Help moving com back

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stay at home dad 13
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Help moving com back

Post by stay at home dad 13 » Fri Feb 12, 2016 8:46 am

I've read the post about weight placement and am still trying to figure things out. Where's my situation, My son picked the design so I'm going with what he wants, not very aerodynamic. But, I've gotten the block down to 3.2 oz with an extended wheel base. Not including glue or paint. So we have some room to add, about 1.6 oz. My question is where to add it to shorten the CM. With the extended wheel base, if I add 1 oz behind the axle it's very rear heavy, should I add the other weight farther in front of the rear axel? Or do I spit the difference and add less behind the axle and more in front to adjust the cm?
Now the short version question
Should I add less behind the axle and more in front of it to get the cm I want, or
stick with the 1 oz behind and move the rest as far forward as needed to get the cm I want?



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Nelvis
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Re: Help moving com back

Post by Nelvis » Fri Feb 12, 2016 9:24 am

If it were me I would just grab three 1/2 oz. weights and balance the car on a ruler or even your finger. Find out where it balances and then play around with the weights by placing them on top of the car. Try to get it to balance about an inch or so in front of the rear axles. As long as it balances around that 1 inch mark you should be okay. If the block weighs 3.2 my guess is that you will end up needing to have more of the weights behind the axle in order to get it to balance at that spot.



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Re: Help moving com back

Post by Nelvis » Fri Feb 12, 2016 9:32 am

Also to answer your other question the weight should be as concentrated as feasible. So putting a heavy weight in back and then needing to put another weight really far forward in order to achieve the com is not ideal.



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Re: Help moving com back

Post by Scrollsawer » Fri Feb 12, 2016 9:48 am

If the unweighted car is 3.2 oz., does that also include your wheels and axles?

I do not know what the car looks like, so cannot really offer up too much in the way of really targeted advice. If you can post a pic. or two of the car (including a pic. of the underneath of the car, that would help with giving you advice. Right off the bat, I have to assume that you should be able to remove some wood weight from the bottom of the car. Routing out certain spots underneath the car can help you get more weight into the rear of the car, so that your COM is improved.

Put the weight behind the rear axles on an extended wheelbase car. Distribute the weight evenly across the rear (along the width) of the car wherever possible. Or at least try to even out the weighting on either side along the rear of the car.

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Re: Help moving com back

Post by stay at home dad 13 » Fri Feb 12, 2016 10:32 am

I having issues with a pic, but I can tell you that I routed out sections for wieghts before and after the axle and some farther ahead. Do I take weight towards the front or middle? Because I'm trying to keep the weights in the rear. But it sounds like I will have to add some to towards the front to adjust the com.



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Re: Help moving com back

Post by Nelvis » Fri Feb 12, 2016 11:23 am

No you shouldn't have to put any weight towards the front. Don't be so concerned with putting a ton of weight behind the rear axle. Just try to get the car to balance on the com location while keeping the weights close together. For example maybe you will need to put half of the weight just behind the rear axle and half of the weight just in front of the rear axle. It sounds to me like you are trying to cram a ton of weight behind the rear axle and then having to compensate by putting a small weight really far forward. This is not ideal.



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Re: Help moving com back

Post by knotthed » Fri Feb 12, 2016 1:41 pm

If your car is 3.2oz without any weight, you need to get it as far back as possible. With a chassis weight that high, you will have a hard time getting a decent com. If you are looking for speed, you should try and cut that weight as much as you can, so you can add heavy metal in the rear of the car.

For reference, most fast people are building a chassis that is around .5oz and adding very close or equal to 4oz of tungsten around the rear axle of the car.



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Re: Help moving com back

Post by birddog » Fri Feb 12, 2016 2:56 pm

knotthed wrote:If your car is 3.2oz without any weight, you need to get it as far back as possible. With a chassis weight that high, you will have a hard time getting a decent com. If you are looking for speed, you should try and cut that weight as much as you can, so you can add heavy metal in the rear of the car.

For reference, most fast people are building a chassis that is around .5oz and adding very close or equal to 4oz of tungsten around the rear axle of the car.
What he said! Knotthed is on the money as usual! With a 3.2oz car without any weight, just put tungsten it as far back as possible. You'll still have a COM that is too far forward, but if you must keep that much wood on the car, you have no choice in the matter.

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Re: Help moving com back

Post by Speedster » Fri Feb 12, 2016 3:41 pm

Our teams always build the same design every year. It is not a Hershey Bar car. We think it has an attractive shape. It is 5/16" thick except for the front where it tapers down. The bare wood weighs 0.5055 ounces. At sometime on Derbytalk it was recommended (OK, they told me to do it) 1 and 1/2 ounces was recommended behind the rear axle slot. 3 Max-v tungsten plates always fit nice there. I never deviate from that. I am then able to put a 2 ounce block in front of the rear axle slot and maybe a tiny, tiny bit more tungsten. The front pocket is big enough the 2 ounce block can be moved around to find the best time when tuning on our Best track. It usually is between 5/8" to 3/4". The weights are all set on Double Face tape. I like this set-up because the scout does it and I keep an eye on him. No matter his age, he understands what he's doing. He likes to start the cars the best. When we get our fastest time the DFW axle is glued in. I am not involved in decorations. When we get to Districts the Inspectors often rave about the Decorations. What in the World is "Minecraft"?
I understand an Extended wheelbase car is being built and the weight amounts and placements are different. Let's be honest. You are doing everything perfect. You two are building a car your son wants. What is his reaction after the race? Is he upset he didn't win or is he satisfied? If he is very upset, then you console him and have a gentle talk with him. Get Troy Thorne's latest book and spend time with him reading it. Stay on Derbytalk and ask lots of questions. These folks can help you go very, very Fast.
Best of Luck.
Cheers,
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Re: Help moving com back

Post by stay at home dad 13 » Fri Feb 12, 2016 4:09 pm

He is great when he doesn't win, last year. There were 2 boys that melted down after they didn't win the first or second race. My son this year is all about the best in show. He wants a tank car with a special gun on top. We are adding 2 extra wheels (not touching anything of course) on either side to give it a more tank like look. The top piece that will be covering up the weights will actually go outside the block of the car, so it could look more like a track, and keeping within spec. With a great 3 coat camo paint job it will look great. I guess the speed part is mostly for me. We'll play with it some more and see what happens. So basically, we'll try to keep as much of the weight in the back as possible to get the com.
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Re: Help moving com back

Post by knotthed » Fri Feb 12, 2016 5:22 pm

SAHD13,

Food for thought, you can make your main chassis a 1/4" plank, then build the rest of the tank with thin plywood or balsa coated with CA glue and probably end up with a faster vehicle and still have the look you're going for. Or hollow out as much as you can.

If you're just going for show, then just slap the weight on it as there isn't really a need to worry about com at that point.



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Re: Help moving com back

Post by Scrollsawer » Fri Feb 12, 2016 5:40 pm

Agreed with the previous post. We have built Hershey bar chassis cars before, with a balsa superstructure on top, and we won both design and speed categories. And if he's going for design, don't worry about the COM on a 3.2 ounce chassis car. Put what you can as far back as you can, and explain why weight placement is one of several important factors for generating potential energy in the Derby.

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Re: Help moving com back

Post by davet » Sat Feb 13, 2016 10:33 am

We built a tank our first year. We cut 1/4" off the back and installed ring gear bolts with large heads in the back to get weight to the rear. We also had some 3/8" bolts inserted in the back. Our COM was right around 1 1/4" IIRC. We hollowed out what we thought we could. Knowing then what we know now we could've shaved a ton more off. Also, the extra wheels between are shaved thin with holes cut in the tread to shave weight. Those wheels don't touch the track. We even hollowed out the cannon. Car was a straight runner on graphite with stock parts. Came in 3rd in Pack and even won a few at Districts. You're not as limited as you think. You're way ahead of the rest of the Tigers I'm sure by coming here so soon in your son's racing career. Good luck.
Image



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Help moving com back

Post by whodathunkit » Sun Feb 14, 2016 9:01 am

devet, I forgot about your tank on here.. I still like the looks of the tank.

I did a little digging around in my photos to look for a tank
that I helped a kid make at one of our very first work shop days.
Image
3rd from the right.
He also placed 3rd in the outlaws that year on a 35' Best Track.
It had 8 wheels with the 2 center wheels lifted .. not as much hollowing out going on as your tank.
Best i can recall we just took a chance with the COM around 1'' placing the weight as far back as he could get it in the tank.


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