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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 |
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#2
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Wheel drag
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. |
#3
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Wheel drag
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! |
#4
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Wheel drag
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#5
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Wheel drag
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 |
#6
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Wheel drag
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. |
#8
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Wheel drag
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 |
#9
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Wheel drag
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 |
#10
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Wheel drag
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