[ausev] VW FOX CONVERSION, GEAR RATIO QUESTION

[Chris Robison]

Good advice. To get an empirical estimate for your motor voltage limit, 
count the minimum number of comm bar separators (the black part between 
the copper bars) visible from one brush to either of its immediate 
neighbors. It's a really rough estimate, but each one of these is 
typically worth about 35V of separation. Add them together to get the 
total number of volts that's safe to put across the motor. Otherwise, 
150V is a good number to go with.

   --chris




loopcat wrote:
> FYI, this is the reply from Otmar at Zilla...
> 
> "Hello John,
> The motor looks like a good one to me.
> I would make sure you have a good clutch in the setup so it does not slip.
> 
> A safe motor voltage limit is usually 150V. I've heard that some GE 
> motors can take much more than that, but it's a good starting point. 
> Current limit is going to limit the torque and therefore protect your 
> clutch and transaxle. I would start off at 400 to 600 amps on the 
> motor and see how that feels.
> 
> You don't mention your batteries. If they are flooded lead acid then 
> you may want to protect them with a 350 amps battery current limit to 
> start and a low voltage limit of about 9V per 12V block. High current 
> AGMs won't need a reduced battery current limit but still you should 
> limit the voltage.
> 
> Once you have it running well, you can contemplate the risks to the 
> batteries, motor and drivetrain of turning up the power.
> 
> -Otmar"
> 
> 
> 
> --- On Sun, 6/8/08, Chris Robison <chris at chrisrobison.org> wrote:
> 
>> From: Chris Robison <chris at chrisrobison.org>
>> Subject: Re: [ausev] VW FOX CONVERSION, GEAR RATIO QUESTION
>> To: "AustinEV News Announcements and General Discussion" <ausev at austinev.org>
>> Date: Sunday, June 8, 2008, 9:52 PM
>> Did you plug in these gear ratio numbers directly, or did
>> you multiply 
>> them by the final drive ratio (the gearing at the
>> differential) first? 
>> Uve's calculator does not provide a separate spot for
>> the final drive 
>> ratio, so you have to do the math yourself as you input
>> each number. A 
>> quick google search reveals that the Fox 4-speed
>> transmission had a 3.89 
>> final drive ratio, and the 5-speed was 4.11.  Multiply 3.89
>> by each of 
>> your gear ratios to get the full ratio for each gear in
>> Uve's 
>> calculator. Remember that your redline with this big motor
>> is going to 
>> be somewhere in the neighborhood of 5000rpm.
>>
>> Beyond that, I'll just say this -- given your design
>> goals as you've 
>> stated in the past, you haven't solved your primary
>> issue. You still 
>> have *way* too much motor for your car. Although it may
>> physically fit 
>> within the Fox's engine bay, you've actually made
>> this fundamental 
>> problem worse in that the Fox was not intended even as a
>> diminutive 
>> sportscar like the NX was. In gasoline terms, you're
>> effectively 
>> cramming a 350 small block into an economy car, which
>> generally 
>> speaking, is something people do if they're aiming to
>> race. What comes 
>> along with that intent are the modifications to the
>> driveline -- bigger 
>> transmission, custom hardened gears, multi-disk racing
>> clutch, enlarged 
>> axles and CV joints, wider and softer tires etc, that are
>> necessary for 
>> a small car to withstand the input from a big powerplant.
>> Without these 
>> modifications, your car is going to be undergoing regular
>> repair.  With 
>> the stock transmission, don't be surprised if you end
>> up needing a 
>> rebuild every 5 or 6 months. The stock clutch will slip
>> under the torque 
>> and will definitely need upgrading and the performance
>> aftermarket for 
>> Foxes may be limited (I don't know).
>>
>> Operating the motor at lower RPMs is actually something to
>> avoid, as 
>> torque will be higher placing higher mechanical stresses on
>> your 
>> transmission, current draw will be higher, cooling less
>> effective, and 
>> generally the motor will run hotter and a bit less
>> efficiently.
>>
>> The way to make this motor work and not tear up your car
>> would be to 
>> keep RPMs high, drive gently and limit the motor current
>> from your 
>> controller, to limit the motor's output torque. Doing
>> so will render 
>> unusable all the extra power the motor is capable of, which
>> means you're 
>> carrying around a lot of extra mass that you won't be
>> using. Worse yet, 
>> much of this will be rotating mass, which reduces
>> efficiency during 
>> acceleration.
>>
>> This motor really belongs in a full-size pickup, van or
>> SUV, in a 
>> direct-drive configuration in a light truck or RWD
>> sportscar, or in a 
>> small vehicle if high performance is desired (and planned
>> for). It will 
>> work in your application, but I think it is far from ideal.
>>
>>    --chris
>>
>>
>>
>>
>>
>> loopcat wrote:
>>> Hello All,
>>>  After my 1st failed attemped at converting a '91
>> Nissan NX (motor too big for car) We had to make a
>> decision, keep the motor or keep the car. So we decided on
>> another donor car, a 1993 vw fox. 
>>> Putting the cart before the horse, last year I
>> purchased a rather large GE DC motor. The demensions/weight
>> are very similar to the famed Warp-11. The motor tag states
>> 16hp at 70v, 214amps, 1822rpm, ~210lbs. From what I have
>> heard, The motor was designed for an air-craft-tug. So it
>> should have a ton of torque. By most accounts the motor
>> will function well in an EV.
>>> The major reason for buying the vw fox was that the
>> engine is latitudinally mounted so my GE-11" motor
>> will fit into the car. 
>>> After buying the fox, I did a bit of research and
>> found some interesting facts on the transmission. The gear
>> ratios are very low numerically speaking. 1st-3.450,
>> 2nd-1.790, 3rd-1.130, 4th-0.830 R+P-4.110. So I was curious
>> how this would perform with the motor. So I went to
>> Uve's EV Calculator and added these transmission ratios
>> to a vw rabbitt, zilla lv, and trojan t-105. The closest
>> motor on the list was a Kostov. I then lowered the volts
>> (120 t-105) and the motor rpm's (3600)  The results are
>> interesting, the motor rpm's were low, but the
>> speed/distance was very repectable, 1st gear was the best
>> with a range of 30 miles between 40-60mph. Top speed
>> ~70mph. At first, I thought the ratio info was bad. Most of
>> the cars on the list have a 2nd gear ratio of about 8.0. But
>> then I saw that the Fiat Palio also had lower gear ratios.
>> So for kicks-and grins, I lowered the volts to 96v. In 2nd
>> gear the rpm's were below 1700 at 50-60mph. 
>>> And this leads me to my question. I have sometimes
>> heard that these old GE-11" spin too slow at lower
>> volts, "tons of torque, but no top-end speed".
>> But if the transmission has lower gear ratios then it
>> should produce the mph's, right? Any insights would be
>> appeciated.
>>> Thanks, John Stuart in San Antonio tx..
>>>
>>>
>>>       
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