A common theme that ran through quite a few emails was: “Is there a difference between the suspensions for an EV and an internal combustion engine (ICE) car?” As you might expect, the answer is “Yes, and No.” Due to the differences in performance and size between an electric motor and an ICE, EV’s present unique challenges but also provide unique opportunities for a chassis engineer.
The Steering Implications Of Ditching The Internal Combustion Engine
Since electric motors are so much smaller than ICEs, there is far more space in a vehicle where the ICE used to be. This space can now be used to package a front trunk or electronic modules that EVs need, but it can also be used to package suspension components in ways not possible before. In particular, the steering gear can now be placed where it is most advantageous, which in most cases is in front of the wheel center. Previously, the engine oil pan dictated where the steering gear could go and for front wheel drive cars with the engine placed sideways (or east/west) this meant that the steering gear was almost always behind the wheel center. Putting the steering gear in front of the wheel center has advantages for steering feel and handling (I’ll talk more about that in a future blog — it’s worth mentioning that rear/mid-engine ICE sports cars like the Ford GT and Porsche 911 have racks mounted ahead of their wheel center lines) so this is a clear advantage of EVs. A good example of this is Volkswagen’s new MEB platform which underpins the ID.3, ID.4, ID.6 and the upcoming replacement for the iconic VW bus (see above). While the previous VW MQB platform placed the steering gear behind the axle centerline (see below), the new MEB platform places it in front. VW clearly saw the advantages of a front placement of the steering gear and took full advantage of the package opportunity provided by the electric motor. Electric motors open up space higher up in the engine compartment as well and could allow for a double wishbone front suspension — which is what Tesla uses — since there are no valve covers or cylinder heads to get in the way. On the other hand, electric motors are larger than a differential so there is generally less space available in the rear of EVs. This can make packaging all the necessary rear suspension parts more challenging in an EV that has a rear motor.
Electric Cars Introduce Surprising Engineering Challenges, Like Nuts Coming Loose
Of course electric motors also have their challenges. One issue we ran into during the development of one of the high powered EV’s I was involved with came as a complete surprise to all of us. We noticed during our testing that the nuts holding the CV joints to the rear suspension kept coming loose. No matter how hard we tightened them, they would still come loose. After a lot of work we discovered that the axle nuts were coming loose due to the high torque reversals happening when going from “throttle” on to “throttle” off. Since EVs use regeneration to recharge the battery during braking and deceleration, there are large reversals in drive torque on the “throttle” vs off the “throttle.” No ICE would ever produce such high reversals of torque so frequently. Our solution was to use a special type of lock washer made by Nord-Lock under the CV joint nut. These washers have a unique design that causes the nut removal torque to be significantly higher than the installation torque. Problem solved! For the most part though, the suspension for an EV can be very similar to an ICE suspension. The same things are important: ride, handling, comfort, and those are not dependent on the type of motor that is used. You will notice that the suspension systems in EVs look a lot like those in ICE cars with one exception.
Death Of The Live Axle?
You may have noticed that there are no EV’s at the moment with live axles. This may change in the future, but there are very good reasons why live axles are not ideal for EV’s. The first is that live axles need a drive shaft that moves up and down with the suspension. Unfortunately, this driveshaft runs down the middle of the car in the same place where it would be ideal to package batteries. Batteries take up a lot of space and are heavy so you want them to be mounted low and as close to the center in the car as possible. Unfortunately, that’s also exactly where the driveshaft is. Having a moving driveshaft would take away too much space for the batteries which would hurt range too much. A perfect example is the new Ford F-150 Lightning. Ford ditched the live axle (shown above) and put in a new independent rear suspension just for the Lightning (see below). I think Ford knew it could never make the Lightning work well enough as an EV without going the extra mile designing a whole new suspension. Believe me, Ford would never have spent that kind of money on the F-150 if it didn’t believe it was absolutely necessary. I think you will see this happen more often as existing vehicles are converted to EV. The other reason is that a traditional differential with the typical ring and pinion gear — a pairing that turns the power 90 degrees — is not a very efficient beast. The ring and pinion depend on a sliding action between the gears in order to minimize noise, and this adds friction. Friction is an energy loss and EV’s are all about minimizing energy losses so that you get as much range out of a battery charge as possible. I will say that there are some companies out there developing electric motors that are mounted on a live axle (see below) but as a suspension engineer, I would be very concerned about the increased mass from these motors. Unsprung mass is the enemy of good ride and handling and is the reason live axles are not in use much anymore. Increasing unsprung mass with the addition of an electric motor to the axle would be the wrong way to go, in my opinion. I don’t think we will see many companies building EV’s with live axles unless they are desperate to get into the EV market with an existing product and they just don’t want to spend the money on a new suspension. I highly doubt the resulting product will be as competitive as it could be. Well, that’s it for this week. Keep sending in your questions to AskanEngineer@autopian.com and we’ll see you again next week. Same Bat time, same Bat channel! (oh boy, I’m really showing my age now). Motor-in live axles may be bad for clean sheet designs, but they seem great for drop-in conversion projects. I believe front or rear steering linkages are different because of the caster angle where a wheel hub attaches to the suspension. I’m thinking the angle between attachment points creates a virtual center the hub rotates around and that placing the steering point in front allows it to be closer to that virtual center while the rear may be physically blocked from being as close. So the front steering linkage would take less torque to move the wheel and have a quicker response no matter how the movement is initiated. To be clear I’m not an engineer and the only source I have for this is imagining suspension parts moving around so a few grains of salt might make it easier to swallow, lol. We’ll see! I have smashed my knuckles many, many times at work undoing nuts and bolts secured with a nordlock. Each time you know it’s going to happen but the abrupt release means you can’t easily avoid it. Due to the AWD, Subaru engines are pushed entirely in front of the front axle. So there’s still room for the rack behind the oil pan, but still be in front of the wheel. Which brings me to today I was again looking at the new Nissan Z and the leading calipers stood out me which they never have before when looking at the car. This lead me to assume the steering rack is behind the wheel center. I then started thinking this must be why most performance vehicles have trailing calipers. A simple detail I’ve overlooked for years now makes sense and stands out because of this article. “We noticed during our testing that the nuts holding the CV joints to the rear suspension kept coming loose. No matter how hard we tightened them, they would still come loose. After a lot of work we discovered that the axle nuts were coming loose due to the high torque reversals happening when going from “throttle” on to “throttle” off. Since EVs use regeneration to recharge the battery during braking and deceleration, there are large reversals in drive torque on the “throttle” vs off the “throttle.” No ICE would ever produce such high reversals of torque so frequently. Our solution was to use a special type of lock washer made by Nord-Lock under the CV joint nut. These washers have a unique design that causes the nut removal torque to be significantly higher than the installation torque.” My solution was the same, with the addition of reducing max regen torque via reprogramming the controller. I have deceleration via regen comparable to the engine brake in an automatic transmission equipped ICE car when I activate the rear brake lever. Currently my rear wheel is unsuspended while my front wheels have air suspension. I designed it to where my weight is centered over the front wheels so its ride quality is quite acceptable without rear suspension, sort of akin to a vintage sports car or a cheap truck. I’m eventually going to hack the rear apart to add a gas shock to it as well. My 90 lb vehicle has a 17 lb electric hub motor in the rear wheel, and you can notice all of the unsprung weight when that rear wheel hits a pothole. Once I have a rear suspension on it, I’m confident ride quality and handling will improve greatly. For those interested, there are pics of the vehicle in my profile. I’m going to build one capable of topping out at 100+ mph when using the motor, and will be aerodynamically slippery enough to reach 45+ mph on flat ground pedaling it with the motor disabled. It’s going to need full suspension, light-duty motorcycle rims, solar race car tires, a roll cage, hydraulic disc brakes, Cotter pin axles, and other changes to be able to reach this speed without killing me. And I’m going to keep the entire vehicle under 100 lbs to retain its pedalability with the motor disabled. 10 kW of electric assist should be good for 0-60 mph in around 7 seconds. The idea is to eventually build a car without any bicycle drivetrain, and shoot for 1 horsepower per pound of vehicle weight, with an electric hub motor in each wheel for AWD without any driveline losses. Such a thing would have ridiculous acceleration. If the vehicle weighs 100 lbs, carries a 200 lb load(rider + tools + luggage), and has about 150 horsepower with AWD, 0-120 mph acceleration could be somewhere around 5 seconds… If I ever build a custom two-seater sports car, it will have offset seating like a VW XL1, an overall size comparable to a Porsche 550 Spyder, and be a streamliner with a Cd in the mid 0.1X region. I’d start by scaling a Milan SL velomobile into a 4-wheeled 2-seater vehicle. The front track would be wider than the rear track, in order to keep the shape needed. Whatever styling there was would ape the Jaguar D-Type, Ferrari 250GTO, 2017 Ford GT, Alfa Romeo Disco Volante, and Alfa Romeo BAT concepts, and function would be the emphasis of the design, not looks. There would be just enough downforce for stability at top speed, and no more, in the interest of keeping drag as low as possible, mostly through ground effects and a rear diffuser. With glass fiber monocque sandwich construction with innegra fiber, it’s possible to have a complete vehicle weighing around 800-900 lbs with front and rear independent suspension lugging around a 20 kWh battery pack, while having mechanicals solid enough to handle 160 mph. It would be light enough to get away with using an overpowered ebike motor, cheap Chinese microcar hub motor, or electric motorcycle hub motor in each wheel. Putting 150 kW to the ground with AWD would be amazing, and it would be light enough that those tiny motors could prove reliable, considering they will each have less load than a single-motored 100 lb ebike or 400 lb e-motorcycle carrying 200 lbs. And it would only need about 40 kW to maintain top speed of 160 mph if it got a CdA of about 0.15 m^2. With an ebike controller powering each wheel motor, instantaneous slip detection and vector control could allow a setting for the driver to lightly press the accelerator for it to maintain constant speed through a corner, maximizing lateral grip. Unfortunately, I don’t know crap about building a monocoque chassis from scratch yet. Even my microcar is a body on frame design. For now, I’ll have to settle with finishing the electric Triumph GT6, which once I have the aeromods in place, will require twice as much power for the same performance as the above hypothetical 2-seater concept. Except the GT6 is not initially going to have anything close to that, maybe 100 kW peak for a 950 kg mass, for 0-60 mph acceleration that should be comparable to a new Miata, so it should still be quite fun. But won’t be embarrassing any supercars. Maybe it will if I later upgrade to a drive unit from a Tesla Model 3. It’s a work in progress and will eventually have working turn signals, windshield, roof, be fully enclosed, and with a much more slippery body based upon a Milan SL velomobile I purchased last year. I want to build the ultimate low-cost transportation appliance, and I also want it to have some very spirited performance. The current iteration accelerates like a sports car from 0-10 mph, like a slow car from the Malaise era from 10-30 mph, and tops out around 50 mph if the battery is fully charged. I measured a 0-30 mph acceleration time of 6.5 seconds, which is quite slow by car standards, but do consider that it only has 2.5 kW peak at the moment. It can lurch ahead of a Porsche Boxter or a modern V6 Mustang at a stop light if I’m careful enough not to lose traction and I won’t get overtaken until about 10-15 mph. It definitely has enough torque to get going, I just don’t yet have enough power to produce that torque to an appreciable speed. It will get a lot faster once I make some necessary upgrades to it. It’s a death trap for sure. The reason that diesel hybrids aren’t more popular is that diesel engines and electric motors don’t complement each other as well as gas and electric. Electric motors are best when gasoline engines are worst, moving the car from a stop. Gasoline is in its power band when electrics are running best when electrics are less efficient. Electric motor and diesel motors are both best in the same part of their power band at the same times, so there is diminishing returns on combining the two. I look forward to the explanation on forward mount steering Some heavy vehicles mount the electric motor behind the rear axle, between the frame rails and use a short drive shaft. This keeps the center of the vehicle open for batteries. How do we edit or delete comments on mobile? “Our solution was to use a special type of lock washer made by Nord-Lock under the CV joint nut. These washers have a unique design that causes the nut removal torque to be significantly higher than the installation torque. Problem solved!” As long as the two halves stay concentric while being torqued! This has caused me a lot of pain/reliability investigation. I thought you were just going to mention that they’re heavier and maybe unsprung weight of the low-mounted battery pack or something. anecdote: the only guy in our group who ran IFS had a Chevy 2500 and was constantly breaking tie-rods. he finally beefed them up so they wouldn’t break and then ripped the idler arm bracket off the frame. I think he made it into one of BSF recovery team’s videos on youtube. With the trend towards cartoonishly large wheels, how does the added unsprung weight of wheels play into the design here? I understand, large wheels can offer clearance for larger brakes, but with regenerative braking, I would imagine EVs could move on from this. Or does the significant weight addition of batteries negate this and reintroduce the need for larger conventional brakes too? Giant wheels just seem to be such a packaging waste needing larger wheel wells and steering clearances.
