# Drive Ratio - Rule of thumb?



## SimonRafferty (Apr 13, 2009)

Is there a rule of thumb to work out what gear reduction will give you a reasonable performance envelope in an EV?

For pushbikes, electric skateboards and other ultra-light vehicles, the following seems to work OK:

Ratio = ((Gross vehicle weight in Lb) x (Wheel diameter in Inches)) / 200 x (Motor Power in Hp)

Something close to that seems to give a reasonable combination of speed, acceleration and hill climbing capability. It's based on PMDC motors by the way.

I wonder if you fancy posting:


Motor Type (PMDC, BLDC, AC, Series Wound etc)
Motor Power (Hp)
Gear Ratio in which you drive most of the time
Gross weight
Tire outside diameter

To see if we can derive something similar for the main motor configurations?

Getting rid of the transmission apart from the diff would provide a reasonable saving in weight and power consumption. This could in many cases be achieved by choosing the right Diff ratio. 

Si


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## Jimdear2 (Oct 12, 2008)

Si,

I think you are also going to want to know the differential ratio for your calculations. then add that to the transmission gear ratio used.

The main reason this probably hasen't been persued much is the limited selection of off the shelf components and the cost of custom made stuff that will stand the loads.

The lightweight stuff you are talking about with the first formula you mentioned has a lot of commercial off the shelf stuff that can be adapted.

I also beleive that I have seen a few people who did run figures on this problem in various threads and forums and their final results seem to come in at about 6.5 to 1 or higher for a differential ratio. This is doable in some RWD vehicles, but most would require a custom rear axle housing made up from a few Ford and GM axles that have those high ratios available. These axles are mostly popular with the drag race and circle track crowd.

Another reason for them settleing on the 6.5 to 1 number is that it is about the highest ratio commonly available and easily installable (I use one in my pulling tractor that I installed with nothing more then a tube of white lead to see gear contact patterens). Anything else is custom (read bring lots of money). 

If you read around you will find that the people who drive mostly in one gear use second gear.

From a few years experiance working on cars professionally (about 40). I can guestimate most 2nd gears ratios in V8, RWD, approx 3000 lbs GVW vehicles are in the 1.8 to 2.2 to 1 range and most differentials are in the 3.0 to 3.5 area. so you see where that 6.5 to 1 comes from.

Of course Motor size, GVW, tire size, battery voltage and all of the other drags and resistances will modify this.

I've often thought that ideally to get rid of the transmission on a RWD you would need a nice small, lightweight, low drag 2 or 2.5 to 1 reduction gear (possibly planetary) unit that was easily attachible to the motor and had a driveshaft yoke and spline on the output side. I think that one of the 13 inch (Warp???) motors has the output yoke and spline so we are half way there. 

FWD . . . . WHOOPS whole new problem. About the only thing I can see is get as close as you can with one gear set in the trans and the diff ratio and gut the rest to reduce drag.

Have a great one,


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## MalcolmB (Jun 10, 2008)

Simon
The nearest I've seen to a rule of thumb is Cedric Lynch's advice on the Agni motors' site here: http://www.agnimotors.com/home/index.php?option=com_content&task=view&id=31&Itemid=67#6

Quote/
Is the motor suitable for my vehicle?
It is suitable for use with a fixed transmission ratio on a road vehicle whose total loaded weight in Kg multiplied by its intended top speed in Km/h come to a figure of 30000 or less, if run on 60V (or 72V, which is safe if the motor is not overdriven above the speed at which it runs without a load). If the weight multiplied by speed come to more than 30000 it will be necessary to use a gearbox (and accept a large reduction of speed when climbing hills) or to use more than one motor. With a fixed transmission ratio it will be possible to maintain near full speed when climbing hills, provided that the battery can supply the necessary power.
/Unquote

Parameters for my motorbike
Motor Type: PMDC, original Etek
Motor Power: 15 hp
Gear Ratio in which you drive most of the time: 5.5:1 fixed
Gross weight: 200 kg (including me)
Tire outside diameter: 24 inches
This ratio gives brisk acceleration and easy hillclimbing.

I've thought (and read) a lot about the need for a gearbox in EVs, both for my motorbike and for a planned mini conversion. The ideal in my opinion would be a compact two-speed gearbox with a ratio difference of 2:1 between first and second. The alternative, which is more readily available at present and sometimes lighter, is to use a larger motor that can deliver twice as much torque as you would need with a single motor/gearbox combo. Alternatively, use two motors instead of one with a gearbox.


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## SimonRafferty (Apr 13, 2009)

I had come to the conclusion that somewhere around 6 to 1 would be good - that (in my case anyway) gives enough torque to climb a 1:1 hill and enough rpm to give about 90mph at 5000rpm.

I had figured on using the rear diff 3.8:1 Plus a Suzuki Samurai transfer box 1.6:1 in high range = 6.08:1. It also allows me to restore the truck to part time 4 wheel drive further down the road.

There are some planitary hub reduction boxes used on tractors which have been used in 4x4's to reduce the ratio. 2:1 is one of the options. They stand a good chance of being strong enough!

Si


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## Grant_NZ (May 28, 2008)

This is quite interesting... is it possible to use just a 2 speed transmission case in place of a standard manual gear box? and have it made possible to be selectable? this of course for an AC system and use electric reverse...


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## Dr. Bill (Oct 17, 2008)

A much overlooked transmission that is easy to find parts for is the chevy 3 speed transmission. It is light weight and has a 1.75 to 1.91 2nd. gear in it, depending on the application. It also has a 1:1 ratio for a road gear. They are still found in about every scrap yard in the country with parts available at all auto parts stores.....Bill


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## Coley (Jul 26, 2007)

I really don't feel that the drag from a 5 speed trans is a big enough deal to try to remove gears etc.

The use of a FWD transaxle is good for a few reasons that are overlooked.

On most of them, the speedometer is driven from it, the alignment and support of the wheel driving axles is already figured out, the mounts are there and mating a motor to it is easy.

I have tried to see how much drag the transaxle has on my car by going down the same hill with or without it in gear and see little difference.

The advantage of the gears is underestimated, until yoiu actually start to drive the car. Direct drive to the rear end seems like a hard way to treat your battery pack.

The only gear I hardly ever use is 1st, as it is a final ration of 15.0 to 1.

2nd gear is 8.250, 3rd is 6.500 (where I drive the most), 4th is 4.000 for highway and 5th with 3.250, is flat road only.

I also use my car to carry fence repair posts and tools out in the fields, where 2nd gear works fine.

Hauling feed and salt blocks home, a flat tire to be repaired, 3rd is still the gear of choice.

I want to get the most use out my EV and need more than 3 speeds to do it.

But that is just my opine.....


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## El Rayo (Mar 23, 2009)

Dr. Bill said:


> A much overlooked transmission that is easy to find parts for is the chevy 3 speed transmission.
> 
> Bill


Could you be a little more specific, Bill? Chevy has had a number of 3 speed transmissions over the years, both stick and auto. The 3 speed sticks I've seen are heavy lumps of cast iron, ranging from 1938 pick up to 1973 Camaro.


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## SimonRafferty (Apr 13, 2009)

Also, what are the peak torque and rpm / power of your eleccy motor?

I've now gone with the Suzuki Samurai transfer box which give me a drive ratio of 6.08 in High and 9.5 in Low range. It has a rated input torque of 360 Nm. My motor (a Siemens 45kw AC) can apparently spit out 450Nm and 10,000rpm but at the voltage I'm using, 6000 rpm seems to be the max - so it will probably only deliver 270Nm.

The Suzuki transfer box, if it works, will be good as it only weighs about 10kg compared to over 50kg for the original gear + transfer box

The whole thing is about half way fitted into a Land Rover Freelander.

Si


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## Dr. Bill (Oct 17, 2008)

El Rayo said:


> Could you be a little more specific, Bill? Chevy has had a number of 3 speed transmissions over the years, both stick and auto. The 3 speed sticks I've seen are heavy lumps of cast iron, ranging from 1938 pick up to 1973 Camaro.


 
The 3 speed stick from an early Chevy or GMC weighs in at about 40-45 pounds which is not as much as a lot of modern transmissions. Mine is a borg warner and they all have about the same gear ratio in them...Bill


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