# Autocross toy



## Bags (Jun 20, 2010)

Hello all –

I am seeking your thoughts on how to configure the following vehicle.

Goals/requirements:

This will be a purpose-built autocross “toy”, to play in the ‘A mod’ class, but not necessarily to win (needs wings to win). Key factor of this class: minimum weight is 900 pounds with driver.

The donor vehicle is an old Formula 500 car, basically an overgrown race kart. It weighs about 550 pounds with its ICE components. Figure the ICE components are about 100 pounds. I am about 215 pounds, though I should be 180, sigh. So the “EV” components must weigh at least 235 pounds, and I expect they will weigh more than that.

For autocross in my area, it needs to run 1-mile courses. It needs to do this 6 times within 75 minutes on a single charge. I am estimating that it is 40% at maximum acceleration, 40% maintaining speed and 20% braking/coasting. Max speed of about 80mph. Targeting 0-60 time of 3.5 seconds or less … and it can’t be a slouch at 60-80 either.

Most importantly for my budget: (1) the motor can’t burn up, and (2) the batteries need to do this (6 runs per session) for 50 sessions/year for 5 years, i.e., for a total of 1500 runs (250 charge cycles). If that’s not feasible, then my budget has to account for the batteries being replaced before then. 

Simplicity of the build, and simplicity of maintenance are important factors. I am expecting/hoping to use a direct chain- or belt-drive in a single gear ratio (there isn’t time nor space for shifting). It is currently driven by a chain from the CVT to the “solid” rear axle. There is adequate space for a motor of any diameter, and up to about 18” in length or a bit longer (mounted transversely, i.e., shaft perpendicular to direction of travel).

I have been assuming that A123 20Ah pouch cells will be the best choice, with a basic BMS. There is adequate space and support for batteries in the “side pod” areas.

There is adequate space for a controller, a small 12V battery, and a radiator.

What configurations of motor, controller and batteries will meet these specs? 

What is the most inexpensive configuration?

And/or given the overall general goals (i.e., going really fast for a total of 1500 miles), what performance trade-off can I make that will have the greatest reductions in cost?

Thanks for any and all advice.


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## rwaudio (May 22, 2008)

Bags said:


> Hello all –
> 
> I am seeking your thoughts on how to configure the following vehicle.
> 
> ...


I have no experience with the type of motor/controller that you would need, but I'm using the A123's and I can say they should do the trick nicely. They are very stiff and with minimal sag should last longer than your requirements if you stick to 10-15C peak.

A Soliton Jr and an Impulse 9 would likely give you more than enough power for a cart of that size.
There are others that could probably give better recommendations though. (while contributing to the weight you need)


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## mizlplix (May 1, 2011)

Racecar 101: Design the car as light as possible and then use ballast to bring it up to weight. The ballast can be placed where you need it for perfect chassis tuning. Otherwise it will not scale properly without going over weight.

How does the CVT work for you? If it is decent, I would try to retain it.
The CVT would give some flexibility to what ever motor you went with.

My $.02-Miz


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## Bags (Jun 20, 2010)

@mizlpix: re: CVT. It works fine for the current ICE engine. But the advice I’ve received about CVT’s is that they go counter to the performance of an electric motor. In any case, it’s probably simpler to have no transmission (single gear). Re: weight. Yes, good point.

@rwaudio: Thanks. How many Wh/mi do you pull under heavy acceleration? Do you limit your C-rate?

To all:

Here is my current thinking:

•	Soliton Jr.
•	Kostov 9” 220V
•	Approximately 80s, 2p A123 20Ah pouch cells, limited to 15C

Do you think this combination can move a 1000lb car from 0-80 in 4-5 seconds or a bit more? (for reference only, not likely to actually do 0-80 all at once in autocross)

Do you think the batteries will last for 1500 miles of up-to-15C (not constant 15C) use/abuse?

Will the motor take 600A at high RPM, for a few tens of seconds, repeatedly within an hour or so?

Do you think there is a cheaper way to meet those goals?


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## McRat (Jul 10, 2012)

First, most autocross courses won't go much past 60mph for safety reasons. 500HP Vettes might touch 70 if the course is too open.
Speed in corners trumps top speed, since very little time is spent at peak speed.
Weight should be low, and center. This allows quicker turns, and less transfer.

I'd start out at 60mph peak. You will need about 40 seconds WOT power for a 60 second lap. You will normally do 3 laps per event, one at a time, per class you're entered in.

Slight toe in at rear, slight toe out at nose, lots of camber.

As far as electrics go? I'm clueless, but have raced a lot of SCCA Solo2 (AutoX).


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## mizlplix (May 1, 2011)

I work for a large fleet owner. Among the things he owns is a John Deere "Gator" . It is electric and has the standard CVT that our Bunker Rakes have, they are gasoline powered with a CVT also. 

It is RPM compensated. That means that what ever spring you are using in the reaction member (rear sprocket) sets the engine RPM it shifts at.

The gas trikes wind up to about 3000RPMs then the ratio changes and the vehicles go faster but hold the motor to that same 3000RPM, until they top out where the motor then winds up to it,s max, about 4400RPM.

The electric Gator does the same thing, but it does it at a lower RPM. It revs up to 2000 or so then gets faster till the CVT stops, then the motor goes up to it's max at 3500RPM.

Still, it might be simpler to just go direct drive electric if you get the top speed you need.

Miz


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## Bags (Jun 20, 2010)

@mcrat: The courses in Packwood, WA, are typically 0.9 miles, and I will usually hit the rev limiter at least once in my bone-stock '06 Civic Si -- that's 57mph. I am usually middle of the pack or lower. FTD is usually in the low 50-second range. I agree 80mph peak is generous/optimistic and unsafe on some courses, but 60mph is clearly too low for my scenario. I'm shooting to make the top drivers work really hard to make FTD. 

@mizlpix: Thanks for the data.


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## Bags (Jun 20, 2010)

PS. Put the weight lower or more centered? I could either sit on the batteries (centering them, but putting me and CG higher), or put them in the side pods (keeping me and CG lower, but putting weight out to the sides).


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## McRat (Jul 10, 2012)

Bags said:


> PS. Put the weight lower or more centered? I could either sit on the batteries (centering them, but putting me and CG higher), or put them in the side pods (keeping me and CG lower, but putting weight out to the sides).


The lower the weight, the less body roll, and the better braking balance. More rubber on the ground essentially. Keeping heavy weight away from the corners of the car allows it to react faster to changes in direction. Less rotational inertia. This is not that necessary for roadracing, but for autoX, quick switchbacks are a plus. 

For big cars, (Camaros, Firebirds, Vettes), I usually run the course in just 2nd gear. Much like an electric motor, they have a lot torque in the low RPM areas, no need to shift. Little cars with pipey engines need shifting.

I'd really think a single speed electric would have a HUGE advantage over ICE engines in AutoX.


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## mizlplix (May 1, 2011)

Back in the 1990's, I worked for the Bondurant Racing School as a chassis Specialist and crew chief'ed for Bob when He drove for the Salleen Team.

For you to drive a car fast, it needs to feel comfortable to you. You need to feel confident in it. The best theoretical axis of yaw is at a line drawn down through the driver's spine into the ground. The car needs to rotate through that exact spot (not necessarily the center-line of the car). 

So, any weight added needs to be low and divided equally around the driver. 

A hard thing to do. And not always practical. Concessions will be made (as long as you can live with them).

You can scale the chassis and use the weight to ballast the chassis, place it where it will do the most good for you. Just keep it close to the yaw axis as possible.

Miz


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## Brute Force (Aug 28, 2010)

Don't add ballast, add batteries.


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## mizlplix (May 1, 2011)

We were talking batteries. The question was where to put them. A scale will tell you where. 

Miz


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## galderdi (Nov 17, 2015)

Try this on for size:

https://endless-sphere.com/forums/viewtopic.php?f=34&t=76189

I have made a few poor decisions based on bad advice regarding batteries. But otherwise it is coming together nicely. The 2017 season should be excellent.


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