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Portsmouth University Study on Bamboo Species
Published: 26th June 2014
Written by: James Marr
We recently helped Portsmouth University with a Mechanical study of the two bamboo species that we use. We have carried out a number of test ourselves with the resources available. Combined with thousands of miles of riding and the experience we have had of teaching building bamboo bikes over the last four years.
The study looks at mainly environment impacts on bamboo. All of our bikes are fully sealed against moisture however with poor maintenance etc. exposure to the elements can occur.
Below is a brief summary of the report, to download the full report and summary poster click the links below
Britain has experienced a cycling boom in the latter half of the 21st century, the number of bicycle journeys made in Greater London has doubled between 2000 and 2012 to 540, 000 per day. Bamboo Bicycle Club aim to create a knowledge sharing community for building robust and safe bamboo bicycle frames. Bamboo is one of the fastest growing plants on the Planet & one of the most common natural building materials used today. With the growing popularity in the use of sustainable materials bamboo is a diverse and robust material.
Bamboo Bicycle Club has provided two species of bamboo for mechanical testing. Both species of bamboo are used in workshops which enable their customers to build their own, hand-made, custom bicycle frames. It is our aim to test samples from their material stock pile and provide them with experimental data which will back up any research they may have undertaken themselves.
This project aims at investigating the mechanical properties and environmental behaviour of two species of bamboo supplied by Bamboo Bicycle Club: Saturated (wet) & Unsaturated (dry) samples tested in Compression, Tension & Three Point Bending. The following objectives have been set In order to achieve these aims.
- Devise an effective method of mechanically testing bamboo through research & experimentation
- Performing accurate tests to obtain the mechanical properties of saturated and unsaturated bamboo samples
- Provide Bamboo Bicycle Club with concise publishable data which meets their business needs
5.1 Water Absorption
From the ambient moisture content calculations, it was found that difference between noded and un-noded specimens for both species was around 1%; Tonkin had a ambient moisture content of 6.7-5.8% Moso was slightly less, from 5.5- 6.4% for noded and un-noded specimens respectively.
The difference in diffusion coefficient (D) for noded and un-noded specimens was almost negligible. Tonkin saturated with a weight increase of 48%; Moso saturated with a weight increase of around double that of Tonkin (around90%) which accounts for these unusual results. Tonkin would be the most desirable species for bicycle application, as it would be the least affected by moisture uptake during rain or from surface moisture. To reduce moisture uptake, several layers of lacquer could be applied to seal the frame.
Tonkin Bamboo has a higher stiffness than Moso Bamboo in all conditions and the performance degrades less than Moso with increased moisture content for noded samples. The degradation of performance for Tonkin is roughly equal whether the sample has nodes or not, contrasting to Moso which degrades more if a node is in the sample. Samples without nodes for Moso bamboo do not appear to degrade. The ultimate tensile strength for Moso with nodes is slightly higher than Tonkin – 253 MPa compared to 240 MPa. Samples without nodes show that the tensile strength is ~40% lower at just 181 MPa where Tonkin is only 2.5% lower at 235 MPa. The ultimate tensile strength reduces a lot for both species for noded samples and samples without nodes for Moso Bamboo. Tonkin samples without nodes does not have a reduction in tensile strength. Both species of bamboo are therefore applicable with Tonkin being the safer option.
In the case of a Bamboo bike frame the ideal species to use would be a Tonkin bamboo piece with no nodes throughout it. This would be the ideal material to use for the down tube of a bike frame due to its high compressive strength and Compressive Modulus. The material is also more consistent when fully saturated in comparison to Moso samples.. The performance of Tonkin when saturated compared to that of Moso shows that Tonkin would be a more suitable species for countries with higher levels of rainfall. Moso is still a useable material for a bike frame but is less consistent in its performance when saturated. Moso bike frames may require thicker columns when being used for core structural tubes. To counteract problems found with saturation, an extra layer of lacquer or a more water resistant lacquer can be applied to Moso tubes.
5.4 Three Point Bending
Due to a bike frame being under many varying conditions, such as multiple loadings, or different running conditions, typical support tubes are not commonly in a purely tensile or compressive state. Because of this, three point bending is an accurate representation of conditions the frame would find itself in, and therefore can be considered a suitable method of giving performance indicators for each species. It should be noted for safety purposes, that unlike other typical bike frame materials such as aluminium or steel where failure tends to be ductile, the tests indicate that bamboo initially fails in a brittle fracture mode. For normal operating conditions these yield values are high enough to not merit any concern, however in a worst case scenario such as a high impact crash, the material could fracture rapidly, potentially causing further injury to the user due to splintering. Therefore greater understanding of frame loads in impact conditions would be desired before progressing with this material selection.