Weight Placement & Testing

[gWebber]

I had an idea for being able to put weights in a car, do some testing and then be able to move the weights with (I think) reasonable ease.

First part of the idea is placing the wieghts so they stay, but can be moved.
I'm thinking this could be accomplished using "Museum Wax" or "Blue Tac".
(Blue Tac leaves less residue I think).

Both are used for tacking an object in place but are removable materials.

Step two is using foil tape to hold while testing , but placing paper over the wieghts so the tape doesn't stick to the wieghts, just to the body ... this should allow for removing the tape, make even tiny adjustments to the wieghts and put back the tape for more testing.

Might even eliminate glueing altogether.

Thoughts ?

 

[gWebber]

Also on the subject of wieghts .. I can get 12 tungsten cubes (2.030 oz) behind the Axle easy, but I could also get another 0.510 more adding a tungsten plate on top of them and even a good bit more stuffing the rear gap with tungsten putty (and still clear the track).

So the question is .. how much is too much behind the Axle?

 

[King Racing]

CN I use(and many others) double side carpet tape in the weight pockets and foil tape to cover the weight.

 

[B_Regal Racing]

I would not put any more than 2.25 ounces behind the rear axle. While it gives you more horsepower down the hill, its harder to carry the speed in the flat. At least, that's a theory of mine...

 

[gWebber]

CN I use(and many others) double side carpet tape in the weight pockets and foil tape to cover the weight.

I just thought the wax would be an easier option for moving weight around during testing, less grip than carpet tape and nearly forever reusable

 

[txchemist]

Let's take some data. First let's load up a typical Pro style thin body with enough cubes to get to 5 oz. OK, we needed 26 cubes.

Now, we use TxW 99.95% tungsten bars and find we have achieved the 5 oz. with just 24 cubes of volume when it is the denser tungsten.

Now let's put our first car on three scales and see what the DFW (Dominant front wheel) weight is, and how much imbalance we have on the back two wheels when the cubes are loaded symmetrically.

We have 12.9 grams on the DFW and one back wheel is 71.6 grams while the other is 56.9 grams. Another way to say that is the imbalance is 14.5 grams.



So after many years, a few clever racers started moving cubes around to see if they could find a "sweat spot" where they got a faster car.



If we move enough cubes to the DFW side, we see that the back imbalance can drop to very low, less than a gram-BUT, the DFW weight went up to 13.89 grams, so with the typical cubes we can improve balance, but it increases the DFW weight- in other words, your COG is slightly worse when you move the cubes and so some racers do not find any additional speed when they look at balance.

Now let's look at the same body, the same wheels and axles, but we use TxW 99.95% tungsten bars to weight the car. We already know we only need 24 cubes of volume, so first we put the bars on to minimize COG.

We notice that the imbalance is only  12.9 grams, and bingo! we have reduced the DFW weight to 11.78 grams. This car should be faster than the car with 26 cubes, no matter how those cubes get arranged.  Can we get even faster? How about we arrange the smaller bars and look for good balance.

We have an imbalance of only 5 grams, not bad, and what happened to our DFW weight? NOTHING!, If anything it went down a bit. to 11.74 grams.



Yes, this car can go faster than than all the cars above. But if we are clever to move the cubes and bars around in front of the axle to look for improvements- why should we not move the weight behind the back axle to the DFW side and see how it performs? So here is a look at the body that produced the fastest Cub time at the Mid American Pinewood Derby in 2018.

We have imbalance down to just over 1 gram, and the DFW weight is only 10.2 grams. As you can see, we have plenty of room to find the right DFW weight to get maximum speed without degrading our good balance. At least with the combination of TxW weights and the TxW body, the serious racer can find some speed.



Our results are summarized in the following chart

 

[gWebber]

After taking some suggestions to trim the body to six in a row in front of the axle, I reset all the other numbers to represent the car as accurately as I could.
P.S I'm measuring the wheel at the hub, not the tread as they are Razor wheels ... is that the correct way?

Unfortunately now the DFW weights nearly two grams more than it did before and no matter how I place the weights it stays very close to this weight.

While I could take the risk of removing wood on a fully painted car to get the DFW weight down, it would unbalance the car and reduce the car's overall weight meaning I'd have to add weight to get to 5oz, then rebalance the car yet again (not easy with one scale) and odds are I'd still be heavy on the DFW after adding more weight unless I gut it completely.

Given that I already cut thought the paint slightly with the x-acto knife, I'm not sure I want to gamble with a dremel tool.

As the car stands it balances on a 1 mm thick ruler edge right where the diagram shows and I'm off center by a tiny fraction.

So at this point I think given the tools I have , it would be easier to build a new car from scratch than to modify this one further.

 

[B_Regal Racing]

In this case, I would race this current car as is and apply what you learned in a car that is designed for it (or at least, that's what I do).

 

["The Fonze"]

In this case, I would race this current car as is and apply what you learned in a car that is designed for it (or at least, that's what I do).

Yeah c’mon George send that thing in already!

 

[gWebber]

Yeah c’mon George send that thing in already!

I need to prep the wheels and axels and tune it. So maybe racing in Sept?

 

[Derek Konrad]

Hey this is a great visual, a few questions, noticed that your Left side where your DFW is looks as if the car body is thinner than the right side? Is that accurate or is that just the photo? If yes, does that introduce a natural steer angle by doing that? Just noticed this

 

[gWebber]

Yes, the body is narrowed on the dominant side.
It doesn’t create any steer as it’s drilled straight.
A bent axle is used for steer.

 

[ScoutDadinVA]

After taking some suggestions to trim the body to six in a row in front of the axle, I reset all the other numbers to represent the car as accurately as I could.
P.S I'm measuring the wheel at the hub, not the tread as they are Razor wheels ... is that the correct way?

Unfortunately now the DFW weights nearly two grams more than it did before and no matter how I place the weights it stays very close to this weight.

While I could take the risk of removing wood on a fully painted car to get the DFW weight down, it would unbalance the car and reduce the car's overall weight meaning I'd have to add weight to get to 5oz, then rebalance the car yet again (not easy with one scale) and odds are I'd still be heavy on the DFW after adding more weight unless I gut it completely.

Given that I already cut thought the paint slightly with the x-acto knife, I'm not sure I want to gamble with a dremel tool.

As the car stands it balances on a 1 mm thick ruler edge right where the diagram shows and I'm off center by a tiny fraction.

So at this point I think given the tools I have , it would be easier to build a new car from scratch than to modify this one further.

View attachment 1016 View attachment 1015

Hi gWebber! What software was this or is this something you designed? I was thinking of this as I was helping my son build his car last week. We balanced the RWs but was tricky to do manually cause I already cut the car into a hour glass shape. Was lightening fast and broke our track record, I think I had 22grams on the DFW. prob .625-.64 COM

 

[Hangtime]

Resurrecting an old thread…

That app is known as jsfiddle. Here’s a link:
http://jsfiddle.net/minionsracing/UUnT5/embedded/result/

 

[ScoutDadinVA]

Resurrecting an old thread…

That app is known as jsfiddle. Here’s a link:
http://jsfiddle.net/minionsracing/UUnT5/embedded/result/

Thank you!!!!! New poster here so this is helpful.

 

[Hangtime]

Welcome. Happy to share. Lots of good info to be found on this site.

 

[ScoutDadinVA]

Welcome. Happy to share. Lots of good info to be found on this site.

Definitely! In one week I feel I have tripled what I know about racing PWD. I have only one question at this point on that JSfiddle, I thought the weight on both rear wheels was supposed to be balanced to get a COM around the center? What am I missing, it is telling me the left weight on scale rear should be 30 grams higher apprx vs right. Confused.

 

[Hangtime]

Jsfiddle assumes a three wheel touching car. You need to offset your weight to the dominant side to balance the X axis. Offset fore and aft to get the weight you want on the DFW. Most here don’t don’t watch COM. But they know how much weight is on each wheel.

 

[ScoutDadinVA]

I get that completely and that is what I did on my last car, ended up with the same weight on both back wheels (around 58g) and yes three wheel car. If I put those values in the Weight on Scale boxes it shows my center (red line) way off to the left. Is this supposed to be this far off center or do weight on scale mean something different. Thanks.

 

[ScoutDadinVA]

I get that completely and that is what I did on my last car, ended up with the same weight on both back wheels (around 58g) and yes three wheel car. If I put those values in the Weight on Scale boxes it shows my center (red line) way off to the left. Is this supposed to be this far off center or do weight on scale mean something different. Thanks.

Nevermind I figured it out, the DFW was throwing it off, I wasn't accounting for that weight. Thanks.