COM question

[AUPatrick]

This may be a stupid question, I seem to be asking a lot lately, and I DEFINITELY appreciate the input. I read a lot on here that COM should be no closer than 1/2 inch in front of rear axle and no more than 1 inch in front of rear axle. I have also read DERBYDAD say that you should put 1.8 oz. behind the rear axle and the rest in front of rear axle.

My car without paint, wheels, axles or weights, weighs in at .7 oz. With my wheels and axles it weighs 1.1 oz. I plan on adding 3.7 oz of weight for a total weight before paint of 4.8 hoping I get close to 5.0 after paint.

I'm doing a car with tungsten cylinder weight (3/8 inch cylinders). I drilled three holes at the very bottom of the car (going side to side) with one hole behind the rear axle and two in front of rear axle. I put 1.5 oz (three cylinders) in hole behind the rear axle, 1.5 oz in second hole (three cylinders) right in front of rear axle, and .7 in the second hole in front of rear axle. Unfotunately, that made by COM a good bit less than 1/2 inch in front of rear axle.

Should I move more weight in the first hole (furthest away) from rear axle? If so, should I remove more weight from behind rear axle or from hole in front of rear axle. Or should I just add more steer and race it and stop asking stupid questions?

 

[STI45]

Was the com measurement done with the wheel and axles installed on the car? I would not expect the com to be that aggressive based on the weight distribution described.

 

[AUPatrick]

no, i didn't measure COM with wheels and axles on the car, I did it without it. Does that make much difference?

 

[Quicktimederby]

Yep!! Install and and I think you will be where you want.

no, i didn't measure COM with wheels and axles on the car, I did it without it. Does that make much difference?

 

[Kinser Racing]

When you put .2 oz on the front of the car the leverage will definitely change the COM.

 

[Guest]

Is placing roughly 1.8oz behind the rear axles advice mainly for BSA? How about

some of the other classes such as Awanas, Royal Ambassadors and the Royal Rangers?

 

[W racing]

You want to place as much weight as possible into the tungsten. This means not very much wood. Enough not to break the car. Some like to place more tungsten behind the axle than in front. However it is almost always possible to add equal amounts of weight behind and in front of the axle to obtain the desired COM. And if you can not do that I would say you have too much wood in the car.

A 5/16" block of wood should be enough for strength. Possibly 3/8" if you are forced to use slots. If you know you have a best track with a proper stop section than you do not need to worry as much about durability. However a scout car can run on brutal stop sections.

 

[W racing]

You are good if you follow the steps here:

http://forums.derbydad4hire.com/post/Keys-to-win-from-the-Pinewood-Derby-king!-5782181

 

[Guest]

/images/boards/smilies/wink.gif

 

[quadad]

AUPatrick, please note that wheelbase (primarily), wheel type, lube, etc., all play a role in 'rules of thumb' for COM. Many racers have a favorite wheelbase and use fairly similar weight placement for some number of cars. They also often allow for moving around ~ 1/4-1/2 oz during testing and tuning. Many times moving the weight around or tweaking axles can have a bigger speed impact than changing drift. Your design should allow this. Best of luck in your racing !

 

[W racing]

You need to copy and then paste all of the below into the web browser:

http://forums.derbydad4hire.com/post/Keys-to-win-from-the-Pinewood-Derby-king!-5782181

We always build a car and weight it with taped in weight and hang unpolished wheels and axles (Which never see track time. They are just there for building.). We measure the COM and then build another car with diffrent axles and wheels and the COm is where we want it.

Of course once you have the design down this becomes less important.

Hello Quadad!

 

[Guest]

Got it

 

[W racing]

Center the weight from left to right on the car. If you are using cubes place the weight as close to the axle as possible. Of course leave a nice area of wood for the axle to sit in. Say 1/4"...

place the weight one row in front of the axle as close as it can go to the axle. Then one row behind as close to the axle and then repeat until the weight is in.

If the rules allow you should raise one of the front wheels. If you are not adding 3.5 ounces of weight or more you have too much wood. I would recomend even more than 3.5 ounces in weight. The weight COM in the vertical direction should be at or below the rear axle (for the added weight).

 

[Guest]

So a 1/4" helps reduce vibrations/movement and/or to retain memory?

Would cutting a 4oz tungsten round in half and placing in such a manner be faster than centering the round exactly between the axles if you were somehow able to beef up axle holes ensuring a solid fit with absolutely no vibrations or movement.

 

[OPARENNEN]

I may be wrong, but I think most of the top ten builders put two rows of 1/4 cubes on the rear as far back as possible (I glue my cubes, to eliminate shutter or bending), then create a 1/4 inch of woood strut for the axels then keep adding cubes in fron of the axle until you reach the preferred weight. I use this for all classes. The only difference is becuse of the reduced weight of SP, ELIM & UNLIM class wheels, you increase the cubes in front. I also often shorten the weight of the car by a single cube, and after painting the body, I bring up the weight to standard with a flat piece of 1/64 or 1/32 flat sheet lead to the 5 ounce limit. With that piece of lead, using tape, you can play with the com by moving the weight back and forth. In my case, I set the "steer" to about 13 inches for SS and SP first, then temporarily secure the lead with tape, test the speed of each lead placement on a test track, until you reach the sweet spot. After that, I play with the steer again to see if I need more or less, based upon further test runs.

Just my opinion. I usually only place between 10th and 6th. So someone does a better job than me

 

[quadad]

SLR, some random observations ... I think W is suggesting 1/4" there to make sure you can keep your axles in place and alignment stays on. People have combined multiple orientations of wood before, not sure if anyone does now. I think more people are inclined to make the weight pockets first and then drill axle holes to take care of some of your other concerns.

Regarding the physics behind it all - while I have great respect for, and have done that kind of work, I believe that while you can work all the equations of motion and dynamics analysis you want, there is something about these little cars and the variable at play here that I think makes building a bunch of cars and experimenting with them on your track a lot more interesting and productive. Though we have learned some things from mathematicians and physics experts, adult league racers take great pride in beating them on the track, and have done so a number of times.

And "W", old buddy, good to hear from you !

 

[Guest]

/images/boards/smilies/smile.gif

 

[5KidsRacing]

Regarding the physics behind it all - while I have great respect for, and have done that kind of work, I believe that while you can work all the equations of motion and dynamics analysis you want, there is something about these little cars and the variable at play here that I think makes building a bunch of cars and experimenting with them on your track a lot more interesting and productive. Though we have learned some things from mathematicians and physics experts, adult league racers take great pride in beating them on the track, and have done so a number of times.

Well said and couldn't agree more...

 

[Guest]

Good deal-

 

[W racing]

Weight placment is critical however it is secondary to rear wheel alignment. If your rear wheels are not aligned you should throw the car away and start over. That is why I like the 1/4" strip of wood for a rear axle.

Ideally I would have a 4 ounce BB sized object and place it in a small hole in the center of the car aligned on the rear axles. Of course that weight does not/can not exist.

I would consider a car sub optimal that has less than 3.5 ounces of added weight. It is very possible to get more than 4 ounces in a scout car. I would not leave a space between the rear axle and the weight. The weight should be as near to the axle as possible.

I use to never place added weight behind the rear axle. My thought being that it would be more stable in the transistion. Weight behind the axle causing the car to do a wheelie. Now I look to even the added weight so that it is equal on both sides of the axle. And as close to the axle as possible. This reduces the rotational inertia of the car in the transition. This is also why I do not like to chase a certain COM by placing weight away from the rear axle and why we build two cars when going for a new design to obtain both the desired COM and desired tight weight placement.