This page discusses the drivetrain of my scion xB EV Conversion.
- 1 Goal
- 2 Direct Drive – Background
- 3 Design
- 4 Next Topics
I wanted a “Direct Drive” setup for my new EV conversion. “Direct Drive” is a bit of a misnomer as what it really means in the EV world is that there is not a multiple ratio gearbox, e.g. a transmission. In theory, this makes for a simpler, lighter, more efficient, and more reliable drivetrain. No clutch, no syncros, no grinding the gears, many fewer moving parts, etc. There is no mechanical neutral gear, and reversing is done by simply running the motor backwards, which in an AC powered EV drivetrain requires nothing other than software.
Direct Drive – Background
Except in very lightweight EVs like motorcycles and E-Bicycles where hub motors are practical and thus an example of “true” direct drive, there is still the need for a torque multiplier, e.g. a single ratio gearbox, belt, chain, or some other method of converting RPM into torque in most vehicles. In addition, any four wheeled vehicle with a single motor driving an axle will still require a differential, to allow the outer wheel to turn at a different speed than the inner when going around corners.
There are been a few attempts to build hub motors that are practical for full size automobiles but these have not yet reached a price/weight/reliability/simplicity combination that is practical for normal driving.
If going with direct drive, the design must allow for both enough low speed torque to get started on steep hills, and also allow for sufficient top speed, usually around 80-100mph. This typically means the drive motor must function efficiently over a quite wide RPM range. production EVs typically have motors that run efficiently over an RPM range of zero to well over 10,000 rpm and which are capable of at least 100 foot pounds (much more for performance models) over much of that RPM range.
All modern Production EVs are direct drive using a single ratio gearbox. Usually the gear reduction ratio is somewhere between 8:1 and 11:1 depending on the weight, power, performance, and other characteristics of the vehicle. The only exception to the direct drive approach I know of in the modern era for “pure” EVs (e.g not range-extended hybrids) is that Tesla originally tried to design a two speed transmission for their roadster, but gave up on it due to technical problems and ultimately went with a single ratio gearbox instead.
Direct drive is less common among EV conversions than is mating the electric motor to the original vehicle transmission. This is largely because many EV conversion drivetrains are designed to be relatively low cost and thus are not powerful enough to support direct drive with sufficient performance either in terms of maximum torque or maximum RPM. Just like with an ICE engine, adding a multiple ratio gearbox compensates for this deficiency at the tradeoff of extra weight, mechanical complexity and somewhat lower efficiency.
EV Conversions that use direct drive require more powerful and thus more expensive motors and inverter/controllers. Either AC or DC direct drive works fine if designed properly.
Usually DC direct drive is found in high performance EV conversions, often utilizing a dual motor with series/parallel switching and a high performance motor controller such as the Zilla. Overall direct drive gear ratio is often around 4:1 taking advantage of the very high torque available from series wound DC motors. Redline RPM on stock series wound DC motors used in EV conversions is usually around 5000 rpm, but properly balanced and prepared motors can go considerably higher than that. Well designed examples of this setup can beat most V8 powered cars in the 1/4 mile, and some are flirting with the 10 second 1/4 mile mark.
AC direct drive for EV conversions is yet rarer. Options in this area are generally to source an OEM designed motor/gearbox combination, such as are available from Metric Mind Corporation, or to find an older Solectria/Azure system. The latter is the route I chose as with a little luck it is possible to find or assemble one of these systems in working condition relatively inexpensively. The metric mind offerings, while very appealing, easily go into five figures. The Metric Mind offerings are available with bare motors, and several have a single speed gearbox available. The Solectria/Azure system was always designed to use an OEM automobile differential for the gear reduction required.
Solectria’s original documentation suggests that an overall gear ratio of 4:1 to 5:1 would be ideal for vehicles weighing up to 5000 pounds using the AC55/UMOC445TF system. I initially designed my 3000 pound Scion xB EV conversion to use a 6:1 overall gear ratio, and later changed that to a 4.8:1 overall gear ratio. The 6:1 ratio gave a top speed around 60mph and enough torque to lay down rubber from a stop on dry pavement. The 4.8:1 ratio seems like about right for the Scion with still good starting torque and a top speed around 70mph. If the car actually did weight 5,000 pounds then in my opinion the 6:1 ratio would probably be necessary to ensure enough starting torque for hills, unless the vehicle operated in a relatively flat area.
Achieving Gear Reduction with Direct Drive
The average automobile differential gear reduction ratio is about 4:1. Differentials can be found with ratios from about 2.7:1 up to 8:1 depending on many factors. Higher ratios are often found on, and are available aftermarket for, four wheel drive jeeps and trucks. Lower ratios are usually found on rear wheel drive automatic transmission passenger cars. Most smaller four cylinder cars which make good EV conversion candidates have a differential gear ratio right around 4:1. Gear ratios can be changed relatively easily on vehicles with drive axles that are physically separate from the transmission. Front wheel drive vehicles with a transaxle that includes the differential are not always possible to change the ratio.
It is theoretically possible to modify a standard manual automobile gearbox to remove all gear ratios except for the desired one and lock it in that gear, thus creating a fixed ratio gearbox. I went partway, removing fifth gear and reverse, in my old Toyota MR2 EV Conversion.
Depending on the arrangement, belt drive, chain drive, or other additional gear reduction can be utilized someplace between the motor and the differential. I ended up doing this in my Scion Conversion, as addition of the belt allowed for both tuning the final gear ratio and for ideal positioning of the motor.
My direct drive design came about as a result of my vehicle choice and motor choice (See Scion xB EV – Motor and Inverter page). I needed a direct drive setup that would mate a Solectria AC55 motor to the original front drive wheels of a 1st generation scion xB. The setup had to fit into the original engine compartment of the Scion. I did briefly consider more exotic arrangements such as placing the motor in a mid-mounted position driving the front axle, but this would have required extreme body redesign or loss of much of the rear battery box volume.
Having removed the original transaxle, I needed to find a replacement differential for the scion. This differential would need to mate to the drive wheels of the scion, and in turn be driven by the AC55 motor. I knew from Solectria documentation that the overall drive ratio needed to be at least 4:1 though living in hilly Seattle, and wanting to be able to tow a small trailer, I wanted it to be higher than that; I initially chose a target ratio of 6:1 to be on the safe side.
I knew this was going to be a pretty highly custom build; in fact I am not aware of anybody else trying this exact approach in an EV conversion. As such, I did spend a fair amount of time researching this, building it, making it maintainable, and making sure it had the best chance of working.
Engine Compartment Subframe
The engine compartment subframe is a steel frame that supports the motor, differential, inverter, vacuum pump, and a few other small components. It bolts up to the original engine compartment shock mounts, so no modification to the structure of the xB engine compartment was necessary. The shock mounts (aftermarket urethane replacements of the originals) isolate vibration and allow for slight flex of the car body.
By mounting the vacuum pump (also on its own smaller shock mounts) on the already vibration-isolated engine compartment subframe it is quieter than it would otherwise be if attached to the vehicle body someplace.
The motor mount allows the motor to slide back and forth several inches and to be bolted down firmly by its flat base. There are tensioning screws to help hold the motor in position when tightening the belt. (much as tensioning screws on the rear wheel of a fixed gear bicycle work).
The frame is made of 1.5″ and 2″ square steel tubing, with some 2″ channel and angle for brackets and other parts. It is a simple but rugged design. The whole subframe probably weighs about 50 pounds. It is supporting a 250 pound motor and a 70 puound differential and about 20 or 30 pounds of other components, mostly the inverter.
Of the 10 original mounting bolt points on the Celica GTS differential, the subframe attaches to 8 of them.
The frame must be very rigid to ensure that the synchronous belt between the AC55 motor and the differential pinion stays in proper alignment, otherwise the belt will wear rapidly, make excess noise, or it could fly apart and cause loss of drive and likely damage.
I spent an inordinate amount of time researching differentials for the scion. I needed something which:
- Had an iRS (independent-rear-suspension) housing design.
- Was commonly available; e.g. found on a wide variety of vehicles.
- Was not ridiculously expensive; e.g. no late models, no racing parts, etc.
- Was sized appropriately for a 3000 pound vehicle with about 100 horsepower.
- Was available with a wide variety of gear ratios, at least up to about 5:1, to support the direct drive gear ratio range I need.
- Had available limited slip carrier options, should I want something like that in the future.
- Would fit in scion engine compartment in position of original transaxle differential.
- Possible to mate with a synchronous belt pulley on the pinion, and mate to original scion drive wheels (via custom halfshafts, if necessary). This basically means a bolt flange on the pinion, and bolt flanges, or a common spline design for the stub shafts.
This list of requirements makes for a fairly narrow range of options. On top of this, differentials are not a terribly common part to mix and match willy nilly, so there is not a whole lot of information available for those wanting to go completely off the reservation, as I was doing. Usually for a given make/model of car, even those people upgrade such as sports cars and 4x4s, there are a relatively few differential options that fit and those are usually available with a few different ratios.
Without going into too much detail, here is the list of candidates I came up with, and their advantages and disadvantages:
|Vehicle||Diameter||Known Differential Ratios||Type||Advantages||Disadvantages||Notes|
|Honda S2000 (AP1 and AP2)||7"||4.1, 4.3, 4.77||Torsen limited slip||high performance vehicle, compact housing with easy mounting points, bolt flanges on axles||moderately rare/expensive||4.77 ratio r&p available from kia sportage front axle. r&p compatible with late model RX8. May be a 5.x ratio available but was unable to confirm.|
|Mazda RX8, Miata (late)||7"||4.1, 4.3, 4.77||Open, LSD||high performance, compact housing||bottom side bolt mounts would complicate mounting system, no bolt flanges on axles||same r&p as S2000; therefore kia sportage front r&p compatible for 4.77 ratio.|
|Mazda RX7/Miata (early)||6.7"||3.9, 4.1||Open||compact r&p||Awkward mounting ears on housing||probably too small as originally designed for ~2000 pound miata and apparently prone to breaking in hopped up cars. Few ratio options.|
|Toyota Sienna AWD||6.7?||3.2||Open||compact, bolt flanges||little info on ratio options, likely no high ratios||AWD rear diff probably not very strong|
|Toyota Rav4 Gen1||6.7?||2.9||Open||compact housing, bolt flanges||little info on ratio options, likely no high ratios||AWD rear diff probably not very strong|
|Nissan 240SX||?||3.x, 4.1, 4.3, 4.6, 4.8, 5.1||Open, LSD||compact housing, many ratios, easy mount||?||extremely common Nissan R200/R200A diff found on many cars and trucks. Short and long pinion options.|
|Toyota Celica (80s)||6.7"||4.1?||Open||?||?||I know a IRS 6.7" Toyota diff existed for some 80s cars but found little info on it. probably too small anyway.|
|Toyota Celica GTS (84-86.5)||7.5"||3.7, 4.1, 4.3, 4.77, 5.28, 5.8||Open, LSD, Locker||Compact housing, easy mount, bolt flanges, many ratios, cheap||GTS IRS housing somewhat rare.||This is the extremely common toyota "F" differential. Many gear ratios available and all types of carriers. The GTS housing is a bit rare but the moving parts are common as dirt.|
|Toyota Supra (80s)||8"||many||Open, LSD, Locker||big and strong, relatively compact housing for its size, bolt flanges||some housings depending on year have awkward mounting points||this is a very strong diff and very common but too big for the scion.|
It is worth pointing out that for window shopping purposes, Ebay is by far the best place to look to get an idea of pricing and fitment of used differentials if you are trying to do something weird. Once you have and idea what you want, then if it is common and cheap local wrecking yards will probably have it, or of course you can buy through Ebay though shipping might be rather steep on large car parts.
Ultimately, I chose to go with the 84-86.5 Toyota Celica GTS differential. The ring and pinion are very common and are available in a very wide range of ratios. You can get LSD carriers. The Differential housing, while somewhat rare, is easy to work with having 10 mounting bolt locations on the main housing. It had easy bolt on flanges for the pinion and stub shafts. it is old enough to be found in the cheap wrecking yards like pick and pull.
If I were to do it again, I might try the S2000 differential as I now know that I do not need extremely high differential ratios in the 5’s like I thought I might need, and the S2000 you can go up to at least 4.77 and you get Torsen (the good, European kind) limited slip technology.
The particular Celica GTS differential I acquired came from a 1984 car from the Arlington Pick and Pull. It is a 4.10 ratio open diff. The car had unknown mileage as the dash cluster was gone (this is often an indicator of relatively low mileage) and the differential is tight with minimal corrosion. Not bad for the age. I bought the differential and halfshafts, though I did not end up using the latter.
A few months later I sourced a second, 3.90 open diff from another Celica GTS at the Tacoma pick and pull. This one is a lot rustier and in overall poorer condition but is still rebuildable and will work as a spare and for bench work like test fitting of parts.
CV (Constant Velocity) halfaxles or halfshafts transmit power from the differential or transaxle to the wheels in an independent suspension car. Every front wheel drive car on the planet has a pair of them. Sometimes they have bolt flanges on the ends, and sometimes they have splined stub shafts, and sometimes a combination.
The CV shafts for the scion have to mate the 1984 Celica GTS differential to the original Scion drive wheels. As these axles need to be made with considerable strength and precision I elected to farm out construction of these parts. The only place I found willing to do the work was driveshaftshop.com out of North Carolina. They build a lot of high end, high horsepower racing stuff, but they are also willing to take on the oddball work. The work did take quite a bit of time owing to several factors, including their being in the middle of moving their shop, and due to the original design plan not working out requiring remote negotiating and agreeing on a new design. However the final parts delivered exactly matched my requirements and were built to a high quality standard for a very reasonable price given the level of custom work needed.
The custom CV axles were built by taking off-the-shelf axle bars and racing grade CV joints, and cutting/grinding off and welding the original scion xB outer stub shafts from its original outer CV joint housings to the outside end, and cutting off 2000 toyota tacoma inner CV joint housing stub shafts, and welding them to the inner CV joint. (the original plan was to keep the stock inner Tacoma CV joint, and the stock outer scion xB joint, and build a custom shaft instead, but DSS did not have the tooling to make the correct splines for the Tacoma end)
I chose the 2000 toyota tacoma front axle CV joint because it has the same “F” code toyota differential as the 1984 Celica, and its stub shafts are compatible. The only difference between the Celica stub shafts and the Tacoma ones are the oil seal race is larger. I ended up finding a Ford rear axle oil seal (National #472747) which is a close enough fit to work. Its OD is .005 too small, requiring the use of a hardening sealant to hold it in place in the Celica GTS diff housing, and the ID is a bit big, but not enough to cause an issue, or perhaps a speedy sleeve or other solution would work if it causes problems. Another solution if you are a good machinist is to mill down the Tacoma oil seal race to the same diameter as the original Celica GTS stub shafts.
i chose to keep the original Celica GTS stub shafts stock instead of allowing DSS to weld them to create the new CV axles. This is so if in the future I need to build a different set of axles I will have the option of building flanged inner CV joints that mate to the original Celica GTS stub shafts.
A synchronous belt is simply a toothed belt. The timing belt found in many modern gasoline engines, and the drive belt on belt driven motorcycles are commonly found examples.
Synchronous belts, as their name implies, do not slip under normal operating conditions. They are available in sizes to handle hundreds of horsepower, if need be. A synchronous belt drive is better than a chain drive for several reasons:
- No lubrication required
Their disadvantages are:
- Relatively expensive
- Require very precise alignment
- Require considerable tensioning
- Limited range of belt lengths and sprocket sizes available
Gates Polychain Carbon GT2
Online design calculator: www.gates.com/designflex/ (requires free registration)
I initially used a 62mm wide, 8mm tooth pitch belt from Gates from their GT2 Polychain Carbon line. I used a 40 tooth driver sprocket and a 60 tooth driven one, for a 1.5:1 reduction on the belt and a 6:1 overall drive gear ratio.
The belt and sprocket combination I chose was rated to handle about 150 horsepower maximum, according to Gates’ drive design calculator.
Mechanically, the Gates belt system worked fine, but it produced a very loud whine with pitch proportional to driving speed, audible even at walking speed. Judging by the wear pattern when I removed it, the belt alignment was decent, but the straight tooth design, hard plastic/carbon belt, and wide width all conspired to make it transmit a very loud, high pitched whine when running at any speed or load. A Gates engineer I contacted suggested that the belt tension might not be enough and thus be causing extra noise. I was unable to measure it accurately enough to say whether this was an issue or not.
Goodyear Eagle NRG “Blue”
Online Design calculator: “Maximizer Pro” (free to use, no registration)
For the noise issue and also to change the overall drive ratio, I chose to change to a different belt type. The second belt type I used was the Goodyear Eagle NRG “Blue” belt system. The color refers to the belt width and tooth pitch; there are several options.
This belt is 37mm wide with a 14mm tooth pitch, rated for about 100 horsepower maximum. This belt is a more standard rubber/kevlar design, but the tooth pattern is more like a tractor tire. The staggered, angled teeth, rubber construction, and narrower width make for a smoother, quieter, constant engagement.
I used a 34 tooth driver sprocket (larger diameter than the 40 tooth gates sprocket owing to the larger tooth pitch) and a 40 tooth driven sprocket, to achieve a 1.18:1 belt reduction ratio and a 4.8:1 overall drive reduction ratio.
The Goodyear belt is still audible, but it is MUCH quieter than the Gates belt was. Also, the goodyear belt gets quieter under load, while the Gates belt got louder.
Goodyear NRG belts have been used in other EV conversions successfully in drivetrains for mating dual motors, mating motors to transmissions, and other applications similar to mine. The “Blue” size seems to be the best choice for most purposes.
I was able to find and purchase the appropriate sprockets for the Goodyear Eagle NRG “Blue” system on ebay at a considerable discount over MSRP. The belt I purchased new, but its retail price was about 1/2 what the Gates belt was.
Custom Installation Components
The synchronous belt components I purchased are designed for installation on large industrial equipment. The sprockets are designed to attach to keyed shafts using taper lock adapters, which the sprockets slide onto and compress onto a drive shaft.
Unfortunately, neither the AC55 motor nor my Celica GTS differential presented standard keyed shafts for mounting the drive sprockets. In both cases, I would need to custom make the necessary adapters to attach the sprockets.
In the case of the AC55 motor, it has an ANSI 32/4 or SAE “C” spline. This is a 1.25″, 14 tooth spline. My solution to this problem was to identify a splined hub (Hub City 0332-00034) which fits the motor shaft and has a machined 2″ outside diameter. I fit this into a taper lock hub of the correct type (“2012” bushing for the Gates 8MX-40S-62 sprocket; and an “SK” bushing for the Goodyear NRG 34 tooth sprocket). In both cases, I simply found the correct depth, welded the bushing onto the spline coupling, then cut the compression slot all the way through. This created a splined taperlock hub that would fit the AC55 motor and the synchronous sprocket. I did machine the ends and the inside diameter of the spline hub to allow the completed hub to fit closely against the motor face.
In the case of the Celica GTS differential, In order to attach the drive sprocket I bought, restored, and had to learn how to use a 1947 South Bend 9A Metal Lathe. Using this machinst’s tool, I was able to manufacture an adapter flange that fit tightly against the original differential pinion flange (where the driveshaft attached) and attached the sprocket on the other end. For the Gates system, I modified a “3020” taperlock hub to become part of the mounting flange, and made an aluminum adapter out of a 3/4″ thick, 4″ diameter piece of 6061 aluminum stock. Bolts went through from the original pinion flange holes to the 3020 hub to hold it all, and the sprocket attached to the 3020 hub in the normal fashion. For the Goodyear system, I had to make an entirely new aluminum adapter flange, this time using 6″ diameter, 1″ thick round aluminum stock. This adapter bolts to the differential pinion on one end, and bolts to the 40 tooth gates sprocket on the other end via 3 holes I drilled in it; bypassing its taperlock attachment mechanism entirely. Both adapter hubs are probably accurate to a few thousandths, which is enough to function successfully though being made by a very novice machinist, they are far from perfect.