Quick (hopefully) weight question

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soccerdad
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Quick (hopefully) weight question

Post by soccerdad »

Hi,
We have to use standard wheelbase and slot position cars. Real basic rules. We run a pretty standard low tech setup, 3 wheel rail rider. We have always used lead (I handle that part). I don't really want to go to tungsten unless really needed. COM is typically 7/8" in front of rear axle. We run on an older (guessing 14 years) BestTrack 43' long. It is in good condition with some minor joint bumps here and there.

We will do our standard axle and wheel prep that is inline with what seems to be standard on this board. The car will be kind of a Hershey bar shape. (but that can be changed since we have not cut yet)

So my questions are: 1) Since we will use lead should we make the car a little thicker to pack the weight close to the rear axle or do we keep it as thin as possible to keep the weight down low? The thinner would spread the weight all the way to the back while thicker would have the weight closer packed but maybe even on top of the rear axle. 2) Since we will be thicker than a tungsten car, regardless of placement, should we carve out or shape the wood in front of the rear wheels to move as much weight back as possible? 3) We are targeting 3/4" COM in front of the axle. Is this a good spot to target?

***EDITED to stay on my topic***

Thanks. This forum has been an awesome source for us.
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Stan Pope
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Re: Quick (hopefully) weight question

Post by Stan Pope »

There are a couple of ways to approach accomplishing the CM target you set. One way is to cut the wood to plan, then place the car with wheels on a pair of scales. Place the weights you plan to add on top of the car, watching the scales to show a weight distribution that equals your target CM location. (The tool http://stanpope.net/cmcomput.htm#Section2 will help you with converting from CM location to weight distribution.) When you get close, remove some wood below the weights and embed as much of the weight as possible in place of the removed wood. Recheck the weight distribution and readjust the remaining weight to correct for the removal of the wood. Finally, glue the remaining weight on top.

A "more scientific" approach is to use a tool such as http://stanpope.net/cmcomput.htm#Section1 to compute the additional volumes of lead needed. Step by step you can simulate the addition of volumes of lead to achieve your target CM location!
Stan
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soccerdad
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Re: Quick (hopefully) weight question

Post by soccerdad »

Thanks Stan
We will look at your calculators. That will be a big help. Which is more important: to have the weight as low as possible or packed as close to the axle as possible?
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Stan Pope
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Re: Quick (hopefully) weight question

Post by Stan Pope »

soccerdad wrote: Which is more important: to have the weight as low as possible or packed as close to the axle as possible?
My understanding is that the improvements in approaching each grow less and less as you get closer. The reverse is important: Deviations from the goals of "low and tight" grow more and more detrimental as the deviations increase. So try to approach both!
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Re: Quick (hopefully) weight question

Post by Speedster »

Maximum Velocity did a test recently and found a little height didn't make any difference.

Here's the way I would build the car using 3/8" lead rod available from at least 2 vendors. Cut the car 3/8" thick from front to back and taper the front a bit. Using a coping saw, cut out a pocket behind the rear axle and a large enough pocket in front of the rear axle so you can move weight around. Leave a 3/16" rail on all sides. Glue a 1/32" piece of Basswood over the holes on the bottom. Put the longest piece of rod you can behind the rear axle slot and the remainder behind that rod to get 1 1/2 ounces. Put as much weight as possible in front of the rear axle slot and see where it ends up to get the CG you want. Add a little more weight behind the rear axle if necessary. Cover the entire top of the car with Basswood to give a nice smooth finish. Concentrating on wheel and axle prep, and of course alignment, is always very important. A 3/4" CG should be safe for your Best track. I have heard when the Best track was first made the angle was too sharp but I don't know when that was.
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Stan Pope
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Re: Quick (hopefully) weight question

Post by Stan Pope »

Speedstger and I agree on a lot of issues, but I disagree with him about using up precious rear volume with low density wood! Instead, I would embed my ballast into the planned volumes with a higher density material such as epoxy. Ideally, the volume is entirely high density ballast, but, unless it is poured hot into the void, there will be some air gaps. Epoxy fills the gaps, gives structural integrity, AND provides the smooth surface.

How to "mold" that flat exterior? Flat wood mold separated from the car body by waxed paper!

Exception: If you are using ballast that you plan to recycle (e.g. due to cost), that is the time to make a "wooden box" to hold the ballast!
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Re: Quick (hopefully) weight question

Post by Speedster »

I agree with Stan. I was just trying to keep the cost down.

No I wasn't. I wasn't smart enough to follow Physics more closely. I do like Stan's idea.
soccerdad
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Re: Quick (hopefully) weight question

Post by soccerdad »

You guys crack me up! Thanks for the tips. I will work on a prototype and then cut my daughter loose on it!
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Re: Quick (hopefully) weight question

Post by soccerdad »

So I saw this on a ladder build thread. Image

Since ours is lead filled, I know that it will be a little different, but with a top and bottom skin AND the lead potted in with epoxy to add density to those nasty air spaces, I think we can have something nice. We would skin it because I obviously don't want to fill the front holes with epoxy.... Am I over thinking this? I found some 1/64" birch plywood that should make great light skins.
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Re: Quick (hopefully) weight question

Post by Speedster »

Nope, you're not over thinking this at all. I would move the cross member in front of the front weight pocket forward so you have more room to adjust weight or simply do away with it altogether.
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Re: Quick (hopefully) weight question

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If you go the "epoxy route," sneak up on the total weight of the car. Perhaps even use the tools in http://stanpope.net/cmcomput.htm to add just enough embedded weight to get the CM location that you want. When that has set, then reweigh to compensate for the added epoxy. Then add a layer of lead on top of the body and centered over the target CM location to bring the car real close to the target weight.
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Re: Quick (hopefully) weight question

Post by Stan Pope »

Another note on adding a layer of wood under the car body:

If you have enough clearance to allow adding a layer of wood under the car body, but elect to epoxy the weight into the body instead, then you should lower the body (drill the axle holes higher) to realize that lowered CM potential!

Theory says that the height of the CM should be zero relative to the underside of the axles, but the effect is really small.
Stan
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soccerdad
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Re: Quick (hopefully) weight question

Post by soccerdad »

OK, here is the prototype. Very rough... The CG is right at 5/8" with about 1.75 oz behind and the rest forward. The blank is 1.145 oz. On the real cars we will add a little styling and tweaking for rail riding, but this is a decent template. The rear of the car is 0.460" with the skins on it. We will curve the nose down some. It is at 4.9 oz the way it sits now.

Image

Image

Image
Speedster
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Re: Quick (hopefully) weight question

Post by Speedster »

I like it.
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