Whilst looking at possible engines for my 7 project, something caught my mind, completely unrelated to my engine choice.
There are a lot of FWD engines/boxes from FWD cars..
If you were to simply turn the engine 90 degrees, and use the existing gearbox driveshaft output flanges, simply run two props one to the front, one
to the rear, would there be any problems of doing that and having a really simple 4wd transmission?
The biggest problem I see is getting the weight of the engine/box central, so the props will have to most likely be offset to one side, but surely
that's not such a massive problem?
Anybody ever heard of this or thought of this before?
You would then have three differentials.
Not sure the gearing would be effective unless the front diff and the rear diff are 1:1, and the gearbox diff is used as the main final drive.
Would put a hell of a lot of strain on the box though.
Don't all awd cars have 3 diffs?
I can see how the gearing can be a bit of an issue, but in terms of strain on the box, I doubt that's a problem if you use it in a car as light
as a seven or other kit car
Pavs
the center diff in most 4wd cars is 1:1 ratio the diff in a fwd gearbox will be more like 3.9:1 which is what will significantly reduce your gearing. .m.
It a layout used in off-road stuff, the gearing works to their advantage meaning they can use large diameter wheels for better ground clearance while
keeping the speed low.
It does mean that the front & rear diffs have a lot of torque to handle though.
I know of a lad that used this idea in an off road buggy, using a Peugeot 309 diesel.
It worked brilliantly, but it was very low geared (which was ideal in this buggy), it's top speed was about 30-35mph. But it would crawl over
anything.
So, if you could lower the diff ratios, it would be a good concept, it's just a case of how much a custom ring and pinion would cost, to get as
close as possible to a sensible road speed.
Maybe it would be easier if you could alter the axles and the centre diff ratios, and spread the ratio through all 3 diffs, instead of putting all the
load on 1 diff alone.
I have thought about that before, would make a simple 4wd mid engine.
Options are:
1)Make the box ratio 1:1 either by gearing or using an inverted tooth chain.
Unlikely to have the room to make 1:1, but might be able to have some reduction of ratio.
2) have 1:1 bevel gearing connected to the diffs, so would require custom gears and a custom enclosure. The prop shafts will need to take 3 times the
torque, so even a modest engine till require a strong prop.
You would need a gearbox with lsd, eg fiat coupe turbo, rs turbo etc.
Darren
Wouldn't you have to lock the differential to make it power front and rear wheels at the same time ? The proper ones use a transfer box not a
differential in the middle so you can select what drive you want.
[Edited on 27/6/10 by Peteff]
quote:
Originally posted by turbodisplay
...You would need a gearbox with lsd, eg fiat coupe turbo, rs turbo etc...
No the diff is open in the centre, as if you did have it locked, the difference in speed will cause wind up, until the wheels scrubbed. It needs to be
a lsd.
Ways 4wd cars cope with speed difference is:
Open and locked in low grip situation
Viscous coupling
Clutch
Open diff with lsd.
Darren
[Edited on 27/6/10 by turbodisplay]
As above - LSD in the centre and at least one other (normally rear), find/build 1:1 front/rear diffs and job's a goodun. You would be stuck with a 50:50 front/rear torque bias though.
50:50 torque bias would be fine in an off-roader, but could be a problem in a road car.
Unless you could find some 1:1 diffs front and back you would end up with just under 8:1 reduction..
Not good unless you want a screaming rock crawler...
quote:
Originally posted by fha772
50:50 torque bias would be fine in an off-roader, but could be a problem in a road car.