Weight Placement

General topics of interest to racers and race coordinators alike.
Shawn Stebleton
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Re: Weight Placement

Post by Shawn Stebleton »

Well, my son's car last year was not the fastest to the bottom of the incline entering the curve in all of its races, but always had the most speed exiting the curve. Alignment was almost spot on (probably the most crucial aspect of the build), but the extra speed coming out of the curve sure doesn't hurt, and he won 1st place in the district. The extra speed, or rather the loss of less speed, was noticeable immediately upon exiting. How much of that is attributable to the alignment and how much is attributable to the concentration of mass is up for debate. Personally, I believe the concentration of mass was the greater factor in the transition. Of course, it had no effect on the ramp (straight incline) or the flat.
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Re: Weight Placement

Post by Speedster »

Shawn, does your Team race on a "Best" track only or is there a circular arc track involved? How long is the "Best" track?
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FatSebastian
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Re: Weight Placement

Post by FatSebastian »

Speedster wrote:does your Team race on a "Best" track only or is there a circular arc track involved?
Earlier, Shawn said he was racing on an older Best Track with a sharp radius. Also, based on the above description "...bottom of the incline entering the curve...", I will assume a Best Track...
Shawn Stebleton wrote:Well, my son's car last year was not the fastest to the bottom of the incline entering the curve in all of its races, but always had the most speed exiting the curve
:thumbup: That the car was lagging down the incline suggests that the DFW was "dragging" the rail relative to the competition. That the car had more speed into the flat suggests that the CoM placement was more aggressive than the competition. Both are reasonable characteristics of a good rail rider IMO.
Shawn Stebleton wrote:How much of that is attributable to the alignment and how much is attributable to the concentration of mass is up for debate. Personally, I believe the concentration of mass was the greater factor in the transition.
:thinking: A favorable CoM placement simply explains the extra speed out of the curve.

We have several tips booklets which claim that raising one of the front wheels results in 25% less friction, when in fact raising the wheel reduces the rotational inertia by 25%. Raising the wheel was still the correct way to go empirically, even if the reasoning was wrong physically. Concentration of mass may very well play some minor performance role, but perhaps for reasons that are different than "pitching" energy lost to transitional rotation as is sometimes claimed?

A competitive CoM requires a relatively low moment of inertia (i.e., the CoM can't be far back if there is too much forward weight, and there is only so much space for weight behind the rear axle, thus precluding "dumbbell" weight distributions). Again, theory predicts the total time lost from pitching rotation v. no rotation is on the order of a millisecond or two, such that the differences from this effect between competitive cars would seemingly be no more than that.

OTOH, in his instructional DVD, Scott Acton experimented with cars having extreme mass distributions and concluded that barbell weighting seemingly correlated with increasing yaw when the car was perturbed. He hypothesized that mass concentration might result in fewer and milder impacts with the rail (he was not rail-riding and not using a Best Track). Without a complete physical explanation to quantify how changes in mass MoI affect performance, it becomes harder to advise Black Fox & son as to why they should alter their design, and by how much.
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Re: Weight Placement

Post by Black Fox »

Gents

Thanks for all good info. However, I have to admit that the closer I get to physic equations, the more my brain hurts. It works better for me if I can see something. So I've added some pictures to help.

First are some pictures of the track we are using. Unfortunately I don't have one that shows the bend but it seems most of you know your tracks well and should be able to figure it out from the pics.

Image

Image

Image

Now here are some of the weight placements that I have. (See pics below). The 3 plates are all I have that will fit vertically. I have others but they are too tall if I stack them that way. I have a lot of 1/4 and some smaller plates and disk. Also I can't stack the cubes on top of one another. They will sit above the body if I do. Oh, I will cover the top once the weights are glued in place.

Now In the first pick I can get 10 cubes in the behind the axle but I need to put a 0.05 oz weight in the front to give me a COG just a hair under 5/8.

Image

Now in the second picture I have all the weight near the axle. But in order to do this I had to remove two 1/4 cubes and put the weight in front of the axle. This still gives me about a 5/8 COG.

Image
Now that you have seen the pictures I have two questions for you.

But first I want to let you know that I want a COG of no less than 5/8. I don't have a track to test these on and I don't want to risk having the wiggles with an aggressive COG. Also, we are dealing with a standard cub scout wheel base.

1. Is it worth placing the 0.05 square tungsten plate under the front wheel if it means I can get 10 1/4 in cubes behind the axle or have all the weight near the axle but with only 8 1/4 tungsten cubes behind the axle?

2. With this type of track. Would I be better off with the weight behind the axle as shown in the pics or push it all the way back? From reading all the post it seems I may depend on the type of track you're on.
Shawn Stebleton
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Re: Weight Placement

Post by Shawn Stebleton »

Speedster wrote:Shawn, does your Team race on a "Best" track only or is there a circular arc track involved? How long is the "Best" track?
Our pack's track is a Piantedosi Classic (wooden) 3-lane track, about 40' in length, but we use the junction of the last two sections as the finish line. Everything is judged by eye. No electronics. It has the gentle arc curve.

The district lately has been using one particular pack's track for its races. It's a Best Track with the old-style sharp curve, and it's set up for 35' + stopping section. There's the start section, the curve seen in the first picture, and three flat sections, and a stopping section that doesn't stop the cars. A blanket does. This particular track is brutal. Lots of cars have hit joints (they're not tapered like the Piantedosi / Microwizard design) and some have even been popped up enough to either get a wheel in-between the rail guides or totally knocked off the track. I've posted these pictures elsewhere, but for your info:


The full curve section -- looks to be about 7' long:
Image
I believe the replacement curve design is only about 4' long. If trying to figure out what type of Best Track you are running on, use this as an example of the old style.


Closeup of the "curve" -- looks to really be several small bends with short straight sections between:
Image

An even closer view of the "curve":
Image


Black Fox, the track in your pics look like a Piantedosi / Microwizard Freedom track, because it looks to me like it has the gentle arc curve of that type and is much thinner so it can flex to make the curve. Also, the two sets of screws in the 2nd picture I believe is from their design. If you saw barbell weights under the curve, then it most likely is.

The Best Track is very thick and must have a permanently-formed curve section. You can see the effect by looking at how much the I-beam is bent underneath in the last picture.
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Shawn Stebleton
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Re: Weight Placement

Post by Shawn Stebleton »

Black Fox wrote:But first I want to let you know that I want a COG of no less than 5/8. I don't have a track to test these on and I don't want to risk having the wiggles with an aggressive COG. Also, we are dealing with a standard cub scout wheel base.

1. Is it worth placing the 0.05 square tungsten plate under the front wheel if it means I can get 10 1/4 in cubes behind the axle or have all the weight near the axle but with only 8 1/4 tungsten cubes behind the axle?

2. With this type of track. Would I be better off with the weight behind the axle as shown in the pics or push it all the way back? From reading all the post it seems I may depend on the type of track you're on.
If it were me, I'd put the weights as you have them in the 2nd option, but for the gentle arc curve it most likely doesn't matter. It isn't a quick rotation then a quick stop of the rotation, like with the Best Track.

If your vertical CoM is at the plane of the axles, or very close, then you should be good.

Some people like it lower for stability, and it can also aid a very small amount on longer tracks with the mass falling just a little bit further to give it more speed, due to the greater potential energy at the starting gate.

Some people like to put the mass up higher if the finish is not very far from the bottom of the curve, as the CoM will then travel a shorter distance. It will get to the bottom of the curve faster, but will exit with a bit less speed. The shorter track will favor this, while on a longer track the lower weighted car would use the extra speed to catch up to and pass the higher-weighted one, all other things being equal.

Just try to make your car more equal than the others! :mrgreen:
Shawn
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Re: Weight Placement

Post by FatSebastian »

Shawn Stebleton wrote:It will get to the bottom of the curve faster, but will exit with a bit less speed. The shorter track will favor this...
:/ This is correct based on physical principles (ignoring more sizable effects such as aerodynamics). Yet even Michael Lastufka, the originator of the "Warp Field" hypothesis, suggests that it is "much ado about nothing" and that "taking advantage of the warp in your track, may be an attempt that gets 'lost in space'." This worked-out example also affirmed that the effect is imperceptibly small.
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Re: Weight Placement

Post by Stan Pope »

Shawn Stebleton wrote:The district lately has been using one particular pack's track for its races. It's a Best Track with the old-style sharp curve, and it's set up for 35' + stopping section. There's the start section, the curve seen in the first picture, and three flat sections, and a stopping section that doesn't stop the cars. A blanket does. This particular track is brutal. Lots of cars have hit joints (they're not tapered like the Piantedosi / Microwizard design) and some have even been popped up enough to either get a wheel in-between the rail guides or totally knocked off the track.
Challenge your organizers to prove that the track is well set up and aligned by sliding a square-edged block of pine along each side of each rail at each joint, moving the block in a "down-track" direction. Anywhere the block "catches", the track is misaligned! (Since there is a mild fillet at the join of rail to track, you might have to relieve (chamfer) the long edge of the block so that the block will ride flat against the track and flat against the rail.)

The challenge is best presented well prior to the start of racing so that there is time for correction and little excuse to not do so!
Stan
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Black Fox
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Re: Weight Placement

Post by Black Fox »

Shawn Stebleton wrote:Yes, Black Fox, the 2nd picture with the weights close together is similar to how I taught my boys to do it.

Also, Speedster is right. Cut off the extra wood in back (if allowed by your rules). We do that. We use a craft stick as a front wing, epoxied in (2-part 5-minute epoxy) for strength, instead of gluing back on the sawed off wood from the back. We have a brutal stop and the craft stick has more strength than regular pine.

Here are pictures of my son's car this year:
Image
Image

And a close-up of the weights:
Image

We used 8 tungsten plates from MaxV, but oriented in a vertical arrangement. In retiring an old laptop computer many years ago, I had saved the screws and nuts. They are size M3, about 30mm long. I had to drill out a very minimal amount of the hole in the center to get it to fit, but it worked fine. The tungsten was 4.03oz by itself. There are several pieces of lead golf tape stuck on the rear end of the weights for 0.07oz, and the weight of the screw and nut itself, also 0.07oz. The rear wheels and axles are 0.23oz. I don't know how much the wood and epoxy weigh, but they aren't 0.00. That's about 4.40oz in the space of the rear wheel. Yes, I know a small bit of the screw extends in front of that, but I'm using that to offset the weight of the wood and epoxy. The CoM is at the nut, which is at the front edge of the rear wheels. The lead tape at the back of the rearmost tungsten plate is just under 15/16" behind the CoM, which means that 4.14oz of the 5.00oz is rotating within less than one inch away from the CoM.

The car body is 5/8" thick at the point where the plates are. That accounts for the plates and the wood top (very very thin). It's minimally thicker than a car with 1/4" cubes with a car body on top of it. The cross-sectional increase is very small. Underbody clearance is 5/16". (No 3/8" requirement--it just had to fit the track in this regard.) The vertical placement of the CoM is just a small bit above the axle plane--about 1/8". Not too bad.

Explanation for the compact weight design:
Think of a see-saw with twins, one on each end, and one of them up while the other is down. How much force does it take to rotate it so that the other twin is up? Now have the twins move to the middle, and start with the board in the same position. Now how much force does it take to rotate it? The Pinewood Derby car rotates from the start to the flat and has to use energy to rotate. That energy comes from the only place it can--the potential energy the car started with. If you lose less energy during the rotation of the car, you have more left over for speed. By spreading the weight out, it takes more energy to rotate.

Note: This car was specifically designed for the old-style Best Track with the 2' radius curve. The rotation starts very quickly and abruptly, the rotation itself is fast, then it is immediately sent to the flat just as quick. I don't know if a car on an arc-curve track like our pack's Piantedosi Classic track would necessarily benefit from this type of design, but we had to design one car, as we could not change the car for the district race. Perhaps weights more spread out and lower would be better for the arc-curve. I don't know. I just know that the cars my son has made the last 4 years, using this weight design, came out of the transition with more speed than the competitors.
Shawn

Thanks for all your help. After seeing your weight placement I did the same but I used Car Pine plates because they were shot enough ti fit in my son's thins body design. I filled the gaps with balsa wood and a little glue. Once it dried, I covered the top with a thin sheet of bass wood. This allowed me to create a smooth aerodynamic body on the top and bottom.

During the race the car had great speed going down and on the straight away but seemed to slow down on the last 4 to 3 ft on our 42 ft track ( and yes it does have a gentle curve. The track is all flat then the rear is lifted to 5 ft high.). So I'm wondering if this weight placement was the cause of the slow down at the very end or something else. Like to hear your thoughts on this.

Anyway, his car still took first but It was close though. In the beginning he was very fast but has a slight wiggle. It got worse as the race progressed. We had issues of the front steering wheel axle being a little lose so we stuck dental floss in it to make the axle hold buy figured it may have been loosing its hold. His speed started decreasing as a result of the increasing wiggles. In the finals (6 there were six races) the first the first one he took second, the 2nd one he took 3rd and I figured at this point he would take 2nd at best. But the suddenly the wiggles went away and he had dramatic speed improvement. That allowed him to take first in the last 4 races. I'm thinking the front drive wheel was some how bumped and improve his steering. It made for a very exciting race.
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