Dismiss Notice
Welcome to
Pinewood Derby Online, a forum for questions and discussions about everyone's favorite gravity powered racers!
CLICK HERE to register as a member today for full access to the forum, it's fast, simple, and absolutely free!

COG

Discussion in 'Ask The Pros' started by cara424, Feb 12, 2019.

  1. cara424

    cara424 Pinewood Ninja

    Messages:
    39
    Likes Received:
    5
    Trophy Points:
    8
    Location:
    Pa
    I know that a COG between 1/2-3/4 is pretty good, but what is the best ration of weight behind/in front of axels? Also no one answered my previous post... how will a car will an agressive COG run on a track with a longer straight run vs a shorter track? (Both aluminum)
     
  2. TRE

    TRE PWD Royalty Pro Racer Pro Race Winner

    Messages:
    1,398
    Likes Received:
    292
    Trophy Points:
    83
    In my opinion i dont think it matters put 1.8 to 2 ounces behind the rear axles and the rest of your weight right in front of the rear axles
     
  3. cara424

    cara424 Pinewood Ninja

    Messages:
    39
    Likes Received:
    5
    Trophy Points:
    8
    Location:
    Pa
    I know this is “the” question. But with 2 oz behind the wheel of a wedge car and good prep of wheels/axels with a car running straight... we only came in 3rd. Anything else I’m not considering for next yr? ..... distribution of weight, slimmer car design, redrill axel slots?
     
  4. TRE

    TRE PWD Royalty Pro Racer Pro Race Winner

    Messages:
    1,398
    Likes Received:
    292
    Trophy Points:
    83
    Are you running 3 wheel rail runner or 4 wheels touching
     
  5. TRE

    TRE PWD Royalty Pro Racer Pro Race Winner

    Messages:
    1,398
    Likes Received:
    292
    Trophy Points:
    83
    If you had 2 oz behind and the rest right in front its not the weight..its something else
     
    SuperSlow likes this.
  6. Darkside

    Darkside District Champion

    Messages:
    105
    Likes Received:
    82
    Trophy Points:
    28
    Gender:
    Male
    Location:
    Maricopa, AZ
    Follow Tre's advice on weight placement. It's not as critical as you're thinking it is. There was most likely something else that was slowing you down.

    You can build everything seemingly perfect and still get a slow car. We built three cars this year, all prepped exactly the same way. Two were buttery smooth and fast straight off the bench. The third was way slow. The solution ended up being rotating one of the rear axles a quarter turn and it was good to go.

    The only way to know for sure if a car will run properly is by testing. Get a test track if you can and test test test.
     
    TRE likes this.
  7. TRE

    TRE PWD Royalty Pro Racer Pro Race Winner

    Messages:
    1,398
    Likes Received:
    292
    Trophy Points:
    83
    You could have a alow set of wheels
     
  8. cara424

    cara424 Pinewood Ninja

    Messages:
    39
    Likes Received:
    5
    Trophy Points:
    8
    Location:
    Pa
    What do you mean?
     
  9. cara424

    cara424 Pinewood Ninja

    Messages:
    39
    Likes Received:
    5
    Trophy Points:
    8
    Location:
    Pa
    How about where the wheel could bump against the axel slot? I did have a very tiny chip on one axel slot
     
  10. TRE

    TRE PWD Royalty Pro Racer Pro Race Winner

    Messages:
    1,398
    Likes Received:
    292
    Trophy Points:
    83
    Not all wheels run the same speed..some you think are fast prep up nice then they are slow
     
  11. CM02WS6

    CM02WS6 Pinewood Ninja Pro Racer

    Messages:
    46
    Likes Received:
    17
    Trophy Points:
    8
    Gender:
    Male
    Location:
    Illinois
    I'm still new to this, but one of the common misconceptions I see thinking as an engineer is regarding whether the car is a 3 wheel or 4 wheel touching car. Exactly what TRE asked. That pretty much double/halves the weight on the front wheels, and the weight on the front wheel is what determines how good of a job it can do holding the car stable. Boiling this down, it seems common that a 3 wheel car can be stable with front wheel weights in the 15 gram range or even lower, which can be done with a COM at 1/2". However, changing that car to a 4 wheeler and going down to 7.5 grams per wheel is not enough for those wheels to be effective. Increasing the front wheel weight to the same range as that 3 wheeler requires a COM at 1".

    With regards to your question about track length, an aggressive COM would be more likely to wiggle on the straights, regardless of how long those straights are.
     
  12. cara424

    cara424 Pinewood Ninja

    Messages:
    39
    Likes Received:
    5
    Trophy Points:
    8
    Location:
    Pa
    So could you effectivley put a little weight right at the front wheels to help control the balance? I noticed that the guys who beat me had some weight placement down the body of the car
     
  13. CM02WS6

    CM02WS6 Pinewood Ninja Pro Racer

    Messages:
    46
    Likes Received:
    17
    Trophy Points:
    8
    Gender:
    Male
    Location:
    Illinois
    Yes, if that was the problem then putting weight at the front would have a direct impact on the front wheel weights.
     
  14. cara424

    cara424 Pinewood Ninja

    Messages:
    39
    Likes Received:
    5
    Trophy Points:
    8
    Location:
    Pa
    I may have .02oz to play with.. depending on the scale at districts. Not sure if rhat would impact it at all, but I can try slapping on some tungsten putty
     
  15. SuperSlow

    SuperSlow District Champion

    Messages:
    144
    Likes Received:
    38
    Trophy Points:
    28
    Gender:
    Male
    Location:
    Harrisburg, Pa
    Your getting way out of shape with this. You really dont want to put ANY weight in the front of your car. If your car is built correctly you should be able to manipulate the weight on your dfw without actually placing any weight up there. As Tre said, you want 2oz behind the rear axle and the rest right in front of it. It is very simple, you just need to keep building cars, learning, and become more experienced.

    You can build 4 or 5 cars EXACTLY the same and they will all run different. Some will need more steer, some will take very little steer and be really clean n stable. Some will be fast some will be slow. You can go down the list and do everything by the book and think you know whats gonna happen and your car tells you something else.
     
    TRE likes this.
  16. Wolfram Racing

    Wolfram Racing Rail Runner

    Messages:
    76
    Likes Received:
    36
    Trophy Points:
    18
    Gender:
    Male
    Location:
    Scenery Hill, PA
    Agree with TRE and SuperSlow...the answer isn't to put weight in the front of the car. Keep it in the rear and distribute it to get the balance and stability you need. It's not ONLY about COM, you also have to consider (particularly for a rail runner) the lateral moment of inertia. Putting weight up front is going to make it turn hard into the rail, even with minimal steer. Think of a broomstick with 10lb weights on the ends and hold it in the middle...hard to turn. Now take those weights and move them to the center right by your hand...much easier to turn. In both cases, same COM but very different MOIs. In the case of the car, concentrating the weight in the rear keeps the MOI low...and that mean less normal force the rail applies to the DFW to keep it on track. Less normal force = less friction.
     
    SuperSlow likes this.
  17. CM02WS6

    CM02WS6 Pinewood Ninja Pro Racer

    Messages:
    46
    Likes Received:
    17
    Trophy Points:
    8
    Gender:
    Male
    Location:
    Illinois
    Totally agree. My suggestion to go ahead and put weight in the front was just a band aid. With only a little bit to play with, and with the desire to increase front wheel weight, then go for it. Implementation of detailed knowledge of COG impact on front wheel weight and Moment of Inertia would require a little bit more experience than I got the sense he was ready for, and would probably require making a new car.
     

Share This Page