weight placement

General discussions for car and semi-truck racers.
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pwrd by tungsten
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Re: weight placement

Post by pwrd by tungsten » Fri Jan 24, 2014 2:34 pm

Sure,

I always start out with a 5/16" slab of wood. You can weigh a full block and then measure its height and determine aprox. how much the 5/16" slab will weigh once it is cut. This is without axle slots. If you need to keep the axle slots you may need to go thicker???

Then I do one of three things:

1) Drill a one inch hole for a tungsten disc either between the rear axles or slightly in front of the rear axles. The heavier the disc the further it goes back on the car. You can calculate this or you can build a test car and adjust from there... With good prep and alignment 5/8 - 3/4" on the COM.

2) Same as above except I cut a one inch by two inch rectangle for the weight pocket. This goes almost to the rear of the car. This holds a custom tungsten weight I had designed that is a 1x1.25 rectangle less than .25 inches tall.

In cases 1 & 2 above I usually cover the top of the body with 1/32 plywood as it covers the hole and paints nicely. However if the disc is very heavy I leave it exposed (If it needs to peep out over the top).

Both of these require cutting the axles. I recommend method three which is used by almost all of the pro racers I know. I am switching over to this as it does not require cutting axles.

3) same as 1 & 2 but you cut a rectangle in front and behind the axle and place cubes (2 rows in back, 2 or more rows in front.). The axle gets 1/4" of wood.

In all these cases you can cut or drill the hole through the 5/8" slab or if you want to do this on the big block and then cut the slab is better. Less cracking in my experience.


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Speedster
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Re: weight placement

Post by Speedster » Fri Jan 24, 2014 4:23 pm

Thank you.
I've always done #3 because I've always thought that was the ultimate. I use a 1/4" car that looks like Derby Evolution's car with 5/8ths behind the rear axle slot. That gives me a little more wood up front which is not against our rules. I glue the top on first so I've got a lot of material to Epoxy in the cubes which helps support the rear axle slot.
Actually, I have close to your rectangle and another one too. OK, they're in 2 pieces but you already knew that. Are you going to go over the top of the rear axles? Wouldn't one on each side of the rear axles give you more of an adjustment? Side to side? With 2 pieces you could get them lower in the car and you have multiple combinations of weight. I haven't tried them yet but they look very promising.
Thanks for answering my questions.
Last edited by Speedster on Fri Jan 24, 2014 5:19 pm, edited 1 time in total.



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Noskills
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Re: weight placement

Post by Noskills » Fri Jan 24, 2014 5:05 pm

PWT,

Yes , thanks for the info. I do the same as Speedster but I bought a 1 1/8 tungsten round which slips nicely in a hole cut by a fornster bit. I made a test car of this but made it thick so the round was above the axels. Never though of cutting the axels. I am sad that I want to build a car with my round disc, the tungsten 3.5 canopy, and a few more with 1/4 in cubes and my son only has 2 more years in the scouts. Guess I can keep crashing the pack meets until they throw me out ;)

Noskills


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Speedster
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Re: weight placement

Post by Speedster » Fri Jan 24, 2014 5:36 pm

Mr. Noskills,
They haven't thrown me out in 29 years and they aren't going to throw you out either. You are a blessing to the derby and the derby needs you. Because of the quick turnover of leaders in the cub scouts all the young new leaders struggle with the derby. You will be a comfort to the new leaders and give the derby continuity. You will watch the young scouts race and then watch their children race. I challenge you to keep a dry eye.
Yours in Going Downhill Fast,
Bill
Yes, I'm older than Stan Pope.



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pwrd by tungsten
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Re: weight placement

Post by pwrd by tungsten » Sat Jan 25, 2014 1:16 am

Speedster wrote: Actually, I have close to your rectangle and another one too. OK, they're in 2 pieces but you already knew that. Are you going to go over the top of the rear axles? Wouldn't one on each side of the rear axles give you more of an adjustment? Side to side? With 2 pieces you could get them lower in the car and you have multiple combinations of weight. I haven't tried them yet but they look very promising.
Thanks for answering my questions.
I would suggest tungsten in the rear separated by 1/4" wood for the axle and then the tungsten in front of the axle. You want the tungsten as close to the rear axle as possible.

Cutting axles leads to problems with loose axles and also wheel gapping as you have to sometimes re-cut the axle to gap the wheels and at that point it is just a pain...

Always place the rear axle as far back as the rules allow. If the rules do not specify a location than 5/8 from the rear is perfect.


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pwrd by tungsten
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Re: weight placement

Post by pwrd by tungsten » Sat Jan 25, 2014 1:18 am

Noskills wrote:PWT,

Yes , thanks for the info. I do the same as Speedster but I bought a 1 1/8 tungsten round which slips nicely in a hole cut by a fornster bit. I made a test car of this but made it thick so the round was above the axels. Never though of cutting the axels. I am sad that I want to build a car with my round disc, the tungsten 3.5 canopy, and a few more with 1/4 in cubes and my son only has 2 more years in the scouts. Guess I can keep crashing the pack meets until they throw me out ;)

Noskills
I am really liking the cubes at this point. The tungsten round are nice and easy except for cutting the axles.


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Speedster
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Re: weight placement

Post by Speedster » Sat Jan 25, 2014 7:00 am

pbt, regarding #2 above and your custom block. I'm guessing a piece that size weighs about 3 1/2 oz. Being one piece you either have to cut your axles or go over the top of the axles. I can't cut axles (rules) so I would have to go over the top. I am now higher in the air and I've got more wood in my 1/4" car. If I use the commercial blocks available I have 3.8 oz or I could get 4 oz. with 2 of the 2 oz. blocks. With your 1 piece block I don't see how you can get enough weight behind the rear axle slot if you have only 5/8" of wood. I need a 2 oz. piece that measures 1/2" x 1 1/2" by 1/4" to put behind the rear axle when the distance is 5/8" from axle to end of car. This would duplicate 2 rows of 6, 1/4" cubes and be a lot easier for a scout to install. I could hold it in place with double face tape and use it over and over again. This size is not available and the vendors won't make it - so far. I could cut the 2 oz. piece in half which would give me the size I want but I don't know if it's possible to cut the tungsten block. If Randy is right that it doesn't make any difference if you go up in the air then the problem is solved. However, if separating the weight a bit because of the rear axle is a better choice per physics then I will need to stick with the 2 rows of 1/4" cubes.
A Final question. From what we know at this time in history, taking a stock wheelbase car, shifting the slots (or not) what is the absolute, positive way to build a car per the Laws of Physics?



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pwrd by tungsten
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Re: weight placement

Post by pwrd by tungsten » Sat Jan 25, 2014 11:59 am

Speedster wrote:pbt, regarding #2 above and your custom block. I'm guessing a piece that size weighs about 3 1/2 oz. Being one piece you either have to cut your axles or go over the top of the axles. I can't cut axles (rules) so I would have to go over the top. I am now higher in the air and I've got more wood in my 1/4" car. If I use the commercial blocks available I have 3.8 oz or I could get 4 oz. with 2 of the 2 oz. blocks. With your 1 piece block I don't see how you can get enough weight behind the rear axle slot if you have only 5/8" of wood. I need a 2 oz. piece that measures 1/2" x 1 1/2" by 1/4" to put behind the rear axle when the distance is 5/8" from axle to end of car. This would duplicate 2 rows of 6, 1/4" cubes and be a lot easier for a scout to install. I could hold it in place with double face tape and use it over and over again. This size is not available and the vendors won't make it - so far. I could cut the 2 oz. piece in half which would give me the size I want but I don't know if it's possible to cut the tungsten block. If Randy is right that it doesn't make any difference if you go up in the air then the problem is solved. However, if separating the weight a bit because of the rear axle is a better choice per physics then I will need to stick with the 2 rows of 1/4" cubes.
A Final question. From what we know at this time in history, taking a stock wheelbase car, shifting the slots (or not) what is the absolute, positive way to build a car per the Laws of Physics?
Rear axle location is key. The further back the better (as long as the rules allow it). This moves your COM back. I would not go over the axle. I would use two rows in the back and then something in the front.

Create an excel spreadsheet to calculate COM

Weight of body (est)
Wheelbase
Weight of all components (5 or 5.04 depending on rules)

Enter zero for rear axles and wheels.
body weight * .5 wheelbase
front axles and wheels * WB
Weight * weights COG in front of rear axle.

The sum of all of these is the weight on DFW.

COM is = (Weight on DFW/weight of car) * WB (Something like this) I may need to think about it...


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Re: weight placement

Post by Speedster » Sat Jan 25, 2014 12:32 pm

OK, you've answered my question. I have to keep the stock wheelbase but I'm allowed to shift it to get the 5/8" of wood behind the rear axle. When you mentioned your custom piece I thought you might have some sneaky trick.



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Re: weight placement

Post by FatSebastian » Sat Jan 25, 2014 1:13 pm

pwrd by tungsten wrote:(Something like this) I may need to think about it...
That looks right.



Speedster
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Re: weight placement

Post by Speedster » Sat Jan 25, 2014 3:08 pm

Thanks for all the info, FS. That was very interesting.
Maybe we'll hear from Sporty and learn how he performed his Magic.



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Re: weight placement

Post by itgl72 » Sat Jan 25, 2014 3:52 pm

Thanks for all the help, looks like we had a good day. I made sure to include my two boys in everything I could in this project. Other than cutting, my 1st and 2nd grader got the idea of how it worked, they picked their design, helped with sanding, lubricating and filing axles, paint, and choosing their decals, but dear old dad still got stuck doing the brunt of it. My 1st grader earned best design, and 1st place for speed among the tigers, and my 2nd grader earned 1st place for speed in the pack.



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pwrd by tungsten
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Re: weight placement

Post by pwrd by tungsten » Sat Jan 25, 2014 9:43 pm

Just read through the other thread. I am in the camp of keeping all of the weight in the rear unless you have a track to tune on. I think that even the folks who add a bit of actual weight on the DFW and see a speedup are not at the optimal weight configuration. I would place it all in the rear...


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