An audiophile and petrolhead's musings: in the hi-fidelity, first class, traveling set - Page 34

Well, as the camper said to his friend as they were running away from the bear: "I don't have to be faster than the bear, I just have to be faster than you."

The Dad's in our scout troop were all rookies - they made the classic mistake of thinking aerodynamics mattered. It was like shooting fish in a barrel...

 

Aerodynamics wouldn't come into play until you broke 60 mph or so, would they? (asks the non-engineer)

 

LOL, he wouldn't let you! 

 

 

Hmm, suprisingly no.  I think I was actually just really tired.    However, I can tell you that they fit extremely easily.  No fussing with insertion or positioning.  It's almost like they just stick on to invisible magnets in my ear holes.  The full review (do to the nature of my reviews) should be out in a couple of weeks.

 

 

See, you say that now.  But I've heard that the road between cush pads and a call to Martin Custom is a fast and slippery slope.  Why should these be any different? 

 

 

Easy peasy. 

I would need to do some calculations to give you a real answer, but in general, you are correct - the speed is far too low and the distance of the race is far too short for aerodynamic forces to be significant. In this application, the various friction forces are much more significant. The total energy in the system is essentially fixed - it is the potential energy based on the height above the ground of the starting position. That means the only force providing positive acceleration is the weight of the vehicle - therefore the max acceleration down the track is constant and is the vector component of the weight. Most tracks have a section that drops the height, then a run-out section at floor level to the finish. All of your positive acceleration must take place during the drop, and your max speed is at the point where the track becomes level. After that, the car will be slowing down all the way to the finish. Any forces acting opposite to that forward vector reduces your acceleration and slows you down. Those forces are friction (axles & wheels) & aerodynamic forces. The aerodynamic force (Drag) is primarily Profile Drag & Induced Drag. Induced Drag is drag caused by the Lift generated by the body, and that is insignificant. Profile Drag is based on the shape, the dynamic pressure & the surface area. The dynamic pressure is 0.5*air density*velocity_squared. So, the Profile Drag varies with the square of the velocity. That's why drag becomes so important as the speed increases. I don't know the relative magnitude of the forces on a Pinewood Derby car, but the axles are by far the largest component of retarding force. If you get those wheels to spin straight and true and smooth as silk on the axles, the car will do well no matter what it looks like. A heavy car with poor axles will be first down the incline, but will then decelerate and be passed on the run-out. A light car with good axles may not have a high enough max speed at the bottom of the incline to make use of those great axles.

Sorry - I kind of geeked-out for a while, and then I had written so much that it seemed a shame not to post it...

 

Nah, that was cool!  Not that we would have known if you decided not to post it... but glad you did!

 

Just had an awesome thought...  all of the world's car manufacturers should do a pinewood derby for fun and bragging rights!

So billy if I understand that right - my block warrior is likely near ideal without custom milling axles or whatever else, since it will pull down the ramp fastest and won't be retarded abruptly by the wheels on the flat section. Aye?

Well, your weight distribution really isn't ideal. From a conservation of energy perspective, you want the maximum amount of weight at the very rear of the car. That makes gravity act on the mass as long as possible, therefore giving you the maximum velocity at the transition point from slope to flat. HOWEVER, putting all the weight at the very back will make the front end too light and the car will be unstable and weave or bounce. That is very, very bad - just like the bobsled & luge, you want as clean a run as possible. The compromise position is usually slightly in front of the rear axle. That puts the weight between the wheels, biased to the rear. I think rule of thumb is ~1" in front of the rear wheels. That's why we had a car that looked like a skateboard - and then we stacked all the weights just ahead of the rear wheels.

So basically rip off what F1 cars do, and ignore aerodynamics?

Yup. I suppose if you really want to get the last possible piece of the puzzle, then reducing the frontal area would be good - but not at the expense of minimizing the rolling friction and maximizing the energy transfer. The deal about having a raised front lip only helps if the start mechanism is a simple peg that is pulled down through the track to start the cars. If it's a fancier starting gate that moves very fast, the lip will be insignificant. On the track we had, you could easily see our car start to move before all the other cars. It was a car length ahead before it was half-way down the slope.

Wow - I just found a guy on YouTube that came up with a brilliant pinewood derby tip:


Edited by billybob_jcv - 9/29/12 at 11:02pm

 

So, a typical Scout dad then... Well at least he won the Pack trophy. 

lol so you are the Chef of the house....

 

But could you kick a football and then shoot it out of mid-air?  That would have been boss!

 

 

Hah, exactly the same thing here!  I have a deal with her.  I cook, she cleans.  I think I gots the better end of that bargain!