Weight placement question

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john-new-11
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Weight placement question

Post by john-new-11 » Mon Feb 25, 2019 6:50 pm

I've cut my car to a flat 3/8" or so and with wheels and axles it weighs 1.5 ounces.

I drilled a few 3/8" holes for cylinders/domes in the rear, with 2 in the very back, 1 about 1/2" in front of the rear axle (I don't recall why I didn't place it closer to the rear axle) and another one in front of the one in front of the rear axle.

If I place a 2.5 ounce dome in the hole that's 1/2" in front of the rear axle, that of course leaves room for 1 more ounce.

To get the cog at between 3/4" and 1", I can:

a) put one .5 ounce cylinder in front of the dome (sticking up halfway) and then two .25 ounce at the very back (as I have 2 holes in the back it makes sense aesthetically to split it this way), which would be fully countersunk into the car. This is easiest as I wouldn't have to do any further drilling.

b) put one .25 in front of the dome and three .25 in the rear (drilling a 3rd hold in between the other 2). All 4 of the .25 would be countersunk, but I would have to drill another hole, but this method puts more in the rear (i.e. .75 in rear and .25 in front, rather than .5 and .5 in method (a)).

c) rework my holes (which I'd rather not do but could if you think it makes a big difference) and just put a 3.5 oz dome slightly behind the hole for the existing 2.5 ounce dome, and call it a day... I would have to increase the size of the existing hole, and then either fill in the other holes no longer being used or just leave them open (if I fill them that would affect the weight and cog so probably just better to leave them open?).

d) drill another hole and move the 2.5 dome in line with the rear axle (so it's farther back than it is now) and then place the other 1 oz in front of the dome to get the cog where I want it.

If sticking with weights in the multiple holes I'd prefer to use the multiple .25 cylinders instead of .5 cylinders (as I can hide them), but if it makes more sense to use larger cylinders (even if they stick up) as this concentrates the weight, I'm happy to do so.

Essentially, I'm not sure how much of an advantage there is to concentrating the weight using multiple smaller cylinders that are a bit more spread out.

Thank you!



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Re: Weight placement question

Post by john-new-11 » Tue Feb 26, 2019 10:30 am

Updated with some images/descriptions. All options put the cog between 3/4" and 1". I'll probably be able to adjust it a little further at the end with some putty, as I'm just under 5 oz (assuming I don't fill in any un-used holes, which of course could change everything...)

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Re: Weight placement question

Post by Speedster » Tue Feb 26, 2019 1:02 pm

I wouldn't use Domes. They're too restrictive. However, if you have a COM of 3/4" in front of the rear axle slot I'd say you're in good shape as far as that goes. If you do everything else following science you should have a competitive car.



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Re: Weight placement question

Post by john-new-11 » Tue Feb 26, 2019 1:07 pm

Speedster wrote:
Tue Feb 26, 2019 1:02 pm
I wouldn't use Domes. They're too restrictive. However, if you have a COM of 3/4" in front of the rear axle slot I'd say you're in good shape as far as that goes. If you do everything else following science you should have a competitive car.
Thank you. I temporarily put on some wheels/axles and with the 2.5 dome where I originally wanted it, I can put two .5 ounce cylinders in the back, and COM is just about 3/4" in from of rear axle, so I'm pretty happy as is.

That being said, do you think there's a big difference in not using the dome and instead either using more cylinders (as I can put those much lower in the body) or possibly carve out space underneath for a bar (though I'd rather not do that at this point). Or I was looking and I found some 3 oz cylinders I could try (I'd of course have to vastly increase the size of the hole, and then reduce the weight in the back, so it might be better to leave as is rather than reducing the weight in the back).



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Re: Weight placement question

Post by Speedster » Tue Feb 26, 2019 3:47 pm

I cut out the back and the front of the rear axle slot and go all the way through. I then glue a very thin piece of basswood over the openings on the bottom. I use a 2 oz block behind the rear axle slot, various tungsten cylinders, 1/2 oz plates, and then finish it off with tungsten putty. My cars are always 1/4" thick and resemble a Hershey Bar. When I'm satisfied with the speed I cover the top with Duct tape and decorate it with striping tape and usually Smiley Faces. I say, my cars, because I build cars to give to others each year.

This isn't for everyone but the cars are always fast.
I just realized what I said. I apologize. I do not give cars to scouts. The cars will always go to an adult.
Last edited by Speedster on Thu Mar 14, 2019 7:10 am, edited 1 time in total.



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Re: Weight placement question

Post by john-new-11 » Tue Feb 26, 2019 3:56 pm

Thanks again. I'm going in circles as to whether I want to just leave it be, or if I should nix the dome and instead start carving out the bottom for a 2 oz plate.

If I carve out the bottom in front of rear axle where the dome currently is and put a 2 oz plate there, I can get 2 advantages; 1) I can add another .5 oz cylinder to the back (as the plate is .5 oz lighter than the dome) and 2) lower the profile of the weight in front of the axle.

Of course this is hard to answer, but do you think that this would yield any noticeable difference vs. just keeping the dome where it is?



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Re: Weight placement question

Post by Speedster » Tue Feb 26, 2019 9:01 pm

I would run your system that gives you the 3/4" COM in front of the rear axle slot. See how it does. If you are using a stock wheelbase (4 3/8") then it has been recommended 1 1/2 ounce be put behind the rear axle slot. Over the years I have had better success with 2 ounces behind the rear axle slot using a tungsten block. You can then use 1/4" tungsten cubes, plates, various sizes of 3/8" cylinders, etc. If you have a track and timer you can move the different weights in front of the rear axle slot into different positions to see what gives you the most speed. If this car is really fast your Team's next challenge will be to build a car to go faster.
Best Wishes to your Team.
Last edited by Speedster on Thu Mar 14, 2019 6:55 am, edited 1 time in total.



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Re: Weight placement question

Post by JohnSeever » Tue Mar 12, 2019 6:37 pm

I cut my daughter's car and it weights 2.5 oz. i need help with placing of weights. the design she picked was this.

Image

Where is a good place to place weights.


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Re: Weight placement question

Post by Darin McGrew » Wed Mar 13, 2019 9:52 am

JohnSeever wrote:
Tue Mar 12, 2019 6:37 pm
I cut my daughter's car and it weights 2.5 oz. i need help with placing of weights. the design she picked was this.

Image

Where is a good place to place weights.
I'm guessing that she's planning to run it with the left end as the front, and the right end as the back.

If you're trying to optimize everything for speed, it's better to have the rear axles as far back as possible. Running that design the other way around (right end as front, left end as back) could be a little faster.

Regardless, put the weight a little bit in front of the rear axle. If you can put a little weight behind the rear axle, then the balance point (the center of mass) can be closer to the rear axles.

And that's what really matters. The point of positioning the weight is to get the balance point (the center of mass) to be closer to the rear axle. An inch in front of the rear axle is safe. Half an inch in front of the rear axle is aggressive. Most cars can probably get away with 3/4 of an inch in front of the rear axle.



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Re: Weight placement question

Post by Speedster » Wed Mar 13, 2019 12:00 pm

I think I can tell you how to do this very easily. It will work the same way if you decide to cut another car and turn it around. When you add the wheels and axles you will have about 3 ounces total. Paint will be about another 3 grams. You need about 2 ounces to reach 5 ounces or 141.75 grams.

Take 3 - 3/8" x 1/2" tungsten cylinders, tape them together and start to tape them to the back of the car where there is enough wood you can drill a 25/64" hole through the side. Don't drill all the way through when the time comes to drill. That will give you 1 1/2 ounces behind the rear axle slot. You need another 1/2 ounce. Take a piece of 3/8" lead rod, 3/4" long and tape it in front of the rear axle slot until you get a balance point of 3/4" in front of the rear axle slot. If you don't have lead rod I think you can buy "Tundra" (3/8" rod) that has the density of lead but is not toxic. Hobby Lobby should have it. Move the weights around until you get the COM you want. Drill the 2 holes, seal them with Bondo or wood filler and sand them smooth. If you go over 5 ounces the lead or Tundra will drill easily. The tungsten cannot be drilled.
You're done.



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Re: Weight placement question

Post by JohnSeever » Thu Mar 14, 2019 7:35 pm

the rear of the car is the right when looking at the picture. i do not have money for the tungsten. i have the 3 pack 1 oz wts and the 4 piece .6 oz wt. the car before paint was 2.6 oz with paint about 2.7 oz. if i add .6 oz, to the rear in the back amnd .3 oz to the front of rear axel would that work?


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Re: Weight placement question

Post by Speedster » Fri Mar 15, 2019 5:38 am

John, here's what I'd like you to do. In your mind, draw a line straight up from the rear axle slot to the top of the car. You'll see you already have a lot of weight in front of the rear axle slot from the shape of the car. Put the wheels on the car just enough to hold them in place. Have your scout hold a ruler on edge and see where the car balances. I do not know the shape of your weights. Whatever amount of weight you still need, or close to it, take it and start moving it up the back of the car. See if you can get the car to balance 1" in front of the rear axle slot.
Let's start there.
What are the shapes of your weights? Cylinders,plates, blocks, cubes?



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Re: Weight placement question

Post by JohnSeever » Fri Mar 15, 2019 8:16 am

I have 3 Round Weights that are 1 oz each, and 4 of the Combo Chassis Weight that are .6 oz available at hobby lobby. I also have 1 that is about the size of what would be between a quarter and a half dollar that is .6 oz as well.


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Re: Weight placement question

Post by Speedster » Sat Mar 16, 2019 4:29 am

John, has the car been painted?



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Re: Weight placement question

Post by JohnSeever » Sat Mar 16, 2019 10:57 am

not yet.


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