making the suspension perform worse. For XC, suspension performance is more important than aero. So, for XC MTB, just get lightest wheels that survive your ride.
Last edited by Hexsense on Fri Apr 12, 2024 1:58 pm, edited 1 time in total.

by Weenie

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by Presing

I am not engineer but I think that due to the width of the tire on mtb bikes, the profile of the rim is not important.

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by js

A number of years back (10-15?), Trek and Bontrager experimented with aerodynamic fairings on their DH wheels. Because of the speeds in DH, that seemed a logical place to look for aero gains. A glimpse at DH runs in the decade since suggests they didn't come up with anything too revolutionary, or that the trade-offs were such that it wasn't worth any gain that may have been present.

As others have mentioned, one of the big issues would be getting the rim wide enough to be effective around a 50+mm tire width. If you could do that, or if there are courses where you could make do with a thinner tire (dropping into the 90+% range as Hexsense suggests), it's not out of the realm of possibilities. But I certainly haven't seen anyone testing data like that or doing the projections to understand what the theoretical gains would be.

Curious to see if anyone else has seen such a study - maybe outside the English-language media?

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by jfranci3

I would look at what Dylan Johnson is doing with MTB tires on his gravel bike. He's windtunnel testing this for his use.
Unless you have extended, exposed road distance, I'd presume compiant rims would give you more of an advantage than aero

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by Upcountry

Marginally successful ultra racer here, and self described aero-weenie... Similarly to another thread on here regarding a lightweight TT bike setup for road ultras, I'm okay with the mental exercise and thoughts, but fear that its easy to get lost in the weeds on something rather minor while missing the big picture.

I have a somewhat "aero offroad" setup on a Mosaic titanium build... Using the 2.1" Thunder Burts, mounted to the Nextie version of the 3T rims you mentioned, measuring 40mm wide. It certainly helps smooth the tire/rim interface, but I don't know that there is much to be gained there, but it looks great... I chose to build a set of wheels with these, as there wasn't much downside. Only 100g of added weight per wheel over a really lightweight setup, and the internal width of 30mm that I prefer for large tires. To really start seeing a benefit, as mentioned above, you need to increase the depth of the wheel as you increase the width of the tire. Flo has done some testing on this for their gravel wheelset. With a 47mm tire, combined with their 55mm deep wheelset, they were able to save 11watts over a box section aluminum rim... So to extrapolate that, you'd likely need something like a 40mm wide rim in a 80mm depth to see a measurable gain at even an unrealistic speed of 20mph. Dylan's testing is interesting, but his 22+ mph gravel races are certainly not comparable to the type of riding being discussed here.

This all goes without considering the average speed of these events. Even Lachlan's TD FKT run, he averaged less than 13mph moving speed(220m/day with 17 hours moving). At this kind of speed, an improvement of 0.001 cda will just not mean much, and certainly be negated by the random junk hanging off your downtube and bags.Rolling resistance is well worth keeping an eye on, as it is a constant energy suck, regardless of the average speed. This is all to say that it's not worth totally writing off aerodynamics, and I definitely keep it in mind for my setups, but we mustn't get too carried away with it. Some things, there is little harm in searching for a marginal gain, but with a really deep wheel, there are indeed tradeoffs. The rigidity of an 80mm rim would certainly add fatigue, will be heavier, and would be miserable in the inevitably windy day.

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by Weltverloren

Thanks a lot for the great input! It's much appreciated. : )

I'm on the go at the moment and will think this through more thoroughly later, but according to some quick napkin math, at Lachlan's 13mph, a 400g weight saving (which is the difference between the 40x40mm deep and the shallow wheelsets I'm looking at) on a route like the GDMBR would cause less of a speed benefit than a power increase of just half a watt.
I'm ignoring a lot of variables here and my "calculations" are based on an average gradient instead of an undulating route, so all of this is more of a guessing game (at least until I'll find the time for a more detailed look), but if the deeper wheels can save even just a watt on average, they might be preferable. Even with the width-to-depth-ratio and the rim-tire interface being far from ideal, this doesn't seem unthinkable.

And yes, I'm well aware that we're talking peanuts here compared to the things that really matter in ultra racing; it's just a thought experiment. : )

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by Weltverloren

Dylan Johnson and Silca did a series of tests at the ARC wind tunnel a while ago which Dylan has now released a video on:

It deals with exactly the question posed here regarding the effect of using deep section rims with wide MTB tires where the rule of 105% cannot apply. The result was that the rims still provided a performance benefit (which albeit marginal is likely going to outweight the advantage of slightly lighter shallow rims on most courses). Check out the video for details, but essentially, Josh Poertner explains that while the 53mm MTB tire won't allow the air to stay attached to the wheel with the much narrower rim, the deep rim still stabilizes airflow by acting like a splitter that reduces the formation of vortices/swirls in the air coming off of both sides of the tire. Simply speaking, air isn't attached to the rim here (as would be the case in a setup following the rule of 105%), but it'll have less chance to create chaotic turbulence. Additionally, deeper wheels make for shorter spoke, which in itself creates a small aerodynamic advantage. The shorter the spokes, the lower the speed at which their outer ends are hitting the oncoming airflow.

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