Losing speed on the flat...

[Stan Pope]

Actually, WW's numbers were 1.22 oz @ 7/8", so I wonder about his measurement.

So are you saying that, by definition, if you get the CM in precisely the right spot, then the weight on the front wheels will always be 1 ounce?

If so, may I then conclude that one way (the best way?) to ensure the CM is placed accurately is to set the front wheels on scale as described above, and tweak the weight placement in the rear of the car until the front weight is 1 ounce?

I think this is what I was trying to ask in the first place...

With some assumptions that I think you made, yes.

You can consult your high school physics books to see the relationships.

Or, you can review the torques about the CM as the car sits quietly on the table (i.e. is stable or, as the physicists would say, in equilibrium). The torques about the CM must balance. If they didn't, the car would flip over. (Well, actually, one end or the other would raise up.)

Looking at the torques just about a horizontal axis perpendicular to the longitudinal axis and through the CM: the downward force by the front wheel(s) times the horizontal distance from CM to the front axle must just equal the downward force by the rear wheel(s) times the the horizontal distance from the CM to the rear axles. (Torque = moment arm length times perpendicular force)

You know the total forces down, the weight of the car. And you know the total of the moment arms, the wheelbase.

A little algebraic slight of hand and you see that things are exactly as you surmised.

Ya?

(Hope ai didn't make too many typos... the print is almost too small to proofread! I may have to borrow Great Aunt Millie's big magnifying glass!)

Now, why would someone measure the location of the CM rather than the weight on the front and rear wheels? Well, it is a practical decision. Not everyone has easy access to a sufficiently accurate scale, but almost everyone has a knife edge. (Actually, it is easier to use the narrow but flat edge of a yard stick, since balanced on a knife edge, the car is not stable.

[Darin McGrew]

(Hope ai didn't make too many typos... the print is almost too small to proofread! I may have to borrow Great Aunt Millie's big magnifying glass!)

That's because the BBS software specifies font sizes in pixels, rather than using the font size your browser is configured to use.

In MSIE, you can fix this by selecting the

Tools > Internet Options... > General > Accessibility... > Ignore font sizes specified on Web pages

configuration setting.

Better browsers like Opera or Mozilla also have a minimum font size setting that will override document-specified fonts that are too small, but not those that are reasonably large. (Where "reasonably large" depends on the user's eyesight, display, etc.)

[MERuhl]

Thanks, folks. I think I've got it now. Come to think of it, I never took physics in high school...no suprise there, eh?

[Barga Racing]

When you said "support the opposite end of the car", I hope you meant that you supported the car by its wheels.

You don't mention tread treatment.

Yes I supported the backwheels at exactly the same height as the scale and weighed the front wheels (actually only one of them touches).

As for tread treatment they were highly polished and coated with graphite. As I have been reading it sounds like that is not a good idea.

I know the physics may say my measurements are not possible but I am telling you that is what I got. Also I determined CM, or CoG as some people know it, by balancing the car on the edge of a ruler and then measuring how far that was in front of the axle. I also forgot to mention that we cut a 1/4 inch off the back and glued it on the front, but a 1/4 in. of pine does not weigh that much.

[Stan Pope]

Yes I supported the backwheels at exactly the same height as the scale and weighed the front wheels (actually only one of them touches).

Excellent! You "done good!" We finally concluded that was probably the case.

As for tread treatment they were highly polished and coated with graphite. As I have been reading it sounds like that is not a good idea.

Yes, that forces you to move CM forward to retain stability. Be like us Cub fans ... wait till next year!

I know the physics may say my measurements are not possible but I am telling you that is what I got. Also I determined CM, or CoG as some people know it, by balancing the car on the edge of a ruler and then measuring how far that was in front of the axle. I also forgot to mention that we cut a 1/4 inch off the back and glued it on the front, but a 1/4 in. of pine does not weigh that much.

The rearrangement of the wood doesn't affect the subsequent measurements.

Please confirm that CM determination was done with the wheels installed.

I think that we concluded that a small variance in the measurement of CM could account for an apparently large variance in the front wheel weight.

Here are some numbers from theory where
Dr Longitudinal Distance from CM to rear axle
Wf Ounces of weight on front wheel tread
Total Wt: 5 oz
Wheelbase 4.375"

Dr Wf
0.875 1.00
1.000 1.14
1.125 1.28
1.250 1.43
1.375 1.57
1.500 1.71

Moving CM 1/8" changes weight on front by 1/7 oz

Darn, now I have to go find your original numbers and compare!

[Stan Pope]

In MSIE, you can fix this by selecting the

Tools > Internet Options... > General > Accessibility... > Ignore font sizes specified on Web pages

Thansk... and if I adjust the view properly, it actually fits on the screen!

The reference to Great Aunt Millie is probably strange to you ... Great Aunt Millie is the counter example for a proposal to eliminate all "inactive" accounts from the SETI@home statistics, fabricated from whole cloth, of course, with a sprinkling of "[censored] and Old Lace". She has been working on her first SETI@home work unit for several months and has hopes of completing it before the curtains are drawn on this phase of the SETI@home project. I have posted details of her activities on the SETI@home Cruncher's Corner message board and has generally become an icon of sorts.

I get occasional inquiries into her progress on that work unit. (for perspective, most participants complete 3 or more work units each day if they leave their computer running 24/7. Of course, Great Aunt Millie only leaves her computer running while performing her daily email activities. And her old 25MHz machine, if left to run, would take many days to finish a work unit!

In the end, it was just Stan having a bit of fun.

[Darin McGrew]

In MSIE, you can fix this by selecting the

Tools > Internet Options... > General > Accessibility... > Ignore font sizes specified on Web pages

Thansk... and if I adjust the view properly, it actually fits on the screen!

Yeah, the design of the HTML and CSS of this BBS leaves something to be desired. My user style sheet includes rules like

Code: Select all

table, tr, th, td {
	max-width: 100% !important ;
}

to try to force fixed-width layouts like this one to stop extending wider than my browser window. But that assumes a browser that supports the max-width property (e.g., Opera or Mozilla, but not MSIE).

[gpraceman]

Yeah, the design of the HTML and CSS of this BBS leaves something to be desired. My user style sheet includes rules like

Code: Select all

table, tr, th, td {
	max-width: 100% !important ;
}

to try to force fixed-width layouts like this one to stop extending wider than my browser window. But that assumes a browser that supports the max-width property (e.g., Opera or Mozilla, but not MSIE).

Software is only as good as the programmers that wrote it (in this case the Open Source Community). Personally, I think this type of information is best communicated via PM.

Let's get back to discussing the topic at hand...

[Barga Racing]

please confirm that CM determination was done with the wheels installed.

I think that we concluded that a small variance in the measurement of CM could account for an apparently large variance in the front wheel weight.

Stan,
I took your advice and double checked my CM measurements. It turns out that the CM is a hair under 1 1/16 of an inch. So either I figured it before with the wheels off or I just plain screwed up. Thanks for setting me straight.

[Stan Pope]

I took your advice and double checked my CM measurements. It turns out that the CM is a hair under 1 1/16 of an inch. So either I figured it before with the wheels off or I just plain screwed up. Thanks for setting me straight.

Thanks for the recheck.

I am always relieved when theory and reality match up. But, I am spurred to action when they don't ... that means that there is an opportunity to learn something.