# Dual motor Differential?



## Batterypoweredtoad (Feb 5, 2008)

I stole this very basic picture from the web: 









I was wondering if anyone had something similar to the differential in this pic designed and for sale that used belts and was nicely encased in something like an aluminum housing? I would like it to be able to use the typical mounting patterns of the WarP motors and have the stub axles terminate in flanges to be able to bolt to custom axles. I saw one a few years ago that held two Curtis/HPEV setups and I know there is a member here with a one off using two DC motors, but I am pretty sure everything was custom.

Thx


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## MalcolmB (Jun 10, 2008)

Something like this? http://www.rimac-automobili.com/upload/tbl_press/rimacautomobili_frankfurt_2_125912.jpg

or this: http://www.greenmotorsport.com/green_motorsport/products_and_services/3,1,388,17,27462.html

I've been looking for a while, but not found anyone who supplies an off-the-shelf unit for fitting your own motors. I don't think it would be all that difficult to get one fabricated. It's just a few bearings mounted in an oil-filled casing. Drive could be transmitted using silent chains, like in a transfer box. I'd like to have a go at building one myself, but don't have the machining resources yet.


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## Tesseract (Sep 27, 2008)

A guy here that goes by "Brute Force" is using twin motors driven by twin Soliton1's with a single throttle signal to both. Assuming that both controllers have good matching of their current sensors and throttle calibration values, the torque delivered by both motors is fairly closely matched (though we obviously do not guarantee such matching between any two or more controllers!).

I would think this would be kind of twitchy, especially at high speeds as even a slight mismatch in wheel torque would make the vehicle want to turn, but IIRC, Brute Force seems to think it works well enough...


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## Batterypoweredtoad (Feb 5, 2008)

Thank you two, that's exactly the info I was looking for. If anyone else does find one that is available to buy, please let me know. If I do it I would like to experiment with running the motors in parallel off of one controller to get a limited slip differential effect. The main problem I see with this idea is getting enough amps to each to have sufficient torque. I guess I should start saving my pennies for a Shiva.


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## rwaudio (May 22, 2008)

Tesseract said:


> A guy here that goes by "Brute Force" is using twin motors driven by twin Soliton1's with a single throttle signal to both. Assuming that both controllers have good matching of their current sensors and throttle calibration values, the torque delivered by both motors is fairly closely matched (though we obviously do not guarantee such matching between any two or more controllers!).
> 
> I would think this would be kind of twitchy, especially at high speeds as even a slight mismatch in wheel torque would make the vehicle want to turn, but IIRC, Brute Force seems to think it works well enough...


Would it be better to implement this using a single controller with the motors in series so that the current is the same but voltage(rpm) could vary slightly between the two motors to allow turning???


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## Tesseract (Sep 27, 2008)

rwaudio said:


> Would it be better to implement this using a single controller with the motors in series so that the current is the same but voltage(rpm) could vary slightly between the two motors to allow turning???


I don't personally have any experience with either way, but I am more wary of wiring the motors in series when driving separate half-shafts because if a wheel lifts off the ground or otherwise loses traction then its RPM will shoot up, and rather quickly if motor current is high at the time.

I just design controllers and stuff; it's up to you guys to actually build things with them!


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## MalcolmB (Jun 10, 2008)

Tesseract said:


> if a wheel lifts off the ground or otherwise loses traction then its RPM will shoot up, and rather quickly if motor current is high at the time.


So how about adding a second motor speed sensor input to the Soliton?


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## Brute Force (Aug 28, 2010)

Nothing twitchy about it. I've had it up to a hair over 100 mph and it's solid under load. Road crown and tire ruts cause more pull than motor torque imbalance. Power boats and aircraft use multiple engines that are not synchronized and no one gives it a second thought. Unless there is a problem and the motors are significantly out of balance (I'd say more than 10%), you'll never even notice.

I used Corvette wheel bearings and stubs for the axle pulley supports. Stock industrial motor pulleys and belts. Had to get custom axle pulleys made. Of course the housing is custom as well.

Don't use a single controller to run two mechanically separate motors, you're inviting disaster. Ask Yabert what happens to a motor when it over revs.


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## Batterypoweredtoad (Feb 5, 2008)

Brute Force said:


> I used Corvette wheel bearings and stubs for the axle pulley supports. Stock industrial motor pulleys and belts. Had to get custom axle pulleys made. Of course the housing is custom as well.


Do you have any detailed pictures of that setup? Anything you would do differently now that you have had it for a while?



Brute Force said:


> Don't use a single controller to run two mechanically separate motors, you're inviting disaster. Ask Yabert what happens to a motor when it over revs.



Wouldn't a pair of motors run off of one controller and wired in parallel attempt to self balance their RPM's?


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## Brute Force (Aug 28, 2010)

It fits really nice in my mid engine kit car. The down side is it was expensive to build and the belts are not exactly silent.

The problem with running two motors off one controller comes when one of the motors looses load. The controllers available only have one tach input. If a belt breaks or one wheel is on ice/water/mud/lifts off the pavement under heavy throttle, the motor RPM will INSTANTLY shoot past red line and blow chunks.


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## EVfun (Mar 14, 2010)

Brute Force said:


> The problem with running two motors off one controller comes when one of the motors looses load. The controllers available only have one tach input. If a belt breaks or one wheel is on ice/water/mud/lifts off the pavement under heavy throttle, the motor RPM will INSTANTLY shoot past red line and blow chunks.


I know of one controller that has 2 tach inputs and can dish out over half a megawatt.


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## MalcolmB (Jun 10, 2008)

Brute Force
That's a really pretty setup! I'm curious about the drive shaft arrangement as I've been trying to work out how to do this using off-the-shelf parts. Looks like you're using standard UJs on the inboard ends, so how do you take up axial movement of the drive shafts? Do you have plunging CV joints on the outboard ends, or do the shafts just slide in the splines?

Malcolm


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## Brute Force (Aug 28, 2010)

The suspension is from a C4 Corvette (1984 to 1996). The half shafts are stressed members of the IRS. They are function as the upper control arms of a 5 link system.


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## Batterypoweredtoad (Feb 5, 2008)

Brute Force: I noticed you put quite a large space between the motors and the halfshafts, was that due to your cars frame design needs or some sort of demand imposed by the use of belts and pulleys?


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## Tesseract (Sep 27, 2008)

EVfun said:


> I know of one controller that has 2 tach inputs and can dish out over half a megawatt.


Are my eyes deceiving me, or is this *spam* for Zilla?! 

That said (and it needed to be said, since this is your second offense in as many days) the only good way to use twin motors as a differential is to drive each motor with a separate controller (ie - like Brute Force did). 

If you wire both motors in parallel and drive them from one controller then they will fight each other in turns and/or when the tire diameters don't match and/or the two motors don't deliver exactly the same torque/amp and rpm/volt.

If you wire both motors in series then as soon as one motor loses traction it will "hog" all of the voltage, spinning itself to destruction while the other motor just sits there, stalled out. 

So the best arrangement is to either use independent controllers for the motors, or else couple the motors together and use the mechanical differential to divvy up the torque. Unless a jackshaft is used (ie - both motors are coupled to the output shaft via a belt or chain), it is otherwise always possible to protect both motors from overspeed with a single tach pickup.

Thus, there is little need for two tach pickups on the motor controller. It sounds like a good idea at first, but if you actually think all the permutations through you find that it's really kind of pointless.


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## steven4601 (Nov 11, 2010)

Just as for an idea to prevent high velocity copper and iron escaping behind Brute force's ears.
An separate module / electronic governor could be used to limit either motor RPM by sending a signal to the motor controller that a motor past its maximum RPM. 

Has this idea been posted before?


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## Batterypoweredtoad (Feb 5, 2008)

Tesseract said:


> If you wire both motors in parallel and drive them from one controller then they will fight each other in turns and/or when the tire diameters don't match and/or the two motors don't deliver exactly the same torque/amp and rpm/volt.


My question for those who are much smarter than me in the ways of Motors and Controllers: How much will the motors actually fight each other? Is it simply a function of how much variation in speed there is between the two motors? Ex: a 3% difference causes the slower motor to try to produce 3% more torque to equalize? Is it a multiple of the differences in wheel speed and torque being demanded by throttle position and terrain? Has anyone actually done it with a powerful enough controller to supply enough power to both motors to make it a reasonable test? I have only experienced the effect in my kids Powerwheels and I am assuming those dynamics don't factor up into full size vehicles very accurately.  

I do like the idea of a "Safety" type of tach signal for this situation where if either motor overspeeds you get a momentary throttle kill to try to save the motor. Figuring out what the controller should do next is the tough part.


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## Tesseract (Sep 27, 2008)

steven4601 said:


> ...An separate module / electronic governor could be used to limit either motor RPM by sending a signal to the motor controller that a motor past its maximum RPM. ...


Brute Force is using a separate controller for each motor, so they can already be independently protected from overspeed. I don't see an advantage in having an overspeed in one motor also limit power to the other motor. Indeed, I see lots of disadvantages in doing that.




Batterypoweredtoad said:


> My question for those who are much smarter than me in the ways of Motors and Controllers: How much will the motors actually fight each other?


I suspect that is only answerable with real world testing.



Batterypoweredtoad said:


> I do like the idea of a "Safety" type of tach signal for this situation where if either motor overspeeds you get a momentary throttle kill to try to save the motor. Figuring out what the controller should do next is the tough part.


Sure - you could use, e.g., two LM2917 to convert the tach signals from each motor to voltages that could be compared to a reference so that when either motor overspeeds it's comparator trips and since most comparators have an open-collector output, they could both be tied together to clamp the throttle signal to the controller to ground.

Me redesigning the Soliton controllers to have two tach inputs to accommodate this would cost substantially more money


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## Brute Force (Aug 28, 2010)

The motors won't fight each other. one will produce more thrust at the pavement. This will cause some amount of torque steer, requiring the driver to make a correction on the steering wheel. Front wheel drive will be more pronounced than rear wheel drive for the same amount of imbalance. I maintain that unless there is a serious problem with the system, it will not be more noticeable than what is caused by normal road conditions.

Right now my EV is disassembled for body work, but this spring/summer when it's back to functional, I will do some testing to lay this issue to rest. I will purposely set the right and left controllers to different current scales to determine where it becomes perceptible. There is already two data points, both motors set the same, which works perfectly both in a straight line and at the autocross. And full throttle with one dead motor, which is very obvious and requires a lot of steering input to drive in a straight line.

It may look like a lot of distance between the axle shafts and the motors, but it's actually pretty tight. Here's one of the issues with building a belt drive: It's tough to get a use full reduction ratio. There is a minimum motor pulley diameter, go smaller and the belt will flex too much as it wraps around the pulley and shorten the life span. Smaller drive pulleys also increase the forces in the belt and may exceed it's tensile strength, also shortening the life span. So you have to use a large driven pulley to get the ratio you want. The best I could squeeze in was 4.10 to 1, the axle pulley is 14.4" in diameter. Add in the motor mount, available belt lengths, belt tensioning, and you'll probably end up with a similar solution.

Each Soliton on my drive has a rev limit implemented, so I'm not worried about centripetally induced ejecta. Molten copper is another matter...


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## MalcolmB (Jun 10, 2008)

steven4601 said:


> An separate module / electronic governor could be used to limit either motor RPM by sending a signal to the motor controller that a motor past its maximum RPM.


I've built a couple of rpm switches for my motors based on this rpm to voltage converter: http://www.ti.com/lit/ds/symlink/lm2907-n.pdf I need to fix an interference problem, but will post details when I've done that. I plan to use the switches to couple a resistor in parallel with the throttle to reduce motor rpm to a safe level.

I'm fascinated by the idea of independent motor drive, so even though Tesseract and others whose advice I respect recommend against it I keep coming back to the idea like a dog with a bone. I have my dual motor front wheel drive setup running on the bench and have played around with it at 12V in both series and parallel.

With the motors in series, if you grab one motor shaft to stall it, the other speeds up and "hogs" all the voltage, just like Tess says. That's why I asked about having two tach inputs and why I've built my own rpm switch. I'm also planning to try out series/parallel switching. The way I see it (foggily perhaps), series mode should be ideal for parking, as that's when you need the maximum speed difference between wheels.

With the motors running in parallel on the bench, when I try to slow one motor down it draws more current and tries to resist me slowing it down. The important bit is that it does slow down, so there is some "give" in the system. Obviously there is a big difference between bench testing at 12V and real life at full pack voltage, but hopefully there will still be enough "give" to avoid problems. As Brute Force pointed out, you compensate for slight differences with steering adjustments. The actual difference in motor speeds in parallel mode should be very small, as this will only be used at higher speeds when travelling in a relatively straight line.

I have thought about using individual controllers, which would be easier in some ways, but aren't you still likely to have similar problems due mismatch in throttle response between the controllers?


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## EVfun (Mar 14, 2010)

Tesseract said:


> Are my eyes deceiving me, or is this *spam* for Zilla?!


Just pointing out to Brute Force that there is an available motor controller with 2 tach inputs. He wasn't aware that such a thing existed. It's not like I get a kickback for each Zilla sold.


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## Brute Force (Aug 28, 2010)

I stand corrected on the dual tachometer inputs. Does anyone have experience with how the Zilla handles an over rev condition with two independent motors? I assume it shuts down both since it cannot control them separately.

The thing to remember about the dc motor controllers used in EVs is that they are not servo drives. The motor speed is not controlled. If you have two (or more) independent motors run by separate controllers, the only thing forcing them to run together is the pavement. They will run at whatever speed they have to to reach equilibrium between their torque and the load applied.


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## LithiumaniacsEVRacing (Oct 9, 2010)

Batterypoweredtoad said:


> I stole this very basic picture from the web:
> 
> 
> 
> ...


This configuration is okay for everyday driving, as long as both controllers are matched. BUT, I would not recommend this configuration for drag racing, if one motor were to drop-off, break, etc....the car could head into the wall.


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## rochesterricer (Jan 5, 2011)

I may have said this on here before, but this makes me wonder how Commuter Cars does this on the Tango(single Zilla 2k with a 9 inch motor at each rear wheel).


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## LithiumaniacsEVRacing (Oct 9, 2010)

EVfun said:


> Just pointing out to Brute Force that there is an available motor controller with 2 tach inputs. He wasn't aware that such a thing existed. It's not like I get a kickback for each Zilla sold.


Not to crap on your promo campaign, but Zilla controllers are a pain in the tail. Too many routes to cause error codes, plus they are built with the minimum parts to do the job. If you want a Mercedes buy from Evnetics, if you want a Hyundai buy a Zilla.


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## LithiumaniacsEVRacing (Oct 9, 2010)

LithiumaniacsEVRacing said:


> This configuration is okay for everyday driving, as long as both controllers are matched. BUT, I would not recommend this configuration for drag racing, if one motor were to drop-off, break, etc....the car could head into the wall.


Another option that would work with this type of system would be to have the axles connect by a spool, this would apply the torque evenly.


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## efan (Aug 27, 2009)

rochesterricer said:


> I may have said this on here before, but this makes me wonder how Commuter Cars does this on the Tango(single Zilla 2k with a 9 inch motor at each rear wheel).


I do have any details, but here is a picture of the actual setup of the motors:
http://www.gizmag.com/go/4048/picture/11089/


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## LithiumaniacsEVRacing (Oct 9, 2010)

More info:

http://www.diyelectriccar.com/forums/showthread.php/tango-drive-arrangementi-17967.html


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## Tesseract (Sep 27, 2008)

LithiumaniacsEVRacing said:


> Not to crap on your promo campaign, but Zilla controllers are a pain in the tail. Too many routes to cause error codes, plus they are built with the minimum parts to do the job. If you want a Mercedes buy from Evnetics, if you want a Hyundai buy a Zilla.


*groan*

Can't we all just get along? EVfun may be an unrepentant Zilla fan club member but he's alright in my book...


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## LithiumaniacsEVRacing (Oct 9, 2010)

Tesseract said:


> *groan*
> 
> Can't we all just get along? EVfun may be an unrepentant Zilla fan club member but he's alright in my book...


Ummm no. It's a new year and a new season, envy will be smoking hot! Relax Jeff, remember I am the bad boy of EV Racing! LMFAO!


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## Brute Force (Aug 28, 2010)

Spools may be great at the drag strip, but not in a street driven vehicle.

I experienced just such a motor failure at the 2010 SORC half mile shootout. Although it did have a tendency to pull to the left, it went down course just fine.


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## rochesterricer (Jan 5, 2011)

efan said:


> I do have any details, but here is a picture of the actual setup of the motors:
> http://www.gizmag.com/go/4048/picture/11089/





LithiumaniacsEVRacing said:


> More info:
> 
> http://www.diyelectriccar.com/forums/showthread.php/tango-drive-arrangementi-17967.html


Thank you. I've always wondered how the drivetrain in that car actually looked. Very interesting.


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## EVfun (Mar 14, 2010)

Tesseract said:


> *groan*
> 
> Can't we all just get along? EVfun may be an unrepentant Zilla fan club member but he's alright in my book...


The east coast/west coast rivalry is Ron's motivator Tesseract. What good is an R2 unit with bad motivator?

I'm still eyeing those Soliton controllers. No external main contactor and no liquid cooling (in a 1200 lb. EV) would make for an even cleaner installation. The thing is, I got 2 perfectly good Zilla controllers and no Soliton.


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## Roy Von Rogers (Mar 21, 2009)

EVfun said:


> The east coast/west coast rivalry is Ron's motivator Tesseract. What good is an R2 unit with bad motivator?
> 
> I'm still eyeing those Soliton controllers. No external main contactor and no liquid cooling (in a 1200 lb. EV) would make for an even cleaner installation. The thing is, I got 2 perfectly good Zilla controllers and no Soliton.


Same here, I'm wanting to put 48 CALB cells in my 2008 Zap PK with a Soliton Jr, but the dang thing was setup for 72/84v, and its going to be PITA to make it work on a 168v. Looks like I'm going to go with 26 160ah cells and put up with the Curtis whine.

Roy


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## EVfun (Mar 14, 2010)

If you could drop back a couple cells you could chose relatively inexpensive Altrax AXE-7245 for less than $600 This would put out 450 amps with up to a 72 volt system and should be silent. When I went looking for information I found the controller was available from Evolve Electrics. I know they are popular with the hot-rod golf cart crowd too.


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## epyon (Mar 20, 2008)

Brute Force said:


> View attachment 11960
> 
> 
> View attachment 11961
> ...


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## iti_uk (Oct 24, 2011)

Brute Force said:


> The best I could squeeze in was 4.10 to 1, the axle pulley is 14.4" in diameter.


Brute Force, did you consider using epicyclical gearboxes instead of belt drives? You could improve on that 4.10:1 ratio without going over-size.

I wonder if the effort and expense of designing and manufacturing the housing and pulleys as you did would be that much less than getting some custom gears cut. Of course, this would lose the fail-safe "sacrificial" belts, protecting the motor and driveshafts from lockup failures, but that's probably not too much of an issue...

Chris


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## Brute Force (Aug 28, 2010)

Truthfully, it didn't occur to me to look into a custom gear set. The pulleys didn't require much design work, there are shops that cut cogged pulleys for superchargers, just tell them how many teeth and what mounting pattern (basically). Other than the housing pieces, everything else is commercial off the shelf available.

Like the OP, the dual belt drive seemed to me to be a simple, straightforward, cheap drive solution. So I penciled up some ideas that looked feasible and started buying parts. By the time I had a viable solution CADed up, it was not simple, straightforward, or cheap. But it's finished, works great, and is a real head turner.


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## iti_uk (Oct 24, 2011)

Fair enough. Off the shelf is brilliant for our builds - I'm trying to find an off-the-shelf planetary 4:1 or 5:1 gearset that will handle up to around 200 Nm (maybe more for a safety factor) output torque (50Nm in on a 4:1 ratio). I am able to get custom gears made, but I'd rather buy things off the shelf for easy replacement and cheaper cost.

Your belt method looks tidy, very nice!

Chris


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## Salty9 (Jul 13, 2009)

I'm only suggesting this from ignorance (unswas) but has a Torsen http://en.wikipedia.org/wiki/Torsen been considered for this application? Wikepedia says it is used in the Humvee. Might be able to handle the torque.


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## Arlo (Dec 27, 2009)

epyon said:


> Brute Force said:
> 
> 
> > View attachment 11960
> ...


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## iti_uk (Oct 24, 2011)

Salty9 said:


> I'm only suggesting this from ignorance (unswas) but has a Torsen http://en.wikipedia.org/wiki/Torsen been considered for this application? Wikepedia says it is used in the Humvee. Might be able to handle the torque.


It could, but that is just a limited-slip variety of differential and requires the same driveline components as any other differential. It would split the output of one input shafts to two wheels.

Limited slip differentials are only useful when you are breaking traction - in a commuter car this is not important, but at the track or off-road, this would be useful.

Chris


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## Brute Force (Aug 28, 2010)

It's well past that planning stage on the power levels. The chassis is fully functional and I've driven it in a few events. Getting bodywork attached is the order of the day. The Solitons are set to limit power at 75kW for each motor (150kW total).

I believe a well designed gear drive would be quieter and have a greater service life. But that's beyond almost all DIYers reach, at least the belt drive has the ring of possibility. I went with machined billets, no reason it couldn't be done with a weldment.


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## meanderingthemaze (Jan 25, 2010)

Are there any issues that come up when putting the motors along a common axis like in attached picture? Does it matter if the motors are the same power/size? Will they fight each other or will they play nicely in this configuration?


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