Short vs. Long wheel base

[cycrunner]

If I remember correctly it is believed, and may have been demonstrated, that a long wheel base car is faster than one with a short wheel base - is that correct? This may be in part due to a longer wheel base car being more stable - less tendency to wiggle? What else favors the long wheel base it this is indeed true?

In visualizing this I think you will conclude that the distance the front wheels of a car with a short wheel base will travel is farther on the straight portion of the track than a car with the front wheels closer to the nose of the car and the starting pin. A longer distance should result in the car taking longer for the car to arrive at the curve transition.

I thing it is also true, however, that our cars will continue to accelerate as long as they are on the 27 degree straight section of the track - this assumes that the cars have not yet reached terminal velocity.

So if the car is accelerating over a longer distance it should reach the transition at a higher velocity. If it is faster at the time it hits the level section of the track it should be faster when it reaches the timer. Well, you might ask, does this higher velocity compensate and overcome the longer distance traveled on the straight section?

To answer this question I did an experiment with one of my cars. I used my Mini Cooper - Slow Motion which is only 3.9 inches overall instead of one of my 7 inch cars. It has a wheel base of 2.7 inches. Because it is this short I could move it back on the starting section of my track - my 7 inch cars are too long to be moved farther back on my start section.

First I ran Sow Motion in the normal mode with the nose on the start gate and with the dominant wheel on the rail and recorded several run times.

Then I moved Slow Motion back 2 inches on the start section so it actually traveled and accelerated for 2 inches farther than normal. I did this by attaching a 2 inch long extension to the inside of my starting gate which swings up and away from the nose of the car. This of course moved the car 2 inches farther back on the start section. I ran this way for several runs and recorded the run times. I made no changes to the car between all these runs. I started the car with the dominant wheel on the rail for all runs. I tried to eliminate as many variables as I could in this experiment.

In looking at all the run times it became apparent that starting the car farther back on my track, thus simulating a shorter wheel base, resulted in an average time of 0.008" faster. Apparently running farther on the slope allowed the car to reach the transition curve at a higher speed and in a shorter time.

I am looking forward to any comments any of you might have.

 

[Kinser Racing]

Cars will run faster when gravity has more time to work with the weight. I would love to see the the difference with the extension on the car. I'm guessing it would be about .010 faster.

The debate of wheelbase has always been somewhat of conundrum for me because I'm not sure that a shorter wheelbase car's weight falls further than a long wheelbase car, as I've always seen it debated.

 

[cygnus]

Trying to picture what you are describing...wouldn't starting your car further up the slope be faster, primarily, because you are raising the height of the COM, therefore creating more potential energy? I think that explains your .008 improvement in speed.

 

[Bullet]

I would think the final distance the weight falls is the same, for the same car/weight set up, no matter of the wheel base. It still needs to get to its position from the start gate to its position on the flat section of the track, the distance h will still be the same in the PE = mgh equation.

 

[pony express]

You're right that the weight will be at the same height and time during the curved section with a seven inch car, but the front wheels will reach the bottom of the slope earlier with the long based car. That of course will start to slow the car sooner than the short wheel base car. It's been a tough debate over the years. I know 5 inches is too long a wheel base for the street stock cars. How short a wheel base you can get away with is the question. Getting the back wheels further back has helped most racers more than changing the location of the front wheels.

 

[LightninBoy]

You're right that the weight will be at the same height and time during the curved section with a seven inch car, but the front wheels will reach the bottom of the slope earlier with the long based car. That of course will start to slow the car sooner than the short wheel base car.

That is a perfect explanation.

 

[LightninBoy]

To answer this question I did an experiment with one of my cars. I used my Mini Cooper - Slow Motion which is only 3.9 inches overall instead of one of my 7 inch cars. It has a wheel base of 2.7 inches. Because it is this short I could move it back on the starting section of my track - my 7 inch cars are too long to be moved farther back on my start section.

First I ran Sow Motion in the normal mode with the nose on the start gate and with the dominant wheel on the rail and recorded several run times.

Then I moved Slow Motion back 2 inches on the start section so it actually traveled and accelerated for 2 inches farther than normal. I did this by attaching a 2 inch long extension to the inside of my starting gate which swings up and away from the nose of the car. This of course moved the car 2 inches farther back on the start section. I ran this way for several runs and recorded the run times. I made no changes to the car between all these runs. I started the car with the dominant wheel on the rail for all runs. I tried to eliminate as many variables as I could in this experiment.

In looking at all the run times it became apparent that starting the car farther back on my track, thus simulating a shorter wheel base, resulted in an average time of 0.008" faster. Apparently running farther on the slope allowed the car to reach the transition curve at a higher speed and in a shorter time.

I am looking forward to any comments any of you might have.

That is a clever experiment. Thanks for sharing. The results seem pretty reasonable too. What I really like about this is that by using the same (presumably stable) car you effectively removed the stability variable.

 

[GravityX]

...shorter wheelbase car's weight falls further than a long wheelbase car, as I've always seen it debated.

I'm pulling this thought over from another thread. It was said the front of the car rises in relationship to the rear of the car as it transitions through the curve of the track. And through this transition rear toe-out is created. To lessen this affect to the car it would stand to reason a shorter wheelbase would be a little more stable through the transition. But as with anything pinewood derby car, there is a happy medium in the build. So many variables...

 

[OPARENNEN]

On a similar/different subject. I only did this on my test track, sitting right at the transition point. But it really seemed like every time, the cars had a significant slingshot appearance to them thru the transition, almost as if they had picked up a bit of speed. Is that possible, or was it only an optical illusion.
Txchemist -- opinion?

 

[pony express]

That transition area is not an illusion. I've seen it myself. If the car is going to wobble it would start here. It's definitely some type of sling shot effect. Getting the most out of this area is tough.

 

[LightninBoy]

If by "slingshot" you mean the car appears to accelerates at a greater rate in the transition than on the incline, then I think what you are seeing is an optical illusion there Opa.

The car accelerates both on the incline and through the transition. So the car is in fact speeding up as it goes through the transition. However, it accelerates at a lower rate in the transition than on the incline.

I think the "slingshot" perception is that of the velocity being converted from a downward angle to the horizontal plane, so speed on the horizontal plane is greatly increased through the transition. And I think our eyes perceive speed better on the horizontal plane than on an angle.

 

[Bullet]

Quote:
Originally Posted by pony express
You're right that the weight will be at the same height and time during the curved section with a seven inch car, but the front wheels will reach the bottom of the slope earlier with the long based car. That of course will start to slow the car sooner than the short wheel base car.

This seems to be most logical engineering explanation for me. Going back to the rotational energy it takes to rotate the car...this small bit taken from the PE gained as compared to continuing on the same path. The extra downward force on the wheels through this transition would also be lost energy to change the direction of the car in space....

I know this has probably been beat to death over the years prior to my new PWD career, but...

Are the tracks with gradual and longer transitions (like Freedom) typically slower, faster, or same as the Best transitions all other things being equal??

 

[Guest]

Please pardon me for asking the obvious here Cycrunner but if you have an extension on the gate then the car not only has more PE but it is also traveling farther that the car resting on the pin.
Right?

It is my belief that the longer wheel base will win out for a couple of reasons.

 

[cycrunner]

Please pardon me for asking the obvious here Cycrunner but if you have an extension on the gate then the car not only has more PE but it is also traveling farther that the car resting on the pin. Right? It is my belief that the longer wheel base will win out for a couple of reasons.

Yes it is traveling further, but I was trying to see if the increase in potential energy by being further back and higher would over come the increased traveling distance. In my test this did happen by a faster time of .008 seconds. I don't see that the longer wheel base would be an advantage here.

 

[LightninBoy]

Please pardon me for asking the obvious here Cycrunner but if you have an extension on the gate then the car not only has more PE but it is also traveling farther that the car resting on the pin. Right?

Not quite. It has more PE because it will travel farther than the car resting on the pin. It is really the same thing.

The point Cycrunner is making is that, taking stability out of the equation, reducing the wheelbase by two inches is the same as moving the car up the ramp 2 inches via a pin extension. Both result in the car falling 2 inches further.

 

[cycrunner]

Please pardon me for asking the obvious here Cycrunner but if you have an extension on the gate then the car not only has more PE but it is also traveling farther that the car resting on the pin. Right? It is my belief that the longer wheel base will win out for a couple of reasons.

Yes it is traveling further, but I was trying to see if the increase in potential energy by being further back and higher would over come the increased traveling distance. In my test this did happen by a faster time of .008 seconds. I don't see that the longer wheel base would be an advantage here.

Here is a related question regarding using a shorter wheelbase car. How far back can the front wheels be before the nose of the car hits the curve portion of a Best Track? Has anyone checked this out?

 

[DerbyDad4Hire]

The wider and heavier the wheel, the shorter the wheelbase.

 

[Guest]

So for the 2 classes of Royal cars (Ambassador and Ranger), extend the WB a bit compared to BSA and Awana cars or was that tip geared more towards Unlimited and Eliminator type razor wheels- it seems the BigDogs run fully extended in those classes.