Wheel base.

[TedBull]

I searched but found nothing. I notice most of you guys run your wheelbase with the fronts set back a bit from the nose. The couple cars I have built I went with the max wheelbase thinking the car would be stable, and faster.

What is the reason and effect of setting the front wheels back? I want to run fenders all around, that means I need to cut a new body. Trying to figure out a good wheelbase to start with.

Ted.

 

[pony express]

One of the reasons the front wheels are back is the trade off for stability with the longer wheelbase with wheels further back favors the longer time the car is on the hill. Longer on the hill means more power. There are so many trade offs in the pinewood derby cars that you must experiment and see if the more power is worth it. It's just the same trade off when you don't put too much weight in the back. The longer wheel base cars are more stable, but they don't stay on the hill as long.

 

[OPARENNEN]

Personal opinion, Back axle back as far as possible, the front best at 5" (5.5" at most). 5 best for same reason as posted above by Pony.

 

[Skippy Kicky]

One of the reasons the front wheels are back is the trade off for stability with the longer wheelbase with wheels further back favors the longer time the car is on the hill. Longer on the hill means more power. There are so many trade offs in the pinewood derby cars that you must experiment and see if the more power is worth it. It's just the same trade off when you don't put too much weight in the back. The longer wheel base cars are more stable, but they don't stay on the hill as long.

I've never thought about that but it makes a lot of sense. Good stuff!

 

[TedBull]

That does make sense. Thanks guys.

 

[IAE Racing]

Quote:
Originally Posted by pony express
One of the reasons the front wheels are back is the trade off for stability with the longer wheelbase with wheels further back favors the longer time the car is on the hill. Longer on the hill means more power. There are so many trade offs in the pinewood derby cars that you must experiment and see if the more power is worth it. It's just the same trade off when you don't put too much weight in the back. The longer wheel base cars are more stable, but they don't stay on the hill as long.

I am not following "trade off when don't put too much weight in the back" Is this simply referring to an over aggressive COM having no control.

 

[txchemist]

Sorry to be a stickler, but "longer on the hill" as a comment to long wheel base can be miss-leading. The original question, I would restate as- two cars, both with back axle as far back as possible, and one with the front axle close to the front, and the other back a bit. Now this can be done with the same COG, but it is easier to keep the COG back with the shorter wheelbase. If both cars have the same COG, they have the same velocity at the end of the ramp regardless of where the front axle is.
Your other consideration is if you want front fenders, a little more room might be useful.

 

[MackayDesign]

Disagree,the front of the car comes up on plain sonner with wheels further forward,common sence

 

[ZZ Racing]

I agree that the wheels further back leaves the car on the hill longer. My favorite wheel base is 5" but I built a new street stock that is running a 4 5/8" wheelbase that is showing good potential. The main thing to remember when running these cars is just because one thing works for one guy doesn't mean it will work the best for you. You as a builder have to try different things with car setup and prep to find the best combination for you and your building style.

 

[txchemist]

I totally realize the front of the car starts to come up a bit earlier with the longer wheel base, but think about the very back of the car, and then think about the COG of the car. In a very short time, both types of cars are rolling on the flat. What matters is the velocity achieved on the ramp. "Longer on the hill" is OK to think about when it was due to a COG set farther back towards the back of the car, but two cars with identical COG even with different wheel bases will have the same velocity, and the back of the car hits the flat at the same time. ( assuming identical in other respects)

 

[Quicktimederby]

I totally agree with ZZ, for the longest time I was able to find my best speed with a longer wheelbase. But with some new designs I'm see better with a shorter WB. You got to figure what works best for you.

I agree that the wheels further back leaves the car on the hill longer. My favorite wheel base is 5" but I built a new street stock that is running a 4 5/8" wheelbase that is showing good potential. The main thing to remember when running these cars is just because one thing works for one guy doesn't mean it will work the best for you. You as a builder have to try different things with car setup and prep to find the best combination for you and your building style.

 

[txchemist]

So, is the "hill" over when the front wheels are on the flat, or is the hill over when all wheels are on the flat?
The answer is when all the wheels are on the flat. So two cars might have a slightly different path going through the curve, but they are identical just before the curve and just after the curve. As far as gravity is concerned, the hill is the height of the COG at the start compared to the flat. So if you start 48" above the flat, and you end up 48" lower, your speed REGARDLESS of the path you take will be 16 ft/sec. right when the COG hits the flat when all wheels are level. That is the same answer as dropping your car and spending almost NO time on the ramp. You hit the ground at 16 ft/sec. (of course this neglects friction and wind resistance), but you get the idea, gravity pulls on the car and continues to speed it up until the car is level.
Now imagine we double the ramp length and adjust a new curve so we still have a 42 ft overall track. The start angle will not be so steep, but the car is again 48" above the flat. So gravity gets to pull this car a lot longer than on our shorter ramp track. When the car hits the flat on this long slope track, it will be going 16 ft/sec. But is there any doubt if we could time the same car on both tracks which overall time would be the fastest.
Regular 42 ft track, or double long ramp 42 ft track with same car height ?

 

[OPARENNEN]

I know I am not the math wizz as TXchem, but my own logic says that there is an immediate slowdown the microsecond that the front wheels hit the curve. So my own logic says that full downhill force is longer if front wheels hit that spot a bit later. My SS car is about 10th on the average, but occasionally in top 5. Red Flag runs with 5" axle spread with wheels moved as far back as possible.

 

[txchemist]

OK, I think I have been attempting to say "It does not matter" to a new member just starting to build some new cars, and a lot of pros chime in with well it does make a difference. Now I did some work, and I think we could agree, if it is a useful effect, it should be easy to see it on the unlimited because with the lighter wheel in front, it is easier to get the COG back no matter what the base length is.
So all you unlimited pros can all chime in. I calculate that the shorter wheelbase would give you a pick up of 0.0002 seconds on the track at Johns. Now that just gets lost in the noise, but I would be interested in what wheelbase the unlimited pros choose.

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[OPARENNEN]

OK, I think I have been attempting to say "It does not matter" to a new member just starting to build some new cars, and a lot of pros chime in with well it does make a difference. Now I did some work, and I think we could agree, if it is a useful effect, it should be easy to see it on the unlimited because with the lighter wheel in front, it is easier to get the COG back no matter what the base length is.
So all you unlimited pros can all chime in. I calculate that the shorter wheelbase would give you a pick up of 0.0002 seconds on the track at Johns. Now that just gets lost in the noise, but I would be interested in what wheelbase the unlimited pros choose.

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I think this is about right. And in that class, that could be the difference of a couple of finishing spots. Look at the difference in Mar between 1s and 10th.

 

[txchemist]

look again, I have another zero in there. It was the difference between 1St and 2nd
and totally swamped by how fast the car next to you was.