Carbon wheels: depth and the point of diminishing returns

[callmeishmael]

This is partly a theoretical question and partly one to inform a purchase.

I’m very likely to put sone Bora WTOs on my forthcoming custom to match the drivetrain (and obviously because they’re good quality and easy tubeless, and I’m very much a tubeless convert).

I was working on the assumption that the 33s would be appreciably slower aerodynamically. Well, perhaps not, if the results below can be extrapolated:
https://mobile.twitter.com/Spragg_Pe...01498680705028

I’ve also done a bit of digging and there seems to be a body of opinion that you’ll get most of your speed gains going from a box section alloy to a 30-something mm carbon, and that after that it’s marginal gains.

To relate this to my personal situation, I don’t race on the road but do do a lot of semi-competitive timed sportives and reliability rides, so while I’m not interested in marginal gains, free speed is always worth having :-)

So- and sorry for the ramble - what’s the Paceline consensus here? Would the difference between the 33 and 45 be anything but marginal (I.e. less than 5w) at real world speeds (average 19-20mph)?

[oldpotatoe]

Would the difference between the 33 and 45 be anything but marginal (I.e. less than 5w) at real world speeds (average 19-20mph)?

Are the routes you are riding flat? Must consider weight, even if the added weight of a 45 is pretty small..in your bike and rider and gear 'package'.

Some info here.

https://glorycycles.com/ride-notes/b...ra-one-review/

[Pastashop]

Good link!

I confess, despite being an engineer and capable of doing extensive literature research, and despite knowing the equations myself and having access to world class aerodynamicists, it is a question that comes up in my mind every time I’m struggling to keep up with a riding buddy who happens to get a new set or carbon rims. 😂

So, to the detriment of my wallet, I’ve bought and used wheels with box section rims, 30 mm deep alloy, aluminum deep section Campy Shamal, Zipp 303, Reynolds 35 mm carbon, Enve 60 mm, clincher and tubular type, on paved and dirt roads, with tires 22-42 mm wide in nearly all possible permutations… speeds and wattage output varying, obviously.

Below 20 mph, there is very little difference IME. Maybe a 0.5 mph faster with 30 mm tires and 60 mm rims compared to box section rims when riding into an oblique wind. Over the last few years, my fastest average times for two “benchmarking” routes on gravel/dirt/strada bianca, (25-50 miles long, about 1.5-2k ft elevation gain), I was averaging ~17-18 mph, and those were on box section rims, 33 mm knobby tires. Coasting on the downhills, my 650b bike (same weight as my 700c bikes) is consistently the fastest — lower rotational inertia helps roll downhill faster.

IME, good eating, sleep, and position completely override any bike upgrades. Clean chain, going from crappy old cartridge bearings with tight seals to well-adjusted cup & come hubs can yield more watts than going 33–>60 mm rims.

Sure it’s expensive to spend $600/watt gain. But getting the right look on the bike — priceless!

[callmeishmael]

Originally Posted by oldpotatoe

Are the routes you are riding flat? Must consider weight, even if the added weight of a 45 is pretty small..in your bike and rider and gear 'package'.

Some info here.

https://glorycycles.com/ride-notes/b...ra-one-review/

I live in the Chilterns in the UK. Very few climbs longer than 2k, but constantly undulating roads, with lots of short (500m-1k), steep (>10%) climbs, so weight is quite important

[callmeishmael]

Originally Posted by Pastashop

Good link!

I confess, despite being an engineer and capable of doing extensive literature research, and despite knowing the equations myself and having access to world class aerodynamicists, it is a question that comes up in my mind every time I’m struggling to keep up with a riding buddy who happens to get a new set or carbon rims. 😂

So, to the detriment of my wallet, I’ve bought and used wheels with box section rims, 30 mm deep alloy, aluminum deep section Campy Shamal, Zipp 303, Reynolds 35 mm carbon, Enve 60 mm, clincher and tubular type, on paved and dirt roads, with tires 22-42 mm wide in nearly all possible permutations… speeds and wattage output varying, obviously.

Below 20 mph, there is very little difference IME. Maybe a 0.5 mph faster with 30 mm tires and 60 mm rims compared to box section rims when riding into an oblique wind. Over the last few years, my fastest average times for two “benchmarking” routes on gravel/dirt/strada bianca, (25-50 miles long, about 1.5-2k ft elevation gain), I was averaging ~17-18 mph, and those were on box section rims, 33 mm knobby tires. Coasting on the downhills, my 650b bike (same weight as my 700c bikes) is consistently the fastest — lower rotational inertia helps roll downhill faster.

IME, good eating, sleep, and position completely override any bike upgrades. Clean chain, going from crappy old cartridge bearings with tight seals to well-adjusted cup & come hubs can yield more watts than going 33–>60 mm rims.

Sure it’s expensive to spend $600/watt gain. But getting the right look on the bike — priceless!

It sounds like we’re very much on the same page 👍

[FlashUNC]

Weight doesn't matter. Aerodynamics does, even uphill. As deep as you can tolerate in crosswinds.

Have Bora WTO 60s and 45s. Prefer the 60s.

[Mark McM]

It takes about 170 Watts to go 19-20mph (on flat ground). Of that power, roughly 80% is aero drag, or about 136 Watts. The wheels' contribution to the aero drag is about 8-10%, or about 11-13 Watts. Going from box section rims to deep aero rims will not save 5 Watts except in high yaw (cross-wind) conditions, so going from 33mm to 45mm rims will certainly not save anywhere near 5W.

Despite the popularity of fancy aero wheels, they are not the most cost effective way to save drag. More cost effective drag savings can be had by selection of clothing, helmet, and tires.

Speaking of tires - at most people's riding speeds, tires contribute more drag than wheels, and the difference in drag between models of tires can be larger than the difference in drag between models of wheels. All too frequently people buy wheels first and then try to decide what tires to use on them, when it should be the other way around - a rider should pick their tires first, and then select the wheels best matched to the tires.

[Michael D]

Mid section wheels are always a good choice because they are almost as fast as deep rims but lighter and much nicer for high speed descents or windy days.

[fignon's barber]

At 19 or 20mph, there will be no noticeable increase in speed between 33 and 45mm rims. If you're not already spending the majority of your time in the drops, you'd making bigger aero gains by improving flexibility and being able to ride more comfortably, and longer, in the handlebar drops.

[robt57]

I'll just add that running aero wheels for 20+ years. The advances in shape for stability in windy conditions is much improved.

I ran some pointy 46mm tubulars [old CX wheels] over winter, when I popped the Aeolus 5/D3 you really notice how little they get tugged on comparatively. Orders of magnitude less.

Point being less watts for 46mm rims that you eat energy fighting to keeping your line becomes counter productive.

Other point would be 20 years ago when common speeds were mid 20 mpg the gains way better than my much slower avg these days. Not a super fast 65 year old, as some are. ;O

[BobC]

As noted previously...Depends on where you live.

Let me give you a different perspective. I live in dead-flat VB but run a 32mm in front due to the constant wind, especially cross winds. Seen too many folks buffeted about in cross winds riding high profile wheels. No thanks.

[m_sasso]

Originally Posted by fignon's barber

At 19 or 20mph, there will be no noticeable increase in speed between 33 and 45mm rims. If you're not already spending the majority of your time in the drops, you'd making bigger aero gains by improving flexibility and being able to ride more comfortably, and longer, in the handlebar drops.

It's been well documented to be faster on the tops than in the drops, with a horizontal forearm. Welcome to the new world of current research.

[dana_e]

Go 60 deep, everyday entry level standard

[Spdntrxi]

40-50mm. good all around depth. Cheeky group rides average 19-21, but there are plenty of sprints that go 30+
Crits 60+.. 26-27mph average, need all the help I can get

[vespasianus]

Originally Posted by callmeishmael

I live in the Chilterns in the UK. Very few climbs longer than 2k, but constantly undulating roads, with lots of short (500m-1k), steep (>10%) climbs, so weight is quite important

Lucky man, I spent many a day riding down from Oxford to ride my MTB in the Chiltern Hills. While that place lacks to mountains or lakes of many other places in the UK, it is a stunningly under appreciated and beautiful part of England.