Still haven't managed to find my tape but using the ruler plus finger method I get between 16 and 17 inches from centre to centre. I'd guess it's an 18 inch frame. I will try and borrow a tape to confirm.
This has been used since July. Someone please take pity on it and take it somewhere muddy. I'll throw in everything from here as well. viewtopic.php?f=2&t=291522