Landracing Forum
Introductions => Formulas => Topic started by: Sumner on October 15, 2013, 08:56:18 PM
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There are different programs on the internet that will help you with the HP needed to run a car at a certain speed. They pretty much require inputs for the frontal area, Cd of the car, the weight of the car if they also take rolling resistance into consideration and of course the speed you desire to run. The problem in using these with a lakester is that if the tires are away from the body to some degree you have the frontal area/Cd of the body and then a different frontal area/Cd for the 4 tires/wheels. Unless you assign one frontal area/Cd for the whole car.
I've have some spreadsheets on my site to help with this but none that were specific to the problem so I made up a new one using formulas from different sources that seem to be fairly well accepted. Here is a view of the spreadsheet with input guesses and outputs for my lakester ...
(http://purplesagetradingpost.com/sumner/Hooley%202013/Lakster%20HP%20Spreadsheet-1.jpg)
You input the rolling resistance value, car's weight, frontal area of the body (not the tires), Cd of the body and the desired speed. Then to the right you input the tire's Cd, height and width for the front and rear tires.
From those inputs you get the HP needed to overcome the rolling resistance, the HP needed to propel the body, the HP needed for the 4 tires and finally a combined RWHP needed to obtain the desired speed.
It also gives you the weight needed on the drive wheels to get the required HP to the ground under different salt conditions.
I made the spreadsheet in Open Office and saved it as an Excel format. I'd really appreciate it if some one opened it in Excel and told me that it works and looks ok in that format.
You can find the link to the spreadsheet here....
http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html (http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html)
The spread sheet should also work for cars, streamliners and even bikes if you enter zero's where the tire inputs are,
Sum
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Sum;
Thanks for posting those spreadsheets.
One point-- in the weight required on the rear wheels for x horsepower-- it probably would help if you point out that the "weight" required on the rear wheels doesn't have to be just ballast weight, rather it is the total of the weight plus the aerodynamic downforce. This can be really significant at high speed.
Regards, Neil Tucson, AZ
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....- it probably would help if you point out that the "weight" required on the rear wheels doesn't have to be just ballast weight, rather it is the total of the weight plus the aerodynamic downforce. This can be really significant at high speed.
Regards, Neil Tucson, AZ
Thanks, good point as that is true. I'll just quote you it that is ok :-),
Sum
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The other thing to remember is that the suspension or axles between the body and the wheels can be significant. Shape counts there.
Pete
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I just tried playing with the spreadsheet a little. To get the numbers to work with what I did at bville this year I ended up with a cd of .35 for my roadster body only. This seems low compared to what I was expecting but that doesn't mean its not accurate. Either way using this for a baseline I used it to try and get a hp for my target for next year and it seems to be about what I was thinking pulling a guess out of my A#$. So my very non scientific opinion is that it works! Thanks for the great spreadsheet.
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Chop shop Eddie thanks for the feed back :cheers:
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Just in case I forgot to say it, dodge it's nice to have you back Sum.
Don
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The other thing to remember is that the suspension or axles between the body and the wheels can be significant. Shape counts there.
Pete
You are right Pete, one has to be realistic about everything. In my case I've inclosed all of the axle/suspension into a 6 inch high enclosure (side pods like Seth's old car) that I hope is very aero in the end and included that in the total frontal area shown in the spreadsheet.
There are lots of variables between the real rolling resistance, the true Cd, frontal area, and 'real' RWHP. Some of these errors might cancel or build. It would be nice to see some data applied from cars that had been in the wind tunnel and had been on a chassis dyno and had a time on the salt.
As I mentioned the formulas aren't mine but came from an aero article in Hot Rod but I've found the same ones in other books on aerodynamics and on the internet. I just arranged them so you only have to input once.
Interesting about the roadster body's .35 Cd. Seems low to me also. Anyone ever had one in a wind tunnel?
Sum
P.S. .....and it is good to be back :-)
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I suppose it could be as simple as thinking of the roadster body by itself. Where as when I typically think of a roadster as pour aero, the formula treats the exposed tires seperately, so the body independent of the tires really isn't too bad. Kinda makes more sense with a little time to think about it.
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Also..most ReMR`s are a lakester entering the air, and a roadster leaving it..... .35? I`d buy that..
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Was it Jimmy Shine's roadster in Hot Rod mag about wind tunnel test in England?
Very low CD from what I remember, it's been awhile.
JL222
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Ahem, Thacker & Shine roadster :-D
http://www.hotrod.com/roadtests/hrdp_0604_thacker_shine_a_v8_roadster_testing/ (http://www.hotrod.com/roadtests/hrdp_0604_thacker_shine_a_v8_roadster_testing/)
Mike
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Re: Spreadsheet
Why is there no air density input?
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Re: Spreadsheet
Why is there no air density input?
Good point but with the HP needed shown one can correct for that or are you saying correct for the air density the car is going through? If the later is the case I'm simply not smart enough to figure that out without a supplied formula :cry:.
Right now I figure there are enough inputs that people are probably guessing about, unless they actually have hard data. Personally I've put in a range of Cd's for my car to give me a range of HP's that I need to possibly shoot for. It can be a reality check,
Sum
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....- it probably would help if you point out that the "weight" required on the rear wheels doesn't have to be just ballast weight, rather it is the total of the weight plus the aerodynamic downforce. This can be really significant at high speed.
Regards, Neil Tucson, AZ
Thanks, good point as that is true. I'll just quote you it that is ok :-),
Sum
Sure-- thanks, Sum.
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Sum, what CD would a car like mine typically have? Backing into it using your calculator, and my fastest run of 189 MPH, I would estimate between a .4 and .45. Does this seem correct?
So, my HP needs look something like this:
CD .4 .45 .5
HP for 189 MPH 482 535 587
HP for 201 MPH 571 635 698
This, of course, assumes my CD estimates are close!
HP Needed for a Specific Speed:
= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
Input Rolling Resistance ======> .024 <== (Note:
Input Car Weight =============> 5000
Input Frontal Area in Sq. Ft. ====> 24.00
Input Cd For Car Body Only====> 0.45
Input Velocity in MPH =========> 201
Body Drag Force = 1064
HP for Rolling Resistance = 37 HP
HP Needed for Car Body = 598 HP
HP For 4 Tires/wheels = HP
For this Speed = 201 MPH
Total RWHP Needed is = 635 HP
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Sum, what CD would a car like mine typically have? Backing into it using your calculator, and my fastest run of 189 MPH, I would estimate between a .4 and .45. Does this seem correct?...
Sorry I didn't see this here...guess that is why you PM'ed me :-). I'm just guessing but would say you are probably in the right range,
Sum