It has a huge affect. When my son was in Cub Scouts, he won the Pinewood derby
It would have more effect on a pinewood car than your car.
There will be minimal difference in a real car not to mention that you have to provide the energy to get the back end back up before the next start.
In theory it will work with a real car, but frankly the actual energy saved, is far too small to measure.
Joe, we are talking about pinewood derby cars here.
“Actually, running two cars back to back decreases the air drag on both cars,”
There wouldn’t be any noticable effects on Pinewood Derby cars and the car and, even if there were, it wouldn’t affect the outcome of the test.
Most of the drag results from the frontal area of the car which is essentially zero for the car in back. Since drag is the primary factor slowing down the cars, the car in front will, most likely, always appear to be the slower car using the test proposed by the puzzler answer.
At auto racing speeds more drag occurs due to turbulent flow at the rear of the car than to high pressure flow at the front. If you do not think that is true for low-speed Pinewood Derby cars, I’d like to see your analysis as to why that is so. “Gut feel” is not an answer.
As I remember Derby racing when I was a kid, the tracks were about a half mile or more in length. The ramp the cars rolled down at the start was about 20-30 feet. Placement of weight, fore or aft, would not seem to be a significant factor.
Only if the mechanism of aerodynamic force at 10 mph differs from the mechanism of aerodynamic force at 90 mph. You have not proven that to be true.
Mechaniker, you are mistaken.
For race cars, at typical racing speeds, the rear car benefits more from the draft. Otherwise “bump-drafting” would be an impossible Nascar strategy.
Since pinewood derby cars operate at speeds closer to bicycles, consider bicycle drafting. The front position is by far the most energy-intensive (speaking from personal experience) to maintain, to the point that the lead position is “rotated” as the lead guy tires.
Mechaniker, there may well be a specialized instance in which the lead position is advantageous, but I cannot think of one. (Aero drag as opposed to downforce/cooling problems that might vex a drafting car).
<font color="blue" face="timrs><i>"When a second car gets very close behind the first car, the air rushes over the two cars as if they were one, which removes the force at the rear of the first car and at the front of the second car. In the lower picture, there are only two big red arrows, so there is less total force working against the [pair of] cars and – voila – they [both] go 3- 5 mph faster. This is the important part: the two cars don’t need to touch [i.e., "bump"] to make this happen. This is plain ole' drafting [as opposed to bump drafting]."
Mechaniker, I didn’t say the lead car derives no benefit from drafting: it (usually) does. I said the MAGNITUDE of benefit is greater for the trailing car.
You strike me as a person with lots of book learning, and zero real-world application of that knowledge.
Again, in addition to several years of bicycle racing, I drove a loaded 24’ U-Haul (top speed: 57 mph) in convoy with a DT-360-powered 24’ box truck (top speed: 70.)
For over 1500 mi, we made better speed with the slower truck “in the pocket” than the other way around.
Ask ANY trucker where the “slow” truck in a 2-truck convoy goes!
At least try to be consistent.
No need to get personal. The bicycle analogy does not fit here; air flow is completely turbulent from the front fork tips back. And kudos for driving a DT-360-powered 24 ft box truck for 1500 miles; I can’t match that. A tip of the hat to you.
There’s no contradiction between those two statements. The first was made in re: two cars in draft; the lead car IS at a disadvantage (relative to the trailing car).
The second clarifies that a lead car (usually*) has an absloute benefit in a draft (relative to no trailing car at all).
*A likely exception would be a teardrop-shaped car trailed by a large flat plate. There’d be little wake behind the teardrop to clean up-and the flat plate would encourage the upstream air separation on the aft portion of the teardrop…producing a draggy “dead air” portion.
Just to defend the validity of the puzzler answer, if the two cars were separated by a car length at the start the aerodynamic effect they had on each other would be negligible, particularly at the beginning, and the result of the tests preserved.
I’m not going to take sides in the drafting discussion, but it points to something else. Viewing a car moving through air as a solid, like a Pine Wood Derby car, if there is a high pressure area at the front and a low pressure area at the back, then some vertical cross section of the body is in tension not compression. I suppose that is where the body would separate if the material wasn’t strong enough to hold it together.
"Only if the mechanism of aerodynamic force at 10 mph differs from the mechanism of aerodynamic force at 90 mph. You have not proven that to be true. "
I’m not sure what you’re saying. I never said anything about how the forces vary with speed.
Drag increases as the square of the speed. But, that doesn’t affect the conclusion that the puzzler answer is bogus.
“Just to defend the validity of the puzzler answer, if the two cars were separated by a car length at the start the aerodynamic effect they had on each other would be negligible, particularly at the beginning, and the result of the tests preserved.”
That wasn’t the answer given to the puzzler as I understand it. And, I don’t think it would work either. As has been pointed out, the car falling from the higher distance will accelerate longer before reaching the level part of the track, so it will probably be going faster at that point. It will, most likey, will have enough time to catch up with the car in front no matter whether it’s really the slower or faster car.
You could run multiple trials and figure out which car is faster, But, that’s not in the spirit of the puzzler. After all, if the speed of the cars is significantly different, you could determine the faster or slower one by viewing individual runs.
As I remember Derby racing when I was a kid, the tracks were about a half mile or more in length.
@meajoe75fan-
Since pinewood derby cars operate at speeds closer to bicycles,
They’re talking about pinewood derby not soapbox…
jadebox is right. The simple way to find out which car is faster is to start them a foot apart and mark the point where the rear car hits the front car, then swap places. The car that gets the furthest without being rearended is the faster car.
When my boy was in Cub Scouts, Pinewood derby track fit in the grade school gym.
Three design elements I’m sure contributed to my victory many, many years ago.
The car’s shape. Very aerodynamic, rocket-like. Not flat or rounded square lines.
Weight—drill out under and behind the 'driver’s seat," fill with lead. We took it to the post office for precise weighing. Exactly 5 ounces.
Most important: In my day, the track was built in sections, and where each section connected to the next, it created a bit of a bump. Some cars went off the track when they hit the bumps. All seemed to feel those bumps and they were slowed down. On my car, my dad created a legal shock absorber. A single cut with a scroll saw, about a quarter inch up from the bottom, starting at the nose of the car and going straight back about 4/5 of the length of the car. The front axle took the bumps and they were absorbed and distributed in a way that put this car so far ahead in every heat it wasn’t even close.
Pinewood derby cars gain maximum speed by reducing friction! Sanding the nails (axles) to remove burrs and sanding the wheels smooth where they touch the car body or nail head is a great way to reduce friction. Also, elevate one wheel 1mm off the ground as a three wheeler will go faster than a four wheel vehicle due to less friction. Have a good race!
Love the discussion about pinewood derby. Having officiated pinewood derby races for 5 years I have a comment. At the last race a friend’s father built a copy of an Formula 1 McClaren race car complete with what looked like the stig from Top Gear. It was lean, mean, painted and appeared to have gps, a clear coat covering, and floor mats. Another child raised by a single mom took the square piece of wood, put wheels on it and stapled a plastic grey rat to the top. He did nothing else. No graphite, no sanding the plastic tire, nothing. That ratmobile whipped the stig’s butt, and every other car entered. It was not close. In fact that year the Formula 1 replica finished in the middle of the pack. So much for graphite, aerodynamics and hiring a professional to build your derby car. Just use a stapler and a block of wood. Happy racing…
Hey Guys,
The answer to why the heavier BSOA derby cars most often win is simple. High school physics! Assume all other things equal … Potential energy, which is proportional to height, gravitation acceleration G, and “mass” is converted to Kinetic energy by the car’s trip down the ramp, which in turn carries it farther along the level part.
I have found that rearward weight distribution increases coasting speed and rolling distance on the flat. I first observed this at age 12 when coasting down hills on bicycles with friends. I applied this observation to pinewood derby cars and soap box derby cars and always finished near the top of the field or first place. I also maximized the weight of the vehicle to the maximum allowed, having also noted that the heavier cars always placed near the top of the field. Road-Apple has a very good explanation why the tail heavy vehicle gets a little more kinetic energy on the down hill section. I can only guess why the roll out is better with the tail heavy vehicle: possibly less rolling resistance with one axle lightly loaded, and/or the front steering or forks absorb more kinetic energy than the more rigidly mounted rear axle, and lightly loaded the front cannot absorb as much? Any other theories?
Gashmore is correct, except for his math. Velocity increases with the square of the height of the drop, so the momentum is increased by about 8.5%, not 4%. Way back when my son was in cub scouts and he made a pinewood derby car, I put the weight in the rear and I think I was the only one to do that because in each race, the cars would come down the incline in a dead heat, but at the bottom, his would suddenly shoot out about 2’ ahead of all the others and win by about 4’ over second place.
He won two years in a row. The second year, I was out to sea so he made his car all by himself, but remembered to put the weight in the rear. Because his cars won by such wide margins, there were protests and each car was carefully inspected to make sure it complied with all the rules, and it did.