# RWD, regen, and direct drive questions



## dtbaker (Jan 5, 2008)

berny said:


> ...a RWD car that weighs about 2,000lbs (709kg) stock. I need it to achieve freeway speeds (70mph, 113kph) and a range of 80 miles (129km) would be nice. ...I don't mind dropping 5-10K on batteries to do it.
> 
> 
> First of all regenerative braking. is more cost effective to extend range by using a DC motor and adding batteries. Is this true?



in most cases, you'll get more miles/$ with DC and forgetting regen. Practical regen requires AC motor and more complex design.... managing regen power to capture as much as possible without toasting batteries, etc.





berny said:


> Also if one were to use regen, wouldn't you need a front wheel drive vehicle?



no.... the 'regen' you can apply in best case is probably going to be significantly less than you'd apply with a hard braking event, so wouldn't affect handling much whether applied to front or rear wheels.





berny said:


> With the EV calculator here http://www.evconvert.com/tools/evcalc/ RWD cars seem to get more miles per charge.



I dunno why that would be.... all else being equal, a FWD might be lighter without long driveshaft. But perhaps a transverse mount is less efficient.





berny said:


> Can one easily make a FWD car with no transmission? All other things being equal how much more range can one expect not having the rotational mass of a tranny eating up power? 1%, 10%?



the mass is not the issue except for acceleration from a stop. transmissions will always have 'some' loss from gear friction, but they are pretty darn good... like in the 90% range, and well worth the mechanical advantage to get decent acceleration AND top end. DC motors have a broad torque band, but to get o-70mph out of 0-5000rpm is not the best use of power. AC motors can rev far higher, but you'd be back to the cost issue.





berny said:


> Also, can one compensate for not having the torque multiplying benefits of a tranny with electronics? Say having two battery packs and switching them between parallel and series for higher torque/ top speed. I have heard of racers doing that with two motors somehow but I can't comprehend how that would work, any hints?



racers do lots of things that might result in amazing performance, but it is usually at the expense of either range or durability....


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## Guest (Jun 27, 2010)

Lots of folks have rear wheel drive vehicles. lots have AC and regen. Lots have DC and no regen. Lots of racers use DC and no regen and rear wheel drive. 

Have a look here and use their search function to look at different types of EV's. 

http://www.evalbum.com/


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## EVfun (Mar 14, 2010)

berny said:


> Also, can one compensate for not having the torque multiplying benefits of a tranny with electronics? Say having two battery packs and switching them between parallel and series for higher torque/ top speed. I have heard of racers doing that with two motors somehow but I can't comprehend how that would work, any hints?


The battery pack is wired in series and the controller converts the available voltage and current into any lower voltage at higher current (volts * amps [input] = volts times amps [output].) Sometimes several packs are used in parallel because in some cases 2 strings of smaller batteries are more able to dish out the current that a single string of larger batteries. Racing is about dishing out the power until something breaks, then making that thing stronger, then trying to go faster until the next thing breaks, and repeat. It is great at finding out what EV parts are durable; it doesn't mean that all the things they are doing are good choices for the street. 

Some racers are using 2 motors and wiring the motors either in series or parallel to get the effect of an electric 2-speed. A series wound DC motor (the most proven choice for EV conversions to date) makes great torque, but the power falls off at higher rpms. More motor voltage allows the motor power to hold on until a higher rpm. The motors torque is directly related to how many amps you can feed it. So, the racers using series/parallel motor switching launch with the motors wired in series so each motor sees the full current the controller supplies. As the motors rev this becomes a problem because each motor only sees half the voltage and starts to "fall asleep." As the power falls off the motors are switched to operate in parallel. Now each motor has full pack voltage available to it, but only half of the controllers current. The voltage increase allows the motors to make power to a higher rpm though not with so much torque as before because they are sharing the current. The effect is about like using a 2-speed transmission with a 2:1 low gear and a 1:1 high gear, without the clutch and transmission problems caused by the extreme torque at launch.


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## berny (Apr 20, 2010)

gottdi said:


> Lots of folks have rear wheel drive vehicles. lots have AC and regen. Lots have DC and no regen. Lots of racers use DC and no regen and rear wheel drive.
> 
> Have a look here and use their search function to look at different types of EV's.
> 
> http://www.evalbum.com/


Thanks for the tip, last time I looked there the search wasn't working properly, and I couldn't find any builds that looked anything like I want, but I will try again.



EVfun said:


> Some racers are using 2 motors and wiring the motors either in series or parallel to get the effect of an electric 2-speed. A series wound DC motor (the most proven choice for EV conversions to date) makes great torque, but the power falls off at higher rpms. More motor voltage allows the motor power to hold on until a higher rpm. The motors torque is directly related to how many amps you can feed it. So, the racers using series/parallel motor switching launch with the motors wired in series so each motor sees the full current the controller supplies. As the motors rev this becomes a problem because each motor only sees half the voltage and starts to "fall asleep." As the power falls off the motors are switched to operate in parallel. Now each motor has full pack voltage available to it, but only half of the controllers current. The voltage increase allows the motors to make power to a higher rpm though not with so much torque as before because they are sharing the current. The effect is about like using a 2-speed transmission with a 2:1 low gear and a 1:1 high gear, without the clutch and transmission problems caused by the extreme torque at launch.


That makes sense. That seems much too complex for a someone starting out with a daily driver. I can see why racers do it but for me it would be easier just to have mechanical gearing if I wanted two different gears. Thanks for the info.



dtbaker said:


> I dunno why that would be.... all else being equal, a FWD might be lighter without long driveshaft. But perhaps a transverse mount is less efficient.
> 
> the mass is not the issue except for acceleration from a stop. transmissions will always have 'some' loss from gear friction, but they are pretty darn good... like in the 90% range, and well worth the mechanical advantage to get decent acceleration AND top end. DC motors have a broad torque band, but to get o-70mph out of 0-5000rpm is not the best use of power. AC motors can rev far higher, but you'd be back to the cost issue.


Awesome, thanks for the help about the regen and handling issues. I can't figure out why the calculator gives consistently higher ranges for RWD but it does, ten or more miles which seems significant to me. Does anyone else have an idea? Is this for real or just a glitch in the calculator?

I'm still torn on the transmission issue, it looks like to go without one I would need to use an AC motor. I will be starting from a stop frequently in my car so it sounds like I could get more range out of it and I don't mind the higher cost. If I went with a AC motor, do you think it would then be a good idea to not have a transmission? or would you still use one? It just seems like a waste to carry around 200lbs that takes 5-10% of my power for only 2 gears if I can avoid it.


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## toddshotrods (Feb 10, 2009)

dtbaker said:


> ...
> 
> I dunno why that would be.... all else being equal, a FWD might be lighter without long driveshaft. But perhaps a transverse mount is less efficient...





berny said:


> ...I can't figure out why the calculator gives consistently higher ranges for RWD but it does, ten or more miles which seems significant to me. Does anyone else have an idea? Is this for real or just a glitch in the calculator?...


I think it's a glitch. Transverse FWD is more efficient, all else being equal, because it doesn't have the 90-degree turn of the ring and pinion. Another clue is that was one of the stated reasons the major manufacturers went so heavily towards transverse FWD production cars. The magazines ranting about how much better the handling dynamics of a rear-drive car are kind of forced them to return to front-engine/rear-drive - even though most people will never drive their vehicle aggresively enough to notice the difference. I laugh in the winter when I see rear wheels on newer upscale cars spinning like crazy to move, while front-drivers (like mine) push through snow and ice with ease. It's about as silly as 400hp to take the kids to soccer practice.





berny said:


> ...I'm still torn on the transmission issue, it looks like to go without one I would need to use an AC motor. I will be starting from a stop frequently in my car so it sounds like I could get more range out of it and I don't mind the higher cost. If I went with a AC motor, do you think it would then be a good idea to not have a transmission? or would you still use one? It just seems like a waste to carry around 200lbs that takes 5-10% of my power for only 2 gears if I can avoid it.


You're going to have a fairly light vehicle, with a reasonable project budget, but I am not aware of a reasonably priced (or even commerically available) AC drive system large enough to run direct drive. Most of the people running direct drive have big (11 or 13") DC torque monsters, or equally torque-monstrous dual 8 or 9" motors. As far as I know, the available AC drive systems are smaller and would be best used with a transmission or really lighweight vehicle. They give you the potential for higher RPM for more top end range, but not the stump-pulling torque to move comfortably with top end gears. Yes, you could gear it pull better down low, at the expense of some top end, but that's in theory. I would want some evidence of how successful it is before spending thousands on it.


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## berny (Apr 20, 2010)

toddshotrods said:


> I think it's a glitch. Transverse FWD is more efficient, all else being equal, because it doesn't have the 90-degree turn of the ring and pinion. Another clue is that was one of the stated reasons the major manufacturers went so heavily towards transverse FWD production cars.


Yeah I was wondering about that myself. I looked online and could only find that manufacturers switch to FWD because it you could make smaller cars lighter cars with the same amount of interior room because there was no drivetrain running under the floor. I found someone that had compared FWD and RWD cars with the same engine and weight and found the mpg to be identical. What are the rings and pinon and why does that eat efficiency? Also they say RWD cars handle better b/c of the weight ratio. With 400lbs of batteries I could make the weight ratio whatever I want. If I had a FWD car with a 50/50 weight ratio would it handle the same as a RWD car I wonder?

So it sounds like for a daily driver having a transmission is preferable. I wouldn't mind getting one of those big torque motors though. 

Since I am going to spend a lot of money on this car and probably have it for a very long time, it would be nice if it was at least moderately quick and fun to drive. I know you can't drive it hard and expect a lot of range, but having the capability and just not using it when I needed the range would be awesome. Could I accomplish this with a RWD car and direct drive? Or is transmission always better? 

Thanks for the responses so far, I am learning lots.


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## toddshotrods (Feb 10, 2009)

berny said:


> Yeah I was wondering about that myself. I looked online and could only find that manufacturers switch to FWD because it you could make smaller cars lighter cars with the same amount of interior room because there was no drivetrain running under the floor. I found someone that had compared FWD and RWD cars with the same engine and weight and found the mpg to be identical...


You can find something to say anything you want it to on the internet.  You have to take a lot of it with grain of salt, and compare it to other viewpoints. I'm not saying the info you saw was wrong, just who knows???

I buy the fuel mileage figures being the same, or at least close. There isn't really a lot of difference anymore, and the better efficiency I referred to with FWD is based on theory and would be microscopic anyway - not a huge difference a person would really notice in comparable vehicles. The more mechanical changes the power goes through the less overall efficiency you'll have.




berny said:


> ...What are the rings and pinon and why does that eat efficiency?


Power in a RWD goes back through the driveshaft and into the differential, where it is turned 90-degrees to turn the wheels. The gears that turn the power are a pinion gear and a ring gear. Before I start babbling Google ring and pinion images. The pinion gears receives the power and turns the ring gear, which turns the axles, which turn your wheels and tires. The hip bone is connected to the thigh bone... 





berny said:


> ...So it sounds like for a daily driver having a transmission is preferable. I wouldn't mind getting one of those big torque motors though.
> 
> Since I am going to spend a lot of money on this car and probably have it for a very long time, it would be nice if it was at least moderately quick and fun to drive. I know you can't drive it hard and expect a lot of range, but having the capability and just not using it when I needed the range would be awesome. Could I accomplish this with a RWD car and direct drive? Or is transmission always better?...


This debate rages regularly here, but the simple answer is it depends on what your building and what you're using it for. I am a direct-drive diehard, but I am also building an EV racer. In your case, I think a transmission would be best. You can build an efficient driver that has selectable modes (gears) for what you want to do at the moment.

I don't care about efficiency. All I care about is going fast. I don't care how much of my pack it uses to get moving with no transmission, just that I get there first. On the other hand, the White Zombie is promising 10-second 1/4-mile, 2.5-2.7 second 0-60mph, and 100-125 mile range this year - it's direct drive, with dual DC motors! If you have the bank account, you _can_ have your cake and eat it two.

So the answer to your last question is, if you have enough money, you can build a direct-drive, RWD, EV that does pretty much everything you need. You'll probably spend that $20K on batteries alone though.


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## major (Apr 4, 2008)

berny said:


> I have done my best to search out the answers to these questions before posting, but there isn't a ton of info on regenerative brakes.


Hi berny,

Seems like there must be a ton on regenerative braking on this DIY site. Have you tried the site search function? http://www.diyelectriccar.com/forums/search.php?searchid=256454 I just got 200 posts with the word _regenerative_.

Regards,

major


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## Guest (Jun 28, 2010)

I don't think there is much on regenerative brakes but tons on regenerative braking. Some DC motors can do regenerative braking and some can't. All the AC motors can do regenerative braking. Most of the DC motors that use that function are sepex motors and are usually small or used on slow moving vehicles. Some on small vehicles that can move fast but not highway vehicles. 

Pete


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## toddshotrods (Feb 10, 2009)

There are large SepEx motors out there, but the controller has to be programmed for it and there are no field maps readily available for them. There aren't any readily available high voltage/high current SepEx controllers either, just the golf cart stuff. I have an 11" GE SepEx from a Hyster forklift. I'm converting to it series and not worrying about regen.


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## berny (Apr 20, 2010)

Yeah I searched for regenerative many times and learned a lot from it, but sometimes you just need a question answered. I didn't mean to sound negative about the site though. This site is awesome, the day I found it was the day I decided building an EV is something I could actually do. 



toddshotrods said:


> You can find something to say anything you want it to on the internet.  You have to take a lot of it with grain of salt, and compare it to other viewpoints. I'm not saying the info you saw was wrong, just who knows???
> 
> I buy the fuel mileage figures being the same, or at least close. There isn't really a lot of difference anymore, and the better efficiency I referred to with FWD is based on theory and would be microscopic anyway - not a huge difference a person would really notice in comparable vehicles. The more mechanical changes the power goes through the less overall efficiency you'll have.


Haha for sure. Id bet you are right that it is close enough to being equal that it doesn't really matter.



toddshotrods said:


> So the answer to your last question is, if you have enough money, you can build a direct-drive, RWD, EV that does pretty much everything you need. You'll probably spend that $20K on batteries alone though.


That is exactly the perspective I was looking for. Transmission it is.

Having dual motors sounds interesting, but probably not something I would try. I am having a hard time picturing how two motors are attached to the same drive shaft.


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## toddshotrods (Feb 10, 2009)

berny said:


> ...Having dual motors sounds interesting, but probably not something I would try. I am having a hard time picturing how two motors are attached to the same drive shaft.


Does this help?  See more good stuff here.


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## TX_Dj (Jul 25, 2008)

That's a hell of a tail housing they've built on that motor. Anyone know if there's a reduction set in there between the aft motor and the tailhousing itself? Trying to figure out what the extra room is for.


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## toddshotrods (Feb 10, 2009)

TX_Dj said:


> That's a hell of a tail housing they've built on that motor. Anyone know if there's a reduction set in there between the aft motor and the tailhousing itself? Trying to figure out what the extra room is for.


That is a $2500-3000 Gear Vendors overdrive unit. I was going to do the same thing on the back of my single 11" motor in the Inhaler (way back maybe in the single digit page numbers of the Inhaler build thread), but decided I didn't need the extra gear enough to justify the cost. Interesting to see someone actually doing it though.

That may be the necessary amount of material needed to mate the GV unit to the motor you're seeing. It was designed to replace the tailshaft of a conventional RWD transmission so they may have needed some additional length to get the motor shaft and GV input hub to mate properly. Just a guess.

If I remember correctly, Jim's dual motors have a long shaft with TH400 splines on the end that sticks out far enough for a driveshaft slip yoke. That would be perfect for the GV.


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## TX_Dj (Jul 25, 2008)

Wow. I would have figured underdrive, not overdrive. Now that you mention it, I see the GV logo on the lower end of the housing.

A single-ratio underdrive with direct for speed driving seems it would be more effective for streetability, but for the strip being able to S/P shift and then kick in the OD once amps start to drop would probably be a great benefit.


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## toddshotrods (Feb 10, 2009)

TX_Dj said:


> Wow. I would have figured underdrive, not overdrive. Now that you mention it, I see the GV logo on the lower end of the housing.
> 
> A single-ratio underdrive with direct for speed driving seems it would be more effective for streetability, but for the strip being able to S/P shift and then kick in the OD once amps start to drop would probably be a great benefit.


Actually you can get it as either an over or under drive unit. Check them out. I like them because they're proven and can be set up from the factory to handle over 1200hp. I can still go that route in the future if I find I need it. My rear motor mount can be re-machined to bolt one right on.


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## TX_Dj (Jul 25, 2008)

Oh yeah, very familiar with GV's under/overs. Just haven't played with one in person, so didn't recognize it until you pointed it out.

They're a great bit of kit, I'd love to add one of their overs to my big truck.


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## Bowser330 (Jun 15, 2008)

toddshotrods said:


> So the answer to your last question is, if you have enough money, you can build a direct-drive, RWD, EV that does pretty much everything you need. You'll probably spend that $20K on batteries alone though.


+1

Whitezombie's pack of kokam's is about 25kwh...

A comparable pack of headway highpower 8AH cells 
(3.2V 8AH 20C 300g 20$each)
1,050 cells 105sx10p = 336V & 80A = 21K$, 700lbs, 26.8kwh, peak battery amps = 1600A


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## toddshotrods (Feb 10, 2009)

Bowser330 said:


> +1
> ...A comparable pack of headway highpower 8AH cells...700lbs... 1600A


Oh well, I was curious about Headway, but hadn't gotten around to doing the numbers on them yet. That definitely wouldn't work for me. A123 rules!


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