Going to districts. Need some advice.

[VADAD]

I'm taking my son to district and have decided to make another car with my daughter for the adult's race.

We have a design that will look like 2 wishbones in opposite directions, one on top of the other. These wishbones will come together for the front axles, and again for the rear. We are using tungsten cubes (1/4 inch), and have 4oz worth. I doubt we will get the wood shaved enough for 4 ounces, but who knows.

Couple questions.

Weights. We plan on a row of cubes in front and behind the rear axle. We will need to either place more weight next to each row, or place more weight in rows on top or middle of the lower rows. Should I do it on the same plane as the first two rows? Or place them above? COG will come into play as far as placement goes, but in general... all low, or stacked?

Aerodynamics. This car will probably have a large hole that air could potentially get sucked into, exiting out the middle and side of the car. The hole will be mostly flat, so I'm not sure if air will go over it or through it. If I reverse the wishbones, I run into the same problem with air getting sucked into the bottom of the car. Any thoughts on this, and which would be better.. top hole or bottom hole. (Remember, I'm trying to make this look cool for my daughter... her design...and make it as fast as I can at the same time.

Bending Axles. The axle holes on my sons car were far from straight. I fixed that by bending the axles with a couple clamps. He got first place in his cub race, but now that we are going to district I want to make sure we don't get DQd for it. You can't tell unless you take the axle out. How picky are the judges about this kinda thing?

[Stan Pope]

I have not seen any rules regarding the straightness of the axles. Some few rules require that the treads sit flat on the track, but most seem to allow negative camber in the rears, allowing the rear wheels to ride on the edge of the tread, a condition which most believe is advantageous over the alternative.

The difficulty with bent axles, whether to get the wheels to ride flat or to ride on the edge, is to get the rear wheels running in the same direction and steering the rear wheels off the rail! Many of us have difficulty drilling holes that are in exactly (or, even, nearly) with the required orientation, and have learned methods to cope with that condition. If you don't know how, someone will surely respond to a request for help.

Purests call for interior voids (through holes) in the body to be enclosed in some manner. In most cases, it is a small issue. If you wish to enclose a void, suggested solutions include clear tape and thin veneers, e.g. 1/64" thick wood. Try to avoid anything that might "flap in the breeze" as that would eat energy.

If you can achieve your CM target with the ballast in one plane, do so. If not, stack two deep.

[Noskills]

I think a picture of your idea would help. When I think if a double wish bone I think of )-( with both being "Cs"

If you plan to put a wheel on the ends of the C with the open part in the middle where do you proposed to put the weight around the axle? With the C look more like the symbol for cents (of the Montreal Canadians logo) where the line through the open part is the axle? In that case put in the cubes as you see others do on this. 2 flat rows of 6 before and after the rear axle.

Noskills

[FatSebastian]

I think a picture of your idea would help.

Agree!

Aerodynamics. [...] (Remember, I'm trying to make this look cool for my daughter... her design...

I would not worry about it then. Of all the potential effects that can rob speed, aerodynamic drag is a lesser one. Other streamlining, and keeping the body thin (1/2" or less), will be helpful even if you have a hole in the middle. I agree with Stan that "in most cases, it is a small issue."

[Topspin.D]

A photo or drawing is probably the best way for us to give advice to you.

That said, I've seen a few cars where there wasn't enough wood that could be cut away and stuffed with weight to get the car up to 5 ounces... The owners sometimes added additional weight (plates) under the car and sometimes glued additional weight on top.

Convention says to add the weight under the car for more stability, but Max V's experiments show that it doesn't matter much if you're on a smooth track.

[VADAD]

Alright, I've tweaked the design a bit to where the "hole" is no longer as big of a problem. My paper cutout in the following pictures doesn't show correct size for axle/weight box, nor does it show what the nose will look like (should come together, flatten, and round off in a wide v shape if looking from the front). Take it for what it's worth.

Next to it is my son's car. It has a giant hole in the front. Looks cool, but iffy on the aerodynamics in a few spots.

[Vitamin K]

Just a quick aside, if you're using imgur to host images (as do I), you can go to the links section of the image and select one of the bbcode options and then paste it directly into the post you're editing. Works well for me.

Looks like my imgs didn't post. Here's are the links...

http://imgur.com/zaeaGFZ" target="_blank
http://imgur.com/zLOAAVZ" target="_blank

[Vitamin K]

So, two things I have questions about:

1) Do you have a copy of the rules to your son's district race? Can you link them here. The derby-experts here should be able to discern whether or not they'd disqualify bent axles (though there are ways around this, as well).

2) Regarding the 'double wishbone' car...is this going to be made from two different pieces of wood that you bend together? Or is it going to be cut from a single block? Either way, I think your woodworking skills exceed mine! It reminds me a little of one of the 'suspension' cars from "Go Ask Grandpa". http://www.goaskgrandpa.com/flex.htm" target="_blank

Alright, I've tweaked the design a bit to where the "hole" is no longer as big of a problem. My paper cutout in the following pictures doesn't show correct size for axle/weight box, nor does it show what the nose will look like (should come together, flatten, and round off in a wide v shape if looking from the front). Take it for what it's worth.

[whodathunkit]

Say.. VADAD,
I like the looks of wishbone looking cars.
The concept of the one seen made with paper and staws is cool looking wishbone design.
Tungsten cubes will be trickey at best to pack into the design as theres not much meat left
as far as wood.
Will she be glueing the cubes in cut outs for weight pockets or stacking them down
on what little wood is seen running down the bottom backside of the design?

T.Dean White,
Did up a cool looking flying wishbone design & car.. that also used t-cubes in the design.
It used a shape in the back of the design for holding the weights..
Kinda: like what is seen on your sons black car very cool looking car design by the way!

I tryed makeing:
T.Dean White's "Flying Wishbone" car design once.
If your daughter would like to see what his wishbone car design looks like.
Tell her to type in Flyin' Wishbone.. useing advanced search to see the design.

And tell her to just have some fun with her.. Wishbone car design.

[VADAD]

BAH! Change in plans.

I got up yesterday to show my daughter the finished design plans for her car. It appears I've gone too far with my revisions of her original concept which started as 3 triangular bars in a triangular pattern, with holes similar to her brothers car. Upon brainstorming with her, we got into the wishbone idea. She thought is was cool. When she saw the idea on paper she wasn't happy . I guess I'll put that design on the back burner.

Here's design #2, again with her input. I'm not looking forward to showing it to her on paper when she wakes up. It's probably to flat for her ... I'm already coming up with car three in my head, but I hope this will work.



This plan is just a simple 3 rail car with triangular beams. The middle rail rises above the side rails as you go towards the rear. Looking down towards the top of the car I probably won't make the weights visible (as seen in the pic). I have no clue whether a .3 inch thick triangle rails are thick enough. That's roughly the thickness of a pencil cut in half lengthwise. One modification I've considered is connecting all three beams together somewhere in the middle with slanted .2 inch beam in between each side rail and the middle rail.

She's a pretty particular 10 year old but if she's showing this much interest in a joint project with dad, I don't feel guilty spoiling her a bit. I'll update soon as to whether this is our final car.

To Vitamin K:

I'll find the rulebook and let you know. Regarding the wishbone car idea... The car was going to be cut from a single piece of wood. It would require tons of dremeling, constant use of a digital caliper to check for thickness, and precision sanding. I know there are easier ways of going about it but I'm using the tools that I've got. The flex car you mentioned is outstanding.

To Whodathunkit:

The paper cutout version of the car was hard to get right. First, it's missing the nose and missing the rear (about 1 inch each nose and 1 inch rear box). Right befor the straw in the front, all three beams join together forming a slanting nose. In the rear, those three beams turn into a box to hold the weight (2 rows of cubes in the front of the axle and 2 in the back). The beams on the wishbone would be about .225 inches thick. The paper wishbones were too long, so the model looks a little stretched especially after adding the nose and rear. Regarding securing weights... I really want to come up with a way to open a small hatch and dump them out as needed, but that a LOT of work. Those weights ain't cheap and I'd love to reuses them in the future. I'm sure what to do yet, glue, or otherwise. Lastly, that wishbone car is amazing wood work. I starting to get the hang of geometric stuff, but I can't get anywhere close to sculpting like that. Wow. It even looks just like a wishbone should look.

[DerbyAddicted]

I'm thinking that your newest design is going to be structurally weak. Normally, .3 would be thick enough, but with you "triangling" them, you're losing close to 25% of your mass in those supports (sorry, algebra and geometry aren't my strong points). And with the extended wheel base, etc., I think if someone mishandles it (grabbing by the nose) and isn't careful, the weight will cause it to sheer in half.

[Noskills]

I agree with DerbyAddicted. If the rails are triangular I would really worry about strength. If they are solid I think you are fine with a pretty cool design!
Noskills

[FatSebastian]

with you "triangling" them, you're losing close to 25% of your mass in those supports

If the rails are triangular I would really worry about strength. If they are solid I think you are fine with a pretty cool design!

Even if the triangular beams as designed are strong enough, wood is a natural product that can warp over time. A triangular beam is an L-beam with the L filled in. An L-beam has less strength than a square beam or I-beam, but it is also less resistant to torsion (twisting). This is a neat design, and I think triangular cross-sections may be workable, but we would probably want to make the beams larger / gaps smaller and steeper.

[VADAD]

Updated design. I think we have reached an agreement on shape.
Now I've got to figure out how to make it structurally sound. I think you guys are right about .3 inch triangles being too thin. Would supports inside the beams cut it, or am I stuck going thicker? If thicker, go thicker on the side rails, middle rail, or all rails?

[DerbyAddicted]

I think thicker is needed. Even with support in the middle, I'm still concerned about the car breaking in half. Also, I'm sure your woodworking skills far exceed mine, but if I were making that, I see a pile of broken and/or repaired bodies before I got it right. lol