by "functionoverfashion" (functionoverfashion)
Published 01/17/2017 at 13:18
by "functionoverfashion" (functionoverfashion)
Published 01/17/2017 at 13:18
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This morning I did an informal experiment in power distribution with my car. Like many people in the Cold North right now, my driveway is a complete sheet of ice. The road, however, is actually down to pavement, or at least very sandy pavement. The car is a 2003 bmw 325xi wagon, 5 speed manual. Here’s the experiment: put some wheels on pavement, others on ice, let out clutch, add throttle, see what happens.
Please note there is no traffic on my road and anyway I’m at the end, so this was very safe. When I put my front wheels on the pavement / rear wheels on ice, I can get the rear wheels spinning pretty fast, the car moving only slowly forward, until all 4 wheels hit the pavement. But when I turn around and put the front wheels on ice / rear wheels on pavement, I can barely get any wheelspin on the fronts before the whole car is up on the pavement - the rear must grab and pull the car before the front has much chance to get power.
I have read that the power is distributed 60/40 or even 70/30 to the rear. Can anyone explain how this is achieved through the transfer case? Shouldn’t I be able to spin the front wheels on the ice nearly as much as the rears anyway? One difference is that I’m in reverse when trying to spin the fronts. Does that matter? I wouldn’t think so. A final note, I tried this with traction control on and off, with similar results. The only difference being the TC (while on) came in to apply brakes to the spinning rears in that scenario. Maybe I need to try this with the fuse pulled?
This does explain the general tendency of the car to behave like RWD in the snow, as far as ease of throttle oversteer. I should also try putting just one rear wheel on the ice. Hmm...
"Smallbear wants a modern Syclone, local Maple Leafs spammer" (smallbear94)
01/17/2017 at 13:33, STARS: 2
I believe the BMW is a RWD-biased system. In other words, in the same way most AWD vehicles are FWD-biased and supplies power to the front wheels until there is slipping detected, the same would apply to RWD-bias except in reverse—the rear wheels being supplied with power primarily and the fronts only being driven when the rears slip.
That said, I don’t understand why the front wouldn’t be getting enough power when the rears were clearly spinning. Unless you had traction control off, the only thing I can think of is that the system is designed to be hesitant in the name of fuel economy.
"HammerheadFistpunch" (hammerheadfistpunch)
01/17/2017 at 13:36, STARS: 3
X drive is kinda funny, if I remember it’s a chain driven transfer case that is rear bias with the fronts engaged with a clutched pto, no center diff, speed and torque bias is accomplished by slipping clutches. It’s only meant to send a small amount if torque forward
"DC3 LS, will be perpetually replacing cars until the end of time" (dc3ls-)
01/17/2017 at 13:39, STARS: 1
1) For the thing about reverse. I wouldn’t think so either. IIRC Reverse is normally a little bit shorter than first, but not much. I don’t think it would make much difference.
2) You get wheel spin when ever there’s to much torque at the wheels compared to the available grip at that wheel. Which is why the fronts don’t spin as much because they don’t get as much torque, because of the AWD setup. Just like how if you start of in 2nd gear you’ll have much better traction, because there’s much less wheel torque going to the wheels in 2nd versus 1st.
"RamblinRover Luxury-Yacht" (ramblininexile)
01/17/2017 at 13:41, STARS: 0
Your center diff is set up to apply more torque to the rear. All other things being equal, the torque applied to the rear will be significantly higher. If the diff is unlocked, the *resistance* encountered by the rear wheels will have to be around twice what the front wheels require to turn for the front wheels to even try - which isn’t the case with them spinning.
There are a number of different ways to do a torque bias - mechanically it’s usually with a planetary spider gear setup or Torsen setup (though that’s a whole other can of worms). It can also be done with uneven spider gear sizing.
"LOREM IPSUM" (lorem---ipsum)
01/17/2017 at 13:45, STARS: 1
Try getting both left or right wheels on ice at the same time, with the opposite side wheels on pavement and see what happens. (With and without TC enabled, for sciencesing)
Anyone wanna take bets whether it will utilize the available traction on the pavement side or just hopelessly spin the icy side?
"functionoverfashion" (functionoverfashion)
01/17/2017 at 13:49, STARS: 0
It’s certainly an interesting demonstration of how the AWD system works in “real life.” I only did the experiment because I’ve noticed when leaving the driveway I get a little unintended wheel spin as I near the top and turn out onto the road. So I decided to be a little more deliberate about it.
"diplodicus" (diplodicus)
01/17/2017 at 13:50, STARS: 1
It will spin only the rear on ice, then apply brake to that wheel and transfer torque to rear wheel with traction. X drive only engages front wheels when both rear wheels slip. 90% sure its all open diffs as well.
"functionoverfashion" (functionoverfashion)
01/17/2017 at 13:51, STARS: 0
On your #2 - that’s true, it’s interesting to see it play out in an extreme case like this, with ice and pavement. It reminded me of the demonstrations of various AWD setups with ramps and rollers, have you seen one of those?
"functionoverfashion" (functionoverfashion)
01/17/2017 at 13:52, STARS: 0
Yeah, and this is a sort-of “early” x-drive I believe. I think what they’re now actually branding as “x-drive” is a little different from my setup. I could be wrong though. The difference could also just be more electronic stability stuff.
"functionoverfashion" (functionoverfashion)
01/17/2017 at 13:56, STARS: 1
It’s a little weird, like you said, that the fronts don’t really engage when the rears are clearly spinning. I guess they engage enough, or I probably wouldn’t have made it up the driveway - this is a very slight incline. I suspect if this car was strictly RWD and I spun the rears on the incline, I would NOT be going forward at all. In this case, I am still able to creep up the driveway and go the required 5 feet or so in about 3 seconds.
"functionoverfashion" (functionoverfashion)
01/17/2017 at 13:57, STARS: 0
I’ll try that - but I’d be across the driveway, pointed at either a) an icy snowbank or b) my mailbox. I’ll look for another place.
"RamblinRover Luxury-Yacht" (ramblininexile)
01/17/2017 at 13:59, STARS: 1
It’s still less intrinsically “spinny” than a traditional RWD, because the total “PUSH CAR FORWARD MAKE GO” force is being split. If your car requires, say, 400lbf to get going, putting 70% of that to the rear and 30% forward is still less likely to exceed your limits of grip. If at any time the difficulty of spinning the rear wheels is ~2x what it takes to pull you forward, your front wheels will start pitching in.
You can see why Subaru made such a good record on pure ordinary open diff - because it *does* work.
"DC3 LS, will be perpetually replacing cars until the end of time" (dc3ls-)
01/17/2017 at 14:00, STARS: 0
No, I’ll be honest I don’t know much about AWD setups lol.
"functionoverfashion" (functionoverfashion)
01/17/2017 at 14:17, STARS: 1
Every once in a while, I’ll run across a video of something like this. Company A wants to show how their AWD setup is better than everyone else, so they put each car up on a set of ramps where any wheel can be given perfect traction, or set on rollers to spin freely - or something like this. You can see where a locking diff would be helpful in this kind of test. Mostly it’s interesting to see how the different systems handle different wheels spinning freely. I don’t have time right now but I’ll bet youtube “awd demonstration” would yield some results.
Cheers
"Textured Soy Protein" (texturedsoyprotein)
01/17/2017 at 15:34, STARS: 0
It’s not really a differential, and most AWD systems don’t use an open center differential because that requires a lot of braking intervention on both wheels of a particular axle to transfer power to the other axle. (Mercedes uses an open center diff on 4Matic but they’re kinda the exception.)
Wikipedia says :
Instead of a permanent torque split (which is featured in earlier systems), xDrive provides variable torque split between the front and rear axles through the use of a multi-plate wet clutch located in the gearbox on the output to the front drive shaft . This setup allows xDrive to modulate the torque split between the front and the rear axles, which is normally split at 40:60 ratio. If wheel slip is detected by the ABS / DSC system , xDrive can react within a tenth of a second to redistribute up to 100% of the engine power to the front or rear axle. [1] The wet clutch is applied through a high speed electric servo motor turning a cam-shaped actuator disc. As the rear drive shaft is hard-coupled to the transmission output, full torque transfer to the front axle can only be achieved if the rear wheels have no traction and are both slipping.
So basically, the rear wheels are always engaged, and to send all the power to the front wheels, the rears need to have no traction whatsoever.
With the fronts on ice, they spin, and the rears are already engaged so the power just shifts rearward.
With the rears on ice, even though they’re spinning, if the ice isn’t totally frictionless, it’s still presenting some resistance to the rear tires, so the threshold to send all power to the front is harder to reach.
"functionoverfashion" (functionoverfashion)
01/17/2017 at 19:20, STARS: 0
Good read, and thanks for the thoughts. An interesting data point is that a few years ago I tried to move my car with the rear driveshaft removed. It did not work, the car would not move at all. I would have thought it would be like having zero traction on both rear wheels but it isn’t quite that simple apparently. My car may also have a slight variation on what is formally branded as “xDrive.”
"uofime-2" (uofime-2)
01/18/2017 at 11:51, STARS: 0
Could be an electronic quirk, or the clutches in your differential are very worn out
"You can tell a Finn but you can't tell him much" (youcantellafinn)
01/18/2017 at 15:02, STARS: 1
After contemplating this last night I think I can explain this sort of reasonably. First off according to Wikipedia xDrive has a nominal 40:60 F:R drive ratio that is variable and controlled by an electronically operated clutch pack. They claim they can redistribute 100% torque to either axle, but from what you’re saying and what I’m reading on xDrive I am skeptical of this claim. To redistribute torque you would increase or decrease pressure on the clutch pack. More pressure on the clutch would result in more torque going to the front axle. It appears to me that the rear axle is driven directly and the front axle uses a clutched PTO to determine how much torque it gets. Assuming the rear is driven directly and the front is clutched theoretically you could open the clutch sending 100% of torque to the rear. How much torque gets sent to the front when the clutch is fully engaged would depend on the exact mechanics of the system. My guess is that generally the system sends most of the torque to the rear and when needed starts moving torque forward. The max torque to the front axle is probably only 40%.
Instead of a permanent torque split (which is featured in earlier systems), xDrive provides variable torque split between the front and rear axles through the use of a multi-plate wet clutch located in the gearbox on the output to the front drive shaft . This setup allows xDrive to modulate the torque split between the front and the rear axles, which is normally split at 40:60 ratio.
Let’s start with pulling out of your driveway forward. Your rear tires are on ice and have nearly zero traction. The front tires are on grippy pavement, but they only get a relatively small portion of the torque. You try to accelerate and the car starts with say 90% of the torque to the rear. Since there is no traction the rears start to spin and torque starts tranferring to the fronts. Eventually (really only a fraction of a second later) enough torque is going through the front tires to get the car moving, but very slowly since it is a relatively small amount of torque being used to accelerate the car. Assuming there is zero traction available to the rear tires on ice, at most you’ve got 40% of your usual torque to accelerate the car.
When you are pulling out of your driveway in reverse you start with the rear tires on grippy pavement. Since they are already getting most of the torque and there is enough traction on the pavement you immediately begin to accelerate out of your driveway since no torque needs to be transferred away from spinning tires.
HammerheadFistpunch has some good write ups on how differentials work and how 4x4 / AWD systems work .
Audi also has a neat video showing how the geometry of their crown gear center differential works to determine the natural torque split. This should start around 3:10 if Kinja behaves.
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"functionoverfashion" (functionoverfashion)
01/18/2017 at 22:08, STARS: 0
This is why I love oppo