Why do wooden coasters get rougher over time?

[Matt N]

Hi guys. As of late (and it seems to have happened very suddenly), a lot of people have said that Wicker Man at Alton Towers has begun to get very rough, despite only being 2 years old. So that did kind of make me wonder; what exactly is it that makes wooden coasters get rougher over time?

I always presumed that it was something to do with the track (or at least, the topper that the trains actually ride upon), as retracking often seems to make rides smoother, but this perplexingly seems to have had the opposite effect on Wicker Man. Do the trains play a big role in it?

So I was only wondering; does anyone know what makes wooden coasters deteriorate over time? Is it to do with maintenance of the track, maintenance of the trains or something else entirely?

 

[Pokemaniac]

I don't know for certain if this is the answer, but if I were to guess, I'd say it has something to do with material properties.

A length of wood isn't a length of wood. It will expand and contract depending on the moisture level in the wood cells. If you've ever laid a wooden floor, you know how you should leave a gap between the edge of the floor and the walls of the room? That's because natural moisture fluctuations can make wood expand around a centimeter for every few metres of board. Worst thing is, this expansion isn't always linear. Go to any hardware store and have a look at the planks left over when a pile is almost picked clean. You've got boards bent like bananas or with the two ends rotated almost 90 degrees on another. That's not because the sawmill delivered them like that. They were sawed straight as a ruler, but moisture changes made them twist and bend over time (Interestingly, there is some research going on about sawing planks into shapes determined by their moisture properties, so that moisture changes will eventually make them straighten out).

So for a wooden coaster, you've got a lot of wooden elements expanding and contracting as the weather changes. Over time, the geometry of its boards will change a little. What was straight will go crooked, what was smooth will go bumpy. The dynamic forces of the trains probably aren't helping the situation either. That's why woodies need such constant maintenance. They're built of a material that refuses to stay the same shape over time.

 

[Howie]

^So what you're saying then, in layman's terms, is:
Because they're made out of wood?
?

 

[HeartlineCoaster]

Why do we all deteriorate over time.

 

[Matt N]

Ah right. Thanks @Pokemaniac; so it’s essentially down to the fact that wood has a lower Young’s modulus than steel and thus it takes less stress to make it change shape? And that’s what causes the wood to bend and deform over time?

 

[Hixee]

The thing to also remember is that wood is a very different material than steel. The notion of elastic behaviour of wood (youngs modulus, thermal expansion, etc) is extremely complex and varies hugely between different pieces of wood.

Wood's strength comes from fibres that are chaotically arranged. It's all these fibres running against each other than generates the strength. Strengthening treatments for wood tend to be methods of increasing the ability of these fibres to hold on to their neighbours. Once these bonds start to fail, the wood tends to splinter - you don't (or rarely) get a ductile failure of wood.

Steel (especially when forged in long drawn sections like coaster tubes) is [ideally] one homogeneous fibre/crystalline structure throughout. Steel fails when the molecules themselves become so distorted they can't hold their shape anymore.

All of that is to say that wood will almost disintegrate after thousands of cycles as the cumulative stress tears the fibres apart. It's something the designers can't do an awful lot about, as the properties of wood are so variable. Wooden coasters are massive structures, mostly because you're relying on that 'average' strength. Engineered timber used in building structures, for example, can be far more 'lean'. Steel is much more predictable, both in its resistance to forces and its methods of failure.

Wooden coasters get retracked when the timber underneath the running rails has deteriorated to the point where it's no longer providing enough support. Couple that with the seasonal changes the wood undergoes (temperature/humidity), means the wood needs replacing.

Of course, there are other factors like the sections you haven't retracked now feeling even worse in comparison, the trains getting worn out and the general day-to-day variations that woodies have that all exacerbate the feeling of wooden coasters getting older with age.

 

[Matt N]

Ah right; thanks very much for the in-depth explanation @Hixee!

 

[Hyde]

The thing to also remember is that wood is a very different material than steel. The notion of elastic behaviour of wood (youngs modulus, thermal expansion, etc) is extremely complex and varies hugely between different pieces of wood.

Wood's strength comes from fibres that are chaotically arranged. It's all these fibres running against each other than generates the strength. Strengthening treatments for wood tend to be methods of increasing the ability of these fibres to hold on to their neighbours. Once these bonds start to fail, the wood tends to splinter - you don't (or rarely) get a ductile failure of wood.

Steel (especially when forged in long drawn sections like coaster tubes) is [ideally] one homogeneous fibre/crystalline structure throughout. Steel fails when the molecules themselves become so distorted they can't hold their shape anymore.

All of that is to say that wood will almost disintegrate after thousands of cycles as the cumulative stress tears the fibres apart. It's something the designers can't do an awful lot about, as the properties of wood are so variable. Wooden coasters are massive structures, mostly because you're relying on that 'average' strength. Engineered timber used in building structures, for example, can be far more 'lean'. Steel is much more predictable, both in its resistance to forces and its methods of failure.

Wooden coasters get retracked when the timber underneath the running rails has deteriorated to the point where it's no longer providing enough support. Couple that with the seasonal changes the wood undergoes (temperature/humidity), means the wood needs replacing.

Of course, there are other factors like the sections you haven't retracked now feeling even worse in comparison, the trains getting worn out and the general day-to-day variations that woodies have that all exacerbate the feeling of wooden coasters getting older with age.

To build this a smidge further; have you ever noticed how modern wooden coasters are painted far less often than woodies through the 1980s? Going back to the wood durability question, this is in part because of advances in pressure-treating wood, would doesn't require white-washing/painting of timbers to protect from weather/UV breakdown. Overall, helps with longevity of the wood - but as @Hixee pointed out, wood will naturally breakdown as a organic material vs. steel.