tube bending question

["paul bland" <paul.blandavon@btinternet.com>]

List, probably not a good time of year to ask questions, but...... can anybody
tell me when bending a tube for say a roll cage how much reduction in wall
thickness is there on the outside of the bend? does it vary depending on
dia/wall thickness and bend degrees? does the type of bender make a difference
(JD squared in my case), thanks in advance and good luck to all on the salt
this year, hope you go fast safely!
Paul in the UK  (35c in the workshop).