Direct drive?

3515 Views 15 Replies 6 Participants Last post by Rich02si, Feb 23, 2011

Rich02si

Discussion Starter · Feb 22, 2011 (Edited)

I am new to EV's. I'd like to get some insight. I'm a racer. Started in motocross, then autocross and road racing. I'd like to build an RWD autocrosser. So obviously first choice is miata. But open to suggestions.

Now to my question...

My goals are light weight-ish (for an ev at least). So my packaging is my main focus. Can I forgo the trans/driveshaft and directly connect the motor to the diff by rotating the diff and having the motor vertical in the trunk or conv top space?

I ask this to delete the weight of the trans and drive shaft and leave more room up front for a lower flatter battery pack.

Any thoughts. I'd need a motor to spin both direction and relatively short but with good torque. And higher rpm power since I won't have a trans to rely on for higher speeds.

1 - 16 of 16 Posts

major

· #2 · Feb 22, 2011

Rich02si said:
I am new to EV's. I'd like to get some insight. I'm a racer. Started in motocross, then autocross and road racing. I'd like to build an RWD autocrosser. So obviously first choice is miata. But open to suggestions.

Now to my question...

My goals are light weight-ish (for an ev at least). So my packaging is my main focus. Can I forgo the trans/driveshaft and directly connect the motor to the diff by rotating the diff and having the motor vertical in the trunk or conv top space?

I ask this to delete the weight of the trans and drive shaft and leave more room up front for a lower flatter battery pack.

Any thoughts. I'd need a motor to spin both direction and relatively short but with good torque. And higher rpm power since I won't have a trans to rely on for higher speeds.

Hi Rich,

I think you will find it difficult to find a reasonable size motor which will work with available rear end ratios. Meaning you will need a high RPM motor and a gear reducer between the motor and diff even if it is direct drive, meaning a single ratio non-shifting set-up. You might look at quick change rear ends which have a spur gear set in front of the ring and pinion. This would also allow you the option of choosing ratio to fit the track. But you're not going to mount that vertically.

Regards,

major

Woodsmith

· #3 · Feb 22, 2011

Hi and welcome,

You can do what you suggest but if you turn a diff nose up you will get a significant loss of lube to the pinion bearing leading to quick failure. Sometimes even turning a hypoid diff upside down to raise the pinion height is enough to have that effect.

Could you house the motor in the tunnel? Depending on motor size you could make a larger section in the tunnel to fit it or place it where the trans was and then run a shaft to the diff.

Reversing a motor is just a matter of some big contactors to reverse the field winding relative to the armature winding on a series motor.

As for speed you can work out with your tyre diameter and diff ratio what speed the motor would need to run to get the speed you need. Usually 4th gear in the trans is a direct drive so you can see what the tacho says.
The tacho is usually matched to the speedo in 4th, from my experience, so the needle positions match. Look at the speedo for your maximum required speed and then see what rpm the tach would say if the needle pointed to the same place on the dial.

Larger motor diameters usually equate to higher maximum torque but lower speed and for smaller motors it is the other way around. However much of it also depends on your controller choice and pack voltage and current capacity.

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Rich02si

Discussion Starter · #4 · Feb 22, 2011

major said:
Hi Rich,

I think you will find it difficult to find a reasonable size motor which will work with available rear end ratios. Meaning you will need a high RPM motor and a gear reducer between the motor and diff even if it is direct drive, meaning a single ratio non-shifting set-up. You might look at quick change rear ends which have a spur gear set in front of the ring and pinion. This would also allow you the option of choosing ratio to fit the track. But you're not going to mount that vertically.

Regards,

major

Thank you for your input. Well I figured I'd have to change the diff gears anyways. But with what ratios are available I wasn't quite sure if I could get close to or around 1:1.

Rich02si

Discussion Starter · #5 · Feb 22, 2011

Woodsmith said:
Hi and welcome,

You can do what you suggest but if you turn a diff nose up you will get a significant loss of lube to the pinion bearing leading to quick failure. Sometimes even turning a hypoid diff upside down to raise the pinion height is enough to have that effect.

Could you house the motor in the tunnel? Depending on motor size you could make a larger section in the tunnel to fit it or place it where the trans was and then run a shaft to the diff.

Reversing a motor is just a matter of some big contactors to reverse the field winding relative to the armature winding on a series motor.

As for speed you can work out with your tyre diameter and diff ratio what speed the motor would need to run to get the speed you need. Usually 4th gear in the trans is a direct drive so you can see what the tacho says.
The tacho is usually matched to the speedo in 4th, from my experience, so the needle positions match. Look at the speedo for your maximum required speed and then see what rpm the tach would say if the needle pointed to the same place on the dial.

Larger motor diameters usually equate to higher maximum torque but lower speed and for smaller motors it is the other way around. However much of it also depends on your controller choice and pack voltage and current capacity.

Thank you for your ideas. If there is room in the tunnel in front of the diff that might be an option. I also just thought possibly fitting it in the rear behind the diff if there is room.

Now concerning rpm and getting the balance of speed and torque. If a motor is longer yet narrower, does that relate to rpm potential because of the reduced rotating centrifugal mass? Could I acheive a desired torque and rpm that way?

Rich02si

Discussion Starter · #6 · Feb 22, 2011

Well here's my goals. Vmax of about 75mph. And max torque between 30 and 50 mph. A range of at least 50 miles. Vehicle weight around 2200-2300 lbs. Probably asking a lot. But that's my benchmark.

WarpedOne

· #7 · Feb 22, 2011 (Edited)

Vmax of about 75mph.

This translates into max ~1500 wheel RPM. At 4:1 fixed gear ratio you need a motor that goes to 6000 RPM. Such things exist, for example Warp motors do that.
AC motors go over 10.000 RPM and you'd need over 10:1 reduction gears.

And max torque between 30 and 50 mph.

With AC you'll have max torque from 0 to about 30 - 40mph, after that tourque will drop sharply toward motor max rpm.
With DC you'll get max (massive) torque at stand-still (0 rpm) which will than drop to the min at max RPM. Expect sharp drop.

A range of at least 50 miles.

This is totally up to the amount of kWh in your batteries. Think about 10 kWh at least.

Vehicle weight around 2200-2300 lbs.

Sounds reasonable.

Probably asking a lot. But that's my benchmark.

It's really hard to say for sure without trying to put together a plan and see what it could do "on paper".

Choose a motor, find its specifications, choose batteries, find specifications, choose a controller and find its specifications. Find a weight of you donor vehicle, substract 300 - 500 punds for al the things you'll pull out of it, add the weight of batteries, motor and controler plus some overhead and you can calculate the "acceleration chart".

Then change the motor and repeat the process. Soon you'll start to understand what can be done, where are the limits and what is a nature of a DC EV and AC EV.

Pulling the transmission might end up being a Bad Idea.

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Rich02si

Discussion Starter · #8 · Feb 22, 2011

Well Im pretty sure for the perfromance Im looking for AC and Lithium are gonna have to be my choices. I dont want to save the money and not be happy enough with the end result.

Well if I put the motor in the transmission tunnel I should have room to put a reduction gearset there since the ev's motors are shorter then a transmission. And still have all that lower part of the engine bay to work with.

I think im lookin around 70kw AC motor and about 8-10 kwh pack. With the right gearing I should be able to keep the torque at a good speed. even if peak drops around 40mph.

Can you give me insight why I "need" the transmission?

Duncan

· #9 · Feb 23, 2011

Hi Rich

DC - cheap unsophisticated
AC - expensive low power (unless you have VERY deep pockets)

I am building a cheap light two seater with a DC motor, I was going to use a gearbox until I calculated that I would be able to spin the tires in top gear - gearbox seemed a bit superfluous then!

Simple calcls - weight on driven axle and tire coefficient of friction (80% for road tires)
give maximum force at tire contact patch
Times tire radius gives Torque - Max torque that you can put down!
Divide it by diff ratio = max motor torque

800 lbs on rear x 80% = 640lbs
My tires are about 2 ft diameter - 640 ftlbs torque

Diff 4:1 - 160 ftlbs motor torque will spin tires ~ 500 amps with most DC motors (very roughly)

My tires are about 2 ft diameter = 6.28 ft/revolution at 1000 rpm (wheel) = 6,280 ft/min = 376,800 ft/hr = 125,600 yd/hr = 71 mph

So with a 4:1 diff I will need 4,000 rpm to get to 70 mph

DC motors have Torque proportional to current and will maintain that torque as long as your controller/batteries can keep feeding the current (until it melts)

If you have a heavy car a gearbox is very useful - if you have a very light car you can get away without

This enabled me to put the motor where the gearbox would have lived

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Rich02si

Discussion Starter · #10 · Feb 23, 2011

Duncan said:
Hi Rich

So with a 4:1 diff I will need 4,000 rpm to get to 70 mph

This enabled me to put the motor where the gearbox would have lived

Curious how you got to the 4:1 ratio?

What motor are you using and how well did it fit in the tunnel?

DawidvC

· #11 · Feb 23, 2011

Have a look at http://www.kansasev.com/evglide-powertrain.html.
This kind of setup will move your motor into the transmission tunnel, but still leaves you with a gearbox. The 2 ratios are generally enough for most electric motors, except if you are way underpowered. Then have a look here, at http://www.diyelectriccar.com/forums/showthread.php?t=45278&highlight=calculator and download the zip file. It is for OpenOffice, btw. Under the geartboxes tab, add an antry for this PowerGlide, Plug in your rear-end ratio, and play around until you pass out, or get the results you need.

PS :- add all the required data for the Miata in there as well, then PM it to Franky. I am sure someone around here will appreciate it.

Regards
Dawid

Duncan

· #12 · Feb 23, 2011

Hi Rich

This is my car
http://www.diyelectriccar.com/forums/showthread.php?t=44370&highlight=Duncan's+Dubious+Device

The diff came from a Subaru - 4.1:1
Since then I have found a Limited Slip Diff - I think its a 4.4:1

The motor is a Hitachi fork lift motor - the overall view is it should be good to 4000rpm - 5000rpm could be risking it
But it only cost $100 !

Woodsmith

· #13 · Feb 23, 2011

Rich02si said:
Curious how you got to the 4:1 ratio?

I think that is Duncan's diff ratio, or at least a close approximation to make the calculations simple in the example.

I have just tested the example with the figures from my trike. It works out that I need 114lbs-ft of torque from my 11" motor to spin the wheel with direct drive through a 2.8:1 chain reduction to the wheel.

WarpedOne

· #14 · Feb 23, 2011

Can you give me insight why I "need" the transmission?

From the techical and "elegance" point of view you don't. Especially if you go with AC drive.

The problem starts to become obvious when you look at the power curve of your AC motor. Power rises up to about half of max RPM and then it quickly drops. So, from standstill power will rise to about 40 mph and then start to drop. This translates into very poor acceleration above 40 mph, where you need increasingly more power just to sustain constant speed.

If you want to take advantage of max power your motor can output at high speed, you need at least two gear ratios. A short one for low speed fun and longer one for high speed madness.

In short: with only one gear ratio you can't get max power at max speed.

DawidvC

· #15 · Feb 23, 2011

I ran a simulation of the tesla roadster through Franky's ev calculator earlier today, using the single ratio transmission they used as against the EV Powerglide (had to hunt the ratios). Using a 2-speed transmission make sense in a tesla if you want to go real fast and real quick.

I would suggest you try the same for your vehicle and see what works best for you.
Regards
Dawid

Rich02si

Discussion Starter · #16 · Feb 23, 2011 (Edited)

I'm thinking about just going with the transwarp 9, and play around with the differential ratio. If I have enough power to that motor(144+ V, ~10kwh pack) I can run a rather high ratio. Maybe 3.3-3.8:1 to get more torque up a little higher in the mph. 0-10 mph is not as important to me. If I can get up to 75-80 mph, great. But where I want that usable torque is in that midspeed range. A lot of people want simplicity for maybe there lack of skill. I totally get that, I'm new to EV's too. But I want simplicity for lack of weight. If I can at least house the motor in the transmission tunnel I'll have all that lower engine bay area to place the pack to keep the weight low in the chassis.

Speed versus RPM Calculator

Engine Speed *5000 RPM
Transmission Gear Ratio 1 :1
Differential Gear Ratio 4 :1
Loaded Tire Radius 11.45* inches

Vehicle Speed 85.2* MPH (- a few for drag)

Based on this calculation I don't need a reduction gear. My acceleration should be based on my power the motor puts out.

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