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#61
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A tire is NOT a circle. It's a toroid. Its rolling circumference can change based on how domed or pushed inwards the outer edge becomes.
As the pressure decreases and the sidewall of the tire relaxes, it moves outward... this draws the tread inward... the area at the contact patch moves inward based on the weight of the car as the tire SPREADS OUT against the ground. The rest of the tire is "doming" at this point to offset the volume of air being displaced at the contact patch. I wish I knew some 3D modeling... I'd throw this together as an animation. Anyway, yes that's the point we were trying to make... the tire does not have to turn the same number of times per distance traveled, it travels more times because the rolling radius is shorter. That's the magic of the wheel, and why mathematicians have been so fascinated by the concept of Pi since the constant was first discovered. Last edited by Porter; 04-06-2003 at 12:16 PM. |
#62
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Thanks for the great posts guys. So now we understand that different sized tires travel at different speeds. Tires that rotate at different speeds tax the differentials. Making turns means that tires are all rotating at different speeds. So is making turns bad for the car? I don't think so, I think that is what the differentials are for, to allow varied tire rotation in the awd system. So then it should not matter if all of your tires are excatly the same size or proper tire pressure. Otherwise everyone would be bucking when driving an awd car around or it would be bad to drive the car because turning taxes the differential.
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#63
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Actually, turning can tax the differentials in an LSD equipped car if you continue it long enough.
The reason that turning doesn't cause issues is that at some point you STOP turning... and the differential in speeds between the sides of the car ceases. The reason that the center diff runs into problems is that in a straight line on a car with differently sized tires the differential in speeds never stops... it continues to heat up the viscous fluid in the center diff and cause partial lockup. If you do donuts continually for several miles, your rear differential will probably be VERY unhappy. In other words, that is the difference between the three types of differentials... A) "Locking" differentials that are totally engaged, which would cause strange bucking sensations as you describe. B) Open differentials, which are in the majority of cars.. these have no lockup whatsoever and will transfer all power to the path of least resistance (the spinning wheel). C) Limited Slip Differentials - i.e. Subaru Viscous Coupling units such as the rear diff on the SVX, or electronically locking viscous coupler units such as the center diff on the SVX. These differentials allow for partial lockup depending on the difference in speeds between the output shafts.... at zero differential in speeds, there is no lockup and the torque characteristics are "open". As the differential in speeds increases, the fluid heats up between the plates in the diff and causes rapidly increasing friction that eventually results in almost full lockup. There are a number of different types of LSDs, including Viscous Couplers, Torque Sensing Planetary Gearsets (TorSen, Quaife, SensaTrac), and clutch based hard locking LSDs used for racing such as the KAZZ differentials. Hope this helps! Last edited by Porter; 04-06-2003 at 02:39 PM. |
#64
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Did you see that honda almost sideswipe you when you pulled off 29? I creased my seat when I saw him veer towards you. He was heading straight for your rear quarter panel Jimmy Spencer style. Glad you made it home safe, we'll have to do that again soon. Oh yeah- you better get your ass to lafayette - you have no excuse not to go! We may not be as crazy as the rex guys, but it's a fun group to hang out with. see ya
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Matt Locker Link 2015 BRZ Limited 6MT 92 Ebony LS-L ECUtune Stage2av1, Z32 MAF, 370cc injectors, TomsSVX intake, BontragerWorks 22mm RSB #003, HID Hi and Lo beams, OT endlink and bushing mods, PWR Aluminum radiator, Harvey's QC shift kit, 2.5" flowmaster 80 exhaust, 17" Michelin Pilot Sport A/S, Poly sway bar bushings, Slotted Bradi rotors, AFBeefcake powdercoated calipers, 97 grill, and a huge set of air horns. 300,000 miles and counting 92 Ebony LS-L. ecutune stage1v4, motorsport 1pc pulley. Garage Queen - sold to Dad in upstate NY 155,000 miles 19 Subaru Ascent Premium - -Hers !. 89 DL 4x4 little red wagon - a.k.a. The immortal suby. 275k R.I.P. |
#65
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Did I miss a good tech talk??????
Gee I'm glad that you all got that worked out.
But I was thinking. Does a flat tyre deliver more torque to the ground than an inflated one? The tyre circumference, over the working radius point is a good one, but it really does lower the gearing as the tyre deforms. Look at a fuel drag car, The slicks at blast off deform to ripple at the bottom, this increases the contact patch, but more importantly reduces the radius leverage of the wheel. It gears the car down, till the speed rises and the centrifugal force expands the diameter of the tyre, to raise the gearing, to give the top speed. Question, How can the gearing change if the rotational speed does not??? Harvey.
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One Arm Bloke. Tell it like it is! 95 Lsi. Bordeaux Pearl, Aust. RHD.149,000Kls Subaru BBS wheels. 97 Liberty GX Auto sedan. 320,000Kls. 04 Liberty 30R Auto Premium. 92.000kls. |
#66
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Quote:
The EFFECTIVE radius of the wheel between the wheel centre and the road governs the speed at which the wheel rotates.
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Trevor, New Zealand. As a child, on cold mornings I gladly stood in cowpats to warm my bare feet, but I detest bull$hit! Last edited by Trevor; 04-06-2003 at 10:31 PM. |
#67
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Good show, fellas.
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#68
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Did anyone test the idea?
I got myself some white-out, found a straight piece of empty road, marked all four tires, and tested the idea. I set all four of my 225/45-17 tires to 32 psi. It took almost exactly 0.5 miles for the front tires to roll one less revolution than the rear tires. I did it twice and got the same result both times. Assuming an average tire diameter of 25", then the loss of a revolution equates to a 0.25% difference in rolling radius between the front and rear tires when the tires are at 32 psi. Now I just need to find out what tire pressures are required to get a 0% difference.
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Mychailo :: 2006 Silver Mitsubishi Evolution 9, E85, 34 psi peak, 425wtq/505whp DJ :: 1995 Laguna Blue SVX L AWD 5MT (sold) Visit my locker SVX Mods: ND iridium spark plugs, Impreza RS fpr, afr tuned to 13.2:1 using a custom MAF bypass, custom exhaust, WRX 5MT w/ STi RA 1st-4th gear & stock WRX 5th gear, Exedy 13 lb flywheel & Sport Clutch, STi Group N tranny & engine mounts, urethane spacers in rear subframe, rear diff mounts, and pitch stopper, SVX Sport Strut Springs (185f/150r), custom 19 mm rear swaybar, urethane swaybar mounts, Rota Torque 17x8", 225/45-17 Proxes 4 tires, Axxis Deluxe Plus organic brake pads. Last edited by mbtoloczko; 08-22-2003 at 04:38 PM. |
#69
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Typically a 3 to 5 psi difference front to rear at usual operating pressures (35-45psi).
I have 51psi max tires on mine, so I run 49f/45r. Remember... if you are running 44psi tires at 35psi, you are underinflated. Bumping the tires up to their optimal range will significantly improve handling and road behavior! I always run 44psi tires at 44f/40r. |
#70
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Re: Did anyone test the idea?
Quote:
Thanks again, guys! You can teach a middle aged dog new tricks.
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Pearl '92 LS-L 179K (Historic 1st 5-speed SVX) Mods: 5-speed, 4.11's, Group-N motor mounts, dual Magnaflows, cone air filter, Kenwood MP-228 CD/Receiver, white-faced gauges, '97 grill, custom window tinting. Ebony Mica '92 LS 80K Oct 2002 - Dec 2004: Victim of theft. She served me well. You can tell the lack of craftsmanship by the wrinkles in the duct tape. |
#71
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Haha, cool man! I think it's fun to have these kinds of discussions, even when I'm wrong (which is a lot)! This particular one is really counterintuitive due to our concept of the rubber being a solid item.
I crossposted with you above about the pressure difference. 3-5psi depending on the range that you're running in... higher pressures will require a slightly higher difference in pressure, because you're adjusting a percentage and not an exact number. The difference is small between the ranges though... 4-5psi at high pressures (40 and above), 3-4psi at lower pressures (35 and below). |
#72
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Quote:
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Pearl '92 LS-L 179K (Historic 1st 5-speed SVX) Mods: 5-speed, 4.11's, Group-N motor mounts, dual Magnaflows, cone air filter, Kenwood MP-228 CD/Receiver, white-faced gauges, '97 grill, custom window tinting. Ebony Mica '92 LS 80K Oct 2002 - Dec 2004: Victim of theft. She served me well. You can tell the lack of craftsmanship by the wrinkles in the duct tape. |
#73
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And so starts another debate!
Gosh, I hope not!
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Pearl '92 LS-L 179K (Historic 1st 5-speed SVX) Mods: 5-speed, 4.11's, Group-N motor mounts, dual Magnaflows, cone air filter, Kenwood MP-228 CD/Receiver, white-faced gauges, '97 grill, custom window tinting. Ebony Mica '92 LS 80K Oct 2002 - Dec 2004: Victim of theft. She served me well. You can tell the lack of craftsmanship by the wrinkles in the duct tape. |
#74
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At one time tires were more round in shape, back in the days of wire spokes and whitewalls.... Tire pressure "common wisdom" is largely a result of those days, and applies little to modern tires with steel belts and rigid tread patterns.
The only part of the tire thin enough to actually deform with tire pressure is the sidewall itself. Your tread will stay flat and wear evenly all the way across at any pressure under about 55 on most tires. Obviously this changes depending on the tire and the construction of the tire, but I can tell you I've had no problems with wear whatsoever, even running 35psi tires at 42 or 45psi for mountain runs where we scrub a lot of rubber. Most "sporty" all season tires wear excessively at the outer edges at 35psi... bumping them up actually makes them wear evenly. Obviously these rules apply to modern, well constructed tires. The same experience may not apply to that set of $25 Sears Weathermaster balloons. Last edited by Porter; 04-07-2003 at 07:28 AM. |
#75
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appropriate pressure...
I'm going to have to disagree with Porter somewhat on tire pressure. If you are using the stock size tire, I think that running pressures near the Subaru-recommended pressure is fine as long as you don't rail every corner at 2x the posted speed limit. I run my 225/45-17 tires at 36f/33r. I don't use the Subaru-recommended difference in pressure of 4 psi because the lower profile tires don't sag as much at the stock 50 series tires.
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Mychailo :: 2006 Silver Mitsubishi Evolution 9, E85, 34 psi peak, 425wtq/505whp DJ :: 1995 Laguna Blue SVX L AWD 5MT (sold) Visit my locker SVX Mods: ND iridium spark plugs, Impreza RS fpr, afr tuned to 13.2:1 using a custom MAF bypass, custom exhaust, WRX 5MT w/ STi RA 1st-4th gear & stock WRX 5th gear, Exedy 13 lb flywheel & Sport Clutch, STi Group N tranny & engine mounts, urethane spacers in rear subframe, rear diff mounts, and pitch stopper, SVX Sport Strut Springs (185f/150r), custom 19 mm rear swaybar, urethane swaybar mounts, Rota Torque 17x8", 225/45-17 Proxes 4 tires, Axxis Deluxe Plus organic brake pads. |
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