Wheel drag

Hi,

A while ago I remember using an online calculator that had lots of
different wheel combinations to test. In particular I am trying to
figure out how many watts a set of Zipp 404's will "save" at 45 kmh (28
mph) over a set of conventional 28 or 32 spoke wheels.

Anyone have any pointers?

Thanks!

Joseph

wrote:
A while ago I remember using an online calculator that had lots of
different wheel combinations to test.

Probably:
http://www.analyticcycling.com/

In particular I am trying to
figure out how many watts a set of Zipp 404's will "save" at 45 kmh (28
mph) over a set of conventional 28 or 32 spoke wheels.

I used to worry about such things, but now I realise, as others point
out regularly, that the time spent comparing different wheels/equipment
is better spent training, and reducing your times, without spending a
fortune.
Alan.

--
To reply by e-mail, change the ' + ' to 'plus'.
http://www.dvatc.co.uk - Off-road cycling in the North Midlands.

wrote:
Hi,

A while ago I remember using an online calculator that had lots of
different wheel combinations to test. In particular I am trying to
figure out how many watts a set of Zipp 404's will "save" at 45 kmh (28
mph) over a set of conventional 28 or 32 spoke wheels.

Anyone have any pointers?

Zipp's website has a lot of test data and info.
http://www.zipp.com/

On the front page are links to a translation of Tour's recent test of
aero wheels. The 808 came out on top (18.1W@ 50km/hr), and the Ritchey
carbon wheels, which I've heard use older 404 rims came second (19.3W).
I think this is for a single (front) wheel. They tested at various
angles, but I don't know what angle was used to generate the numbers
for comparison... it wasn't zero, though. I also am not sure if they
added in the power to spin the wheel (it isn't mentioned).

In the text they say that at 50 km/hr with a moderate cross wind
(probably 12.5 degrees, the 808's best point), a "standard" wheel takes
48W, a Ksyrium 32W, and the 808 only 17W.

After looking at some of Zipp's test data
http://www.zipp.com/tech/aero.shtml, it does appear that Tour did *not*
measure the power to spin the wheel... so the values they list are only
the translational drag numbers converted to watts. If you look at
Zipp's tech info on aerodynamics, they have separate graphs of drag and
power to spin... and the power to spin alone is around 40-50W for their
wheels @30mph (48.28 km/hr).

If we look at Zipp's data the drag @ 30mph-12.5 degrees is about .23lb
for the 808, .53lb for the Ksyrium, and .77lb for the 32 spoke GL330...
so the proportions are fairly close to what Tour got. To convert to
power, just take force times velocity.

30mph = 13.41m/sec
1lb= 4.448N

so, Power (W)=59.6*drag (lb)

808= 13.7W
Ksyrium= 31.6W
GL330= 45.9W

.... so these numbers agree pretty well with Tour... but then we have to
add in the power to spin the wheel. From Zipp's graphs:

808= 39.5 + 13.7 = 53W total
Ksyrium= 47.5 + 31.6 = 79W
GL330= 62 + 45.9 = 108W

Remember, this is at 12.5 degrees resultant wind... which would be the
equivalent of a 6.7mph wind from the side @30mph ground speed...
probably more than what most riders would normally experience at wheel
level. But... wow!... 55 watts for one wheel! (comparing 808 to GL330)

If you want to know the power values for other speeds take the ratio of
speed cubed... not perfectly correct, but close enough. For instance,
to determine the difference between the 808 and GL330 at 20mph:

Delta P@20mph = 55* (20/30)^3 = 16.3W

If you'd like to know the effect that wheel drag has on speed on a flat
road, a decent approximation would be the ratio of total power raised
to the .37 power, which is greater than 1/3 to account for rolling
resistance. For instance if you can go 30mph with 808's with a 6.7mph
sidewind, how fast could you go with Ksyriums? First you need to get a
ballpark number for the total power it takes for that case. There are
several online calculators if you don't have your own. It's about 380W
for somebody with a very good CdA of .215m^2. Another factor is, I want
to know what it is with two wheels, not just one. Well... for lack of a
better alternative, I'm going to say it's double for two (even though
the rear wheel drafts a bit)... so it's (79W-53W)*2 = 52W.

((380W-52W)/(380W))^.37 * 30mph= 28.4mph... a 1.6mph decrease. Of
course the difference would be less at a lower speed or less of a cross
wind... but I'll leave that as an exercise for you...

So, I hope you have enough info now to have all sorts of fun with
Zipp's graphs!

wrote:
Hi,

A while ago I remember using an online calculator that had lots of
different wheel combinations to test. In particular I am trying to
figure out how many watts a set of Zipp 404's will "save" at 45 kmh (28
mph) over a set of conventional 28 or 32 spoke wheels.

Anyone have any pointers?

Thanks!

Joseph

Se below for a objective look at such things. Remember tho, it assumes
all things being equal, one wheelset to the other, rider output wise
and that rarely is.

Ron Ruff wrote:
wrote:
Hi,

A while ago I remember using an online calculator that had lots of
different wheel combinations to test. In particular I am trying to
figure out how many watts a set of Zipp 404's will "save" at 45 kmh (28
mph) over a set of conventional 28 or 32 spoke wheels.

Anyone have any pointers?

Zipp's website has a lot of test data and info.
http://www.zipp.com/

On the front page are links to a translation of Tour's recent test of
aero wheels. The 808 came out on top (18.1W@ 50km/hr), and the Ritchey
carbon wheels, which I've heard use older 404 rims came second (19.3W).
I think this is for a single (front) wheel. They tested at various
angles, but I don't know what angle was used to generate the numbers
for comparison... it wasn't zero, though. I also am not sure if they
added in the power to spin the wheel (it isn't mentioned).

In the text they say that at 50 km/hr with a moderate cross wind
(probably 12.5 degrees, the 808's best point), a "standard" wheel takes
48W, a Ksyrium 32W, and the 808 only 17W.

After looking at some of Zipp's test data
http://www.zipp.com/tech/aero.shtml, it does appear that Tour did *not*
measure the power to spin the wheel... so the values they list are only
the translational drag numbers converted to watts. If you look at
Zipp's tech info on aerodynamics, they have separate graphs of drag and
power to spin... and the power to spin alone is around 40-50W for their
wheels @30mph (48.28 km/hr).

If we look at Zipp's data the drag @ 30mph-12.5 degrees is about .23lb
for the 808, .53lb for the Ksyrium, and .77lb for the 32 spoke GL330...
so the proportions are fairly close to what Tour got. To convert to
power, just take force times velocity.

30mph = 13.41m/sec
1lb= 4.448N

so, Power (W)=59.6*drag (lb)

808= 13.7W
Ksyrium= 31.6W
GL330= 45.9W

... so these numbers agree pretty well with Tour... but then we have to
add in the power to spin the wheel. From Zipp's graphs:

808= 39.5 + 13.7 = 53W total
Ksyrium= 47.5 + 31.6 = 79W
GL330= 62 + 45.9 = 108W

Remember, this is at 12.5 degrees resultant wind... which would be the
equivalent of a 6.7mph wind from the side @30mph ground speed...
probably more than what most riders would normally experience at wheel
level. But... wow!... 55 watts for one wheel! (comparing 808 to GL330)

If you want to know the power values for other speeds take the ratio of
speed cubed... not perfectly correct, but close enough. For instance,
to determine the difference between the 808 and GL330 at 20mph:

Delta P@20mph = 55* (20/30)^3 = 16.3W

If you'd like to know the effect that wheel drag has on speed on a flat
road, a decent approximation would be the ratio of total power raised
to the .37 power, which is greater than 1/3 to account for rolling
resistance. For instance if you can go 30mph with 808's with a 6.7mph
sidewind, how fast could you go with Ksyriums? First you need to get a
ballpark number for the total power it takes for that case. There are
several online calculators if you don't have your own. It's about 380W
for somebody with a very good CdA of .215m^2. Another factor is, I want
to know what it is with two wheels, not just one. Well... for lack of a
better alternative, I'm going to say it's double for two (even though
the rear wheel drafts a bit)... so it's (79W-53W)*2 = 52W.

((380W-52W)/(380W))^.37 * 30mph= 28.4mph... a 1.6mph decrease. Of
course the difference would be less at a lower speed or less of a cross
wind... but I'll leave that as an exercise for you...

So, I hope you have enough info now to have all sorts of fun with
Zipp's graphs!

Nice! Just what I was looking for. Enough info to get me on my way. My
first estimate has Zipp 404's allowing for 40 km/h with the effort
required to go 37 km/h with 32 spoke GL330's with the 6.5 mph sidewind.
I'll play some more now.

I use a .35 CdA for simulating being in the drops. Is that reasonable?
I'm working on a way to measure frontal area using a bitmapped b/w
photo with a histogram, but until then I use .65 sq meters.

Thanks!

Joseph

A.Lee wrote:

I used to worry about such things, but now I realise, as others point
out regularly, that the time spent comparing different wheels/equipment
is better spent training, and reducing your times, without spending a
fortune.

Unless you are actually in danger of winning a competition, it makes
sense to me to just ride what you have and calculate what the fancy
stuff might have done for you. If it looks like you might have won if
you'd had an upgrade, then spending the money would make more sense. If
it is just a matter of finishing 20th instead of 30th in a TT, then it
doesn't really matter to me.

On that note, you can build (or have built) a decent set of aero wheels
for much less than what the boutique models cost. A 30mm deep aluminum
rim (Velocity Deep, Sun TA1, Speedcific Niobium), with oval spokes
(Wheelsmith AE15 or XE14, DT Aerolites, Sapim Cxray), should be quite
fast, durable, and serviceable.

wrote:

I use a .35 CdA for simulating being in the drops. Is that reasonable?
I'm working on a way to measure frontal area using a bitmapped b/w
photo with a histogram, but until then I use .65 sq meters.

That is probably a good guess for the GL330 case (CdA is the thing that
actually changes when you swap wheels)... as I recall, you are larger
than the average cyclist. I've been using .32 to model myself (6ft,
160lbs). Of course it will vary quite a bit depending on how
aerodynamic your body is, and how low you get in the drops.

It amazes me that elite cyclists can get CdA values of less than .20
with the best equipment.

You can get a good estimate of your sustainable power if you have a
good climb with a known grade... aero effects will be small. Then you
can translate that power number to riding on the flat and get a guess
at CdA. Or, you could also estimate CdA by taking measurements while
coasting down the hill. IHPVA has info on how to do this.

http://www.ihpva.org/

Ron Ruff wrote:
It amazes me that elite cyclists can get CdA values of less than .20
with the best equipment.

That *is* amazing! Unless you're talking about faired HPVs, of course.
But 0.20 on a diamond frame setup?

--
Dave
dvt at psu dot edu

Ron Ruff wrote:
A.Lee wrote:

I used to worry about such things, but now I realise, as others point
out regularly, that the time spent comparing different wheels/equipment
is better spent training, and reducing your times, without spending a
fortune.

Unless you are actually in danger of winning a competition, it makes
sense to me to just ride what you have and calculate what the fancy
stuff might have done for you. If it looks like you might have won if
you'd had an upgrade, then spending the money would make more sense. If
it is just a matter of finishing 20th instead of 30th in a TT, then it
doesn't really matter to me.

On that note, you can build (or have built) a decent set of aero wheels
for much less than what the boutique models cost. A 30mm deep aluminum
rim (Velocity Deep, Sun TA1, Speedcific Niobium), with oval spokes
(Wheelsmith AE15 or XE14, DT Aerolites, Sapim Cxray), should be quite
fast, durable, and serviceable.

There is no danger of me winning my main event, but the 4 events I am
going to concentrate on next season are all over 250km. These are more
or less big team time trials (without aero bars) where up to 30 riders
start per team, and the best 10 riders' times count toward the team
competition. Folks get "used up" along the way, but I'd like to be one
of those who finish with the lead group. In the 540km event, the 404's
theoretically could save me 1 hour. I won't finish an hour sooner, but
I might be able to stick it out the whole distance at speed.

Many months ahead to find some money for these wheels...

Joseph

wrote:

There is no danger of me winning my main event, but the 4 events I am
going to concentrate on next season are all over 250km. These are more
or less big team time trials (without aero bars) where up to 30 riders
start per team, and the best 10 riders' times count toward the team
competition. Folks get "used up" along the way, but I'd like to be one
of those who finish with the lead group. In the 540km event, the 404's
theoretically could save me 1 hour. I won't finish an hour sooner, but
I might be able to stick it out the whole distance at speed.

Many months ahead to find some money for these wheels...

Joseph

Well, now that we know you're not talking about differences between
wheels during a TT-style event, the differences become even less
important. The benefits of aero wheels are greatly reduced when riding
in a pack. If your team of riders isn't riding 'on the rivet' as they
say, in full team time trial mode, you'll probably recognize no
appreciable benefit whatsoever.