Very loose rear end (Handling)

[Gforce]

Originally Posted by Tcoat

My excerpt was directly from the FRS manual. Apparently at least the engineers that wrote it think it cares.

I see nothing in your quote to indicate that the Torsen will be adversely affected. It cannot be. The operation of the will be unaffected by the use of the space saver spare. The handling of the vehicle will be, which is why the space saver should be fitted to a front axle, especially on a rwd vehicle. The space saver cannot develop as much traction as the stock tire, in any direction, obviously.

I have not bothered to calculate the difference in rolling circumference, if any, between the space saver spare and the stock tire size but the list of items affected would indicate there may be a slight size difference.

Those provisions were more likely written by lawyers than by engineers, just btw.

From an engineering perspective a Torsen is not a limited slip differential, it is a torque biasing differential, as you would quickly discover if you ever drove with traction control switched off. If you have traction control on the Torsen has nothing to do.

[Tcoat]

Originally Posted by Gforce

I see nothing in your quote to indicate that the Torsen will be adversely affected. It cannot be. The operation of the will be unaffected by the use of the space saver spare. The handling of the vehicle will be, which is why the space saver should be fitted to a front axle, especially on a rwd vehicle. The space saver cannot develop as much traction as the stock tire, in any direction, obviously.

I have not bothered to calculate the difference in rolling circumference, if any, between the space saver spare and the stock tire size but the list of items affected would indicate there may be a slight size difference.

Those provisions were more likely written by lawyers than by engineers, just btw.

Written by engineers with input for the lawyers. You can disregard it all you want it is there in black and white.
There is more at play than just circumference and to isolate it and say "same diameter so all is good". just does not fly (unless of course you just driving in a perfectly straight line I suppose).

[gramicci101]

Originally Posted by Gforce

Take all the time you think you need. You still won't understand.

Confirmation bias.

Look it up.

That's hilarious coming from you. Confirmation bias is the only thing you've mentioned on this forum that you actually have experience with.

[Ultramaroon]

Originally Posted by Gforce

From an engineering perspective a Torsen is not a limited slip differential, it is a torque biasing differential, as you would quickly discover if you ever drove with traction control switched off. If you have traction control on the Torsen has nothing to do.

Wow. No on all except inconsequential nomenclature. There's no point in making a distinction between limited slip and torque bias if no explanation is given.

Traction control versus Torsen? Nothing to do? Bullshit.

There's a great deal of sliding contact going on inside a Torsen. The greater the difference in reactive torque between the outputs, the greater the shear load between the contacting surfaces.

@Kronos, in those miles driven on the donut, your diff got HOT. Imagine driving 50 miles of constant cloverleaf on and off ramps with no break in between to allow things to cool off. That's what happened. I'm sure your diff is fine. The prudent thing to do would be to replace with fresh OEM lube and enjoy the peace of mind.

[Gforce]

Originally Posted by Tcoat

Written by engineers with input for the lawyers. You can disregard it all you want it is there in black and white.
There is more at play than just circumference and to isolate it and say "same diameter so all is good". just does not fly (unless of course you just driving in a perfectly straight line I suppose).

How you go from "may not operate correctly " to bad for your differential is a tad hard to follow. You may not actually be just a blowhard but you make a convincing imitation of one. Are you 90 years old or something? You're like an old dog on a bone.

Before you add to the growing body of incorrect information you have already posted on this site maybe post to a link supporting your claim that running a space saver spare on the rear drive axle of a BRZ (or any car) or some sort of explanation in your own words as to how this is even possible.

You cannot do that, just btw.

[Tcoat]

Originally Posted by Gforce

How you go from "may not operate correctly " to bad for your differential is a tad hard to follow. You may not actually be just a blowhard but you make a convincing imitation of one. Are you 90 years old or something? You're like an old dog on a bone.

Before you add to the growing body of incorrect information you have already posted on this site maybe post to a link supporting your claim that running a space saver spare on the rear drive axle of a BRZ (or any car) or some sort of explanation in your own words as to how this is even possible.

You cannot do that, just btw.

May not operate properly can very often equal bad for. That is exactly why you don't want things to operate wrong. Most basic concept in engineering.


Can't eh?
"If a Temporary/Compact Spare tire is used in a driven-wheel position, any differences in tire revolutions per mile require the vehicle's driveline to compensate continuously, increasing heat, wear and the possibility of failure. This is especially true for four-wheel and all-wheel drive vehicles, as well as any vehicle equipped with a limited-slip differential.
For example, the tires fitted as Original Equipment on 2013 Honda Accord sedans roll about 800 times every mile, while the Accord's shorter diameter Temporary/Compact Spare tire rolls about 850 times per mile (a difference of 50 tire revolutions per mile or about a 6% increase). Driving at higher than recommended speeds or for prolonged periods of time can result in excessive heat leading to driveline noise, wear or failure."
https://www.tirerack.com/tires/tiret...jsp?techid=237

[Gforce]

Originally Posted by Ultramaroon

Wow. No on all except inconsequential nomenclature. There's no point in making a distinction between limited slip and torque bias if no explanation is given.

Traction control versus Torsen? Nothing to do? Bullshit.

There's a great deal of sliding contact going on inside a Torsen. The greater the difference in reactive torque between the outputs, the greater the shear load between the contacting surfaces.

@Kronos, in those miles driven on the donut, your diff got HOT. Imagine driving 50 miles of constant cloverleaf on and off ramps with no break in between to allow things to cool off. That's what happened. I'm sure your diff is fine. The prudent thing to do would be to replace with fresh OEM lube and enjoy the peace of mind.

OK, Torsen cannot limit tire slip which is the definition of an LSD. A Torsen biases torque delivered by the pinion gear at some ratio other than equally (that's the bias part, an open differential always divides the torque equally). If a tire over speeds the Torsen tries to overdrive the other slower turning drive wheel. Works the same way if fitted into a transfer case as was the case in earlier Audi in line Quattro systems. The torque applied to the faster moving drive wheel is transferred by forward directed torque on the slower moving wheel, the differential worm gears (referred to as helical in non patented Torsen type diffs) cannot reverse direction relative to the spinning wheel, that's how the torque gets biased on the first place. If there is no torque resisted by the faster moving wheel the faster moving wheel just spins and no limit is placed on the differential. You are probably unaware of the poor man's LSD: the handbrake, much less how that useful technique works. With traction control active the BRZ Torsen leads a very lazy life because all the limited slip function is handed by the rear brakes.

Torsen works by using the non reversing principle applicable to worm drives. Friction is there alright but the movement you suggest is not. Your suggestion that a differential subject to constant differentiating induced by moderately different sized tires would get hotter than normal is ludicrous. Most heat in the diff comes from the pinion gear which turns far faster and far more constantly than the differential gears of any design. Use some common sense man.

You are sort of correct to suggest there is a wear item in Torsen differentials but the service life of the thrust washers you must be referring to is longer than the bearing seals.

I repeat. You cannot wear out a Torsen differential by differential action any more than this would occur for an open differential. Clutch type LSD do wear unusually.

[mav1178]

christ.

[Ultramaroon]

Originally Posted by Gforce

I repeat. You cannot wear out a Torsen differential by differential action any more than this would occur for an open differential. Clutch type LSD do wear unusually.

No, I was referring specifically to the worm gears which are loaded up because the torque IS being resisted by the faster moving wheel. I would agree with you if there is no torque applied to the input.

It's less severe than with a clutch type but it's still there. You'll feel it shudder accelerating through a tight turn.

Thanks for the well-worded explanation of the difference. Much better than I would have done.

Please try to be less douchey, though.

[Gforce]

Originally Posted by Ultramaroon

No, I was referring specifically to the worm gears which are loaded up because the torque IS being resisted by the faster moving wheel. I would agree with you if there is no torque applied to the input.

It's less severe than with a clutch type but it's still there. You'll feel it shudder accelerating through a tight turn.

Thanks for the well-worded explanation of the difference. Much better than I would have done.

Please try to be less douchey, though.

No problem. My main point was to reassure the poster who seemed concerned about the use of the space saver spare. (The Chris Harris video of using four on that Mercedes had me practically in stitches. Now he's quite the wheelman).

If you have a clutch type LSD then you need to be concerned about wear from different sized drive wheel/tires. Not with a Torsen though.

Anyone using a space saver also needs to be aware that the traction characteristics of those tires are markedly different from any regular tire. The speed limit for those tires is 50 mph /80 km/hr for good reason. The wheel speed inputs will likely be off and that may affect a lot of driver aids some drivers rely on. Hence the warning in the owner's manual.

That shuddering you experience from the Torsen in the BRZ results from the high bias ratio. At low speeds in tight corners the Torsen reaches the limits of its capability to differentiate and you feel the slower moving tire being actually over speeded just like a four wheel drive with locking hubs. Only it's a rear tire that is getting pushed faster than the car is travelling. In effect forcing the tire to "slip."

[Gforce]

Originally Posted by Tcoat

May not operate properly can very often equal bad for. That is exactly why you don't want things to operate wrong. Most basic concept in engineering.


Can't eh?
"If a Temporary/Compact Spare tire is used in a driven-wheel position, any differences in tire revolutions per mile require the vehicle's driveline to compensate continuously, increasing heat, wear and the possibility of failure. This is especially true for four-wheel and all-wheel drive vehicles, as well as any vehicle equipped with a limited-slip differential.
For example, the tires fitted as Original Equipment on 2013 Honda Accord sedans roll about 800 times every mile, while the Accord's shorter diameter Temporary/Compact Spare tire rolls about 850 times per mile (a difference of 50 tire revolutions per mile or about a 6% increase). Driving at higher than recommended speeds or for prolonged periods of time can result in excessive heat leading to driveline noise, wear or failure."
https://www.tirerack.com/tires/tiret...jsp?techid=237

Driving at higher than recommended speeds on space saver spares can kill you. You will have to concede that even tire rack can't say running the space saver will wear out components if you stick to the recommended speeds. And tire rack is just making that stuff up anyway, as they often do. They should stick to tire advice.

[Gforce]

Originally Posted by gramicci101

That's hilarious coming from you. Confirmation bias is the only thing you've mentioned on this forum that you actually have experience with.

Apparently, you do not know what that is.

[Tcoat]

Originally Posted by Gforce

Driving at higher than recommended speeds on space saver spares can kill you. You will have to concede that even tire rack can't say running the space saver will wear out components if you stick to the recommended speeds. And tire rack is just making that stuff up anyway, as they often do. They should stick to tire advice.

The issue isn't that driving with a doughnut is dangerous it is whether you can run it on the drive wheel of a LTD. You said I would find nothing. I found about 100 that all say the same. Of course linking them would be a waste of time since you would just say they were all made up anyway. The recommended speed is on the non drive wheel.

[Ultramaroon]

Originally Posted by Gforce

No problem. My main point was to reassure the poster who seemed concerned about the use of the space saver spare. (The Chris Harris video of using four on that Mercedes had me practically in stitches. Now he's quite the wheelman).

If you have a clutch type LSD then you need to be concerned about wear from different sized drive wheel/tires. Not with a Torsen though.

Anyone using a space saver also needs to be aware that the traction characteristics of those tires are markedly different from any regular tire. The speed limit for those tires is 50 mph /80 km/hr for good reason. The wheel speed inputs will likely be off and that may affect a lot of driver aids some drivers rely on. Hence the warning in the owner's manual.

That shuddering you experience from the Torsen in the BRZ results from the high bias ratio. At low speeds in tight corners the Torsen reaches the limits of its capability to differentiate and you feel the slower moving tire being actually over speeded just like a four wheel drive with locking hubs. Only it's a rear tire that is getting pushed faster than the car is travelling. In effect forcing the tire to "slip."

Ok, I'm sold. ...more or less. I don't sweat a donut on an open diff because involutes. I also see your point regarding service life and the difference in contribution to heat dissipation.

I'll change my tune with appreciation for some good honest schooling and maybe even sketch some free body diagrams to convince myself. I may come back here with more questions.

Your point I emboldened is probably the most important observation for the casual reader.
@Kronos, nevermind the fluid change but do heed the owner's manual's guidance in the future. It may not be tragic to drive in a constant turn but it certainly isn't doing anything to extend the life of the diff.