VIDEO: Limited slip differential Lego style
Lego open differential - click image above for Lego LSD video
After spending enough time searching YouTube to make us reconsider our personal path in life, we're slowly coming to the conclusion that there just aren't other old-timey awesome vids like the one on the differential. Which, while a pity, isn't the end of the world. It's not even the end of videos. Case in point: today we learn how you tackle the issue of slip when playing with Legos.
First, let's a define our terms. A differential is a set of gears that allows drive wheels to turn independently of each other. Otherwise you'd snap an axle going around a turn on dry pavement. Slip is what happens when one wheel loses power (via the differential) and the other lose traction. Put another way, with an open diff if one tire is touching pavement while the other is on ice or in mud, the car won't go anywhere. Why? Because the differential allows 100% of the torque to be transmitted to the wheel that can spin freely, i.e. the wheel with slip. What you need then, is a way to limit the slip.
Now, in a real, non-Danish plastic brick-mobile, the solution proposed by the video wouldn't work so hot. For one thing, it would be very heavy and bulky. On real cars however, there's no one best solution -- or least no one can agree on a best solution. And really depending on the application -- road racing, suburban commuting, Baja 1000 -- one size doesn't fit all.
Dominant types include clutch-type (where clutches on the half-shafts compress based on torque input -- a spinning wheel would cause the clutch to engage, binding it to the non spinning wheel and transfering torque), geared (where after a predetermined torque split is achieved -- say 75% to one wheel, 25% to the other -- worm gears spin both drive shafts), viscous (increased heat from increased torque makes special oil have more friction, thereby tying the wheels together) and electronic (
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Reader Comments (Page 1 of 2)
bvz 6:13PM (8/11/2009)
Cool video...
but oh God, make that music stop!
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jwer 9:39AM (8/12/2009)
Having trouble finding your mute button?
bvz 12:34PM (8/12/2009)
touche :)
Henreh 6:13PM (8/11/2009)
Yeah, that's not a limited slip, because it allows no slip at all...
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groundZ 6:39PM (8/11/2009)
How so? The two camshafts have to make just under 180 degree rotation to engage (depending on the thickness of the shafts) which allows for quite a large amount of slip.
The fault I see with this being put into production is addressed, it would occasionally lock up if you turn in one direction a lot. For example, On my route to school I have to make several right turns but only one left. I would always be slightly paranoid and would probably end up changing my route to make it even out a bit more.
Also something I thought of while typing this out, it would be significantly worse to make right turns than left, since right turns are considerable sharper than left (assuming you live in a country where you drive on the right side of the road).
EXP Jawa 6:53PM (8/11/2009)
He's right - it's not an LSD, its a locker. A limited slip reduced (limits) wheel spin, but it cannot outright prevent it all the time. A locker goes from open to full lock - which is what this design does.
The drawback to this specific set up is that it allows very little differentiation to occur before lock. Less than one rotation can occur, where in the real world you'd want to ideally be allow for limitless differentiation below, say, 30 or so RPM, but be capable of going to full lock above that speed difference. That allows a vehicle to manuever normally around turns without tire scrub, while still transfering all available torque to the high-traction tire when needed. This device can't do that - it locks under most any turn condition. That works fine for Lego, but in the real world, not so much. You'd be scrubbing tires anytime you turned the steering wheel.
Dr. Spaceman 7:36PM (8/11/2009)
After he added that stuff, I wondered ... why even have a differential at all?
Mike P. 8:05PM (8/11/2009)
There's a more elegant way to accomplish this with Lego. I've used it several times in my bots.
Just use the regular "full-slip" differential, but also connect the two axles to each other using a pulley. Very little power loss when going straight, enough "give" to turn tightly, and when one wheel slips it transfers enough torque to the gripped wheel. If you're sophisticated enough, you can even rig it up to change the elastic tension depending on wheel speed so that it essentially engages only at low speeds.
sparrk 6:17PM (8/11/2009)
Lego rules !
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Farmboy 6:25PM (8/11/2009)
Yes they do. Little known fact is that Lego started out making wooden vehicles.
Joe 6:28PM (8/11/2009)
"the differential allows 100% of the torque to be transmitted to the wheel that can spin freely"
More accurately, the diff transmits power 50/50 all the time, but is limited to however much torque the wheel with the least resistance can put down. So, if one wheel can put down 0 ft-lbs, both get 0. If one can put down only 10, the wheel with grip also gets 10.
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EWong 6:36PM (8/11/2009)
That isn't a limited slip.. its a locking rear end.
Might as well be a welded diff
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henrykrinkle 6:44PM (8/11/2009)
Even with zero wheel slip, unless your amount, length and degree of left turns is exactly the same as right turns the cams will eventually lock against each other in one direction or another.
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Rocketboy 9:38AM (8/12/2009)
And how would you reset the cams as well? Once you're locked, you're locked.
Lemon 6:57PM (8/11/2009)
This is basically an open diff until all of a sudden, WHAM! Straight axle. If you were to turn one direction long enough, the diff would lock up and things would get ugly
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EXP Jawa 6:59PM (8/11/2009)
Or more accurately, it allows a nominal torque distribution of 50/50. It can, however, instantaneously go from 100/0 to 0/100 of available torque, for the reasons you describe. The only torque that's ever delivered (or created) is how much the tires can handle. So realistically, available torque (the 10 lb-ft in your example) is the only torque there is. If 100% is on one side, the actual torque distribution is 100/0 (or 0/100).
Since it's locked, the differential can support that switching back and forth just as fast as the traction changes. As a result, in a real vehicle, a 50/50 split is only a baseline and only occurs under relatively ideal conditions. The instantaneous torque split is constantly changing as traction changes.
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EXP Jawa 7:00PM (8/11/2009)
Was supposed to be a reply to Joe. Silly comment system!
chuck 9:42PM (8/11/2009)
Joe is correct. An open diff splits torque 50/50 in all conditions. If a wheel is on ice or snow and when it slips the dynamic friction only results in 10 lb-ft of torque, then the wheel on pavement also only gets 10 lb-ft of torque even though the surface-tire friction limits are much higher. Torque split is 50/50 while wheelspeed varies based on the surface conditions and throttle input.
In contrast, a locked diff always has equal wheel speed. Wheel torque will vary depending on the surface conditions. If you have a vehicle with one tire in the air with a locker, both wheels turn at the same speed but 100% of the torque will go to the wheel touching the ground (up to the friction limit).
EXP Jawa 11:06PM (8/11/2009)
Chuck, your second paragraph says the same thing I said. When locked, either side could see 100% of available torque. Joe didn't specify that it was an open diff that split torque 50/50 (which of course it does), and I didn't mention an open diff either. He said that this Lego diff - when locked was my interpretation - split torque 50/50. And that isn't quite accurate, as you and I have both stated.
Joe 2:15AM (8/12/2009)
I was commenting on the article, which was discussing an open diff. Open was implied.
Also, this thing isn't impressive at all. I couldn't watch the video at work (stupid media blocker...) but now that I have... lame.