Car's Weight on the Ramp

[joe]

Stan, Teeman, other physicists/engineers, etc...people who know more than me: I got to thinking that my technique for measuring the weight on front wheels with elevated back end was not correct. While it showed that a car with a COG higher than axles had more weight on front wheels, the front wheels were on the flat and so were the elevated rear wheels, and the nose of the car was not being supported by a "starting gate." So I set up a little test with my scale, "accurate" to .001 oz. whereby the front wheels were supported on a 30 degree ramp on the scale, rear wheels independently resting on 30 degree ramp (same plane) off scale, nose of car resting on independent "startgate" which is off scale also. Teeman is correct, the start gate is supporting some weight, and the angle of the start gate in relation to the track also makes a difference. If the startgate is angled towards the nose of the car, the weight on the front wheels increases, if the startgate is angled perpendicular to the track, the weight on the front wheels decreases -- and I assume weight supported by gate increases.

The scale is extremely touchy in this configuration and tiny shift in any portion of test ramp/gate will change weight reading (why is that by the way?). Nevertheless, this is what I found with perpendicular startgate-- suspect method and all: On our cars that have COG at or below axle plane - we have 3 of these -- the weight on front wheels stayed about the same with elevated rear, possibly a bit less, but would be less than .1 oz. in any case. On cars with higher COG (1/4" to 3/4" above axle plane) have 4 of these that I measured -- the weight on front wheel(s) was REDUCED from 2.26oz to 1.81; 2.3 to 1.75; 1.38 to 1.1; and 1.85 to 1.6. The larger reductions had higher COG. Do you see anything applicable to a derby race here?

Better yet, do you see thing(s) wrong with my method?

[Stan Pope]

Cool study! Good to actually test theory once in a while!

On the "why is that by the way?" question, I think even if the component slopes do not change, there will be enough linkage between the car nose and the starting pin that relative motion between the pin and slope may drag the nose of the car toward or away from the track, resulting in an increase or a decrease in the measured "weight."

One not very useful application of the knowledge is to understand why the braking force of wheel/axle friction seems so much more significant on the flat than on the slope. The "braking force" is (approximately) proportional to the force perpendicular to the track. Your tests show it to be reduced when the track is sloped.

I think that the actual reduced weight should be calculatable as proportional the cosine of the slope. Many tracks slope at 30 degrees, and cosine of 30 degrees is 0.866. Oh, oh, I think that I recall another senior moment in which I refered to that fraction as 1/2g ... which, of course. is the forward acceleration down the slope, sine of 30 degrees!

Your measures of "track normal force" for flat and sloped sections with elevated CM cars show ratios less than 0.866, which suggests that weight was not transferred to the front wheels when the car orientation changes. What could be preventing the car from adding that weight to the scale? Do you get a different reading if you
1. place the car on the device with low slope, then increase the slope to 30 degrees, or
2. set the slope to 30 degrees and then place the car on the scales and allo it to run down to the pins?

It is possible with "1" above that adhesion between the car nose and the pin prevents the car from sliding down the pin. The lighter the front-end of the car, the less likely the car is to slide down the pin, too.

[joe]

Cool study! Good to actually test theory once in a while!

On the "why is that by the way?" question, I think even if the component slopes do not change, there will be enough linkage between the car nose and the starting pin that relative motion between the pin and slope may drag the nose of the car toward or away from the track, resulting in an increase or a decrease in the measured "weight."

This "linkage" seems to make sense -- because moving any part of the apparatus, or even pushing down hard on the table seems to affect the weight, and this is easily explained by minute movements of my "startgate" which of course is touching the nose of the car. Indeed, when the slope is adjusted, the weight varies, and it has to considering if the ramp were in theory rotated to 90 degrees, then the entire weight of the car would be supported by the startgate.

I think I will reconfigure with the entire slope and car resting on the scale, and try to see what component of the weight is supported by the startgate at different angles of the ramp.

[WarpSpeedINC]

Stan, Teeman, other physicists/engineers, etc...people who know more than me: I got to thinking that my technique for measuring the weight on front wheels with elevated back end was not correct. While it showed that a car with a COG higher than axles had more weight on front wheels, the front wheels were on the flat and so were the elevated rear wheels, and the nose of the car was not being supported by a "starting gate." So I set up a little test with my scale, "accurate" to .001 oz. whereby the front wheels were supported on a 30 degree ramp on the scale, rear wheels independently resting on 30 degree ramp (same plane) off scale, nose of car resting on independent "startgate" which is off scale also. Teeman is correct, the start gate is supporting some weight, and the angle of the start gate in relation to the track also makes a difference. If the startgate is angled towards the nose of the car, the weight on the front wheels increases, if the startgate is angled perpendicular to the track, the weight on the front wheels decreases -- and I assume weight supported by gate increases.

The scale is extremely touchy in this configuration and tiny shift in any portion of test ramp/gate will change weight reading (why is that by the way?). Nevertheless, this is what I found with perpendicular startgate-- suspect method and all: On our cars that have COG at or below axle plane - we have 3 of these -- the weight on front wheels stayed about the same with elevated rear, possibly a bit less, but would be less than .1 oz. in any case. On cars with higher COG (1/4" to 3/4" above axle plane) have 4 of these that I measured -- the weight on front wheel(s) was REDUCED from 2.26oz to 1.81; 2.3 to 1.75; 1.38 to 1.1; and 1.85 to 1.6. The larger reductions had higher COG. Do you see anything applicable to a derby race here?

Better yet, do you see thing(s) wrong with my method?

Funny thing Joe,
After hearing comments last week over wiggle on flat being related to weight being less on the front after transition, and after thinking the cars always seem lighter on the front during staging, I performed the same experiment.
I set my scale up with a wood dowel epoxied to the scale 90 degrees to the pad surface. I then mounted another peice of flat stock just above the readout face for the rear wheels to sit on.
Starting on a level surface, I zero'd the scale, set the car on and recorded the reading. I then removed the car, set the scale at 30 degrees, re zero'd scale, then "staged" the car as normal. The weight on the front was allot less.
Of course I have lost the paper I jotted this stuff down on, but my findings seem consistant with yours. Which only stands to reason. The steeper the slope, the less percentage of weight that is on the wheels, but the more percentage of weight that is pushing the car. If there was no change, or opposite change, the car wouldn't know it was on a hill, and would just sit there.

Right?

Warp Speed

[Stan Pope]

Indeed, when the slope is adjusted, the weight varies, and it has to considering if the ramp were in theory rotated to 90 degrees, then the entire weight of the car would be supported by the startgate.

If you actually performed the 90 degree rotation, I think that there is a good chance that the scale would still show deflection. The reason is that friction between the start gate and the car nose would resist the scale's attempt to push the car away from the scale.

I think I will reconfigure with the entire slope and car resting on the scale, and try to see what component of the weight is supported by the startgate at different angles of the ramp.

As a matter of procedure, you possibly should recalibrate the scale in the new orientation. Many such scales compensate for the weight of the pan, and the force of the pan changes when the scale is tipped.

Also, consider using a 0.01 scale, which should give adequate measures but not be so sensitive to extraneous environmental factors.

[Stan Pope]

If there was no change, or opposite change, the car wouldn't know it was on a hill, and would just sit there.

Right?

Warp Speed

Love it!

Nonsensical tangent: If I built a "really dumb" car, might it be "incapable of knowing" and, in its ignorance, just sit there even as the slope increased up to 90 degrees?

Seriously, I think that Joe is assessing two aspects:

The first is the track normal force independent of shifting the CM over the supporting wheel base, but simply due to the track slope.

The second aspect is the effect on track normal force as the CM gravity vector moves forward over the wheelbase.

We observe the first aspect clearly from his experiment so far.

The second aspect seems to be masked a bit by friction noise in the measurement technique.

In analyzing, it is helpful if all location measurements happen relative to the intersection of the track and the start pin. That will avoid some red herrings (and other false trails). For instance, the CM gravity vector moves forward relative to the wheelbase so long as the CM is above the track. And it would move backward if the CM were below the track surface. (I think that I incorrectly referenced the axle plane earlier.)

[Stan Pope]

This is another critical case in analysis: Suppose that the CM were raised above the axle plane so far that when the car is tipped 30 degrees the CM is exactly above the point where the front wheels contact the track. Under those conditions, all weight has been transferred to the front wheels, the car is balanced on its front wheels, and the scale under the rear wheels should read zero!

From this analysis, it should be easy to see that as CM is lowered in the car, less weight transfer occurs.

[joe]

First I am going to see how various angles of incline and the position of COG affect the overall weight of the car -- I don't think this has any real application other than satisfying my curiousity. But my technique is different from yours Warp. I did not tilt the scale but built instead a tiny ramp for front wheels on the scale so the scale remained flat. Are we measuring the same thing with the two different methods?

Do you see any possible application other than theoretical? For example, would a car with a very light front end and high COG in the rear (ala BatWing) become even lighter on front wheel and possibly lose ability to steer on the ramp? Stan, could there be implications here regarding your 4 oz. rear weight/1 oz. front weight depending on vertical location of the COG? Looks like the low COG might let you go further with this on the ramp, although granted, most of the problems manifest themselves on the flat when the car "reloads." Interesting!

[Stan Pope]

First I am going to see how various angles of incline and the position of COG affect the overall weight of the car -- I don't think this has any real application other than satisfying my curiousity. But my technique is different from yours Warp. I did not tilt the scale but built instead a tiny ramp for front wheels on the scale so the scale remained flat. Are we measuring the same thing with the two different methods?

To study friction while the car is on the ramp, you must use the normal force (i.e. the force perpendicular to the track) at that point. When it is at the starting pin, the weight vector (vertical, due to gravity) divides between the force perpendicular to the track and the force perpendicular to the starting pin. (This separation follow the rules of vector addition, and you could end up with a one ounce car showing 0.7 oz against the track and 0.7 oz against the starting pin if the track were sloped at 45 degrees! Yes, sounds crazy, doesn't it? But it is true.)

Your scale, to measure this force correctly, needs to have its sensor oriented in the correct direction and calibrated in that orientation.

Do you see any possible application other than theoretical? For example, would a car with a very light front end and high COG in the rear (ala BatWing) become even lighter on front wheel and possibly lose ability to steer on the ramp? Stan, could there be implications here regarding your 4 oz. rear weight/1 oz. front weight depending on vertical location of the COG? Looks like the low COG might let you go further with this on the ramp, although granted, most of the problems manifest themselves on the flat when the car "reloads." Interesting!

The whole car measures "lighter" when it is on the slope according the the cosine rule earlier in the thread. A higher CM car will shift more of the remaining "weight" from the back wheels to the front wheels.

Might be useful implications, but my past study of the issue yielded none to me... except for the "Cubmaster's Car" which runs normally down the slope, but once on the flat "pops a wheelie" and grinds to a stop as its back end drags.

[joe]

Stan, then my method is wrong, and I need to use Warp's technique. Is this as simple as pressing the tare button once my scale is at 30 degrees?

[Stan Pope]

Stan, then my method is wrong, and I need to use Warp's technique. Is this as simple as pressing the tare button once my scale is at 30 degrees?

Quite likely not that simple. Pan mass is the problem. But, give it a try and set up a test using a known weight and a simple computation angle, e.g. 60 degrees. Support the known weight by a string that is parallel with the pan surface.

[WarpSpeedINC]

Stan, then my method is wrong, and I need to use Warp's technique. Is this as simple as pressing the tare button once my scale is at 30 degrees?

I found once I zeroe'd the scale at 30 degrees, it read pretty consistant.
At 30 degrees, gravity is acting on the load cell just as it would against the car and track at 30 degrees, thus the sensed weight of the tray changes. Once this is compensated for (tare mode), and if the pin/stop is 90 degrees to the tray, readings should be real world.

?????

Warp Speed

[docbar]

Most scales/balances need to be level to read accurately. In this case the difference probably doesn't matter.

[Teeeman]

Here's a graphic of what we are trying to discuss I think:




I also did a theoretical calc for this year's track (measured the track day of race) and the results were interesting:







My son's car ran a fastest run of 2.808 or so... so we were getting very close to the perfect numbers.
This was odd for us to be .2 slower this year vs. last year also, same track (this was consistent for the whole field).
I think the track transition slope relaxed (new track last year) and the ramp was more shallow.

-Terry

[Teeeman]

One more shot just for giggles, I thought this looked cool so I figured I'd throw it in here instead of the photo section (since it is sorta related):




-Terry