Weight Behind Rear Axle OK?

[ZiggyDude]

Greetings!

We have been putting a lot of weight behind the rear axle to try and max out the potential energy. Under the plates behind the axles are round holes drilled with tungsten - two or three per car and go as deep as possible.

But when I see a lot of other pro cars it seems that they tend to put the weight in front of the rear axle. There is little room for weight behind in the body.

My CG is about 3/4" forward of the rear axle.

I was curious on thoughts about this weight placement.

The cars dominate the pack - but we were outclasses at the districts. (Wood vs. metal track was part of that I think)

My thanks in advance.

- Ziggy -

 

[TRE]

in fat wheel cars i dont put more than 2 ounces behind the rear

 

[ZZRacing]

Most if not all pro cars are 1/4 inch thick bodies. I run 12 to 14 1/4 inch tungsten cubes behind the rear axle and another 12 to 14 in front depending on the class of car. I don't check com I use three scales but for some reason last night I did and that car with full fenders had a 1/2 " com. What type of cars beat you at the district races? I would have to bet it's thin 1/4 inch thick bodies.

 

[Skywalker Racing]

I'd agree with ZZ here. Your profiles are definitely robbing you of speed.

Don't worry about COM. Squeeze as much weight as far back as you can unless the tracks you're running on are in terrible condition.

 

[DerbyDad4Hire]

If you want top speeds then you have to sacrifice stylish lines. Thin planks tapered in the front are the fastest cars.

 

[Pacfanweb]

Need to put some tape over those holes as well.

 

[Crash Enburn]

Greetings!

We have been putting a lot of weight behind the rear axle to try and max out the potential energy. Under the plates behind the axles are round holes drilled with tungsten - two or three per car and go as deep as possible.

But when I see a lot of other pro cars it seems that they tend to put the weight in front of the rear axle. There is little room for weight behind in the body.

My CG is about 3/4" forward of the rear axle.

I was curious on thoughts about this weight placement.

The cars dominate the pack - but we were outclasses at the districts. (Wood vs. metal track was part of that I think)

My thanks in advance.

- Ziggy -

Ziggy - for what you've got, you're doing fine. As you refine your builds, you'll start getting to much closer tolerances and will get the weights completely enclosed in the bodies.

You are correct in that the pros all have weight behind and in front of the rear axles. The pro bodies are all approx. 1/4" high and weigh less than 10 grams. This lets them get more weight densely located at the back of the car (lower in the body is better than higher). The typical weight opening behind the axle is 1.5 inches across and 1/2" to the axle beam — this means that the sides of the car are 1/8" thick, and the axle beam being 1/4" across, with another weight pocket in front of it.

In future cars for your kids, you can keep the same shape freedoms, and yet make the bodies shorter (less tall). Using lead instead of the zinc Revell/Pinecar weights will get you more weight in a smaller area (lead being 1.6x as dense as zinc). Tungsten, the weight of choice, is another 1.6x denser than lead — 24 1/4" cubes weigh ~4 oz., so all the weight fits into a 1.25" span in the back of the car.

Lead, btw, can be gotten cheaply if not free (ask at a tire shop) , be melted over a camp stove, and then poured into whatever opening in the car (a 1/4" hole is too small to pour into). Tungsten is more expensive ($20 for 4 oz.) and cannot be manipulated easily.

 

[gdurban]

Need to put some tape over those holes as well.

+1

you will gain speed by covering those voids to reduce turbulence under the car.

 

[slipstream]

I have found if you do lead you need to cut the back deeper about 2 cubes stacked 1/8th to get close to 2 oz behind the rear axle (I get 1.6+). In front of the axle you can do 1/4 deep (this one is) 5-1/2 wide x 3-1/4 cubes long (2.3+) to get it up to weight if your body is under 15 grams. This puts the COM around the 3/4 mark. I will post pictures of the current one I'am working on for my brothers kids. Right now its not sanded and weights 5.15 maybe a bit more.
Side note: Do NOT pore lead into a drilled hole on a thin car the hot lead WILL crack the body. I have not cracked a body with weight pockets and drill axle holes last. if you look close you can see the weight of the lead i wrote on it.

 

[ZiggyDude]

My thanks to all for your input. Yes, the fastest car in the race was a flat like you are all talking about. It was shaped like my red car with the center removed. Some of the faster cars were also lower but not all of them. We ran the blue and green car in the Webelos 1 division.

We were using a combination of tungsten cylinders in drilled holes and zinc flats put over the hole. I will get the tungsten plates for next year.

Since the inside of the car is hollow - how do you keep the cubes from falling out? Glue would not seem strong enough and something like duct tape would worry me as too soft.

 

[Skywalker Racing]

I will get the tungsten plates for next year.

Since the inside of the car is hollow - how do you keep the cubes from falling out? Glue would not seem strong enough and something like duct tape would worry me as too soft.

1/4" tungsten cubes work great and are easy to install.

http://derbydad4hire.com/Tungsten-Cubes-S-04.htm

You can glue them in or use a double sided tape to hold them in the car body. Once you have them where you want them, cover them up with a foil duct tape or muffler tape. You could cover the entire bottom with one long piece of this tape to improve your aero.

 

[Crash Enburn]

Since the inside of the car is hollow - how do you keep the cubes from falling out? Glue would not seem strong enough and something like duct tape would worry me as too soft.

Surprisingly, white glue does a remarkable job of holding the cubes — they do not come out easily, and not without removing wood from the car. I don't recommend it as even after you've got the cubes out, you then have to file the cubes to get the glue and wood back off.

Use double-sided tape to hold the cubes to the car, and then cover the bottom of the car (cubes) using aluminum tape for ducts. This way, the cubes are held securely enough, but you can take them back out with little effort.

I have found if you do lead you need to cut the back deeper about 2 cubes stacked 1/8th to get close to 2 oz behind the rear axle (I get 1.6+). In front of the axle you can do 1/4 deep (this one is) 5-1/2 wide x 3-1/4 cubes long (2.3+) to get it up to weight if your body is under 15 grams. This puts the COM around the 3/4 mark.

This is a common misconception. The idea is not that the CoM has to be 3/4" in front of the rear axle (wherever it is), but rather that the CoM is as far back in the car as you can get it (and stays at least 1/2" or more in front of the rear axle, lest the car become a wheelie machine).

Using lead, you're not going to have as dense a weight area to work with. If you make the car and the weights thicker, to keep the weight more densely located around the rear axle, you're giving up aero. If you go with a thin car, the lead is going to have to be spread out, and your CoM won't be as far back in the car. There's also the moment of inertia to consider (the car rotating through the transition); I don't know which combination is best: thicker, but closer to the axle, or more aero, but spread out.

One of the last lead cars I built is 1/4" thick. I put the rear wheels at 5/8", and routed pockets fore and aft of them. The back 2.5" of the car is lead-filled, and the CoM is at 1" in front of the axles.

 

[ZiggyDude]

One of the recurring points that people have made is the need for aerodynamics. I think that I was surprised by that in some ways as there are so many people (not the pros on this board) who think aero does not mean anything as long as your not a block. I have seen races won by cars that were in no way slippery - though that does not mean that they beat tough competition I guess.

I do trust the collective wisdom of this board and am working on ways to lower my profile for next year. Currently every thing my son and I build is based on the standard BSA wedge body. The highest point of the body is about 1/8" higher than the wheels. So, I guess I need to cut half of that off.

If you look at the pictures above, the center red car also removes wood from the outsides of the body while the other two are more like a plank. Sort of like an Indy or GP car. I don't see a lot of people removing that wood. Is there a downside to that?

Thanks

 

[MrRoadWarrior]

Ziggy, here are my district old timer (winning) cars from 2015-17. My kids love the look of cutting the sides off, but it's an aerodynamic disadvantage. The design pushes air into the wheels and creates more surface area. Cutting the center out and covering the body ($2 Short, left side) is a faster design. There is a lot of good information on the ladder cars thread on this website.

 

[jatofau1]

You might want to measure your COM with the wheels installed. This will more accurately reflect the COM.

 

[ZiggyDude]

I noticed that $2 and other cars moved the wheel base to the back of the car. I found that interesting

 

[B_Regal Racing]

One of the recurring points that people have made is the need for aerodynamics. I think that I was surprised by that in some ways as there are so many people (not the pros on this board) who think aero does not mean anything as long as your not a block. I have seen races won by cars that were in no way slippery - though that does not mean that they beat tough competition I guess.

Judge for yourself whether aero plays a part in derby racing. Watch the following two videos:

 

[ZiggyDude]

Impressive!

I find it interesting that adding weight to take the car from 4.67 to 4.80 made almost no difference while the fenders did. I am not sure that our rules would allow them due to ground clearance though.

But this certainly proves the height of the car should be kept low!

I am convinced!

 

[jatofau1]

What are your clearance rules? Most clearance rules are put in place to make sure the car clears the center rail of the track. Always run fenders if you can. You can make the fenders or just buy the GEE fenders from John. I buy Johns fenders and put them on my kids scout cars because I do not have time to make fenders. My kids cars have won the last three years in the pack and done very well at the district race,

 

[MrRoadWarrior]

I noticed that $2 and other cars moved the wheel base to the back of the car. I found that interesting

We have to use the stock wheelbase so we started cutting off the block 5/8" behind the rear axle and attaching it to the front. I think it looks funny but it places the weight back. We're still 3/4" COM at that wheelbase.