Aero penalty of gravel bike

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robeambro
Posts: 1619
Joined: Sat Jul 07, 2018 6:21 pm

by robeambro

SwissSide tested only a 20w difference in aero resistance at 30km/h between a *full gravel setup, including knobbies, shallow wheels, cables everywhere* and a *full on aero optimised beast, no cables, v-shaped cockpit, aero everywhere and deep-af wheels*. And these were "bike only" numbers - with a rider and various equipment on board, chances are the difference would be even less (provided they achieve an identical position).

Image

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

I have no proof to doubt these home-made tests, but I am somewhat skeptical. 1.5km/h would be 20w or more in aero savings, depending on speed - which is a number comparable with what found by SwissSide. Howeve - on bikes that should be roughly as aero, on similar wheels, similar tyre width.. Seems a bit much to me. If I had to make an educated guess, I would say that the great majority of the difference found in these home-made tests comes from rider position.

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warthog101
Posts: 587
Joined: Wed Sep 25, 2013 10:05 am

by warthog101

I am running wider tyres on the Revolt. 28f 32r vs 25 f & r on the TCR now.
27k km on the TCR and 8k km on the Revolt but not that much of it on the bitumen.
Yes the bars are a bit lower on the TCR as you can see from the photo.
It is what it is. Were I to road race again no way would I do it on the Revolt.

That test is of the aero drag and rolling resistance of wheels and tyres only.
The TCR isn't exactly aero but no way am I seeing the chunky Revolt as being more aero. Yes my position is probably slightly less aero but I'm tipping the whole bike is too.

TobinHatesYou
Posts: 10949
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

robeambro wrote:
Fri Dec 09, 2022 8:45 am
SwissSide tested only a 20w difference in aero resistance at 30km/h between a *full gravel setup, including knobbies, shallow wheels, cables everywhere* and a *full on aero optimised beast, no cables, v-shaped cockpit, aero everywhere and deep-af wheels*. And these were "bike only" numbers - with a rider and various equipment on board, chances are the difference would be even less (provided they achieve an identical position).

20W at 30km/h for a bike-only, is, in fact, quite a huge difference.

I am probably putting out 120W at 30km/h on my Madone. And as you mentioned, that's 120W including my big squishy cylindrical body.

robeambro
Posts: 1619
Joined: Sat Jul 07, 2018 6:21 pm

by robeambro

TobinHatesYou wrote:
Fri Dec 09, 2022 10:08 am
robeambro wrote:
Fri Dec 09, 2022 8:45 am
SwissSide tested only a 20w difference in aero resistance at 30km/h between a *full gravel setup, including knobbies, shallow wheels, cables everywhere* and a *full on aero optimised beast, no cables, v-shaped cockpit, aero everywhere and deep-af wheels*. And these were "bike only" numbers - with a rider and various equipment on board, chances are the difference would be even less (provided they achieve an identical position).

20W at 30km/h for a bike-only, is, in fact, quite a huge difference.

I am probably putting out 120W at 30km/h on my Madone. And as you mentioned, that's 120W including my big squishy cylindrical body.
That *is* huge, I said "only" since it's roughly the same aero saving that the two other posters reported with their setups where the two bikes are mostly set up the same way.

Would you say that it's realistic to find a 1.5kph speed gap at "normal wattages" between a gravel frame and a road frame, all else equal*? If I take SwissSide's data at face value, I'd say absolutely not - if I take the two homemade experiments reported above at face value, I'd say absolutely yes.

*incl. rider position.

TobinHatesYou
Posts: 10949
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

robeambro wrote:
Fri Dec 09, 2022 12:40 pm
TobinHatesYou wrote:
Fri Dec 09, 2022 10:08 am
robeambro wrote:
Fri Dec 09, 2022 8:45 am
SwissSide tested only a 20w difference in aero resistance at 30km/h between a *full gravel setup, including knobbies, shallow wheels, cables everywhere* and a *full on aero optimised beast, no cables, v-shaped cockpit, aero everywhere and deep-af wheels*. And these were "bike only" numbers - with a rider and various equipment on board, chances are the difference would be even less (provided they achieve an identical position).

20W at 30km/h for a bike-only, is, in fact, quite a huge difference.

I am probably putting out 120W at 30km/h on my Madone. And as you mentioned, that's 120W including my big squishy cylindrical body.
That *is* huge, I said "only" since it's roughly the same aero saving that the two other posters reported with their setups where the two bikes are mostly set up the same way.

Would you say that it's realistic to find a 1.5kph speed gap at "normal wattages" between a gravel frame and a road frame, all else equal*? If I take SwissSide's data at face value, I'd say absolutely not - if I take the two homemade experiments reported above at face value, I'd say absolutely yes.

*incl. rider position.

The 37.5kph values are pretty telling. Bike-only = 38W difference. Let’s put road wheels/tires on and assume that brings the difference down to 30W. For me, 37.5kph is maybe 230W, and I would certainly be moving in excess of 39kph by upping the power to 260W.

Hexsense
Posts: 2970
Joined: Wed Dec 30, 2015 12:41 am
Location: USA

by Hexsense

TobinHatesYou wrote:
Fri Dec 09, 2022 12:48 pm
robeambro wrote:
Fri Dec 09, 2022 12:40 pm
TobinHatesYou wrote:
Fri Dec 09, 2022 10:08 am
robeambro wrote:
Fri Dec 09, 2022 8:45 am
SwissSide tested only a 20w difference in aero resistance at 30km/h between a *full gravel setup, including knobbies, shallow wheels, cables everywhere* and a *full on aero optimised beast, no cables, v-shaped cockpit, aero everywhere and deep-af wheels*. And these were "bike only" numbers - with a rider and various equipment on board, chances are the difference would be even less (provided they achieve an identical position).

20W at 30km/h for a bike-only, is, in fact, quite a huge difference.

I am probably putting out 120W at 30km/h on my Madone. And as you mentioned, that's 120W including my big squishy cylindrical body.
That *is* huge, I said "only" since it's roughly the same aero saving that the two other posters reported with their setups where the two bikes are mostly set up the same way.

Would you say that it's realistic to find a 1.5kph speed gap at "normal wattages" between a gravel frame and a road frame, all else equal*? If I take SwissSide's data at face value, I'd say absolutely not - if I take the two homemade experiments reported above at face value, I'd say absolutely yes.

*incl. rider position.

The 37.5kph values are pretty telling. Bike-only = 38W difference. Let’s put road wheels/tires on and assume that brings the difference down to 30W. For me, 37.5kph is maybe 230W, and I would certainly be moving in excess of 39kph by upping the power to 260W.
It's hard for less aero riders to understand us.
I also in a camp that can easily do 20mph solo ride with 140 watts average without conciously trying to be aero. So, a 20 watts at this kind of speed is huge.
However, I know a friend of the same height who need 170 watts to average 20mph. And many taller riders who need 200w+...
For them, the bike aero diff is much less impactful.

robeambro
Posts: 1619
Joined: Sat Jul 07, 2018 6:21 pm

by robeambro

I'm not sure I follow - what I meant was that given the similarity in setups between e.g. the Wilier & the 3T, and the Revolt & TCR, I wouldn't have expected a 20w aero penalty in those instances. I wasn't trying to start a circlejerk on who's more or less aero.

warthog101
Posts: 587
Joined: Wed Sep 25, 2013 10:05 am

by warthog101

robeambro wrote:
Fri Dec 09, 2022 6:43 pm
I'm not sure I follow - what I meant was that given the similarity in setups between e.g. the Wilier & the 3T, and the Revolt & TCR, I wouldn't have expected a 20w aero penalty in those instances. I wasn't trying to start a circlejerk on who's more or less aero.
I am slightly less aero on the Revolt. I can't get the bars as low as I have them on the TCR. It's OK though as it is a bit better on the rougher gravel being a touch higher. Most of the gravel I do is fast gravel road stuff so I like to try to be a bit aero.
The tyres are larger on the Revolt and the wheels are wider.
As I said the gearing is possibly a bit less efficient too. Plenty of time spent on the small cassette end on road. Dunno how much difference that makes. The Revolt is also ~1kg heavier.
Have only just got they vector3 pedals and xc bodies for the Revolt. They aren't in that photo. I will run both bikes on the same loops over the next month or so and check the power.
With the riding I have done so far, on similar perceived effort, the Revolt is about 1.5kmh slower.
Only taken it on one fastish bunch ride and it was definitely harder to roll through and do turns as the speed crept up.

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