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Aerodynamics of Road vs. Cross bikes

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Aerodynamics of Road vs. Cross bikes

Old 05-06-19, 08:39 AM
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Abe_Froman
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Aerodynamics of Road vs. Cross bikes

Not a gripe, just curious. I've been kicking around the idea of buying another bike. Not just because of 'slowness' issues, moreso its just, well, I like buying bikes.

Anywho...the reason (most recent one anyway...) I've been contemplating it...I ride in group rides on the weekend, using my CX bike (GP4000 tires...I'm not huffing and puffing trying to keep up running knobbies or anything truly stupid...). I'm one of the stronger riders in the group. However, I HAVE noticed that I definitely do not have the same top end speed as a few of the strongest riders. I'm wondering how much of this is due to me just being less fit (eminently possible and extraordinarily likely), but also how much of a handicap I've given myself when out in the wind and moving over 25mph or so (when I really start to feel like I can't hang with these strongest guys).

Obviously this is all going to be conjecture. I'd be curious for some input as to how much of a disadvantage people think I'm at. I've got a cross bike, fairly aggressive position, 28mm gp4000 tires, shallow rims. What kind of difference between that and say, a standard road bike with deep section wheels and aero cockpit?
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Old 05-06-19, 08:48 AM
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Originally Posted by Abe_Froman View Post
Obviously this is all going to be conjecture. I'd be curious for some input as to how much of a disadvantage people think I'm at. I've got a cross bike, fairly aggressive position, 28mm gp4000 tires, shallow rims. What kind of difference between that and say, a standard road bike with deep section wheels and aero cockpit?
I think most of the difference is going to come down to position. Most cross bikes have a more relaxed position so it may be difficult to optimize your position relative to a road bike.

Fat tires and shallow rims could cost you 10-20 watts, an aero frame might gain 5-10.
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Old 05-06-19, 09:00 AM
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I'm going to agree that it's mostly (if not entirely) a matter of riding position. Deep section wheels really aren't all that beneficial -- certainly not beneficial enough to make the kind of difference you're talking about. (The marketeers have interesting ways to make miniscule benefits look significant.) "Aero" bikes and cockpits aren't really of much benefit when compared with body position and clothing and helmets.

On bikes, surface area is a big deal. A slightly more upright position adds DRAMATICALLY to surface area. So does added tire width.
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Old 05-06-19, 09:40 AM
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Do you wear a race cut jersey? Tuck elbows in?

Handlebar width makes some difference,

& # of spokes as well as rim depth, so wheels do make a difference,

but even if you gain 3%, there will always be riders who are 20% stronger.

That said, I don't take the cross bike on fast group rides because it's more work & I have little hp to spare.
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Old 05-06-19, 09:41 AM
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Biggest aerodynamic Drag is the rider on the saddle ..
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Old 05-06-19, 09:51 AM
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A friend has a cross bike and an older aluminum Cervelo Soloist, and a couple of years ago he did some field tests on them. His "standard hoods" positions were different enough that there was a considerable difference in CdA between the two. IIRC, his CdA on his cross bike was almost .05 m^2 higher than his CdA on his (then old but now ancient) Soloist. There must have also been some difference in Crr but I don't recall what they were. He didn't look at CdA in the drops position.
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Old 05-06-19, 09:51 AM
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I'll second what others have said about riding position, but the gearing truly will also make a big difference. It sounds like you're fit enough to reap the benefits of a true road bike. Aerodynamics will be a factor to a lesser degree than (and mostly because of) riding position, gearing and stiffness of a road bike that makes it jump on acceleration and maintain speed more easily than a CX bike. Depending on the geometry of the kind of road bike you're looking at, it will probably also descend more stably = faster.
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Old 05-06-19, 10:00 AM
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+1 for position and kit, that is the bulk of your aero. But when in doubt, always get another bike, right?

I've seen a couple of the really strong guys bring their cx/gravel bikes out to a fast group ride, it's a bit demoralizing watching them breakaway on 40mm knobbies.
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Old 05-06-19, 10:02 AM
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Well for the purposes of discussion...I think we can exclude rider position from (at least this..) the discussion. That's something that is adjustable, and not tied to the particular bike. Cross bikes are roughly a centimeter higher, centimeter shorter at the front. I've had a professional fitting...any road bike I got would have the exact same setup as my cross bike right now in terms of rider position...wouldn't want to go any longer or lower. Not at the moment, anyway.

Also...gearing is not a factor either. I'm in Chicago...it's not like we have descents haha

So really...most things could be fairly standardized...wheels, tires, position, etc. The things specific to a cross bike that would slow the bike down IMO would be the width of the frame...big wide fork, round tubes, but round downtube, wider chain and seat stays...

I've seen some fairly large differences in independent testing of AERO frames...20 watts plus at around 30mph. That sort of leads me to believe an aero frame benefit relative to a totally UNaero cross bike could be upwards of double that. But of course I'm just wildly speculating.
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Old 05-06-19, 10:44 AM
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Iíd look at power data if I were you. If the pack is cruising along at 25mph (assuming flat terrain) that shouldnít be too hard, assuming you position yourself properly. Letís say youíre putting out 225-240 watts, but you know you can hold 275 for 20 min, you should be fine. Usually there will be lulls in the pace when you can recover. OTOH if youíre redlining a new bike isnít going to save you. Iíd look at position on the bike, tires, kit/clothing/helmet, and finally, your frontal area. If youíre round line a huge Krispy Kreme donut, Iíd invest on fitness first. But if you just like to buy bikes and have the means to experiment that way, why not? Itíll be the shortest way to answer your own questions. And if it gets you motivated more power to you. Good luck!
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Old 05-06-19, 10:48 AM
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Originally Posted by RChung View Post
A friend has a cross bike and an older aluminum Cervelo Soloist, and a couple of years ago he did some field tests on them. His "standard hoods" positions were different enough that there was a considerable difference in CdA between the two. IIRC, his CdA on his cross bike was almost .05 m^2 higher than his CdA on his (then old but now ancient) Soloist. There must have also been some difference in Crr but I don't recall what they were. He didn't look at CdA in the drops position.
I had a Cervelo Soloist. Great bike. My last aluminum frame
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Old 05-06-19, 10:52 AM
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Originally Posted by Abe_Froman View Post
Also...gearing is not a factor either. I'm in Chicago...it's not like we have descents haha
Gearing is 100% the factor. I've known plenty of riders around here that do great riding with others but those gears don't pay the bills when it comes to actual speed. They're not supposed to.

Also cross bikes handle like crap at road speeds. The longer stays and higher bottom bracket places you higher up.

You can ponder it all day or get on a road bike and go faster. There's a reason we have them and there's a reason we have road gearing.

Doubt me? Enter my race. It's at Fox River Grove on May 19. I will comp your entry. I mean it has a hill so your low gears will be an advantage right?
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Old 05-06-19, 10:52 AM
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Originally Posted by Abe_Froman View Post
Well for the purposes of discussion...I think we can exclude rider position from (at least this..) the discussion. That's something that is adjustable, and not tied to the particular bike. Cross bikes are roughly a centimeter higher, centimeter shorter at the front. I've had a professional fitting...any road bike I got would have the exact same setup as my cross bike right now in terms of rider position...wouldn't want to go any longer or lower. Not at the moment, anyway.

Also...gearing is not a factor either. I'm in Chicago...it's not like we have descents haha

So really...most things could be fairly standardized...wheels, tires, position, etc. The things specific to a cross bike that would slow the bike down IMO would be the width of the frame...big wide fork, round tubes, but round downtube, wider chain and seat stays...

I've seen some fairly large differences in independent testing of AERO frames...20 watts plus at around 30mph. That sort of leads me to believe an aero frame benefit relative to a totally UNaero cross bike could be upwards of double that. But of course I'm just wildly speculating.
I have a buddy who races his Trek Boone for both CX and road. His road set up is basically just changing wheels (Zipp 404s with 24mm tubulars) and brake pads. Stem is slammed so he can get pretty low, but I give him a hard time about how many watts the arms of his canti brakes are catching in the wind.

[edit to add photo]


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Old 05-06-19, 10:53 AM
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Originally Posted by GreenAnvil View Post
Iíd look at power data if I were you. If the pack is cruising along at 25mph (assuming flat terrain) that shouldnít be too hard, assuming you position yourself properly. Letís say youíre putting out 225-240 watts, but you know you can hold 275 for 20 min, you should be fine. Usually there will be lulls in the pace when you can recover. OTOH if youíre redlining a new bike isnít going to save you. Iíd look at position on the bike, tires, kit/clothing/helmet, and finally, your frontal area. If youíre round line a huge Krispy Kreme donut, Iíd invest on fitness first. But if you just like to buy bikes and have the means to experiment that way, why not? Itíll be the shortest way to answer your own questions. And if it gets you motivated more power to you. Good luck!
Well I don't have any issues keeping up with the group. I'm strong enough to pull (for short stretches...) at 25-27mph when I need to. But not NEARLY as long as the few noticeably faster guys, and I also get outpaced when we're both in the wind sprinting.

I'm not too concerned about how this affects cruising in the paceline...besides, the proportion that rolling resistance accounts for rises significantly when you're drafting anyway.
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Old 05-06-19, 10:55 AM
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Originally Posted by Psimet2001 View Post
Gearing is 100% the factor. I've known plenty of riders around here that do great riding with others but those gears don't pay the bills when it comes to actual speed. They're not supposed to.

Also cross bikes handle like crap at road speeds. The longer stays and higher bottom bracket places you higher up.

You can ponder it all day or get on a road bike and go faster. There's a reason we have them and there's a reason we have road gearing.

Doubt me? Enter my race. It's at Fox River Grove on May 19. I will comp your entry. I mean it has a hill so your low gears will be an advantage right?

Hah, jokes on you! I was planning on coming to that race anyway

See you there
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Old 05-06-19, 10:58 AM
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Originally Posted by Seattle Forrest View Post
I had a Cervelo Soloist. Great bike. My last aluminum frame
Me too. It had such a short head tube that you could get long and low. And damn was it stiff. Awesome crit bike, but I could barely ride it for more than 90 minutes on the roads around here because it would just beat you up.

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Old 05-06-19, 10:59 AM
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Originally Posted by Abe_Froman View Post
Hah, jokes on you! I was planning on coming to that race anyway

See you there
Jokes on you - road is dead and no one is racing anymore so you might be the only one.
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Old 05-06-19, 11:01 AM
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Originally Posted by Psimet2001 View Post
Jokes on you - road is dead and no one is racing anymore so you might be the only one.

BUT, I'm bringing a cross bike, so I can just go ride in the grass lol
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Old 05-06-19, 11:10 AM
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Maybe see if you can rent a more aggressive road bike for your next outing? Maybe it'll tell you before splurging on your own.
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Old 05-06-19, 11:11 AM
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Originally Posted by Psimet2001 View Post
Doubt me? Enter my race. It's at Fox River Grove on May 19. I will comp your entry. I mean it has a hill so your low gears will be an advantage right?
This is really awesome. I give you crap sometimes (because we're all passionate about this bike stuff and disagree at times) but I've got to say, you're a stand up guy.
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Old 05-06-19, 11:13 AM
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Originally Posted by Seattle Forrest View Post
This is really awesome. I give you crap sometimes (because we're all passionate about this bike stuff and disagree at times) but I've got to say, you're a stand up guy.
Thanks - I try. It's all there is that's left in this industry. It's all doom and gloom and if I'm going to drive myself into the poor house and destroy my own and my family's future financially I might as well help a few people have fun riding bikes.
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Old 05-06-19, 11:19 AM
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Originally Posted by Seattle Forrest View Post
This is really awesome. I give you crap sometimes (because we're all passionate about this bike stuff and disagree at times) but I've got to say, you're a stand up guy.
Originally Posted by Psimet2001 View Post
Thanks - I try. It's all there is that's left in this industry. It's all doom and gloom and if I'm going to drive myself into the poor house and destroy my own and my family's future financially I might as well help a few people have fun riding bikes.

Except, I'm going to pay anyway and thwart his attempt at good will. That'll teach to be a spreader of good will
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Old 05-06-19, 11:36 AM
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On nice road tires, my cross bike cost a ton more energy to keep up on the fast flat rides
No problems in the hills, even with the extra 2lb convenience fee
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Old 05-06-19, 12:24 PM
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Originally Posted by Abe_Froman View Post
Well I don't have any issues keeping up with the group. I'm strong enough to pull (for short stretches...) at 25-27mph when I need to. But not NEARLY as long as the few noticeably faster guys, and I also get outpaced when we're both in the wind sprinting.

I'm not too concerned about how this affects cruising in the paceline...besides, the proportion that rolling resistance accounts for rises significantly when you're drafting anyway.
Gotcha. Well, you seem to have a decent engine. Unless youíre riding with P/1s and the hammer is really really going down I donít see how you wonít be able to eek out a few extra slippery watts and hang a bit longer with those guys with a properly well-fitted aggressive road bike. Pull a Nairo and suck wheels for dear life until you really have to face the wind or they yell at you. If the guys are just stronger then thatís it, but at least youíll have another bike youíll enjoy. You wonít know until you try.
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Old 05-06-19, 12:37 PM
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For what it's worth, the OP basically spent the whole winter on a trainer doing erg mode intervals.

Saying this because it's relevant to the question.
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