Understanding weight placement and center of gravity

[firestone]

IS there any consensus on the true ideal center of gravity? I've seen/heard/read about 1" in front of rear axle, but with how heavily weighted some cars look with cube weights all the way to the back, I can't imagine those cars have COG that far forward.

Also I'm getting the feeling that cubes are the way to go. My son has been running cylinder tungsten above rear slot and just in front of it, and does ok at the Pack level, but we're looking to really step it up next year. His goal is to get back to the NYC event next year and be competitive.

 

[flockshot]

One of the variables in how far rearward you can place the center of gravity, is the type and condition of the track you will be running on.
I am not the guy to define what is right, but I see many who use 3/4 inch, and I don't see many suggesting otherwise. However, those who say nothing on the subject, may know best of all.
I am building a track for me and my grandsons to play with, and then I can do my own experimentation.

 

[Darkside]

I don't so much worry about the actual location of the COM. I go by how much weight is on each wheel. Sure you can calculate the location of the COM from that, but why bother. Six to one, half dozen to the other... A good safe starting point would be 15 grams on the DFW and the remainder evenly distributed between the rears. You can then experiment by adding or reducing weight on the front as well as from side to side on the rears from there to see what distribution works best for you.

 

[electric sheep]

I follow the same approach as Darkside and check the DFW weight. I noticed my kids cars had better speeds when I made this change. I haven’t had issues with this approach.

 

[Carl ODay]

I don't so much worry about the actual location of the COM. I go by how much weight is on each wheel. Sure you can calculate the location of the COM from that, but why bother. Six to one, half dozen to the other... A good safe starting point would be 15 grams on the DFW and the remainder evenly distributed between the rears. You can then experiment by adding or reducing weight on the front as well as from side to side on the rears from there to see what distribution works best for you.

Did some googling and cant find a scale that will measure individual wheels... Does such a thing exist?

 

[Jimmy & his 2 Kids]

Did some googling and cant find a scale that will measure individual wheels... Does such a thing exist?

Well yes and no... folks just use 3 very small scales for each corner. However Chuy over at NPWDRL is quite the engineer and has created a 3 point single scale for this very purpose. You can see it here:
http://npwdrl.boards.net/thread/2577/3-wheel-scale
Personally I have just always done the same... racing Awana with no axle location restrictions I have just enough room behind rear axle for 2 fulls rows of 6 cubes each, then the rest goes right in front of the axle with bias on the DFW. I do this with my Awana cars and league.
Though the fastest league racers do pay closer attention to weights at each wheel I am sure when testing and tuning.
Hope that helps some. Good luck!
Jimmy

 

[Carl ODay]

Well yes and no... folks just use 3 very small scales for each corner. However Chuy over at NPWDRL is quite the engineer and has created a 3 point single scale for this very purpose. You can see it here:
http://npwdrl.boards.net/thread/2577/3-wheel-scale
Personally I have just always done the same... racing Awana with no axle location restrictions I have just enough room behind rear axle for 2 fulls rows of 6 cubes each, then the rest goes right in front of the axle with bias on the DFW. I do this with my Awana cars and league.
Though the fastest league racers do pay closer attention to weights at each wheel I am sure when testing and tuning.
Hope that helps some. Good luck!
Jimmy

Chuy does like his Arduinos! Thanks for the info.

 

[Jimmy & his 2 Kids]

Chuy does like his Arduinos! Thanks for the info.

Sure thing! Yes he amazes me with what he does with them.

 

[firestone]

The thing I keep reading about is this fear that too much weight towards the back of the car could actually flip it or raise up the front wheels off the track. Seems highly unlikely to me, even if you went out of your way to get weight all the way to the back edge of the car.

Has this actually happened or has anyone witnessed a car flip because of weight placement?

 

[flockshot]

I have seen the 'wheelies' on videos but have not seen it myself.
There is however, a little more to it I think. There is the stability of the car to consider, so if your car us running 'true' with no wiggle with extra weight in the back, I would say you are good.
The pro's will have a better answer than I do for sure.

 

[Charles Studer]

Jeez......take it easy. You should be good with 12 cubes plus tight to the axle spar with good cross weight. Nice tight gaps and a good alignment and then push your boundaries. Fast BASXally first then dance to find YOUR SPEED

 

[Darkside]

The thing I keep reading about is this fear that too much weight towards the back of the car could actually flip it or raise up the front wheels off the track. Seems highly unlikely to me, even if you went out of your way to get weight all the way to the back edge of the car.

Has this actually happened or has anyone witnessed a car flip because of weight placement?

I have seen this. It happened to my son's car last year at his scout race. What caused it, however, was not the center of mass being towards the rear of the car. If the center of mass is in front of the rear wheels, the car can not physically wheelie on its own as there is no torque being applied to the car to lift the nose. What happened in our case was that the pins on the start gate were angled back towards the cars by about 5+ degrees with respect to the track. Because the pins are spring loaded and rotate about an axis to release the cars, the initial movement of the point on the pin that contacts the nose of the car was not only forward but slightly upward as well. This upward movement is very slight and had no effect on the cars weighted more towards the front, but it was enough to pop the noses of the rear weighted cars up off the track. My sons car, at 15 grams on the DFW, was the most aggressively rear weighted car and was effected the most. The nose of his car not only got popped up but it was kicked sideways as well on three of his four runs. So on those three runs his car went down the track sideways with the DFW riding up on the center rail. The one run that it actually came back down in the lane, it set the fastest time of the day.

So basically rear weighting the car will not be a problem as long as the track is in good shape and the start pins are perpendicular to the track or angled slightly away from the cars. So if your track is rough and your start pins are angled towards the cars you may want to weight the car more towards the front. Otherwise, game on. Push it back.

 

[Brian Stanley]

Darkside, was this on a BestTrack? We have Scouts coming up soon we were planning a 12-15 gram front. Now I'm worried . We don't have this issue on my test track, but the Council's track condition is questionable.

 

[Jimmy & his 2 Kids]

Regarding getting squirrelly, If your front end (DFW side)is too light it will decrease your DFW’s “bite”. Then you run into needing more steer to keep the wiggles at bay than should be nec. In a close race that could be the difference between winning and loosing.

 

[Darkside]

Darkside, was this on a BestTrack? We have Scouts coming up soon we were planning a 12-15 gram front. Now I'm worried . We don't have this issue on my test track, but the Council's track condition is questionable.

No, it was on a derby magic track. They had some issues with the start gate due to years of use and storage and had rebuilt it or made a new one. I'm not sure which. But in any case when it was installed on the track the pins were angled back. I had seen this before the race, but the angle was so small that I didn't think it would be an issue... It was most definitely an issue.

I don't think the track type would be an issue though. I have derby magic track and we had tested that car on it and there were no issues with the front of the car lifting. My start gate is original and in good shape though, and the pins lean slightly forward of perpendicular. Just make sure the start pins are perpendicular to the track or slightly forward of perpendicular and you should be good.

 

[DerbyBoyz]

I say build the car and give it what it likes. All cars are not the same, and I have seen cars with a lot of steer and are fast.