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Comment by InclinedPlane | Hacker News Reader

You'll notice though that a differential doesn't solve the problem of one of the drive wheels not having any traction (mentioned early in the video as a problem of one wheel drive cars). In that case the differential will spin the wheel with no traction while not sending any power to the wheel that does have traction (in the same manner as it would if you happened to be taking a very sharp turn that left one of the wheels nearly stationary). An excellent video otherwise, but I thought that was an odd aspect.

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> In that case the differential will spin the wheel with no traction while not sending any power to the wheel that does have traction

Boy do I ever get reminded of this every winter. It's a shame that cars that have the option to lock the differential gear are a lot more expensive.

And for more on limited slip differential:

http://en.wikipedia.org/wiki/Limited_slip_differential

Thanks! Now I know what to look for. It seems the "electronic limited slip differential system" is what I want, as my main trouble is getting out of icy parking spots.

Not necessarily (but availability probably depends where in the world you are). You can get a Subaru Impreza with a locking center diff.

A limited slip differential solves most of this problem, and you don't have to worry about unlocking the diff when you are on dry road. (having a locked diff means the tyres will have to slip over the road when turning. On a dry road they won't slip as much and this can cause significant damage to various components)

2006 (and later) Honda Civic Si's come with a limited slip differential, and are fairly affordable, from both a purchase and a maintenance perspective. While the LSD is mostly for traction under hard cornering, as accelerating out of a corner is a huge weak point of front-wheel-drive, it does work to increase traction in wintry or wet weather by distributing torque over both wheels.

I'm not much of a car person, but did limited slip differentials exist at the time this video was made? To me it would seem more odd if an advertisement explained the mechanics behind something that the product it was advertising couldn't do...

Limited slip differentials were apparently invented in the 1930s but were relatively exotic (reserved for race cars) until about the 50s. I suspect the makers of the film were just a little confused and didn't fully appreciate the problem.

Also, I can't resist posting a link to this video of a working differential built using legos: http://www.youtube.com/watch?v=zOijFK7uh6U

But the torque applied to both wheels will always be equal in theory, so the spinning only happens because of the 'dynamic friction is often less than static friction' factor. This does mostly solve the problem.

The torque will be equal but will be equal to that of the spinning wheel, which does not help much.

And it's worse than dynamic vs static friction, in cases where you really need it it's likely the wheels are on different surfaces (one in snow, the other on pavement, for example) and then both wheels will have the torque equivalent of being on snow...

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