Re:How do vacuum hubs work?

Looks like Les has already answered my question whilst I was typing… 

Welded front diff? That's a crazy idea! 

I've been running a welded front diff for over a year now in my daily driven Jimny. From my experience I can tell you you don't need to switch the hubs individually to achieve what you want to do.
In a cross axle situation drive will already be going to the wheel that's on the ground. When you want to turn, both front wheels will be actively pulling the car in the direction they are pointing so the car has no to choice to do anything but turn. And obviously in a straight line it will go straight. Combined with fiddle brakes on the rear axle you will have a very capable and highly manoeuvrable jimny. It's only when you lock the rear diff that you lose manoeuvrability.
The only accompanying mod I've done is to bypass the 4wd switch on the transfer case, instead operating that circuit from a simple on-off switch inside the car. This gives me control over when the hubs engage and disengage which is obviously very important for quick disengagement when you transition onto high grip surfaces or any other time where drive to the front wheels is not required.

It's also worth noting that the hubs work just as well with pressure as they do a vacuum. 5-10psi is enough to operate them (the engine vacuum is within that range). Just be aware that the hub seals (specifically the one behind the wheel bearing and the one in front of the CV joint) are critical to the system working and aren't designed to work with pressure. It may be prudent to glue them into place.  A cheap 12v compressor with a pressure cut-off would be more than sufficient for operating the hubs in this manner. 

Thanks Busta!

I’ll weld the Diff up before rebuilding the front axle then… cheapest traction mod available! 

The vacuum is not required once the hub is engaged as magnets normally hold the position ( subject to grease, dirt and sods law)

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Thanks Martin 

Been giving this lots of thought today…

Decided after looking at all the very expensive options for front drivetrain upgrades, to indeed weld up the front Diff, and use standard front shafts, albeit with new CVs as they seem reasonably priced…

So I’ll order a couple of them for when I get parroted and can get on with finishing and rebuilding the two narrowed front axles… so 26 spline TG shafts in the back with an open Diff and fiddle brakes, and the above in the front with a 12v vacuum pump(s).

THANKS for all your valued input!

Dave