Derby Talk

My boys wanted to try something new this year so we are going with the Puma weights



http://www.derbymonkeygarage.com/mobile ... x?id=37977

If we want a low COM car (5/8" or less) given the puma weights are at the axle, is it likely the rest of the weight (tungsten cubes in our case) will need to be behind the axle?

Notes:
- standard wheelbase car with rear axle 5/8" from back
- puma pair weight about 1.75 oz. so we likely need to add another 2+ oz

We are wondering how much accommodation there might need to be for weights in front of axle and are wondering if even any at all?

Thanks for any thoughts!

I did numerous tests this past summer with a car I was able to move weights in different positions. With a stock wheelbase car (4 3/8") I think Sporty is correct. 1 1/2 ounces behind the rear axle slot is recommended. I have a wood track with a Micro Wizard timer so I don't have a track as perfect as the new aluminum ones. However, the car turned the fastest time with 1 1/2 oz. behind the rear axle slot.
However you decide to place your weights, I would be eager to learn the results.

davidwilkie wrote:My boys wanted to try something new this year so we are going with the Puma weights

...

If we want a low COM car (5/8" or less) given the puma weights are at the axle, is it likely the rest of the weight (tungsten cubes in our case) will need to be behind the axle?

Notes:
- standard wheelbase car with rear axle 5/8" from back
- puma pair weight about 1.75 oz. so we likely need to add another 2+ oz

We are wondering how much accommodation there might need to be for weights in front of axle and are wondering if even any at all?

Thanks for any thoughts!

Spend a few minutes experimenting with http://www.stanpope.net/cmcomput.htm. To get CM of .625" with a 4.375" wheelbase and assuming a total of 4 oz ballast required (body+wheels+axle = 1 oz), adding the PUMA wts moves the CM to 1.14" ahead of the rear axle. I need to add 2.25 more oz of ballast. The CM of the additional ballast would need to be 0" from the rear axle center! Not gonna work if I want to keep the the body at 1/4" thick! Since the sintered tungsten cubes weigh in at about 6 per ounce (I'm remembering right, aren't I?), that would be about 13-1/2 cubes to add. To keep the CM of the added cubes at 0", some need to go ahead of the axle and some behind! Since only 6 cubes fit across the car body, there is an odd cube to deal with, and symmetry won't tell how to arrange them. Back to cmcomput.htm! Alternatively, place 6 in front, 6 behind, both sets adjacent to the rear axle and add equal amounts of tungsten putty just beyond the cubes to bring the total to 5 ounces.

If, however, the wheelbase is extended to 4-5/8", then the ballast geometry changes and the CM of the cubes needs to go behind the rear axle by about a quarter of an inch. cmcomput.htm helps with that computation as it will with even more extended wheelbases!

Caveat: These computations assume a specific configuration of the unweighted body and the axle locations. Work out your own solutions for your specific case!