I'm building a pedal trike and I've bought some lovely CrMo tubing. I'm having a bit of trouble bending it. A while ago I made a plywood tube bending frame that would put a 250mm diameter bend in 1" tube. I bent some mild tube in it very nicely. It was surprising how little force it took to bend the tube. Now I've tried to do the same thing with the CrMo but it all went wrong. First it took a whole lot more strength to get it to go but then it buckled rather than bent nicely round the former. Any tips for making it bend more easily or do I need a redesign?

I was speaking to a guy at Autosport about this (his company supplies 4130 tube) and he said that it won't bend the same way T45 does, that you will struggle to achieve a 90 degree bend. Shallow angles are achievable but the inability to properly heat treat the welded fabrication of a space frame will result in cracks on and around the welds. Apparently, the drag boys don't paint their cages/frames specifically so that they can see where the cracks appear. What sort of bender are you using? as I suspect you will struggle with anything other than a mandrel bender.

I am afraid CrMo tube is a step too far for the home workshop as, as said, it is very difficult to work with.

Have heard but never tried it myself. Filling up the tube with dry sand then trying to bend it may help. Only a suggestion.

Regards,

Arnie

I used to build drag bike frames from 4130 - I used a record plumbers pipe bender (with a long brace welded across the base and an extended handle....)
The largest tube was 1 1/4 .062 wall , and that would bend through 90 degrees no trouble - the only thing to watch was the temperature of the former (too cold and they would split !)

Sounds like a hasty redesign is needed. Its very strong stuff, though, for thin wall tube.