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Beer Geek
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Discussion Starter · #1 ·
Yes, corner weights.

Using the process I nailed down and posted up at the end of the last thread, I corner weighted my car twice on Sunday. Once in the AM, once in the PM. The first time through I came up with 110.0 square inches of contact patch. The second time I came up with 109.6 square inches of contact patch. Petrol sender values were 132/255 and 130/255, the only variable.

I believe I am getting an acurate picture out from under the car, better than I hoped for going in. Quarter inch grid graph paper is close enough for me, and whatever mistakes I am making there seem to be consistent as well.

So with some faith in my data collection, I threw a variable in the system tonight. I bought a dummy. I bought three 50# bags of dog food and 1 case of Coca Cola (24 12 ounce cans). Total dummy weight 168# plus packaging, total swmn weight is 168# plus clothes.

So I set the dummy 168# up in the passenger seat and corner weighted the car tonight, empty driver's seat, 127/255 petrol sender value. I got 116.9 square inches of contact patch, biggest single gain on the RF corner. So far so good, right?

FWIW I have very little faith in either of my two tire gauges. For all the 'good' data I have collected since Sunday AM, gauge A reads 40psi on all four tires and gauge B reads 36 psi on all four tires. Everytime. Exactly.

I went ahead and calculated the weight of the dummy using square inches of contact patch and psi. I need a very stiff drink.

According to gauge A (40psi), my empty GTO with half a tank of gas weighs 4400#, and my dummy weighs 275.2#. According to gauge B (36psi) my empty GTO weighs 3960# and the dummy weighs 247.7#.

So I went back and looked at it the other way, I know the dummy weighs 168#, I picked up 6.88 square inches of contact patch. That means my actual tire pressure is 24 psi, and my empty goat with half a tank of gas weighs 2640 pounds. So why can't I run mid tens to low elevens with no mods?


Aaack!!

20-20-24 hours to go-o-o-, I wanna be sedated;
Nothing to do, no where to go-o-o,
just get me to the (Liverpool) airport, put me on a plane...
and let Mike White handle it...

[we now return to our regular thread]

I'll just slide the data in here all smooth and put what I think I am looking for as a response to the original post so as not to cloud your thinking.

Lobotomy!! Lobotomy!! Lobotomy!! Lobotomy!!

There are no pounds in this data. The mm's are ride heights measured the usual way. sq-in is square inches of contact patch. The percents are percent of contact area at this tire compared to total contact patch under the car during this event.

Empty car, 132/255 petrol sender value:

LFxxxxxxxxxxxxxxxxxRF
30.38sq-inxxxxxxxx30.88sq-in
27.61%xxxxxxxxxxx28.07%
585mmxxxxxxxxxxx583mm

LRxxxxxxxxxxxxxxxxxRR
25.25 sq-inxxxxxxxx23.50 sq-in
22.95%xxxxxxxxxxxx21.36%
582mmxxxxxxxxxxxxx580mm



And now, bring on the 168# dummy, stick it in the passenger seat, empty driver's seat, empty car, petrol sender value now 127/255:

LFxxxxxxxxxxxxxxxRF
30.00 sq-inxxxxxx33.44 sq-in
25.71%xxxxxxxxxx29.09%
582mmxxxxxxxxxxx572mm

LRxxxxxxxxxxxxxxRR
27.81sq-inxxxxxx25.63sq-in
23.79%xxxxxxxxx21.93%
574mmxxxxxxxxx571mm
 

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dude, swmn...you need a long vacation somewhere without cars and plenty of chick:drink:
 

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You're a true romantic, swmn, to pursue this discovery. The quandaries of divergent measurement systems adds to the romance and quaintness. A guy with a 12 ton shop press in the grand room ... and no strain gauges !

Wal Mart/Home Depot and the like often feature $6.99 digital tire gauges. If you were to procure two of those, you might reduce your margin of error ...

I'm on the verge of sending you an ANSI A +/-1.5% 0-60PSI bourdon gauge for gratis, but I want to enjoy a few more of your writeups first before I ask for a corporate endorsement for an item that fixes your adventure ;-)

WzT
 

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Beer Geek
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3,713 Posts
Discussion Starter · #4 ·
wango-z-tango said:
A guy with a 12 ton shop press in the grand room ... and no strain gauges !

WzT
Yup. What I really need is to get loose with a fresh suspension upgrade.

:gears:

With some Ramones ripping. Right now I am at 60k with shot rear springs (q.v.) and front struts holding on by a thread.


I might just complete the cycle, move the dummy to the driver's seat, weigh that, and then empty the car and weigh it again to see if I get 110.00 square inches of contact patch again.

If I do I might skip the Norco 76412G and SS strain gauge. I might just load up the springs and count the number of jack strokes to reach a pre-determined height. I have a set of 4 springs, I just don't know which one of the fronts is stronger than the other, and I don't know which of the rears is stronger than the other. Once I finish the dummy cycle I would at least like to have a good idea of which spring goes where.
 

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Beer Geek
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Discussion Starter · #5 ·
It is pretty clear to me that contact patch area versus weight on the tire is not a linear relationship.

I am thinking besides moving the dummy to the driver's seat I should also move the dummy to the trunk and the sheet metal in front of the radiator. My neighbors already know I am a lunatic, so no worries there.

So Tuesday driver's seat, Wednesday trunk, Thursday radiator shell, then Friday I'll be headed to the gas station for $4.68 or whatever of premium to get back up to 131/255 for empty. Assuming I don't have any long days at work, bad assumption that.
 

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Have Bar, Will Travel
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10,804 Posts
What you have discovered is something I was unsure of. Radial tires are not flexing in the proper way to get an accurate patch to pressure ratio.

I was hoping it would work out differenly. Also getting a more accurate tire gage is not going to completely fix the issue.

By any chance are you running larger rims and lower profile tires?

Here's to drinking back your sanity.
 

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Beer Geek
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Discussion Starter · #7 ·
Napalm said:
By any chance are you running larger rims and lower profile tires?
Stock/OE 17" rims, stock sized General Exclaim's with under 500 miles on them.
 

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Beer Geek
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Discussion Starter · #8 ·
I think what I need to do is quantify how much 1/16 sq-in weighs when the contact patch is around 30 sq-in and when the contact patch is around 20 sq-in.

So I bought another 50# bag of dog food.

I am going to go once with 50# over each axle, and then again with 100# over each axle to see how much each contact patch grows when it gets a known (balanced) amount heavier.

I suspect the tire load v- contact patch area graph is actually a fairly simple curve, but I don't know if there is an apex (or a minimum) somewhere between ~20sq-in and ~30 sq-in.
 

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Ludicrous Speed!
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1,041 Posts
I guess I took the easy way out... I own a set of Intercomp SW500 digital corner weight scales. Seems easier than making my head a-splode with your equations... heh
 

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Beer Geek
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3,713 Posts
Discussion Starter · #11 ·
wango-z-tango said:
What are you gonna do with all that dog food after the experiment, swmn ? Find a way to burn it as fuel or something ?
I think I am going to donate it to the SPCA just in time for a year end tax write-off. The Coke-Cola is getting mixed up with plenty of rum when I put this thing to bed.

If Mike White ever posts up on doing a suspension on an 04 with the interior smelling like Purina...
 

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Beer Geek
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3,713 Posts
Discussion Starter · #12 ·
Crud, there is more than one variable loose in here.

I got my tires inflated to 40psi per gauge A, 36 psi per gauge B for all this. having sampled the fill chucks at three local gas stations now I am confident gauge A reads quite a bit higher than actual, gauge B is probably within one or two pounds.

I got 110.0 and 109.6 square inches of contact patch under the empty car on back to back attempts.

So with the 168# dummy in the passenger seat I got 116.9 sq-in. With the 168# dummy in the driver's seat I got 117.4 sq-in.

So my rears are running 23 to 29 sq-in each, the fronts 30.3 up to 33.4 sq-in, I see the relationship between contact patch area and pounds corner weight is not linear and set out to define the curve.

So I put one 50# sack of dog food on the radiator shell and one 50# sack of dog food in the trunk. My total contact patch is now 108.6 sq-in.

Then I go back and do it again with 2 50# sacks of dog food in the trunk and 2 50# sacks on the radiator shell. 108.8 sq-in. Yeah, 108.6 and 108.8.

All the shapes coming out from under the car are blobs, sort of egg or elliptical shaped with the narrow ends cut off by the sidewall.

With the 168# dummy between the axles the dimples in the paper from the textured concrete work surface are uniform height across the contact patches.

With the known weights outside the axles the dimples in the paper are taller/deeper near the edges and about average uniform height (see above) in the middle. Like the tires are carrying more weight on the edges of the contact patches. More noticeable on the fronts that were bigger to start with. Sort of like the contact patch is trying to make an hour glass shape, but is constrained by the sidewalls.

The technique has been to load the car, jounce each end separately, and then roll the car at walking speed or slower, 18-30" onto the paper scales.

If I am setting up some kind of standing wave in the tire by loading outside the axles how far or how fast do you reckon I will have to roll the car to power through the wave, or is this a geometry issue I am just stuck with?

Thanks
S
 

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Nice work.
 

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Registered
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I think at this point, only Isaac Newton could respond...:)

BTW, thanks for your concern about my situation.

Mike.
 

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swmn ... I think the only solution to this quandary, is to add a modified Galileo Barometer, with a Schrader valve, to your arsenal. (j/k, smile bro).

With mercury as the measurement medium, you can more correctly ascertain actual tire pressure in the actual prevailing barometric conditions. Even the ANSI A bourdon gauge I am about to send you, can't compare to a properly constructed Galileo device, which is self-correcting on barometric and has no case pressure problems.

Another useful vein of discovery might be achieved through photographic paper, if your venue supports manually rolling a car in the dark onto 8x10s and garage safelights.

You're one of a kind, swmn .. you've definitely earned beers gratis next time you're in Niagara.

WzT
 

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Nobdy
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195 Posts
I just read your threads, you are one of a kind. I didn't see anyone mention tire flat spotting, could this cause a problem in your calculations. Also would it be easier to use floor jacks to lower your car onto the paper?
 

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Premium Member
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13,016 Posts
Wow......I admire your creativity.

I, however, am not that patient. I'll have this pretty soon.

 

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GMX 341
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386 Posts
wango-z-tango said:
You're a true romantic, swmn, to pursue this discovery. The quandaries of divergent measurement systems adds to the romance and quaintness. A guy with a 12 ton shop press in the grand room ... and no strain gauges !

Wal Mart/Home Depot and the like often feature $6.99 digital tire gauges. If you were to procure two of those, you might reduce your margin of error ...

I'm on the verge of sending you an ANSI A +/-1.5% 0-60PSI bourdon gauge for gratis, but I want to enjoy a few more of your writeups first before I ask for a corporate endorsement for an item that fixes your adventure ;-)

WzT
I know its been damn near a month but I like that answer.
 

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Ignoring the Speed Limit
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Dude. You need help. :nuts: :gr_jest:

Next time you're in the neighborhood, let's take it to Guldstrand and let them weigh it.
 
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