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How 4WD for mid engined cars?
geoffreyh - 28/1/06 at 09:40 PM

I'm wondering how the transmission and diffs are working to get 4WD with a mid mounted engine.

How is the front axle connected to the transmission behind the engine?


Geoff

zilspeed - 28/1/06 at 09:46 PM

The transmission is often in front of the engine -which is still ahead of the rear axle line. RS200 - Metro 6R4 - Andy Burton's Peugeot Cosworth.

PS - let's not do that whole mid engined thing again

geoffreyh - 28/1/06 at 09:58 PM

But how are they doing it for Lamborghinis or the new Bugatti Veyron?

It looks like lambo is working with a primary and secondary axle.

[Edited on 28/1/06 by geoffreyh]

mark chandler - 28/1/06 at 10:28 PM

If you want to do it cheaply and easily simply pick a FWD car as a donor and keep the gearbox.

Only limitation is top end, from memory a v6 Rover engine with auto FWD box in a landrover tops out at around 85mph with 3.54 diffs.

A fwd box from a diesel golf spins the engine at under 2000 rpm in 6th at 85mph so this could possibly be a good choice with some low diffs at each end.

Regards Mark

[Edited on 28/1/06 by mark chandler]

TheGecko - 29/1/06 at 04:49 AM

quote:
Originally posted by geoffreyh
But how are they doing it for Lamborghinis or the new Bugatti Veyron?

It looks like lambo is working with a primary and secondary axle.
Lamborghini has used much the same drivetrain layout for their flagship cars since the early '70s. The Countach started it with the V12 arranged inline between the rear wheels driving forward to the gearbox which is then effectively between the seats. The gearbox output shaft then runs backwars, through the engine sump, to a differential in a housing which is part of the sump (not with shared oil though). It's obviously a pretty easy step from there to having a centre diff at the forward end of the gearbox driving forward and backwards. Here's a quick rough diagram that may help the explanation:[img][/img]

Hope this helps a little,

Dominic

cornishrob - 2/2/06 at 06:37 PM

I have thought long and hard about this one and the answer only came to me a couple months ago.


The cheapest easiest way to do it in my eyes is to get a transverse 2 wd drive engine and gearbox system. like a ford fiesta

Mount this mid chassis instead of having drive shafts going to hubs have a prop running in either direction to a diff.


this is where everyone slates me that you have two final drive reductions making it very low geared!

True of a normal diff

BUT i have had another moment of clarity thinking back to my celica GT4


the celica GT4 has four wheel drive. but unlike sierras, the engine is mounted transverse in the front with a nomal style gearbox powering the front wheels, BUT the drive shaft power the drivers wheel is put through a non detachable transfur box to put power arse end. At the rear is a rather ordinary looking differential with prop in drive shafts out.

BUT, because the front passenger driveshaft is soley goverened by the gearbox then the final drive reduction must be in the gearbox.

the only way that the diff will be anything other than a 1:1 ratio will be if there is an opposite ratio in the transfur box.


I cant see this being the case as it would cause the prop to spin faster, more likely to fail and rally cars all all about robustness.

TheGecko - 3/2/06 at 03:22 AM

quote:
Originally posted by cornishrob
the celica GT4 has four wheel drive. but unlike sierras, the engine is mounted transverse in the front with a nomal style gearbox powering the front wheels, BUT the drive shaft power the drivers wheel is put through a non detachable transfur box to put power arse end. At the rear is a rather ordinary looking differential with prop in drive shafts out.

BUT, because the front passenger driveshaft is soley goverened by the gearbox then the final drive reduction must be in the gearbox.

the only way that the diff will be anything other than a 1:1 ratio will be if there is an opposite ratio in the transfur box.


Unfortunately, your assumptions about the insides of a GT4 gearbox, based on external appearances, are wrong And if you think about it, there's no way it could possibly work the way you describe, with one front wheel being fed from one side of a diff and the other three wheels from the other side!

The GT4, in common with most transverse AWD boxes, has the centre diff (which proportions power between front & rear) mounted co-axially with the front diff (splitting power between left and right front wheels). The centre diff to front diff connection 1:1 ratio (usually) and the centre diff to tailshaft ratio is the inverse of the rear diff ratio. This also makes sense when you look at it because the centre diff is on a transverse axis and is therefore driving the tailshaft through a crownwheel and pinion arrangement. To be 1:1 the pinion would have to be the same size as the centre diff crown and it's nowhere near it.

The drivers side half shaft drives through the hollow middle of the centre diff, which is a planetary style from memory. This is where it's possible to misinterpret things from the outside.

Sorry to burst the bubble


Dominic

cornishrob - 3/2/06 at 05:34 PM

someone with the internal knowledge bursts the bubble


back to the theoretical drawing board


I have to say though with the ammount of time i spent under that car i would never have guessed that the workings were arranged like you described