Greetings! Is rear-most weight placement more desireable than a tapered rear car and rear fenders? In looking at Lightnin Boy's video on Youtube (
) it apprears that a car will get held back by turbulence. On my sons current car we do not have rear fenders and the COG is around 3/4" I'm guessing that would move up to a bit over an inch if we tapered the back of the car and had rear fenders. Not sure which is better..
Weight placement vs Aerodynamics
- Thread starter Matt's Dad
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When in doubt, watch what the Pros do. There have been cars with trailing rear fenders built before (see Mid America cars and World Record Track cars), but I don't see any Pros currently running them in league races, which leads me to believe there is no significant advantage to be had - at least not one that outweighs the potential energy lost by moving that weight up.
They may make more sense in a scout race where the quality of the track is in doubt and you'd want to move the COM up anyway for stability. In that case, it may not hurt to try them.
They may make more sense in a scout race where the quality of the track is in doubt and you'd want to move the COM up anyway for stability. In that case, it may not hurt to try them.
One reason most of the pros don't run rear trailing fenders is because we generally don't have room behind our rear wheels. Cub Scout axles, if the wheelbase has to remain 4-3/8", are moved forward on the rear of the car some, leaving more room for a rear trailing fender.
The scouts I worked with last year did not have rear trailing fenders, but one has asked for them this year, so we made a set for his car. Looks pretty sharp... will know how it races in a couple of weeks. It is still in the construction phase right now.
Good point. I forget some districts still require stock wheelbase.
The reason there's no room behind the rear wheels on our Pro cars is because we don't have to run stock wheelbase. The reason we don't chose to run stock wheelbase is because there's more speed in pushing those rear axles (and weight) back as far as possible before losing stability. I wouldn't do this on most of the poorly maintained Scout tracks I've seen.
Our council let's us drill our own holes which is nice. I do worry about losing stability with too aggresive a COG at our pack race. It seems there's always someones car flying off.
I appreciate the answers - I think I'll forget about the rear fenders.
I appreciate the answers - I think I'll forget about the rear fenders.
That sucks. We may need to rebuild all our router jigs.
https://en.wikipedia.org/wiki/Wheelbase
We kept the distance the same but moved it back a touch; to me this is still "stock wheelbase" but I sure don't want my boy DQ'd over my aggressive rules interpretation.
https://en.wikipedia.org/wiki/Wheelbase
We kept the distance the same but moved it back a touch; to me this is still "stock wheelbase" but I sure don't want my boy DQ'd over my aggressive rules interpretation.
Wheelbase does technically mean distance between wheels, but when we talk about Stock, it usually means location as well.
Rules vary from pack to pack. Our rules allow for any wheelbase. Some actually require that you use the original slots. Sounds like your rules may not be the best ever written.
Rules vary from pack to pack. Our rules allow for any wheelbase. Some actually require that you use the original slots. Sounds like your rules may not be the best ever written.
Totally your call, but I would ask them to review that rule before I had my scout stretch the wheelbase to meet it. Not really a massive speed gain to be had by the extra 3/8" of wheelbase and they could DQ your son if they don't decide in your favor on race day. Technically, you'd be correct but that's obviously not the length they meant when they wrote "Wheelbase may not be changed from the body kit distance........." . It needs to be brought to their attention so it can be corrected though. If they don't want people to alter the stock wheelbase location then they need to add a measurement from the nose to rear axle as well. Our rules state "i. The maximum allowable distance from the front of the car to rear axle is 6 1/16 inches. "
The quote above was from the 2016 rules. I asked the scout leader to forward the actual rules for this race to see if there were any changes, and the rules he re-sent to everyone were pretty much the same as last year, except these rules omit even mentioning axle location or distance. We like building cars, so I think we will build a backup car with stock wheel base and location, but I don't feel like there is any rule in place right now that prohibits our current axle location.
Here are the actual rules that were sent:
Car Dimension Rules
The overall length of the car shall not exceed 7 inches.
The overall width of the car shall not exceed 2 ¾ inches.
The car must have 1 ¾” clearance between the wheels.
The car must have 3/8” clearance underneath the body so it does not rub on the track.
Derby Car Weight Rules
The car shall not exceed 5.0 ounces.
The official race scale that is used at car check-in shall be considered final.
Car Modifications Not Allowed
The official pine wood block must be used. The block may be shaped in any way that is desired.
Official BSA wheels must be used. The wheels may not be cut, drilled, beveled or rounded. You may remove the seams and imperfections from the wheels.
The axles may be altered, polished and lubricated.
Other Pinewood Derby Race Rules
Once a car passes inspection and is entered into the race, only race committee members can touch it.
If the car loses a wheel, or is otherwise damaged, the racer shall have 5 minutes to make repairs.
Each car must pass inspection by the official inspection committee before it will be allowed to compete.
The Inspection Committee has the responsibility to disqualify those cars that do not meet these rules.
Car Dimension Rules
The overall length of the car shall not exceed 7 inches.
The overall width of the car shall not exceed 2 ¾ inches.
The car must have 1 ¾” clearance between the wheels.
The car must have 3/8” clearance underneath the body so it does not rub on the track.
Derby Car Weight Rules
The car shall not exceed 5.0 ounces.
The official race scale that is used at car check-in shall be considered final.
Car Modifications Not Allowed
The official pine wood block must be used. The block may be shaped in any way that is desired.
Official BSA wheels must be used. The wheels may not be cut, drilled, beveled or rounded. You may remove the seams and imperfections from the wheels.
The axles may be altered, polished and lubricated.
Other Pinewood Derby Race Rules
Once a car passes inspection and is entered into the race, only race committee members can touch it.
If the car loses a wheel, or is otherwise damaged, the racer shall have 5 minutes to make repairs.
Each car must pass inspection by the official inspection committee before it will be allowed to compete.
The Inspection Committee has the responsibility to disqualify those cars that do not meet these rules.
I emailed our district to make sure I could move back the rear axle as long as I kept the stock 4-3/8" wheelbase. I'm printing that email out and taking it with us in case the judge that morning sees it differently.
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