Causes, Prevention and Fixes

Causes:

Splitting of bamboo is caused mainly by rapid changes in moisture content of the bamboo. This

can be caused by rapid changes in relative humidity of the atmosphere, or the bamboo being dried

unevenly, or getting soaked and then drying out too quickly. Some bamboo species are more

prone to splitting than others. And thinner walled bamboo is more susceptible to splitting. Lastly,

the septums of the bamboo (those little walls at each node) can cause splitting even if the bamboo

dries out slowly. The septum does not shrink along with the rest of the bamboo when it dries and

it resists, causing a split right at the node. That’s why we drill out the septums.

Another cause of splitting is uneven wall thickness, which is caused by the bamboo leaning at

more than 45 degrees while it grows. Those sections are found in the lower half of the “leaner”

and can split because it dries out unevenly – similar to “reaction wood” in wood working.

Prevention:

The best way to prevent the bamboo from experiencing rapid changes of moisture content is to

properly dry the bamboo and later, seal it with a waterproof coating. The best coatings are two

part automotive clear coats. But special equipment and a practiced technique are required for

those. Epoxies also work but as a water proof outer coating, will turn yellowish from solar UV

exposure and won’t protect casting tape, which also breaks down under UV exposure. Of the rattle-

can sprays, the UV resistant Krylon clear seems to hold up the best. Before coating, it is very

important to sand or peel the outer skin off the bamboo because that is made of a waxy material

that resists any coating.

With bamboo bicycles, riding in the rain can allow water to accumulate inside the seat tube if you

ride without a rear fender (mudguard). Water can splash up to the back side of the seatpost and

drip down into the slot of the seat binder, making it’s way to the bottom of the seat tube, where it

can collect and eventually soak into the bamboo. Preventing water from entering the seat tube

with a silicone sealant at the slot can help. Another method is to allow the water to enter but

provide a drain hole at the bottom of the seat tube for it to exit. The professional method is to fill

the lower half of the seat tube with an expanding polyurethane foam and paint the inside of the

upper portion with a waterproofing sealer. This also seals up any passages between the seat tube

and the down tube.

Fixes:

There are three levels of splitting: minor, small and severe.  

A minor split is a very small crack that one can catch with a fingernail. They generally don’t

penetrate to the inside of the bamboo. These can be easily fixed with cyanoacrylate adhesive

(super glue) which wicks its way into the deepest part of the crack. A razor blade can be helpful in

getting the glue into the ends of the crack.

A small split is about half a millimeter to a full millimeter wide. These can sometimes penetrate

through to the inside of the bamboo. They can be fixed in a two step process: First with super

glue as above, that is let dried for a few hours and then the wider sections are scraped out with a

razor blade. The second step is to fill the wider portions with a strong epoxy based adhesive like

JB WoodWeld. This may not work on longer splits.

Severe splits have two main methods of repair: The first is to open the split as wide as possible and

fill the inside of the bamboo with two­-part expanding polyurethane foam. Don’t try to compress

the bamboo to close the split. Just let it close down naturally but prevent the expanding foam from

expanding the bamboo by using zip ties or hose clamps. After the foam has hardened, dig out the

foam from the split area and fill in the wall thickness with epoxy soaked hemp fiber or other strong

fiber.

The second method for severe splits starts with filling the bamboo with the expanding foam but

instead of trying to fill the crack with fiber/epoxy, you overwrap the whole tube with an epoxy

soaked cotton fabric, this covers up the bamboo but can offer an interesting style with various

printed patterns.