Electric Vehicle Conversion/Chassies, suspension, and running gear
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.
Unit body rust
Rust problems in a vehicle can be expensive and difficult to correct. Recent vehicles have been substantially improved in their corrosion resistance, using zinc plated metal and full dip priming. Older vehicles do not have this quality of build and may be subject to rust. This is especially important in unit body construction, where thin sheet metal provides the primary structure of the vehicle. This is another reason to convert a light truck - almost all are built with a sturdy frame to which the battery load may be transferred.
Tires and wheels
A selection of wheel and tire combination must respect both the weight and intended use of the vehicle and its expected loads. A low rolling resistance is desirable, obtained usually by choosing an appropriate belted radial passenger car tire. For light trucks this means using a tire intended for a rather heavy sedan, rather than one designed for a light truck, assuming that the vehicle is intended to be used on improved roads. For a light vehicle it will be necessary to use wheels and tires suitable for this weight, which may not be available as standard equipment, and suitable tires may not fit the wheels that are on the vehicle. As more popular vehicles will have a wider choice of aftermarket wheels and tires this might influence the choice of vehicle to convert.
The converter should resist the urge to add a wide stance to the vehicle by selecting a wheel with a greater rim offset. As with ICE vehicles this added offset can upset the carefully designed steering geometry and imposes additional stress on spindles and axles. On top of this, the conversion will carry weight near the maximum vehicle capacity at all times, imposing additional stress on the suspension components.
The additional weight of the batteries will cause compression of the springs which in most vehicles will change the suspension geometry. This will also reduce the space available for suspension travel. Such compression might be rarely encountered in an ICE vehicle as it corresponds to the maximum load. In an electric vehicle this load will be present at all times, so the suspension travel and geometry should be restored by various modifications.
It may be desired to restore normal ground clearance, especially important if the batteries are between the frame rails of a truck or similar vehicle. Under other circumstances such as the creation of a sports electric vehicle a low ground clearance may be desirable as it can reduce aerodynamic drag and reduce rollover tendencies. A low ground clearance may also be considered as a styling element.
Spring rate and spring force
Spring force is the amount of force applied by the spring at is normally loaded condition. If weight is added to the vehicle the springs will compress until the necessary force is obtained or the suspension "bottoms out", by resting on the "snubber pads" that prevent metal–to–metal contact at full travel.
Spring rate is the additional force applied by the spring for each unit measure of spring compression. Additional spring rate is used to prevent excess compression of the suspension due to road bumps and to reduce the tendency of the vehicle to lean in turns. Additional spring rate is especially important if the suspension travel is reduced as part of the vehicle's styling and handling modifications.
Additional spring force may be obtained by several means:
- Air adjustable shock absorbers.
- Coil spring over shocks. ("coilover shocks")
- Stronger coil springs.
- Air bladder within coil springs. These are adjustable with air pressure but require that the shock absorber is mounted outside of the coil.
- Additional leaf in leaf springs. This is a common modification to truck suspensions.
- Coil spring shims. This does not increase spring rate as do the methods above. Note that full suspension travel plus bump pad compression must be available, so this may not be a practical solution)
Shock absorber damping rate
When the spring rate is increased then the damping rate applied by the shock absorbers should also be increased. If the suspension modifications are by use of coil-over or air adjustable shocks then the appropriate damping may be obtained by proper equipment selection. Some high end shock absorbers allow the damping rate to be adjusted statically (by mechanical adjustments on the shock absorber).
McPherson struts are commonly used in the front suspensions of lower priced modern sedans and coupes. These struts use a robust coil–over shock absorber to perform four functions:
- To maintain suspension geometry by controlling camber and caster by its relationship to a single lower A–arm
- To provide suspension travel
- To provide suspension damping
- To allow steering motion of the front wheels
In some suspensions the coil spring is separate from the strut and bears on the lower A–arm.
A similar device, the Chapman strut is used in the rear of some sports cars (such as the Fiat X1/9). The ability to obtain specialty racing equipment replacements for these struts should be considered when selecting a light car for conversion. Only a limited number of performance vehicles will have high rate or adjustable struts available. Where such components are available it will likely be possible to also obtain high performance brake components.
Since the vehicle should never be driven if the total vehicle/passenger/cargo weight is above the manufacturer's GVWR, it might be considered that is no need for brake upgrades. Without energy recovery or dynamic braking there is not the retarding force of engine compression, so greater brake wear is to be expected, and brake overheating on a long downgrade is a possibility. For this reason higher performance brakes are preferred in an EV conversion, specifically the use of disk brakes at the front of the vehicle.
Vacuum brake boost system
With the heavy additional weight associated with the batteries it is important that heavier vehicles be equipped with power brakes. As these are (typically) operated by engine intake manifold vacuum a conversion will require a vacuum pump and a chamber to act as a reservoir. Additional control circuitry with vacuum switch(s) and relay(s) to regulate the pump motor is also required and is documented in another section of this book. Pumps may be obtained for EV use from specialty suppliers and some older production ICE vehicles used factory installed pumps.
For a list of production ICE vehicles with vacuum pumps see this site.