Unless the final drive ratio in the transaxle can be altered drastically, it would need approximately 1:1 axle ratios front and rear. The tailshafts would be slow and 3 times the torque of most RWD tailshafts. The engine mounting arrangement would need to resist typical FWD torque - 3 times the torque of a typical RWD.

je suis charlie

So it could be done by reducing the final drive ratio?

Could this be used to advantage by balancing the front and rear and center diff ratios?

It's been done many times. It's not even close to a new idea. Obviously, you didn't do any kind of internet search before posting here.

It would not be suitable for racing because the double differential reductions give it a very low top speed.

All you've really done is taken a conventional Range Rover or Explorer style layout, moved it further back in the car, and added more reduction in the transfer case. Open centre diffs are fine for road use.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

You will be stuck with 50/50 front-rear torque distribution.

While theoretically you could play with this by making the axle ratios at front and rear unequal, in practice this will result in the front-drive transaxle's left and right output shafts having significantly different speeds (the front-drive transaxle won't know the difference). The differential in most applications will not be long for this world if you do that. The spider gears inside the diff generally just spin on the cross pin, no bearings or bushings.

You could, of course, install one of a number of limited-slip differentials inside that transaxle, but then quite obviously the front and rear reduction ratios have to work out identically.

Many years ago, a cover article in one of the engineer fish wrappers featured a lightweight AWD vehicle that featured a transverse mid engine, with timing belts instead of driveshafts, and four wheel steering, so it didn't need a center diff.

AFAIK, it never went into production. I'm not sure that all-wheel steer with fore-aft symmetry can be driveable. Harley's transition to timing belts for final drive came much later, so it was ahead of its time in that sense.

The structure comprised mostly a single longitudinal rectangular aluminum tube that housed the final drive belts and carried the chain pull loads to the transmission. I can't remember if the tube was offset to one side of the driver, or if it was central between a driver and passenger, but I do remember it being straight, and simple enough to form on a press brake, so it wouldn't do well in a crash test, but it sure looked like a fun thing to build.

Mike Halloran
Pembroke Pines, FL, USA

Wow thanks everyone for your responces.

From what i can make out it would be impractical as the driveshaft rpm front and rear would be significantly higher than the tail shaft rpm from the center diff due to the ratios.

What if the center diff could be significantly under driven, say 0.5:1?

Or would it be easier as Greglocock mentioned to take the traditional engine/gearbox layout and move it rearward?

In what way would you propose to "under drive" the center diff - which is an integral part of the transaxle on a transverse-engine front-drive car?

There's another thing to think about. The overall drive ratio situation might not be as bad as you imagine. What's the rolling diameter of your wheels, and what's the rolling diameter of the wheels of the front-drive econobox that the drivetrain came from?

Some of the axle ratios from the tail end of the "malaise era" (1970s - 1980s) were pretty tall (low numerical ratio) - this was still in the era of rear-axle-drive cars, after the 1973 fuel crunch, but before overdrive automatic transmissions became commonplace, and before front-drive took over ...

Minor google searching finds a reference to a Chrysler rear end with 2.24:1, Ford 8.8 inch with 2.26:1.

These will not be the rear-ends that the drag-race shops sell ... they'll be the rear-ends that the drag-race shops scrap.

Under driving the center to me would be a case of a lower final drive ratio in the transmission which acts as the center diff.

The wheels would be around 14-16", which is standard for the donor car i believe.

So what you're saying is that a lower value ratio on the front and rear diffs would enable higher speed even without altering the center diff ratio?

What effect would this have on the acceleration of the car though?

You have to crunch the numbers. I am talking about the overall rolling diameter of the TIRES, not just the rims. The OD of the TIRES is what counts, not the rims.

Figure out how many revs per mile your actual wheels and tires will turn, multiply that by whatever the axle ratio is that you are going to use for the front and rear axles, multiply that by whatever ratios are inside the transmission including the final drive inside the transmission. Each of these factors counts just as much as any of the others. The overall drive ratio is what matters.

Of course the acceleration will be affected. That's why your transmission has multiple ratio choices inside it. The lowest ratio has to be low enough to start off from a stop and go uphill, the highest ratio has to be enough to achieve whatever speed you want to be able to do while keeping the engine revs in a plausible range.

You are talking about adding naturally heavy final-drive diffs to a naturally heavy transaxle.
All that weight will have a huge effect on acceleration.

Note that racing parts are not necessarily durable; they are often instead just barely strong enough to survive for a limited time, e.g. one race, lightweight, and easily replaceable.

You need to spend some time in a junkyard moving diffs and transaxles around.

Note also that you don't need much structure to carry a lone driver, and you don't need much power to accelerate a light vehicle. You might want to spend some time in an ATV store, looking at the size of the driveline parts, and do some comparative anatomy with street car stuff.

OR, do some actual engineering.

Mike Halloran
Pembroke Pines, FL, USA

Yes it definitely seems like i have some calculations to do.

The main reason i was thinking AWD rather than RWD was because i wanna use it for rally on gravel roads.

And yes i know one can use FWD or RWD on gravel rally but AWD is just so appealing. Plus i love the building side of things rather than just buying something like an impreza and rallying it.

So having the extra weight of multiple diffs and and tubes doesn't bother me as much as the reliability of parts and engineering.