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Son's first derby

Discussion in 'Ask The Pros' started by Rewind, Feb 26, 2020.

  1. Rewind

    Rewind Lurking

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    Hi all,

    I've recently discovered this forum in preparation for my son's first derby, as I'm hoping we can have a good year one showing. We're attempting a rail rider (although his pack's rules of no bent axles and four wheels touching the ground have proved challenging for our RR build), and I have a few questions which I am hoping the more experienced builders can assist with. I searched the forum for these specific questions, but did not locate any results.

    Axle hole finishing:
    We're running Dynasty Rage wheels with BSA Elite Axles from DD4H, and my son is wanting to spray paint his car. For axle hole prep, our race does not allow washers and I've read mention of using super glue or nail polish around the drilled axle holes to provide a slicker surface for the wheel to ride on when in contact with the body. Are one of these the best strategy for axle hole finishing?

    Weight distribution:
    The DFW is the right wheel, and we're aiming for a 0.75" COM with 4" drift over 4ft. What is the best weight distribution per wheel? For the non-dominant front wheel (since it has to be touching per his rules) I'm aiming for just a few grams, or as little as humanly possible. Is it best to attempt to equally distribute the weight between the back wheels if possible? We're using 3/8 tungsten cylinders, and I'm curious if I should drill all of the holes skewed toward the DFW side of the car. Would that have a positive impact on speed?

    Thanks in advance!
     
  2. Thinkin'Bout Racin

    Thinkin'Bout Racin National Contender

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    Welcome,

    First off, I am not a Pro, but have been helping out Cubs for a few years now with decent success. So take my information with that into consideration.

    There is a LOT of GREAT information here at this site! By reading what you have written and the fact that you found this site, means you are already ahead of the curve! Keep reading, then read some more, and then read even more yet! I know that sounds like I am trying to blow you off, but there is SOOOO MUCH to learn, that you don't even know what you don't know yet. By reading, you will get a better understanding of the physics involved and you will be better able to ask questions that will be of the greatest benefit to you and your son.

    For your questions:

    There are a number of threads that address the 4-wheel touching rule. Keep searching ...try different search words. There is also a good thread on this located in the premium section. Basically you want (in your case) the right front (DFW) to have the most weight of the two fronts. Try to have only a gram or two (at the most) on the NDFW. You are on target with that.

    CA glue or nail polish ...it is a builder choice. Both will work. I would suggest trying whatever you are thinking on a spare piece of wood first. There is a few tricks you will pick up by practicing this a couple times before trying it on your car. Your method will be slightly different than someone else's. You need to figure out what works for you. Just be sure to polish it smooth after it dries and use a junk axle in the hole to keep it open.

    Most don't use COM at this level. They use the weight on each wheel instead. Weight bias is another thing that varies by the builder and car design. Track condition along with your competition should be factored into how the car is weighted. Your thoughts of a 0.75" COM is a decent starting point for a beginner build and a scout race. I would suggest a bit more steer. I recommend 5"-6" over 4' on all of my scout builds. Track condition along with an unknown in the lane next to you can cause a wiggle. Each wiggle that the rears touch the center guide will cost you about a hundredth of a second. Most of the time if a car wiggles, it will wiggle 2-3 times or more. One or two inches of extra steer will only cost you about 2 or 3 thousands of a second. Rear weight bias is another area that each builder has their own preference. Try to balance them out some. I have seen recommendations of a few grams to about 14-16 grams bias on the NDRW.

    One thing you didn't mention is wheel/bore prep. I would strongly encourage you to look at this! There is speed to be had in doing so!

    Enjoy the time bonding and teaching with your son . Teach him how to use both his hands and his mind. explain the physics to him in simple terms that he will understand.

    Good Luck!
     
    Last edited: Feb 27, 2020
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  3. Rewind

    Rewind Lurking

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    Thanks so much for the reply. I've found a lot of good info in this forum, and my son and I have had great fun implementing what we've read. I will aim for the 5-6" of steer over 4' instead of 4". Thanks for the suggestion! Before I had found DD4H and saw the black ice kit, I had already purchased some novus 2 for polishing the wheel bores and was going to then burnish the bore with some graphite. For the axles I'm planning to go 1,000-10,000 grit and finish with polishing and then sailkote. As for wheel weight, I did purchase 4 small scales for weight distribution measuring and attempting to balance the weight. Until you mentioned it I was thinking that COM was in addition to the wheel weight distribution, but now it makes sense that COM is unnecessary if you know what your target weight per wheel is. I'll do some digging for wheel weights that have yielded good results. Thanks again.
     
  4. Thinkin'Bout Racin

    Thinkin'Bout Racin National Contender

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    It sounds like you are well on your way to becoming addicted. ;) There are a number of ways to burnish in graphite. Most ways listed here will do a decent job. Again, that is something that each builder does a little different. It also depends upon how many runs might be required before you are able to re-lube.

    Remember that when you are looking at DFW weight, that 14 grams on a 5" W/B will yield a different COM than 14 grams on a 4 3/8" W/B. Much like trying to determine the correct amount of steer, you need to know a little about your track and it's condition. I normally like to start out a little on the safe side because a DFW that is too light can become a problem on a rough track or with cars bouncing in the lane next to you. You will have a much better idea of track condition and the level of competition after your first Derby. You can then make adjustments based on what you have learned.

    Good Luck
     
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  5. Thinkin'Bout Racin

    Thinkin'Bout Racin National Contender

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    Here is a link to something to play with. I find it useful to help teach the Cubs about what happens when you move weight around. I like it because it is visual ...not quite as nice as if you have a test car mocked up on 3 or 4 scales, but still a good teaching tool. It also is useful to help a person understand the relationship of the COM (both the X and Y axis) and the relative wheel weights.

    Here you go: http://jsfiddle.net/minionsracing/UUnT5/embedded/result/
     
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  6. Rewind

    Rewind Lurking

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    Thanks! This is an impressive site.
     
  7. davet

    davet League Racer

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    That link was key for us to build consistent cars. It also helped us know if we were being more aggressive with weight bias from year to year.
    We bought 2 cheap, identical, $14.00 digital scales online and used them to get the numbers to input into the formula.
    Keep a record of all these numbers for each race you run and record the times from the races for that setup.
    You will most likely run on the same Pack track and possibly same District and Council track each year.

    I would say drill the holes for the tungsten as low and as deep as you have to to get it centered. Epoxy it in then when it dries it will be structurally strong. Then, you can sand the body down to the tungsten.

    We were initially using nail-polish. Never tried superglue here due to being messier I guess.
    I tried 4 or 5 different ones including the much sought after Sally Hansen which I can't remember the type right now.
    The one that showed less wear marks when we spun a wheel against it was the cheapest clear my wife had on hand at home. I think the key for us was to put it on at room temp. My wife keeps her polish in the fridge for some reason. It goes on uneven when cold. Take it out the day before and put a real thin layer on while you have junk axles in the holes to keep the holes clean. We let it dry for a few hours then put another coat on. Let that dry for several days then polish with wax before you install wheels.

    I'm now precutting kits for church workshops and I use nailpolish too.
     
    Last edited: Mar 7, 2020
  8. Rewind

    Rewind Lurking

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    Thanks to all! There were 42 cars entered in my son's derby this year, and we managed to take 1st place with our average time being 0.04 seconds faster than the second place winner. Great to see my son's hard work pay off, and we couldn't have done it without all of the great info on this forum. Now on to districts!
     
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  9. Thinkin'Bout Racin

    Thinkin'Bout Racin National Contender

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    Great job guys! Sounds like you won by a bit over a car length.

    Be careful on that ground around the rabbit hole ...it may be unstable, collapse and you might fall in! ;)

    Now, the question will be can you make changes to the car before districts? and if you can, do you want too?

    Again, Congratulations!!! I am glad to hear that your son learned a valuable lesson about hard work and perseverance along with learning about how to build a fast car.
     
  10. Peregrine Racing

    Peregrine Racing Pinewood Ninja

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    Congrats!
    We had our district race yesterday and my boys finished second and third. The final times screen showed their cars to be .04 and .02 faster than the 1st place car’s fastest run, but they both had one run each that was .3 slower than their fastest run. I figured it was just an uneven lane transition, but they never lost a heat, so it couldn’t have been just a bad run. It was run outside on a 32’ freedom and they had some issues with the timer sensors, so I’m guessing it may have been that? Not sure, as usually when there was an issue, the timer just kept running even after all cars finished as opposed to randomly stopping after the fact. Second time in four years an issue like this has happened. Just use it as another life lesson and be thankful we got something with a time that far off averaged in.
     
  11. Follow Me

    Follow Me Hammering Axles

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    Raced my first race last month but the rail riding/running wasn't a success. The track is homemade out of wood and the car bounced back and forth 6 times on the way down (I did a slow motion video) and the center strip is painted wood. It's kinda rough. Do I build a non steering car and hope for the best or make it a heavy steering car? Like 6 inches or more of steer over 4 foot? The rough center strip has got to cause a lot of friction for a rail rider/runner.

    I posted a thread called "new and addicted" the other day and towards the end I detailed my build if you want to read it. I suppose there could be another reason it was so bouncy that I'm unaware of. Tia
     
  12. Mojo Racing

    Mojo Racing National Contender Pro Racer

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    You need more steer than 3 inches over 4 feet on a wood track. A better wood track in our area, we still race with no less than 5 inches. And we make sure we have a little more weight on the dominant front wheel as well.
     
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  13. Follow Me

    Follow Me Hammering Axles

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    Thanks, to get more weight on the DFW can you just move the COM forward or something more complicated? If COM, how far in front of rear axle?
     
  14. Mojo Racing

    Mojo Racing National Contender Pro Racer

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    For the cubs I coach that have to race on the wood track, we go about about 0.55-0.58 oz if you weigh the front wheel. I don't know what that measures in inches, but if I guessed, 7/8"-1" maybe.
     

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