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Old 11-24-2014, 08:48 AM
 
Location: Prosper
6,268 posts, read 12,101,075 times
Reputation: 9325

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Quote:
Originally Posted by Drover View Post
Straight from the Aerostar owner's manual: "WARNING: The tire size on the E-4WD must be maintained." And no it doesn't only send power to the front wheels when it detects slip; the default torque split 33/67 F/R. And depending on what model year it is, the OEM tire size may be 70 or 75 aspect ratio.

OP, if you want to prematurely wear out your transfer case, go ahead and put different-diameter tires on your car. Otherwise, keep them the same size.
Care to post that page from the Aerostar owner's manual?

As I said before... it isn't going to wear out the transfer case, because this is NOT a mechanically linked system.

3% difference is perfectly within limits.
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Old 11-24-2014, 09:52 AM
 
8,711 posts, read 8,906,804 times
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Quote:
Originally Posted by PDD View Post
Why would anybody call a dealer who actually services the auto in question when you can get on the internet and get free "expert" advice from anomous persons with no liablity for damages caused by bad advice?

Dealers aren't always the brightest bulbs either.

Not saying trust a stranger, but on occasion you do run into someone who know's the particular vehicle like the back of their hand.
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Old 11-24-2014, 10:13 AM
 
8,711 posts, read 8,906,804 times
Reputation: 12181
Quote:
Originally Posted by MckinneyOwnr View Post
Yes, you can.



Normally what you say is correct for AWD vehicles, but not in this case. The Aerostar employed an electronically controlled magnetic clutch for the front wheels, so there is no mechanical limitation by having front/rear wheels rotate at different speeds.

The newer 911 turbos use a similar type electronic system, so staggered tires can now be run whereas they could not on previous generations (993, 996.)

The aerostar's system is MUCH more primitive however.

The Aerostar's Dana TC28 transfer case is a planetary center diff with an electromagnetic clutch. Because of how plantary gearsets work, when the clutch is "open" you still get 30% of the power transmitted to the front wheels. However, when the clutch is "locked" you get a 50/50 split. Those are the only two modes and it can't really adjust to vary power in between. There are two speed sensors that detect slippage. If the speeds are the same, the transfer case is "open" and the front wheels receive some power , but not directly coupled to the rear wheels (and the transmission). If the rear wheels slip, then the transfer case locks and you get 50/50 split of power. It's just a really basic, simply AWD system. This is where having different wheel sizes would hurt things.


As far as i know, the AWD porsches run staggered widths, not staggered diameters. ANY awd system that fully locks for 50/50 power split cannot run different size tires. It completely defeats the purpose of 50/50 split of power if the TC needs to slip to account for different rolling diameters. Infiniti's AWD system is very similar as the transfer case is a magnetic clutch that can vary anywhere from 0 to 100% fully engaged. The tire diameters still need to be the same size just for the brief times that the TC does fully lock. Without it, something needs to slip. Either the tires, or the TC.
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Old 11-24-2014, 10:33 AM
 
Location: Prosper
6,268 posts, read 12,101,075 times
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Quote:
Originally Posted by BostonMike7 View Post
The aerostar's system is MUCH more primitive however.

The Aerostar's Dana TC28 transfer case is a planetary center diff with an electromagnetic clutch. Because of how plantary gearsets work, when the clutch is "open" you still get 30% of the power transmitted to the front wheels. However, when the clutch is "locked" you get a 50/50 split. Those are the only two modes and it can't really adjust to vary power in between. There are two speed sensors that detect slippage. If the speeds are the same, the transfer case is "open" and the front wheels receive some power , but not directly coupled to the rear wheels (and the transmission). If the rear wheels slip, then the transfer case locks and you get 50/50 split of power. It's just a really basic, simply AWD system. This is where having different wheel sizes would hurt things.


As far as i know, the AWD porsches run staggered widths, not staggered diameters. ANY awd system that fully locks for 50/50 power split cannot run different size tires. It completely defeats the purpose of 50/50 split of power if the TC needs to slip to account for different rolling diameters. Infiniti's AWD system is very similar as the transfer case is a magnetic clutch that can vary anywhere from 0 to 100% fully engaged. The tire diameters still need to be the same size just for the brief times that the TC does fully lock. Without it, something needs to slip. Either the tires, or the TC.
What you are saying is correct... only if the torque split is handled mechanically. I do not believe that to be the case with the E-4wd models. A google search brings up many examples of people who have completely removed the 4wd module, and they no longer have any front wheel drive. That would not be possible if the system was mechanically splitting torque. Possibly, their magnetic clutch has failed and they don't know it.

Yes, Porsche has always run staggered widths. But now, Porsche's system allows for staggered sizes to be run, though I can't tell you the limits of their e-system, mine is the 996. But the tires run by the 996 turbo had to be the same diameter front/rear, the stock tires on the 997 turbo on the other hand, are 235/35/19 and 305/30/19... the fronts are .75" shorter in diameter (a 3% difference already) and there are people running 325 width tires (4.5% diameter difference) with no problems.

Regardless of whether the system is mechanical or not though, I stand by my statement that a 3% difference isn't going to damage anything.
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Old 11-24-2014, 10:56 AM
 
Location: Chicago
38,691 posts, read 86,783,990 times
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So it comes down to this OP: You can trust what the manufacturer says and specifies in your owner's manual, or you can trust some random guy on the internet who doesn't even understand how your AWD system works. It's up to you.
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Old 11-24-2014, 11:08 AM
 
Location: Prosper
6,268 posts, read 12,101,075 times
Reputation: 9325
Quote:
Originally Posted by Drover View Post
So it comes down to this OP: You can trust what the manufacturer says and specifies in your owner's manual, or you can trust some random guy on the internet who doesn't even understand how your AWD system works. It's up to you.
You could be helpful and post that page from the manual, since you seem to have it right in front of you.
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Old 11-24-2014, 01:57 PM
 
8,711 posts, read 8,906,804 times
Reputation: 12181
Quote:
Originally Posted by MckinneyOwnr View Post
That would not be possible if the system was mechanically splitting torque.
Electromechanical clutches operate electrically, but transmit the torque mechanically.

With that clutch fully engages when the rear wheels slip, there's a direct mechanical connection betweek the front and rear wheels. With different rolling diameters, somethings gonna strain.


My Inifniti's AWD system is similar. Electromechanical clutch to vary the torque to the front wheels, but when it's fully engages, it's a direct mechanical connection between the front and rear wheels. Tire diameters need to be the same size.
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Old 11-24-2014, 06:11 PM
 
Location: MN
2,734 posts, read 2,572,787 times
Reputation: 2100
Why does the OP want bigger rear tires? We're not talking about a performance vehicle by any means.
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