Interesting. Did you mean variations in CdA, not Crr? I thought you wrote that you took Al's number x 1.5.

BTW, at what speed did you calculate your "total drag" numbers above?

Also, is the pic for the Aero TT tire on the Bontrager website correct? I'm not seeing any "wings" on that thing...

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http://bikeblather.blogspot.com/

I think Damon is referring to the two entries in Al's (AFM's) Rev. 6 list for "Zipp Dimpled Clincher"? One was for a new tire and the other for a used tire (~ 60 miles).

Rik

Josh: I for one appreciate all of the time you spend here to explain your testing results and data, and walk us through the testing methodology and specifics. You refer to one thing that has bothered me about HED's data on their website regarding the HED3. The drag data for the H3 (aluminum) is listed as with a 22mm tire, but the data for the H3C (carbon) is listed as with a 19mm tire. But the drag numbers are listed as worse for the H3C at 10 degrees and above on HED's website!

Do you have any testing data on the Flashpoint 80? Or barring that, what would your estimate be of how it would stack up against your published data of the 808?

Rik

I agree, the pic isn't the best, but I also think you may be looking in the wrong place...based on the description (and Damon's comment above), the Bontrager tire apparently has a small wing/lip/flare to help ease the transition from the tire casing to the rim (just like some Michelin tires had back in the 1990s). You could therefore take the data he's posted as an endorsement of the idea of using silicone caulk to fill that gap.

Nope...that's where I was looking, i.e. along the bead/rim interface.



I'm still not seeing it...

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http://bikeblather.blogspot.com/

FWIW, the little 'wing' on the old Michelin tires was pretty small (2-3 mm wide?), such that the tire still curved in before flaring out again to cover the horiztonal portion of the rim/brake track. I *think* I see the same thing in that pic...but it could be that I'm just imagining it because of what I expect it to look like.

I'm not seeing it in that photo either, but here it is on my bike:



Bontrager Aero TT 19 mm tire

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Damon Rinard
Engineering Manager,
CSG Road Engineering Department
Cannondale & GT Bicycles
(ex-Cervelo, ex-Trek, ex-Velomax, ex-Kestrel)

Modolo Kronos front brake?

Close! Weinmann Delta.

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Damon Rinard
Engineering Manager,
CSG Road Engineering Department
Cannondale & GT Bicycles
(ex-Cervelo, ex-Trek, ex-Velomax, ex-Kestrel)

does it brake better than a modolo kronos?

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=====================================
S�rgio Marques
When it hurts is when it feels good ;-)
Sergio-Marques.com

Much better. I definitely see it there. Thanks.

OK, so what does this "19C" tire really measure? On Al's chart, it looks like he measured it to be actually more like 22mm wide.

Also, in case you missed it above, what speed did you use in your "total drag" calcs you posted?

Thanks again!

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http://bikeblather.blogspot.com/

"tripwires"

Bad Crr notwithstanding, my old Conti Supersonic 20c's had 3 little "wires" on each shoulder. Tripwire effect would be the only reason I can see to put them there...they also measured a true 20mm (matched up well with my Trispoke), and I had some very fast TTs on them back in 2004 and 2005, before I started looking closer at Crr data and switched to Michelins & Vittorias.

While I don't ride Zipp wheels on my tri rig (although I did just add old 340s to my cross rig!) and am a self admitted skeptical and completely cynical bastard, emphasis on the bastard for sure, I have to say I hope you're able to slough off crap like this from people who know very, very little to nothing about truly testing. It's clear to most (I think) people that you guys test the bejeebers out of your stuff and while I have to say that you clearly don't go out of your way to pick test results that make your product look bad, you do seem to go out of your way to not overly inflate your numbers and to go through some pretty darned extensive testing.

Anyway, all I'm trying to get across is what others have said, while we'll all still be watching your figures, don't worry too much about aberrant misfires who are convinced against all evidence that you are making up numbers and make an inferior product. As the old saying goes, you can please some of the people all of the time or all of the people some of the time, but most of the time some people are jerks. That might not be a direct quote but I'm pretty sure I have it about right.

-Andrew Saar

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-Andrew Saar
It is better to do the right thing and be paid poorly,
than to do the wrong thing and be rewarded richly.

Ah, I dreamed of owning a Kronos when I was a kid and they were new. But alas I've never ridden a bike equipped with them, so I can't compare , sorry!

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Damon Rinard
Engineering Manager,
CSG Road Engineering Department
Cannondale & GT Bicycles
(ex-Cervelo, ex-Trek, ex-Velomax, ex-Kestrel)

I meant variations in Crr. Pretty sure Al has two entries for that tire. Though CdA varies too of course. ;-)

Drag data is at 30 mph (no, I'm not that fast over long distances - I race pursuit :-) )[/reply]

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Damon Rinard
Engineering Manager,
CSG Road Engineering Department
Cannondale & GT Bicycles
(ex-Cervelo, ex-Trek, ex-Velomax, ex-Kestrel)

aaah, i have one as front brake in my trek :P

they...well, slow you down, especially when the replaced brake was a bontrager speed limit witch is amazing

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=====================================
S�rgio Marques
When it hurts is when it feels good ;-)
Sergio-Marques.com

I measured mine at 20.2 mm mounted on my old Dupont 3-spoke and inflated to 120 psi.

I also just saw Al measured Crr for a new tire; I think he's found Crr drops after a few training rides. Might have to send him a couple of my used ones.

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Damon Rinard
Engineering Manager,
CSG Road Engineering Department
Cannondale & GT Bicycles
(ex-Cervelo, ex-Trek, ex-Velomax, ex-Kestrel)

If I understood our tire guys correctly, this ridge(s) is sometimes designed into tires to "catch" the edge of the tread rubber during molding to prevent a cosmetically wavy edge between tread and sidewall.

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Damon Rinard
Engineering Manager,
CSG Road Engineering Department
Cannondale & GT Bicycles
(ex-Cervelo, ex-Trek, ex-Velomax, ex-Kestrel)

Yeah, I was amazed too when I put those on my bike for the first time! Very confidence inspiring for me.

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Damon Rinard
Engineering Manager,
CSG Road Engineering Department
Cannondale & GT Bicycles
(ex-Cervelo, ex-Trek, ex-Velomax, ex-Kestrel)

I misread the Vernier when I measured the first time adding a bonus 0.100 " to the width. I remounted and remeasured the new tire inflated to 120 psig at 19.4 mm on a Velocity Aerohead Rim. I see tire Crr typically drop ~ 0.00015 after ~ 50 miles or so. The width also goes up possibly due to assuming a more circular cross section due to "road softening" or stretching of the tire or both ??

Al

The 2 entries are for the same tire new and used. I've seen the Crr drop typically ~ 0.00015 after ~ 50 miles possibly due to softening or stretching of the tire ?? The width also typically increases a bit.

Al

Thanks for the info Al!

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Damon Rinard
Engineering Manager,
CSG Road Engineering Department
Cannondale & GT Bicycles
(ex-Cervelo, ex-Trek, ex-Velomax, ex-Kestrel)

[/reply]

Thanks Damon. I think there may be a danger in just giving a "total drag" number at just one speed. This is because the "drag" force related to Crr is constant (or nearly so) and the aero drag force varies with the square of the apparent wind speed. That means that a tire such as the Aero TT, even though it's great aerodynamically, may not perform as well at lower speeds due to the higher constant "offset" due to it's high Crr.

Here, I "reverse engineered" your tire data over a range of speeds from 1 to 35 mph and plotted the results. I was curious as to what the "crossover point" would be for the various tires where the better rolling resistance would win out over the better aerodynamics of the Aero TT tire.



As you can see, on this wheel, both the Zipp dimpled 21C clincher and the Race X lite 21C actually have less total drag below 20-21 mph, while the 23C Race X Lite Pro is faster overall at speeds all the way up to 28 mph.

It's also interesting how the 23C RXL Pro is better than either of the two 21C tires listed all the way up to 35mph, even despite it's somewhat worse aerodynamics. That really says a lot about how valuable a low Crr tire can be.

NOW do you see why I was asking about having the wings put on a casing like the RXL Pro?? ;-)

I'm also thinking I may have to apply a bead of RTV to my VF Record front tire on my TT wheel....:-)

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http://bikeblather.blogspot.com/

Thanks Damon. I think there may be a danger in just giving a "total drag" number at just one speed. This is because the "drag" force related to Crr is constant (or nearly so) and the aero drag force varies with the square of the apparent wind speed. That means that a tire such as the Aero TT, even though it's great aerodynamically, may not perform as well at lower speeds due to the higher constant "offset" due to it's high Crr.

Here, I "reverse engineered" your tire data over a range of speeds from 1 to 35 mph and plotted the results. I was curious as to what the "crossover point" would be for the various tires where the better rolling resistance would win out over the better aerodynamics of the Aero TT tire.



As you can see, on this wheel, the both the Zipp dimpled 21C clincher and the Race X lite 21C actually have less total drag below 20-21 mph, while the 23C Race X Lite Pro is faster overall at speeds all the way up to 28 mph.

It's also interesting how the 23C RXL Pro is better than either of the two 21C tires listed all the way up to 35mph, even despite it's somewhat worse aerodynamics. That really says a lot about how valuable a low Crr tire can be.

NOW do you see why I was asking about having the wings put on a casing like the RXL Pro?? ;-)

I'm also thinking I may have to apply a bead of RTV to my VF Record front tire on my TT wheel....:-)[/reply]
hi tom,
what mass, rho and percentage of raw roller Crr did you use for the plot? Definitely the very best way to look at it for one's personal case!