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Author Topic: Nomograph relating CdA, BHP and MPH  (Read 16303 times)
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landracing
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« on: March 21, 2008, 08:19:29 AM »

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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isiahstites
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« Reply #1 on: March 21, 2008, 08:34:39 AM »

How do you figure what your coefficient of drag is?

Scott
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tortoise
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« Reply #2 on: March 21, 2008, 08:50:01 AM »

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.
« Last Edit: March 21, 2008, 08:58:02 AM by tortoise » Logged
narider
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« Reply #3 on: March 21, 2008, 09:02:08 AM »

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
« Last Edit: March 21, 2008, 09:04:07 AM by narider » Logged
landracing
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« Reply #4 on: March 21, 2008, 09:07:27 AM »

Woops darn late nights see what it does to you.. Ill get that fixed tonight.

Jon
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willieworld
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« Reply #5 on: March 21, 2008, 10:41:31 AM »

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
« Last Edit: March 21, 2008, 10:47:01 AM by willieworld » Logged

willie-dpombatmir-buchta
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« Reply #6 on: March 21, 2008, 11:05:43 AM »

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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« Reply #7 on: March 21, 2008, 01:13:54 PM »

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
« Last Edit: March 21, 2008, 02:21:56 PM by willieworld » Logged

willie-dpombatmir-buchta
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« Reply #8 on: March 21, 2008, 02:19:23 PM »

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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willie-dpombatmir-buchta
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« Reply #9 on: March 21, 2008, 02:28:56 PM »

I run naked and faired both(no crutch though).






MPH and FA fluctuate(between the 2 classes and fuel tanks ran) anywhere from 136-140mph and 3.9-4.4sf with 62hp


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
  
« Last Edit: March 21, 2008, 02:31:34 PM by narider » Logged
Sumner
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« Reply #10 on: March 21, 2008, 02:57:21 PM »

I run naked and faired both(no crutch though).






MPH and FA fluctuate(between the 2 classes and fuel tanks ran) anywhere from 136-140mph and 3.9-4.4sf with 62hp


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  cool, maybe an unfair advantage  evil,

Sum
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« Reply #11 on: March 21, 2008, 03:26:29 PM »

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 grin ) very flat up front).



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) -  undecided
Todd 
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« Reply #12 on: May 30, 2008, 02:35:18 PM »

There is some good info on wind tunnel testing at

www.A2WT.com
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The answers are blowing in the wind...
www.A2WT.com
http://www.facebook.com/pages/AERODYN-A2-WIND-TUNNEL/259986785465
@A2WindTunnel #A2WindTunnel
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« Reply #13 on: June 04, 2008, 01:15:49 PM »

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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ddahlgren
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« Reply #14 on: June 04, 2008, 02:12:57 PM »

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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