Landracing Forum Home
October 22, 2017, 09:40:45 PM *
Welcome, Guest. Please login or register.
Did you miss your activation email?

Login with username, password and session length
News:
BACK TO LANDRACING.COM HOMEPAGE
 
   Home   Help Search Calendar Login Register  


(Note: Donations are not tax deductible)







Live Audio Streaming and Archives of Past Events
Next Live Event: TBD
Pages: [1] 2   Go Down
  Print  
Author Topic: HP needed for speed for a lakester (will work for cars also)....  (Read 7604 times)
0 Members and 1 Guest are viewing this topic.
Sumner
Global Moderator
Hero Member
***
Offline Offline

Age: 73
Location: Blanding, Utah
Posts: 3961


Blanding, Ut..a small dot in the middle of nowhere


WWW
« on: October 15, 2013, 07:56:18 PM »

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 ...



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

The spread sheet should also work for cars, streamliners and even bikes if you enter zero's where the tire inputs are,

Sum
Logged

manta22
Hero Member
*****
Offline Offline

Age: 78
Location: Tucson, AZ
Posts: 2984


What, me worry?




Ignore
« Reply #1 on: October 15, 2013, 08:04:37 PM »

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
Logged

Regards, Neil  Tucson, AZ
Sumner
Global Moderator
Hero Member
***
Offline Offline

Age: 73
Location: Blanding, Utah
Posts: 3961


Blanding, Ut..a small dot in the middle of nowhere


WWW
« Reply #2 on: October 15, 2013, 08:36:57 PM »

....- 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  smiley,

Sum
Logged

Peter Jack
Hero Member
*****
Online Online

Age: 74
Location: Calgary, Alberta
Posts: 3441





Ignore
« Reply #3 on: October 15, 2013, 08:51:17 PM »

The other thing to remember is that the suspension or axles between the body and the wheels can be significant. Shape counts there.

Pete
Logged
Eddieschopshop
Full Member
***
Offline Offline

Location: Sacramento, California
Posts: 144




Ignore
« Reply #4 on: October 15, 2013, 09:10:36 PM »

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.
Logged
SPARKY
Global Moderator
Hero Member
***
Offline Offline

Age: 75
Location: Phoenix
Posts: 6236




« Reply #5 on: October 15, 2013, 09:28:50 PM »

Chop shop Eddie thanks for the feed back  cheers
Logged

Miss LIBERTY,  changing TKI  to noise, dust and RUST!!!

The # 1 issue is: TO KEEP THE REPUBLIC      
   Center for Self Governance            tncsg.org     mrspowell.org

"Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than outright exposure. Life is either a daring adventure or nothing."   Helen Keller
salt27
Hero Member
*****
Offline Offline

Age: 60
Location: S.W. Orygun
Posts: 1233





Ignore
« Reply #6 on: October 15, 2013, 10:00:39 PM »

Just in case I forgot to say it, dodge it's nice to have you back Sum.

  Don
Logged
Sumner
Global Moderator
Hero Member
***
Offline Offline

Age: 73
Location: Blanding, Utah
Posts: 3961


Blanding, Ut..a small dot in the middle of nowhere


WWW
« Reply #7 on: October 15, 2013, 10:57:52 PM »

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  smiley
Logged

Eddieschopshop
Full Member
***
Offline Offline

Location: Sacramento, California
Posts: 144




Ignore
« Reply #8 on: October 15, 2013, 11:18:04 PM »

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. 
Logged
Kiwi Paul
Sr. Member
****
Offline Offline

Posts: 494




Ignore
« Reply #9 on: October 16, 2013, 09:56:24 PM »

Also..most ReMR`s are a lakester entering the air, and a roadster leaving it.....  .35? I`d buy that..
Logged
jl222
Hero Member
*****
Offline Offline

Location: Clovis calif.
Posts: 2523




Ignore
« Reply #10 on: October 17, 2013, 05:00:16 PM »


  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
Logged
4-barrel Mike
Hero Member
*****
Offline Offline

Age: 69
Location: Portland, OR
Posts: 2776


Any fool can drive a V8




Ignore
« Reply #11 on: October 17, 2013, 06:29:02 PM »

Ahem, Thacker & Shine roadster  grin

http://www.hotrod.com/roadtests/hrdp_0604_thacker_shine_a_v8_roadster_testing/

Mike
Logged

Mike Kelly - PROUD owner of the V4F that powered the #1931 VGC to a 82.803 mph record in 2008!
Interested Observer
Sr. Member
****
Offline Offline

Posts: 328




Ignore
« Reply #12 on: October 17, 2013, 06:59:25 PM »

Re: Spreadsheet
Why is there no air density input?
Logged
Sumner
Global Moderator
Hero Member
***
Offline Offline

Age: 73
Location: Blanding, Utah
Posts: 3961


Blanding, Ut..a small dot in the middle of nowhere


WWW
« Reply #13 on: October 17, 2013, 08:24:05 PM »

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
Logged

manta22
Hero Member
*****
Offline Offline

Age: 78
Location: Tucson, AZ
Posts: 2984


What, me worry?




Ignore
« Reply #14 on: October 18, 2013, 07:48:17 PM »

....- 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  smiley,

Sum


Sure-- thanks, Sum.
« Last Edit: October 18, 2013, 07:50:10 PM by manta22 » Logged

Regards, Neil  Tucson, AZ
Pages: [1] 2   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Simple Audio Video Embedder
Valid XHTML 1.0! Valid CSS!


Google visited last this page September 19, 2017, 12:42:14 PM