I need a clarification on where COM should be measured from? From the center of the rear axle or from the front edge of the rear wheel or from the absolute rear of the car?
Center of Mass
- Thread starter Flamethrower
- Start date
this is what I was taught...
[font="courier new, courier, mono"]The center of mass position is found by balancing the car on a sharp edge and measuring the distance from the body center to the balance position. The body center is 3.50" or 8.89 cm from each end of the 7" long car body. Multiply inches by 2.54 to get centimeters (easier math). The COM must stay somewhere between the axles or else the car would do a “wheelie”. You may not get a perfect balance but find a spot where it’s just about as likely to tip either way. Make a soft lead pencil mark at the center of the body and make another soft lead pencil mark just above the balance point. The distance between these is, the COM position relative to the car center.[/font]
[font="courier new, courier, mono"][/font]
[font="courier new, courier, mono"]The center of mass position is found by balancing the car on a sharp edge and measuring the distance from the body center to the balance position. The body center is 3.50" or 8.89 cm from each end of the 7" long car body. Multiply inches by 2.54 to get centimeters (easier math). The COM must stay somewhere between the axles or else the car would do a “wheelie”. You may not get a perfect balance but find a spot where it’s just about as likely to tip either way. Make a soft lead pencil mark at the center of the body and make another soft lead pencil mark just above the balance point. The distance between these is, the COM position relative to the car center.[/font]
[font="courier new, courier, mono"][/font]
COM is taken from the center of the rear axle, if the balance point is 1" in front of the rear axle then your com is 1". the farther back it is the faster the car will be but the harder it will be to stabilize. i usualy try to get a 5/8" com, but i have run a 1/2' com before. it requires more steer to make it right. I have heard of guys running a 3/8" com , but that is beyond me.
Both answers are correct,
Most will give you the distance from the back axle to the balance point and it will be 1/2 to 1" with most between 5/8 and 3/4". Now, if you try to do a physics model of the car, you need to get the distance from the body center as chief says..
So you could have two cars with COM of 1/2" ( meaning from the back axle), but one is a street stock and one is a street pro and the REAL COM is different because it is the distance from the center of the car body. This is a case where common usage is incomplete until you know the distance of the back axle to the back of the car. Example: I say I have a COM of 1/2", and my axle is 7/8" from the back of the car, that puts my real com 1 3/8 " up from the back of the car, subtract that from the car center of 3.5", and the real physics definition of COM or COG ( same thing) is 2 1/8".
Why is determining COM or COG so critical ? Does your COM or COG have anything to do with weight placement ? Sorry, dumb rookie question but gotta know. Thanks
yes, you want as much weight in the back as possible without the car doing a wheelie.
it will allow gravity to do a better job.
i do not worry much about it anymore, after a few cars you get a feel of where the weight is and where you are comfortable running it.everything else being equal a low(3/8" to 5/8") com is faster than a 3/4" to 1" com. but the farther back a com is, the harder it is to stabilize the car. Meaning you will need more steer to keep it straight. I do not know if there is a point where the excessive steer looses more speed than the low com adds.??
anyone feel free to correct me, as i am far from the fastest here.
it will allow gravity to do a better job.
i do not worry much about it anymore, after a few cars you get a feel of where the weight is and where you are comfortable running it.everything else being equal a low(3/8" to 5/8") com is faster than a 3/4" to 1" com. but the farther back a com is, the harder it is to stabilize the car. Meaning you will need more steer to keep it straight. I do not know if there is a point where the excessive steer looses more speed than the low com adds.??
anyone feel free to correct me, as i am far from the fastest here.
regarding COG VS. COM, cog is center of gravity, and usually refers to the distance from axle to the front to back balance point.
com is center of mass. and usually refers to the exact center of weight front to back AND left to right.
while both have a place in pinewooding, most are just woried about the cog. to find the com, you need at least 2 scales, and depending on the class, 4 is better. 1 scale under each wheel and the numbers will tell ya how far off your com is. just remember, the front scales will read differently than the rear as most of the weight should be in the rear.
as always, anyone out there feel free to correct me.
com is center of mass. and usually refers to the exact center of weight front to back AND left to right.
while both have a place in pinewooding, most are just woried about the cog. to find the com, you need at least 2 scales, and depending on the class, 4 is better. 1 scale under each wheel and the numbers will tell ya how far off your com is. just remember, the front scales will read differently than the rear as most of the weight should be in the rear.
as always, anyone out there feel free to correct me.
Does anyone have an opinion on weight placement height ? As in is it more effective to have weights higher on the car than lower ?
lower seems to be better. most rout out a couple of pockets on the underside of the car. 2 rows of 5 -1/4" cubes behind the axle, the rest in front. but thats just me and my last few cars i actually put a cube between the axles,
- Nov 24, 2011
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Yep, DNA has is covered...here's a pic of the weighting of a car with 1.0 g wheels and 3/4" COG, with a 5/16" thick body. I don't route quite so close to the edges any more though...rows of 5 cubes are fine. I've found that room for 4 rows in front of the axle and 2 rows behind it allows me to hit all the COG's I'm targeting. Make sure you leave room for BB's or some smaller weights to get the mass exactly 5.0 oz.
Like "Seekinspeed asked", wouldn't weight placed higher in the back of the car, "fall further", and therefore go faster when it hits the flat? Most of the FAST cars look like flat, slivers of wood with wheels. How high is too high? Would a sliver run faster, with the weight suspended above? Like a "SPOILER" or "FIN" made of Tungsten?
Sorry, I'm new here, I've built a few Pack winners, but "You Guys" take it to a whole new level!
Thanks in advance to any answers you can give me... I usually run a "high in back,slim wedge shape", with weight placed 'on top' of a hollowed-out/underneath front body. 5/8" COM.
Our rules are strict. 5 oz. BSA wheels only. No raised, 4 on the floor. No Canted wheels. No rounded wheels. No oil. Graphite only. Etc.
No timed runs, but wins every race(in the Pack)!
I also, "Polish" the wheels inside, and out with graphite, so if the car doesn't roll... it will still slide down, just as fast...
I "rag-on" a pile of graphite, while spinning a wheel into a cloth. When done, ("They look FAST!" Quoted from EVERYONE!) They change color, and become- SLICK!
Sorry, I'm new here, I've built a few Pack winners, but "You Guys" take it to a whole new level!
Thanks in advance to any answers you can give me... I usually run a "high in back,slim wedge shape", with weight placed 'on top' of a hollowed-out/underneath front body. 5/8" COM.
Our rules are strict. 5 oz. BSA wheels only. No raised, 4 on the floor. No Canted wheels. No rounded wheels. No oil. Graphite only. Etc.
No timed runs, but wins every race(in the Pack)!
I also, "Polish" the wheels inside, and out with graphite, so if the car doesn't roll... it will still slide down, just as fast...
I "rag-on" a pile of graphite, while spinning a wheel into a cloth. When done, ("They look FAST!" Quoted from EVERYONE!) They change color, and become- SLICK!
The COG is one of the four tenents of pinewood derby building. 1. Get the back holes perfectly drilled. 2. COG and weight placement for maximum stability of these back engine powered car with their inherit over steer. 3. Lubrication or getting all the friction out of the car. 4. Aerodynamics.
The lower the weight the more stable the car to a point. The new wheel weights have put a new wrinkle in the weight height issue. We might find that too much weight out there and it gets too high. I think some of the advantages of the lateral placement of the wheel weights is the ability to balance the rear lateral moment of inertia. It's like the tight rope walker. Would he rather carry a bowling ball or a long pole?
The lower the weight the more stable the car to a point. The new wheel weights have put a new wrinkle in the weight height issue. We might find that too much weight out there and it gets too high. I think some of the advantages of the lateral placement of the wheel weights is the ability to balance the rear lateral moment of inertia. It's like the tight rope walker. Would he rather carry a bowling ball or a long pole?
What if-(???) Hypothetically... NOT thicker.(WOOD)
Quite slim, with a weight/ ABOVE the car, suspended by a curved wire,(From behind)(or straight up) resembling a spoiler, putting the proper weight at a higher COM, on an Aerodynamic car. Best of Both Worlds, kind of thing... not work?
Higher, falls faster.
Aerodynamic, fights wind resistence.
Lube fights friction.
Steady,wins the race.
"IF" my weight is 1/8" HIGHER, THAN-on a duplicate car, would that weight fall farther, FASTER?
All things being equal?
Quite slim, with a weight/ ABOVE the car, suspended by a curved wire,(From behind)(or straight up) resembling a spoiler, putting the proper weight at a higher COM, on an Aerodynamic car. Best of Both Worlds, kind of thing... not work?
Higher, falls faster.
Aerodynamic, fights wind resistence.
Lube fights friction.
Steady,wins the race.
"IF" my weight is 1/8" HIGHER, THAN-on a duplicate car, would that weight fall farther, FASTER?
All things being equal?
Over the years I've seen some of those cars with weights rigged high in the air. None of them were very fast. Keep the weight as low as possible without getting a bad body roll couple.
Surprisingly, weights closer to the track fall farther. I don't have the quick graphic ability of TxChemist to show it, but you can verify it with some simple geometry (on a BestTrack aluminum track, the hill is ~27°). The extra fall doesn't amount to very much at all, something like 1/32" for a weight 1/2" lower than the elevated weight if I remember right.
Thin to win.
Re: COM. It depends on your track. From the sounds of it (no timing), you're on a wooden track, which tends to be a bumpier ride, and requires a less aggressive COM. Wood track: COM @ ~ 1 1/4". Aluminum track: COM @ ~ 3/4"
Re: Lubing/shining the wheels: Don't put graphite on the treads of the rear wheels — you want those stable, not sliding around.
(PS - I got all excited when I clicked the link and saw that Chief had responded. Bummer.)
Thin to win.
Re: COM. It depends on your track. From the sounds of it (no timing), you're on a wooden track, which tends to be a bumpier ride, and requires a less aggressive COM. Wood track: COM @ ~ 1 1/4". Aluminum track: COM @ ~ 3/4"
Re: Lubing/shining the wheels: Don't put graphite on the treads of the rear wheels — you want those stable, not sliding around.
(PS - I got all excited when I clicked the link and saw that Chief had responded. Bummer.)
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