# Deadly fast street legal sandrail upgrade



## Jayls5 (Apr 1, 2012)

My electric Sandrail is currently running an AC-20 motor. The vehicle weighs ~1000 lbs or so. It accelerates briskly in 2nd gear to 70 mph.


Well... I bought an AC-35x2 motor. Through trades, I have acquired another Curtis 1238-7501 controller.

Punchline: Along with my previously owned 1238-7601 controller, I can deliver 1200A to my new AC35x2 motor. Current pack voltage is 103.6v nominal.

The new motor has an Auxiliary rear shaft that matches my current setup, so I MIGHT be able to use my old coupler for the AC-20.

I actually bought the motor to go into a full sized car... but once I realized the aux shaft might work... I thought about having some fun with a crazy fast setup.

Does anyone have any estimates of how fast this will be? I have a feeling ~235 ft-lbs torque and ~130 hp in this little thing will be a rocket.


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## dcb (Dec 5, 2009)

mind the traction limits of the tires. You might need to start in 4th


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## jacksdad (Apr 13, 2015)

So my setup will weigh about 1200 lbs and has 35x2 motor it isn't running yet but I worked out <3s 0-60MPH theoretical with a 4:1 ratio so reaching the knee around 60-70 MPH as i have no gearbox.

Kiwi EV put a 35X2 in his sonic which i think was around 1400 lbs or more and got 3.2 s for 0-100 KPH with a 3:1 differential direct drive 

http://mevowners.proboards.com/thread/5290/australia-ev-sonic?page=6

I'm guessing the issue is traction that kind of acceleration is close to 1G so requires a Mu of 1 at the tyres which is tough without any downforce - it'll be interesting to see


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## dain254 (Oct 8, 2015)

With the end to end AC35 and that much power my guess is it will wheelie uncontrollably, which will be awesome! stack your batteries in the nose!


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## Jayls5 (Apr 1, 2012)

jacksdad said:


> So my setup will weigh about 1200 lbs and has 35x2 motor it isn't running yet but I worked out <3s 0-60MPH theoretical with a 4:1 ratio so reaching the knee around 60-70 MPH as i have no gearbox.
> 
> Kiwi EV put a 35X2 in his sonic which i think was around 1400 lbs or more and got 3.2 s for 0-100 KPH with a 3:1 differential direct drive
> 
> ...


Awesome link! Gives me an idea of what to expect 

I'm a little worried about programming this thing. I only just saw it needs a motor encoder isolator. I have no idea if that's on the motor wiring itself already or if it's something I have to buy.


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## madderscience (Jun 28, 2008)

Can your batteries and wiring handle 1200A?

mind you, it sounds like lots of fun, but there aren't too many EV cells out there that can put out 1200A without being damaged. There is no LiFePO4 that can do that (typically 3c) in a size likely to be found on a dune buggy. Not sure what the C rates are for things like leaf and volt packs. I know they are higher but a non-paralleled leaf pack would have to be rated at 20C to give 1200A.


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## Jayls5 (Apr 1, 2012)

madderscience said:


> Can your batteries and wiring handle 1200A?
> 
> mind you, it sounds like lots of fun, but there aren't too many EV cells out there that can put out 1200A without being damaged. There is no LiFePO4 that can do that (typically 3c) in a size likely to be found on a dune buggy. Not sure what the C rates are for things like leaf and volt packs. I know they are higher but a non-paralleled leaf pack would have to be rated at 20C to give 1200A.



90AH 103.6v nominal LIPO (28 cells in series).

They're 20C continuous rated & 30C burst. 1200A will be 13C peak discharge.

At 650A discharge it's currently being run at, individual cells are seeing <0.2v of sag. I'm fairly confident it will tolerate 1200A momentary peak loads.

Wiring runs are fairly short, since the battery box is right above the motor. It's 0 AWG.


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## jacksdad (Apr 13, 2015)

its not going to be 1200A for very long with that torque/weight ratio you would reach > 5000 RPM and tail off current after a few seconds, 20C continuous seems a lot to me do you have a link to the spec ??


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## dcb (Dec 5, 2009)

it won't start off at 1200 either... but it will "level off" at 1200 for a spell during the "constant horsepower" mode, basically as soon as the torque starts falling off (and acceleration...)


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## dcb (Dec 5, 2009)

a more concrete example from evtv


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## jacksdad (Apr 13, 2015)

those are cool graphs 

i'd like to have some guages showing power voltage torque and current


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## Jayls5 (Apr 1, 2012)

jacksdad said:


> its not going to be 1200A for very long with that torque/weight ratio you would reach > 5000 RPM and tail off current after a few seconds, 20C continuous seems a lot to me do you have a link to the spec ??



I've got these packs wired 18P7S:

https://hobbyking.com/en_us/turnigy-5000mah-4s1p-14-8v-20c-hardcase-pack.html


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## Jayls5 (Apr 1, 2012)

I'm starting to think this is a bad idea. All the weight is in the back, and this new motor will only add even more with triple the torque. It sounds like a recipe for disaster.


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## dcb (Dec 5, 2009)

what does it do now if you launch it hard in 1st gear? does it either spin the tires or lift the nose or neither?


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## D a n n y^ (Aug 4, 2015)

More importantly, where are you located so that I can see this in Action?!


Sent from my iPhone using Tapatalk


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## Duncan (Dec 8, 2008)

I wonder what this would be like compared to my Device

http://www.diyelectriccar.com/forum...dubious-device-44370p15.html?highlight=duncan

https://www.youtube.com/watch?v=LUpyAY2sFWw


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## JCDavis (Mar 23, 2017)

I like what you guys are doing here  

Electric burnouts and cars doing wheelies are my idea of a good time


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## Jayls5 (Apr 1, 2012)

dcb said:


> what does it do now if you launch it hard in 1st gear? does it either spin the tires or lift the nose or neither?


Nah, the smaller AC-20 motor only puts down about 80 ft-lbs of torque. First gear is kind of long (~45 mph), and the tires are pretty big. Max acceleration kind of just lurches you in the back of the seat up to 35 mph then tapers off past 5000 rpm. It's definitely not a racecar in current form. The only time it breaks traction is if I initiate WOT on a turn from a dead stop.

This AC-35x2 would be an entirely different animal.



D a n n y^ said:


> More importantly, where are you located so that I can see this in Action?!


Hampton Roads, VA



Duncan said:


> I wonder what this would be like compared to my Device
> 
> http://www.diyelectriccar.com/forum...dubious-device-44370p15.html?highlight=duncan
> 
> https://www.youtube.com/watch?v=LUpyAY2sFWw


Yours is way faster as of right now. No question about it.


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## jacksdad (Apr 13, 2015)

i dont think the front wheels will lift off it all depends where you COG is, using some A-level mechanics :



so if the whole mass of the car and driver is around 1400 lbs ?? then as long as the COG is > 12 inches in front of rear axle the front wheels shouldnt lift off assuming around 4:1 max gear ratio. Looking at your car pictures and splitting the car into components - chassis (~250lbs), battery case(~200lb), motor(~170lbs) and driver(~180lbs) i think your cog will be about 18-24 inches in front of rear axle 

You can also work out the acceleration roughly from this as 1400 Nm torque at wheel and 16 inch (0.2 m radius wheel) ?? gives 5 x 1400 N at ground = 7000 N giving 12 m/s/s or 26 mph/s (assuming the wheels don't slip) so 0-60 mph in just over 2s !!


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## Jayls5 (Apr 1, 2012)

jacksdad said:


> i dont think the front wheels will lift off it all depends where you COG is, using some A-level mechanics :
> 
> 
> 
> ...


I'm incredibly surprised how accurate your weight guesses were. Battery case for the ~9 kWh of LIPO, necessary wiring, low voltage lithium battery, and misc gear probably adds up close to 200 lb.

I'm going to take the car down to the recycling center this week for a free weigh-in of my vehicle. It will only be total weight, but nice to get concrete numbers.


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## dain254 (Oct 8, 2015)

Drive on the scale with your fronts - get a weight. Then put the back wheels on the scale, get a weight. Now you have a total AND can figure CG without having to guess! JacksDad has most of the answer there, but weight transfer due to acceleration is unnaccounted for. Simply lifting the tires based on the torque of the tire relative to the center of mass isn't the full story.


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## jacksdad (Apr 13, 2015)

Duh! thats a good point once it moves at all their will be an extra torque from the inertia of the car if we assume an acceleration of 10m/s/s then this torque is the mass 550 kg x 10 x hight of COG = ?? 0.25 m = 1500 Nm so would about double the torque lifting the wheels so your COG will need to be twice as far forward as calculated so 2 feet in front of axle


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