How deep do wheels need to be for aero bennies w/ gravel tires?
 

I keep finding myself doing my longest rides on gravel rather than pavement, and since I’m too big and not fast enough uphill for the A group, I also find myself in no-mans-land a lot, riding solo. Sure, I could fall back and take up with the next group on the road, but this is also a great excuse, er, opportunity, for a wheelset upgrade, so I’m going with that for now!

Do those of you running deep section rims have any thoughts about ideal depth for 32-42mm gravel tires? I was looking at the Flo G700s and reading about Flo’s thoughts on aero gravel, and while their G700s are 55mm deep and 25mm internal, the set weighs nearly 1900g, and I’m just not sold on strapping on an extra 1lbs plus to get the gains. I’m down with aero trumping weight on the road, but not totally sold for the same on gravel, at least not at the cost of adding ~500g…

I’m currently running 1475g Spinergy GX, which are 24mm x 20mm deep box section alloy, so I’m wondering if I’d see benefit just moving to an aero profile rim of any depth, or if as Flo claim, the depth is super important to getting the gains with gravel tire widths and treads. I love the Spinergy’s and would stick with the fiber spoke thing, except they don’t offer a proper aero rim with both the width and depth.

If I got it right, I think Flo is saying that since rims don’t match the width of gravel tires (unlike road tires) creating that smooth, laminar flow between tire and rim, having the rim depth is important for flow reattachment. Do you think that has merit? Do you think it has enough merit to warrant pushing 6lbs (i.e. wheel and tire) of rolling stock?

So where does the depth vs. weight thing shake out for you on gravel?

If you consider the flow reattachment as a horizontal cross section of the wheel viewed from the side, a deeper wheel will have more of its "height" that has sufficient depth to reattach for such a wide tire, and as you add up/integrate those aero gains there is probably some optimal depth that would help. IIRC the only published data I've seen for gravel widths is the swissside data that shows ~2-5w

https://www.swissside.com/en-us/blogs/news/gravel-report

Are you really looking for an aero advantage while riding gravel ?. How fast are you trying to go ?. My recollection is that aero gear like an aero bar, aero wheels, etc.. won't really give much advantage unless up near 20 mph or so. Can you maintain 18-20 on a dirt road ?

I am no aero expert, but I do love my aero tech.

Coming from a "road" background I have always thought that deeper = better, but perhaps with wider gravel tyres that's not true. I actually went for lightweight and shallower 30mm carbon wheels for my gravel bike upgrade, as a lot of my gravel routes are very climb packed. Perhaps if you have faster, smoother gravel routes where you can run narrower tyres, the aero wheels would help? I don't know.

This article is a good one: https://www.renehersecycles.com/aero...-gravel-bikes/

"An aerodynamic teardrop shape has a ratio (length to width) of 3:1. With 25mm tires, that works out to a 50mm-deep rim (25mm tire + 50mm rim = 75mm deep). However, the trailing end of the rim should be rounded a little. So you get a minimum depth of 45mm for an aero rim for 25 mm tires. Once you go to a 42mm tire, it’s easy to do the math. You’ll want a rim that’s 42mm wide and about 75-80mm deep."



Yes... Yes I am... 🤪

Not sure it will give you any noticeable benefit, but one thing about carbon wheels is they are more durable, so that’s an advantage off road.

Even if the aero wheels don’t help very much, they’ll look cool, right? That’s good enough for me.

There’s no crossover speed where aerodynamics suddenly go from giving minimal advantage to giving enough to be worthwhile. The advantage of aero equipment is always small compared to rider ability, but it’s also always there. And of course, even a rider who can’t average 20 mph solo over an entire course will still find themselves going that fast, or faster, at points on the course and they will reap the bigger returns at those speeds.

To each their own, I don’t think Jan Heine is a particularly credible source on aerodynamics, wind tunnel testing not withstanding. If I’m asked to pick between Heine and Specialized on the benefits of aero wheels for gravel, I’ll pick Specialized every single time. It is a good point, though, that the benefits of aero wheels will be blunted by a lot with wide tires, and also that what is more aero can be surprising. Also, that even without an aerodynamically designed frame, there are lots of little optimizations you can make to reduce your drag coefficient. That said, there’s still something kind of hysterical about going to the trouble to develop aero tire fairings and then installing them on a round-tubed steel bike with accessories and cables sticking out in every direction. At least he got rid of the exposed brake cables for this bike.

Thanks for that. The Swissside work does make it seem as though depth is critical (assuming same internal and external rim widths), just as we see on the road.

So the same optimization rules apply— get rim as close to tire width as possible, go as deep as possible— but what’s really interesting to me is that the effect of aero optimization are more consequential on gravel than the road in terms energy savings and loss simply because of the literal scale of the rubber. It reinforces my concerns arising from the realization that I tend to do longer (both time and distance) rides on the gravel bike than the road bike, and therefore, any gains have more time to accrue and become meaningful.

The weight piece is still a wildcard, if only because talking about a 1lbs variance is scary!

First, are there even any gravel wheels wide enough for an aerodynamically advantageous transition to 38 mm or 40 mm wide gravel tires.

Second, at least for my riding, there are more accelerations and decelerations while riding gravel.

So, being a wimpy roadie with a newfound interest in gravel, the increased weight and decreased rolling resistance of gravel tires (compared to road tires) mandate the lightest wide wheels I can afford: Roval Terra CL Wheelset | Specialized.com.

I am, yeah, and I’ll go as fast as I can! Aero gains, though small, actually build up and become more consequential (in terms of time saved) the longer one rides, so they’re actually bigger gains the slower one goes…to a point. If you look at the Swissside tests linked upthread, you’ll see they tested at 30kph, which is 18.6mph, a speed that’s admittedly high for my ride averages, but does that mean there are no bennies at 17.5mph? It could be, but probably not, and 18mph is well below the fast finishes I do on some group rides, segments where I’m happy to have any advantage available that allows me to run near the front (i.e. not come in last!).

Yes, I think this is true for me, too, at least it feels that way. It seems I can be hustling along pretty good on a dirt road and maybe it’s something like a rough patch of washboard combined with the heavier, draggier tires, but I lose speed really quickly, like more quickly than on pavement, and I have to throw down more watts on the dirt to regain speed than I do on the road.

That’s why I’m having a hard time believing going in for 55mm deep but 1900g heavy Flo wheels makes sense. I think I do want at least 45mm depth to try to get some sailing effect when conditions are right, but I don’t want to have to accept more than 100g in added weight.

I did just look at Light Bicycle and they have the WR65, which is 25mm internal and 65mm deep, which is pretty attractive. They can be built up to 1610g with Bitex hubs and CX-Ray spokes, making them temptingly close enough to an acceptable weight.

Similar to high school physics, the spirit of gravel considers air resistance to be negligible.

NerdAlert/GeekWarning recently discussed the 105 rule as it relates to wider tires and they said what you mentioned in your first post, that some/much of the aero benefit can be still be realized with a deeper rim depth.
Anyways, I quoted the above comment because its what I was going to suggest- look at LB or BTLOS for a wheel build that will come in lighter than the porky Flo option you cited.

Aero looks cool on gravel until you accidentally gash your expensive carbon wheel on a sharp rock. So I would keep that in mind when choosing your rim depth. I think anything under 40mm depth looks good with a fat gravel tire without being overly exposed.

Also I would look into areas you can cut weight on your gravel bike. If you're climbing, I'm a big proponent for weight reduction and getting your gravel bike as light as possible without compromising durability.

This.
The 105% rule is pretty well documented and accepted as a design guideline for aero wheels. This was developed in the early 2000's by Josh Poertner while doing wind tunnel testing for Zipp. He's spoken about it many times on his Marginal Gains podcast. You can read about it here: https://silca.cc/blogs/silca/part-5-...d-aerodynamics

The Rule of 105 states that the rim must be at least 105% the width of the tire if you have any chance of re-capturing airflow from the tire and controlling it or smoothing it.
“The critical point is that subtle variations in rim shape can and will change aerodynamic drag as well as handling, but none of it is possible unless the rim is at least 105% of the tire width.”

In order for a wheel to provide aero benefits with a 40mm tire, it would need to have a 42mm wide external dimension.

Knobby tires also disrupt air flow - and would further negate any aero benefits.

Josh Poertner isn't the only voice in aero. HED and FLO have done their own CFD and wind tunnel tests that show some benefits of deeper section wheels with gravel tires, but these are far lower than the numbers you see posted for 25-28mm road tire sizes. Enve says that once you get above 33mm tire size, there is no aero benefit on their SES wheels.

This article on Pro's Closet website seems to sum up the conversation pretty well:
https://www.theproscloset.com/blogs/...-carbon-wheels

All that said - I run 45mm deep Zipp 303S for gravel (and cyclocross). I doubt I'm getting much in terms of aero benefits, but the 23mm internal rim width works well with wider tubeless tires and these wheels are lighter than my OEM alloy wheelset. The deeper section wheels also shed mud a lot better than box rims.

I ride dirt roads which are pretty tame, so thankfully I don’t think I need to worry about rock damage. Thanks for bringing that risk up though; I hadn’t considered it. Perhaps building in durability to resist such damage is why the Flo G700 are comparatively heavy.

And speaking of heavy, that’s exactly my problem! I’m quite heavy and not at all interested to drag around any additional, unnecessary weight. My complete bike is 19lbs, which is acceptable to me, but if I can gain some long ride efficiency for little weight gain, I want to consider it.

I never understand this question- I would imagine most people of decent fitness can maintain 18-20 on a paved road, the same is true for gravel (depending on steepness, etc.)

Just get some 303s and call it a day

When you get older, that speed as a steady pace would be harder to maintain, requiring a pace of 20 or so, with the average at the end being 18-19 or so. I can no longer maintain that and do not try, I ride over 2000 miles a year as well as other fitness activities. When I was in my 40’s and racing as a beginner I could do 40 miles in 2 hrs. No longer. Such is getting old.

In my experience, gravel and dirt road riding is slower than asphalt and I would be surprised if that’s not a typical experience.. Thus my question as to how hard and fast the OP expects to be riding where deep aero wheels would be beneficial for the aerodynamic advantage. I don’t think they are, but perhaps the OP is much younger and fitter, possibly races, can’t say. When I was facing and doing triathlon team races, I recall the general consensus at the time was an aero bar had less benefit at speeds less than 20 or so. That thought carried over to any aero gear. Possibly the thought process has changed, but thus question what kind of speeds the OP can maintain and will deep aero wheels really provide any advantage.

I suspect Poertner’s 105 rule was formulated in a very particular context, and one totally specific to road bikes, and so is not the same thing as saying there is no aero aero benefit for gravel tires. It’s something that needs to be specifically tested, particularly since those who did do gravel-specific testing, e.g. Flo, did find benefit.

Interestingly, Flo claim to have found that when testing wider 28mm tires which broke the 105 rule, that their combined drag number (i.e. Cd and Crr) was lower than it was for 25mm tires that met the 105% rule on that same rim: https://blog.flocycling.com/aero-whe...ls%20at%20Zipp.

At the very least, I find there are no easy answers; it’s all quite complicated!

Anyway, I’m not getting too deep in the murk myself, as I usually run slick 35mm gravel rubber, the
exception being winter slop, when I move up to 42mm knobbed. Winter aero I’m not worried about at all, as I pretty much look like the Michelin Man when in winter gear.

303 S? They’re a little narrow and not really deep enough by my reckoning, but not unattractive.

They stand up to a ton of abuse, I wouldn’t mind an additional 2mm IW

My gravel wheels have an internal width of 25 mm -- pretty wide by current standards -- and are 30 mm wide externally, which is still about a good 10 mm narrower than the tires mounted thereon. At that difference, regardless of whether one believes in the 105% rule-of-thumb or not, one cannot expect much aerodynamic benefits. Maybe just barely better than a box section wheel?

Isn't that Flo testing chart still showing more aero drag when 28mm tires are used? The chart seems to show a significant aero advantage on the 25mm tires with greater yaw angles.
It's only when they factor in the lower rolling resistance that the total numbers start to even out.

This also seems to confirm that a 29mm wide external rim with 28mm tire would be significantly faster.

I actually run a narrower front 38c gravel tire and a 43c in the rear. It's not much, but I like running the narrow front tire when on road for whatever minimal aero gains. You could also mix-n-match wheel depth (deeper in the rear). I'm not sure if you deal with heavy crosswinds in your area, but it's also something to factor in. Even as a heavy rider, I think there are still benefits to keeping your gravel bike reasonably light. The less weight I've cut from my gravel bike, the easier it is to spin up those spicy climbs and still conserve energy in the process.