# Why a gearbox?



## Coley (Jul 26, 2007)

For ease of mounting the electric motor. By using the gearbox, you already have mounts in place.
If front wheel drive, it makes it easy to mount the motor, run the speedometer, run the shafts to the wheels.
Also, having a gear box lets you use the motor at its best rpm while climbing hills etc.
After driving mine for 5 years, I can't imagine NOT having the gearbox. It just make the conversion easier, plus reduces the strain on the battery pack.


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## Duncan (Dec 8, 2008)

Hi Fireman
I will answer this as somebody who had planned on a gearbox and is now planning not to have one

A gearbox does one things
It multiplys torque by reducing revs 

An old style four speed has a top gear of 1:1 which is the same as direct drive

First gear is about 3:1

Then you have a diff, mine is 4.1:1

Your motor will have an amount of torque - I think mine will have about 135 ftlbs at 350 amps

This means that in first gear there is 3 x 4.1 x 135 ftlbs available = 1660 ft lbs
my wheel/tyres are about 2 ft in Diameter

So there will be 1660 lbs force to push my car along

BUT at about 30mph I will run into the rpm limit on my motor!

I am building a very light car, my target weight is 650Kg, 1430 lbs
of which 50% will be on the back driven wheels

I am going to use run of the mill tires so with 715 lbs on the back tires there is no way that I can put down 1660 ftlbs of torque (drag tires could) 

In top gear 1:1 I will get 135 ftlbs x 4.1 = 553 ftlbs

With only 715 lbs weight on the back tires that is about as much torque as my cheap tires can handle 

So I decided that I didn't need a gearbox - in about a years time I will be able check my reasoning!

The Tesla
The Tesla uses a motor that is good for 12,000 rpm so it can use an 8:1 diff and still run to 120 mph without blowing the motor

The tires on the Tesla are a lot better than mine but they are still very close to their grip limits
From what I can see the Tesla is tire grip limited until about 60 mph and the power curve is so flat that a gearbox would only make a tiny bit of difference

Back to your issue, most diffs are around the 3:1 to 5:1
most motors are limited to about 5000 rpm

So anything except a real light weight can use the extra torque in the lower gears
If my car weighed 1400kg I would definitely need a gearbox and use the lower gears to take off.


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## gor (Nov 25, 2009)

also: on steeper hills you'll need about 4 times more torque than on flat, same with acceleration - sharp vs. smooth
to match torque demand, tranny allows trade-in torque to speed (rpm); w/o transmission you'll need same times more powerful motor 
what better - smaller motor +tranny or more motor less tranny - weight, cost, work, parts, ease of conversion, built (fit) consideration
since it comes with conversion ICE car already - for most applications it only make sense to leave it and use it, even if you might need some of the gears very rare


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## peggus (Feb 18, 2008)

The reason is simply cost. You can design an EV that does not need a gearbox but in case of DC motors you need a bigger motor and beefier controller to supply enough torque for takeoff and hill climbing. 

The gearbox came free with car so why not use it.

That said, my EV is fairly light weight and I find myself driving around in 4th gear a lot which is 1:1. Only occasionally do I use 3rd, so I'm thinking of getting rid of the gearbox and upgrading from a Curtis to a Zilla. My gearbox is very noisy, which is why I'd like to get rid of it.











Fireman_Mike said:


> I'll get into more details of my project in due course, but trust me it's well weird!
> 
> For now I have a basic newbie question which I haven't found answered in the wiki or in the basic searches I've done: Why a gearbox? I've seen the clutch vs. clutchless stuff, but everything I've seen implies that in most/all homebuilt EVs the motor is hooked up to a conventional gearbox. Why? As far as I'm aware, electric motors frequently used for automotive applications produce peak torque at stall and throughout their speed range, and can even be reversed electrically, so why would anyone want to change gears? Diesel-electric locomotives certainly don't, and AFAIK the Tesla won't. So there must be some good reason why you don't all throw the gearbox out with the engine...
> 
> ...


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## Fireman_Mike (Mar 2, 2010)

Thanks for the thoughts... I still feel, for mechanical simplicity, direct drive is the way to go; it appeals to my engineering aesthetics. FYI this is my project:





















Bonus points to anyone who has the slightest clue what it is!  And don't say 'a Land Rover'! 

I'll post more about it in the appropriate forum shortly.

Mike


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## Thaniel (May 25, 2008)

Fireman_Mike said:


> As far as I'm aware, electric motors frequently used for automotive applications produce peak torque at stall and throughout their speed range, and can even be reversed electrically, so why would anyone want to change gears?
> Thanks
> 
> Mike


Short answer. 
Horsepower

A flat torque curve is great but if you really want to make something move quickly you need HP. With the same torque turning at a higher RPM the motor will produce more HP. If you can keep max HP coming out of the motor then the car will accelerate quicker. This is true for ICE's as well as electric or any other type of shaft driven power plant. Ever wonder how those puny 4 cyl motors can get big HP numbers. They just turn them faster 

I've seen EV dragsters that do not use a gearbox. Shifting does take time and the ones without gearboxes have huge motors. For my motor, controller and battery setup the gearbox will definately yeild faster acceleration times.

Suppose accleration may not be an priority for some. I just hate the stereotype that EV's are ugly and slow. 

Thaniel


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## Thaniel (May 25, 2008)

Fireman_Mike said:


> Thanks for the thoughts... I still feel, for mechanical simplicity, direct drive is the way to go; it appeals to my engineering aesthetics. FYI this is my project:
> 
> Mike


 
That is one freaky looking vehicle.


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## gor (Nov 25, 2009)

peggus said:


> The reason is simply cost. You can design an EV that does not need a gearbox but in case of DC motors you* need a bigger motor and beefier controller* to supply enough torque for takeoff and hill climbing.
> 
> The gearbox came free with car so why not use it.
> 
> That said, my EV is fairly light weight and I find myself driving around in 4th gear a lot which is 1:1. Only occasionally do I use 3rd, so I'm thinking of getting rid of the gearbox and upgrading from a Curtis to a Zilla. My gearbox is very noisy, which is why I'd like to get rid of it.


true, more voltage - more barrery (same Ah) - things to conceder


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## gor (Nov 25, 2009)

Fireman_Mike said:


> Thanks for the thoughts... I still feel, for mechanical simplicity, direct drive is the way to go; it appeals to my engineering aesthetics. FYI this is my project:
> 
> 
> 
> ...


sure not Lend rover! - thats Moon rover though looks like Mars rover similar to Moon tractor
where's my bonus points?!


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## bliksem (Aug 3, 2009)

It's a Zamboni


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## Fireman_Mike (Mar 2, 2010)

gor said:


> sure not Lend rover! - thats Moon rover though looks like Mars rover similar to Moon tractor
> where's my bonus points?!


Actually... it IS a Land Rover. A very limited production one... points still available 

I'll give you another clue... full-frontal this time, it's in storage in the UK, yes that's a license plate, it's one of only three that are road-legal...












See that in your rearview mirror and weep, oh Hummer drivers!

Mike


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## John (Sep 11, 2007)

If you're going to run without a gearbox you need a much more powerful set up to achieve the gradability, acceleration, and top speed you'll require and that means spending quite a bit more. I agree that ditching the gearbox makes for a more elegant solution. Also without a gearbox when comes to hill climbing you could find your self labouring your motor at very high power and low rpm's making the motors inbuilt fan ineffective at cooling your motor when it needs it most. You might need to fit a blower. More cost.


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## Fireman_Mike (Mar 2, 2010)

Thanks for the further contributions, for those wanting to know more I've started a formal thread in the 'conversions & builds' area:

http://www.diyelectriccar.com/forums/showthread.php?p=169222

Mike


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## Woodsmith (Jun 5, 2008)

I do know that that is a Judge Dredd 101 Land Rover.
Fantastic looking vehicles but not well thought out and a pain to use as it was really only a film prop.
The standard 101 could be described in the same way but as a British army prop!

The standard 101 is very desirable but hard to live with. It was loosing a 'tug of war' with one at a charity show that made me decide to turn my Series 3 Safari from this:









Into this.
























6x6, super low gearing and 3 speed transfer box.

Good luck with your project.


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## gor (Nov 25, 2009)

Fireman_Mike said:


> Actually... it IS a Land Rover. A very limited production one... points still available
> 
> I'll give you another clue... full-frontal this time, it's in storage in the UK, yes that's a license plate, it's one of only three that are road-legal...
> 
> ...


right, they might think it's Volks Wagen minivan!
am i close? 
do you giving any bonus points coupons?


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