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FWD to AWD conversion

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Rokita

Automotive
Jun 13, 2022
7
i have FWD car in which engine is mounted in transverse, im planning to make AWD conversion there are available AWD gearboxes that fit my engine but those use electrical clutch before rear differential to turn on AWD only when needed
thats not what im looking for
i need all wheels to be powered all the time since this gearboxes dont use any central differential running this setup with "welded" clutch on rear diff will probably destroy the gearbox


obraz_2022-06-13_113754715_o6slvj.png

my idea was to take existing gearbox and mount engine longitudinally
obraz_2022-06-13_113844503_cfr24c.png

obraz_2022-06-13_114005055_xp1trx.png


what are your thoughts can it work or trying to is just waste of time?
because for me it sounds like it really could work
im not concerned about weight balance because i would adres it when fitting a roll cage and since driver is on left side it evens the balance enough
gearbox linkage will be changed to hydraulic
 
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How much of the vehicle's unibody structure and suspension are you willing to change?

Anything can be made to fit anything else with enough cutting, fabrication, and welding. This certainly is not going to be a bolt-on job. You will have to change everything.

Is it a waste of time? Most hobbies are...

It will be better to sell this vehicle and buy a different one that already has your preferred powertrain layout as factory equipment. Audi (real one, not VW platform with Audi badge) and Subaru come to mind.
 
when it comes to unibody im not worried because a lot of ppl already build rwd swaps you can even buy kit for that
there is enough space to run shaft i just need to cut some metal on the rear so i can mount differential the only issue would be the exhaust but i plan to run it trough fender or bonnet

when it comes to mounting the engine that pretty much is solved too

i came here to discuss using fwd gearbox as central differential with axles coming out as shafts that attach to front and rear differential

there are awd builds of the same chassis using as i mentioned awd gearboxes that came in SUVs but these gearboxes dont use central differential what will cause a lot of stress on gearbox when turning
i wouldnt worry about that if this build was meant for offroad but its meant for tarmac

i know race cars need servicing often but i just to not want an obvious failure point
 
Two problems you're going to have:

-packaging an additional differential in the front. Differentials are not small, and especially the type of differential you would need to use here (not an oem design that's integrated into the front powertrain already). Don't forget that you don't have to just package the front diff itself; you also have to package enough structure for the diff, rotated engine, and original transmission to live in very close proximity and be mounted strongly enough to handle all of their various loads. If this is a Macpherson strut car in the front (which I'm assuming it is... ) you can't just cut out whatever you want. Inner fenders are a structural component in a macpherson car.

-You'll need to sort out the existing differential in the current transmission. You're effectively turning the existing front diff into an AWD center diff; what works in the front is not going to work well in the center. What options you have will depend heavily on what specific transmission you're dealing with.

I think you'd be much better off adapting an existing AWD transmission system designed for front transverse mounting (ie Mitsubishi) to your existing engine. It'll be a lot less work and you'll be using components which are designed to be mounted the way you want to mount them.
 
It perhaps might help to know which vehicle the original poster is talking about here.

Another nuisance is that the final drive ratio of the transverse transmission is built into the transmission itself. Can you get a hypoid-gear-and-differential assembly with a 1:1 (or close) final drive ratio? It has to be the same ratio front and rear, also.

I have run across longitudinal-rear-drive conversions of front-drive bodyshells before. Invariably they are using a rear-drive transmission meant for longitudinal installation and without its own final drive ratio contained inside it, so that the rear differential can have a final drive ratio that is in the conventional range (3:1 - 4:1 or thereabouts).

There are transmissions for transverse installation which include a "center differential". There is a version of the ZF 9HP automatic transmission used in the Jeep Cherokee that has such a thing. There may be others.

Various random thoughts, most of which arise from not knowing exactly which vehicle the original poster is proposing to convert.

Most front-drive (transverse) transaxles have the diff as close as possible to the center of the vehicle. Turned longitudinally, that puts the diff pretty far forward. Where's your front-axle centerline relative to what would normally be the right-hand differential output flange?

Pay attention to where the steering rack is, and whether it's going to interfere with your plans.
 
Wait a couple of years and just use two electric drive assemblies. No need for a front-to-rear drive train connection.
 
BrianPetersen said:
It perhaps might help to know which vehicle the original poster is talking about here.
Civic ej8 1996 Coupe
obraz_2022-06-14_152605370_gcwnit.png

BrianPetersen said:
Most front-drive (transverse) transaxles have the diff as close as possible to the center of the vehicle. Turned longitudinally, that puts the diff pretty far forward. Where's your front-axle centerline relative to what would normally be the right-hand differential output flange?
obraz_2022-06-14_152859280_cgldau.png


model I posted is very close to reality
the right axle is much shorter than left one
what means that center diff ends up just before firewall
obraz_2022-06-14_152402644_txaslo.png


BrianPetersen said:
Pay attention to where the steering rack is, and whether it's going to interfere with your plans.
Right, I will adjust the design

gruntguru said:
Ouch! - those driveshaft angles.
I know...
 
I am not 100% sure, but I think that generation is the last one before Honda started using a full-perimeter subframe around the whole drivetrain. It has what looks like a "subframe" for the lower control arms and the steering rack. Minor digging reveals that "subframe removal" does not entail engine or transmission removal, which suggests that it's a steering-rack and lower-control-arm subframe but is not involved in engine or transmission mounts.

I suspect that your proposed driveshaft to the rear is going to be going right through that area.

What differential (with the aforementioned approximately 1:1 final drive ratio) are you proposing to use?
 
Well, with the open diff in that transmission, and with the same final-drive axle ratio in both the front and rear (which had better be the case to avoid burning up the diff in the transmission), it's going to be locked at 50% front and 50% rear torque split, whether that's what's needed or not. It will act as any other open diff does. If the rear wheels are on ice, the front wheels won't deliver any torque. (And vice-versa.)

There's reasons that normal rear drive units on transverse-powertrain vehicles use a viscous clutch or a controlled clutch of some sort rather than an open center diff. Those are the ones that have variable front-rear torque split. (Normally 100% front 0% rear unless there is slippage of the front tires, and some systems can achieve almost 100% rear torque bias by locking that center clutch so that the rear powertrain is driven no matter if the front wheels are completely slipping or off the ground)
 
From what I can tell - He's wanting to use the normal differential that's built into the (normally front-drive, transverse) transmission as the center diff, and then use the now-forward-pointing halfshaft output to drive the front diff and the now-rearward-pointing halfshaft output to drive the prop-shaft to the rear and the rear diff. So the original-equipment final-drive diff that's built into the transmission will now be the center diff.
 
GregLocock said:
And how are you going to cope with the random front/rear torque split?
There are "fake LSD" that are made for these gearboxes
obraz_2022-06-15_100057284_v6w3hc.png

its made out of two plates that have springs pushing them againts spider gears - preload
locking pin that goes trough differential is also being pushed against cut out in the plate that is also puting more force on spider gear locking it more when loaded - locking angle
so by changing the angle of the cut for one of the plates I can actually change torque distribution
I could even change torque distribution for breaking
essentially it is horribly simplified LSD
obraz_2022-06-15_100427310_j9jtnh.png

its not a good design by any means for FWD car that often runs axles at different speeds but in central diff where we want pretty consistent speeds - I think it will hold up
I also redesigned it a bit so there will be a small clutch disc between the plates and spider gears
it also includes changing a metal washer under the spider gear for something with a bit more grip - clutch pack
I talked with some offroad guys and a heard that a lot of them used these "fake LSD's" in their AWD hondas
of course they used it in front and rear diff because honda design is missing center differential but I got a proof that it is kinda working

because the front differential will be almost directly connected to center diff the torque will be split a bit more to the front - I can cope with that

BrianPetersen said:
From what I can tell - He's wanting to use the normal differential that's built into the (normally front-drive, transverse) transmission as the center diff, and then use the now-forward-pointing halfshaft output to drive the front diff and the now-rearward-pointing halfshaft output to drive the prop-shaft to the rear and the rear diff. So the original-equipment final-drive diff that's built into the transmission will now be the center diff.
yes, exactly what I'm talking about

BrianPetersen said:
What differential (with the aforementioned approximately 1:1 final drive ratio) are you proposing to use?
I will probably go for a shorter gearbox, and find diff that will fit
I know ppl have used gearboxes from 1.3 1.4 and 1.5 D series engines
gears are much shorter in some of them
i totally forgot about it and didnt though of it until couple days ago so I did not yet prepared the exact parts I would fit

BrianPetersen said:
There's reasons that normal rear drive units on transverse-powertrain vehicles use a viscous clutch or a controlled clutch of some sort rather than an open center diff.
in case of honda trasverse awd gearbox fron wheels are powered all the time and controlled clutch only engages rear differential so by this design most we can get is probably 45% rear torque
 
You can get 100% rear torque split if the front wheels are on ice and the clutch to the rear is up to the task.

...which most of them aren't. Look at how skinny the rear halfshafts are and how small the rear diff is on a lot of them. It's just enough for the manufacturer to claim it has all wheel drive.

... but this is not a limitation of the layout, only of the components selected.
 
Rokita said:
I will probably go for a shorter gearbox, and find diff that will fit
I know ppl have used gearboxes from 1.3 1.4 and 1.5 D series engines
gears are much shorter in some of them
i totally forgot about it and didnt though of it until couple days ago so I did not yet prepared the exact parts I would fit

He's talking about your front and rear differentials - not the trans.

The final reduction in the FWD gear box is built into the gearbox, meaning in your setup what is normally the final reduction is BEFORE the split to front and rear; if you use a relatively standard reduction ratio in your front and rear differentials, your total gearing is going to be EXTREMELY short. For the gearing to be remotely sensible, if you use the same trans, you need the reduction at your front and rear diffs to be as close to 1:1 as possible.
 
He's talking about your front and rear differentials - not the trans.
transmission gearing actually affects the speed

rn my final ratio is 3.8 i can find 3.2 gearbox and fit longest gears aviable in it
then even with keeping 3.2 final ratio on differentials it wont shorten the gearing that much
engine will also rev much higher than its stock
so gearing will be fine probably
i didnt calculate that exactly
 
I think you had better calculate your overall drive ratios (and figure out what RPM it's going at what road speed in each gear) based upon available components.

I think you are in for a rude awakening unless you find appropriate 90-degree final drive and diff assemblies with extremely tall (ideally 1:1) final drive ratios.
 
Rokita said:
engine will also rev much higher than its stock
so gearing will be fine probably
i didnt calculate that exactly

Unless this is a tractor pulling rig where you need 10,000,000 lb ft at the wheels, or 'much higher than stock' means 20,000 RPM, you have a lot of important work to do.

The gearing will not be fine unless you calculate what your true ratios are and plan accordingly. I can assure you, as setup with back-to-back 3.2:1 reductions is not going to work.
 
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