Thank you to the guys on this forum...Now sharing a secret

Secrets, tips, tools, design considerations, materials, the "science" behind it all, and other topics related to building the cars and semi-trucks.
againstthegrain
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Thank you to the guys on this forum...Now sharing a secret

Post by againstthegrain »

5 kids, 14 years of Pinewood Derby. Perennial Pack and District champs, competitive at, and sometimes Council champs.

We are finished with Pinewood derby.
Thanks to the guys on this forum we went from 16th in our Pack the first year to Council champs.
Thank you!
2 of my kids are also going to be engineers. I credit their interest in science and engineering to our many, many hours of learning the principals behind what makes a Pinewood car fast...aerodynamics, friction etc. etc.

Here are some pictures of two of our cars (bottom side) illustrating our weighting technique that provided stability and speed (3 wheel rail rider).
Dominant side and low weighting for stability and speed, rear weighting for speed. All weight is tungsten.
Make sure also to use a black marker or paint on the underside nose of your car to make sure the sensor senses you crossing the line!
I am sure this weighting technique has been posted before, but here goes:

Image

Image

Image

We are selling all of our pinewood stuff as well, check out the Trading Post section of the forum here:
http://www.derbytalk.com/viewtopic.php?f=35&t=7506" target="_blank


Again, thanks to all of you for sharing so much info and experiences.
Last edited by againstthegrain on Wed Jan 29, 2014 12:57 am, edited 1 time in total.
Selling all my Pinewood gear...
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DerbyAddicted
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Re: Thank you to the guys on this forum...Now sharing a secr

Post by DerbyAddicted »

Thanks for the tips. I assume the tungsten rounds are for incremental adjustments?
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whodathunkit
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Re: Thank you to the guys on this forum...Now sharing a secr

Post by whodathunkit »

Is this what you would call.. off center weighting?
What type of automobile can be spelled the same forwards & backwards?
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Re: Thank you to the guys on this forum...Now sharing a secr

Post by Topspin.D »

Thanks for giving us a peek behind the curtain. What's the COM of these cars?
againstthegrain
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Re: Thank you to the guys on this forum...Now sharing a secr

Post by againstthegrain »

whodathunkit wrote:Is this what you would call.. off center weighting?
I like to think of it as on center weighting. When dealing with only 3 wheels touching the more weight you can put in the triangle formed by the three wheels the better. I know this has been discussed on this forum many times. I am not a physicist or an engineer but the way I understand it is the following... You want the rear wheels to carry an equal amount of weight to keep the friction between the wheels and axles equal on both sides. Since the touching front wheel is carrying some of the weight on that side, and the lifted front wheel (and axle and wood) also tends to lift the weight off the back wheel on the touching side like a teeter-totter, you need to shift more weight on to that rear touching side wheel. My personal experience has been that this weighting pattern will eliminate wobble. You also have the added benefit of being able to move the center of mass closer to the rear because it tends to wobble less, being side weighted. Also you can use less aggressive steering on the front wheel because more weight is carried on that side.

I also like to put the tungsten cubes in the cavity of the car and then attach tungsten plates directly below the cubes to make the car bottom heavy. Perhaps in theory the COM should be on a plane just as high as the axles, but in reality the tracks we race on are not cleaned and polished perfectly smooth between races. So any bumps or jolts to the car will tend to affect a top heavy car more than one that is bottom heavy... like difference between a top heavy hay wagon and a low slung sports car.

Attaching the cubes and plates with a squishy type material that absorbs shock also helps to deaden the blows. ;) It also allows you to remove the cubes for next year's car.

If your car is fairly symmetrical it does not hurt to point out the front end on the bottom. We have been staged backwards 3X!

Get as much weight off the lifted front as possible. Do you need a full length axle up there?

This is what worked for us on a long aluminum track set-up where a stable, non-wobbling quiet car will shoot past others in the flat.
Last edited by againstthegrain on Wed Jan 29, 2014 12:53 am, edited 2 times in total.
Selling all my Pinewood gear...
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againstthegrain
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Re: Thank you to the guys on this forum...Now sharing a secr

Post by againstthegrain »

DerbyAddicted wrote:Thanks for the tips. I assume the tungsten rounds are for incremental adjustments?

Yep.

Also the pics I shared did not show all the tungsten cubes we used, I just wanted to show the pattern. Our COM was fairly aggressive. On a long track it is very important to stay on the rail and have a "quiet" car. Alignment is far more critical than COM on long tracks. If your nose is floating in the flat, you are dead. Side weighting also helps in this regard.

This is just some of the advice I picked up on this forum over the years that has worked for us. If the real pros on this forum find that my posts are in error please correct me.

againstthegrain
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Re: Thank you to the guys on this forum...Now sharing a secr

Post by DerbyAddicted »

againstthegrain wrote:
DerbyAddicted wrote:Thanks for the tips. I assume the tungsten rounds are for incremental adjustments?

Yep.

Also the pics I shared did not show all the tungsten cubes we used, I just wanted to show the pattern. Our COM was fairly aggressive. On a long track it is very important to stay on the rail and have a "quiet" car. Alignment is far more critical than COM on long tracks. If your nose is floating in the flat, you are dead. Side weighting also helps in this regard.

This is just some of the advice I picked up on this forum over the years that has worked for us. If the real pros on this forum find that my posts are in error please correct me.

againstthegrain
It's interesting, because visually, I wouldn't expect your COM to be aggressive.
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Re: Thank you to the guys on this forum...Now sharing a secr

Post by againstthegrain »

It's interesting, because visually, I wouldn't expect your COM to be aggressive.
You are probably correct about the cars in these pictures. The cars I pictured here are have our least aggressive attempts at side and rear weighting. Not all the cubes are placed in the car here; I usually use 14+ ;) cubes, 3 plates and tungsten beads, paced in a stair-step triangular pattern. Also, I will tend to shave the wood of the forward weight pocket closer to the dominant side wheel and slightly closer to the axle than pictured. The plates will also be pushed back flush with the back end and as close to the dominant side as possible. We will also usually remove as much weight as we can from the front end, especially from the non-dominant front wheel side (metal and wood). We only tried for a front/rear COM of 1/2 inch in front of the rear axle once. A side and bottom weighted COM, and a front/rear COM of 3/4 to 7/8 proved optimal for the long track we raced on. You want those rear wheels carrying as close to the same amount of weight as possible, of course the ratio of weight carried by each wheel changes as the car races (resting on pin, initial downward slope, transition curve, and flat.). Low and side weighted cars required less toe in to maintain rail riding, which allowed us to shoot past others in the flat, We were always pretty good out of the gate, but we looked like we had overdrive after the transition from the curve into the flat. On a shorter track we may not have fared as well. Alignment and making those bumps as uneventful as possible is key to maintaining speed in the flat.

The side to side COM, and the horizontal COM are pretty aggressive in the cars pictured as well. ;)

againstthegrain
Last edited by againstthegrain on Wed Jan 29, 2014 1:35 pm, edited 2 times in total.
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Re: Thank you to the guys on this forum...Now sharing a secr

Post by DerbyAddicted »

Yes, in the finals of my son's derby, there was one car that was neck & neck with him on the downward slope, but once they hit the flat, my son's car was able to pull away.
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Re: Thank you to the guys on this forum...Now sharing a secr

Post by DerbyAddicted »

Yes, in the finals of my son's derby, there was one car that was neck & neck with him on the downward slope, but once they hit the flat, my son's car was able to pull away.
againstthegrain
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Re: Thank you to the guys on this forum...Now sharing a secr

Post by againstthegrain »

Yep,
Speed out of the gate / off the starting pin has more to do with wheel to axle friction (An object at rest will remain at rest).
So good axle and hub prep will be apparent at that point of the race.
If your alignment is poor in the flat you will quickly lose any momentum you built up.
We spent as much time aligning our cars each year as we did on building.
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Re: Thank you to the guys on this forum...Now sharing a secr

Post by Stan Pope »

againstthegrain wrote: We spent as much time aligning our cars each year as we did on building.
What is your alignment procedure?
Stan
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againstthegrain
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Re: Thank you to the guys on this forum...Now sharing a secr

Post by againstthegrain »

Stan,

First let me say a huge thank you to you for all your years on this forum (and the previous forum). Your posts helped me and my children think about the how and why a certain modification might or might not work. You never seemed to give away the store with any of your answers, challenging our minds by dangling a carrot. We had to work out for ourselves (by reading other's questions and answers) why rail riding might be better than strait alignment, why moving the COM farther to the rear (or over to the side, or lower) might increase or decrease potential energy or stability, why rolling friction is better than sliding friction when considering the wheel to track interface, etc. etc. In the same spirit of encouraging intellectual curiosity I posted these pictures of a possible pattern of weight placement...incomplete of course!.
We have used several procedures for aligning our cars. They have all been posted on DT. The first procedure we used was your download from years back. For those who don't own a section of track, a flat table with a stripe of masking tape down the middle is a good starting point to observe movement of the wheels on the axles, toe in or out, possible binding, adjusting turn etc. etc. As you well know it is not just about turning 1 inch over 3-4 feet.

Again, thank you for stirring the intellectual curiosity of myself and especially my children over these years. God will richly bless you for your work on this forum.
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Re: Thank you to the guys on this forum...Now sharing a secr

Post by Stan Pope »

Thanks, atg,

Asked because 5th Grandson is coming by tomorrow to install his wheels and align for upcoming racing. This is my Son's 2nd time around and this Tiger Grandson's first try at building a car. Word so far is that it "looks good." We'll see what 10 minutes of alignment will do for it! When done, we'll know that the rear wheels are tracking together and that they will stay off the rail. Hopefully, too, Son will understand the current alignment technique and be able to apply it next year if I'm not able to help. We won't know if the DFW toe is optimum for their track, but it will be "in the ballpark." I'm sure that it will run better than Son's first car, though! :)
Stan
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Re: Thank you to the guys on this forum...Now sharing a secr

Post by againstthegrain »

Stan,

If those rear wheels are tracking together and staying off the rail, and the DFW stays on the rail, you will probably be ahead of 99% of the Tigers. It was our personal experience that on the long tracks we raced on that the DFW toe could be less aggressive (with our setup) than the toe stated by many others on this forum. Good luck on your races!
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