swing between birches beam dimension?

[Kristofer_Skoglund]

I have 2 birch trees about 4m apart. I plan to hang up a pressure-treated beam with angle bracing and use carriage bolts to attach the swing. However, it's a pretty long span at 4m, so I'm wondering what beam I should use to ensure it holds up. I was thinking of 45x145mm, which I think is sufficient for vertical support, but it might be wobbly horizontally with just 45mm, especially considering that it's for swinging. So I thought of buying 2 pieces and gluing and bolting them together to make it 90x145. Does anyone have any tips and ideas?

 

[MathiasS]

45x145 sounds excellent.

I have noticed that it is the ropes that break when the kids have fallen to the ground, not the rule...