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My non-aero gravel bike optimized for 38mm tires...
Last Post 02/24/2021 09:04 AM by christopher behrens. 18 Replies.
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huckleberry

Posts:645

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02/22/2021 04:11 PM
I use my canti-brake Ibis Hakklaugi for winter riding and trade back and forth between wheelsets with either 28 mm tires or 38mm tires. My ride out to the midpoint is fairly flat with 500 feet of climbing. I time myself, not racing but a similar effort each time as the return ride is the fun part with plenty of climbing and dirt connections. Anyways, I time myself on that first half going out and it's odd that I am consistently 7 to 10 minutes faster on the 20 mile stretch with the 38mm tires than the 28mm. Same wheels, same gearing, same perceived effort. Could the larger tires break the air in front of the bike and lead to better aerodynamics? Or is it the rolling weight helping with the momentum, even though the weight difference in the tires is not huge - 28mm Conti GP 4000 S2 vs 38mm Panaracer Gravelking - 210 grams difference for the pair. I did not intend to compare the tires for speed, but this is a regular ride and I switch the tires up for fun and possible explore routes on the return ride and I happened to take note that I was faster on the 38's.
smokey52

Posts:351

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02/22/2021 04:36 PM
I don't know anything about riding on gravel. I would check the tire marks in a patch of looser gravel. If the 28mm is deeper than the 38mm, maybe sinking into the gravel a bit more adds some resistance.
huckleberry

Posts:645

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02/22/2021 04:38 PM
Thanks, Smokey -

I should have been more clear - the timed ride to the midpoint is all on pavement.
smokey52

Posts:351

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02/22/2021 04:53 PM
Are the treads and pressures comparable?
huckleberry

Posts:645

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02/22/2021 05:03 PM
Contis have a smooth center surface where the Panaracers have a subtle file tread.

95/90 on the Contis and 75/70 on the Panaracers.
huckleberry

Posts:645

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02/23/2021 09:55 AM
Found this interesting article...

https://www.renehersecycles.com/12-myths-in-cycling-1-wider-tires-are-slower/
Cosmic Kid

Posts:3399

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02/23/2021 10:28 AM
My guess is it is related to rolling resistance, but 7-10' for such a relatively short stretch is a huge difference. Hard to sat for sure w/o a power meter on....it could be that you are actually pushing more wattage w/o realizing it...a difference of 10w can feel relatively the same RPE-wise in an endurance zone, but noticeably faster.
Just say "NO!" to WCP!!!!
huckleberry

Posts:645

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02/23/2021 10:34 AM
What are your thoughts on that Rene Herse article, CK?
Cosmic Kid

Posts:3399

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02/23/2021 10:35 AM
Haven't read it yet....i'll try and take a look at it this afternoon.
Just say "NO!" to WCP!!!!
huckleberry

Posts:645

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02/23/2021 10:40 AM
The rolling resistance makes sense, and the roads around here are not smooth, AND lots of winter debris on the shoulders - gravel, sand, etc which feels much better with the wider, softer tires.

I also wonder if there is some kind of effect where the wider tire breaks the air in front, somewhat like the praying mantis aero position where the hands break the air in front of the head, BUT in this case the tire is breaking the air around the forks and downtube - I know, maybe a bit too reachy in theory ; )
longslowdistance

Posts:2240

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02/23/2021 01:21 PM
The retro nerd Jan Heine (sorry if spelled incorrectly) from Bicycle quarterly is a big fan of fat tires, and applies some science lite to back it up. The fat road tires he sells roll great but are a tad fragile to keep the weight down.
Orange Crush

Posts:3238

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02/23/2021 02:11 PM
This is a clear case of uninformative soft data and no inferences can be made other than the fact that the 28 mm tires are superior in that you are out on the bike longer. That is, unless you don't want to be out on the bike longer.
Cosmic Kid

Posts:3399

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02/23/2021 02:34 PM
Read the article and mostly agree....I think he is downplaying the aero issue a bit, but worth noting that rim choice and how big the tire is vs. the rim is a big factor in the aero.

but his points re: how something can "feel" faster when it isn't, along with his points re: rolling resistance are spot-on. But IIRC, above 32mm tires tend to slow down on the road.
Just say "NO!" to WCP!!!!
79pmooney

Posts:2575

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02/23/2021 02:44 PM
On aero - the match between the tire and the rim matters a lot. Air resistance is the frontal area times the coefficient of drag. Ie that coefficient is just as important as the frontal area.

What rims are you using? Maybe you found the magic combo. Or maybe those rims and the 28c Contis are really bad.

Of course the other side of this is your speed. That resistance varies with speed squared. This takes me back to calculus and derivatives that I was up on 44 years ago but has gotten pretty rusty. It would be fun to develop the equations for time lost/gained based on air resistance but that is (for this little brain) a matter for a different time (if ever).
huckleberry

Posts:645

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02/23/2021 03:20 PM
I did spend time in Texas...
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