Drivetrain Thoughts

Still in hospital, but filling my time thinking about and sketching lil-bugga ideas… most recently the drivetrain… you’ll hopefully have read and remember I have already narrowed two Jimny front axles –

I’d already decided the one for the rear will be an ”open” Diff, ie. stock with no traction aids, but with tougher Trailgear HD shafts and CVs as most of the drivetrain strain will be to the rear axle, particularly if it’s steering at the same time, and will also be having fiddle (cutting) brakes as well… I already had these at the start of the decision to build lil-bugga, they were destined for the front of The Boogie my road legal greenlaning vehicle (www,jimnybuggy.com), but Rus’ at JimnyBits very kindly agreed to swap the long shaft for another short shaft to suit this build.

TrailGear 26 spline HD shafts in the ”rear” Diff

But i still needed to decide on the front setup before I eventually get released from hospital and get back to work in the garage rebuilding the axles…

I spent lots of time looking at the alternatives –

  • “Open” diff with standard shafts and CVs (all of which I’ve already got in my initial pile of parts from James Priestly who breaks the odd Jimny and amassed the parts for me)… so no additional cost there…
  • TrailGear 22 spline HD shafts – £543*
  • TrailGear 26 spline HD shafts – £684* + 26 spline side gear £42* so £726*
  • Quaife Front LSD – £774* in 22 or 26 spline versions, so plus either of the above TrailGear HD shafts
  • Lockright / Lunchbox “Mechanical” (un-) Lockers NOT AVAILABLE for the front diff…
  • ARB Air Locker £930* + Compressor £201* = £1,132* plus advisable to run the TrailGear HD driveshafts above…
  • Weld the diff up, and use standard shafts and fit new CVs £60 for a pair (from J&R Driveshafts) as a precaution, not knowing the history of the ones I’ve got…

*Prices correct at time of writing, 25/04/22 from JimnyBits’ website

So, guess what I decided to go for, with this being a ”budget” build… Yup, the last option in the list above…

Weld up the diff… with it not being fitted into the axle yet I’ll weld the side gears to the sun gears to make it a permanetly locked diff / axle, and then rebuild the axle with the standard 22 spline shafts I have (photo below) but swap the CVs for replacement new ones which are aftermarket versions from J&R Driveshafts at £30 each delivered at the time of writing…

The middle two in the above photo, the bottom two are the 26 spline TrailGear HD Shafts from JimnyBits going in the rear axle, and the top shaft is the longer passenger (in the UK) side shafr for comparison, hence how and why the axles can be narrowed by 13”.

BUT, the first question that pops into you minds, is what’s the steering going to be like with a permanent filly locked front diff?! Ahhh….

That’s what I’ve been mulling over…

The Jimny runs vacuum operated free wheeling hubs which can be temple rental with the ECM making the decisions whether to switch them on or not, the vacuum tank in the wing, the vacuum switch block, and the hubs themselves possible suspects when problems occur, so most offroad drivers decide to take the hubs off and swap them for manual switched aftermarket hubs, but…

What if you had a motorbike engined offroad buggy with none of the computer decisions? But also no vacuum source either…

So the (re)search began, starting with how the vacuum hubs work? For this I turned to Google and the invaluable BigJimny resource which includes a Wiki, Discussion Board / Forum and an online shop too! There I amassed the knowledge that the hubs work with just a burst of vacuum pressure between 5-10psi to pull the internal drive gear (the same as a manual switched hub) over the driveshaft to engage drive to the wheels… The opposite happens to switch them off, if the ECM decides it’s appropriate, by using abother pulse of vacuum pressure to the other port on the hub to suck the gear cog selector off the driveshaft and disengage drive to the wheels…

So, going back to my application… for now I’ll weld up the diff and rebuild the axle with vacuum hubs, paying particular attention to the vacuum (air) passages through the hubs, and add (a) vacuum pump(s) later to control the hubs switching on and off manually with switches close to hand, maybe even individually so that I could have OFF / ON / LEFT / RIGHT on demand… Considering running a locked axle can affect steering – making the vehicle tramline straight ahead – I could feasibly just engage drive to one wheel, whichever i chose in a given situation… saving strain on the CVs / shafts / diff at the same time…

More as the build eventually progresses, just explaining my thought processes at the moment…

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