VEHICLE DYNAMICS - WHAT IS IT ALL ABOUT ?
Altering the suspension a good idea? What will happen if you alter, try to improve the ground clearance or want to put a fancy or bigger and wider tire? Or more on the latest trend of ICE to EV conversion which alters the vehicle dynamics if not played well?
Let's talk about few things to be considered and things to keep in mind before carrying out any modification to an OEM design.
I will be talking more in terms of suspension, increase or decrease in ride height. We can always relate with the kind of modification we have carried out and then compare the scenario.
Suspension
Motorcycle's suspension serves a dual purpose: contributing to the vehicle's handling, braking and providing safety, comfort by keeping the vehicle's passengers comfortably isolated from road noise, bumps and vibrations.
The typical motorcycle has a pair of fork tubes for the front suspension, and a swingarm with one or two shock absorbers for the rear suspension. Altering the length of the fork, raises/lower the front of a motorcycle and thus increase or decreases its head angle. Lengthening the fork would have the opposite effect on the rake of a motorcycle, since rake is measured in the opposite direction.
Under braking condition, the front fork arrives to support up to 80% of the bike weight and the braking forces: the static load can be tripled. Whilst doing this, it will have to keep its ability to absorb road shocks, in order to ensure the maximum braking efficiency.
Shock absorber/Damper
A damper is simply an energy absorber. This energy loss is necessary to prevent uncontrolled oscillations in the suspension. The damping force is generated inside the cartridge, using the resistance of oil passing through valves and orifices.
Camber angle
Camber angle is the angle between the vertical axis of the wheels used for steering and the vertical axis of the vehicle when viewed from the front or rear. If the top of the wheel is farther out than the bottom (that is, away from the axle), it is called positive camber; if the bottom of the wheel is farther out than the top, it is called negative camber.
Camber angle alters the handling qualities of a particular suspension design,
Caster angle (Rake angle)
Caster angle is the angular displacement of the steering axis from the vertical axis of a steered wheel in the vehicle, measured in the longitudinal direction. In automobiles, the caster angle may be adjusted to optimise handling characteristics.
Excessive caster angle will make the steering heavier and less responsive. Lesser caster angle will make the steering quick and calls for sharper controls. When the front suspension of a vehicle is aligned, caster is adjusted to achieve a self-centring action in the steering, which affects the vehicle's straight-line stability. Improper caster settings will require the driver to move the steering wheel both into and out of each turn, making it difficult to maintain a straight line.
Result: Improper caster settings will require the driver to move the steering wheel both into and out of each turn, making it difficult to maintain a straight line.
Rider effects: Lesser rake will make the steering quicker which actually means the steering will be more eager and sharper for inputs. Lesser experienced riders will be caught unnoticed. Higher rake will make the steering slower will be more reluctant in turning in.
Ride height (Ground clearance)
Ride height is the amount of space between the base of an automobile tire and the lowest point (typically the axle, centre stand or exhaust header in this case); or, more properly, to the shortest distance between a flat, level surface, and the lowest part of a vehicle other than those parts designed to contact the ground.
A higher ground clearance means that the centre of mass of the vehicle is higher, which makes for less precise and more dangerous handling characteristics. Higher ride heights will typically adversely affect aerodynamic properties also. A lower ground clearance is exactly the opposite and make the underbody of the vehicle vulnerable to damage.
Result: Handling characteristics will change. So, when we increase the suspension height, it will be difficult to control the vehicle. Shorter riders will find the seat height cumbersome. The higher clearance has direct repercussions to vehicle dynamics. Decreasing the suspension height will provide more stability, but the rider will be more cramped and obvious problems of scrapping the underbelly.
Rider effects: Taller seat height of 15% have direct effects on ride ability. Ground clearance is a definite bonus by sacrificing vehicle dynamics.
Roll centre
The roll centre of a vehicle is the notional point at which the cornering forces in the suspension are reacted to the vehicle body. The location of the geometric roll centre is solely dictated by the suspension geometry.
Result: Turning radius will change due to the increase or decrease in the height of the suspension. The results will clearly show the lesser rake angle or higher rake angle will make the turning radius smaller or larger respectively.
Rider effects: Lesser turning radius of say 5% will catch the rider surprisingly as the steering as well as the vehicle will be more eager to dive into the corner and make U-turns difficult. The sudden eagerness of vehicle if unattended will end up in a fall. Larger turning radius of say 5% will also yield a similar result as the vehicle will be more reluctant to turn in and might go wide if not controlled properly.
Centre of mass (Centre of gravity, CoG)
Manufacturers try to design vehicles so that its centre of mass is lowered to make the vehicle handling better, that is maintaining traction while executing relatively sharp turns. Centre of mass is that point at which a system or body behaves as if all its mass were centred at that point. Where the weight, and also all accelerative forces of acceleration, braking and cornering act through it.
Moving up the CoG we have more weight transfer in braking and accelerating, but less leaning angle at a certain speed. Moving forward the CoG we have more weight in the front: less stability of the rear wheel when braking, less wheelie in acceleration, but even less traction on the rear tire. Moving backward the CoG we have more weight in the rear: more stability when braking, more wheelie in acceleration, more traction on the rear wheel.
Result: The alteration of suspension or the increase or decrease of ride height will shift the CoG higher or lower respectively which already altered the vehicle dynamics and braking dynamics.
Rider effects: Up to 10% variation in CoG won’t be noticeable to novice riders, though aggressive riding and advance riders will meet it with an unpleasant dynamic. If the CoG have moved upwards, the vehicle will be top heavy and in turn will assist the sharper rake angle and make the vehicle more eager to drop into corners. The top heavy nature will force the vehicle to lean further on a turn and the vehicle will no longer be nimble.
Roll moment
In a vehicle suspension, roll moment is the moment of inertia of the vehicle's sprung mass (the portion of its weight supported by the suspension).
The roll moment is the product of the sprung mass and the square of the distance between the vehicle's roll centre and its centre. If the vehicle is subjected to centrifugal forces, such as in a turn, the roll moment will cause the body to rotate (lean) towards the outside of the turn.
Result: Stability of the vehicle around corners and at high speeds will be less if we increase the ride height.
Sprung mass
Sprung mass in a vehicle with a suspension such as a motorcycle is the portion of the vehicle's total mass that is supported by the suspension, including in most applications approximately half of the weight of the suspension itself.
The sprung mass typically includes the body, frame, the internal components, passengers, and cargo, but does not include the mass of the components at the other end of the suspension components.
Rider effects: Minor change in the sprung mass up to 5% which may go unnoticed.
Un-sprung mass
The un-sprung mass is the mass of the suspension, wheels or tracks and other components directly connected to them, rather than supported by the suspension.
Un-sprung mass includes the mass of components such as the wheel axles, wheel bearings, wheel hubs, tires, and a portion of the weight of drive shafts, springs, shock absorbers, and suspension links. The un-sprung mass of a wheel offers a trade-off between a wheel's bump-following ability and its vibration isolation. Bumps and surface imperfections in the road cause tire compression, inducing a force on the un-sprung mass. The un-sprung mass then reacts to this force with movement of its own.
Result: The larger the ratio of sprung mass to un-sprung mass, the less the body and vehicle occupants are affected by bumps, dips, and other surface imperfections such as small bridges. However, a less sprung mass to un-sprung mass ratio can also be deleterious to vehicle control.
Rider effects: Minor change in un-sprung mass around 5% which may go unnoticed. Lesser the unsprung mass, better the handling.
Weight Distribution
Weight distribution is the apportioning of weight within a vehicle. Typically, it is written in the form x/y, where x is the percentage of weight in the front, and y is the percentage in the back.
In a vehicle which relies on gravity in some way, weight distribution directly affects a variety of vehicle characteristics, including handling, acceleration, traction, and component life. For this reason, weight distribution varies with the vehicle's intended usage.
Result: Modification of suspension or alteration of ride height of the vehicle will alter the weight distribution towards the rear or front which makes the front-end loose traction easily or the rear to step out in braking.
Rider effects: A change of 10% F/R ratio will make the steering lighter by 10% which will also reduce the traction by 10% which will also reduce the combined braking performance up to 15%. A similar weight distribution to the opposite direction will also have a similar impact.
Cornering force
Cornering force or side force is the lateral (i.e., parallel to the road surface) force produced by a vehicle tire during cornering.
Cornering force is generated by tire slip and is proportional to slip angle at low slip angles. The rate at which cornering force builds up is described by relaxation length. Slip angle describes the deformation of the tire contact patch, and this deflection of the contact patch deforms the tire in a fashion akin to a spring.
Result: Due to the altered suspension design or the changed ride height, cornering will became more stiff or soft respectively.
Slip
Slip is the relative motion between a tire and the road surface it is moving on. This slip can be generated either by the tire's rotational speed being greater or less than the free-rolling speed (usually described as percent slip), or by the tire's plane of rotation being at an angle to its direction of motion.
Result: Contact with the ground is minimum and therefore the control and road holding is less.
Slip angle
In vehicle dynamics, slip angle is the angle between the direction in which a wheel is pointing and the direction in which it is actually traveling.
If the ratio of front to rear slip angles is greater than 1:1, the vehicle will tend to understeer, while a ratio of less than 1:1 will produce oversteer. Actual instantaneous slip angles depend on many factors, including the condition of the road surface, but a vehicle's suspension can be designed to promote specific dynamic characteristics.
Result: The slip angle is go less than 1:1 because of the rake angle if the rear is lifted or having a longer suspension travel. The vehicle will show the tendency to oversteer. The slip angle will go greater than 1:1 again because of the rake angle if the front side is lifted or having a longer suspension travel. The vehicle will show the tendency to understeer.
Body flex
Body flex is a lack of rigidity in a motor vehicle's chassis. It is often something which will be avoided by the manufacturers as higher levels of body flex is a sign of structural weakness, and means that the vehicle's suspension cannot work as efficiently - the body takes up some of the 'slack', rather than the parts of the vehicle which were specifically designed for this purpose. A chassis that flexes may be prone to fatigue and further "softening" with use will eventually result in failure.
Result: Altering the suspension travel compared to the OEM design will have minor to major shift of stress concentration, and more loads are transferred to the chassis or vice versa. This will eventually may result in fatigue and early failure.
Rider effects: 1 out of 50 tests by many testing authorities have showed a failure which puts the failure as high as 10% which have to avoided at any cost before implementing the design modification.
Body roll
On wheeled or tracked vehicles, body roll is the load transfer of a vehicle towards the outside of a turn.
When a vehicle is fitted with a suspension there is compliance between the mass of the vehicle and the vehicle's contact with the ground. Body roll is the noticeable deflection produced when load transfer acts on the compliant elements of the suspension.
Result: There will be a noticeable deflection on the vehicle when load acts on the vehicle if there is an alteration on the ride height or suspension and the handling of the vehicle gets troubled immediately
Rider effects: The body roll around 7% more is countered by the stiffer springs in some case, so any change is negated though the rebound characteristics make the vehicle unstable. This part requires a lot of calculation to come to the perfect set up.
Directional stability
Directional stability is the stability of a moving body or vehicle about an axis which is perpendicular to its direction of motion.
Stability of a vehicle concerns itself with the tendency of a vehicle to return to its original direction in relation to road surface which when disturbed (rotated) away from that original direction. If a vehicle is directionally stable, a restoring moment is produced which is in a direction opposite to the rotational disturbance. This "pushes" the vehicle (in rotation) so as to return it to the original orientation, thus tending to keep the vehicle oriented in the original direction.
Result: Changing direction quickly resulting from a sharper rake angle will result in loss of vehicle control. And with some calculations, there is a 5% chance of miscalculating & falling from vehicle.
Rider effects: The rider has to be aware of quicker steering if the rake is made sharper. In high speed riding conditions, a steering damper is fitted to take care of the sharp movements.
Ride quality
Ride quality refers to a vehicle's effectiveness in insulating the occupants from undulations in the road surface (e.g., bumps or corrugations). A vehicle with good ride quality provides a comfort for the driver and passengers.
Result: If the suspension is made stiffer, the change and settings will definitely hamper the ride quality to a greater extent. The tradeoff to this set up is the vehicle will become less wallow and will stick to the line better in time of cornering. It’s the trade-off for stiffer suspension
Rider effect: Minor shift in stiffness won't create an immediate noticeable effect, but it may also transfer the road shocks to joints, bolts etc and eventually create premature failure of parts.
Speed wobble
Speed wobble is used to describe a quick oscillation of primarily just the steerable wheels of a vehicle. Initially, the rest of the vehicle remains mostly unaffected, until translated into a vehicle yaw oscillation of increasing amplitude producing loss of control.
Result: The top five influences on wobble have been found to be lateral stiffness of the front tire, steering damper, height of bike centre of mass, distance of bike centre of mass from rear wheel, and cornering stiffness of the front tire etc.
Rider effect: A 10% increased chance of speed wobble is can be quantified.
Now seeing all these, one may think OMG, modification is a strict no no. To be honest; yes, the engineers at OEM must have done so much calculations and combinations to come up with a final solution and have tested it till failure. There is a reason why our authorities are against modification to the vehicles. Minor changes can be made as there will always be a 5% factor of safety kept by the OEM on every parameter, but to understand which one alter what parameter requires a professional in after market space.
So do wisely and stay safe !
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