How thin of a car is ok

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Rpenning1970
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How thin of a car is ok

Post by Rpenning1970 »

I was going to build a car and was making it a rail rider with the extended wheelbase. How thin of a car is ok to run, i want it as thin as possible but not so much that it is weak and can break. I am building rail riders for my two kids also but they have there won car designs. last COG, with an extended wheelbase car axles ar 5/8 of an inch from the end of the block on each end total length is 7 inches what should i be aiming for.
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Stan Pope
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Re: How thin of a car is ok

Post by Stan Pope »

I'd aim for at least 0.063 sq. inches of cross-section area at the thinest point between the axles. (That cross section area can be made up from two side rails of 1/4"X1/8" dimensions.) You can usually get by with less, but I've never seen one with this large of rails get "broken in combat."
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Re: How thin of a car is ok

Post by 5kidsracing »

Rpenning1970 wrote:I was going to build a car and was making it a rail rider with the extended wheelbase. How thin of a car is ok to run, i want it as thin as possible but not so much that it is weak and can break. I am building rail riders for my two kids also but they have there won car designs. last COG, with an extended wheelbase car axles ar 5/8 of an inch from the end of the block on each end total length is 7 inches what should i be aiming for.
You can easily run a car that is 1/4" in the rear and tapers to 1/16 in the front as long as it is full width. It kinda depends on what weight you are using. If you are using, say, tungsten cubes you can make an 1/4" thick car. If you are using other less dense weight then you could be 3/4" think in the back to have enough real estate to hide the weight. I would shoot for a 5/8"-1" COM, nothing less than 5/8" though.
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Re: How thin of a car is ok

Post by john4840 »

1/4 inch would be the thinest I would go.

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FatSebastian
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Re: How thin of a car is ok

Post by FatSebastian »

:welcome: Rpenning1970
Rpenning1970 wrote:How thin of a car is ok to run...
FWIW, here's a related topic which asks (almost) the same thing.
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Re: How thin of a car is ok

Post by PeterT »

My son's current year car is a little over 1/4" tall, but we beveled the sides in on the bottom edges of the car between the front and rear wheel which makes the car look about 1/8" tall when viewed from above. Since most derby cars are viewed from above, it makes it look like a ridiculously thin car. Since it was an afterthought we didn't measure to take the bevel far enough to be hidden by the wheels. I'm sure you can execute this even better than we did if you're really going for that super thin look.

We've had cars thinner than 1/4" either break or crack in testing/racing, so that's about as thin as we will go. You could also possibly go thinner if you shave your pine way down and then glue laminate basswood - or something else nice and strong - on top. We've tried this and found it to be a colossal waste of time.

Here's a picture of the current car; the beveled bottom edges are just barely visible from this angle. Try clicking on/enlarging the
photo and it will be more obvious.
Image
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Re: How thin of a car is ok

Post by idpwdnut »

Here is the "thinnest" car I have built so far.
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Side View
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My digital caliper has a dead battery, so these are kind of rough. Thinnest part is just under 3/16" and thickest is 5/16".

It was a very high quality wood block to start off with. Some blocks would not allow it to be so thin. It has faired very well through two seasons of racing and now going into its third. It has been a tough competitor. In fact I dropped it tonight taking the pictures for it. If it every breaks, I'll just cut out the weights and build again. Win-win for me, but not if in the final rounds of a race.

Good luck on your build.
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sporty
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Re: How thin of a car is ok

Post by sporty »

You can go very thin and still not have flex issues.

see some of my cars.

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All of these cars are only 1/4 thick where the weight was placed, the rest of the car is thinner.

they key is the wood to way less than 10 grams once the weight pockets have been removed.

use 4oz of weight, use monokote.not paint, to get the most out of the weight placement in the car.

I pick wood that has grain lines that run the length of the wood (7inches). atleast 8 to 12 grain line in the wood. This allows me to just go so thin, and You have to stay wide, for stiffness. but can flair the sides.

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pwrd by tungsten
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Re: How thin of a car is ok

Post by pwrd by tungsten »

I like 5/16" thick. We have a rough / brutal stop section so that plays into things a it. However even on a nice track I like 5/16" with tapered nose.
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Re: How thin of a car is ok

Post by Derby Evolution »

1/4" is thin enough. Most of my cars are 1/4" in the rear and taper to the front. By making the car much thinner then that, you are putting yourself at risk for the car breaking. It is possible to make them thinner, but it is not really needed. I can make car bodies that are 1/4" that weigh in under 10 grams painted, so the need to go thinner is not there.
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Duane
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Re: How thin of a car is ok

Post by Duane »

Someone please remind me: What's the point of very thin car bodies, or hollowed-out bodies, other than for exotic looks? The thin front end does not help with air drag. Making the front thinner and lighter means that the weighted back end then needs to be heavier and thicker, so air drag is increased by the thin front end. A thin or hollowed front end does help with moving the com rearward, which helps the total energy available. Does the changed com provide enough energy to overcome the added air drag?

On the original question, I've raced a car with 3/16" thickness, with a narrowed waist. Was rigid and worked fine with the right wood block. And built an unraced car with 3/32" thickness, that seems sturdy enough but slightly twistable.
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Re: How thin of a car is ok

Post by Rukkian »

I am not sure how a thin front end causes more air drag as that does not make sense to me.

For my 2 sons this year, both cars are completely flat - OS was 3/8" and YS was 1/4" with thin rails between the axels and behind the rears with the weights inside surrounding the rear axels. We then glued 1/32" balsa wood on top and bottom to seal it in and used a wrap on each one, as that is what they wanted (probably would not do it again). I have not checked either one now, but the balsa and skin did not add much. With the multi-layer they seem very solid and unlikely to bust, but we shall see.
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Re: How thin of a car is ok

Post by rpcarpe »

For new parents/cub teams I recommend 1/4" or less in front, 1/2" or less in the rear. This gives them a good start. Many go thinner once they realize how little wood they need. In workshops, I recommend a car body weigh in under 1 oz (that's heavy compared to the 10g's from Sporty)

Rpenning, how thick is your current design?
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sporty
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Re: How thin of a car is ok

Post by sporty »

I want to comment on Duane'scomments.

But first:

The thinner cars do a few things.

1- the more weight you can get in that car, the more advantage you have, compared to someone doing the same thing. Same with monokote. Less weight and provides a good smooth slick surface without all the work of paint. and they can look very nice.

2- Aero does come into play, for a few reasons.

A- The lower the profile, the more the air drops low., creating less drag. My cars also use raised axle wholes. so the underside of the car is also low to the track, preventing air flow from getting under the car and effecting speed.

B- The flatter and smoother the surface, less turbulance is created, allowing for more cleaner air, more streamlined air.

C- Duane, the low front end nose, on my cars, are designed from my wind tunnel testing. The lower nose helps keep the air low, the raised up wood where the axle are, so the wood is higher their. then slopes right back down. The air goes right back down also.

The lower the air drops, the less it effects those forward spinning wheels and can even help you if you have it drop down in a way over the wheel to almost give the wheel a easier forward spin, but how it effects the wheel rotation from behind. ( i never perfected that type design).

These small amounts add up. Every little bit you do adds up.

3- I have not had one car ever break at a stop section, had some hard hits, flips and even up in the air and my son drop the car. The key as I mentioned is wood with tight wood grains.

4- The nose on most of these cars are very aero. They were done like that for a reason.


Derby evolution and pwrd by tungsten are both very good builders and very top notch racers.

But I have to add that you can go plenty fast with a 1/4 thick car and be under 10 grams. pinewood derby wood, I hope is used for scout races.

When I bought a bunch of 1/4 thick popular wood car designs, from Lester racing. I also wanted to come up with something that could compete and win against those designs. I was able to do this, once upon a time.

But those same cars that were able to do that are basically these designs of car pcitures I previously posted in this topic.

Here is what we used for wind tunnel testing on allot of different cars, The results from that , is why I went to this type of car style also.


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Here we used a fog machine to see the wind.

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I wish I had better pictures, but when we were testing, dang I forgot to bring my camera and all I had was my phone. and Did not think to do video either.


Now I have made thinner cars-
This clip, Is the thinnest I have gone and do not recommend going this thin, it's just not needed.
Now you can tell, that the wood is thinner than the 1/4 thick tungsten.

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Sporty
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Re: How thin of a car is ok

Post by FatSebastian »

Duane wrote:Making the front thinner and lighter means that the weighted back end then needs to be heavier and thicker, so air drag is increased by the thin front end.
To maintain 5.0 ounces, making the front thinner and lighter means that the weighted back end needs to be heavier, but it doesn't need to be thicker. In a high-performance wood car, the builder is usually replacing low-density pinewood with ultra-dense tungsten. Having almost 40 times the density of wood, W can very nicely tucked into a 1/4" thick body. Thus, the total cross-sectional area is not necessarily increased because the wooden front is tapered / streamlined.
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