DrIsotope

My Ritchey Swiss Cross is, on average, about 2.5mph slower than my Cervelo R3. But my Cervelo has tubeless 25mm tires on it, and the skinny tires on my Ritchey are 700x35 Hutchinson Overides. The Ritchey also spends a lot of time on fire roads and horse trails. It's certainly not aerodynamics that's slowing the CX bike down.

Reynolds 

What is a "fast (ish...) road group ride" for you? If it's under 35-40kph I doubt you'll note any aero difference.

Hellgate

The difference is so minimal it doesn't matter. If you're worried about aerodynamics of the bike itself, you have bigger issues.

A CX bike traditionally has a the bars 1cm up and back, BB up a cm or so. Add cantis with 30mm tires and go ride.

Honestly, people over think this way too much.

Ride and be happy...

Abe_Froman

Yea cruising between 20-25 mph, or 35kph or so.

Though I’d argue you’ll notice aero gains more when solo at 18 than in a paceline at 22.

GlennR

If you're riding 16mph into a 8mph headwind you're air speed it 37kph

I sometimes hit 12mph/19kph headwinds down at the beach.

Just saying.

Abe_Froman

Well like I said, I’ve already made my decision...I’m getting the CX bike; just too functional to worry about the minute road performance losses.

Thisnis just a mental exercise more than anything. I find the physics behind bijes interesting.

Abe_Froman

Try 25 the mph headwinds here

GlennR

If I could only have one bike... i'd keep my CX. It's a "swiss army knife" and does 90% of everything I need.

Reynolds 

That's true, but only with 0º headwinds. Most headwinds are in fact crosswinds, where lateral aerodynamics also matters.

Doge

If I read your OP correctly the only racing is for cross. So optimize for cross.
That bike can do fine for most other things. It is a road bike. It is not suitable for road racing, but you didn't say you want that.

Depending on you aero road speed what you would loose. But a rider that can do 30mph on an aero road bike might do about 23 on an optimum cx setup on the same road and power. Lots of stuff goes into that.

I'd guess
a 25 aero road would be 21 on cx
a 23 aero road would be 20 on cx
a 20 aero road would be 18 on cx

But they are not close on the road for racing as mentioned in your OP.

Iride01 

How fast do you guys average in a CX race that aerodynamics plays a noticeable and quantifiable part?

12strings

I'd say your over estimating things a bit here...maybe if you are thinking with off-road tires...30mph down to 23mph?

I'd even say your "not suitable for road racing" is not entirely accurate.

We had a riser who regularly showed up on his cx bike for road rides, and did fine, even when he occasionally kept his cx tires on put of laziness... He also raced cx, as well as criteriums on the cx bike (with road tires).

Besides, the categories are so broad that the differences really need to be addressed between specific bikes..
For example, I ride a modern, steel, "endurance ROAD bike.". I guarantee there are top level cyclocross bikes that are 5 lbs lighter, with more aero tubing, and a more aggressive geometry than my road bike. Faster in every way.

Also, my "road" bike can take 35mm tires in the front, 30mm in the back, and I race it in cyclocross races...go figure. :-)

Abe_Froman

I’d say the majority of a cat 4 or 5 race is 13-16 mph. A bit higher on the straightaways.

Thoug note...it’s much more rare to have someone blocking the wind for you though...it’s not like there’s an offroad peleton to hide in.

Also...I’m more interested in the aero differences for road and gravel use.

HTupolev

Those numbers seem basically impossible. In order for the road bike to be seeing double the speed returns for each given power increase through the 20s, the aero cost of the cx bike would have to be roughly like dragging around an entire second bicycle rider through the wind. But if that was the case, the difference at the low end of the range should already be much larger than 18mph vs 20mph... which would seem to suggest that the road bike is running extremely slow tires compared with what the CX bike is using.

Doge

It is always a guess on what you want to give up. I don't know why anyone would race on a bike that costs them mph. I'd be surprised if your cx bike riding buddy was doing crits at 25PSI in his cx tires. Maybe he was, but I put the line " an optimum cx setup on the same road and power" to mean as raced in cx vs as raced on road. That generally means low cx racing pressure, mudder tires and not so aero rider or frame setup.

I frequently see racers holding 30+ solo on the road. I don't see them (or the same racer) doing 23+ in cx races, on the paved parts.

That that same cx bike can have smoother dry tires, pumped up, and lower more aero road setup and a rider that can hang with less powerful road riders does not surprise me. But the same rider (I get to see that in several of the collegiate kids) on 25PSI mud tires, in an upright cx race setup are significantly slower than when I see them crit racing.

kbarch

In some ways it depends how cross-specific you make it. For instance, the bars on my TCX are a bit wider and flare outward slightly. This (along with a less agressive saddle/bar drop) can result in significantly less aero positions than one may be used to. Also, tires aside, wheels designed for cross aren't usually so aero, either.

Doge

@HTupolev and @12strings, at first read you are both disagreeing with me, but it seems @12strings is saying that a cx bike is faster than I listed relative to a road bike, and @HTupolev is saying it is slower. So I'm in the middle with my guess, or I have no idea which way I'm off.

So...
Same rider, same power, on same flat pavement is race optimized (PSI, components/tires, position for their respective race) for
-Road (100PSI+ 25mm tires, 14# bike, aero bike, near flat back) and goes 30MPH.
-cx racer (25PSI, 33mm wide mud studded tires, discs, 18# bike, higher position) will go?

Doge

Re-reading - it is not just aero drag. A 33mm heavy rubber tire at race 25PSI is creating a lot of friction drag compressing and flexing. The road bike has minor tire Crr increases with speed. Flexing the 33mm mud tire at 23mph takes work. And the aero part of the bike. And the position part of the rider.

My comments were simply based on observation and I listed the reasons why.
I was at collegiate nats in Reno watching many of the same kids I saw, and will see in Grand Junction on the road. I see them in MTB too and local races.

HTupolev

I'm not saying either. I think that the relatively performance characteristics in your example don't make sense for any realistic pairing of road bike and CX bike.

I don't think it's realistic that you have the road bike increasing speed twice as fast with power increases than the cx bike, but then also only being 2mph faster at the low end of the listed numbers, going only 20mph when the cx bike is at 18mph. The only way to make those numbers add up is to assume that the slower bike has lower resistances than the faster bike with respect to drag sources that are more dominant at lower speeds (such as rolling resistance). The opposite is obviously usually true.

//========================

As far as what tends to be the case in reality, I think it depends. Hard to put numbers down when "optimum cx bike" can vary so wildly by course.

If severe mud conditions demand super-gnarly tires and a beach cruiser fit, then yeah, any road bike will obliterate the cx bike.
If the cx race is on a fairly non-crazy course and it's dry, a more road-like fit and considerably less-aggressive tires might be optimal. And then the differences will be much less striking.

12strings

Yes, I see I may have miss-read some comments.

I was speaking of a cc bike, with road tires. In this set-up, I think there is a very small difference.

With cx tires, the difference will be greater.

Abe_Froman

Obvoously tires make a dofference. But in regards to the OP, I’m only worried about frame differences. I’ll have a seperate set of road wheels/tires to use when i’m on the road...

Doge

OK - for JUST frame (and fork and discs). Assuming same tires, and same bar height and seat height - just an aero road frame vs a cx frame with disc brake maybe 27mph vs 30mph and 20 vs 19 and maybe 16.5 vs 16.9.
I'm guessing folks. But frame alone is not that big a deal. My ref is my son's two cx bikes vs his (nevermind) road bikes. The front everything is bigger. Forks etc. I am aware of fork / tire clearance is not a bad thing. The reality is if the rider is doing both cx and road, it will be more than the frame alone.

Abe_Froman

Not really, assuming you have a road wheelset. I mean sure, the bars will be about 1-2cm less aggressively placed, but that’s the only other difference. And that still puts you in endurance road bike territory.

Doge

Well what really is it then?

The KOM on this segment is https://www.strava.com/segments/709325 32.2 mph on my kid's aero road bike - ridden many many times on both. He can't hold 29 on a non-aero road bike (a very fast crit race bike). So somehow a cross bike is not really 3 mph slower?

Abe_Froman

I’m not saying the cross bike isnt slower than an aero road bike. I’m saying other than the frame, there isnt really any difference.

I mean what else IS there on a bike other than frame, wheels, tires, components, and rider? If the wheels, tires, and rider are standardized...that leaves just components. But cross bikes use standard road groupsets...so therenisnt really any difference there.

So yea...it’s just the frame.