# Direct Drive



## toddshotrods (Feb 10, 2009)

You need some type of slip yoke and the CV joint that is made to plug into the FWD transaxle is probably your easiest way to accomplish it. Then the CV joint will provide some slip for the driveshaft to work back and forth as the suspension moves. Of course this is assuming you're connecting to a solid rear end. If it's a solidly mounted IRS differential, you don't need anything like that.


----------



## Woodsmith (Jun 5, 2008)

Even when mounting the motor and diff in the same cradle there is scope for misalignment. When you add the length of a transmission case as well then it become even more difficult to get it aligned and then keep it there with body flex while driving.
It is then easier and better to just have a very short shaft, with UJs or CVs and a sliding component between the two to absorb any movement.

I still have a propshaft I made for my 6x6 Land Rover that went between the gear box and the transfer box. It is literally two UJs back to back with a slip joint in the driving flange in the transfer box. Even though both units were very unlikely to move relativ to each other the shaft allowed for chassis flex.

ETA:
Or are you talking about the driveshafts from the transaxle/diff to the wheels in an independent rear suspension system as opposed to a live beam axle?


----------



## sunworksco (Sep 8, 2008)

Ladogaboy said:


> I just wanted to bounce some theoretical designs/ideas around, so let's assume that we have a motor system with sufficient power and rpm to use a direct-drive system (think Tesla Roadster).
> 
> Anyway, I've seen some projects where someone gutted an existing ICE FWD transmission, keeping only the 1st gear and and the final drive in order to create the desired reduction ratio. The transmission and mated motor was then mounted to a RWD car's mounting bracket (i.e., mustache bar).
> 
> ...


Here is a Tzero Acpropulsion.com direct-drive using a Honda trans.It only uses the top gear with the other gears removed.
Tesla would still be dragging some knuckles if it were not for ACP!
Regards,
John


----------



## Coley (Jul 26, 2007)

If you are thinking of using a front wheel drive trans for the axle positioning, why not leave all of the gears in it also?

Once you drive the car with gears, you will be glad you left them in.


----------



## sunworksco (Sep 8, 2008)

ACP motor is ac and needs no shifting.
Regards,
John


----------



## Duncan (Dec 8, 2008)

Hi Ladogoboy

You are asking about fitting a FWD transaxle in place of a RWD system

This is common for mid engined kit cars, 
You won't be steering the rear wheels anymore but as your suspension moves the drive-shaft angles will change, your CV's will still be required but at much smaller angles 

You need something to cope with the angle change, the only way not to have to is to use a "live axle" - heavy and primitive!


----------



## Ladogaboy (Apr 25, 2010)

Coley said:


> If you are thinking of using a front wheel drive trans for the axle positioning, why not leave all of the gears in it also?
> 
> Once you drive the car with gears, you will be glad you left them in.


As someone else already pointed out, this is designed for AC motors, which don't require the extra gears. 

As far as I understand it, the extra gears are removed in order to both reduce weight and simplify the drive line. I have to imagine that the entire transmission might also be less durable when it's not fixed to a single gear.

Thanks, all, for the information on the CV joints. I'm guessing that the difficultly would really come in mating everything up so that the car's suspension geometry doesn't change too much. Probably just a matter of making sure that the spindles in the transmission, CV joints, driveshafts, etc. match up, right?


----------



## Duncan (Dec 8, 2008)

Hi Ladogaboy

The chances of being able to match all of the splines and dimensions up is ZERO!

What you would normally do is remove the old diff and rear suspension and fit the entire front subframe from the FWD car in its place

These days most cars use strut suspension with the bottom arms mounted to the subframe

Strut suspension is quite forgiving about strut top position

This means that you use the complete front suspension - upright, wishbones, driveshafts, CV joints in the rear of your car

You will need to make up something to stop it steering!

You can modify the suspension - own wishbones, modified struts or short-long arms or whatever fairly easily

Modifying the driveshafts/ CV joints/ uprights is a LOT more difficult


----------



## toddshotrods (Feb 10, 2009)

Ladogaboy you still didn't provide any specifics of what type of rear-drive setup you plan to use. Solid (stick) rear end, IRS...?





Duncan said:


> ...The chances of being able to match all of the splines and dimensions up is ZERO!...


That's not true. You would be surprised how much mixing and matching you can do (within certain brands). I used to be heavily into Fieros and there are parts from S10s, Corvettes, Cadillacs, and many FWD cars that can all be mixed and mingled to create a custom setup.

The biggest issue here, once again, is we are answering a vague open-end question and trying to give specific information. I could tell him a hundred ways to do something like this with GM parts, but can't suggest one without knowing what he actually plans to do.






Duncan said:


> ...What you would normally do is remove the old diff and rear suspension and fit the entire front subframe from the FWD car in its place...


That is how the Fiero was created. It used a Chevy Citation (FWD) powertrain in back. The cradle (subframe) was a model-specific part but still similar to a FWD cradle.

I still think it's a mistake to tell people how easy this is, and how forgiving the geometry is. Have you ever experienced snap-oversteer in a mid-engine car? It's called *snap* oversteer because that's what happens: you're going forward and suddenly the back of the car decides to take the lead! I've been there twice, and even though it was fun and funny for a thrill-seeker like myself, it was dangerous. Once I _literally_ did a 360 off an exit ramp onto the next highway. Luckily for me, there wasn't any traffic that day.

Maybe for an NEV the geometry isn't so critical, but we don't know how the person intends to use the car. Both times this has come up recently the intended purpose hasn't been disclosed - even when prompted - leaving us guessing at what will work and what won't. I would rather err on the side of safety in that case.

Mayabe the difference is I've seen what people will cobble together. Having owned and worked in shops that did this type of work, I've also had to fix and undo a lot of unsafe work. You have to understand what can happen when you simply say "go for it, it's easy!"






Duncan said:


> ..."live axle" - heavy and primitive!


We drag racers take offense to that statement!  I can also show you some stick axle cars that will embarass expensive sports cars on a road race track. It's that geometry thing again - works wonders...


----------



## DavidDymaxion (Dec 1, 2008)

Something else to be careful about: A FWD system is designed for the front of the car, to provide understeer. If you just blithely plop a FWD suspension it into the back of a car the vehicle is then designed to oversteer under duress!

Be careful about calling a solid axle heavy! On a Mustang, the car is 100 lbs lighter with the solid axle than with the independent rear suspension! Unsprung weight is greater, but overall the car can be lighter with a solid axle.

At SCCA solo Nationals a solid axle Mustang took 1st place in it's class -- and would have taken first or trophied in every other stock car class! Yep, it beat the majority of stock Corvettes, Porsches, etc.! A solid axle car can do surprisingly well in corners, and is often preferred for drag racing and land speed racing.

Generally I think IRS is faster for cornering, but you don't have to give up much with a solid axle.


Duncan said:


> ... You are asking about fitting a FWD transaxle in place of a RWD system
> 
> This is common for mid engined kit cars,
> You won't be steering the rear wheels anymore but as your suspension moves the drive-shaft angles will change, your CV's will still be required but at much smaller angles
> ...


----------



## Duncan (Dec 8, 2008)

Hi Todd, David,

The fastest track car I have driven had a live axle - and I still say its heavy and primitive - because it is!

On the track they are difficult to beat, on the road the lower unsprung weight of an IRS is better

"Snap Oversteer" - When a mid engined car transitions from understeer to oversteer it tends to happen fast - not because of anything wrong with the suspension but because of the low polar moment of inertia

Everybody I have known with mid engined cars has spun them - luckily only two hit anything and nobody was hurt

It is a characteristic of high grip / low inertia - when it goes it goes - snap - why am I going backwards?

Quote
_Something else to be careful about: A FWD system is designed for the front of the car, to provide understeer. If you just blithely plop a FWD suspension it into the back of a car the vehicle is then designed to oversteer under duress!_
I'm sorry David but I disagree, understeer/oversteer is due to:
Front rear balance of mass and stiffness
Roll steer geometry
When fitting a FWD unit in the back both of these are variables that the converter has to set,
FWD/RWD does not matter

Nobody is going to do this level of modification without doing a lot of homework but hundreds of people have

Sensitivity of rear strut tops
I have done a lot of work on this for several of my vehicles including my current build
I have been surprised at just how much I can move the strut tops without making any signifigant change to the geometry, on my first strut car I had to move the strut tops inwards by 180mm (7inches) to achieve the desired change in geometry
(that also required modifications to the strut bottoms to achieve the correct wheel camber)
The modifications worked like a dream - on the track and road


----------



## Ladogaboy (Apr 25, 2010)

toddshotrods said:


> Ladogaboy you still didn't provide any specifics of what type of rear-drive setup you plan to use. Solid (stick) rear end, IRS...?


The reason I didn't provide specifics is because I wanted to keep the discussion more theoretical for now. Given that this is a DIY forum, I was hoping that we could come up with some cost-effective and safe ways for people to create their own direct drive systems. From what I've seen, most of the people who have done so have an extremely high-level of knowledge and experience, and someone who attempts to replicate the process could get themselves into serious trouble. 

Either way, this is the purpose for such a drive system:



Daily driveability. This is supposed to be a solution for a performance-minded street car, but that means no snap overseer and stable suspension geometry. Basically, I want to be able to "toss the keys" to a competent but uninitiated driver and expect them to survive a test drive.
Lightweight (hopefully unsprung too, so probably no live axle) in order to facilitate more battery capacity.
 Cost-effective/cheap; otherwise, we should just work through AC Propulsion's engineering team.
It would be really nice to be able to preserve the RWD car's original rear suspension, but that might be too much to ask. I wondering whether fabricating a new, rear subframe wouldn't be easier than trying to adapt the FWD car's front subframe and suspension. An AC motor and striped-down/modified transmission would weigh significantly less than the FWD ICE and transmission, so I can't imagine that it would need the same type of structural support. I would think that a tubular subframe would suffice.

Either way, I'm sorry if I can't provide more particulars. I'm just trying to compensate for a huge gap in technical knowledge/expertise that a DIYer would need to have before tackling this kind of undertaking.


----------



## toddshotrods (Feb 10, 2009)

Ladogaboy said:


> The reason I didn't provide specifics is because I wanted to keep the discussion more theoretical for now. Given that this is a DIY forum, I was hoping that we could come up with some cost-effective and safe ways for people to create their own direct drive systems...


I get your discussion goal, but the problems is there are so many variables and possible ways to skin the cat that it's hard to have a meaningful discussion without details. You'll end up with a bunch of facts that don't add up to anything meaningful. Usually the thread dies with no conclusions, just a lot of debate about nothing. On the internet, carefully constructed, narrowly focused questions develop into threads full of helpful information.

For example if you ask, how can I connect a whatchamacallit FWD transaxle to a doohickey IRS differential, there might be part numbers, measurements, people who have done it explaining the hurdles they had to overcome, etc. The next person who wants to do something similar can receive the suggestion to use the same setup, and a link to the thread. Example: Google "how to put mini tubs in a 69 Camaro". You'll get lists of threads detailing the entire process, with pictures. Google "hot to put fat tires on my car" and you'll get a wide variety of responses that will probably leave you scratching your head.





Ladogaboy said:


> ...I'm just trying to compensate for a huge gap in technical knowledge/expertise that a DIYer would need to have before tackling this kind of undertaking.


To be honest, those people shouldn't be attempting this type of extreme work yet. It would be better/safer to start with some smaller project and/or pre-engineered systems to learn _how stuff works_ first...



Ladogaboy said:


> ...From what I've seen, most of the people who have done so have an extremely high-level of knowledge and experience, and someone who attempts to replicate the process could get themselves into serious trouble...


...so that doesn't happen.


----------



## toddshotrods (Feb 10, 2009)

Duncan said:


> ...Nobody is going to do this level of modification without doing a lot of homework...


That's cute - so optimistic, so much faith in people.


----------



## DavidDymaxion (Dec 1, 2008)

If it's the fastest, call it heavy, call it primitive, call it whatever you want, just don't forget to call it fast!









Duncan said:


> Hi Todd, David,
> 
> The fastest track car I have driven had a live axle - and I still say its heavy and primitive - because it is!


I agree, if it gets bumpy my solid axle car skitters and hops over the bumps under very heavy cornering.


Duncan said:


> On the track they are difficult to beat, on the road the lower unsprung weight of an IRS is better


Don't worry, we are agreeing. Let's make sure we agree on the definition: I would say snap oversteer is when a car very suddenly and severely oversteers. Yes, things can happen quicker than a driver can react to in a mid engined car (I used to have one). No, generally that is not due to the suspension -- Pontiac, Fiat, Ferrari, Toyota, Porsche, et al have done their homework and set up the suspension properly.

My comment was if you just transplant a FWD suspension to the rear of a car, without tuning the suspension, it well could exhibit snap oversteer. Older Mustangs had snap oversteer problems due to poor rear suspension design. Putting drag radials on the rear and cornering tires on the front can lead to snap oversteer, too.


Duncan said:


> "Snap Oversteer" - When a mid engined car transitions from understeer to oversteer it tends to happen fast - not because of anything wrong with the suspension but because of the low polar moment of inertia
> 
> Everybody I have known with mid engined cars has spun them - luckily only two hit anything and nobody was hurt
> 
> ...


----------

