Electric Vehicle Conversion/Powertrain

Note the presence of hazardous materials and conditions that must be approached with proper precautions and procedures to avoid damaging, injurious, or even fatal consequences.

Motor cooling

DC motors will usually include an internal fan on the driveshaft end to move air through the motor from the commutator end. This will usually be sufficient for most vehicles. When such a motor is placed within a rear drive that is enclosed (such as a Volkswagen Type 3 with battery boxes around the motor), this can lead to overheating since the warm air is exhausted in the forward part of the motor compartment and so may be recycled through the motor. In this case, some form of partitioning and venting (perhaps using a motorized fan) is required.

As the motor is cooled by an internal fan attached to the mainshaft the motor should not be placed in neutral to coast downhill after an uphill run.

Motor to transmission installation

This is one of the key elements in creating a successful EV conversion. A poorly made adapter plate will quickly wear the transmission's front bearing, which will produce most of the driveline noise and which may lead to oil seal and/or gear failure.

The adaptation of the electric motor to the transmission uses two elements, a stationary adapter plate to connect the motor housing to the transmission housing and a rotating coupler to connect the motor shaft to the flywheel/clutch plate/pressure plate assembly (manual transmission) or the flex plate/torque converter assembly (automatic transmission).

The adapter plate

The adapter plate provides the union between the electric motor and the transmission. Some installations such as for the Volkswagen Beetle and similar use such a simple machined adapter that it is easily obtained at low cost from a commercial supplier. Adapters for more complex or less popular vehicles may be quite expensive. It is possible for the builder to construct such a plate using simple tools as outlined below, but a great precision in layout and fabrication is required. For this reason most converters purchase an adapter plate and coupling as a package from a specialty converter or have the plate fabricated by a professional machinist. If a flat plate is used, some means of properly spacing it must be employed, typically by using hollow tubular or billet stand-offs. More common will be the use of a stack of flat plates, forming an adaption between the large diameter of the flywheel and the smaller diameter of the motor, with interior volume to accommodate any projection of the flywheel forward of the face of the transmission (or bell housing if a separate unit is used).

There are only a few critical dimensions in an adapter plate. The overall length (the distance between the motor aft face and the transmission forward face should be accurate to within 0.25 millimeter. This is less critical than some other dimensions but the proper distance is required so that the clutch will operate correctly and so that the pilot shaft will enter but not bind on the pilot bearing. The most critical dimensions are those that determine the alignment of the transmission center to the electric motor/flywheel center. The error here should be no more than 0.001 inch.

Manufactured

For some installations, such as an early Volkswagen or Porsche, the adapter can be made as a very simple lathe turning with a few precision holes for mounting bolts, while other vehicles such as the Volkswagen Rabbit may use a single thickness of low cost aluminum plate.

For other more typical vehicles a more complex adapter is required A few specialty manufactures will manufacture a motor to transmission adapter plate by using a numerically controlled milling machine to carve an adapter from a thick billet of aluminum. Owing to a high materials cost and extensive milling time the purchase of a plate will be a substantial portion of the cost of materials for a conversion.

Fabricating from scratch, using two or three flat plates with spacer blocks

The adapter is the blue object between the motor and the transmission

By careful layout and simple machining an adapter plate may be fabricated at relatively low cost compared to milling a thick plate. By using multiple plates with spacers a substantial reduction in materials and milling expense can be obtained. This method is suitable for a person with machinist's layout skills and with access to a substantial drill press. As an alternative to the layout method shown below it is also possible to have a machine shop properly locate the center and to mill the motor locator hole using precision mill equipment. To do this a machine shop will require an empty transmission forepiece or bell housing.

Note: this method requires manipulation of the transmission and so it should be first drained of oil. Since residual oil will drain out through the tailshaft seal the end of the transmission should be bagged.

Some of the components may require either a lathe or mill, and this work can be performed at modest expense by a machine shop. The most difficult operations to do without a lathe are the fabrication of uniform and accurate spacer blocks and the boring of the motor locator hole. Most of the remaining tasks can be done with a drill press and a bandsaw or sabre saw, often obtainable at low cost as used equipment. If a used drill press is to be used the condition of the quill bearing will be critical to the accuracy of the machine, and so also to the accuracy of the resulting adapter plate.

The exact design will be determined by the specific bolt patterns of the motor and transmission and the distance between the motor and transmission. This latter distance will typically be in the range of 75 to 100 mm.

close up view shows the three plates - motor locator, transfer plate, and a thin transmission dust cover plate. These are separated by simple cylindrical spacers. Rings cut from plastic buckets (metal would be better) complete the enclosure to keep gravel, mud, and water from the clutch

The design described below uses three plates that are spaced by simple cylindrical spacers cut from one inch (25mm) round stock. These must have parallel faces and perpendicular holes some of which carry the alignment dowels while others carry simple bolts.

Three aluminum plates are used. The material should be a high strength alloy such as 6061.

Motor locator plate - this has a locating hole (typically about 100 mm) or so diameter and 4 mounting holes for bolts that fit into threaded holes in the motor body. A 5mm thickness is appropriate for this plate as it is not highly stressed.
Transfer plate - this is between the motor plate and the transmission plate and has bolt patterns of both the motor plate and the transmission plate. It should be sufficiently thick to not flex under stresses between the motor and transmission, with 6mm being appropriate and this must be made from a good alloy.
Transmission locator plate - this has the bolt pattern drilled that matches the transmission. This should be about 2.5 mm thick for stability under milling or drilling operations, and if milled (and thin) should be clamped below the transfer plate, If made from stock that is too thick then the locator dowels will not protrude enough on each side to locate the transmission and the corresponding spacer.

These plates are separated by aluminum spacer blocks milled or cut to a precise dimension from a billet of square or rectangular solid stock. Alternatively. the sides of the adapter may built up from strips of plate but this will be more difficult to fabricate to high precision unless a "sandwich" of multiple cut-out plates is used - a process that can consume a great amount of material while producing a large amount of scrap.

Two of the spacers between the transmission dust cover and the central transfer plate use hollow locator dowels at each end. This is the transmission end, with the dowel projecting through the dust cover to enter precision holes in the bell housing or transmission fore-piece. A similar set of locators must be used between the transfer plate and the motor plate unless other precision locating means are provided.

The motor plate dowel is flush with the plate as this will be covered by a steel washer.

The motor in an ICE vehicle is typically kept in precise alignment with the transmission by two hollow dowel pins through which two of the mounting bolts are passed. These are usually press fitted into the motor block. It is the accurate locations of these dowel pins and the motor locator hole that are critical to the accuracy of the motor adapter plate. New hollow dowel pins (a set of four, rather than the two usually used in the ICE vehicle) are obtained from the parts department of the automobile dealer and are used to ensure precision alignment, both when drilling the matching pieces and when the components are assembled to the transmission.

Only the turning and boring of the cylindrical spacers and the fly-cutting or CNC milling of the motor mounting hole need be done using professional shop services; the remaining work may be done using simple but accurate machinist layout methods and a substantial drill press. Only the brave, cautious, patient, skillful and otherwise unemployed should consider making the motor hole to conform to an accurately scribed line using basic hand tools (this is possible, but not practical for most people).

This fabrication requires the ability to accurately lay out and drill various holes. This layout is done by coating the portion to be marked with machinist's blue, which is then scribed with a pointed tool. (If no machinist's blue is available, candle smoke or whiteboard pen may be used if the resultant powder is "fixed" with a light lacquer such as common hair spray. The principle layout tools required are a good quality straightedge, a scribe, a divider, and either a large divider or a trammel scribe (a bar with a fixed pivot point and an adjustable scribe. These tools need not be purchased as they may be made by the constructor. They do not require accuracy, but rather rigidity, adjustability, and the ability to hold a setting. A good machinist's scale can be useful but is not required.

Glue-up to ease assembly. Note dowel pins and cut-away spacer to clear clutch actuator hydraulic slave cylinder.

There are only a few critical points of layout and cutting and drilling. These are the relative distance between the dowel pin receivers and the position relative to these of the central point. This central point must be aligned with the design center of the transmission drive shaft. This shaft is an extension of the transmission mainshaft that mounts about half of the gears within the transmission. The forward extension of this shaft carries the clutch drive splines and also has a small cylindrical protrusion called the pilot shaft that normally fits into a bearing carried in a receiver hole in the ICE crankshaft.

It is often the case that with a used transmission that the transmission drive shaft will have some play or wobble due to bearing wear, which is a good reason to replace this bearing or to have it replaced if it is practical to do so, along with the front bearing seal which should always be replaced when the transmission is out of the vehicle. Note that much of the noise produced by an electric vehicle conversion will come from this front transmission bearing and the transmission gears. A newly rebuilt transmission is recommended to obtain a quiet installation.

It will be best if the three critical points are laid out using the manufacture's design dimensions. If these cannot be obtained then the following procedure may be used:

First the approximate location of the three critical points are determined. The location of these relative to the stock used to make the plate is not critical, but the locations should allow the transmission plate to have some extra material on all sides of the transmission face.

An approximate location for a clearance hole for the transmission drive shaft may be determined by simple measurement from the dowel pin holes. Using a simple hole saw a clearance hole for the shaft is made, but only in the plate marked for the transmission. If the pilot shaft extends beyond the transmission face this hole will allow the pilot shaft to protrude when the transmission plate is against the transmission face. If the pilot shaft does not extend beyond the transmission face this will allow the pilot shaft to be viewed through the hole.

Build-up of plates and spacers on a spare bell housing (a transmission fore-piece, actually). It is very handy to have such a part as it is light and easy to handle and can be directly measured by a machinist to obtain the critical dimensions.

Determine an appropriate orientation for the plate, center the pilot shaft in the hole, and securely clamp the plate to the transmission with some C clamps.

Place some scrap wood pieces on a bench (if required for clearance of the protruding pilot shaft) and place the transmission with the clamped plate tailshaft up on on the bench. Using some spray machinist's bluing, lightly spray into the holes that carry the dowels. This is now an approximate marking for these holes. After the bluing dries, unclamp the transmission and set it aside. If you have not got a reasonable impression of the location of the dowel pin holes, clean the workpiece and repeat the procedure.

Scribe a line joining and crossing the centers of the two dowel pin locations scribe a perpendicular line across the center of one of one of the holes. Centerpunch this location.

The next scribe and punch mark establish a critical dimension requiring high accuracy. If not done accurately the transmission will not fit properly on the dowel pins that are to go into the holes determined by this mark. Place the dowel pins into the transmission. These pins will usually be beveled on their rim. Determine an appropriate reference place to measure on the pins Place a machinist's scale across the pins and determine the extreme distance D₁ between the outer edges of the pins and the smaller distance D₂ between the pins. Be sure to use the same concentric reference line relative to the pin - these may be the extreme measurements on the inside hole of the pin for example. The average of these measurements (D₁ + D₂)/2 is the distance between the centers of the dowel pin holes. Transfer this measurement to the workpiece and scribe the location of the second hole. Do not centerpunch at this time.

Place the two workpieces together, with the transmission side of the transmission plate upward. Clamp at several places. With the drill press drill a pilot hole for one of the holes through both workpieces. Redrill with the proper size for a press fit of the dowel pin. Mark the outside of the second workpiece (the surface facing down) as "motor side". Unclamp and separate the pieces. Carefully press the dowel into the "transmission side" workpiece from the transmission side, allowing it to protrude through both sides. Placing this on the bench with the spacers you may now place the transmission on the plate with the dowel mating and the pilot shaft through the hole. You may then pivot the transmission on the plate to bring the second punch mark main scribe line to the center of the second dowel pin hole. The intersection of the scribe marks should be in the exact center of this hole. If not, rescribe as necessary and recheck. Now centerpunch the second hole. Clamp the work pieces with inside faces together , being sure that the dowel pin is engaged in both pieces. Pilot and finish drill for the second dowel and insert as with the first. The joined workpieces should now mount cleanly on the transmission using the dowel pins. If not, consider re-doing the second hole with a 90 degree re-orientation of the workpiece.

Next: Laying out the center.

Blue an area along the center of the transmission plate face from edge to edge on both sides and around the pilot shaft hole on the inside face. Erect a perpendicular to the base line joining the dowel pin holes. This may be done from reference punch marks offset the same distance from each dowel pin on the base line. Strike several sets of arcs with the trammel or a large divider. Join the intersections of these pairs with a scribed line. Transfer this scribed line to the inside face of the transmission face. Alterntatively, you could drill a small (1.6 mm or smaller) hole at each of the two punch marks and scribe only on the inside face.

The goal is to find the center for the large motor locator hole on the motor side workpiece. This will be done indirectly by layout from the inside face of the transmission plate. To do this most easily prepare a fixture to hold the transmission vertically, or place it into a large box with blocking. (The transmission should first be drained of oil.) Spray or brush machinist's blue around pilot shaft hole on the inside face of the transmission plate. Place the transmission plate, inside face up, on the transmission, aligning with the dowel pins. You should observe that the pilot pin center lies very close to the scribed centerline. As previously noted, there will be some wobble in the transmission drive shaft, so in the following it is important that the shaft is biased consistently for each scribe by applying the same pressure in each of several appropriate directions. Note that the front oil seal (especially since it is new) may apply a spring force before the bearing limit is obtained. We want to be working against the bearing limit, and so must first override any seal force. The end of the pilot shaft will have a small conical center hole, used to align the shaft as it was initially cut on a lathe when it was manufactured and when the splines were milled or broached. This center hole will form the reference for one leg of the dividers used to make the scribe marks. Now it may be that the point of the divider may be pushed or pulled to one side or the other of this hole. As with the wobble, it is important that all scribes be made consistently by pulling the scribe to the side of the hole toward which the pilot shaft is pulled. We will assume here that this is in the direction of the scribe line.

Set the dividers to a fixed distance of a few centimeters.

Pressing the pilot shaft, and puling the dividers, strike an arc at each of the cardinal directions - up, down, left, and right All scribe marks must use the same setting of the dividers. Remove the workpiece from the transmission. Lightly punch the intersection of the up and down arcs with the center line.

Blue the motor face of the motor mount plate at the center and let dry.

Now we will establish the center point on the motor mount plate. This will be done with the plate order reversed but not flipping the plates. The normal order would be to have the transmission face of the transmission plate down, the motor face of the motor plate up. Orienting the plates this way with the dowel pin holes aligned, shuffle the plates, aligning on the dowel pins and secure the plates with clamps. What is now seen is the inside face of the transmission plate its centerline and the pilot shaft scribe marks made in the preceding steps. Through the central pilot shaft hole is seen the unmarked center of the motor plate, recently blued. Set the dividers a bit longer than was used to scribe from the pilot shaft but not so much as to prevent the following scribing. From each of the light punch marks on the transmission plate scribe and arc on the motor plate through the hole. Separate the plates and mark a straight line across the intersections of the arcs. Reassemble and extend this line across the transmission plate to where it intercepts the left and right pilot shaft arcs. Lightly punch the intersection of these arcs with the line and from these punch marks scribe a pair of arcs onto he motor plate. Disassemble and join these arc intersections with a straight line. The intersection of the two lines is the desired motor center. Centerpunch this location.

Trial fit - bolted to motor using Grade 8 high strength bolts. Oversize nuts act as spacers to get proper thread depth in motor

Trial fit with transmission dust cover in place

Trial fit of the adapter. This is the orientation of the bolts (front to back) for initial transmission mating. The upper bolts are reversed at final assembly to allow easy inspection of the nuts for security.

Quality check: The motor center location should be consistently centered on the centerline scribe marks of the transmission plate when that plate is flipped, provided the transmission line center points are accurate.

This will form the center point for a flycutter or a simple CNC milling operation. You may want to have this work done at a machine shop. The other work that you may want to have done at a machine shop is lathe cutting of the spacer blocks.

Before cutting the motor hole, layout the bolt pattern for the attachment bolts. A typical motor will use four such bolts. The holes for the bolts may be of lesser precision that that for the main motor hole but should not bind against the bolts. The bolt pattern must be aligned properly so that the motor cabling is presentable in the proper orientation. Typically a square pattern (relative to up) is used, not a diamond pattern. With this layout complete you may drill the mounting holes or this may be postponed until after the major hole is machined.

The motor mount hole must be cut to a precise diameter to fit the alignment boss on the motor. If it comes out undersize it will be necessary to enlarge it while retaining its accurate positioning, which can be difficult. If it comes out oversize this may be corrected by appropriate precision shims which are a nuisance to maintain in position. Obviously it is far better to get it right the first time.

Before cutting the center motor hole, it is now appropriate to drill the remaining transmission mount holes in both plates. Be sure to mate the two plates in the proper orientation, to each other. You may want to mount the plates to the transmission, bolting at the dowel pins and clamping elsewhere. You may then scribe inside the holes or use a cut and sharpened shoulder bolt, twirled to make a small circle. Removing the plates, clamp them together and through pilot drill and finish drill with the drill press. Remount and check clearance by inserting shoulder bolts in every hole.

Upon completion of the motor hole this may be used to scribe a hole mark on the transmission plate. This may then be rough cut on the outside of the line using a sabre saw. The purpose of this hole is for clearance around the coupling. You may want to make this hole substantially larger as it will facilitate access to the motor attachment bolts. The outline of the transmission face may be marked on the appropriate side of the transmission plate and the excess metal removed, largely for appearance. This removal may be required in only a few specific locations. A similar removal may be performed on the motor side plate

(More detailed procedure is to be provided here)

For this type of adapter plate there will be open space between the stand-offs. To keep gravel, water, and mud from the clutch this area is closed off by selecting plastic buckets of appropriate size and cutting rings to fit in between the plates.

Upon completion of the face plates and the spacer billets the bluing may be cleaned off and the plates assembled to the transmission using bolts. A thin adhesive such as cyanoacrilate (CA, or "super glue") may be flowed between the spacers and the face plates to hold everything in place (do not glue the transmission to the adapter). When the bolts are removed the entire adapter will hold together in one piece - a convenience when assembling the motor-adapter-transmission unit.

If the motor with the coupler mounted may now be mated to the adapter plate. Then the flywheel and clutch assembly is attached to the adapter. If the clutch plate has been out of the assembly it must be aligned using a pilot shaft tool to alight the plate when the pressure plate is bolted to the flywheel. This is required so that the pilot shaft and splines will properly engage the clutch center splnes when the transmission is inserted. The transmission may now be mounted - it may be necessary still to slightly rotate the flywheel to obtain engagement of the clutch splines. The transmission may now be bolted up against the adapter using bolts of appropriate length plane washers and lock washers on both sides.

After mounting the transmission, do not forget to replace the oil!

The coupling and pilot bearing

The coupling transmits the motor torque to the flywheel or to the automatic transmission flex plate. This must be strong and accurate and is almost always purchased complete or made by a professional machinist rather than fabricated by the builder. Most couplings do not provide a pilot shaft bearing since this is usually incorporated into the flywheel. While some clutchless adapters have been designed without a transmission pilot shaft bearing, this bearing is critical to long transmission life and should always be included, either as part of the coupling or as an adapter that is part of the coupling assembly.

The pilot bearing in an ICE vehicle is usually a needle bearing, as this must take many turns as the engine idles. In the electric conversion, this bearing may be made of lubricated brass or bronze, since there is far less relative motion between the flywheel and the transmission input shaft.

The flywheel

Motor side of lightened flywheel with pilot bearing installed. Angle iron is bolted on transmission side to prevent rotation while tightening coupling bolts.

Edge view shows a conservative and low cost reduction for this application. Material removed with a lathe. Unused bosses on the pressure plate side could also be removed by milling.

Angle iron (cut from part of a bedframe) bolted to the flywheel to prevent rotation when bolts are torqued.

For vehicles using a manual transmission, the motor drives a flywheel, a large disk. For the electric vehicle conversion the flywheel is needed only to mount the clutch, if a clutch is used at all. The outer rim of this disk will have a gear ring shrunk or welded on, intended to be driven by the starter gear. The after face of the flywheel mounts the clutch and forms one face of the two surfaces driving the clutch disk. The flywheel will have substantial weight to allow the ICE motor to idle smoothly at low speed. Rarely, this flywheel may have been balanced as a unit with the crankshaft, in which case it may be considerably out of balance for this new application. The extra weight required for ICE smoothness reduces acceleration and the energy to spin it cannot be easily recovered, adding to brake wear. For these reasons it is appropriate to lighten and balance the flywheel, which is done with specialized tools at an automotive machine shop. This may be lighted considerably more than is the case for ICE performance enhancement. Further, the starter ring is no longer used so it may be removed for a further weight saving except where this contributes to flywheel strength. Do not remove so much material that the flywheel becomes weak. Weight removed at the periphery will be more effective in enhancing performance and will also reduce loading on the inner portion, which may be made correspondingly light. If the outer portion that contains the treads for the pressure plate mounting screws is thinned excessively it will be necessary to use nuts to secure the bolts that hold the pressure plate to the flywheel. In this case it must be possible to access these nuts in some way in order to complete the assembly of the components. An appropriate access port in the forward face of the central transfer plate should be provided, or an access from the side via a penetration of the cylindrical dust shrouds.

The clutch

Some electric vehicles do not use clutches with their manual transmissions. Such installations offer greater acceleration, lacking the rotational inertia of the clutch disk, pressure plate, and the flywheel mounting the pressure plate. Such systems require accurate shifting to avoid damage to the transmission and are not recommended for a casual use vehicle.

The clutch should have sufficient closing spring force to avoid slippage in normal operation. It should be practical in most cases to use a stock clutch and springs. Other considerations apply to a high performance vehicle for racing, some employing triple copper plate racing clutches.

Using an automatic transmission

An unmodified transmission is usually not recommended for an EV conversion owing to the inefficiencies and rotational mass associated with the torque converter and a missmatch between the normal operating speeds of an ICE engine and the replacement electric motor. Furthermore, automatic transmissions typically depend on the engine's idling rotation to maintain the transmission in a ready state. Older automatic transmissions will use a considerable amount of energy to operate the transmission's oil pump. This pump produces pressure to operate hydraulic pistons that engage the clutch bands around the planetary gearsets. Some automatic transmissions have been converted by using a properly machined splined spool to replace the torque converter, in effect forming a permanent converter lock-up condition. A particular advantage to using an automatic transmission is the presence of a parking pawl, which prevents vehicle motion when the vehicle is not operating, as a supplement to the parking brake. Shifting should be performed manually at reduced throttle since there is no longer the cushioning effect of the torque converter.

The motor mount

The motor must be securely fastened to the frame. Depending upon the original mounting configuration it may be possible to use or adapt the existing mount supports to the motor support band. It is not necessary to use the highly flexible mountings that were used with the ICE as the electric motor does not produce the periodic vibrations prevalent in an ICE. On the other hand, hard mounting a DC motor can transmit transmission noises to the frame which may be effectively absorbed by a softer mounting. Note that the transmission can produce high twisting forces and so some portion of the motor-transmission unit must be secured against rotation. This may require a separate torque arm or a mounting using extensions on the adapter plate.

See also: powertrain

Electric Vehicle Conversion Index