Alignment & COG height - what are they worth?

[Go Bubba Go]

Thought I would let you all in on a little experiment I have running. I don't have a test track yet, so I'm running it "on the road" (WIRL North Coast Open & PWDRacing.com May 5th race).

My older son built a rail car this year using Tungsten cubes. The car was thin enough that we could run it either side up. He decorated one side in Green featuring "Luigi" and the other side in Brown featuring "Donkey Kong" (see below).



Given that wheel prep, axle prep, horizontal weight placement, hub/body interface etc. are identical whether the car is running in Luigi mode (green side up) or Donkey Kong mode (brown side up), the remaining significant variables which do differ based on orientation are as follows:

Slight barge nose: Oriented downward in Luigi mode, upward in Donkey Kong mode
COG height: Slightly below axle plane in Luigi mode, slightly above axle plane in Donkey Kong mode
Alignment: Aligned straight and pretty well in Luigi mode, a little out of kilter (will get the specs when it gets back home) in Donkey Kong mode

Care to speculate about which orientation will run faster and, more importantly, why?

Got one data point from Lucky this weekend from the North Coast Open. Next weekend Mr. Slick and Warthog are gonna run it down 3 times in Luigi mode and 3 times in Donkey Kong mode to add a few more data points. It will be interesting to see the difference and magnitude...

p.s. Best experiment I could contribute at this time "sans practice track"

[pwdarchitect]

I know but I'm not sayin'

[Lucky13]

I know but I'm not sayin'

Me too !!

[PWD]

We will be interested to see the results. We were going to do something similiar this year but the problem is riding the rail. You can't do this test and ride the rail the same way.

Also any car, even with perfect alignment, is going to hit the rail. If the car has one wheel raised then I believe the faster side will be the side that the car veers into the rail towards the raised wheel (dominate wheel hits).

[Go Bubba Go]

We will be interested to see the results. We were going to do something similiar this year but the problem is riding the rail. You can't do this test and ride the rail the same way.

Also any car, even with perfect alignment, is going to hit the rail. If the car has one wheel raised then I believe the faster side will be the side that the car veers into the rail towards the raised wheel (dominate wheel hits).

Your comments brought to mind another thread where we first encountered the idea for a "2 sider".

Don't think I ever saw any details on how one side ran vs. the other in the case of Bob's car.

Our 2-sider was built as a 4 wheeler to fit our Council specs. Might have to build a 3 wheeled version next year for the Open Class adult races.

[Teeeman]

I'm going with whichever way the alignment works best will run best...

I place COM height as a "2nd order" variable...


(taking bets... taking bets... OK, who's running the betting pool on this one?)


-Terry

[PWD]

I totally agree with Teeman.

A better test would be to build the best car you can build with the ability to move the weight up and down. Ideally you might also allow for moving the weight horizontally in addition.

Personally I have not been convinced that lower weight really makes any difference. Yes we do design our cars with the weight as low as possible. But I have never seen any evidence that it makes a difference.

[Teeeman]

I totally agree with Teeman.

A better test would be to build the best car you can build with the ability to move the weight up and down. Ideally you might also allow for moving the weight horizontally in addition.

Personally I have not been convinced that lower weight really makes any difference. Yes we do design our cars with the weight as low as possible. But I have never seen any evidence that it makes a difference.

I rate wheel prep (including lube) and alignment and rear weighting as 1st order effectors.
(wheelbase shorter can make alignment harder so it kinda falls in this)

I rate CMy and focusing the mass of the weight as 2nd order.

I put barge noses and aerodynamics as 3rd order (in the noise).

Coriolis... if an effector at all... at least 4th order, maybe 15th order... (grin!).

I have to agree it would be cool to build a car with a peg hole system for just testing different weightings in a snap... we actually had talked of building just such an animal after we finish our test track which has stalled as we are wrapping up CMB for the year.



-Terry

[Go Bubba Go]

...A better test would be to build the best car you can build with the ability to move the weight up and down. Ideally you might also allow for moving the weight horizontally in addition.

At some point we are considering building a thin car like this for which we could mount some Tungsten Plates on top, run the times, then remount the Tungsten Plates on bottom. Maybe build a set of wood plates that resemble the shape of the Tungsten plates to mount on the opposite side of the car and cancel out any changes in aerodynamics / clearances.

In that way I believe we would cancel out the nose configuration issue (it would be the same regardless of the weight mounting, we would actually not be flipping the car) and pretty well cancel out the difference in alignment (it might differ slightly as a result of the shift in weight placement top to bottom, but I would think it would be very, very slight).

Anybody with a test track care to take that test on for us? We don't have ours (yet...).

p.s. After the test with the plates above and below in the same orientation (say 3 x 2 in one layer), then we would move on to changing the orientation of the plates when mounted on top (stack them high, spread them out, shift them from side to side, etc.). Keep the same front to rear weight placement, but change around the height and left/right density and placement.

[Teeeman]

You're welcome to use our track when we get it done...

just drop by, we're in Hunstville, ALA-BAMA! (Forest Gump voice here)...


-Terry

[Go Bubba Go]

Well, here's some data for you (we) data types.

On the PWDracing.com 42' Best Track (I believe that's their setup):

Luigi side up (in seconds):

3.0689 Heat 1
3.0631 Heat 2
3.0729 Heat 3
3.0647 Heat 4
3.0705 Heat 5
3.0626 Heat 6

3.0671 Avg
0.0042 Std Dev

Donkey Kong side up (in seconds):

3.1581 Heat 1
3.1694 Heat 2
3.1550 Heat 3
3.1688 Heat 4
3.1664 Heat 5
3.1664 Heat 6

3.1640 Avg
0.0060 Std Dev

Luigi side up the clear winner by 0.0969 seconds average.

How much is due to better alignment, and how much due to slightly lower CM, more work required to determine. My gut is that the lion's share is due to better alignment.

These results are very similar to the results Lucky13 saw when testing both sides the night before the North Coast Open.

Next stop - MaxV's shop for an "evaluation".

[PWD]

Bubba, thanks for the times. The difference is pretty major for just a difference in height of COG, IMO.