Wheels & Times ?

[Momentum Racing]

What would be the approximate time difference between 1.4g and 2.0g wheels ?

Also, generally speaking, what % of the speed difference can be attributed to merely adding the extra 2.4g to the rear of the car,and not the MOI of the wheels?

 

[txchemist]

You can't add another 2.4 gr to the back of the car if you built it correctly in the first place- you would be adding it in front of where your weights already were, so most likely your net COG is infinitesimally better as it moves the net 2.4 gr from the midpoint of where the axles were to just above the weights already in the car.
So COG change will be close to 0.0005 sec. so not any difference at all. The whole story is the change in MOI. This is not a published value and varies with vendor. For most of us that means the fastest will be from DD4H.

If you do not have a super good prep, you will get some speed improvement but maybe not as much as a top 50 racer would get.

 

[Momentum Racing]

Thanks for the response Tx. I was thinking more of COM, I could manipulate both COM and COG by where I placed the 2.4g freed up by swapping the 2.0g wheels for 1.4g wheels . Not only limiting the extra 2.4g to ballast weight, but the possibility of utility weight in the form of something like fenders or air shields. Understanding the MOI of the lighter wheels will make the car quicker, I'm wondering what percentage in time reduction can be attributed to the lesser MOI of the wheels, and what % can be tied to fine tuning the extra 2.4g of "free" weight in the form of ballast or utility features.

I have a hunch the better MOI probably accounts for 95+ percent of the speed gain, in fact I would bet the lighter wheeled car would beat the heavier wheeled car even if you didn't add the extra 2.4g back into the lighter wheeled car.

Do you have any data on the values of lighter wheels being linear in speed increase? I suspect the values will not remain linear as the wheel gets lighter due largely to different profiles, but perhaps something like for every 1/10g in wheel reduction you can expect to see a.005 in time reduction.

 

[txchemist]

The terms "center of mass" and "center of gravity" are used synonymously in a uniform gravity field to represent the unique point in an object or system which can be used to describe the system's response to external forces and torques.

By a car built correctly, I meant one that had fenders if allowed - even with heavy wheels fenders will drop time enough to be used, so giving up a bit of COG to drop wind resistance is followed all the time. You also will not see a top car go back and stack two 1/4" cubes to move COG back because the drag with the thicker body is worse than a thin body with less aggressive COG.

When I said the whole story- I should have said 99.9% speed increase is lower MOI.

It becomes harder and harder to make a faster wheel because most of the easy gains are first removed-i.e. the plastic farthest from the hub. So going from 2.4 to 2 gr. is a greater speed increase than 2.0 down to 1.6 unless you allow for a thinner wheel along the way.
So not linear-just exponentially harder to squeeze more speed out of anything in a gravity car.
Just like it takes more and more energy to approach the speed of light- it takes more and more effort to approach the speed of gravity.

 

[TRE]

from my tests there isnt much if any speed difference on 2 gram vs 1.4 gram

 

[Momentum Racing]

For the practicality of gravity racing purposes I think it's wise to differentiate the two. Most refer to COM for longitudinal balance, and COG for vertical balance.Competitive cars might have a COM of 1/2" with a COG of 5/8".

I'm not sure anyone with an understanding of aerodynamics or friction coefficients would ever stack two cubes high, but one might take the extra 2.4g available in the form of tungsten and stretch it into a 1/32" H x 1/8" W x 1.75" L slab and place it across the back of the car. The distance behind the axle will effect COM, placing it on the top or bottom of the car will effect COG.

 

[gWebber]

Aaaaarrrrggggg, science, brain hurt, brain hurt

 

[Vitamin K]

Out of curiosity, is there any standard way to calculate the moment of inertia for a given wheel? Measure the amount of spin you get when a standard amount of force is applied to the wheel?

 

[Momentum Racing]

Out of curiosity, is there any standard way to calculate the moment of inertia for a given wheel? Measure the amount of spin you get when a standard amount of force is applied to the wheel?

I think Randy at MV did such a test comparing different types of lubes. I'm sure you could do something similar with different weight wheels, as well as equal weight wheels that are weighted differently.

I would like to see a chart graphing the optimal distance lighter wheels have a positive effect. Racing to the bottom of the hill the difference is marginal, at 35' it might be moderate, at 42' they may be nearing their optimal gains, at 49' possibly at their pinnicale, at 60' past their prime, and by 70' the heavier wheel wins.....at some point the heavier wheel will surpass it.

 

[Momentum Racing]

from my tests there isnt much if any speed difference on 2 gram vs 1.4 gram

That's interesting, and gives me hope /images/boards/smilies/smile.gif....a 30% loss in wheel weight with minuscule gains. Have you found much difference in something like a BASX 2.4g wheel and a SS 2g wheel ,which is less than a 17% weight reduction?

 

[txchemist]

I am sorry- but just because you draw a line on the bottom of your car where it balances and call that COM does not make the COM actually drop down to the bottom of the car. The COM is a point. The line is a way to discuss it, and over time the correct meaning starts to disappear from misuse, but COM and COG are absolutely the same thing, a point somewhere in the car body. It has X,Y and Z coordinates When you attempt to redefine them because you like to think of them differently-that is fine for you- but don't confuse others with incorrect terminology.

 

[B_Regal Racing]

Here is a hint:

I would suggest you no long worry about COM, but instead, concentrate on the weight on each wheel. I know you can calculate COM from the weight on the wheels, more or less, but, you can make your life so much simpler just with knowing how much weight is on each wheel.

 

[gdurban]

Out of curiosity, is there any standard way to calculate the moment of inertia for a given wheel? Measure the amount of spin you get when a standard amount of force is applied to the wheel?

See lectures 5 and 6 here: http://www.pinewoodderbyphysics.com/lectures.shtml

 

[Vitamin K]

See lectures 5 and 6 here: http://www.pinewoodderbyphysics.com/lectures.shtml

Thanks!

 

[Momentum Racing]

I am sorry- but just because you draw a line on the bottom of your car where it balances and call that COM does not make the COM actually drop down to the bottom of the car. The COM is a point. The line is a way to discuss it, and over time the correct meaning starts to disappear from misuse, but COM and COG are absolutely the same thing, a point somewhere in the car body. It has X,Y and Z coordinates When you attempt to redefine them because you like to think of them differently-that is fine for you- but don't confuse others with incorrect terminology.

Of course there is only one COG in the car. Derby racers refer to COM and COG to differentiate the two axes most are concerned with, little pay homage to the third axis. It's the common vernacular of the sport, not my interpretation. Your understanding of a line being a means to discuss it should clear up any misconceptions that may plague you in this area.

Most understand when a racer states their COM as 5/8" ,they are referencing the balance point in front of the rear axle, not the actual cars true COM, as you could not calculate that with only one value.

Additionally, COG and COM are not always the same, lest you confuse others./images/boards/smilies/wink.gif

.

 

[Vitamin K]

Most understand when a racer states their COM as 5/8" they are referencing the balance point in front of the rear axle, not the actual cars true COM, as you could not calculate that with only one value.

This has always kind of bugged me. Not great for newbies just getting into the hobby. I wish we could come up with a better term for it.

 

[Crash Enburn]

"Derby racers ... common vernacular..." I've been here for four years, and I don't recall anyone else making that particular 'distinction', Mo.

And when talking about a car that is ~1/4" thick, using weights that are essentially vertically centered in that body, your "CoG" is going to be right there in that waistline. There's no point in even mentioning it.

If you're talking about a 2" thick scout car, then the weight placement (high or low) is going to make a difference. But in that case, the kid doesn't really care about the minutiae of how high or low his CoM is, just that 'lower is better' and it is positioned such that his car won't be doing wheelies off the track after the transition.

VK - the typical conversation is 'how far in front of the rear axles the CoM lies', so the reference arose from that.

I suppose that one could start referring to the "ATC" ([rear] axle to CoM) distance. But even then we'd still have to explain CoM. /images/boards/smilies/biggrin.gif

 

[Momentum Racing]

"Derby racers ... common vernacular..." I've been here for four years, and I don't recall anyone else making that particular 'distinction', Mo.

Differentiating COM and COG ?

If so I will have to search, I know I've seen it numerous times here or over on DT.

 

[Momentum Racing]

This has always kind of bugged me. Not great for newbies just getting into the hobby. I wish we could come up with a better term for it.

I agree, it can be misleading to individuals just delving into Pinewood and the associated physics. However, I do feel it's easier than referencing the X,Y,Z, axes for most folks, which is probably why it came about to begin with.

 

[Momentum Racing]

Crash...when counting .0001's I feel fractional differences in the cars vertical COG can come into play. Weight placement in even a 1/4" thick car can matter....for example, how does the vertical COG change just by canting the rear axles ?... by positive cant in the front wheel ?...by adding the Basswood layer to the top or bottom of a ladder frame ? by adding a thin tungsten plate or putty to the top , bottom, or middle of the car...etc.

.