As an approximate way to compare chassis designs a truss analysis program has been used to run
comparative theoretical torsion tests.

The GRAPE GBW32 (Version 4.0) analysis program was used to run these tests.

For each chassis a 3D DXF file was created in AutoCAD defining the particular chassis geometry.

This was loaded into GRAPE and materials, loads and constraints were applied.

The image below shows the configuration of loads and constraints. The four corners of the lower rear
chassis frame were constrained to be fixed in space. Two loads were applied to the top front frame.
These were each 1000 lbs in magnitude, but applied in opposite directions. The result was to apply a couple
to the front of the chassis (the two applied loads act to produce a pure torsion). The specific material
properties for the tubing being used was loaded into the GRAPE material
library, then applied to the respective chassis elements.


The practical application of this to a real chassis is to mount the rear of the chassis firmly to the ground then
apply an offset load to the front frame and measure the deflection of the front frame under the applied load.
This is the basis procedure used to test the torsional strength of real chassis. A number of example
chassis designs were tested in this way. A brief summary of the results is as follows...

 

Chassis Design Torsional Stiffness Notes

1
1X
2
3
3X
3Y
5
7A
7B
8A
8B
8C
9A
9B


1219
1129
1846
1787
2175
2161
1205
1787
485
127
161
152
187
200


  Locost 7 with all square tube
  Locost 7 Standard Chassis
  All 25 mm square tube
  All 25 mm square tube
  Vertical RHS tube added
  Horizontal RHS tube added
 
 


 
 
  Backbone chassis
  Backbone chassis

  lbs.ft/degree  


The material assumed for these tests was C350 grade mild steel with the following
mechanical properties,

Property Value

Young's Modulus of Elasticity  
Density  
Poisson's Ratio  
Minimum Yield Strength  


  290 x 10^3 lb/sq.in.
  0.2836 lb/cu.in.
  0.25
  50,762 lb/sq.in.

The Type 7, 8 and 9 chassis were part of a design exercise to see the effect of different
layouts. These generally followed a path between the typical Clubman spaceframe chassis and a backbone
chassis, as might be found in a Cobra-replica. All the designs used 25SHS tubing. The outcome of this test
is that a backbone chassis would be about twice the weight of a Clubman chassis for a similar torsional
stiffness. The benefit of the backbone chassis is a body with functioning doors can be used while a
Clubman requires entry  over the side panels. A practical backbone chassis will require heavy gauge
tubing in order to achieve acceptable torsional stiffness (much larger than the 25SHS tubing assumed here).

 


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