Load Paths in Structures - 3, Bicycle Example by Kotur Raghavan
An earlier article (https://www.fembestpractices.com/2020/10/load-paths-in-human-body-by-kotur.html
) focused on the load paths in human body for two different cases of loading
conditions.
A related article focused on the
importance of load path. The components in the load path get stressed. In a
human body the stress manifests as ‘pain’.
(https://www.fembestpractices.com/2020/10/human-body-further-structural-insights.html)
In the present article we will
load path in respect of a commonplace thing, a bicycle.
Fig. 4.1, A Bicycle,
Component Parts and Classification.
A typical bicycle is shown in Fig
4.1. Items labelled L are all load bearing parts. They are the seat, the central
frame (letter L with yellow fill) along with the wheels (including spokes), the
handle and the pedal. Brake cables and chain drive form the functional part.
Mud guard and the chain guard are safety features. The parts of the bicycle are
assembled in such a way that the functional and safety components do not
support load in proper usage.
Fig. 4.2 Load Path
In order to have more clarity on
load path, let us consider the example of a Bicycle with a single rider of
weight W. Let us also assume that the only load of reckoning is the weight of
the rider (W). We will understand the concept
with reference to Fig. 4.2.
There are three locations of
physical contact between the rider and the bicycle – the seat, the handle and
the pedals and they form the load application points. The total weight of the
rider gets applied on to the bike in three parts WS, WH and WP and the sum of
them will be equal to W. Their relative proportions depends on the sitting
posture and the style of riding. For example, during slow relaxed riding we can
expect that most of the weight acts on the seat and WS will be relatively be very
high. At higher speeds or while climbing a gradient riders tend to leave seat
and thereby apply more load directly on the pedals WS may be zero. Likewise the
relative values of reactions RR and RF are also dependent on the posture and
riding style.
SIMULATION MODELS
A simple working structural model
will be as shown in Fig. 4.3.
Fig. 4.3, Working
Structural Analysis Model.
All the load carrying components
are included. One-dimensional beam elements will simulate well the structural
behaviour of the main frame. Here it is assumed that a single spoke in each
wheel transmits the load from the bearings (BF and BR) to the road contact
points RF and FF. In reality the wheel assembly is quite complex because of the
spokes are in pretension. In any case, the model is realistic up to the
bearings BR and BF. One can analyse the bicycle for its structural behaviour
using the model shown in Fig. 4.4.
Fig. 4.4 Simpler
Structural Model
Here the model is truncated at
bearings and the points are constrained. It is to be noted that the reaction
forces at bearing locations will be same as those at ground contact points in
the first model. Such simplifications are frequently introduced in structural
analysis. They call for engineering judgment and a feel for the behaviour of
structures.
It will perfectly alright to carry out an
independent analysis of the wheel assembly using the reaction forces at BF and
BR and support at the ground level.
Key Takeaways:
1. 1. Further
insight into load paths in structures.
2. An insight into development of simulation models. The importance of engineering judgment is emphasized.
Sir,
ReplyDeleteGreat and very simplest load transfer schematic.
One query I have.
Load transfer from BR to FR & BF to FF is tensile or compressive load?
Spokes of the wheel takes only tensile load as per the assembly of a typical bicycle.
Thank you.
Thank you.
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