Derby Talk is a forum for Pinewood Derby, Awana Grand Prix, Kub Kar Rally, Shape N Race Derby, Space Derby, Raingutter Regatta and other similar races where a child and an adult work together to create a race vehicle and a lot of fun and memories
I noticed that you can get two rows of 7 1/4" tungsten cubes behind the rear axles by simply cutting off the rear 1/2" of the car, then using laminate sheets on the top and bottom of the car to secure the weights (see picture).
My COM for this car is approx. 3/8". I got a total of 3 3/4 oz. of weight (22 cubes) packed adjacent to the rear axle. The weight of the car prior to painting is 4 7/8 (heavier than I would like, but I can still Scrollsaw wood weight off the car before drilling my axle holes, if need be.
Has anyone else tried this weighting approach? Am I being overly aggressive here? Opinions are very welcomed.
The issue I ran into, was to much weight, began to slow the cars performance. by adding to much weight and pressure on the rear axles / wheel bore added friction.
there was some serious debates ad info by stan pope and FS a few years ago on this.
It also required to much drift for rail riding to get it tuned right to stick to the rail, which slowed the car times.
But stick with it and maybe you can find a cure for the speed times going backwards, instead of forward.
I have done the 2 rows of 7 behind the axle before, but I did it by leaving a small shelf of the block and gluing the weights onto it.
I like your method a little better since it makes for a cleaner looking design and would allow for better COM tuning by gluing on the bottom piece first, arranging your weights to get desired COM and then adding the top piece. However I agree with sporty that a 3/8" COM is pretty aggresive and is actually harder to make fast than a more mild COM.
I can certainly see your point on the heavier weight causing slowness due to weight on the friction-bearing surfaces. I read somewhere that a good goal is to get 3.5 oz. around the rear axles. I am only increasing it by .25 oz., so I'm hoping the relatively small incremental added weight (.25 oz.) will not torpedo us during race day. From the reply post pic. I saw, it was fuzzy, but it looked like there were several ore cubes than 22 packed in around the rear axle. I may be wrong on this, but I thought I saw about 4-6 more cubes in your pic.
The rail riding feedback is quite interesting. I can see how the added weight again translates into added friction on the FDW, thus making the car slower. Seems like I'd need a less aggressive drift to try and compensate for the added friction, but again, we're only talking .25 oz. over what folks like 5KidsRacing thinks is a good weighting goal (3.5 oz.).
I bet you guys are right. I am somewhat seduced by the idea of the additional .25 oz. of weight, higher up on the track.
3 to 3.5 is the sweet spot goal area. some of that depends on designs of the car, wood thickness, ect.
If you have the skills, you can do it. Its just allot harder to get right for the average builder and it was trickier for me too. thats why I found the best luck with 12 in back and 12 in front. 1/4 cubes.
I tried this weight ratio a few times, and i was always slower than the other way. Mostly cuz I had to add more drift to keep the car stable. But some really good sidewall prep on the rail rider wheel may buy you a few good timed runs, before the friction begins to increase.
Scrollsawer wrote: From the reply post pic. I saw, it was fuzzy, but it looked like there were several ore cubes than 22 packed in around the rear axle. I may be wrong on this, but I thought I saw about 4-6 more cubes in your pic.
Scrollsawer.
Actually there are just 22 cubes and the rest is tungsten putty. sorry for the blurry pic. here's another angle, zoom in on the back and you can see the weights pretty well.
As I said earlier though, your method is much more elegant.
iI'll add in, when Woodworx did this, he had a problem with the stop section, after a few runs the tungsten broke free. not sure what glue you used, I think he used gorrilla super glue. He always did the thin tops like you did on you your car.
for some reason, they would break or crack right at the axle wood.
those cars were used at pwdr and they had a good stop section.
Maybe make sure whatever glue steve used is the same as what you used. I never had a issue with 2 part epoxy. but I stayed with the 12 in the rear.
I have ran 3/8th balance point many times. but not with the more weight in the rear, as I mentioned I ran into to much drift and slowed times some. I just ran a lighter front end and less wood and I was able to win with that style in 2010 and beyond.
Thanks guys. Appreciate the feedback. So, Sporty did you say you had your best luck with 12 cubes behind and 12 in front of the rear axle? That's 4 oz. If my math holds up. I understand the value in keeping the weights on either side of the axle balanced, makes perfect sense. But the 12 & 12 is way beyond the 3-3.5 oz. Goal you mentioned earlier. Maybe I'm missing something??
I used Loctite Professional Bond Super Glue to glue the cubes to the rear of the pine, then to the bottom layer of plyboard paneling, then to the top layer of plyboard paneling. I then secured in my woodworking vise for an hour, even though 50% strength is attained in the first 5 minutes. I would be really surprised if the cubes broke free, but I suppose anything could happen (I am a novice after all). They are glued in on three sides.
Sounds like you should be just fine with the glue. Ya just never know with what the stop section is like. Thats the only reason I even brought it up. I know I would not fret over it.
Yes i ran 4oz and sometimes 3.7oz.
I thought you were refering to the sweet spot zone with the sweet spot weight amount. So I was confused or mis understood what you were refering too.
I prefer 1.8oz of load weight on each rear wheel axle, evenly. Which is to me the sweet spot, give or take a .1 of a oz.
thats like 3.5 to 3.6 oz total weight on the rear axles. but I have seen 3.2 to 3.5 load on the rear axles work just fine too.
So, I perhaps should have added more info there for you. as I made the asumption, that's what you were refering too. Sorry about that.
I usually drill canted axle holes right up front. If you get those wrong, your brilliant build will not work.
I've had the same problem as Sporty... 3/8" COG leaves too little weight on DFW to rail ride. If you're doing a edge guided track, might get away with it.
Check if you can get a test run on the track before final gluing of weights.
Good Luck!
Reed
My wife started a new support group... Widows of the Pinewood Derby.
Scrollsawer wrote:Am I the only one who has attempted this weighting approach?
We tried something similar but don't recommend it. Although tempting, as Sporty said, we found that two rows of seven cubes behind the rear axle never worked as well as two rows of six cubes in an extended wheelbase configuration.
Weight issues aside, I like the neat presentation by using the veneers.
Something I considered experimenting with, but won't get to because this is my son's swan song year, is gluing a thin carbon-fiber veneer to the top. Carbon fiber is very strong, comes in thin sheets, and the webbing pattern looks cool. If you have problems with the weight breaking free, you might experiment with carbon fiber.
