Author Topic: CP vs CG  (Read 102260 times)

0 Members and 1 Guest are viewing this topic.

Offline Richard Thomason

  • Sr. Member
  • ****
  • Posts: 421
    • http://www.dannyboystreamliner.com
CP vs CG
« on: December 13, 2014, 01:39:39 PM »
I read with great interest the Bonneville Racing News interview with George Poteet concerning his unfortunate crash this past year. A great tip of the hat to George and the whole team for everything they have accomplished. I wish them well in their coming endeavors, whatever they may be. I too understands George's concern regarding father-time and its relentless movement. Ed and I are not getting any younger either, and building our new car is taking longer than expected. We retired the original Danny Boy, because it was 25 years old and we were starting to push up over 350 mph and were concerned about the ravages of time and salt on the chassis. We had seen several older cars crash in the previous few years and just didn't want to repeat history. Thus the decision to embark on a new project.

The article about George's crash got me to thinking about our earliest days at the salt and getting started.
In 1979, when Ed Tradup and I were considering a build for the salt flats, we did a lot of research and investigation. I contacted my dad's cousin (Clinton Wilkinson), who was a very high up engineer with Boeing. When I explained what we were attempting to do, there was a long silence, then he said I'm only going to tell one thing because you're probably going to kill yourself. Make sure the CG is ahead of the CP! And bang the phone slammed down. Pretty sage advice. Anyway, that was one the driving factors that led us to build the Danny Boy as a front motored, front wheel drive car and eventually with a huge vertical stabilizer. 

Offline Bob Drury

  • Hero Member
  • *****
  • Posts: 2599
Re: CP vs CG
« Reply #1 on: December 13, 2014, 02:32:44 PM »
  Richard, we all need more input from folks like yourself, Tom Burkland, George Fields, et al who have learned from their own experiences and listening to the advice  and "real life" experiences both on and off the Salt and Dirt of El Mirage.
  I also want to thank George Poteet for sharing what He learned and would do differently if ever in a similar crash to His recent one.
  With a site like this one "newbees" and "oldbees"  have a chance to both learn and to build safer cars that go faster.
  I would urge you to share more with the rest of us and consider posting on our BUILD forum.
                                                                                         Hopefully speaking for everyone on this site............
                                                                                               "One Run" Bob Drury

Bob Drury

Offline Sumner

  • Global Moderator
  • Hero Member
  • ***
  • Posts: 4078
  • Blanding, Ut..a small dot in the middle of nowhere
    • http://purplesagetradingpost.com/sumner/sumnerindex.html
Re: CP vs CG
« Reply #2 on: December 13, 2014, 02:33:15 PM »
....In 1979, when Ed Tradup and I were considering a build for the salt flats, we did a lot of research and investigation. I contacted my dad's cousin (Clinton Wilkinson), who was a very high up engineer with Boeing. When I explained what we were attempting to do, there was a long silence, then he said I'm only going to tell one thing because you're probably going to kill yourself. Make sure the CG is ahead of the CP! And bang the phone slammed down. Pretty sage advice. Anyway, that was one the driving factors that led us to build the Danny Boy as a front motored, front wheel drive car and eventually with a huge vertical stabilizer. 

You certainly got some good advice way back then  :cheers:,

Sumner

P.S.  In 4 years you will be as old as I am now.  Wish I could hang around for you to catch up with me but time marches on and is dragging me along with it, or something like that  :-)

Offline John Burk

  • Sr. Member
  • ****
  • Posts: 695
Re: CP vs CG
« Reply #3 on: December 13, 2014, 03:19:22 PM »
Richard thank you for posting that . Anybody with any doubt about the CP of their car make a 1 or 2 ft wooden model of your car and with it hanging from a thread and aim your blow gun at the nose . Staple a thread to the nose with washers to make it hang level . If it doesn't point into the wind move the thread forward till it does . If the model is cut roughly accurate on the bandsaw and smoothed up a little that's good enough . Don't be surprised when the CP you find is way forward of the traditional center of side area . The CP model of my streamline was at about 40% from the nose .

Offline panic

  • Sr. Member
  • ****
  • Posts: 845
    • My tech papers
Re: CP vs CG
« Reply #4 on: December 13, 2014, 07:34:18 PM »
If a simple analysis of a 2-dimensional (elevation, profile, silhouette) area of the car is good enough, an accurate scaled drawing will produce good results.
E-mail me for details.

Offline RogerL

  • Full Member
  • ***
  • Posts: 111
Re: CP vs CG
« Reply #5 on: December 13, 2014, 08:04:07 PM »
Good comments Richard. I choose to ignore the one about us all getting older, but you and George are correct.  I also read the article in TBN. Interestingly I trashed my previous ‘liner for pretty much the same reasons. The car was a rear wheel drive, weight biased to the rear, turbo motor. I was on a warm up run (around 270)when the turbos came on about the time I ran into to some damp salt. The rear tires spun, car went side ways and that was the end of that. I knew the car had the CP/CG relationship backwards but it had never been a problem, until ……When we started designing the current car, we discussed the CP/CG issue and the folks at Riley Tech and Bob Riley insisted we build a model and do a wind tunnel test for stability, including yaw. We did that and proofed the design. The other issue was to go with all wheel drive. When running for the higher speeds, I am convinced AWD is a necessity for not only acceleration but for stability reasons as well. In describing the problem, I suggest to folks they take a pencil and put it between their two index fingers (inline) and push, no problem. now move one finger out of line and stability is no longer in the cards. One finger can be viewed as the driving force and the other the aero force. Now think about having a driving force at the same end as the aero force pushing in the opposite direction. Pretty simple. I will also offer that George and Ron’s car was not the only one with that problem running at Bonneville, there are others and it is only a matter of time as speeds increase and salt conditions continue to be marginal that the issue will persist. My incident was in 1990, really surprises me we are 24 years past and the issue is still around and being discussed like it is some new revelation. That said, it what makes Bonneville so cool, we all get to do our way.

Offline Speed Limit 1000

  • Hero Member
  • *****
  • Posts: 1396
Re: CP vs CG
« Reply #6 on: December 13, 2014, 09:20:52 PM »
Roger, 24 years. I remember it like it was yesterday. Happy you are still with us and involved in LSR. Since Ben Jordon got me into LSR, I understand it can be a life long adventure.

John
John Gowetski, red hat @ 221.183 MPH MSA Lakester, Bockscar #1000 60 ci normally aspirated w/N20

Offline desotoman

  • Hero Member
  • *****
  • Posts: 2815
Re: CP vs CG
« Reply #7 on: December 14, 2014, 01:43:28 PM »
Some great advise from many people.......

Hopefully it will change some peoples mind set......

Some people feel the longer the dart the less likely it will happen......

In the end that is what makes Landracing so great, everyone has there own ideas, some work, some not so well.

Tom G.
I love the USA. How much longer will we be a free nation?

Asking questions is one's only way of getting answers.

The rational person lets verified facts form or modify his opinion.  The ideologue ignores verified facts which don't fit his preconceived opinions.

Offline jl222

  • Hero Member
  • *****
  • Posts: 2955
Re: CP vs CG
« Reply #8 on: December 14, 2014, 02:14:33 PM »

  Anyone know the CP vs CG of a top fuel dragster,funny car,formula 1 or Bville lakester like the DMR?

   What changes does the CP-CG have when the spoilers and wings add downforce?

  How come Street roadster spin more often when their CP is better than other roadsters?

  In our case with the 222 Camaro, adding weight to front of car for better CP would increase tire spin.

                  JL222



   

Offline tortoise

  • Sr. Member
  • ****
  • Posts: 685
Re: CP vs CG
« Reply #9 on: December 14, 2014, 05:22:32 PM »
 How come Street roadster spin more often when their CP is better than other roadsters?
What moves the street roadster CP rearward relative to a roadster?
Quote
 In our case with the 222 Camaro, adding weight to front of car for better CP would increase tire spin.
Was this added weight in front of the front axle?

      



  
« Last Edit: December 14, 2014, 05:25:55 PM by tortoise »

Offline Stan Back

  • Hero Member
  • *****
  • Posts: 5879
Re: CP vs CG
« Reply #10 on: December 14, 2014, 06:01:37 PM »
Street Roadsters have less engine set-back than (Altered) Gas and Fuel Roadsters.

Street Roadsters also have to put the driver in the original "cockpit" where Gas and Fuel Roadster can move them rearward as long as they stay in front of the rear end.  And the cage goes with them.
Past (Only) Member of the San Berdoo Roadsters -- "California's Most-Exclusive Roadster Club" -- 19 Years of Bonneville and/or El Mirage Street Roadster Records

Offline jl222

  • Hero Member
  • *****
  • Posts: 2955
Re: CP vs CG
« Reply #11 on: December 14, 2014, 08:16:44 PM »
 How come Street roadster spin more often when their CP is better than other roadsters?
What moves the street roadster CP rearward relative to a roadster?
Quote
 In our case with the 222 Camaro, adding weight to front of car for better CP would increase tire spin.
Was this added weight in front of the front axle?

      



  


  Like Stan says the engines are not allowed the setbacks other roadster classes are.

  Not adding weight forward and upsetting weight on rear wheels of 222 Camaro.

  I would like more answers on the static vs dynamic down force on CP that no one is talking about.

  Indy and formula 1 didn't go to rear engines because they were worried about CP. What is their CP-CG?

  I can see Airplanes designed with correct CP because of landing and flying straight, but rear wheel driven cars especially short wheelbase cars have different problems.

    JL222




 

 


Offline tortoise

  • Sr. Member
  • ****
  • Posts: 685
Re: CP vs CG
« Reply #12 on: December 14, 2014, 10:58:41 PM »
 I would like more answers on the static vs dynamic down force on CP that no one is talking about.
You're mixing 2 questions. The CP/CG relationship concerns whether aerodynamic forces  act to turn the vehicle. Downforce concerns whether the wheels can produce steering and tractive force. If the wheels never break loose, a bad CG/CP relationship may never show up as a problem.
« Last Edit: December 14, 2014, 11:01:20 PM by tortoise »

Offline jl222

  • Hero Member
  • *****
  • Posts: 2955
Re: CP vs CG
« Reply #13 on: December 15, 2014, 01:40:50 AM »
 I would like more answers on the static vs dynamic down force on CP that no one is talking about.
You're mixing 2 questions. The CP/CG relationship concerns whether aerodynamic forces  act to turn the vehicle. Downforce concerns whether the wheels can produce steering and tractive force. If the wheels never break loose, a bad CG/CP relationship may never show up as a problem.

  That's what I'm trying to get going on the thinking. Just because airplanes are designed that way, don't be moving weight forward to improve CP and make thinks worse by increasing chances of wheel spin.

  Streamlines and a few other classes can design in CP and CG but others are limited.

  I still would like to know the CP vs CG on a top fuel car at 330 mph when the wing is putting hundreds of lbs. of down
force on rear tires.

           JL222

Offline John Burk

  • Sr. Member
  • ****
  • Posts: 695
Re: CP vs CG
« Reply #14 on: December 15, 2014, 07:57:44 AM »
If the traction is good enough the CP can be at the push bar . Salt and dirt traction is poor so the CP location matters to us .