Weighting and center of gravity on a short track vs. longer

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Ignacious
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Weighting and center of gravity on a short track vs. longer

Post by Ignacious » Sun Jan 12, 2014 5:08 am

Greetings, this is my first posting so bear with me. I am relatively new to pinewood. This is my third year with my oldest son and first with my youngest. Call it beginners luck but we managed to win Fastest of the Pack for a fairly large and fairly competitive rural pack, right out of the gate against more experienced racers, for the last two years in a row. Now we want to move up to the annual district race. I have already researched some differences. They use a wooden center rail track and our pack uses a plastic track with no center rail. I've been told the district track is considerably shorter than the one our pack uses. I have been told ours is roughly 40' but I'm not sure and that the district track is roughly 32'. You folks would have a better idea as to the accuracy of these guestimates.

The question I have is that given the difference in track length, do I need to adjust the weight and COG to reflect these changes? Currently we use a 3/4" COG. I have been told by people who have raced district that the cars that win at the district short track would not win on ours because they would slow down before the finish. The same for our cars on the district track. A short track diminishes the "speed up effect" when hitting the flat part with an aggressively weighted car. Personally I think this is hogwash and maybe sour grapes among some who did well at pack level but not at district level. If the car is fast on a 40' track, there is absolutely no reason it should not be just as fast on a 32' track given that once you hit the flat you start decelerating regardless of where your weight is placed. I am no math or engineering genius, this just seems like common sense. But I defer to your wisdom in this arena.

Thanks.



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Stan Pope
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Re: Weighting and center of gravity on a short track vs. lon

Post by Stan Pope » Sun Jan 12, 2014 11:41 am

Welcome to the fray, Ignacious! Congrats on your success so far.

The "general rule" is that longer tracks benefit slightly from less aggressive weighting. Possibly this is due to "second order" factors in the friction equations OR due to the fact that less aggressive weighting requires less toe-in on the dominant front wheel in order to maintain control, and that reduced friction has more track in which to overcome the slight reduction in initial energy. The latter is evident.

It is sad that your pack chose to run with a track that differs so greatly from that which is used for district races. The "trough" or "hot wheels track" (which is what I think you described) requires a greatly different alignment setup for success. Cars that guide by the center rail have a clear advantage on tracks with a center rail, so that setup change on your car is necessary.


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Re: Weighting and center of gravity on a short track vs. lon

Post by TXDerbyDad » Sun Jan 12, 2014 7:16 pm

Stan Pope wrote:Welcome to the fray, Ignacious! Congrats on your success so far.

The "general rule" is that longer tracks benefit slightly from less aggressive weighting. Possibly this is due to "second order" factors in the friction equations OR due to the fact that less aggressive weighting requires less toe-in on the dominant front wheel in order to maintain control, and that reduced friction has more track in which to overcome the slight reduction in initial energy. The latter is evident.
Quoting general rule has my interest piqued. Is there something I can read up on about this? It would answer a question of why our more aggressively weighted cars have never been our fastest cars. Our fastest cars were modified wedge designs (wedge with only center section between front and back full sections) and under 3 seconds, while our thin candy bars have done well and close to 3 seconds flat on a 40 ft track, but never below it.
It is sad that your pack chose to run with a track that differs so greatly from that which is used for district races. The "trough" or "hot wheels track" (which is what I think you described) requires a greatly different alignment setup for success. Cars that guide by the center rail have a clear advantage on tracks with a center rail, so that setup change on your car is necessary.
It could be financial. A 40 ft four lane Besttrack is around $2k with a timer and shipping. Maybe they should invest in fundraising for one or building a wooden one with center strip. I think our wood 4 lane 40ft track was ~$300 in materials.



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Stan Pope
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Re: Weighting and center of gravity on a short track vs. lon

Post by Stan Pope » Sun Jan 12, 2014 8:17 pm

TXDerbyDad wrote:Quoting general rule has my interest piqued. Is there something I can read up on about this? It would answer a question of why our more aggressively weighted cars have never been our fastest cars.
The relationship has been mentioned at various times here on DT. I don't recall seeing the explanation that I gave anywhere.


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pwrd by tungsten
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Re: Weighting and center of gravity on a short track vs. lon

Post by pwrd by tungsten » Thu Jan 23, 2014 4:32 am

The two tracks in question are close enough.

If a track were 100 feet or so (mostly flat) then a center weighted car "might" be optimal. Also at that point a 4 wheel car might out perform a 3 wheel car. Weird things happen. If you make a center weighted 3 wheel car you should offset the weight to the DFW side.

That being said your tracks are similar enough. For a well aligned car a COM in the 5/8-3/4 is perfect. we just competed in a race with 3/4" COM. Car just happened to work out that way. Would have preferred 5/8". A rule of thumb might be 5/8"-7/8" depending on axle, wheel prep and alignment. Folks new to the sport might pick 7/8" if they are not canting the rears and riding the rail at which point they could get more aggressive.

Cars always slow down on the flat. It just seems like your car is accelerating because everyone else is slowing down at a faster rate.


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Re: Weighting and center of gravity on a short track vs. lon

Post by Speedster » Thu Jan 23, 2014 5:55 am

I'll give you our experience at District in 2013 but I can't say why the car ran like it did.
We first raced 29' from Pin to Timer on a Best Track. Races were close, sometime winning by only 2". Top 8 cars then move to a 40' Freedom Track. Our car ran all 8 races a car length ahead of everyone. We were running a "sort of" railrider. Rules state wheels have to be straight up and down but they give you a little angle. It gets more confusing. I worked with an Autistic boy from a different Pack. We did a good bore prep but that was about it. We even used the box axles when we were allowed to use the China axles. I don't know if it ran straight because I didn't check. He was in 2nd place behind Josh on the Freedom track. I think it was Heat 6 out of 8 his car flew off the track at the start. He picked up 4 points by not finishing and I don't know what might have happened to the car. He ended up tying for second, got beat in the run off, and got the 3rd place trophy. He was happy.
A friend of mine from yet another different Pack did the same thing. Their car was in close races on the Best track and when they reached the Freedom track they too had a greater distance for the win. He too was running a "sort of" railrider. The long circular arc seems to give an advantage to our cars.



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Re: Weighting and center of gravity on a short track vs. lon

Post by Shawn Stebleton » Fri Jan 24, 2014 11:50 am

For the Best Track profile, you want the weight compressed front-to-back in as little area as possible. It has to do with rotational inertia of the entire car as it transitions from the slope to the flat. If you can get the car hollowed out (routed or otherwise) everywhere underneath, except for the axle area, and put weight in a very compact form around the rear axle so that the center of mass (CoM) of the car is in that 5/8 - 7/8 of an inch range in front of the rear axle, then you will be doing well. The car will have a lower "moment arm" (I think that's the term--it's how far away from the axle the effective center of the weight rotates about that axle) and use less energy for the rotation, leaving more for speed.

My boys understand the concept well. The last 3 years we've put 4+ ounces of tungsten and a bit of lead tape completely within the space between the rear wheels. The front edge of the weight is just behind the front edge of the rear wheels, and the back is a good bit in front of the rear edge. It sits taller, but has the effect of shooting out of the curve at the bottom. The emcee of the district race used comments like "the car is pulling away on the straight," indicating that it is going faster coming out of the curve and/or slowing down less. This is in spite of being even or just behind when entering the curve. We also "dog-trot" or "crab" our cars, as you can read elsewhere.

We use the same car weighting for the pack race even though it is on a Piantedosi Classic wood track (gentle arc curve), as the lengths are comparable (42 ft vs 40 ft). My boys must enter the district race with the car that qualified at the pack race. I would suggest designing the car for the tightest curve you will experience. It should do well on a more gentle curve track. If you can make the weights adjustable, set them a little more forward (CoM at maybe 7/8") for longer tracks and further back (try 5/8") for shorter ones.


Shawn

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