using a thin body design- Please help with weight

[clydesdale]

We are going to use a thin design, but I could use some help deciding on how to weight the car. Tungsten or lead? The car will be thin. Do most people use plates or tungsten rod and then a little tungsten putty to dial it in? I want to have the weight about an inch or so in front of the rear axles, but do I screw plates or do I cut out a cavity and put lead or tungsten. Please advise.

[ichiban]

I'm using plates this year on our bsa derby cars as I don't have my cavity cutting routine down pat. I cut out a space at the rear, underneath for the plates. Since I have some weight the length of the car for design purpose, I'm limited to using 5 of the 6 tungsten plates at the rear and will be using tunsten beads to get near 5 oz. During prototyping I discovered stacking all five plates at the rear end moved the COM back further than distributing the plates into two rows (one in front of the other) and not having the stacks so high. As a result, I had to increase the height of our wedge from 1/2" to 3/4" at the rear so the bottom most plate was flush with the bottom of the car. The cars only have 1/8" thickness of wood hanging over the plates so I glued the plates as a stack and glued it into the space; no screw was used.

My first entry cars into league racing will be wing style. For simplicity and since I'm behind schedule, I'll use tungsten rounds. All I have to do is drill a 1" diameter hole and glue the round into it. Part of the weight will be exposed as the wood will be no more than 1/4" in height but I've seen very fast cars with this design.

I don't know why you want the actual weight an inch in front of the rear axles as your COM will not be at a competitive location.

[clydesdale]

From reading on this forum, it seemed that 1 inch of front of the rear axles was a good location. What do you suggest?

[speedie4u]

I suggest tungsten cubes and or the plates, both might be needed. depending on how you cut it out or place the weight.

1 inch in front of the rear axle is a starting point. I would go with atleast a 4oz on the rear, 1oz on the front. with using two scales to be able to get this.

Thats the basic numbers.

[clydesdale]

When you say 1 oz on the front, do you mean behind the front axle? Please describe. Thanks.

[ichiban]

Jan 18, 2009 18:42:32 GMT -6 clydesdale said:

From reading on this forum, it seemed that 1 inch of front of the rear axles was a good location. What do you suggest?

I think you misworded your previous post or misunderstood what you read in other posts. The actual weight does not sit an inch in front of the rear axle, the 'Center Of Mass' (COM) can sit an inch in front of the rear axle. A simple way to find the COM of your car is to take a ruler and hold it balanced on its side. Then sit your prototype with all its weight, wheels, axles (in their proposed location) on top of the ruler perpindicular to the length of the ruler. Slowly move the car left or right of the rulers edge until the car is balanced. The point directly above the rulers edge is where the COM is currently located. Try to get this point a distance of 3/4" to 1 and 1/4" in front of the rear axle. From what I've read 1" should be a safe but competive location for the COM.

The next thing to consider is how much actual weight is on the front wheels and how much is on the rear wheels. This is why speedie4u recommended two scales but if you only have one scale, you can create a block of wood that is the same height as the scale. Then weigh the car by the front wheels while the rear wheels are on the block and vice versa. From what I've read 25% of the cars total weight on the front wheels is an acceptable distribution. I'm assuming with a distribution less than 20% on the front wheels, the car could wheelie and/or become unstable during the race.

[khouse]

From my little experience and what others have shared with me I have came up with a little chart.
Over 1 inch - very conservative
7/8 to 1 conservative
5/8 to 7/8 aggressive
3/8 to 5/8 very aggressive
I'm making a 3/8 COM car to see if it stays on the track. I'm pretty sure it will because I have a secret plan!

[JustaDad]

Feb 26, 2009 21:00:49 GMT -6 khouse said:

From my little experience and what others have shared with me I have came up with a little chart.
Over 1 inch - very conservative
7/8 to 1 conservative
5/8 to 78 aggressive
3/8 to 5/8 very aggressive
I'm making a 3/8 COM car to see if it stays on the track. I'm pretty sure it will because I have a secret plan!

Secret plans and good ideas is what K-man does best ;D

[khouse]

It's no secret that I'm slower than most here. But I always have a plan. I'm running my secret car next week. I don't know if it will work or not. But if it does I'll share the secret. If it doesn't work I'll just try something else..... Maybe Soapbox Derby! LOL!

[derbydominator]

As far the weighting is concerned, we use the 1/4" cubes in most of our cars but had some success in our latest Pro Stock with the disc. The cubes allow for a more dense placement of the weight in the rear of the car and better customization such as off-center weighting. The disc allows for a better moment of inertia, but you are somewhat handicapped by the axle line. You can still get aggressive with the disc set up by using cubes or other weight behind the rear axle though. Plates work well alone or in combination with the cubes. I hope that helps.

[derbydominator]

I'll add one thought to the comments from KHouse on the COM's. I believe he is quite accurate in his categories, just that if you are going to incorporate a COM more aggressive than 7/8", then I would strongly suggest a rail rider set up to avoid speed wobbles.