How Diffs Work

Rupunzell

Rupunzell

Bernice Loui
Found this on Jalopnik... a well produced film about how an open diff works from the 1930's..

Keep in mind an open diff works pretty well as long as both wheels have similar grip (think constant torque), once one wheel looses a significant amount of it's grip then the open diff sends most of it's torque to that wheel causing even more loss of traction. It's why "limited slip" diffs were invented to deal with this problem.

[ame="http://www.youtube.com/watch?v=Em1zQToa_a8"]YouTube - Around the Corner[/ame]
 
Just awesome.

What a great demonstration. At first, I couldn't figure out what the motorcycles were in the video for. :)
 
Great stuff, Bernice!

Both GM and Ford produced highly educational service training material for their respective technical people world-wide, back in the good ol' days!!
Not sure if the same degree of effort goes into supporting their products these days!!

Thanks for posting this - great reminder of our GM Service Training schools down in this part of the world. :)

cheers, Ian - NZ
 
It was the discussion on LSD that got me thinking about posting this here. The way this film presents the concepts is simply brilliant and to the point.

It's also entertaining to the mind set / culture of 1930's America too. It's a real time capsule.

ianlawson said:
Both GM and Ford produced highly educational service training material for their respective technical people world-wide, back in the good ol' days!!
Not sure if the same degree of effort goes into supporting their products these days!!

Thanks for posting this - great reminder of our GM Service Training schools down in this part of the world. :)

cheers, Ian - NZ
Click to expand...
 
Clairification

Rupunzell said:
once one wheel looses a significant amount of it's grip then the open diff sends most of it's torque to that wheel causing even more loss of traction. It's why "limited slip" diffs were invented to deal with this problem.

YouTube - Around the Corner
Click to expand...

Great old film, Bernice!

I hate to nit pick but an open diff does not send more torque to the slipping wheel. Theoretically, the torque is 50:50 all the time limited by the wheel with the worse traction. If you can get 500Nm to the "bad" wheel before it slips then you can only get 500Nm to the "good" wheel even if the contact patch at that wheel is capable of delivering more. LSD were developed to take full advantage of all the traction that is available which entails tha ability to split the torque non-symetrically across the diff.

In reality, even open diffs have some friction in them and that friction supports some amount of torque across the differential. The bias ratio of an open diff is typically around 1.2:1 or so. That means that an open diff will actually send slightly more torque (~55% which is [1.2/[1+1.2] ) to the high traction wheel because of friction. Limited slips are usually in the 2:1 - 4:1 range.

Hope this helps clairify.
 
Thanks Paul for the details.

As always, it's the details that make all the difference..:hmm:

This also says an open diff works pretty darn well under "most" conditions. Which brings up the interesting topic of just how much bias is optimum for any given car/tire/wheel/ road conditions?

Much like Eric's Rotary powered X that currently has plenty of power, but needs to put that power on the ground for it to be effective.

Bernice

Paul Valente said:
Great old film, Bernice!

I hate to nit pick but an open diff does not send more torque to the slipping wheel. Theoretically, the torque is 50:50 all the time limited by the wheel with the worse traction. If you can get 500Nm to the "bad" wheel before it slips then you can only get 500Nm to the "good" wheel even if the contact patch at that wheel is capable of delivering more. LSD were developed to take full advantage of all the traction that is available which entails tha ability to split the torque non-symetrically across the diff.

In reality, even open diffs have some friction in them and that friction supports some amount of torque across the differential. The bias ratio of an open diff is typically around 1.2:1 or so. That means that an open diff will actually send slightly more torque (~55% which is [1.2/[1+1.2] ) to the high traction wheel because of friction. Limited slips are usually in the 2:1 - 4:1 range.

Hope this helps clairify.
Click to expand...
 
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