Bending a 4130 chromoly frame with heat question

pedro5189 · #1 10-08-2009, 08:16 PM

Hello,

I have a 24" wheeled jump frame that is 4130 chromoly, my plan is to modify it some what to fit a standard 135mm mountain bike rear hub in because as it stands it has a BMX width of 110mm. I have decided the best way to do this is to bend out the rear triangles at the point that they have been shaped for the tyre clearance as I believe this is the best way to keep the frames strength.

My question is, will it be ok to heat the tubing to red hot at the place I want to bend it?

Will the tubing loose much strength?

Will I need to quench it or just allow it a slow cool down?

Any other suggestions?

Thank you in advance

Venice Motor Bikes · #2 10-08-2009, 08:48 PM

The problem with heating & bending tube, is that it can kink if you dont use a proper tube bender.
Also... slow cool, ALWAYS!

DOC BOLM · #3 10-08-2009, 08:51 PM

I would cut a 2x4 the length you want to spread the rear end and wedge it in with a hammer.Leave it in and heat your side rails just the steel changes color,then bend them.Use a magic marker and ruler to set up your bends and heat on your marks.H.D

Kevlarr · #4 10-08-2009, 09:07 PM

Make a spreader out of two short stubs of pipe, threaded rod, two washers and two nuts.

pedro5189 · #5 10-08-2009, 09:11 PM

Quote:

Originally Posted by DOC BOLM

I would cut a 2x4 the length you want to spread the rear end and wedge it in with a hammer.Leave it in and heat your side rails just the steel changes color,then bend them.Use a magic marker and ruler to set up your bends and heat on your marks.H.D

so do you mean have the frame already pried appart and then heat at the bend points?

i have a bit of stud bar that i cut to length with nuts to go inside and outside of both the rear dropouts, would you recommed i pri the frame to point half way between these two sizes (122.5mm) then heat one side, allow to cool then up the size to 135mm and heat the other side?

bairdco · #6 10-08-2009, 11:35 PM

WAIT WAIT WAIT!!!!

don't heat it, don't beat it with a hammer, and don't try to pry it apart (i mean, unless you really want to...)

but here's a better, easier, safer way...

go here, page down a bit, and do what it tells you:Bicycle Frame Spacing

jug98 · #7 10-08-2009, 11:42 PM

The proper way to bend tube without getting kinks is to fill the tube area with sand, then heat and make bend. I do my bending on a bending table at work. Filling a bike frame takes alot of tapping and shaking to get the proper fill. Once it is filled, you can bend it into a pretzel with no kinks.

pedro5189 · #8 10-09-2009, 09:35 AM

thanks chaps, looks like sheldon has the answer

Easy Rider · #9 10-09-2009, 10:12 AM

Quote:

Originally Posted by bairdco

WAIT WAIT WAIT!!!!

don't heat it, don't beat it with a hammer, and don't try to pry it apart (i mean, unless you really want to...)

but here's a better, easier, safer way...

go here, page down a bit, and do what it tells you:Bicycle Frame Spacing

Now that's good info!

pedro5189 · #10 10-11-2009, 12:05 AM

Well, I bent the frame using sheldons advice and it worked. The frame however was a stubborn little bugger so i grafted in the help of my old man, we ended u having to use a 4 foot lever and my 190pounds at one end and his 200 pounds at the other to gently coax it into submission. The result is a frame that was straighter than when it came out of the factory.

Thanks again chaps