Weight placement question

[John-newbie11]

I've cut my car to a relatively flat 3/8" or so and with wheels and axles it weights 1.5 ounces.

I currently have a 2.5 ounce dome about 1/2" in front of the rear axle, leaving room for 1 more ounce.

To get the cog at between 3/4" and 1", I can either put .5 ounce in front of the dome and then 2 x .25 ounce at the very back, or I can put .25 in front of the dome and .75 (3 x .25) in the rear.

I'd like to try and use the .25 if possible as I can hide those cylinders in the height of the car. But if it makes more sense to use larger cylinders (even if they stick up) as they concentrates the weight, I'm happy to do so.

Alternatively, if I rework my holes (which I'd rather not do but could if you think it makes a big difference), I can just put a 3.5 oz dome slightly behind the hole for the existing 2.5 ounce dome, and call it a day...

OR, should I move the 2.5 dome in line with the rear axle (so it's farther back than it is now) and then place the other 1 oz in front of the dome to get the cog where I want it?

Essentially, I'm not sure how much of an advantage there is to concentrating the weight using multiple smaller cylinders that are a bit more spread out.

Thank you!

 

[Castoro Racing]

I've cut my car to a relatively flat 3/8" or so and with wheels and axles it weights 1.5 ounces.

I currently have a 2.5 ounce dome about 1/2" in front of the rear axle, leaving room for 1 more ounce.

To get the cog at between 3/4" and 1", I can either put .5 ounce in front of the dome and then 2 x .25 ounce at the very back, or I can put .25 in front of the dome and .75 (3 x .25) in the rear.

I'd like to try and use the .25 if possible as I can hide those cylinders in the height of the car. But if it makes more sense to use larger cylinders (even if they stick up) as they concentrates the weight, I'm happy to do so.

Alternatively, if I rework my holes (which I'd rather not do but could if you think it makes a big difference), I can just put a 3.5 oz dome slightly behind the hole for the existing 2.5 ounce dome, and call it a day...

OR, should I move the 2.5 dome in line with the rear axle (so it's farther back than it is now) and then place the other 1 oz in front of the dome to get the cog where I want it?

Essentially, I'm not sure how much of an advantage there is to concentrating the weight using multiple smaller cylinders that are a bit more spread out.

Thank you!

Where is your cog now and did you drill your alignment at all?

 

[Castoro Racing]

Where is your cog now and did you drill your alignment at all?

Posting a picture may help also

 

[John-newbie11]

I don't really have a cog yet as nothing is in place. My original plan on this car (I think, but honestly I'm not even sure as I'm helping my whole family build cars...) was to drill a bunch of holes, and then when everything was all set to go I could just plug and play weights in different holes to get the perfect cog. But then I started reading about concentrating the weight in one location (or tighter locations anyway) rather than splitting it up, even if it's in the back...

I have not done alignment yet.

Here's where I'm at:

w/o weights (obviously), and please ignore the slight incline at the bottom of the rear (I wasn't paying attention and starting angling the rear of the car...):

the following all place the cog between 3/4 and 1":

2.5 dome, .5 in front, 2 x .25 in back:

2.5 dome, .25 in front, 3 x .25 in back (would need another hole (which might also affect the cog slightly though I assume the amount of wood being removed is minimal):

2.5 dome moved back closer to axle, 2 x .5 in front:

3.5 oz dome slightly behind the existing hole for the 2.5 dome:

 

[Castoro Racing]

I don't really have a cog yet as nothing is in place. My original plan on this car (I think, but honestly I'm not even sure as I'm helping my whole family build cars...) was to drill a bunch of holes, and then when everything was all set to go I could just plug and play weights in different holes to get the perfect cog. But then I started reading about concentrating the weight in one location (or tighter locations anyway) rather than splitting it up, even if it's in the back...

I have not done alignment yet.

Here's where I'm at:

w/o weights (obviously), and please ignore the slight incline at the bottom of the rear (I wasn't paying attention and starting angling the rear of the car...):

the following all place the cog between 3/4 and 1":

2.5 dome, .5 in front, 2 x .25 in back:

2.5 dome, .25 in front, 3 x .25 in back (would need another hole (which might also affect the cog slightly though I assume the amount of wood being removed is minimal):

2.5 dome moved back closer to axle, 2 x .5 in front:

3.5 oz dome slightly behind the existing hole for the 2.5 dome:

I think how aggressive you get with weighting depends on how good your alignment is especially in the rear. Are you placing the axles in the slots by hand or are you drilling the axle holes. If you have a decent jig or way to drill the holes you can get more aggressive. Either way I would put wheels and axles on then start measuring based on where you place the weights. You can just put set of unprepped wheels and axles so you can get a true measurement. I know most on here dont measure cog
Is this a rail runner and how many wheels touching?

 

[Charles Studer]

I like to hollow out the main body and throw weight as far back as possible... remember if you are using cog the wheels and axles weight count too....

 

[Thinkin'Bout Racin]

remember if you are using cog the wheels and axles weight count too....

Not only do the wheels and axles count ....the longer the wheelbase, the longer the lever and ultimately, the greater the effect on the COG.

 

[John-newbie11]

I think how aggressive you get with weighting depends on how good your alignment is especially in the rear. Are you placing the axles in the slots by hand or are you drilling the axle holes. If you have a decent jig or way to drill the holes you can get more aggressive. Either way I would put wheels and axles on then start measuring based on where you place the weights. You can just put set of unprepped wheels and axles so you can get a true measurement. I know most on here dont measure cog
Is this a rail runner and how many wheels touching?

We have to use the slots, no drilling. 3 wheels touching, this will be my first attempt at railriding.

 

[John-newbie11]

Not only do the wheels and axles count ....the longer the wheelbase, the longer the lever and ultimately, the greater the effect on the COG.

Standard wheelbase, we can't extend.

 

[Thinkin'Bout Racin]

Standard wheelbase, we can't extend.

Do you axles have a specified location in the block? Or is there a specific web length?

 

[Thinkin'Bout Racin]

Do you axles have a specified location in the block? Or is there a specific web length?

Man, I hate auto correct!!!!!

Do your axles have a specified location in the block? Or. Do the rules require a specific WB length?

 

[John-newbie11]

Do you axles have a specified location in the block? Or is there a specific web length?

I just checked and our rules say that the wheelbase can't exceed 4 1/4.

Also, no solid or straight through axles. I'm wondering if this doesn't mean that I can't drill holes, as long as I use regular axles.

 

[Castoro Racing]

I just checked and our rules say that the wheelbase can't exceed 4 1/4.

Also, no solid or straight through axles. I'm wondering if this doesn't mean that I can't drill holes, as long as I use regular axles.

You can always drill into the slots if you need to use the original slots. It may be helpful to fill them first.

 

[Thinkin'Bout Racin]

Sometimes it is easier to post your rules ... I would take that as no straight thru (completely thru the body) axles, or one solid piece.

Probably too late for this car, but next time move the rear axle center to 5/8" from the rear.

My comments about WB length were directed at the affect that a different WB would have on the COM. You need to have the wheels and axles in place (loosely is fine) when measuring COM.

 

[Thinkin'Bout Racin]

You can always drill into the slots if you need to use the original slots. It may be helpful to fill them first.

Correct me if I am wrong, but I believe that the stock slots are 4.325".

 

[John-newbie11]

Thank you all for your input so far, but any further thoughts on whether it's better to keep the weight 'tight' (i.e. as close together as possible, maybe even just using the one 3.5 ounce dome) vs. spreading it out little?

And/or does it make sense to have a little more behind the 2.5 dome rather than in front, or .25 here or there is negligible as long as the bulk of it is right around the cog, and the cog is in the right place?

 

[Thinkin'Bout Racin]

I don't really have a cog yet as nothing is in place. My original plan on this car (I think, but honestly I'm not even sure as I'm helping my whole family build cars...) was to drill a bunch of holes, and then when everything was all set to go I could just plug and play weights in different holes to get the perfect cog. But then I started reading about concentrating the weight in one location (or tighter locations anyway) rather than splitting it up, even if it's in the back...

I have not done alignment yet.

Here's where I'm at:

w/o weights (obviously), and please ignore the slight incline at the bottom of the rear (I wasn't paying attention and starting angling the rear of the car...):

the following all place the cog between 3/4 and 1":

2.5 dome, .5 in front, 2 x .25 in back:

2.5 dome, .25 in front, 3 x .25 in back (would need another hole (which might also affect the cog slightly though I assume the amount of wood being removed is minimal):

2.5 dome moved back closer to axle, 2 x .5 in front:

3.5 oz dome slightly behind the existing hole for the 2.5 dome:

Back to your original question... it is difficult to give a specific answer, because we don't have all of the information needed to do so.

So generally, it is better to have the weight tightly grouped. Not a requirement, but when two options are available with minimal impact on other aspects, then yes, tight is good.

We know your body weighs about 42 grams. Most people here get those weights down to under 15 grams with many at 7-8 grams. If you could, it would be benifiical to hollow out some of that weight from underneath. This could still be done on this car or on the rest that you are building. Making the body lighter allows you to put the weight where it is most beneficial.

So we don't know for sure how far back the weight needs to be placed. The best way to determine that is with the weight on each wheel. Secondarily, knowing how the car balances (actually balancing the body with wheels, axles, and different weights) gives us a reference starting point.

So this is a guess, based on what I can see in the pictures and the information you have shared so far. If you have a very good alignment, I don't think you could get the weight too far back.

If this was mine, I would mock up the third option (with 3 behind the canopy) and also slide the canopy as far rearward as possible. I would also slide the weight in front of the canopy to the DFW side. My best guess is this setup will produce a 3/4" - 7/8" COG.

 

[Thinkin'Bout Racin]

I've cut my car to a relatively flat 3/8" or so and with wheels and axles it weights 1.5 ounces.

I currently have a 2.5 ounce dome about 1/2" in front of the rear axle, leaving room for 1 more ounce.

To get the cog at between 3/4" and 1", I can either put .5 ounce in front of the dome and then 2 x .25 ounce at the very back, or I can put .25 in front of the dome and .75 (3 x .25) in the rear.

I'd like to try and use the .25 if possible as I can hide those cylinders in the height of the car. But if it makes more sense to use larger cylinders (even if they stick up) as they concentrates the weight, I'm happy to do so.

Alternatively, if I rework my holes (which I'd rather not do but could if you think it makes a big difference), I can just put a 3.5 oz dome slightly behind the hole for the existing 2.5 ounce dome, and call it a day...

OR, should I move the 2.5 dome in line with the rear axle (so it's farther back than it is now) and then place the other 1 oz in front of the dome to get the cog where I want it?

Essentially, I'm not sure how much of an advantage there is to concentrating the weight using multiple smaller cylinders that are a bit more spread out.

Thank you!

John,

I went back and reread your original post/question. I am not a pro, but will offer this ....location of weight is more important than concentration of that weight. Once you have the location addressed, further gains can be had from having that weight tightly grouped. Again, location is MORE important. Having weight further rearward increases the potential energy when the car is placed on the hill.

Just so you know, alignment is probably the #1 factor in a fast car!!! COG is also important, but less so than getting the alignment correct. A car with good alignment and a 1" cog will beat a car with a 1/2" cog and fair alignment every time. Running straight and true is IMPORTANT!!!!

 

[DerbyBoyz]

I would move the weights back like in picture four, but I would move the round weights back too. Then I would take the middle part out of the car with forstner bit and then use some tape or something to cover.

 

[John-newbie11]

I'll read all of these answers in a moment (and thank you for them), but also wanted to post one more follow-up; I've been reading more and is it not better to have the weight lower rather than higher? If that's the case, is the dome a bad idea as the weight is a bit higher than if I were to drill more holes for cylinders which would put all of the weight in the car/slightly above (rather than the bulk of it well above the car)?