I'm sick with fear. My hands are locked onto the grips so tightly that, afterwards, I'm unable to straighten them. The wind on my bare, vulnerable skull is a hideous feeling.
I'm a student of statistics, and I deal with fear by rationalising it away. The adventure sports industry is built on the appearance of danger. In Croatia I gritted my teeth and jumped. I like to think that I'm smart rather than cowardly.
But rationalising makes this, a new type of fear, much worse. I'm riding a small Thai motorcycle down a steep, winding road - and I'm wearing no armour at all. I can clearly hear the voice of my hard-bitten riding instructor in Adelaide: "You do not throw your leg over a bike without your gear on".
Rationally, I enumerate each part of my body that would be damaged in a fall. I'm going to lose a lot of skin, some of it irreplaceably. I'll lose teeth. I might shred my hands. As our speed climbs above 50km/h - Lyn's speedometer is mercifully broken, but mine works just fine - I might smash my skull and spend the rest of my life drooling.
Fifty metres ahead of me, Lyn is no better protected. I'm wondering if it'll be her or me, and glad that I don't have to choose. She's a better rider but a more aggressive one, and rationally, two bikes doubles our chances.
The jungle whips past us. Traffic is light. But this is not liberating. This is sickening.
At the end of our ride, down to the Treetop Adventure Park in Ko Chang, I meet the other type of fear: irrational fear. I've never liked heights, and these jokers have ziplines, tightropes, and ladders twenty metres in the air, with stunning views of the beach if you can bear to look.
I have my rationalisation toolkit ready to rock - for all the good it does against vertigo - but I don't need it. The French owner gives us our safety harnesses and, as soon as I'm clipped in, I'm fine.
This works even if I'm not clipped in to anything particularly solid.
We blitz the course, leaving the Austrian couple who are sharing our guide in our dust. Even the 150m flying fox, 15 metres up, doesn't cause me more than a second's hesitation. I'm so confident that I accidentally re-shave my head on the steel cable while turning to look at the view.
I think I broke my fear of jumping in Croatia. Once you've leapt from a ten metre rock into an unknown river pool, something changes. Now, I've broken my fear of riding without armour: arriving in Sihanoukville, we opted for two motorcycle taxis instead of a tuk-tuk, to save five dollars. It's totally irrational. Lyn even commented afterwards on how "safe" her rider was. He's not safe. He's slightly less hideously dangerous.
On each of our trips, both of us have consciously sought out our fears, engaged them, and beaten them. But while irrational fear cripples you, rational fear protects you. I fear that my victory is friendly fire.