Cub scout build- is 9/16 in. CoM too aggressive?

[bluhorshue]

Fist wanted to say thanks to this forum. My son won his pack race last year using the tips I read here. We built his car for this year using a 10 gram ladder body, covered with monokote trim 2 oz weight behind back axles, and 2 oz right in front of them. Rail runner design with the back wheels 5/8 inch from the back and front wheels 3/4 inch from the front, so about 5 5/8 wheelbase. I suspect this car could be about a gram heavy on the official scale which would be OK. Anyway, the CoM looks to be about 9/16 inch in front of the rear axle.

I'm worried this will be unstable because at a previous district race we flew off the track on 2 different runs though it was stable at the Pack race, and that car wasn't a ladder body car and had a less aggressive CoM. I think the Pack Race is an aluminum BestTrack, but it is either shorter or extremely fast because the fastest cars are under 3 seconds. While the district race uses a longer and faster track that is an aluminum racing surface on a wood ramp, and the winners are usually just under 4 seconds.

While I am asking- if I do need to move weight around, is there an easy way to break up a brick of 12 tungsten cubes epoxied together without wrecking the car?

Thanks in advance.

 

[txchemist]

A few questions about that District track- How many other cars flew off? did you have trouble on different lanes? What surface does the DFW ride up against, the side of the aluminum strip, or two wood strips as dividers on the outside edges?
That track may be simply a hazard due to imperfections that kick your car off. If it uses outside dividers, a rail runner will need to be modified to run that sort of track. Can you make another car for Districts?

 

[bluhorshue]

I think the district track is probably ok, my son's car was the only one that flew off and it was probably build quality that was at fault since we were pretty new to this. Two of the lanes did seem to cause that car problems though. The district track has two aluminum rails that go between the wheels of the car like a normal track, but I think it may be because it is longer, taller and faster it may have pushed the car beyond its limits of stability. That car was 3/8 in tall, and this one is only 1/4, so I'm assuming a lower center of gravity on this car. The DFW is the right front wheel and it curves left. I could build a new car or alter the current car (assuming it can stay on the track and qualify at the pack race). Thanks for taking the time to offer an opinion.

 

[txchemist]

Do you have a two low cost small scales to weigh your back wheels and then the DFW to discover the loading on each? Keep car level when you do this. Much easier than COG and you can see what your last year car was on DFW weight, let's assume 15 grams. If you want to be safe, bump it up to 18-20 grams. If your back wheels were way different, that amplifies any wiggles that start so move weights around to get better distribution.

 

[bluhorshue]

Thanks for the help. Rearranged some weight to stay conservative, and moved two blocks. I'd like to think that we did a pretty good job on the rear wheels- even built a jig out of lego blocks to try to get the same hole placement on both sides, but I'm also sure we are nowhere near the precision of a pro build. I don't think it is possible to learn how to do this properly building a car a year.

 

[LightninBoy]

. I don't think it is possible to learn how to do this properly building a car a year.

Very true.

 

[LightninBoy]

While I am asking- if I do need to move weight around, is there an easy way to break up a brick of 12 tungsten cubes epoxied together without wrecking the car?

Yeah ... DON'T USE EPOXY. Holy crap that stuff is the worst when you are trying to reclaim the tungsten.

 

[SuperSlow]

Even if you get them out you pretty much have to file the epoxy off the cubes to re-use them n get them to stack in correctly.

 

[txchemist]

A s a seller of tungsten I encourage epoxy. As a user of tungsten, I first cover the weight with metal tape, then I epoxy to the removable tape. I can, with some effort, get the tungsten back and then clean off the metal tape adhesive with Dawn. I just do this with the weights touching the back axle wood because after the epoxy cures, I then drill my axle holes ( set body and weights in a silver bullet covered with shipping tape or plastic wrap to get weight and wood exactly level before the drill) ( obviously a much easier job if using 2x6 and 1x6 bars) The rest of the 1x2 and std. cubes are just put in place on sticky tape, and when done cover with metal tape.
For Cubs, I just cut a notch out of the back of the car, add a low cost semi-pro bar 2x6, and then use 3M VHB tape to put two tungsten plates on the bottom to finish it up. No work at all.

or, use the 3M tape like this for all weights

after the weights, remove any extra tape, and later cover with metal tape so edge of plate does not mess up stop section. Note we have a weight all the way over to DFW side for good balance.

 

[bluhorshue]

Just wanted to thank everyone again. My son won the pack race, though afterwards we raced it against our car that couldn't hold the track at districts and were not quite as fast. We'll likely build another car body for districts since we think we can do better, and we know that this car isn't going to be good enough. What's interesting is that the old car that can't hold the track is just over 1/2 inch thick (used 1/2 inch tungsten cylinders), with 25 grams on the front wheel, so we must have stumbled on better alignment or steering or something. We prepped wheels and axles the exact same way, except the bent front axle. In theory the new car should be more aerodynamic and has more weight placed near the back if everything else were the same. The quest to squeeze a few extra .001's out of a car may never end.

 

[speedwriter2]

Even if you get them out you pretty much have to file the epoxy off the cubes to re-use them n get them to stack in correctly.

Speedwriter here. I have had great success reclaiming tungsten cubes with a small propane torch. Of course wear a mask and do it outside on a paving stone or such. The epoxy will burn right off, only thing is it discolors the cubes but who cares.

 

[aircooled]

Speedwriter here. I have had great success reclaiming tungsten cubes with a small propane torch. Of course wear a mask and do it outside on a paving stone or such. The epoxy will burn right off, only thing is it discolors the cubes but who cares.

 

[aircooled]

If you put the cubes in nail polish remover with acetone the cubes will cleanup like new.

 

[LightninBoy]

A s a seller of tungsten I encourage epoxy.

Ha! Pretty much sums it up right there.

 

[Pacfanweb]

I just put a small strip of 2 sided tape in the weight holes, stick the weights to that, and cover with foil tape. You can use needle nose pliers and pull them right off the tape later.