Weight placement question
- Thread starter John-newbie11
- Start date
The short answer is yes. There will be small differences in the amount of potential energy based on the height the weight is above the surface. There are MANY other things to perfect along the way (like alignment and wheel prep) before the small difference in height of the weight will make a significant difference in the performance of your car.
So, what I am saying that moving forward, you might want to consider placing the weight inside the body behind the rear axle ( 1.8-2.1 ounces). You first need to get the body weight down so you can add the weight where you want.
By placing the weight inside the car, you also gain a small amount of aero advantage .
Depends on what you’re going for here. If you’re just wanting to win Cub Scout races, your setup will be fine. If you want to win league races, you’ll have to change how you do your weight. I’m sure if you search I’ve posted pictures of my cars and how I put weight in them. The thickness of the car was as thick as a tungsten cube. I would stick I believe 12 behind the rear wheels and have about 12-16 more in front with putty to even the weight out. Weight is another trick of the trade as far as placement and your COG. Your COG will effect the tuning of your car. The further forward you COG, the more aggressive rail running you can have. The further back COG, less aggressive. But then you’re just toying with the wiggles and finding the sweet spot. I always like 1/2” COG to 5/8”.
To clarify, the car actually weight 1 ounce (28 grams or so) and the wheels/axles are another .5 or so (hence the 1.5 total before adding weight). I would rather not start hollowing out too much underneath, as the car is very thin (for me anyway, as in the past we just did simple wedge). Anything I cut is by hand, either with a cheap router tool I bought (which could work as I can set the depth) or drilling by hand (so I could accidentally go through the car). Of course, I could purposely go through the car and have lots of holes (I'm ok with that design as long as it doesn't interfere with wind resistance).
In any case, I went ahead and loosely put on wheels and axles, and I'm able to keep the 2.5 ounce dome where it is, and put two .5 ounce weights in the back, and the cog is at about 7/8". I think it will come down slightly as I should be able to add some putty at the back when all is said and done.
Do you think I'm pretty good where am I at (in terms of weight placement/cog), or would you still try to slide the canopy further back? I'm hoping I can get the alignment to be decent, but won't be going crazy with it.
This is just for our local pack race, nothing crazy...
Personally, I don't like to have any weight exposed outside of the body. I do try to keep my weight lower, but if you put something underneath the car, beware of the stop section.
I beleive they are 4 3/8". (I just checked the NYC rules and stock is quoted as 4 3/8. That may be the most clear part of their rules)
I read that stock slots are needed and the wb needs to be minimum 4 1/4". If stock slots are needed you can always drill into them. If not then extending the wheelbase and pushing rear axles back a bit is better.
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Thank you (to everyone). This is just for cub scout races, nothing more. We have another car whee we're putting cubes underneath in hallowed out sections, for this one I'm ok with not hiding them and, given that it's just for a scout race, I'll probably leave the dome as is (even if it's a bit higher than weight should probably be).
If you can't add the third cylinder in back, then, if it is easy, I would suggest moving the canopy rearward. If you don't, the car will run fine. I was just trying to point you in the right direction for the fastest option with what you have.
You won't know how fast you need to go until you see how you stack up against the competition. You may already be faster than most there, or you may need to step it up next year. No matter what, you have found a great place to ask questions.
Also, be sure to cover any holes or openings with tape or clear packing tape. It does make a difference.
Work on getting the alignment the best you can, it will pay dividends!!!!!
Thank you. Can you point me in the right direction (tutorial, specific thread that you recommend...) to look into alignment? We've done pretty well in the past w/o doing much (if any) alignment, so I'm curious how much better we can do if we actually try to do it right. However, I don't want to drill any axle holes or otherwise go crazy (not that that's crazy in your guys world I'm sure). I'm attempting to rail ride on this car and will go for the 1" (I think) over 4'.
Go with what you have and have fun. If you where asking which is better as in your dome weight you have, I would pick a flat bar like aTX weight every time. Which sets lower as that weight you have which is what I was meaning. Every minimal speed gain you do adds up to be come big gains.
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There is a video on you tube called the physics of Pinewood Derby. Actually there are about three by the same person. They each have a different run time based on the edited version. I believe that the BSA even has a link to it on their web site. It does a good job of explaining a number of things about the physics for a newbie. It gets a few things wrong, but overall it is a decent place to start. Especially for a Cub and parent that have not done a rail runner yet.
Ignore what he says about graphite and the amount of steer needed. There is a difference in graphites if that is what you are using. Also, with a 4 3/8" WB, you are going to want at least 4- 5" of steer over 4 feet in a Cub Scout race.
I am not trying to Open Pandora's box here, as he uses bent axles to achieve rail running. Yes it can be done that way, but it is more work to set up the car correctly and even more work if you ever want to reprep the car and run it again. However, using bent axles is a lot cheaper that buying a drill jig.
Try searching Lighting boys alignment method. He has a couple of really good videos on checking your alignment. If you choose to use bent axles, pay close attention to his speed square method as you will want to use that for your set up.
Higher rear weight reduces stability out of the hill ....but with the less aggressive com of say 1" that should be easily tuned out with a touch more steer..IMO
You could throw three wide rear of axle and cover with dome..... but it will need a skilled tune
As I'm now re-thinking using the dome, but don't want to cut out a slot for a bar (I don't think), what are you thoughts on just drilling for more 3/8" cylinders? .25 can be fully sunk, .50 ounce would stick up half way (but still have half down in the body). I would need five .50 to equal the 2.5 dome (or some combination of .25 and .5).
Or is this getting over complicated and should I just stick with the dome?
Or is this getting over complicated and should I just stick with the dome?
Yes, over complicated. Stick with the smaller dome and push rearward if possible. Add more weight lower in the car as far and rearward as possible.
With your COM being at least 0.875", the car can be stable. I still like to build lead/graphite cars because I see it as a challenge to compete with the local tungsten/oil cars. My lead wedges are about 0.650 tall in the rear because of using lead. So they probably have a higher COM than what your canopy weighted car has. With good alignment, and a 4 1/2" WB, I get the COG down to around 0.465".
Ideally, your next car should have the weight inside the body.
In thinking about it further, I could replace the dome with a 3 oz cylinder that's only 1/4" high. I'd need to drill a 1 1/8" hole around where the 2.5 dome is. However, I could then only have .5 in the back vs. having 1 in the back with the 2.5 dome.
So... is it better to have the 2.5 dome with 1 ounce in the back (but the dome sits high) vs. 3 ounce in front of rear axle (but nearly fully counter sunk) with just .5 ounce in the back?
Ok, your overthinking this ....with as heavy as your body is (I know it probably seems light to you), it is more important to put the availabile weight toward the rear of the car. Having a dome will cost you only 3-4 thousands of a second. While that is a lot at the pro level, it is an acceptable loss at most Scout events. Put your efforts more into alignment and wheel prep. Then on your next build continue to make improvements not only on wheel prep and alignment, but also on where to place the weight.
A decent rail rider can gain you 2-3 tenths of a second vs. a car that wiggles going down the track.
A decent rail rider can gain you 2-3 tenths of a second vs. a car that wiggles going down the track.
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Stick with the dome and move it back a bit. The amount of speed you would gain by moving the weight beneath the car will be trivial, and the amount you will gain by moving it back will be significant.
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