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Trailing arm has its own advantage which makes it different from other suspension arms. When it
goes into droop wheel base decreases and when in bump wheel base increases. This is because the
wheel moves in circular arc about the pivot axis when seen in side view. Semi trailing arm is parallel
or with some angle to the cars longitudinal axis. It is of two type –
Semi trailing arm
Trailing arm – When arm is parallel to the longitudinal axis of vehicle.
Semi Trailing arm - When arm is making some angle with longitudinal axis.
These are used as rear suspension of vehicle. It is basically an arm which has one end that swivels on
the sub frame attached to a spring, kind of like what you get on a motorbike's rear wheel. Its arm is
quite bigger than other type of suspension arms. It is very much rigid at the wheel point which stops
its false movement. There is no chance of camber gain or loss. It has low unsprung mass and made up
of aluminium. When it is compared with double wishbone then, adjustments are definitely possible
with either suspension type, only difference it makes is that it is much easier to adjust A-arms over
trailing arms. Trailing arms get tricky if you want to be able to adjust camber, castor and toe.
Pure trailing arm suspensions have a roll centre at ground level and instant centres at infinity resulting
in as mentioned before no camber change and also a large amount of tire scrub. A semi-trailing arm
suspension allows you to place the roll centre wherever you desire as well as placing the instant centre
in a desirable place. This will allow you to have camber change and somewhat tune the camber curve
based on analysis. Both the camber change and tire scrub will stay constant. The roll centre can also
be placed in a position which allows for reaching a roll gradient that you desire. The semi-trailing arm
maybe a little more complicate then a pure trailing arm but it is simpler than a double wishbone. Both
pure trailing arm and semi-trailing arm suspensions put large loads of the connections between the
chassis and the arm and if the bushing or bearing can deform, the result will be oversteer.
For bushing Baja vehicles have always used Delrin Nylon for all the suspension bushings due to its
durability and ease of machining. In the past it has been proven that Delran Nylon can withstand wear
and provide very smooth connections. Different cheaper material like HDPE (High Density
Polyethylene) bushings starts wearing resulting in compliance in the suspension mechanism
connections. In order to avoid any risks, Bronze “oil impregnated” is used to make the bushings, this
material has a much higher wear resistance yet at the same time it is much heavier.
Another bushing material used is POM (Polyoxymethylene). This material properties are-
High abrasion resistance
Low coefficient of friction
High heat resistance
Good electrical and dielectric properties
Low water absorption
High tensile strength
stiffness and creep resistance
Significantly outperform higher resistance
Allowing for thinner and lighter part design
Offers outstanding low and high temperature performance
Good mating with metal and other polymer.
EQUIVALENT ARM ANALYSIS
The load on the rear axle is transferred equal to
𝐖𝐫 = 𝐖(
b = Longitudnal distance from front axle to centre of gravity
ax = acceleration in x direction
h = centre of gravity
L = wheel base
W = Weight of vehicle
δ1 and δ2 be the tensile and compressive force respectively. δ1 & δ2 is calculated by formula-
δ 𝟏 =
𝐅 𝐱 𝐙 𝟐
𝐙 𝟏 𝐜𝐨𝐬 𝛉 𝟏
δ 𝟐 =
𝐅 𝐱 (𝟏+
𝐜𝐨𝐬 𝛉 𝟐
𝐭𝐚𝐧 𝜽 𝟏 =
𝒁 𝟐 + 𝒁 𝟏 − 𝒆
𝐭𝐚𝐧 𝜽 𝟐 =
𝒆 − 𝒛 𝟐
The maximum and minimum compression and tension was checked in trailing arm.
If you have a single rigid trailing arm instead of two (upper and lower) control arm the intersection of
forces 1&2 is taken as virtually pivot point. And the realation becomes -