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Old 02-21-2013, 11:29 AM   #51
KC
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Originally Posted by kyoo View Post
you know though, regardless of what car is winning/losing etc, from an objective point of view, if an STU/X rwd car can upgrade to whatever LSD it wants, is it not fair to ask that AWD cars are able to do the same for ONE of their LSDs?
LSD: Limited slip differential. If there's slip, it transfers (some) power to the wheel that isn't, through various methods, or in the case of a locker, 50/50 all the time.

Open diff: Power to both wheels on an single axle occurs when a single wheel on that axle regains traction.

Ok?

There are many complexities that make up differing AWD systems. I'll stick to Subaru for the time being in this discussion. Specifically their MT full time AWD (with or without a VLSD on either front or rear diff). I don't believe the center, even in the 09+ is 'open', but Viscous. Meaning, it will adjust power delivery to the front and rear depending on lockup.

A completely open F/R diff, AWD car (Subaru anyways, and hopefully I don't over simplify)

Should both front wheels retain traction, they both get power.
Should both rear wheels retain traction, they both get power.

They operate independently of each other. Depending on the center diff, anywhere between 10% and 50% (or more in some instances) could be sent to the rear at any point in time.

Should a front wheel lose traction, that one gets the power, but there's still (some) power being sent to the back in the chance that one or both wheels have traction back there.

Should a rear wheel lose traction, that one gets the power, but there's still (some) power being sent to the front in the chance that one or both wheels have traction up there.

In a worst case scenario, the inside front and inside rear will spin up. The 1st axle to regain traction will have power to both of it's wheels. As soon as the other axle regains traction, both wheels now have traction.

It's autox. And there's a constant game of shifting power going front to back and side to side and back to both as each wheel loses and then regains power as the car dances through the cones. In only rare circumstances will a WRX have less than 1 wheel getting power at any point in time. It's more common to have at least one full axle (either front or rear) with both wheels getting power at any point in time. I'm not saying it doesn't happen, but they can go from 1 to 2 to 3 to 4 wheels of traction rather quickly (or any combination thereof).

In the case of loss of grip in an open diff RWD car, it will send all it's power to the wheel with the less grip until such time both tires have grip. No power at all to the front wheels, at any time, under any circumstance, not even a chance at getting 10%.

Lift the inside rear wheel on a RWD open diff car because the front nose dives? Zero power anywhere. AWD? Odds are great you still have traction/power in the front wheel(s) still.

Essentially, by giving an AWD car an allowance for a limited slip on one axle, you give a car that generally has power going to 2 wheels all the time (whether they be slipping or not), and make it so it's 3 wheels all the time.

And that is the reason why RWD cars can have a REAR LSD. Because both wheels, on the only axle that can get power, will power going to them at all times. Much like AWD with open diffs F/R will have more opportunity to have at least one wheel with power and then 3 to 4 as soon as both axles are settled.


IOW:



--kC
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Old 02-21-2013, 11:56 AM   #52
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Originally Posted by KC View Post
LSD: Limited slip differential. If there's slip, it transfers (some) power to the wheel that isn't, through various methods, or in the case of a locker, 50/50 all the time.

Open diff: Power to both wheels on an single axle occurs when a single wheel on that axle regains traction.

Ok?

There are many complexities that make up differing AWD systems. I'll stick to Subaru for the time being in this discussion. Specifically their MT full time AWD (with or without a VLSD on either front or rear diff). I don't believe the center, even in the 09+ is 'open', but Viscous. Meaning, it will adjust power delivery to the front and rear depending on lockup.

A completely open F/R diff, AWD car (Subaru anyways, and hopefully I don't over simplify)

Should both front wheels retain traction, they both get power.
Should both rear wheels retain traction, they both get power.

They operate independently of each other. Depending on the center diff, anywhere between 10% and 50% (or more in some instances) could be sent to the rear at any point in time.

Should a front wheel lose traction, that one gets the power, but there's still (some) power being sent to the back in the chance that one or both wheels have traction back there.

Should a rear wheel lose traction, that one gets the power, but there's still (some) power being sent to the front in the chance that one or both wheels have traction up there.

In a worst case scenario, the inside front and inside rear will spin up. The 1st axle to regain traction will have power to both of it's wheels. As soon as the other axle regains traction, both wheels now have traction.

It's autox. And there's a constant game of shifting power going front to back and side to side and back to both as each wheel loses and then regains power as the car dances through the cones. In only rare circumstances will a WRX have less than 1 wheel getting power at any point in time. It's more common to have at least one full axle (either front or rear) with both wheels getting power at any point in time. I'm not saying it doesn't happen, but they can go from 1 to 2 to 3 to 4 wheels of traction rather quickly (or any combination thereof).

In the case of loss of grip in an open diff RWD car, it will send all it's power to the wheel with the less grip until such time both tires have grip. No power at all to the front wheels, at any time, under any circumstance, not even a chance at getting 10%.

Lift the inside rear wheel on a RWD open diff car because the front nose dives? Zero power anywhere. AWD? Odds are great you still have traction/power in the front wheel(s) still.

Essentially, by giving an AWD car an allowance for a limited slip on one axle, you give a car that generally has power going to 2 wheels all the time (whether they be slipping or not), and make it so it's 3 wheels all the time.

And that is the reason why RWD cars can have a REAR LSD. Because both wheels, on the only axle that can get power, will power going to them at all times. Much like AWD with open diffs F/R will have more opportunity to have at least one wheel with power and then 3 to 4 as soon as both axles are settled.


IOW:



--kC
very nice explanation, thank you for that -

in cases of cars lifting a rear wheel, isn't that cause by dive from braking + cornering load, typically on corner entry? we should make a distinction between grip and torque to the wheels - the car may not have torque going to the wheels, but it will have grip in the other three tires. so unless you're trying to get on the power as the car is under braking and corner load simultaneously, that particular scenario shouldn't matter for rwd (depending on certain cases of extreme suspension setup) should it? And that scenario only plays out for STU RWD cars that don't come with LSD from the factory - which I can't think of any off the top of my head..

it also doesn't consider awd cars w/ LSDs compared to the ones that already have an LSD from the factory. If you're trying to limit LSD in AWD cars for that reason, then ceteris paribus it just skews people towards the cars that do come with LSD from the factory - i.e., at least the earlier WRXs had viscous in the rear, compared to open in rear for 08+

i definitely see your point though. most of the common rwd cars in STX already come with lsd, ie rx8s, 350zs, but the older 3 series (which won STX last year) do not. WRXs will always have power going to at least 2 wheels, unless they're up on two wheels - i don't see when they could have power going to a single wheel in the scenario though.


edit::
furthermore, this "rwd lifting rear tire" scenario can be ignored, since STX and STU rwd cars are allowed LSDs - I never argued against that. Considering that, I guess your argument is that rwd cars always get power to only two wheels, and awd cars get power to always at least two or three, and sometimes/usually 4? that's the difference between rwd and awd cars isn't it..? but the point is not to get them to have as few powered tires to the ground as possible, which seems to be what this LSD rule for awd cars does.

i understand that things should be fair, etc. and we're trying to limit the advantages of awd, but awd cars with open diffs, IMO, do not have any advantage compared to a rwd with LSD, and are in fact disadvantaged.

Last edited by kyoo; 02-21-2013 at 12:54 PM. Reason: additional thoughts
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Old 02-22-2013, 07:45 AM   #53
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Originally Posted by kyoo View Post
very nice explanation, thank you for that -
I try. It's been a while since I've studied the varying and ever changing Subaru AWD systems (they've has like 5-6 different ones between AT/MT over the past decade or so).

Quote:
in cases of cars lifting a rear wheel, isn't that cause by dive from braking + cornering load, typically on corner entry? we should make a distinction between grip and torque to the wheels - the car may not have torque going to the wheels, but it will have grip in the other three tires. so unless you're trying to get on the power as the car is under braking and corner load simultaneously, that particular scenario shouldn't matter for rwd (depending on certain cases of extreme suspension setup) should it?
Power/Torque = Same thing. If I wanted to be pedantic, I could have said "energy". It 'shouldn't matter' to the person that's not a left foot braker (FLB). However, someone that does have the penchant for LFB will more than likely still have their foot on the skinny pedal applying power to the wheels that can get them, even though the brakes are on and the car is slowing.

Quote:
And that scenario only plays out for STU RWD cars that don't come with LSD from the factory - which I can't think of any off the top of my head..
I understood the point of the discussion was "if RWD can swap/add diffs, why can't AWD?" It doesn't matter what comes from the factory on a RWD car. They're open for them:

Quote:
For AWD vehicles that did not come with any type of limited slip differential (including center differential or transfer case), a single aftermarket LSD may be added. 2WD vehicles may use any LSD unit.
I don't get what your above statement had to do with what I was saying, understanding that all RWD cars can use any rear diff, no matter what came from the factory. I was comparing open diff cars only. Regardless, RWD can apply power at any point in time to only the rear axle. Where AWD has power going to both the front and rear.

Quote:
it also doesn't consider awd cars w/ LSDs compared to the ones that already have an LSD from the factory. If you're trying to limit LSD in AWD cars for that reason, then ceteris paribus it just skews people towards the cars that do come with LSD from the factory - i.e., at least the earlier WRXs had viscous in the rear, compared to open in rear for 08+
This is where choosing the car for the class is pertinent. When you have what seems to be an outlier of a car, you have to make due with what the rules allow.

Also, while most Subarus have open front and rear diffs, they're still getting power to another axle that RWD simply cannot. At that, they have a form of an LSD in the center diff to control which axle gets more power.

Quote:
i definitely see your point though. most of the common rwd cars in STX already come with lsd, ie rx8s, 350zs, but the older 3 series (which won STX last year) do not. WRXs will always have power going to at least 2 wheels, unless they're up on two wheels - i don't see when they could have power going to a single wheel in the scenario though.
Bingo.

Quote:
edit::
furthermore, this "rwd lifting rear tire" scenario can be ignored, since STX and STU rwd cars are allowed LSDs - I never argued against that. Considering that, I guess your argument is that rwd cars always get power to only two wheels,
... with at least 1 having contact enough to the ground to apply power in turns to keep the car moving forward.

Quote:
and awd cars get power to always at least two or three, and sometimes/usually 4? that's the difference between rwd and awd cars isn't it..? but the point is not to get them to have as few powered tires to the ground as possible, which seems to be what this LSD rule for awd cars does.
Where RWD can only use at least 1 and at most 2 in a corner, and AWD "generally" will always have at least 2, one in the front and one in the rear (unless extreme circumstances has both inside wheels unloaded to where it matters in time), and more often than not, 3. The advantage is definitely to the AWD, especially when it comes to powering out of a corner using the front wheels to pull the car around where the rear wheels may be unloaded.

Quote:
i understand that things should be fair, etc. and we're trying to limit the advantages of awd, but awd cars with open diffs, IMO, do not have any advantage compared to a rwd with LSD, and are in fact disadvantaged.
This is the disconnect.

RWD cannot power the front axles in a left foot braking situation to have the car pulled around the corner where AWD can. Simple as that. In a section on the course where both AWD and RWD may not have any traction in the rear at all, the AWD may have traction in the front, and power going to it.

Think every time you apply the gas on a hard turn waiting for weight to transfer to the rear. A RWD will always be at a disadvantage in the time it takes for power to get back to the rear axle to propel the car forward should there be enough grip to do so. Meanwhile, the front axles on an AWD will be pulling the car while power and weight is transferring rearward (unless you're in a power slide and that's just bad driving because you're exceeded the slip angle). Open diff on the front or rear doesn't matter. If both front tires have enough traction, power goes to the full axle and pulls the car in the meantime.

AWD will have more power going to an additonal axle more times than RWD. If you don't have the right car for the class, that's not a rules problem.

--kC
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Old 02-22-2013, 10:35 AM   #54
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Quote:
I try. It's been a while since I've studied the varying and ever changing Subaru AWD systems (they've has like 5-6 different ones between AT/MT over the past decade or so).

Power/Torque = Same thing. If I wanted to be pedantic, I could have said "energy". It 'shouldn't matter' to the person that's not a left foot braker (FLB). However, someone that does have the penchant for LFB will more than likely still have their foot on the skinny pedal applying power to the wheels that can get them, even though the brakes are on and the car is slowing.

I understood the point of the discussion was "if RWD can swap/add diffs, why can't AWD?" It doesn't matter what comes from the factory on a RWD car. They're open for them:


I don't get what your above statement had to do with what I was saying, understanding that all RWD cars can use any rear diff, no matter what came from the factory. I was comparing open diff cars only. Regardless, RWD can apply power at any point in time to only the rear axle. Where AWD has power going to both the front and rear.
Sorry let's be clear ab point of discussion - I'm considering this rule as a whole - You are totally right in terms of that specific question: yes, open diff rwd cars, there will be times where there is no power distributed vs. open diffs f/r awd cars will usually have power going somewhere (let's not get into a topic of what mods certain cars "need" ).

I'm talking about the current scenario, and make a more or less natural assumption that at a nationally competitive level, we have: STX RWD cars have diffs, and STX AWD cars from the factory may or may not (08+ open f/r, 07 down open front viscous rear), and not get into center diffs just yet.

Quote:
This is where choosing the car for the class is pertinent. When you have what seems to be an outlier of a car, you have to make due with what the rules allow.

Also, while most Subarus have open front and rear diffs, they're still getting power to another axle that RWD simply cannot. At that, they have a form of an LSD in the center diff to control which axle gets more power.
I mean.. power to the front wheels is what AWD is.. the center diff is a diff. power distribution for most AWD cars is 50/50, less audis, sti w/ dccd etc. Center differential, a viscous (which, as mentioned before, will eventually degrade to an open diff) for the WRX, will vary lockup between the front and rear wheels. so if the front is traveling faster than the rear, and they are clamped together, front slows and rear speeds up, respectively. However, this doesn't matter for this scenario:

Quote:
Bingo.

... with at least 1 having contact enough to the ground to apply power in turns to keep the car moving forward.

Where RWD can only use at least 1 and at most 2 in a corner, and AWD "generally" will always have at least 2, one in the front and one in the rear (unless extreme circumstances has both inside wheels unloaded to where it matters in time), and more often than not, 3. The advantage is definitely to the AWD, especially when it comes to powering out of a corner using the front wheels to pull the car around where the rear wheels may be unloaded.
The advantage of RWD + LSD vs AWD + open diffs does NOT go to AWD - our disagreement is here. So, bringing back your scenario where the car will lift a rear wheel (even that is rare for rwd, common for awd/fwd, but anyway), you've got the rwd car which lifts its rear wheel, and regardless will distribute torque between the rear wheels depending on the diff type and lock up strength, but will send power the the wheel it needs to.

Then you've got the AWD car, which lifts its rear wheel with open diff - in this situation, power does NOT go to one in front and one in rear - the car effectively becomes FWD. Power is shuffled all the way to the lifted rear wheel, which is obviously in the air. Power distribution is normal in the front, albeit an open diff.

So we've got this mid corner situation - then throw in the fact that, for AWD cars, as mentioned before, the power distribution is 50:50 - so the power the car is putting down is effectively half of what it would in a full traction scenario - here's where the viscous center diff comes into play - it will sense the rear wheel spinning faster, and (slowly for viscous) attempt to clamp the front and rear speeds together, effectively distributing torque to the front (I may not be 100% on the mechanics, but roughly this should be correct). Obviously, torque distribution will not be anywhere near 100%, especially for a viscous - so let's just say half for illustration's sake. So we have a RWD + LSD, distributing power in whatever aftermarket fashion it wants, vs a FWD car with 75% of the power that it normally has, AND an open differential in the front - you still want to say the AWD car has the advantage here?

Quote:
This is the disconnect.

RWD cannot power the front axles in a left foot braking situation to have the car pulled around the corner where AWD can. Simple as that. In a section on the course where both AWD and RWD may not have any traction in the rear at all, the AWD may have traction in the front, and power going to it.
I don't want to get too much into driving technique, though obviously it has it's heavy implications. The purposes of LFB are for body/weight transfer/human traction control, mitigating turbo lag, and ensuring a smooth transition between braking and accelerating (among others I'm sure I've missed) - if you are using LFB in AWD, I don't think you are doing it to count on the front wheels to pull you through the turn when your outside rear is in the air, and compared to a RWD car that is using the technique (which may or may not be all that common) where the rear is lifting (again, I've pretty much never seen this) but HAS a limited slip, more or less mirrors the previous scenario.

Quote:
Think every time you apply the gas on a hard turn waiting for weight to transfer to the rear. A RWD will always be at a disadvantage in the time it takes for power to get back to the rear axle to propel the car forward should there be enough grip to do so. Meanwhile, the front axles on an AWD will be pulling the car while power and weight is transferring rearward (unless you're in a power slide and that's just bad driving because you're exceeded the slip angle). Open diff on the front or rear doesn't matter. If both front tires have enough traction, power goes to the full axle and pulls the car in the meantime.
In terms of "waiting," that depends on your drivetrain setup - for example, evos send all the power to the transfer case first, and then send the power out - half the power will go to the front and half the power will go to the rear - So, in similar terms of delay (though theoretically awd cars will have more "delay"), a RWD car will send all its applicable power to the rear, and AWD cars will get half and half. Exceeding traction, and (typically) RWD cars will power oversteer and AWD cars will power understeer. Just before then, yes, AWD cars will have more traction - see: any very snowy day, like what we got yesterday lol. Open diff on front or rear DOES matter. It changes the distribution of applicable torque, and the handling characteristic of the car. I don't want to get into a pure RWD vs AWD, but for this scenario, I just plain have to give it to RWD. In autox, I think ceteris paribus the benefits of AWD and RWD as a drivetrain layout are nicely balanced. << obviously pending what kind of situation the differentials are in

Quote:
AWD will have more power going to an additonal axle more times than RWD. If you don't have the right car for the class, that's not a rules problem.
Yes, AWD cars will have power going to the front wheels sometimes, and RWD cars cannot.. Not sure where you were going with that, but I think the discussion above covers the differences for drivetrains. And you're right - it may not be a 'rules' problem but I think it does create a problem - everyone targeting the same cars, on and on.. for me, this doesn't exactly paint the greatest picture for the future of this class.

Thanks for the thorough post! Good discussion going, I think.

Last edited by kyoo; 02-22-2013 at 10:42 AM.
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Old 03-21-2013, 12:54 AM   #55
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^ while I never got a response to my post above countering some of the points/false information, I did get one from the SCCA:

#10334 Differential Allowance Proposal

The STAC believes the current restrictions on limited slip differentials for AWD vehicles are appropriate.


So, there you go. GL to all this year!
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Old 03-21-2013, 09:27 AM   #56
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<~~~ agrees with STAC on that.
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Old 03-22-2013, 03:39 PM   #57
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I wish there was something i could do to be more competitive in stu, i always feel like no matter how good i drive i will always be out classed. I drive an 09 wrx.
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Old 03-23-2013, 09:43 AM   #58
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I wish there was something i could do to be more competitive in stu, i always feel like no matter how good i drive i will always be out classed. I drive an 09 wrx.
Such is the fate of not owning "the car to have" for a given class.

...and no matter how you write the rules, there will almost always be a car or two that dominate. It's extremely difficult to equalize performance potential across a wide range of cars, when .001 matters.
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