# direct drive or gearbox/transmission



## onegreenev (May 18, 2012)

I would retain the clutch and would not just drive in 4th. With a good setup you might be able to forgo 1st gear and for the most part maybe even 2nd. You can go clutchless but you won't be able to shift fast if you want and its hard on the transmission. Just because you can does not mean you should. My last VW Bug pretty much did great in 3rd and 4th and never in 1st. Most builds are low voltage meaning 144 volts or less and usually retain the stock transmission. With the motor rpms being what they are with DC motors it is a good idea to keep all your gears available and keep your clutch so you can better keep the motor within the proper range of rpm for the best possible outcome. Do not LUG your electric motor but don't go too fast either. Find the sweet spot and try to keep it there by using the gearing in your transmission. If you plan on AC then depending upon what setup you have you might even get away with 3rd as your driving gear as the AC motor can run faster. I'd stay away from 4th unless your just cruising on the freeway. Most of your driving in town will be best used in 2nd and 3rd. Use what your car has to your advantage.


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## onegreenev (May 18, 2012)

If I did it different I'd raise the voltage of my system. I'd keep the clutch and use the gears available and maybe even change the gears to better utilize the power and torque band of the DC motor. Thats using DC of course. I am intrigued by the new monster BLDC motor and Scott drive that may soon be available. That of course may change the game.


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## cts_casemod (Aug 23, 2012)

onegreenev said:


> If I did it different I'd raise the voltage of my system. I'd keep the clutch and use the gears available and maybe even change the gears to better utilize the power and torque band of the DC motor. Thats using DC of course. I am intrigued by the new monster BLDC motor and Scott drive that may soon be available. That of course may change the game.


I forgot to tell, Im using AC with a DC bus of 650v
For now I will have max power at 1500 (100nm) and peak power from standstill of 200nm. Later On plan to upgrade and have max power up to 4350rpm. 
I ask 4th because I am looking to see if I can go Direct Drive (that's what 4th is)

By the way my motor looks almost the same as that one on the picture but it's induction. 
Regards


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## Duncan (Dec 8, 2008)

cts_casemod said:


> I forgot to tell, Im using AC with a DC bus of 650v
> For now I will have max power at 1500 (100nm) and peak power from standstill of 200nm. Later On plan to upgrade and have max power up to 4350rpm.
> I ask 4th because I am looking to see if I can go Direct Drive (that's what 4th is)
> 
> ...


My question would be
Can you spin the tires in top gear from standstill??

- will 200Nm (which is torque and NOT power) spin the tires

200Nm x final drive (4:1??) = 800Nm
Tire radius = (300mm??)
Force at contact point = 800/0.3 = 2666N

Weight on front wheels = 400Kg 
Coefficient of friction of tires - 80% for sporty road tires

80% of 400Kg x G = 3139Newtons

so in your case (repeat with your numbers) you could not spin the tires in top

If you could then you would not need to use the lower gears
As it is - you would need to increase the torque or use lower gears to be a hooligan


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## tylerwatts (Feb 9, 2012)

I don't fully understand Duncan's logic but I think with your motor acceleration will be slow with direct drive. Not poor as you'll have max from 0rpm, but not like the ice. I understand your constraint is space, so maybe losing the clutch and getting the motor as far into the bell housing as possible is the best compromise. 

Can you not run your ac motor to higher rpm? This would let you select a lower ratio, say second, and effectively double your rpm with still maintaining a reasonable top speed, then if driving for a long period at higher speeds, select say 4th gear to bring the motor back into rated torque rpm range.


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## Duncan (Dec 8, 2008)

Hi tyler
I was going to use a gearbox - had even gone and bought one!
Then I calculated that I would be able to spin the tires in top gear
So I didn't need a gearbox
Except for reversing
Without a gearbox the motor went where the gearbox would have - so i could put batteries where the motor would have gone

Understand the logic?


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## PStechPaul (May 1, 2012)

I don't think the ability to spin tires is the main criteria for the suitability of direct drive or driving in 4th gear. A vehicle may occasionally need to negotiate a rather steep incline such as a driveway or speed bump, or be able to climb out of a pothole or deep rut. Such situations require a great deal of torque at essentially zero speed. Depending on weight distribution, and whether or not both drive wheels are involved, it could be the equivalent of a 100% grade (45 degree angle) where the thrust may be as much as 1/2 the vehicle weight. So for a typical 3000 lb car with 24" diameter tires, the torque required is 1500 lbft. A 40 HP 1800 RPM motor has about 117 lbft torque, and maybe twice that at peak. With a 4:1 gear reduction you might get 936 lbft. So you need at least 60 HP, or an additional gear reduction. And BTW, to reach 60 MPH with that gear ratio the motor would need to be overclocked by 2x to 3600 RPM.


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## mizlplix (May 1, 2011)

As a purely personal observation, I would not even consider direct drive in any vehicle over 2,000LBs.

A large enough motor would require a large, expensive pack to run it.

Miz


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## Duncan (Dec 8, 2008)

PStechPaul said:


> I don't think the ability to spin tires is the main criteria for the suitability of direct drive or driving in 4th gear. A vehicle may occasionally need to negotiate a rather steep incline such as a driveway or speed bump, or be able to climb out of a pothole or deep rut. .


There is a fallacy here
you can not put any more torque through your tires than that which will spin the tires

Lower gears don't help when the tires spin

If you can spin the tires in top you don't need lower gears


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## PStechPaul (May 1, 2012)

Duncan said:


> There is a fallacy here
> you can not put any more torque through your tires than that which will spin the tires
> 
> Lower gears don't help when the tires spin
> ...


But once you break traction the force and torque is reduced, so all you have to do is get a sudden surge to make the tire slip. You can get enough torque to do that if you rev the motor and pop the clutch, using flywheel action for additional instantaneous torque. For direct drive there may be enough free play in the U-joints and differential gears to have a similar effect.

Also, with an AC motor, the torque may not be as great under locked rotor conditions or low RPM, and it may not be as well controlled. It will also probably be very inefficient and overheat. A series wound DC motor may have different characteristics under such conditions and may be more suitable for direct drive.


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## cts_casemod (Aug 23, 2012)

PStechPaul said:


> But once you break traction the force and torque is reduced, so all you have to do is get a sudden surge to make the tire slip. You can get enough torque to do that if you rev the motor and pop the clutch, using flywheel action for additional instantaneous torque. For direct drive there may be enough free play in the U-joints and differential gears to have a similar effect.
> 
> Also, with an AC motor, the torque may not be as great under locked rotor conditions or low RPM, and it may not be as well controlled. It will also probably be very inefficient and overheat. A series wound DC motor may have different characteristics under such conditions and may be more suitable for direct drive.


By the contrary if you have a properly setup AC Motor with encoder you can actually make a very healthy torque with nothing more that the nominal power. I've seen 600%. 
This torque reduces once the motor gains speed, as the VFD keeps the iron saturated. So torque x speed = HP. = Very high Efficiency. You just need to make sure the cooling is sufficient as the internal FAN is operating too slow...

With DC Motors you simply pump more current. While you can make a crazy amount of torque (More that the AC) the brushes will melt and the efficiency is low.


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## mizlplix (May 1, 2011)

Some of the members cars will actually drive in high gear alone, but The battery current draw would be excessive and not conducive to good mileage.

My main two criteria s for a direct drive decision would be :

1- Does it perform well?

2- Is the current draw acceptable?

Miz


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## cts_casemod (Aug 23, 2012)

mizlplix said:


> Some of the members cars will actually drive in high gear alone, but The battery current draw would be excessive and not conducive to good mileage.
> 
> My main two criteria s for a direct drive decision would be :
> 
> ...



Well... Thats what I started asking in the first post!! What are people experiences, not theory.
Cant believe with so many Ev'ers no one can share their experiences... Or is everybody just looking and trying to convert?

I Know at least one guy that converted his smart and says it works, he actually was a long time without the shifter on the car (No reverse either!!!).

His Smart weighted 750KG which is not much different from my Polo.


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## PaulS (Sep 11, 2012)

The number one rule for building in efficiency, performance and economy is:
Get rid of as much weight as you can.
Will your car perform without the weight of a transmission or flywheel?
How fast do you have to turn your motor to get to the speed that you want to have.
If you don't need a differential then get rid of it - they are heavy! Use a transverse mounted motor and jack shafts turning your wheels. (or better yet use hub motors)
The weight of a gas engine car is rarely below 2000 pounds unless it is built up and even then you are looking at close to 16-1800 pounds. An electric car should weigh in at around 1000 to 1200 pounds with all the bells and whistles. That is how you get efficiency, mileage and performance.


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## tylerwatts (Feb 9, 2012)

Paul and Miz are roughly there, and it boils down to efficiency. If you're rarely below 30-40mph then you mioght be able to live with the inefficiency at these speeds, but if your speed range varies alot, particularly like commuting with ICEs with clutches, then you need to consider a lower ratio to allow maintaining some efficiency at lower speeds. Maybe you wouldn't need a clutch, my view is you should only need 2 gears really, a low speed ration, and a high.

Miz mentions this often and I agree. CTS's Polo is a light car and should remain so after the conversion so any more than 2 gears should be a waste, and his AC motor will have a nice wide torque range to use both ratios well.

CTS, consider getting a stronger trans and stripping out all but the 2 gears you need, a low and a high, and losing the clutch. You could even mod the input to allow your motor to be mounhted as far inboard to the bell-housing and give room for a bigger/stronger trans also. Try use the donor drive shafts also. I'd recommend a diesel, mine has nearly ideal ratios since the diesel has a wide torque band, and limited top rpm of ~5000rpm also.

My 2c


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## PaulS (Sep 11, 2012)

Tyler,
having built autos for lots of folks for a bit over 40 years, mostly for performance street applications but also for serious drag racers, I can say without hesitation that it would be a monumental task to "remove" gears from any auto transmission. The powerglide is the all time champ for light-weight and strong transmissions. In second place is the Ford C4 with three gears. The powerglide is better supported in the aftermarket with super strong cases to shafts and everything you need to make a transmission stand up to any amount of power you can apply. The C4 is limited to about 1000 hp but stands up well to that level of competition.
If you decide you need an auto then I would say (off the record) "Get a powerglide". There is none lighter (the C4 weighs about 25 lbs more) and you can make them shift any way you want.
I don't believe that any transmission is necessary or recommended for an electric car but I will concede that most gas conversions to electric may be better off with one because of the weight of the vehicles. Electric Forklifts weighing 5-10000 lbs don't use transmission but they do have a final drive of 27:1 and a top speed of 17 mph. (along with 5-600 AH 36-48v batteries) They run for 6-8 hrs between charges though. and that is all with old technology SCR and transistor controllers. The newer systems are using AC and independently excited field motors that are a bit more efficient.


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## tylerwatts (Feb 9, 2012)

Thanks PaulS

I was only saying remove unwanted gears from a manual trans to reduce drag.

Re autos then, do you know of any front longitudinal 4/AWD trans that handle good torque? My first thought was a Subaru STi box, but that's manual. Don't know what Audi's S models use but they might be decent, bigger auto models have V8s normally with at least a couple of turbos too.

Reason for longitudinal trans is it opens up space in front of the box for pretty large dia motors, whereas my transvers 4wd box is very limited due to the proximity of the transfer box, ~9" is the max and only DC motors are generally that small.

Thanks

PS, like to keep 4wd since the UK cannot handle snow in winter and rwd is hopeless, my poor miata was a mess even with brilliant winter tyres, the roads are just not designed for it, and it gets worse each year!


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## Duncan (Dec 8, 2008)

tylerwatts said:


> like to keep 4wd since the UK cannot handle snow in winter and rwd is hopeless, my poor miata was a mess even with brilliant winter tyres, the roads are just not designed for it, and it gets worse each year!


Having lived in Scotland and England for nearly 40 years I would agree that RWD is occasionally a problem

However FWD is very nearly as good as 4WD

The only time I have had problems with FWD is when I had insufficient ground clearance

I also spent 4 years in the USA where I was entertained every winter by the sight of dozens of SUV's in the ditches as I drove past in my (FWD) Sundance

My current vehicle is 4WD (Subaru Legacy) - but because of the small population (1 million on an Island the size of England) we have a lot more unmade (dirt or gravel) roads here and 4WD is useful then


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## tomofreno (Mar 3, 2009)

cts_casemod said:


> ok guys here's the thing, I am building my polo and lots of different issues are coming. I would like to hear your opinions about driving in 4th gear only.
> what's the performance
> what's the motor you have
> what power is your system?
> ...


 In order to use direct drive and have both relatively fast acceleration and relatively high speed, you need a motor/controller combination with high peak torque, peak torque out to relatively high motor rpm, and relatively high power. Power can be limited by the controller or battery pack.

Consider my car with 2260 lb ev weight. It has an HPEVS AC50 with 1238-7501 Curtis controller (130V max, peak current 550A) and nominal 115V pack. Peak torque is 90 ft-lb, greater than the 70 ft-lb of the original ice, so it has fairly good acceleration in first gear (13.45:1 overall), 5.8 mph/sec (measured). With that pack it has peak torque out to about 3900 rpm, or a bit less than 20 mph in first gear. Second gear is 7.16:1 overall, so a bit more than half the wheel torque as first gear for a given motor torque. Measured acceleration rate drops to about 3.4 mph/sec, and peak torque is out to a bit less than 35 mph. Third gear is 4.85:1 overall, so a bit over one third the wheel torque as first gear. Measured acceleration rate drops to 2.5 mph/second, and peak torque is out to about 50 mph. Top speed, if only limited by max 8k motor rpm would be about 80 mph in second gear, but actually it is limited by pack voltage/controller power to about 65 mph. This could be increased if the controller had a higher max input voltage. I’m guessing acceleration in second and third are a bit higher compared to first gear than the gear ratios would suggest due to starting from a stop with un-rev’ed motor in first gear. 

From this you can see that I would require a motor with much higher peak torque (about 3x) to have similar initial acceleration starting in third gear. If I did, and it had peak torque out to the same rpm, I could carry that higher initial acceleration out to about 50 mph before torque started falling off with higher rpm, and have high top speed and acceleration using only third gear. But of course if the motor were wound for that higher peak torque, it would require more voltage from the controller to maintain peak current out to the same 3900 rpm, so a higher power controller and higher voltage battery pack would be required. 

It is not enough to have a motor with high max rpm. It must have sufficient torque at higher rpm to deliver the power required to move the car at that higher speed, and accelerate it quickly to that speed. So with both AC and DC motors what matters is how high the peak torque is, and to what rpm it can be maintained.  That will depend on the pack voltage and max input voltage of the controller.

Most series DC motors have a max rpm of around 5k. You can estimate what top speed that would permit knowing your wheel radius and overall gear ratio. You can easily get very high peak torque with such a motor and a controller such as a Zilla 1k or Soliton1. Both of these can be used with higher voltage packs (over 250V), so can deliver plenty of power also. The only question would be if the top speed is sufficient at that max rpm. An AC motor with similar power will be very expensive, which is why most people use DC. Or, you can do as I did and purchase a lower torque and power motor/controller and use a transmission. My car's performance could be considerably better if the controller had higher max voltage. Hopefully this illustrates some of the considerations.


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