# Direct line or tranny for Autox/handling?



## ferd (Dec 1, 2015)

The group I help has built several EVs with and without transmissions. We haven't run autocross per se, but have tested handling performance during some EV challenge meets. Our cars were designed to be basic transportation - not racers - so I'm not sure my experience relates to your questions. But here's what we've found.

Electric motors have such strong and flat torque curves (compared to ICE) that a transmission isn't absolutely necessary - as long as your motor and battery pack are large enough. The transmission can extend your battery range or shrink the minimum size of your battery pack, due to the gears allowing you to operate your motor closer to its peak efficiency (which will vary as road conditions vary its loading). Because of the differences between electric motor and ICE torque curves, you have to think about and use transmissions in these two applications differently. For an ICE the transmission compensates for a relatively narrow and peaky torque curve. A transmission also helps overcome road speed limitations imposed by one gear ratio vs. the r.p.m. limits of your motor / controller.

I have run autocross with ICE cars, and yes gear shifts can be critical for best lap times. But an electric motor does not run out of torque as quickly as an ICE does. I’d expect that I’d need to modulate the throttle differently due to that, and wouldn’t lose time caused by shifting. Both systems can over-power the tires, but react differently since available torque is different. 
Which is best? That’s probably most influenced by the design of your vehicle and how you drive it. Maybe some others who have driven autocross in an EV can weigh in?


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## Duncan (Dec 8, 2008)

Hi 
My car is direct drive and is used for sprints - and drags as well as a road car
Only three real events so far + a couple of drags,
Great fun - not very competitive as my top speed was too low for the straights 
My new battery should fix that

The other issue is the driver, it used to take me too long to get up to speed with a track when I had a powerful mini - and that was nearly 30 years ago - I don't think I will be able to fix that

http://www.diyelectriccar.com/forum...-dubious-device-44370p2.html?highlight=duncan


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## mizlplix (May 1, 2011)

Have driven A/C in 2 cars. One about 1900 Lbs and another at 3,100Lbs. 

Given that experience, I would like an Atom chassis EV with two 9" DC motors, no tranny, but a way to do "quick change" to the final drive ratio to match weather and varying track conditions.

Auto crossing in a street car is OK, but a purpose built car is another fun factor altogether!

Miz


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## John Metric (Feb 26, 2009)

Direct Drive is fine. OF course, I would skip the full spool rear end or you will squeal around every corner.


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## Chris Pincetich (Nov 30, 2015)

If you plan to autocross with race tires and try for top time, then you'll want a Limited Slip Differential to get the power to the pavement. Plan accordingly


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## mizlplix (May 1, 2011)

AutoX = 4Wd + 2-9" MOTORS + QC PACK + space frame + fiberglass body
(QC Pack = 2 or 3 packs on drop out frames) 

Miz


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## miscrms (Sep 25, 2013)

I'm not an expert of any sort, but it seems like it really depends on the motor, the weight of the vehicle, and the options you have to get gear ratios and power to the wheels. It seems to me that a lot of folks that leave the transmission in are mostly doing it to get the right gear ratio, not necessarily to shift gears a lot. In a RWD it can be tough to find a rear end with a high enough ratio by itself. In a FWD you usually need the transaxle anyway.

An exception might be using an under-powered motor/controller in a heavy car. Even my little saab which was probably 2200lbs with the lead in it needed to start in first with its 11hp 6.7" dc motor and 300A current limit. In second or third it would absolutely crawl off the line.

So far in modeling the Leaf motor in the Saab, I can't really find any advantage to adding a 2 speed option in terms of acceleration and 1/4 mile times. I suspect this is because power output is basically constant from ~3k rpms all the way to the 10k limit. When modeling the dual AC-35 in the same car, there was a significant advantage to a 2 speed. Similarly I suspect this is because the AC-35 seems to have a distinct power peak, and then declines at higher rpms.

Of course I only really have the option of a fixed single speed at a favorable ratio because it came with the motor. Otherwise I'd probably have been mating the motor to the existing transmission/transaxle.


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## mizlplix (May 1, 2011)

I have a 1900# car and have driven it both ways, direct and with a powerglide. 

In an area like Phoenix, it is basically flat with no long up or down grades and that is important. With it in direct drive, I could keep up with traffic nicely and it was really quiet. But if I wanted to shine, the powerglide in low would allow me to jet away from the stoplight easily.

The low gear did give a few miles in range too.

Miz

PS: My Ford 8.8 rear axle has a 6.14:1 gear set.....


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## galderdi (Nov 17, 2015)

I am currently completing a khanacross car in Aus. It is similar in a lot of ways to your Autocross. Although its possible to run the car without a tranny it really depends on your top speed. Mine has a full gearbox and khanacross is limited to around 60 mph. 

When I run it in motorkhana (a bit tighter and more technical than autocross) I only need 1st gear and I am seeing around 35mph.

If you need any more top end than 40mph or so from a single gear you would start having to compromise between top speed and initial launch.

This is with a 500kg front wheel drive car running an ADC 9inch and Curtis controller.

I am thinking of another project which would be only for motorkhana. For this I would make the total weight a lot lighter (around 300kg) and it would be direct drive.

Here are the videos from my events so far. Keep in mind it is a work in progress so I am yet to achieve adequate performance in my opinion:
https://www.youtube.com/results?search_query=galderdi+2016


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