Cristóvão Gomes Soares

MSc. Student e-mail: cristovao.soares@ist.utl.pt

Structural Validation of Motor-in-Wheel Prototype for Electric Vehicles

The present thesis intent is to evaluate the concept of ‘one assembly fits all’

and the structural integrity of the prototype developed prior to this document. The

concept is based on integrating several systems inside a wheel, so adding to the

conventional brake system, it will add electric traction, steering and suspension, so

this module could be used as an alternative to the conventional traction on a

conventional vehicle.

The evolution of the motor in wheel technology will be presented,

emphasizing the current market trends.

The concepts involved in the different systems integrated in the wheel are

evaluated.

Forces concerning the operating conditions are estimated through the

approximate static equilibrium condition, based on the solutions presented for the

computational model plate prototype, identifying the most severe scenario.

It is described the motivation and preparation made for the use of sensors

and evaluation of variables of interest in the prototype, concluding with the

expected objectives of the experimental phase, although it was not possible to

perform the experimental evaluation as planned at an early stage. A methodology

for developing a structural part of interest is also presented (belonging to the set of

unsprung weight).

By reviewing the main concepts involved, it was possible to identify the key

issues related with the prototype, concluding that it did not satisfy the conditions to

which it was proposed. Independently to the expected operational characteristics

of the prototype, three options for improvement were successfully developed, (for a

single structural element considered of interest).

Keywords: Drive by wire, steer by wire, motor in wheel, Arduino.

1. Introduction

On present days we assist to a progressive entrance

of hybrid and electric vehicles on the market, this goes to

show how important the alternatives to conventional

energy source to power vehicles are becoming.

In the context of seeking solutions to the processing /

replacement of existing vehicles that mainly use fossil

fuels, a module has been developed for the purpose of

agglomerating the mechanical functions independently of

the existing vehicle construction in which insert.

The developed module, reference (1), (prior to this

document), on the base of the concept ‘one assembly fits

all’ will be evaluated on the dynamic characteristics

expected if implemented on a conventional vehicle, with

the purpose of answering the question if it’s reasonable to

do further improvements on the concept seeking a final

commercial product.

1.1. Objectives

Review of concepts individually affected by the

integration of multiple systems within a wheel, by

comparison to when they are present on a conventional

vehicle. With that information it will be possible to discuss,

in a final phase of this work, if the expected features are

reasonable for the advantages presented.

Independently to the concept evaluation, the

structural integrity of the proposed prototype is also

assessed, taking into account the expected operating

scenarios.

It’s intend to develop / design a structural part, which

could resist the operational scenarios proposed, essential

for the integrity of the module under the estimated

operating conditions.

2. State of the art

Early in car history, around 1898 the very first front

wheel electric drive is sent to market, which used the

motor-in-wheel, build by Ferdinand Porsche (2). Other

electric vehicles have been sent to market at the time with

a similar approach, the case of The Electrolette, built by

Luis Antoine Kriger in 1903 (3). However the fast

development of the oil based propulsion car, mainly

attributed to Henry Ford and his production system,

allowed unbeatable prices on this type of cars. Soon the

electric car technology, mostly batteries, were too

expensive and since then (1908) until recently, the electric

cars were out of mainstream selling.

Currently with the concern of using renewable

energy sources, a window of opportunity has arisen to

hybrid or pure electric cars.

This process of going back to electric is being done,

leading to the arrive of the first hybrid car to Europe in

2000, the Toyota Prius, and only recently, in 2012, was

sent to market the ‘plug-in’ version. Other similar cars

were sent to market by others car makers, but not adding

any innovation relative to Toyota worth mention.

A growing concern on finding an alternative to the oil

based propulsion car, lead to a new interest on motor-in-

wheel technology. In 2008, Michelin revealed a wheel

prototype including an active suspension and an electric

motor inside (4). The predicted market release date for a

mass production car was 2010, which didn't materialize.

Tesla Company is currently the reference brand on

construction and sales of luxury electric cars, with the

Nissan Leaf leading the sales within the medium range

electric cars. Neither of these automotive brands uses the

concept of motor-in-wheel, but both present their electric

cars as an alternative to the standard car (5), (6).

The Shaeffler company developed, in partnership

with Ford Europe, a motor-in-wheel module in a pre-

production vehicle, however, it's in an initial testing phase

without any prediction for a production of the module (7).

The number of companies developing concepts of

motor-in-wheel has been growing, but Protean Electric

stands out with the advanced stage in the development of

their prototype (8). This company, pioneer in the

production of this kind of solution, describes the

importance of this modules with the possibility to include

them in the production structure from a wide variety of

vehicles.

3. Concept revision

The characteristics of the three systems built on the

wheel (traction, steering and suspension) are evaluated,

while in their conventional way.

The main objective that was achieved with the

prototype was the combination of various systems built

inside a 15.5in wheel. Figure 1 through figure 3 show an

embodiment of the proposed architecture where some

components are labeled for clarity.

Figure 1 – Proposed concept.

Figure 2 – Brake and traction system.

Figure 3 – Suspension and steering system.

In the case of an independent steering system

dynamics the opportunity for improvement is referred to

when the lateral acceleration is moderate to high, with low

lateral accelerations an independent steering system does

not provide any additional advantage relative to a

conventional steering system.

The concern from the car makers with the dynamic

characteristics of the vehicles has been increasingly

integrated into all ranges of vehicles, with the choice of

parameters for the different variables being a complex

project.

It should be noted that in evaluating the

performance of a suspension it is necessary to take into

account three objectives: vertical movement of the tire

relatively to the road (or road holding), vibration isolation

and suspension travel (movement relative to the vehicle

body relative to axis of the wheel).

The search of the structure is justified, (belonging

to the unsprung weight), which could resist to the

proposed operating conditions with the intention of

keeping the weight to a minimum.

In a project where several variables are not

favorable, in the case of the prototype under study,

enunciating the suspension travel, figure 4, and the high

unsprung weight, the resulting project is necessarily a

compromise between safety and comfort.

.

With a reasonable inclination of 10°, a vehicle with

1200Kg and 18kW available, can achieve a maximum

speed of 8.8 m/s, or about 31km/h.

In the case of this module being applied to a

conventional car, this module cannot be applied to the four

wheels and a main traction system must be maintained.

Since the main purpose is reducing energy

consumption in the vehicle, one should use the electric

motors at low speeds.

Using only two modules it is reasonable to assume

that the maximum speed will be approximately 30km/h

speed at which a main traction system would come into

action.

The optimum transmission ratio taking into account

previous assumptions is 18.7 when the ratio is 2.4 in the

prototype. With low torque being a characteristic of the

motors, when they work below the reference speed an

high efficiency loss is expected, with less power being

available when the electric motor rotates below 4968 rpm.

4. Computational model-

Simulations

4.1. Curve scenario

It is necessary to distinguish between inner wheel

and the outer wheel to bend the curve. Assuming the car

is in equilibrium with a maximum lateral acceleration the

loading is resumed in the table 1.

Figure 4 – Suspension travel.

Table 1 Load scenario

Loading

Direction Vertical Horizontal

Inner

wheel 1440 N 1440x0.8= 1152 N

Outer

wheel 4560 N 3648 N

The convergence of the tension in the control

point is verified in both conditions, in the case of the outer

wheel condition the tension changed from 765MPa to

736MPa when the used number of finite elements

doubled, in the case of the inner wheel condition remained

the same tension value of 266MPa from the initial mesh

used.

4.2. Braking scenario

To estimate the maximum braking torque available we

begin by admitting the friction factor between the tire and

the asphalt is 0.8, but with this assumption it is also

necessary to estimate the increase in vertical force on the

front wheels when braking, using a rigid model, figure 5,

with the assumption of static equilibrium for the scenario of

maximum braking the loading is resume in table 2.

Table 2 - Forces in brake scenario

Simulation scenario 1

Estimated

values

Vertical force per front

wheel 3384 N

Horizontal force 2707 N

Applied to

the model

Vertical force per front

wheel 3384 N

Additional vertical force 866/0.13=

6662 N

Vertical force

transferred to brake

caliper support

-6662 N

Horizontal force

transferred to the

wheel shaft

2707N

After the process of refining the distribution of

tension in the area of interest is set 250MPa.

4.3. Scenario of extreme lateral

force

The specific scenario, in which the vehicle weight is

supported by two side wheels, figure 6, is removed from

the rollover condition. In many accidents the car rolls

without major damage occurring, and then it is important

for the wheel structure to support the process of

positioning the car in the normal position.

Figure 5 - Center of gravidity in the average distance between the axes

Figure 6 – Side wheel supporting half the weight.

After simulation, it is possible to understand that the

tension and displacement values found do not represent

reality, since the structure would come into plastic

deformation invalidating the results, however, for

comparison between scenarios, these values are

important, showing the need for a geometry modification

of the part in question so it can withstand the loading

estimated

For the improvement project this most critical

scenario, is the basis for the development of the new plate

geometry.

4.4. Electric engine torque

It was previously referenced that the power of an

electric motor is insufficient to tow a vehicle with an

average weight 1200Kg. However the methodology is

made to analyze the forces involved so the structure that

supports them can be designed.

The contact forces at the engagement of the model

are -20.6N in the x direction and 116.9N in the y direction.

As the magnitude of the applied forces is much lower than

for the remaining scenarios, the evaluation of

displacement and tension due to these forces is referred

to the chapter on the proposed improvement of the plate.

5. Improvement of current

structural solution

5.1. Material selection

The strategy of choice of materials can be made

taking into account multiple objectives, the piece is part of

a structural function where minimizing weight means

better combination of safety (road holding) and comfort

(power transfer to the carrier).

The aim racing course with the lowest weight

solution is the solution of lower price. For the choice of

material to be used in the prototype board is necessary to

establish the order of priority of the objectives, so a

definitive choice can be made.

Three scenarios will be chosen for weighting: 90%

of importance for weight and 10% for the price, 50%

between weight and price and finally the material that suits

the weighting of objectives with 10% significance for

weight and 90% for price.

The resulting materials from selection were

Aluminum A206, Low alloy steel SAE 4130, and Cast iron

BS EN 1562:1997.

5.2. Proposed geometry

evolution

The allowable tension to be used in the project is

due to the safety factor specified. The safety factor to be

used in the design will be defined with the value of 2,

considered reasonable by the author.

With the safety factor defined in the design, the

tension admissible in the simulations must be smaller to

approximately half the fatigue limit tension for the material

in analysis. Figure 7 through figure 9 is presented an

example of stress evaluation for different conditions.

Figure 7 – Stress evaluation on plate.

Figure 8 – Stress evaluation on plate, on the assembly.

Figure 9 – Stress distribution for torque related to traction.

6. Final geometry

It was possible to separate areas of the plate to

the various loads to which they are exposed. The

analysis zone from the scenario identifies himself as

braking zone a), the design for the scenario of

extreme lateral force is dependent on the thickness

defined in the zone b) and the area of the structure

that must support the efforts made by electric motors

is defined as a zone c).

For the aluminum plate the final geometry is

presented the thickness values are 13mm for zone

a) 31mm for zone b), and finally 4mm for zone c).

.

Figure 10 - Aluminum plate.

For the cast iron plate the final geometry is presented,

thickness values are 11mm for zone a), 24mm for zone b),

and finally 2.5mm for zone c).

Figure 11 - Cast iron plate.

For the Low alloy steel plate the final geometry is

presented, thickness values are 9mm for zone a), 19mm

for zone b), and finally 2.5mm for zone c).

Figure 12 - Low alloy steel

Price and weight were evaluated and are presented

on figure 13.

Figure 13 Price vs. Weight.

Conclusions

One can understand that due to high costs of

research, the brands that produce electric cars do not use

the modules 'motor in wheel'. The bet has been building

compact transmissions but still within the main volume of

the vehicle. The fact that manufacturers are conservative

about the technology they use, in principle, it is also the

need to keep this return of electric vehicles to the market

by avoiding criticism, even if sometimes unfounded.

With regard to variables within the alignment in

the prototype, it is not possible to separate the angle of the

steering axle from the camber angle, as these results in a

center of rotation that depends on the basic module be

more or less compact. As this result was not taken into

account in selecting the material, the final proposal

consists of three material choices, ie, it is necessary to

consider the consequences before making a final

choice.

The steering system in the prototype presents a

pillar with a squared section which will have a vertical

movement relatively to the 'sleeve' (which are secured to

the plate), the choice of material for the pair should be

carefully considered since it is not proposed any form of

lubrication. In practice, the pillar is a structural element of

high importance where are expected wear problems of

difficult to assess.

It appears to be possible to effectively control the

steering by a servo, provided that it is chosen correctly

there should be a parallel system to prevent catastrophic

failure, namely a failure of one element in the chain of

action in the steering system causes the complete failure

of the steering system. Concepts of reliability are needed

to evaluate and quantify the current security level of the

steering system, however it is understood that the

system is not acceptable since the first car to be

marketed with independent management has parallel

systems of action and a completely mechanical system for

last resort.

It is noted that the selected transmission ratio is

unsuitable for the operating conditions with the

characteristics of the motors used, noting that it is limited

by the distance between shafts.

It can be stated that the study has a prototype

with an expected unsatisfactory performance in the

changed systems built in the wheel.

The work ends with the presentation of three

plates with different combinations of weight and price, any

one of the plates is reasonable to be used as targets.

1. Future work

The sensitivity analysis of the results implies a

high combination of variables, where accounting for how

each variable affects the results is the goal, with the

possibility to adopt the procedures for computational

evaluation, verifying whether or not the considerations are

on the safety side.

On the evaluated concept of 'one assembly fits

all', as was found not be the best approach, is mainly

based on the problems directly related to the steering

system implemented, it is proposed to abandon this goal.

The geometry better suited to meet the needs of

the toroidal torque is, as can be seen in figure 14 (9).

Figure 14 – Torque related to engine geometry (9).

This finding reveals a way forward, because

conventional motors have low torque, thus, for the future

development of an engine for integration into a wheel, this

information should be taken into account.

Still based on the reference (9) it is possible to

verify the need for a braking system, assuming it is

possible to integrate around the suspension, the concept

intended to be the future has the characteristics shown in

figure 15, where the toroidal motor occupies most of the

space available inside the wheel.

Figure 15 – Space management within wheel (9).

Protean Electric has followed this concept, presenting their

module into production this year (2013), which does not

invalidate that research is done, (independently of Protean

Electric), in the form of construction of the two systems

following the same concept.

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