Weight distribution

[Wrath]

First time poster here....I found this forum last year while helping my 8 year old daughter compete in her first Girl Scout derby race. This site has been an amazing resource, so I wanted to start off by saying thank you for all the pros and experts whom have spent so much of their free time helping others! After using the tips I learned here, we took 1st place in her Girl Scouts service region last year. This year's race is a few weeks away so I wanted to get some feedback about our build for this year...trying to improve on our build from last year.

I'm having a tough time getting 12 blocks behind the rear wheels and 12+ blocks in front of the rear wheels. We are required to use the standard derby kit (block, wheels, axles) provided to us, and we are required to use the existing axle slots.

The body weight is about 14 grams (without wheels and axles), so by the time I add 10 tungsten blocks behind the rear wheels, 12 blocks in front of the rear wheel, and wheels/axles the total body weight is about 4.80 ounces (see attached pic). Then I add tungsten putty to make up the total car weight of 4.97 ounces.

Because I don't have a track to test on, I tried to keep the COG around 3/4 inch in front of rear axles to be conservative/safe. When I use 3 scales to figure out the weight distribution on each wheel, I get 47 grams (dominant rear wheel - left side), 67 grams (non-dominant rear wheel - right side), and about 27 grams on the DFW.

I feel my weight distribution for each wheel seems off, but I don't know how to fix it and still keep my COG around 3/4 inch. Is there any advice on what I could do better? Should I push the COG back to 5/8 inch, shave off more of the body so it's lighter than 14 grams, etc...

Thanks in advance!

 

[Castoro Racing]

Could you take a pic of the whole bottom and let us know more about how you built the car. Is it 3 wheel touch, railrider, how did you set up alignment, stock wheels. I think there are some major variable that will help get you in the ballpark or correct weighting.

 

[Castoro Racing]

How are you cutting the weight pockets would it be easy for you to get off some more wood in the rear.

How is the track, did you notice nicks or cuts in your wheels last year or does it run smooth?

 

[Wrath]

Hi Castoro,
Thanks in advance for any advice. Attached is a pic of the whole bottom view. I cut the body using my band saw and the body is thin about 5/16 inch. Then I cut out the 1/4 inch pockets using a Dremel and router bit.

It is built w/ 3 wheels touching, and I cant the rear wheels at 3 degrees and the bent the front wheel to get about a 3-4 inch steer over 4 feet so it's a railrider. However, I am not allowed to trim the DFW side down 1/16 inch because our rules say the minimum body width between wheels must remain at 1.75 inches to accommodate for the track. The track is a 42 foot Best Track, and seems to be in pretty decent condition (if I recall correctly from last year).....didn't notice any major issues with nicks or cuts in my wheels.

Our rules require us to use the derby kit provided, which includes standard stock wheels and axles. Also required to use the pre-cut axles slots and graphite only.

Thanks again for any advice any pros/experts have!

 

[KyleBCostco]

I would route out the front compartment some more so you can move the weight back another 3/8" also try to widen it so you can fit 6 cubes across. Just my $.02

 

[Castoro Racing]

Did you drill the rears with a jig?

If the rears are at least good you can be more aggressive with weighting

 

[Castoro Racing]

You can trim the DFW since you would have more than 1.75 inches as the inner hubs will add spacing. Unless they truly meant the body needs to be 1.75.

 

[Castoro Racing]

How competitive is the pack and how did you guys do last year.

Before I try an estimate (want to fill in the last few major veriable) I agree w Kyle to try to get more space in that front weight pocket. Moving it back a little allow more flexibility in weighting.

 

[Wrath]

I use a #43 drill bit with pin vise, and a Derby Worx pro axle guide tool to help drill the axles since I have to use the existing slots. So my wheel alignment is not an exact science since it's dependent the existing axle slots. Is there a better way to do this?

The rules state the body between the wheels must be a minimum of 1.75 inches so I didn't want to break the rules (trying to set a good example for my daughter since it's Girl Scouts...haha).

I initially built the car to be a little sturdier since my 8 year old (and the folks managing the race) are jostling around the cars.

The Girl Scout group here is competitive but not compared to what you guys face. We won last year, and the top 3-4 cars were all pretty close...we won by 0.02 second average. So I wanted to try to improve my build from last year.

I agree that I probably need to take a look at pushing the weights back a little more...I have about a 1/4 inch wiggle room behind the rear wheels to drill out, but I was still concerned about getting 12 blocks behind the rear wheels. How important is maintaining a 3/4 inch COG to you guys? I was concerned that putting 12 blocks behind the rears would make my COG too aggressive, especially without a track to test it out on. Also, does pushing the weights back help with the weight distribution across the 3 wheels? I currently have about a 20 gram difference between my rear wheels.

Thanks again for your help!

 

[Castoro Racing]

I agree with that rule interpretation dont trim the body!

If you can I would fill the slots and redrill them. This will help you get a more accurate drill. I like clear epoxy but have used lightweight bondo also. If you have 3 scales I would not think about the COG. IMO COG will let you see if weighting is in the ballpark but is not as precise.

The more space you have in your weight pockets the more flexibility you have to shift around the cube to adjust weighting.

If you can get the slots filled and have more room in that rear pocket I would aim to get your DFW around 20gs and the rears much closer in weight. Cars can have less weight on that wheel but I think there are enough variables where I would play it safe. There is a chart by TXchemist somewhere on this forum that shows a correlation between wheel weight and COG and has some good range suggestions.

 

[Wrath]

Thanks! I hadn't considered filling in the slots and redrilling them...good idea. I'll also move the weights behind my rear wheels back another 1/4 inch and try to get 2 rows of 6 blocks.

Couple of last questions:
1) Is there a general consensus on what the weight difference should be between the rear wheels if the DFW is around 20 grams?

2) Where do you recommend putting the 12 blocks (2 rows of 6 cubes) in FRONT of the rear axle...would it be 0.5 inch in front of the rear axle to match the weight placement behind the rear wheels?

Again thanks for the help.

 

[KyleBCostco]

With a decent track and a decent drill, I wouldn't be worried about how far back you can put the weight. My son's stock wheelbase (axle slot) cars we haven't been able to balance with our hand with the wheels on and they were fast and stable..

 

[Castoro Racing]

There are 2 schools of thought I have heard of on rear weight distribution, which are to get then even or have them offset a bit. A 20g difference is way too much and they should be much closer.

I would make more room in the weigh pockets than move weights around till you get the weighting that you want.

Kyle-there are alot of variables I bet you ironed out on your cars where you can be more aggressive. There are still a ton here so I would say be a bit conservative.

 

[Jeff H.]

you might try to get a better balance between the rear wheels. you might take out more wood out and move the weights over so they are flush with the DFW side of the car (which would let you get 12 behind the rear axle if you wanted to). This let my son get the balance between the rear wheels to within 6 grams this year.