Forklift motor direct drive to diff (4k rpm)?

7196 Views 7 Replies 5 Participants Last post by  brian_
Hi all, I'm looking at starting a budget conversion that is capable of 65+ mph. I found a possible donor car (BMW e30), and I can get a diff for it that is a 3.73:1 ratio. The car is an auto, so I can't really use the transmission. If I were to use a 10 or 11in forklift motor is it possible to get that kind of speed?
I did some calculations, and to get 65mph, the motor would have to be spinning at about 4k rpm. Not sure if a forklift motor can stand up to that kind of speed continuously or not, and I haven't found and conclusive info in that respect...
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MattsAwesomeStuff

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Hi all, I'm looking at starting a budget conversion that is capable of 65+ mph. I found a possible donor car (BMW e30), and I can get a diff for it that is a 3.73:1 ratio. The car is an auto, so I can't really use the transmission. If I were to use a 10 or 11in forklift motor is it possible to get that kind of speed?

But more importantly, you're asking the wrong question.

It's not "is this motor able to rotate that fast" that is the challenge, "How much power can a motor provide?" is the challenge. So, technically yes, if you load down the motor down it won't rotate fast enough, but that's a power constraint, not a "is it safe to rotate this fast" constraint.

4000 rpm is perfectly within the rotation range of most electric motors. The bigger the motor actually, the lower the RPM has to be because the outer parts of the motor are under more centrifugal load and more likely to tear off.

What you're looking at is a 1200kg (2640lb) smaller car.

You'll probably need 15-17 or so horsepower to travel at 65mph (105km/h). That means a motor that can handle 12-15kW continuously. Plus some extra for acceleration and hill climbing (up to about double).

The cross-secitonal area of the car tells you roughly how much power it'll require to travel a given speed. The weight of the car tells you how much power it'll require to accelerate or climb hills, and how much energy you'll waste starting and stopping.

A 10 or 11" motor is plenty. A 9" is probably fine, power-wise.

Even the smaller impulse 9", is good for 30hp continuously. Probably double what you need.

brian_

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You'll probably need 15-17 or so horsepower to travel at 65mph (105km/h). That means a motor that can handle 12-15kW continuously. Plus some extra for acceleration and hill climbing (up to about double).
Double 15 to 17 horsepower is 30 to 34 horsepower. Is anyone happy with the acceleration of a 34-horsepower car? This is the power of a 1950's VW Beetle, stuck in one gear... in a heavier car. Now if "some extra... up to about double" means 34 hp continuous and 100 hp for 15 seconds at a time, that's more reasonable.

I've seen a few of those lists of how much car can be handled by what motor. I've yet to see any of them provide performance data to go with it.

onegreenev

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Double 15 to 17 horsepower is 30 to 34 horsepower. Is anyone happy with the acceleration of a 34-horsepower car? This is the power of a 1950's VW Beetle, stuck in one gear... in a heavier car. Now if "some extra... up to about double" means 34 hp continuous and 100 hp for 15 seconds at a time, that's more reasonable.

I've seen a few of those lists of how much car can be handled by what motor. I've yet to see any of them provide performance data to go with it.
9" GE Series Motor attached to a VW Ghia and a Synkromotive controller at 96 volts and 850 amps pushed the little car to 85mph and had spritely acceleration and it was way better than the stock 1500 VW engine. An AC-35 or AC-50 with a 96 volt 650 amp controller in a Bug is even better than my old GE 9" Series motor. HP! no idea. But I like how they perform.

Minimum for any DC Series setup in my experience is 9". So like a nice GE 9" or Warp9 or something like that. Some use 8" DC motors but those are for light weight vehicles. Not heavy ones. A solid 9" should do fine in the BMW. Just feed it enough volts and amps and be sure to advance the brushes if you go above 120 volts.

Pete

I have a nice little 9" Series DC motor that was in an older BMW 2002. Custom Built by Jim Husted. Nice motor.

Duncan

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Hi Electric

I'm using an 11 inch motor - direct drive
"Duncan's Dubius Device"
I've just killed my first motor so I'm sorting out another one \$200

I'm using 1200 amps and 340 volt - gives me close to 500 hp

At the end of the 1/8th mile it's at 85 mph and 4700 rpm

You will need at least a 9 inch motor - here (NZ) 11 inch motors seem to be more common

MattsAwesomeStuff

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Is anyone happy with the acceleration of a 34-horsepower car? This is the power of a 1950's VW Beetle, stuck in one gear
Do you remember when the New Beetle came out, they had all these ads making fun of the old Beetle?

"Introducing new features, like, heat."

"0-60? Yes!"

Etc.

Of course no one was happy with it, but that's not a fair comparison. A 30hp gas engine is not comparable to a 30hp DC motor, especially when comparing off the line.

I was too lazy to run the numbers, but, let's do that.

http://www.enginuitysystems.com/EVCalculator.htm <-- Calculator.

When I punch in 1300 kg (car + driver), I get 12kw to travel 65 miles per hour.

http://www.procato.com/convert/ <-- 0-62mph in 12 seconds is 0.23g, or 2.31 m/s^2.

Punching that into the previous calculator gives... 100kw.

So, you're right, to have a 0-60 that isn't sluggish takes 100kw.

On the lower end you'll still be beating gas engines off the line, it's the upper end where power requirements fly and gas engines are in their power band.

You wouldn't feel sluggish in city driving, but past, oh, 40mph is where you'll start to feel a bit slow.

(Edited to add) - The 30hp I mentioned above would make an 86 second 0-60 time . Again, because of the torque curve of a gas engine it would still seem quick off the line, but would fade fast.

brian_

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9" GE Series Motor attached to a VW Ghia and a Synkromotive controller at 96 volts and 850 amps pushed the little car to 85mph and had spritely acceleration and it was way better than the stock 1500 VW engine.
If it was 850 amps at 96 volts (that is, at the same time), that's 81.6 kW or 109 hp (less inefficiency, so perhaps 90 hp output). That's at the peak power point - any lower speed and the voltage would be lower; any higher and the current would drop off. That's a whole different world from 34 hp, it's going to be much quicker than a stock 1500 VW, and the difference is why I commented that 34 hp continuously and a hundred horsepower briefly would be very different from just a maximum of 34 hp.

I was too lazy to run the numbers, but, let's do that.

http://www.enginuitysystems.com/EVCalculator.htm <-- Calculator.

When I punch in 1300 kg (car + driver), I get 12kw to travel 65 miles per hour.

http://www.procato.com/convert/ <-- 0-62mph in 12 seconds is 0.23g, or 2.31 m/s^2.

Punching that into the previous calculator gives... 100kw.

So, you're right, to have a 0-60 that isn't sluggish takes 100kw.
That's all consistent, confirming real power requirements. Now if only there were real motor power data for these old brushed DC motors.

brian_

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People's standards for performance vary. Pete, have you measured an acceleration time, 0 to 60 mph (or anything else more practical)? I have no doubt that it works well for you, but one driver's "sprightly" is another's "slow like a slug", and over the past few decades expectations for performance have escalated, so that a vehicle which was good in the 1960's and considered perfectly functional in the 1980's couldn't be given away as a new car today. The weakest gasoline E30 ever built put out over 100 hp, and not just for a few seconds before it heats up and the power needs to be backed off; even that was over three decades ago and BMW wouldn't sell anything like that now.

My first car had a 68 horsepower engine, and I enjoyed driving it, even in competition... in the 1980's. The equivalent model from the same manufacturer today has a 106 hp engine and yet almost no one would be happy with it. My current car (which is probably 50% heavier) has 160 hp, and is much quicker... quicker than I need, but now considered slow by many enthusiasts' standards.

To compound the problem of comparisons, most drivers with manual transmissions don't effectively use their engines, so they have little idea of the car's actual performance. They're driving with the engine in the bottom half of its operating range, so they never find out what performance is actually available. Shifting through the gears, an engine can be kept high in it's power range; when used with only one ratio, a typical brushed DC motor is far from its peak power output most of the time. When shifted for performance, my ordinary 6-speed car spends only a second or two below the engine speed for torque peak, so high electric motor torque from standstill is not an advantage.

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