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  #31  
Old 08-26-2003, 04:52 PM
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ensteele ensteele is offline
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Are you going to go until the tires are one rotation off, or are you going to go a certain distance and see how much of a rotation they are off? Just wondering. I am working on my pressures and heights now.
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  #32  
Old 08-26-2003, 05:16 PM
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if the static size is as well correlated to dynamic rolling circumference as I "think" it will be, I would have to have the electronic measuring equipment Trevor mentioned - otherwise, how would I know I hadn't lost count comparatively between front & back (open to ideas). Right now I'm planning about 1000 feet and see how much of a rotation difference there is.
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  #33  
Old 08-26-2003, 07:06 PM
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Lee, you say,

ÒUnfortunately, 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?Ó

I say you are the best example of what Ò whiz kids Ò should be i.e. you have knowledge which you are able to put to use based on good practical sense. The idea of using a signal from the ABS is brilliant but taking advantage of this could be expensive when related to your amateur research but I will think on it.

Some may say you are wasting time, money and energy but I would not agree as you are achieving satisfaction from your efforts and the results will be very interesting. Who knows the subject may not have previously been researched on a practical basis !

I will be monitoring progress with profound interest.
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  #34  
Old 08-26-2003, 09:49 PM
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Lee,

I have a suggestion to easily quantify the difference in the rolling circumference between the front and rear tires.

1) Find a piece of straight road that's at least 1/4 mile. 1/2 is better.

2) Write down the odometer reading for your car.

3) Mark the sidewall of all four tires at point on the sidewall that is closest to the ground.

4) Drive the car 0.1 miles. Get out, and note how much the front and rear wheels are out of sync. If your tires are as close in rolling circumference as you think, there will be virtually no difference after 0.1 miles. Drive another 0.1 miles and check.

I did this and found that after 1/2 mile, the front wheels had rotated only 1/2 more revolution than the rear wheels. This was with 37.5 psi in the front and 35 psi in the rear. With this information and a rough measurement of the rolling circumference (for my wheels its 6.4 feet), its possible to calculate the difference in the rolling circumference of the front and rear wheels. For the numbers above, the difference in rolling circumference between the front and rear wheels is 0.25% which equals 0.2 inches. Pretty darn small.
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  #35  
Old 08-27-2003, 12:41 AM
mile-hi-bri mile-hi-bri is offline
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Hi Guys, GREAT thread! Glad to see other people find this sort of thing a "fun" way to challenge/test the brain power. Maybe it has something to do w/SVX FEVER.

Couple of additional thoughts in the rolling diam. When the vehicle hits a bump(during the test measurements, and obviously in real life), doesn't that effect the footprint of the tire.(lenghth X width ratio)? Wouldn't that change the rolling diameteer because the sidewall has 'flexed' more than on smooth road?

And, all tire manufacturer's list a rolling circumference and a rolling radius in their tire literature. You will see that these specs are slightly different (even though tire size is the same) based on perhaps, tread style, tread depth and even speed rating,(stiffer side walls, rolling radius change?).

Also, as a car turns(hope this doesn't sound like a brainfart!), the rolling distance has changed, front to rear, side to side.

I don't remember, do we have a diffy that is effected by this difference, like a posi or limited slip axle diffy is? (NEVER run different size tires on same axle!).

Obviously, ABS needs to 'adjust' for different 'rolling distance', to be able to accurately understand varying 'speeds' at the wheel sensors. (maybe a percentage of difference is 'allowed')

Didn't SOA probably figure this same 'concept' as a potential problem in the trans diff?

Hope this all made sense! Just more food for thought! Thanks, Mile-hi-bri
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  #36  
Old 08-27-2003, 08:28 PM
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mile-hi-bri,

I'll take a stab at some answers and let others jump in. Numbering your questions/comments/paragraphs as 1 through 6....

1. Yes, but these "should" average out and given we can't vary tire pressure in real-time, a compromise would be to ignore this variation.

2. Yes again, but their spec is at one pressure and one load value and is therefore not transferable to specific vehicles. Consider (a little silly I know, but stick with me) putting a 225x50x16 tire on a go-kart vice the SVX, obviously the SVX mounted tire will have more flex in the sidewall and hence a smaller rolling circumference.

3. Yes, and given enough time I would like to quantify just how much difference there is when in a turn.

4. yes, yet again (at least as I understand it). The transfer case on the US version AWD system uses clutches that slip. Minimizing the tire rolling circumference will reduce wear on those clutches, reduce heat induced from the slippage and therefore extend tranny life to some small degree.

5. ABS is in braking, and for the moment I'm limiting myself to transmission issues only - my limited capabilities are stressed now (taking two MBA courses at night, full time job, wife & 3 kids at home, etc sorry for the whining there, I'm all better now ).

6. Got me.

thanks for the thoughts.
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  #37  
Old 08-27-2003, 08:35 PM
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Quote:
Originally posted by mbtoloczko
Lee,

I have a suggestion to easily quantify the difference in the rolling circumference between the front and rear tires.

1) Find a piece of straight road that's at least 1/4 mile. 1/2 is better.

2) Write down the odometer reading for your car.

3) Mark the sidewall of all four tires at point on the sidewall that is closest to the ground.

4) Drive the car 0.1 miles. Get out, and note how much the front and rear wheels are out of sync. If your tires are as close in rolling circumference as you think, there will be virtually no difference after 0.1 miles. Drive another 0.1 miles and check.

I did this and found that after 1/2 mile, the front wheels had rotated only 1/2 more revolution than the rear wheels. This was with 37.5 psi in the front and 35 psi in the rear. With this information and a rough measurement of the rolling circumference (for my wheels its 6.4 feet), its possible to calculate the difference in the rolling circumference of the front and rear wheels. For the numbers above, the difference in rolling circumference between the front and rear wheels is 0.25% which equals 0.2 inches. Pretty darn small.
Gosh, seems almost too easy I might get to finish this in my lifetime

I've got straight, flat roads that go on for miles. I can think of one exceptional stretch inside the gate at Cape Canaveral. Now if I can just convince the security people that I'm really doing a scientific test for a group of car nuts.....

How did you do the rolling circumference rough measurement? I could easily do something like 10 turns of a chalk marked tire....
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  #38  
Old 08-28-2003, 01:03 AM
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ensteele ensteele is offline
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Quote:
Originally posted by mbtoloczko
Lee,

I have a suggestion to easily quantify the difference in the rolling circumference between the front and rear tires.

1) Find a piece of straight road that's at least 1/4 mile. 1/2 is better.

2) Write down the odometer reading for your car.

3) Mark the sidewall of all four tires at point on the sidewall that is closest to the ground.

4) Drive the car 0.1 miles. Get out, and note how much the front and rear wheels are out of sync. If your tires are as close in rolling circumference as you think, there will be virtually no difference after 0.1 miles. Drive another 0.1 miles and check.

I did this and found that after 1/2 mile, the front wheels had rotated only 1/2 more revolution than the rear wheels. This was with 37.5 psi in the front and 35 psi in the rear. With this information and a rough measurement of the rolling circumference (for my wheels its 6.4 feet), its possible to calculate the difference in the rolling circumference of the front and rear wheels. For the numbers above, the difference in rolling circumference between the front and rear wheels is 0.25% which equals 0.2 inches. Pretty darn small.

That is easy enough for me to do! I love this thread. It has been a real eye opener for me!
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  #39  
Old 08-28-2003, 02:03 AM
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Quote:
Originally posted by lee

... How did you do the rolling circumference rough measurement? I could easily do something like 10 turns of a chalk marked tire....
That's how I did it except I went 4 revolutions because I could measure that distance with a 25 ft tape measurer. :-)

Let us know what you measure!
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  #40  
Old 08-28-2003, 02:45 AM
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Maybe this is why the Trans go so often?

Maybe the tire Pressures were off all this time ?

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  #41  
Old 08-28-2003, 03:20 AM
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The trannys really aren't that bad. After Subaru corrected the problem with the clogging ATF line inlet on the radiator cooler, the trannys have proven to have a longevity similar to other auto trannys give or take the odd part that breaks on some people's SVX trannys. My '95 SVX had 90K miles on the stock tranny, and its working great.

Anyhow, a large difference in rolling circumference between front and rear wheels would increase the slippage of the center clutch which would generate heat, increase the wear rate on the center clutch, and introduce debris into the ATF. Sounds bad, but I wonder how this compares to the amount of slippage that must occur on a daily basis to accommodate driving the car around corners.
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  #42  
Old 08-28-2003, 05:36 PM
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Quote:
Originally posted by mbtoloczko
Lee,

I have a suggestion to easily quantify the difference in the rolling circumference between the front and rear tires.

1) Find a piece of straight road that's at least 1/4 mile. 1/2 is better.

2) Write down the odometer reading for your car.

3) Mark the sidewall of all four tires at point on the sidewall that is closest to the ground.

4) Drive the car 0.1 miles. Get out, and note how much the front and rear wheels are out of sync. If your tires are as close in rolling circumference as you think, there will be virtually no difference after 0.1 miles. Drive another 0.1 miles and check.

I did this and found that after 1/2 mile, the front wheels had rotated only 1/2 more revolution than the rear wheels. This was with 37.5 psi in the front and 35 psi in the rear. With this information and a rough measurement of the rolling circumference (for my wheels its 6.4 feet), its possible to calculate the difference in the rolling circumference of the front and rear wheels. For the numbers above, the difference in rolling circumference between the front and rear wheels is 0.25% which equals 0.2 inches. Pretty darn small.


Hi Mychailo, You beat me to the punch with the chalk! I am an advocate of the "KISS" principal in designing and this is a good example.

Your measurements of 1/2 a turn out after 1/2 mile equates to the center clutch slipping 1.77 turns in about 445 revolutions, or about 1.4 deg per revolution.
Considering that the clutch has very little pressure on it when cruising, this amount of slip would produce very little heat or wear.
It should make it easy to equalise the diffidence front to rear and also side to side. The camber of the road could affect the side to side roatation.

Harvey.
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  #43  
Old 08-28-2003, 09:20 PM
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Clutch Slip.

Harvey,

Please explain how you calculate a figure of 445 clutch revs in half a mile using MychailoÕs figure of 6.4 feet per wheel rev and a final drive ratio of 3.545 : 1 , the latter confirmed by your figure of approx. 1.4 deg slip.

My calculations show a clutch slippage of 0.436 degrees.
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  #44  
Old 08-30-2003, 08:10 PM
oab_au oab_au is offline
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Re: Clutch Slip.

Quote:
Originally posted by Trevor
Harvey,

Please explain how you calculate a figure of 445 "clutch" revs in half a mile using MychailoÕs figure of 6.4 feet per wheel rev and a final drive ratio of 3.545 : 1 , the latter confirmed by your figure of approx. 1.4 deg slip.

My calculations show a clutch slippage of 0.436 degrees.
Read again, I did not say clutch revs.

1/2 mile = 0.81 km = 445 revs of wheel. (225/50X16 = 550revs per km)
180 degs in 445 wheel revs = 638 clutch degs per wheel rev / 445 wheel revs = 1.433 clutch degs per wheel rev.

Harvey.
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  #45  
Old 08-31-2003, 04:04 AM
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Harvey I have read your words again and again. I see no inference towards Ò wheel revs Ò, or any suggestion of same. I quote verbatim : -

Your measurements of 1/2 a turn out after 1/2 mile equates to the center clutch slipping 1.77 turns in about 445 revolutions, or about 1.4 deg per revolution.

Ò Your measurements of 1/2 a turn Ò surely refers without doubt to MychailoÕs figure, to whom your post was addressed.

The figure he stated was 6.4 Ft. based on practical measurements and based on this figure figure wheel revs equate to 412.5 for 880 yards. Tyre specifications have no relevance as practical measurements is what this thread has been all about and the figure under discussion was defined by Mychailo.

Ò the center clutch slipping 1.77 turns in about 445 revolutions Ò surely means the revolutions it ( the clutch ) has completed.

Your logic in quantifying clutch slip makes no sense and is incorrect. Clutch slip is understood, expressed and quantified as just that, i.e. slip occurring between the friction surfaces of the clutch.
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