I have seen a few pics here and there of rear wheels drive cars that run accessories off of the rear end. there seem to be a few advantages to this. its frees up space in the engine bay, it sucks less power from the engine, and depending on the car in question it can result in better weight transfer.

i would like to toss around some ideas as to how this can be done. i have a miata and i am considering doing this.

since the differential is a sealed unit i cant imagine how you would actually run it off of the diff itself. it seems to me that the best way to do it would be to have a pulley mounted to the part of the axle that connects to the transmission and does not flex (the housing of the cv joint). i would cut the floor pan of the trunk section to allow a belt to enter the trunk area where it would wrap around the accessories which would be mounted to a special bulkhead made for this purpose.

thats the concept im considering. thoughts? ideas? is there a better part of the rear end to mount the pulley to? discuss/ thanks.

 

It's a unique idea, I just don't think that mounting those things to a driveline component behind the tranny would work.
What happens when the car is at a light? The benefit of being attached to the engine is that there is a minimum rpm it will see, and a set maximum.
It is always being driven as long as the car is running(when you need them),
and also exposure to the elements is reduced by being under the hood.
Some more thought needs to be put into this, but the extra room would open alot of doors.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 9bells &raquo;</TD></TR><TR><TD CLASS="quote">It's a unique idea, I just don't think that mounting those things to a driveline component behind the tranny would work.
What happens when the car is at a light? The benefit of being attached to the engine is that there is a minimum rpm it will see, and a set maximum.
It is always being driven as long as the car is running(when you need them),
and also exposure to the elements is reduced by being under the hood.
Some more thought needs to be put into this, but the extra room would open alot of doors.</TD></TR></TABLE>


hmmm. good points. the accessories would be dead at idle. i hadnt considered that. i have only seen this on track cars and i suppose that would be the reason.

well lets consider which accessories need to be driven constantly. the power steering pump wouldnt be needed at idle. the alternator could shut off and i could run off of battery power while at idle. the only thing that would really suffer would be the a/c compressor.

does that sound right?

 

If anything you need the power steering pump more at low speeds than when moving. By moving accessories to be driven off the rear differential you also add the complexity and weight of additional plumbing, wiring, etc.
The setup works well on dedicated, track only RACE CARS. Specifically on cars with transaxles where accessories are run at engine speed. The corvette c5r and c6r are examples of this.
Your energy losses would be GREATER by driving the accessories from the back of the car. You still have the belt drive mechanism, but now you have greater frictional and head loss for any kind of pump and increased resistance for any electrical accessory. You also have additional energy loss due to friction in the transmission (both in bearings and from fluid shear between gear teeth), energy loss in the driveshaft, energy loss in the differential etc. before that energy makes it out to the accessories. So you end up having to run things faster or size them bigger, negating the power you hoped to free up. Basically if it takes 5hp to drive the accessories you need, but due to driveline loss that 5hp is down to 4.5 at the rear diff your engine now needs an extra .5hp to make up the difference or you slow down.
Basically the laws of thermodynamics screw you. If you do happen to find away around those laws, please do tell because you would pretty much revolutionize everything in the world for the rest of time.

It's a neat thought excersize, but unless you are building a very specific type of car for a narrow range of applications, it's not of benefit.
The easier and more robust solution would be to underdrive your existing accessories and find ways to increase thier efficiency, improve thier packaging, or if you need more space around the engine, possibly fit out electrical power steering, etc.

 

I have thought about this for the Alt. But people say it wouldnt work. I still say it would and will one day try it when i have a running car.

Get a universal water pump kit, somethign usually for a sbf or sbc. You remove you alt. hook up the electric motor to the alt. and run it to a switch of some sort. Then it will turn on when you turn the car on and will run off the battery. A battery is usually only used to start the car, so should hold enough juice to start the car and then the car will run off the alt that is ran by the alt when the car is running. I believe this will work and then you can mount it anywhere.

Thats my idea. I will get it to work one day.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Niles &raquo;</TD></TR><TR><TD CLASS="quote">If anything you need the power steering pump more at low speeds than when moving. By moving accessories to be driven off the rear differential you also add the complexity and weight of additional plumbing, wiring, etc.
The setup works well on dedicated, track only RACE CARS. Specifically on cars with transaxles where accessories are run at engine speed. The corvette c5r and c6r are examples of this.
Your energy losses would be GREATER by driving the accessories from the back of the car. You still have the belt drive mechanism, but now you have greater frictional and head loss for any kind of pump and increased resistance for any electrical accessory. You also have additional energy loss due to friction in the transmission (both in bearings and from fluid shear between gear teeth), energy loss in the driveshaft, energy loss in the differential etc. before that energy makes it out to the accessories. So you end up having to run things faster or size them bigger, negating the power you hoped to free up. Basically if it takes 5hp to drive the accessories you need, but due to driveline loss that 5hp is down to 4.5 at the rear diff your engine now needs an extra .5hp to make up the difference or you slow down.
Basically the laws of thermodynamics screw you. If you do happen to find away around those laws, please do tell because you would pretty much revolutionize everything in the world for the rest of time.

It's a neat thought excersize, but unless you are building a very specific type of car for a narrow range of applications, it's not of benefit.
The easier and more robust solution would be to underdrive your existing accessories and find ways to increase thier efficiency, improve thier packaging, or if you need more space around the engine, possibly fit out electrical power steering, etc.</TD></TR></TABLE>

really? maybe you wouldnt see many gains in higher gears but in lower gears where the torque is multiplied tremendously, i cant see how it would suck more power. the drivetrain loss and everything that you mentioned is there whether i do this or not. but, unless i am missing something, driving the accessories after the torque has been multiplied is the way to go.

i agree with a lot of what you said but i disagree with that.

you mentioned an electric pwr steering pump. id be down for trying that any day of the week and i read a good write up in this forum about taking the electric pwr steering pump from an mr2 and this was one of the things i was considering. any ideas on how to electrify my a/c compressor? i suppose i cant go that route with my alternator. something about a perpetual motion machine.

 

Torque is COMPLETELY irrelevant. ENERGY is what is relevant here. The drivetrain loss issue must have been explained poorly. If you loose energy driving your accessories off one side of the engine, and the wheels off the other, that's the regulatr situation. Now if you move your accessories so they come after the transmission, or after the transmission and rear differential, you loose energy BEFORE you drive the accessories (in the drivetrain), and you still loose energy driving them. The ammount of energy to drive them doesn't go down because you moved them, torque multiplication isn't free, it comes with a loss of power. So if it took 5hp to drive all yoru accesories and you lost 1hp driving the drivetrain, now you're down horsepower at the wheels because your engine has to work harder to make a little more power to overcome the extra losses in the driveline before the energy could get to the accessories.

You also loose MORE than you would have driving them off the front of the car, because most of the accessories hook back up to things in the front of your car and whatever fluids they pump/compress/etc or electrcity from the alternator needs to flow back up front through more hardware, so now it takes more energy to drive them, meaning your engine has to make a little more power again, or your down that much power at the wheels again.
Using electric power to run a power steering pump means you need to 1)have your engine running, ok, 2) drive an alternator, and loose energy in the belt drive, bearings, and conversion of mechanical energy to elecrical energy 3) run wiring to the power steering motor, loosing energy in the form of heat due to the resistance in the wires, 3)drive the motor, loosing energy due to the conversion of electrical energy to mechanical energy 4) drive the pump off the motor, loosing energy in the rotating parts, and THEN the ENERGY (which is what's important) gets to working the power steering.
Electric power steering is just power steering using electricity instead of a hydromechanical system.
Some of those losses are trivial, others add up. What you find is that by having alot of little sources of loss, either your engine works harder, or your accessories don't work as well. If you want to do this just to be different, go for it. But you will actually hurt your cars performance. Very dedicated purpose cars have thier reasons for doing it, and it isn't in any way related to being more efficient or faster.
MR2's use their complicated steering ssytem because it was either that, or an even more complicated, less reliable standard style system (more complicated and less reliable because of the extra plumbing back and forth from the engine bay). the NSX used electric power steering instead to avoid the complexity.

mreg you seem like a guy who learns something about cars and trys to think outside the box. that's a really good thing and it's how cars get better. In this case though the laws of thermodynamics are against you.

Just for reference, Power=torque*rpm/5252 (the number is 5 something, if you look at a dyno chart it's the rpm where torque and horsepower always cross, i can never remember the actual number) So if you multiply torque and get more, your rpms go down, in the real world this sucks twice as much because you also lose power in the conversion due to friction, resistance, etc.

 

It's basically like a 401k and taxes. Your 401k is pre-tax money, so you want to put $10k in it then it's best to take it out pre-tax. If you put that $10k in with post tax money, it's going to cost you $13k gross.

Damn, who knew you could explain thermo and mechanics with taxes.

 

Sounds like fun. Put the turbo in the back, put the accessories in the back, and then put the engine in the back. Or buy a Porsche.

 

Electric power steering has been proven over and over again to improve fuel economy about 5%. That's why so many new cars are coming with it, not just to reduce complexity (though it helps a lot with that too; no more pump and high-pressure hydraulic hoses).

EPS would be stupid and a waste if you spent most of your driving driving slalom courses in parkings lots. However, most people don't drive like that: they drive at higher speeds, and usually in a straight line. Over 35mph or so, you don't even need power steering. So you're basically wasting a lot of energy just so you don't have to strain your arm muscles so much when you're in a parking lot or turning out of your subdivision. The rest of the time, that PS pump is wasting energy for something you don't need.

EPS just uses power when you actually need steering assist. The rest of the time (like when you're driving 80 on the highway) it turns off, saving you energy and fuel. Who doesn't want 1-2mpg higher highway mileage?

For other things, your point about efficiency is valid. You don't turn off the A/C on the highway, nor do you turn off your water pump. That's why things like this are run directly by the engine instead of electrically. But steering is LONG overdue for conversion to electricity, like they did with radiator fans decades ago. Who still uses a belt-driven radiator fan? There's a good reason they moved to electric fans.

 

i've considered doing away with power steering altogether but i have different offset wheels than that of the zero scrub setup as the car comes from factory. in short, my weak little baby arms cant handle a parking lot without power steering. lol.

i am going to look into the electric power steering thing more seriously.