Talk:Six-wheel drive

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Intermediate Differential -- How does it work?

In a 6x4 or 6x6 truck, where both rear axles are driven, the rear-most differential will be a conventional one, but the 'middle' differential has to have prop shafts connected front and back. How does it work inside? I haven't seen a diagram of such a diff.

Would be useful if this page tackled this issue.

-- EdJogg (talk) 23:53, 22 January 2014 (UTC)[reply]

There are a few ways of ways of doing this. Originally ('20s-'30s), heavy trucks would use worm gear final drives rather than bevel gears. The worm shaft passed above the crown wheel, so (unlike a conventional bevel) both ends of it could emerge from the axle casing. A short prop shaft between the axles then drives the rear axle from the front axle. The downside of this is that the shaft is short, so limits articulation between the axles, especially for an off-road vehicle. It was used for some steam wagons (and petrol) as heavy-haulage on-road wagons, where they needed the weight carrying but not the off-road capability.

In off-road practice it was only really usable for bogie suspensions (like the Scammells) where the two axles had a fixed relationship to each other. I guess it would work with independent suspension too, but I doubt if the two technologies overlapped.

Having mentioned them, Scammells didn't work that way. The Scammell system is really a pair of "walking beams" rather than a bogie. Each side pivots independently of the other (this is why Scammells have such good articulation) and there are no "axles" as such. It looks like a "H" from above. There is one short "axle" in the middle and just one diff. At the ends of this short axle are a pair of see-saw beams containing a geartrain that links their centre pivot (on the axles) out to the ends where the wheels are. The beams can each pivot around the axle half-shaft (lovely animation here).

Another way is to have a pair of propshafts from somewhere else, such as an intermediate transfer box, one to each axle. One is longer than the other and passes over the first axle. I believe this was a popular system in the USA, especially for 6×4s.

A modern system uses a front axle with a pass-through shaft, even though it uses hypoid bevels. As many such trucks have two-speed rear axles, the input shaft can be arranged off to one side of the pinion shaft, enough to allow for the pass through. It still has the short propshaft articulation limit, but for construction site dump trucks it's worthwhile (as these also need highway speeds, they have two-speed axles).

With independent suspension it gets easier. Backbone chassis can have the diffs fixed centrally (avoiding the articulation limit) and then the wheel stations off to each side.

Having already mentioned "H"s, then H-drive is something else again. It's a DAF invention: a single central diff splits the drive into left and right driveshafts, which each run fore and aft inside the bottom corners of a welded punt chassis. At each wheel station (there are no axles) there's a bevel box driving the half shaft out to the wheel. It's probably best known through the Alvis FV600 system, (Stollie et al), and their problems of driveline windup owing to having no differential action fore-and-aft. When used for equally spaced axles the front two usually both steer, one less than the other, and so there's a lot of wind-up in the bevels – with the infamous results for the Stollie. Andy Dingley (talk) 01:14, 14 February 2014 (UTC)[reply]

Thank you, Andy, very interesting. It's tempting to just copy this lot straight into the article, but I know we ought to have the odd reference or two!

-- EdJogg (talk) 10:43, 14 February 2014 (UTC)[reply]