Landracing Forum
Introductions => Formulas => Topic started by: landracing on March 21, 2008, 09:19:29 AM
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Like most spreadsheets and online calculators, milage may vary.
This was developed from an article done for Cycle Magazine in the late 1970's.
Jon
http://www.landracing.com/formula/nomograph.htm
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How do you figure what your coefficient of drag is?
Scott
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In your example inputs, don't you have the drag coefficients shown as frontal areas and vice versa?
As to Scott's question, if you have good horsepower data as set up on a previous run, (corrected for atmospheric conditions), and know the frontal area and speed, you can try various drag coefficients until the powerr calculated matches the power you believe you had.
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I just punched in my knowns until the HP was correct and came up with a CD that is less then .02 of what my old Willie P. calculator told me my setup was 4 years ago(although it allows adjustability of both air density and mechanical drag as well). And the frontal increase combined with the speed increase of my dinner-plate-fairing lowered my CD .08 which is only .01 from what my other calc showed and right inline with my speeds obtained.
More accurate then I could of imagined(especially considering the lack of info) and more then enough to verify my other data as well as reason to continue using those numbers as a baseline of my future changes.
Thanks,
Todd
PS: Yes, the example data is backwards in data entry
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Woops darn late nights see what it does to you.. Ill get that fixed tonight.
Jon
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punched in my info and it said i needed 129 rwhp my bike only makes 112 hp rear wheel on the dyno its .8 cd--7.3 sq.ft. frontel area --715 lbs--fuel record 141.296 that equals 112 rw hp check out this site and check the bonneville aero calcularor and many others and for you bike guys there is a rake and trail calculator willie buchta
i run one of there air fuel ratio guages a great product
http://www.rbracing-rsr.com/calculations.htm
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I show a less then 1.5hp difference between the RB calc and Jon's, although I'm not sure what either is basing the calc's on as far as air-density numbers.
I do find it odd that I run over 140mph in a mile with 62hp at Maxton, yet all the numbers seem to jive pretty close in both these calcs... Hmmm?
Todd
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todd what is your cd and frontel area ---my bike is an open (no streamlining) WITH A SIDE CAR willie buchta
el mirage is dirt bonneville is salt lots of tire slippage wasted hp plus isnt your bike partially streamlined
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let me add something else here ---im not saying any formula is right or wrong im just saying this is the one i use for myself --it doesnt matter which one you use as long as you use the same one all the time and use it for comparisons only--where the hell is dolan he is much better with words than i am see you in the dirt willie buchta dont ride a horse backwards upstream -- i think i got it jack miss you dude
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I run naked and faired both(no crutch though).
(http://i122.photobucket.com/albums/o244/narider/LSR/76%20XL%20-%20LSR%201300/1300onTheLineC.jpg)
(http://i122.photobucket.com/albums/o244/narider/LSR/76%20XL%20-%20LSR%201300/100_3022.jpg)
(http://i122.photobucket.com/albums/o244/narider/LSR/76%20XL%20-%20LSR%201300/halftucked1300c.jpg)
MPH and FA fluctuate(between the 2 classes and fuel tanks ran) anywhere from 136-140mph and 3.9-4.4sf with 62hp
(http://i122.photobucket.com/albums/o244/narider/LSR/76%20XL%20-%20LSR%201300/BooFrontaljpg.jpg?t=1206127485)
Bigger pic----> http://tinyurl.com/2rcbj8
Agreed on using the same all the time(although I haven't been fast enough to make any little effective changes yet).
It's the track and the vehicle we run that tells what the formulas will put out, not the other way around.
Missing Jack too Willie... luckily, nothing's forever! :wink:
Todd
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I run naked and faired both(no crutch though).
(http://i122.photobucket.com/albums/o244/narider/LSR/76%20XL%20-%20LSR%201300/1300onTheLineC.jpg)
(http://i122.photobucket.com/albums/o244/narider/LSR/76%20XL%20-%20LSR%201300/100_3022.jpg)
(http://i122.photobucket.com/albums/o244/narider/LSR/76%20XL%20-%20LSR%201300/halftucked1300c.jpg)
MPH and FA fluctuate(between the 2 classes and fuel tanks ran) anywhere from 136-140mph and 3.9-4.4sf with 62hp
(http://i122.photobucket.com/albums/o244/narider/LSR/76%20XL%20-%20LSR%201300/BooFrontaljpg.jpg?t=1206127485)
Bigger pic----> http://tinyurl.com/2rcbj8
Agreed on using the same all the time(although I haven't been fast enough to make any little effective changes yet).
It's the track and the vehicle we run that tells what the formulas will put out, not the other way around.
Missing Jack too Willie... luckily, nothing's forever! :wink:
Todd
Todd just curious if you have ever run with that partial fairing tipped up a little more in the back? You aren't a big guy, but Willie is even skinnier than you 8-), maybe an unfair advantage :evil:,
Sum
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Sum, I did some tufting here at the shop (both with a squirrel cage and on the road) and best I could tell(results varied largely) is that my least amount of back draft was at that angle (not too mention tilting it up, put my last minute headlite replacement(a Twin Cam Derby cover :-D ) very flat up front).
(http://i122.photobucket.com/albums/o244/narider/LSR/76%20XL%20-%20LSR%201300/100_4136.jpg)
Remember, the only reason I put this $10 fairing on was to qualify for a PS class.... I actually remember telling Scott that I installed something that I think will make me a bit slower, but the speed I'm trying to obtain is slower as well so it should all even out(from the sound i heard over the phone, he just shook his head at me) - :|
Todd
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There is some good info on wind tunnel testing at
www.A2WT.com
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one thing especially for the bike guys to remember, if you are using a dynojet your rear wheel numbers are inflated by a good 15% from SAE horsepower numbers... that might account for the slight error margin in some of the above comparisons.
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It also changes with how hard you strap it down and how much air is in the tire.. Pretty useless in my mind..
Dave
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How do you figure what your coefficient of drag is?
Scott
Pay your bucks to the wind tunnel or do a roll down test. I believe Jack D has experience in doing roll down tests.
I would have to dust off a book or two or do a bit of googling, so you may see what you can find on roll down tests. In theory it's supposed to be easy IIRC.
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I would have to dust off a book or two or do a bit of googling, so you may see what you can find on roll down tests. In theory it's supposed to be easy IIRC.
Uh, see "Coast Down Formula", in "Formulas", this site.
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Jack started his tests at the local bowling alley -- and progressed to engineering wonders.
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There is some good info on wind tunnel testing at
www.A2WT.com
cool link
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Hello, I just went to open the nomograph, one of my favorite things, and it appears the link is broken. Is there another way I can get access to it?
Thanks, Ralph Hudson
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^^^
I was wondering the same thing?
Is there another calculator to use that has the cd numbers?
Jon
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I suspect this may be an issue the web elf needs to sort out, the link might have gotten broken when they did the upgrade.
Larry
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The RB Racing link in #5 still works...