The next project - very early planning stage (homebuilt car)

So early this year as the bulk of the work on my Toyota wrapped up, I started thinking about what my next project would be. This next car will be built over 15-20 years and will be the last performance car I own, the budget will be $30-$40k. This will actually be the second car I've set out to design, the first was a tube-framed mid-engined twin-turbo rotary supercar I was designing when I was a teenager...yeah that would've lasted real long and was totally affordable There were quite a few other questionable design choices made. It was rollin' on twennies because I wanted that concept car look, and the roof height wasn't much more than a Ford GT40's. But anyway, that's getting off-topic. I hardly shared that design with anyone but I will share this one.

I won't even start work on this project within the next 5 years for sure. Hopefully I'll have it finished well before human-driven cars have to putter on the soft shoulder with a Slow Moving Vehicle sign on the back.

Overall concept:

It's going to be a track-oriented midrange 2-seater sports car. Design philosophy like the NASA/Elan NP01, styling like something between an AW11 MR2 and an Aston Martin Vulcan. There will be pop-up lights

Specs I've worked out so far:

Chassis: Spaceframe, total weight target under 2700lbs. Fairly small car. Rear-midship lateral layout (like an Evora). I'll try to make the chassis meet as many cage requirements as practical.

Wheels: 18x12, 315/30/18 tires. 18s are the most popular diameter for performance tires and will likely increase in popularity. I like wide wheels and I cannot lie, I considered 18x14 but the selection in that size is terrible and probably won't improve much.

Suspension: SLA w/ pushrods, center-linked heave spring & damper & mini sway bars. Would like FRICS, but there's a budget.

(So far very similar to that last design, except it was a mid-longitudinal car with slightly less fancy suspension)

Engine:...or I should say motor 400~600hp via dual electric motors, ideally AC induction type. A Tesla S rear motor pair would work just fine. For the record I chose this back when I thought me and the 4AGE would live happily ever after, but if I hadn't, my new ICE suddenly ruining itself could've made the decision for me.

Battery: To be intentionally chosen as late as possible, controllers chosen to suit, but I plan to use OTS or junkyard parts, preferably with distributed battery management modules. Two packs, stuck low down against the front and rear firewalls. Will have a minimum number of spare modules that can be brought online in case of a failure or as an emergency reserve. Solar panels under front & rear glass for idle climate control & trickle charge. Maybe also a supercapacitor to improve efficiency. There will be room for an APU/range extender but it probably wouldn't be very useful. If I had to choose a battery right now I'd use the same LiNiMnCo cells used in the e-Golf.

Drivetrain: Combining gearbox going into a 4~6spd lateral dogbox transaxle with helical diff, custom sequential shift system, straight cut gears for strength and sound No reverse gear necessary. There is a weight and reliability penalty for using an actual gearbox instead of just running the electric motors into a diff (or even straight into the axles), but it should pay off in performance and will pay off in fun! No clutch. It will work sort of like a pneumatic sequential shift system with spark cut & throttle blip does in an ICE. A computer will control the motor speed and power application to allow for flat shifts controlled by paddles. Electric actuation if possible. Finding a gearbox to start with looks like it could be the hardest parts-sourcing job involved. Plenty of longitudinal racing gearboxes could do the job, but I don't want the power to turn 90deg for no reason, that's a waste of power and space. Considering some kind of compact front motor setup. It will be hard to keep up without AWD in the future, but I want to keep weight and complexity down, and I especially want to keep parts under the front of the car to a minimum.

Brakes: Plain-jane iron vented discs, same all-around for cheapness, dual MC hydraulic system, hydraulic handbrake setup for the lolz. Plus computer-controlled regen braking with auxiliary load dump resistor bank (can be used to increase regen braking force, or give you some if the battery is full). Maybe an ABS system.

Cooling system: Battery type affects cooling system type which affects packaging greatly, but there will be a ton of space in the rear of the car for batteries. The trouble will come with using (actual) air cooling or worse yet direct refrigerant cooling on the front pack...that could take up a lot of space where there won't be much available. A non-cooled battery pack (often rather misleadingly called "air-cooled" such as on the Leaf, where the pack is a fully sealed metal box!) is the best wherever you can use it, but it may not be possible depending on battery type and pack size. Liquid cooling is almost as compact but is complicated, expensive and heavy. Air cooling takes up a lot of space and some energy but is cheap and simple. Direct refrigerant cooling is like air cooling but more complicated and draws a lot of energy...basically you're air-conditioning the inside of the battery packs. So non-cooled (unlikely) or air-cooled would be best.

Aero: Flat bottom with splitter & big diffuser, retractable canards, active spoiler.

Interior/entertainment: Plain-jane race car stuff other than a custom digital dash and a Noble-style dual-harness system. The car is a dumb platform that I can bolt new technologies onto, I'm not integrating anything more tightly than necessary. Very different from that last design.

Body: Wet CF panels and coated lexan windows. Body panels will be somewhat minimalistic and it will have plain-jane doors that open outwards, a removable hardtop panel like an Elise, and a window into the guts through the back glass. I'll take some inspiration from PC case modding to show off the controllers' innards through there.

And that's it, comments are appreciated but there might not be any updates for a year or two Next steps are concept sketches and laying things out in a solid modeling program.

Spec tightening:

Drivetrain: I've decided to go with a direct-drive front motor for sure, making the car AWD, with a 3-position switch that lets me choose between FWD/AWD/RWD. I'm aiming for a top speed of 300kph at most, so if the motor is good for the revs involved there's no problem. This will make the car more fun and a good bit faster, but also a bit heavier and much more cramped in the front - I can say goodbye to any idea of a frunk now! There may be space for an Elise-like bin in the rear above the transaxle. Aiming for 100~200hp to the front wheels. A compact combined motor/inverter unit should fit, these are a new technology that's just about to reach production cars. If I had to choose one right now, I'd go with the Siemens Sivetec MRS 7701 found in the limited-production Volvo C30 EV. The Sivetec MRI series (these are AC induction type) looks like a better choice to me but I don't think they've been put into any production cars. These are liquid-cooled so this leans the cooling choice toward liquid.

For the rear, I've been researching the Tesla S drivetrain more. What looks like 2 motors is actually a motor and inverter. The exact same motor (and diff, but I'm not interested in that) is used in some other EVs like the Mercedes B-series. The control system for this has already been reverse-engineered, so a Telsa S motor & inverter could work just fine. Both use liquid cooling so the battery cooling choice decision is tilted further. The motor & inverter should basically last forever so these should be easy to source in a junkyard...if there's any trouble, it'll be from junkyards immediately sending these nigh-unsellable parts full of copper and cast aluminum straight to a recycler!

I still have no idea what to use for the lateral transaxle. I'd hate to have to go longitudinal, again it would be a waste of power and space.

Well you did say very early planning stage! Sounds good, I'll report back in 20 years.

Love the FWD/AWD/RWD switch.

For a lateral transaxle, would something like a FWD trans work? Super common and cheap, and some are pretty beefy.

You could even use the SAME trans in both front and back for service-ability.

EDIT: btw, I have some drawings I made for a car I want to build (but I want to make my car out of wood and epoxy). I started trying to translate them to a free CAD software, but boy am I bad at CAD software anymore. I used to be ok with solidworks, but its been a WHILE!

A FWD transaxle technically could work (if you can find one that would handle like 500ft-lbs at least!), but I'd prefer something racing-oriented to save me the cost and effort of switching to straight-cut dog-tooth gears (most racing gearboxes already have these). I also have no use for a reverse gear, which some racing gearboxes don't have. Furthermore, most FWD transaxles have the axle outputs at a lower level than the rest of the gearbox, which is bad for keeping the CoG low. If anything I might like the rest of the gearbox to sit lower than the axle outputs.

On the other hand, a longitudinal racing transaxle would make the motor sit right in the middle of prime rear-pack real estate, the tail would stick out into prime diffuser real estate, and it would turn more energy into heat with the ring-and-pinion gear. But it would require less mods, would definitely not blow up, and it should keep the CoG lower.

Of course longitudinal racing transaxles are bloody expensive, so a compromise might be something like a modded WRX gearbox. It would still require a lot of mods and have a vestigial reverse gear, but it would be much cheaper.

I'm not using a trans in front, to save weight and space. If I use a combined motor/inverter unit, internally that's a small motor going to each front axle.

Dumb question- what is stopping you from getting an Elise chassis and building off of that?

I know it's been done before- but that guy is trying to turn that work into a bigger company.

Seriously, though, back when there was the 100 MPG challenge program, I thought the Elise would be a good chassis to start with.

Crushing Import duties, if I build it here. But it's not a bad idea, technically. Aesthetically, I want the car to be 100% my design. Elises are a 7 or 8 to me, I want this car to be a solid dime IMO

The tube frame should be more repairable than the Elise's bolted & bonded aluminum extrusions too.

Space could also be pretty tight in an Elise, forcing me to move the battery packs upward, and making front drive packaging even more difficult. This car will be longer, again more like an Evora, with the extra length used for battery packs and controllers instead of rear seats (cabin will be further forward than an Evora).

I'm just thinking of the aluminum space frame, not the body. Or the rear, or any of the suspension (although, that may be handy).

So if you could find a crashed one, where the tub is still decent...

And you can make it as long as you want.

I might very well use Elise suspension arms. There seems to be a good suspension aftermarket for S2s which would cut down the need for custom parts. The front would require some mods to allow for front axles, a custom upright & different hub at least.

A frame from a crashed car could make sense. I'd want to extend the wheelbase for sure, closer to Venom length than stock length. There are a few S2s driving around here, I'll have to keep a close eye out and see if any get crashed I think the trickiest part might be making room for the front axles, a channel would have to be cut into the "rails."

Would you need a reverse gear in the transaxle? It seems it would be easier to run the electric motor in reverse.

This has almost the same fun factor with much less work.

914Driver wrote: This has almost the same fun factor with much less work.

It's not solely about fun vs effort, otherwise I'd just buy an Exocet I do want bodywork even though it's a lot of work. The powertrain(s) will be about the same amount of work overall. There will be more electronics & programming work involved with the EV powertrains but the motor and inverter units are simple and reliable things I'd probably never need to open. It'll pay off in performance and reliability.

Wall-e wrote: Would you need a reverse gear in the transaxle? It seems it would be easier to run the electric motor in reverse.

That's right, no reverse gear needed.

Found a gearbox that could work: the Toyota EA60 AKA the Aisin BG6.

This is the same model used in the Evora and some common Toyotas like the RAV4 and Auris. There are no straight-cut gear sets available for this model but it should hold together in stock form (Lotus pushes roughly the same amount of power and torque through it in some Evora variants) and it's attainable, which the straight-cut sequential dogbox idea increasingly seems not to be. The gear sets alone for even the more popular FWD transaxles commonly run over $4k.

I could get sequential shift function by servo-driving the shift levers, this wouldn't be as fast and I wouldn't get that sweet gear noise but it would do the job. However without dog-tooth gears this requires me to choose between bringing back the clutch or building an automated gear-floating system and accepting the risks involved. Either could work but both have downsides.

It also has a reverse gear which I don't need and it weighs over 190lbs, but any gearbox that might stand up to this power is going to be heavy.

It's a decent fallback option if nothing else.

Helical diff for EA60 found:

http://www.momentummotorsportshop.com/qdf27e-ea60-bg6-quaife-atb-helical-lsd-differential/

I've been weighing the pros and cons of auto gear float vs. adding a clutch, and decided on auto gear float...the clutch adds a good bit of weight and complexity, and its only function would be to ensure safe shifts. It's actually the more complicated solution because it requires all the same programming, sensors and actuators that the auto gear float system would have plus more for the clutch.

To build the auto gear float system would require a motor/input shaft speed sensor if there isn't one built into the motor already, of course a gearbox output/diff speed sensor (wheel sensors and some code could work too), some actuators on the transaxle's shift lever (preferably electromagnetic but the more common pneumatic actuators could work), a position sensor on the shift lever (similar to the one I'd like to put on my Corolla for data-logging purposes), and a custom control system between the E-throttle and motor controller. When I hit a shift paddle, it would:

1. stop relaying my e-throttle value to the rear motor controller

2. cut power to the rear motor and put the gearbox into neutral.

3. When the shift sensor confirms neutral position (maybe a lack of gear engagement + a small delay, since neutral is vague) blip the throttle (or hold it at zero longer for upshifts) to match motor revs with wheel speed for the next gear

4. engage the next gear, and once the sensor confirms it's gone in, resume relaying the e-throttle value to the rear motor controller (I may not use the raw value, I might use a hint of smoothing to reduce shock loads on the gearbox). If the sensor says it hasn't gone in, go back to neutral and throw an error. Then I have the option of retrying or switching to FWD mode to limp home.

Hopefully all of that can be made to happen in well under a second.

The program for the clutched system would be the same, just starting and ending with clutch disengagement and engagement.

The stock synchros are meant to take the weight of the gearbox's input shafts. On this gearbox that's probably around 40lbs of rotating mass, just guessing based on the weight of the whole transaxle. The electric motor doesn't have any compression or acceleration effects at zero power, it's basically a heavy flywheel - in other words, on the clutchless system I'd just be adding weight to the input shafts. The whole Tesla S motor weighs 70lbs, probably less than half of that is the rotor assembly, so I think the synchros should handle well-matched shifts no problem, especially considering that the weight of the ICE flywheel and clutch assembly is gone.

Updates on what's been swirling around in my head:

Body - a major problem with a lot of mid-engined cars, both in terms of performance and looks, is a lot of front overhang. Worst of all there's usually nothing up there but bodywork and a splitter, and maybe a radiator (which I won't need much of). I've been thinking of using a nose shaped something like the Mazda Furai's to get around this problem. It's very short and it looks good. All that's going to be up there, beyond the usual stuff like front suspension, steering and MCs, is the front motor, controller/inverter and a small radiator stack, maybe a charge port & controller (which is small). Only downside is it will make fitting pop-up lights more tricky. I really want those, even if they're not needed and take way more effort to put in

Front suspension - having your coilovers actuated by a bellcrank on a front suspension with axle-driven wheels is tricky. The two potential solutions I've seen are bent or forked pushrods like Porsche uses, and bellcranks integrated into the upper control arm like Pagani uses (even though their cars are all RWD). The former requires more design effort (especially if you want lots of lock angle) but the latter greatly reduces adjustability so I'm leaning towards the former.

Rear powertrain cooling - on the Tesla S the diff housing doubles as a coolant manifold for the motor and inverter, and has beefy bus bars built in to move power between the two, seen here (the dark gray cylinder is the motor and the light gray cylinder is the inverter):

I'd have to get a custom piece made to do the same, right now I'm thinking about putting the inverter behind and slightly above the motor, just above where one of the drive axles would come out. I want to leave the area between the transaxle and firewall as wide-open as possible for the battery pack. If there's going to be any room for a cargo bin, it'll be above the body of the transaxle. The cooling system for the rear powertrain will be completely separate from the front, the rear radiator(s) will be at the very tail of the car, fed by ducts ahead of the rear wheels and on the top of the tail.

Steering - No idea which rack I'll use yet, but I've decided I'm not adding a hydraulic system for it, so it will have to be direct-drive electric PS. If it were a RWD car I could get away with an unpowered system, but being AWD I could use the assist. Ideally I'd like to put it low down and ahead of the front axle line.

Looks like an all-electric gearbox actuation system is possible, ran across this system used on Rob "Chairslayer" Parsons' car:

http://mastershift.com/p_street_manual.html

So no need for a compressed air system now. I'll probably copy that system rather than buying one...ideally just use the same servos to drive the gearbox's shift levers directly and eliminate the cables.

a few thoughts on shifting without the clutch:

for each gear change 1-2, 2-3, 3-4, 4-3, 3-2, etc there should be one or more positions during a full rotation of the output shaft where the shift collar can slide from one to the other with no restriction. The key is getting it to slide at exactly the right time. You may be able to get a good output shaft position sensor and then manually find all the spots where a shift is possible for each shift. Then your programming just has to see the shift called for and reference the available shift output shaft positions, reduce throttle, wait for the next correct position of the output shaft, then bang, then back to throttle pedal throttle.

The output shaft will be rotating quickly so you should never have to wait any noticeable amount of time for a shift to happen.

Here's the harder thing though, shifting when not moving. If you come to a stop in anything other than 1st gear, its very possible that you stop the car in an output shaft position that will not allow 1st to be engaged. With a clutch, the synchros solve this, but without a clutch downshifting to 1st while stopped may be a royal pain. (move engine in neutral very slowly until input shaft matches output shaft for a shift).

I think trying to get the gears to engage on a particular rotational position might not be possible, I doubt any actuator has the movement speed, low latency, or maybe even consistency to hit such a fast-moving target.

From what I understand about how synchros shift, the relative rotational position of the shafts has a miniscule effect (once both shafts are spinning) compared to the relative rotating speed of the shafts. The gear can engage at just about any angle (at regular small intervals) once the teeth on the synchro line up with the gaps in the gear. A dogbox has much bigger teeth and is more picky about angle but the effect of angle is still relatively tiny from my understanding, since the angles are always changing and it will mechanically "brute-force" a workable angle in a tiny fraction of a second.

So if I can match speeds closely, cut power to the motor so it's freewheeling, and push into gear, the synchro should do its thing and act as a clutch to bring the input side of the gearbox (which now includes the rotating parts of the electric motor) to the exact same speed as the output side at which point the gears will engage.

I think turning the motor very slowly while going into 1st at a stop should work as a solution to that problem. I can program the computer that if output speed is 0, give the motor a tiny blip of power then push for 1st, if the shift fails repeat once or twice then throw an error.

Factory Five 818

I've seen it, I want something prettier and more DD-able This car will have a roof, windshield with wiper(s), side windows, and AC.

I'm not sure why you need a gearbox at all. If you're running the front motor direct drive, what would be wrong with running the rear motor that way as well?

GameboyRMH wrote: I've seen it, I want something prettier and more DD-able This car will have a roof, windshield with wiper(s), side windows, and AC.

How about the coupe version then:

http://www.factoryfive.com/galleries/project-818/sinister-black-818c/

jstein77 wrote: I'm not sure why you need a gearbox at all. If you're running the front motor direct drive, what would be wrong with running the rear motor that way as well?

Indeed I could, but you can get better performance with a gearbox, same reason they're used in Formula E. It's also more fun to drive this way!

mainlandboy wrote:

GameboyRMH wrote: I've seen it, I want something prettier and more DD-able This car will have a roof, windshield with wiper(s), side windows, and AC.

How about the coupe version then: http://www.factoryfive.com/galleries/project-818/sinister-black-818c/

That looks nice! I especially like the new headlights. Didn't know they'd made a version with a fully enclosed cabin.

But the proportions of that car are pretty far from what I have in mind, and I'd have to redesign the front end structure and suspension to take the front powertrain.

The reason I'm hesitant to just stuff my powertrain ideas into an existing car is largely because I actually want to design the car myself from scratch, and the powertrain is just a fraction of that. I want to end up with a car where I can't look at any part of it and think "I would've done that differently" or "this could've looked better with a little change here" or "I'm not sure how this thing works." A car that only has the compromises I want. You could say I'm building it for the sake of making it exactly how I want it.

What car could make you more happy or proud than one made exactly how you want?

Wow... and I thought planning my ratrod woody build, from a derelict 26 chevy cowl was ambitious

I understand the draw of having something that's all yours... Good luck!

Ran across something that will save me a lot of work and open up plenty of parts options, the UMC Drive, an open-source control device for AC inverters & DC controllers:

http://www.diyelectriccar.com/forums/showthread.php/any-ac-motor-any-inverter-umc-152234.html

This will take a programmer-friendly set of CANbus commands or just plain 'ol PWM signals and run just about any combination of inverters/controllers and motors. Using this with the Tesla S powertrain will save me plenty of programming work and make the vehicle easier to modify or upgrade. This opens up more options for the front where I can perhaps get a more compact powertrain by mixing and matching different parts.

Speaking of the front powertrain, I've been thinking that a BMW i3 unit could be a decent choice. It's a hybrid DC/induction unit and it's fairly compact:

Power is 170hp(130kw)/184ft-lbs which is still sensible to send through the front tires. They're good for over 11krpm, but here comes the big sticking point: They don't come set up for direct drive, they come attached to a diff with a 9.7:1 ratio (which is bonkers by ICE standards but common in EVs) which is why the i3 is limited to 150kph, to prevent an overrev. Add the fact that this car's wheels are actually smaller in diameter than the i3's, and I'd want a drive ratio less than half of that, closer to a low ICE diff ratio, so a custom diff box may be required there. The higher ratio also means acceleration from the front wheels will be reduced, but this would be acceptable since most of the power will come from the rear powertrain and the front is mainly there to keep rear wheelspin in check.