Losing speed on the flat...

[MERuhl]

Last year was my son's first AWANA Grand Prix. His car did well (5th in racing, 1st in design), but I think it could have placed higher if I had done a better job with the weight distribution. Since that was my main contribution to the design (I strongly believe the kids should do as much of the work as they're able), I'd like to get this sort out before this year's race on Nov. 6.

Last year we consistently beat everyone down the ramp, but lost acceleration as soon as we hit the flat. We did not carry our speed well.

So here's my question:

What's the best way to weight the AWANA car if you don't plan to move the axles? I've read elsewhere that the center of gravity should be located more toward the front on cars with shorter wheelbases. This is consistent with my observations from last year: the top two cars both used the Pine Car under-body tapered weight system, while we had drilled in from the rear of the car and had most of our weight over and slightly in front of the rear axle. I'm thinking we had the weight too far back for the short, stock wheelbase.

Any suggestions? If we go to a long wheelbase, will that more than compensate for any slight misplacement of weight? Or should we stick with the stock wheelbase and get the weight placement right?

[MaxV]

Even with the short wheelbase Awana cars, I target a balance point of 1 to 1-1/4 inches in front of the rear axles. This takes advantage of back-weighting while still leaving enough weight on the front wheels.

If you can, do use an extended wheelbase car. They are generally far superior in performance to the Awana wheelbase (in fact, it is really not fair letting them race in the same event). Then you will not likely have a weighting problem - but still use the balance point measure from above.

If you must use the stock wheelbase, then use the axle slot closest to the end of a block as the rear axle, and weight the car to hit the balance point mentioned above. This will require most of the weight in front of the rear axle.

There could be other factors that slowed down your car, such as alignment, lubrication, etc. So I recommend working on improving those factors as well as the weight.

Good luck!

[MERuhl]

Randy - thanks for the reply. Last night my son and I decided to go with the longer wheelbase, based on our research here, at your website, and others.

As I mentioned earlier, we had done a good job of wheel alignment and friction reduction. As a kid, my Dad and I won our Pinewood Derby twice, and were Regional Champs our last year. So I had the basic concepts down from years ago. Last year's car had it's front right wheel raised, and wheels lubed with graphite powder. It rolled true. But it lost ground on the flat to the same 3 or 4 cars consistently, so I knew the difference had to be the weight distribution.

Here's another couple of questions:

How do you check the balance point? Do you actually sit the car on a raised edge and make sure it balances where you want it to?

And how much weight do you put in the front of the car, and where do you locate it? I noticed in another thread something about putting .8 to 1 ounce of weight over the front wheels (but not in front of them - another mistake I made last year!).

Thanks for your help!

[Splinter Sprinter]

Also what I think is equally important for weight distribution is what kind of track you will race on. We race on 2 totally different types of tracks.

One for RA’s which is a continuous slop down, with very little if any flat at the bottom and what you might consider as a traditional s-drop BSA track with a long flat at the end, about 28 feet of flat.

We learned the hard way that the same long wheel base car wouldn’t perform the same on both tracks. We back weighted (at center of gravity) the car for the BSA track and cleaned up. We won all the way through Council. Took the same car to the RA’s races with the continuous slop and got schooled. Our times were slower and we lost speed in the second half of the track for the same length track. We borrowed the first place car (friends of the family) and together went to our BSA track with the reverse happing. For continuous slop tracks we found that more center weighted (between the two axles) cars run faster if all is equal.
I think Michael Lastufka (Pw Engineer) or Stan Pope wrote something on weight distribution and different track designs, maybe they could add something to this.

So now we “Purpose Build” a different car for each type of track.
Good Luck!

[Stan Pope]

Here's another couple of questions:

How do you check the balance point? Do you actually sit the car on a raised edge and make sure it balances where you want it to?

And how much weight do you put in the front of the car, and where do you locate it? I noticed in another thread something about putting .8 to 1 ounce of weight over the front wheels (but not in front of them - another mistake I made last year!).

As you are getting ready to assemble the parts, trial assemble the wheels/axles to the car. Weigh all of the parts to assure total weight as planned (e.g. 5 ounces). Set the front wheel(s) of the car on the scale, the rear wheels on an elevated surface so that the car body is level. (I use a screwdriver handle, since the ribs on the handl help keep the car from rolling.) Arrange the weights so that the scale registers approx. one ounce.

Note that 1 ounce on the front wheel tread is very different from adding one ounce of lead over the front wheels!

If you run 3 wheels and the wheels don't have slick treads, then you can go short on the one ounce target. If you run 2 wheels or the wheels have highly polished tread, then aim for somewhat more than an ounce.

I think Michael Lastufka (Pw Engineer) or Stan Pope wrote something on weight distribution and different track designs, maybe they could add something to this.

Stan did. I don't know if Michael did.

[MERuhl]

SS - That's exactly what we're addressing here: the specific requirements of running Awana Grand Prix cars on the stock Awana Grand Prix tracks. My experience as a kid (30+ years ago) was with BSA tracks of that era, which, as I recall, were continuous slopes. Using my old back-weighted cars as models last year got me the same results you experienced.

I've seen the article you mentioned about the differences between performance on tracks of varying design. It's consistent with my observations from last year - the few cars that beat us were more forward-weighted than ours. Hopefully, this year will be different.

[MERuhl]

Note that 1 ounce on the front wheel tread is very different from adding one ounce of lead over the front wheels!

I get it! That sounds simple enough, but combining that technique with isolating the center-of-gravity at 1.25" ahead of the rear axle seems kind of like patting your head and rubbing your stomach at the same time.

[MERuhl]

Here's a link to Stan's discussion of track types:

http://members.aol.com/standcmr/lbw2.htm#i_II1a

[Stan Pope]

Note that 1 ounce on the front wheel tread is very different from adding one ounce of lead over the front wheels!

I get it! That sounds simple enough, but combining that technique with isolating the center-of-gravity at 1.25" ahead of the rear axle seems kind of like patting your head and rubbing your stomach at the same time.

Yup! It's two ways of approaching the same concept.

To stick to the track, you need to have reasonable sliding friction (at least sideways) between the tread and the track and you need enough weight pushing the tread and track together to keep the car from coming loose. YMMV, depending on the track surface, the tread surface and any treatment you give the tread. One ounce on one turned front wheel seems to work well on a varnished Piantidosi track (28' start to finish). More is needed on longer tracks and slicker tracks (e.g. aluminum)

For maximum speed on a Piantidosi track, you need CM as far back on the car as is stable.

There is some interplay. For instance, moving the front wheels back from the front edge of the car increases the weight on them (it), and moves the CM backward. Both are beneficial results!

Here's another piece of CM info that comes into play: Raising the CM (above the wheelbase) reduces the distance that the CM must travel as the car goes from start line to finish line. If some important oscillation factors are ignored, the raised CM car would appear to be advantageous, at least on simulation. In real life, the raised CM car appears to suffer if any imperfections (track or wheels) cause torques about the longitudinal axis, and the cars rarely perform up to the owner's expectations. Similar torques affect your car, especially at its higher speed, even if the CM is low in the car. But since the moment arm is smaller, the effect is less. This could contribute to unexpected loss of speed on the flat.

[MERuhl]

Thanks again, Stan. After posting last night, I re-read a lot of the stuff on your website.

It occurred to me later on that measuring the downward force on the front wheels will actually assist me in fine-tuning the center of gravity. So at least I was able to sleep last night.

[Barga Racing]

With everyone talking of CM and weight on the front wheels I was curious about my boys cars from the last 2 years. Two years ago our first pinewod car was very fast and extremely stable. I knew it had a CM of 1.25 inches but did not know the eight on the front wheel. Last years car was extremely fast but very unstable. About half the runs it wold get the death rattle about halfway down the track. Sometimes it would not and then it was really fast. I knew the CM was 7/8 on that car. I took both cars to work and weighed them on an accurate, calibrated scale. I made a stand the same height as the scale to properly support the opposite end of the car that was being weighed. The car with 1.25 CM had 1.44 oz and the 7/8 car had 1.22 oz. It seems to me that to only have 1oz on the front wheel I would have to make the CM about 5/8 which woud be extremely unstable. So how do you guys get 1.25 CM with only 1oz on the front wheel? We are using the standard BSA wheelbase. Is everyone talking about 1oz on an extended wheel base? This year we are goingto make a car with adjustable weight system to play with to determine the best CM for our track.

[Stan Pope]

So how do you guys get 1.25 CM with only 1oz on the front wheel? We are using the standard BSA wheelbase. Is everyone talking about 1oz on an extended wheel base? This year we are goingto make a car with adjustable weight system to play with to determine the best CM for our track.

Well, since you ask, I pulled out a fresh kit.

When you said "support the opposite end of the car", I hope you meant that you supported the car by its wheels. That is important. Your numbers don't match with my measurement cited below. On the other hand, I might have received a "nonstandard block" in that BSA kit! (I only open one of the several kits up in my cabinet.)

The wheelbase on this block is 4 3/8". 4 oz vs 1 oz ratio would put the CM 1/5 of the wheelbase forward of the rear axle. Doing the math in my head, 1/5 of 4 3/8" is just under 1 1/8". (Check me, someone!)

You don't mention tread treatment. Turns out, that is very important. It is also very important that the wheel/axle pairs have very comparable friction coefficients and that the car have excellent alignment. Failure on any of these counts nails you! Differeing Cf increases the car's tendency to get "out of shape", high polish on the tread allows it to do so. The first brush with the rail assures it.

[MERuhl]

4.375 divided by 5 = .875, or 7/8". So Wooden Wonder's 7/8" car was dead on as far as Stan's CM calculation goes.

This is only speculation, but since it appears to have maximized the Stan's CM calculations, maybe it was so fast that it revealed problems with other areas, like wheel alignment or camber. I dunno.

[Stan Pope]

4.375 divided by 5 = .875, or 7/8". So Wooden Wonder's 7/8" car was dead on as far as Stan's CM calculation goes.

Hmmm! I should know better than to do fractions division in my head! Lessee 8 times 4 is 40 + 3 is 43 divided by 5 is ... um hum??? I see my error!

Actually, WW's numbers were 1.22 oz @ 7/8", so I wonder about his measurement. Probably it was the measurement of CM location, since this is the hardest to do with accuracy, and a small error would make a large variance in weight.

Probably his rear wheels were supported properly during weighing. Supporting behind the rear wheels would probably have messed up the weight much more.

[MERuhl]

Actually, WW's numbers were 1.22 oz @ 7/8", so I wonder about his measurement.

So are you saying that, by definition, if you get the CM in precisely the right spot, then the weight on the front wheels will always be 1 ounce?

If so, may I then conclude that one way (the best way?) to ensure the CM is placed accurately is to set the front wheels on scale as described above, and tweak the weight placement in the rear of the car until the front weight is 1 ounce?

I think this is what I was trying to ask in the first place...