Earlier this year I, while I was giving rides in a C7 Corvette at a racetrack, I was asked if I could give a stunning young blonde a ride around the course. Naturally, I agreed, because every middle-aged man in a Corvette wants a girl who looks exactly like that one sitting next to him. Even if we’re both wearing helmets for the whole time. Imagine my surprise when it turned out that she was a BMW auto mechanic who knew quite a bit more about cars and engines and welding and whatnot than I did.
This video was on her FB page this morning. If you don’t understand how and why a differential does what it does, it will be of nontrivial assistance. You can skip forward to the three-minute mark if you just want the gritty details.
As for my friend — I’d like to tell you that she’s single and just waiting for some nice young man to win her heart by doing something totally romantic, but of course she’s dating some handsome young bastard. Them’s the breaks!
I have seen this film before, it’s a great “Differential 101” As for the female BMW mechanic, lucky for you a helmet doesn’t cover everything
Joke forming in head…9-inch…hot blonde…rear end…pre-load
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pinion bearing?…Dammit! Lost it!
Ring AND pinion.
Is it also true that if you’re on ice and one wheel is stopped and one wheel is spinning the spinning wheel is rotating at 2x the usual RPM?
The spinning wheel is getting 2x the usual torque through essentially a gear reduction, so I suppose it would.
I’m not sure it works that way, but normally the carrier gears aren’t spinning or working when going straight. When one wheel is stopped, the carrier gears go into action.
I’d explain it better, but we’re here to talk about Hot chick that can weld, set your cam gear backlash, etc.
But if I’m dragging a car backwards that’s locked in park, one wheel will roll along as normal, while the other spins backwards at the same rate/speed.
I know these chicks exist, but I’ve only seen the Not so Hot variety in the flesh.
If one wheel is stopped, then the other is moving at twice the average speed for your rpm and gear. The moving wheel is not under any significant load, so thinking in terms of road speed isn’t very useful. At least not until the spinning wheel gets traction and the shock breaks something expensive in your drive line. Then you can tell your friends that the wheel was going “160 mph” when your transmission output shaft stripped.
Thanks Jack, great video.
Watching it really makes me feel like I’m back in grade school. Nothing was better than a cool technical movie substituted for regular instruction, often because the teacher wasn’t up to teaching that day.
Doesn’t matter what generation you grew up in–when you saw the teacher rolling in some form of film equipment, be it a slide projector, film projector, CRT TV, or digital projector, you knew you wouldn’t be having a bad day.
http://i.imgur.com/CAU82nq.gif
Unless, of course, it was Drivers Ed.
A picture would have been nice.
And as I am older than Jack, I prefer ‘women of a certain age’ such as the Queen of the Nürburgring.
https://3d-car-shows.com/bbc-top-gear-star-sabine-schmitz-races-porsche-962-at-the-classic/
i think the limited slip differential is one of those things the true enthusiast will want to understand and he will fully know why an open diff is rubbish and why electronically controlled open diffs are rubbish and why the torsen and viscous diff work and how 1 and 1.5 and 2 way diffs are… different… detroit lockers, quaifes, eaton truetrack, KAAZ…
the LSD makes things happen… two black marks.. ‘elevens’… controlleable powerslides
LSD… lot more fun than lysergic acid
Amen.
Lsd ‘s are great right up until the point you spin both rear wheels and lose directional control. What works on a racetrack is not necessarily what works best on the street.
Am I the only one around here who finds burnouts incredibly juvenile? Waste of tire rubber!
Why would you spin both wheels with an Lsd? Unless you intended to.
The point is, you can accelerate faster and harder coming out of a turn or stopped, before one or both wheels lose traction. But it’s not just about performance and street racing.
Open diff, non Lsd cars leave you stranded when trying to make it up a snowy, muddy or steep dirt road. Even on a flat street you will be eventually stranded. Pull off the road for a second and it’s sandy soil, you’re stuck.
Non Lsd cars also get stuck just going through a driveway entrance at an angle. Take 1/2 the weigh off of one wheel and you’re stuck. Get out your AAA card. And ARPP.
Never driven on an icy road? I’ve owned a bunch of cars with traditional LSD and am well aware of their advantages for performance driving. But on icy snowy roads, they are a double-edged sword. Great for getting going, but once you are going they are a liability. Volvo had the best solution for foul weather – the auto-locking diff. Spin a wheel at less than 19mph and the diff locks solid, then unlocks at 19mph. Otherwise, I’ll take an e-diff in the snow every time over a traditional LSD.
An Lsd just limits the slip. E-lockers are more of a liability when 100% locked. And an Lsd loosens it’s grip once you’re under way. Unless you still have it floored.
My Miata with LSD and good tires is an extremely capable vehicle in snowy, icy conditions. The BMW 1-series loaner that I was driving the morning after a early-season snow dump with summer rubber and an open differential was an unmitigated POS. I’m convinced that an LSD would have been more beneficial to me than better tires.
Real, mechanical limited slip differentials are performance equipment that enhance regular commute-duty driving.
You would be wrong. Tires make far more difference than an LSD on slippery surfaces. All having a LSD would do for you on a 1-series with summer tires in snow would be having both rear wheels spin and zero directional control. BTDT, have the cold-sweat stained t-shirt. I got caught out in my LSD-equipped ’91 318is by a very early October surprise snow storm while it had the summer performance rubber still on – forecast was for just rain. I parked the car and got a ride home.
With snow tires that car was fantastic in the snow. But my 328i with e-diff is MUCH better, even with the traction control off. The cost is added brake pad wear, of course. But you can buy an awful lot of brake pads for what BMW gets for an LSD. An LSD is more FUN in the snow, but you have to be a lot more careful too.
Yeah, well, I think you’re wrong.
krhodes1 is right. If one wheel has no traction, then adding a second wheel with no traction doesn’t help much. Last winter, I drove my buddy’s Infiniti G35S with viscous coupling LSD – the same type as a Miata uses, I believe – out of the garage and around the block to park it in his driveway. It’s normally just a summer car, with a Legacy GT on Hakka7s taking care of winter duties. But we needed to shuffle some vehicles around. The road was covered in a bit of hardpack snow and I was riding on Falken summer tires with decent tread depth. The lack of traction was hilarious. I tried to do some donuts at the cul-de-sac at the end of his crescent but there was a very slight grade and I ended up sliding down it. I needed to take a few runs just to get out of there.
On the other hand, I drove a Chevette with an open diff and studded winter tires for a few winters during high school and university and it performed very well. It was actually a lot of fun to drive in winter and I was never at a loss for traction.
I do prefer to have an LSD that engages smoothly and progressively – such as a torsen-type or viscous coupling – over an open diff or anything electronic that might limit my ability to spin both wheels anytime I choose to do so. But differential type has nothing on tire selection.
Not my experience, the Boston ice got a couple of my fwd and rwd open differs stuck even on winter tires. You need both tires to make it up a hill sometimes. My LSD equipped 2wd cars did much better on snow tires.
Krhodes I think your experience is out of date any of the modern stock lsds whether helical, clutch, or viscous are very “weak” so to speak they don’t have the quirky behavior of the old lockers and aggressive clutch diffs like old gm cars. In fact the helical torsen is great for this due to the way it works also useful for performance since no slip is needed to apportion torque.
I don’t think anybody is saying that an open diff provides as much traction in traction-limited situations as a limited slip.
Try it next year with summer tires, Power6. It’s something everyone should experience once, with no plans to actually get anywhere.
Car and Driver did a test a few years ago comparing winter, summer, and all-season tires. In the 0-50 mph snow test, the winter tire took 14.2 seconds while the all-season tire took 17.6 seconds. The summer tire was only able to get to 11 mph.
http://www.caranddriver.com/features/a-tire-for-all-seasons-feature
I can’t wait to find out this winter. I previously owned a Volvo 960 with LSD and it was a handful on occasion. I had proper snow tires on all four corners and one snowy day I tried to make it a long gradual hill. I got about 1000 feet into it before I rapidly went ass-backwards up and over a small center divide and ended up in the other lane facing the proper direction–to go right the hell home. I did have a few times going up steeper snowy hills where I expected that I might lose traction and then just plodded up the hill with nary a slip. I think it’s perhaps the expectation that you’ll hold ground and then the unfortunate surprise when both rears come loose and do so rapidly. With the open diff, you just get the one tire fire, but at least the lazy wheel won’t lose lateral traction as easily as a pair of spinners.
This year I have an open diff but no hills. This could be fun or a huge pain in the arse.
BTW, KRhodes1, what’s up with stealing old posts from the e90 forums: e90post.com/ forums/showthread.php?t=766512
@zamoti
Well, I am nothing if not consistent!
Your experience is the same as mine. An LSD is GREAT, right up until it (usually quite suddenly) isn’t great anymore. And then you will find yourself facing the way you came much more quickly than you would think possible. You give it that one more lb-ft of torque than you have traction and weeeee! Around she goes. Modern traction control just works way better in the snow, it reacts MUCH faster than you can, and on individual wheels.
On a racetrack, an LSD is just about mandatory. Unfortunately I don’t drive on racetracks very often.
+1000, TonyJZX
And burnouts are only juvenile if you think dancing, singing, and spending time with friends are juvenile. Overwhelming traction with torque is one of life’s great joys, and one of the joys that can be enjoyed in greater measure with age.
Sorry, but I don’t see it. You are turning expensive tires into smoke. And adding wear and tear to the drivetrain. Dancing, singing, and spending time with friends are productive, not destructive activities.
Finally! After more than 47½ years of driving, now I understand differentials… or at least 95% of them!
I’m still working on the other 5%…
Thanks Mr. Baruth for sharing this video with us.
Here is another one, along with additional videos indicated on the right side that may shed some light on further mysteries of differentials.
Wow someone made a lot of models for this film. My favorite was probably the one of the unenclosed driveshaft running through the passenger space!
Interestingly, one of the main problems with one-wheel-drive that the film cited as a reason for including a differential was that if the driven wheel does not have traction, then you’re stuck as all power is transmitted to the driven wheel which has no traction.
But a standard differential does nothing to solve that problem! Even though a car with a differential is 2WD (when both wheels have traction), in a scenario like in the film where one of the drive wheels has no traction, a standard differential will route all power to the drive wheel that has no traction, and you’re still stuck. Anyone who’s ever attempted to power a car out of a snowy/icy parking spot knows this all too well.
You would need a limited-slip or locking differential to overcome that problem.
If you are stuck in snow, slight left foot braking is a passable substitute for a limited slip differential. It’s a little tricky to get the balance right; you can typically slow-down the spinning wheel just enough to get the wheel with traction moving, but not so much that all your wheels are locked.
This is what a modern traction control system does – brakes the spinning wheel. Some also reduce throttle.
The Lsd with traction control gives you best of both worlds. As long as you have the option to turn off the TC.
If it’s rear wheel drive, you should use the hand brake.
I thought the big drums attached to those rear wheels that could take the weight of people running atop them were mighty cool, too. My oldest shop teachers were some of the kids who watched these films when new and who breathed the air of a machine-obsessed America.
Now that we’re mostly mechanically illiterate, squooshy blobs of report writing or policy profusing eloi, I think back very fondly upon them and the smell of grease, burnt steel, solder and solvents. Especially solder.
I love these GM technical films from the 30’s. Especially this one that shows one of GM’s novel early approaches to independent front suspension.
http://www dot youtube dot com/watch?v=mZzfIFBXx5Q
Thanks for sharing that video too, danio.
In the 7th minute, the voice over says: Even under that terrific test, the driver still has full control of the car. How’s that for safety?
That’s the kind of safety attitude I appreciate, as opposed to all the new intervention systems.
That video got me really hot & bothered.
There’s something about real, mechanical, limited slip differential that I can’t do without (in my present vehicle or future ones).
Was someone saying something about an attractive blond BMW mechanic of the female chromosome type?
I don’t know who Jam Handy is, but he (or she?) makes a fine technical instruction film.
Jamison Handy was his name…his company made employee films for GM for decades. There’s a lot on the internet – and this site – about him.
Jam Handy is well known to Misties.
I recall another one that he did about how the Venturi Effect atomizes fuel in a carburetor that bore an uncanny resemblance to sperm entering the uterus.
I did a post for TTAC about Jam Handy and his Organization.
https://www.thetruthaboutcars.com/2013/12/automotive-wayback-machine-the-jam-handy-organization-archive/
…cool video!…
…i know TTAC had nothing to do with this video, but i’d love to see a similarly elucidating follow-up for limited-slip-differentials, including explanation of the tradeoffs involved and why LSDs aren’t preferred in all applications…
I’ve always appreciated a woman who knows cars.
Any woman possessing even a passing knowledge of the physics & mechanics behind limited slip differential is going to be a real spinner..err..keeper.
I kept waiting for something to explode or transform. Then I realized this was not a Micheal Bay General Motors film :)
Great old film! I learned how differentials worked by building them out of LEGO. Same with pistons, crankshafts, steering racks and suspension. LEGO has several sets that make functional model cars in which these mechanical principals can be explored. I thought they even made a set with a simple 2 speed transmission.
There is a youtube video of a guy that built a Lego Technic 911. It had working diffs, 6 speed gearbox, and, IIRC, 4 bar mechanisms for the trunklid and hood.
edit: search lego porsche 911 (997) turbo cabriolet
Have you seen the pneumatic engines some crazy talented AFOLs have built on youtube? Lego has made all sorts of pneumatics, gears and even sets with linear actuators that almost approximate real hydraulics, but nothing compared to what some of these guys have dreamed up. Some of them even have automatic transmissions made with rubber bands!
Sweet! I just changed the fluid in my differential last weekend -may have been it’s first change ever (RWD ’74 Peugeot 504), now I know why.
The arrangement on a FWD car is somewhat different no?
It’s the same principle, just packaged a bit differently. On front-engine front-wheel-drive cars the differential is incorporated into the transmission housing rather than being a separate unit. These are properly called transaxles rather than transmissions.
And on rear-engine rear-wheel-drive cars and probably mid-engine rear-wheel-drive cars too.
There are a wide variety of ways that FWD cars package things. From the transmission and diff in the motor’s oil sump and sharing the engine oil like on an original Mini, to chain-driven separate transmission under the engine with a mostly sort-of separate diff like a Saab 900, to the transmission on the end of the engine with the diff behind like on a VW Golf to every variation in-between.
A similar video on how manual transmissions work can be found here:
http://youtu.be/JOLtS4VUcvQ