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First attempt of gravity

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  • First attempt of gravity

    Let's just say the results are (sigh) less than stellar.

  • #2
    What issues are you having? Looks like an acceptable design.

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    • #3
      Great effort. I like how you've handled the pickup shoes. Is it safe to say that it's running too light in the front?

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      • #4
        Gives indications that it is light in front AND rear. Granted, I need appropriate tires (using Supertires currently).
        I have played with the spring tension on the pickups... but can't seem to find anything that gives adequate tension. I will explore getting heavier ones... (have tried .007, .008, and .009). I have much to learn. One of the biggest struggles I had was being spoiled by years of popping axles in and out, willy-nilly on conventional chassis. Having to deal with fixed axle and gear set, really...how do I say this nicely? SUCKS!

        Anywho, thanks to everyone in this forum for the information you have provided.

        -Jeff

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        • #5
          Gravity cars can be difficult to work on. More weight in the back might be helpful as well as some thin lead sheet on top of the chassis. Gravity cars generally work better if you use silicone on sponge tires. The ones made for magnet cars often have a bulged tread, for gravity cars you need tires with a perfectly flat tread.
          You may have to adjust the pickup shoes so they make better contact if you are running on a braided track. With a gravity car too much shoe spring tension will lift the front of the car, so usually braided or leaf style pickups are used on those.
          Last edited by RichD; 05-29-2019, 04:32 PM.

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          • #6
            Thanks Rich! I am going to try a pair of silicone/sponge tires next. From what I have read regarding tires for gravity cars, even the soft version of these is going to be a bit too firm, but until I can get a hold of Mr Derosa, I will see how they perform. The current .270 rims have undersized tires on them, so I guess that makes them even harder. If only I had the patience to wait until I get all the proper parts.... but my fabrication ADHD kicks in! For my next iteration, I think I am going to try the same G style pickup system, only reversing it...so that the pickup shoes trail. Thoughts?
            The current weight of this build is almost identical to that of a SS Viper...I will play around with weighting it down a bit more in places.

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            • #7
              Gravity cars are a lot like a boat. Funny dat, at first glance I thought the shoes looked like they were toed down (ploughing). See the hard part of the burn, where the rail is already starting to cleave out the material right behind the upward toe roll on the front of the shoe? Typical of distortion at the bend which causes just a bit to hang below the level line. IMHO, that forward portion needs to move upward fractionally, but the rest of the patch needs to stay right where it is; in order to get you on "plane".

              Other than that, you have a pretty good scour going along the rest of the contact patch. For that reason, I think you're very close, so I wouldnt radically alter the setting until you move that "hard burn" out of the picture. I see the effect of this "concavity" quite a bit with thinly stamped shoes, where the burn is either hard on the toe or the heel, but you cant hit center; regardless of which way you pitch the contact patch (fore or aft). If anything, I prefer to err the contact patch to the convex side of things; so that the toe isnt ploughing, and the heel isnt dragging.

              For dinking with shoes, I have a pair of smooth forceps to hold the shoe. To execute the bend I use the old Aurora Vibrator tool. Basically a screw driver with the head cut off and longitudinal slot sliced into the end of it. This allows one to make very subtle changes in the geometry of the shoe at precise locations along its length. Id be lost without the silly thing.

              Dont sell yourself short. That is an excellent first pop. Hats off to you!
              Last edited by model murdering; 05-29-2019, 08:23 PM.

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              • #8
                Thanks Mr Murdering. I will take your advise and go in for a deep dive of the shoes. And thanks for the props, I ain't giving up.

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                • #9
                  Definitely get some weight on the car. From my experience you want to be at least 20 grams. Super tires won't give you the traction you need. Get some Quicker Gravity tires. Also, you want to keep the spring tension on the pickup shoe springs as light as possible. The car looks great. Building is only part of the process. Now the tuning starts. I have cars that I've been able to improve lap times by a second by testing and tuning.

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                  • #10
                    One more thing, most of the motors that are available will have too much power at 12 volts. You may have to reduce the voltage somewhat by adding a resistor in series with the motor or putting diodes in series with your controller. It would probably take five or six diodes to do the job. I have a "choke box" with six diodes connected to a rotary switch, that makes it easier to dial in the voltage.

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                    • #11
                      Originally posted by HObro View Post
                      Thanks Mr Murdering. I will take your advise and go in for a deep dive of the shoes. And thanks for the props, I ain't giving up.
                      Cool... his name is MR. Murdering! E :>)

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                      • #12
                        Naaaaw Ernie ... "Mrs" Murdering is by far the coolest. She lets me play with lil cars and big cars.

                        I vowed to sign my submissions as Ed discussed a while back, such that we used to do; and of course I failed to do so.

                        Shamefully,

                        Bill Hall



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                        • #13
                          IMG_0747-1.jpg

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                          • #14
                            Wow, I think you have some potential here. First, add weight on the sides between the body mounts and on the nose. As previously recommended, 20 grams is a good starting point. The recommendation to slightly concave the pickup shoes is spot on. In regard to the pickup shoe springs, try 0.006" springs and try cutting off a turn at a time. It's a fine balancing act to find enough pressure to keep the shoes on the rail/braid yet low enough that you don't lift the front end of the car off of the track. Sili-foam rear tires are a must; The ones intended for gravity racing are wider, softer, and have a flatter tread as compared with sili-foam tires intended for magnet car racing. I've experimented with Super Tires and they just don't have the needed grip, especially if running at 18 VDC. The diode trick works well. Each diode will drop 0.7 VDC. I find that three diodes work well when running on 12 VDC and suspect that at least six would be required at 18 VDC.

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                            • #15
                              By the way, I think the way that you handled the front axle/pickup shoes is pretty nifty. Nicely done!

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