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  #16  
Old 08-22-2003, 04:46 PM
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mbtoloczko mbtoloczko is offline
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I hope lee doesn't mind too much, but I'm going to chime in here. What I think is most important here is the rolling "circumference" of the tires. Static loaded radius is ok, but in my experience, static loaded radius does not exactly correlate to rolling circumference.
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  #17  
Old 08-22-2003, 04:55 PM
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OK Larry, you have seriously confused me.

I have, so far, only been considering trying to determine how to match the number of times my front tires rotate with the number of times the rear rotates. Are you suggesting I measure tread width as a part of this test? Or are you saying this is what I should be measuring instead of radius?

I went and looked back on some old threads to see if I could get a grip back on what I was doing. Although it took me awhile, a light bulb finally went on for me in this thread:

http://www.subaru-svx.net/forum/show...&threadid=9785

As a result of my re-reading, I was wrong at a guess of measuring at the front of the contact patch. However, I believe (based on the referenced thread above) that I am right at measuring in the middle of the "flat" of the tire to the center of the hub - and that's where I have been taking measurements for this experiment. Let me explain the physical measuring process.

I take a ruler, and holding it vertical, I put one end against the floor next to the tire, then measure the distance to the top of the wheel. This is an alternate to measuring to the center of the hub cap as I found it easier to locate the top-center vice the center of an unmarked hub cap. All it then requires to get rolling radius is to subtract half the wheel diameter.
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  #18  
Old 08-22-2003, 05:14 PM
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Quote:
Originally posted by mbtoloczko
I hope lee doesn't mind too much, but I'm going to chime in here. What I think is most important here is the rolling "circumference" of the tires. Static loaded radius is ok, but in my experience, static loaded radius does not exactly correlate to rolling circumference.
hey, the more the merrier...I'm just trying to figure this out and it wouldn't be the first time I was either partially or even totally wrong. But, I believe you are 100% correct.

What I hoping to find is that the static loaded radius is very highly correlated to the rolling circumference. Then it becomes an easy thing to check while I'm checking pressure levels. In fact, if correlation would turn out to be equal to one, then all I need do is set my preferred pressure (either front or rear), then add/subtract air in the other until the radius matches.

If, as you suspect from experience, it is off somewhat, then perhaps a constant factor could be applied, i.e., front static radius should be rear + 1/8" or whatever is appropriate. That is one reason I was asking others to measure as well.

did any of that make sense?
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  #19  
Old 08-22-2003, 06:12 PM
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Lee,

I meant that you measure the diameter of the tire horizontally across the tire and hub from tread to tread. I did not mean to measure the width of the tread.

I still have some reservations about youre taking vertical measurements. but I'd like to see your measurement numbers and conclusions.
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  #20  
Old 08-22-2003, 08:14 PM
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Larry

I think that the measurement that Lee is taking (vertical) will change with the change of tire pressure. That is part of what he is trying to determine. If you measure a horizontal distance, I don't think you would get any difference with even a 10 psi difference. I could be wrong. The idea is to see what is needed to get a more accurate way to get the same rolling radius instead of using the 4 psi difference that the manual uses. Different tires will respond differently with pressure. I will stop now, because my head is starting to hurt.
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  #21  
Old 08-22-2003, 08:42 PM
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Lee,

You are correct in basing your figures on the effective diameter of the wheel and tyre (tire) centre (center) to the road. This is the nearest figure you can measure static.

You say quote : -

" I take a ruler, and holding it vertical, I put one end against the floor next to the tire, then measure the distance to the top of the wheel. This is an alternate to measuring to the center of the hub cap as I found it easier to locate the top-center vice the center of an unmarked hub cap. All it then requires to get rolling radius is to subtract half the wheel diameter."

I presume that the wheel diameter you subtract is that measured across the same opposite points you establish as the " top of the wheel ", rather than the specified size for the wheel. This is indeed a good method because as you say it is hard to establish the exact centre point.

In motion the variable factors are near beyond accurate calculation. Probably the most important being temperature which will increase pressure and the effective diameter and therefore rolling circumference. Load carried and the load ratio front to rear is also an important factor. Minor variations will be constantly occuring due to drivng conditions altering vertical and lateral loads.

All that said, surely these variables in motion are within the tolerance the transmission has been designed to accept. But again having said that I am glad that my car, ex Japan, has the proper set up i.e. a differential to adjust front to rear variations rather than slip and friction.

You could drive yourself up the wall trying for the exactly exact ! However what you are doing is full of interest and therefore worthy of the effort. Keep reporting your findings.
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  #22  
Old 08-23-2003, 09:02 AM
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Quote:
Originally posted by Trevor
...snip...

I presume that the wheel diameter you subtract is that measured across the same opposite points you establish as the " top of the wheel ", rather than the specified size for the wheel. This is indeed a good method because as you say it is hard to establish the exact centre point. ...snip...
Yes, I am measuring as you suggest - from rim edge to rim edge, not the nominal 16" that the wheel is labeled. FYI, the rim to rim measurement on my car with OEM rims is 17-3/8". So, when I say I measure from floor to top edge of rim and get 20.5", that means I have a static rolling radius of:

20-1/2" - (17-3/8")/2 = 11-13/16"
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  #23  
Old 08-23-2003, 10:31 AM
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Lee

Then are you trying to adjust the pressure so that all 4 will have the same rolling radius? That is what I had understood before. It that is the cold tire, then you will drive to heat them up and measure them to see what the hot tire radius is. Correct?
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  #24  
Old 08-23-2003, 11:52 AM
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Quote:
Originally posted by ensteele
Lee

Then are you trying to adjust the pressure so that all 4 will have the same rolling radius? That is what I had understood before. It that is the cold tire, then you will drive to heat them up and measure them to see what the hot tire radius is. Correct?
you have correctly understood my goal.

first, I want to see if the rolling circumference is highly correlated to static loaded radius (I suspect it is).

then, using that info, I'm hoping to find a pressure setting that will retain the correct size relationship front to back as the tires go from cold to hot.

If this works, and I change tire brands (and I will on my next set) I should be able to easily replicate the scenario to minimize stress on the transfer case assembly.
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  #25  
Old 08-24-2003, 02:38 PM
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Interesting Information

I usually listen to a car talk show on the radio every Sat. morning out of Seattle, WA. The guy is has been a car talk radio host for 17 years and seems to really know his stuff. He can usually diagnose problems with information given by the car owner in a short period of time.

Now for the interesting stuff: This Sat morning he mentioned that one of the biggest problems with AWD cars on the road is uneven tire size or diameter. This is because of not rotating tires, uneven tire pressure, mixed tire types or brands, or variations of tires that are the same brand. He has measured new tires after being mounted and found up to 1/4" difference in diameter!!!! Quality control is a problem with some brands.

Anyway, he said that the problem with all of these things is that people are tearing up their transmissions and differentials because of these problems. This is becoming one of the most common problems in the repair industry. He said that the differential is meant to take care of the difference in rotation when turning or other small variations, but if there is a difference of 1/4" between tires, it is like going around a corner all of the time and this will tear it to pieces in a period of time. The more the difference, the quicker the problem will occur.

This gets use to where Lee has brought us. Measurement of the tires and wheels.

A friend of mine (SVX owner) and I measured his OLD Toyo Proxes which have become very hard with age.

Cold tires
20 1/4 inches all the way around

Front pressure 40 psi (both)
Rear pressure 28 psi (both)

Hot after 30 miles of Freeway driving
20 1/4 inches all the way around

Front pressure 43 psi (both)
Rear pressure 34 psi (both)

I will do mine next and see what kind of info I can get. I just measured two of my cars and found 1/8" to 1/4 " difference between front and back (floor to top of wheel).

This may be why the transmissions and differentials are having problems. It may be more critical with these high performance cars and the speed in which they are driven.
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  #26  
Old 08-24-2003, 02:41 PM
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I also moved this to the Technical Forum. It is technical in nature.
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  #27  
Old 08-24-2003, 05:56 PM
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Thanks Earl this is getting more & more interesting.

Which model Proxes are on you friend's SVX.
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  #28  
Old 08-24-2003, 10:12 PM
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I'm not sure, but they are the round ones. I will check which ones they are.
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  #29  
Old 08-25-2003, 02:07 AM
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Lee,

You appear to be really getting into this thing and what you have posted to date makes sense. In order to dispel all doubt it would be great if you could measure wheel revs completed for a given distance, front compared to rear.

Where I live i.e. Auckland New Zealand it is possible to hire specialised measuring equipment of a sort not worth owning by individual short term users. Portable counters are available which sense using various types of input. There are optical devices which will operate using a contrasting patch on the moving element e.g. a paint mark or sticker on a tyre. Some can count the teeth on a moving metal gear. A more crude device might use a small adhesive magnet operating a transducer.

If you were to hire one such instrument you could take comparative measurements over a test length of road, or if you shelled out for two instruments simultaneous records would be possible. The fun/interest factor may or may not make my suggestion viable.
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Old 08-26-2003, 04:35 PM
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Trevor, et al,

Unfortunately, in my little town there would be no source for this equipment; therefore, I would have to design something, maybe a tap off the ABS sensor wheel - that is likely beyond my electronics ability. Any electronic whiz kids out there?

Given time, I also want to check while driving a constant radius turn as well.

As an update.... for my car & my tires I have found a sweet spot where the hot and cold static radius stays coupled. Details follow (using my floor - top of rim scheme):

Cold pressure/size
front: 44PSI, size 20-5/8"
rear: 31PSI, size 20-5/8"

Hot pressure/size
front: 47 PSI, size 20-3/4"
rear: 32 PSI, size 20-3/4"

for the scientific types, my cold pressure is measured in my garage, ambient temp at 85 degrees F. Hot tire temp was obtained after a driving loop of about 15 miles with the last five above 60mph.

My plan to check rotations while moving is to warm up the tires, check for static size once more, then drive down the block I live on. Each lot is surveyed at 100ft widths and I intend to coerce one of my kids to count the chalk marks I will make on the side of a set of front & back tires. Probably can't accomplish before next weekend - it's pretty hard to tie down a teenager for a stupid Dad experiment.
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