Any ideas how to get out of this muddle?

Trevorleach · 2026-03-22T09:13:48+0000

Truck bodies built on two new chassis.
Problem we are having is the right hand side has an arch in the middle of the side rave, this is despite having the rear of the chassis jacked up high enough to create a downwards bow prior to be fully welded, thoughts being weld it stressed up and it will relax down when jacks released.
This worked on the first chassis and whilst is not perfect is pretty damned good.
It also worked on the left side of the second chassis, but for some reason the right side on second chassis still has this arch.
The side raves are rolled, and we think they might have built in stresses from the rolling process and when we cut out the wheel arches it gets interesting!

Longditudal body chassis runners channel top flanges 10mm thick web 4 maybe 5mm thick
Body cross raves 100x50x3mm box
Side raves 200x50x5mm channel
Front and rear body cross raves 100x100x5mm box, rear is 8mm thick.
Chequer plate over the wheels ai 6mm.

addjunkie · 2026-03-22T09:30:27+0000

In simplistic terms…when you cut something out you should replace the cut out with equal amounts. So for example, when you cut a hole through a beam on a ship for a pipe, you put extra steel around the hole to replace whats been removed. Obviously its all calculated mathamatcally. So in your example over the wheel arches, it should be cut, and have metal added, before being built into the frame.

Agroshield · 2026-03-22T09:40:39+0000

Would any kind of heat shrinking work?

AIUI, heat locally and quickly and then cool quickly. The surrounding metal staying cool stops the longitudinal expansion of the heated bit, so it can only expand to get thicker. Then you cool it quick and that creates a longitudinal pull, reducing the arch slightly.

Would welding on the (underside of the) top flange of the channel cause it to shrink (in the same way we use to remove a bearing race)?

Trevorleach · 2026-03-22T10:03:49+0000

addjunkie said:
In simplistic terms…when you cut something out you should replace the cut out with equal amounts. So for example, when you cut a hole through a beam on a ship for a pipe, you put extra steel around the hole to replace whats been removed. Obviously its all calculated mathamatcally. So in your example over the wheel arches, it should be cut, and have metal added, before being built into the frame.

Sorry I forgot to mention the side raves are double plated between the cross raves where the arch is cut out, they are only tacked in until the body is bent then they are fully welded along with the rest off it, all overhead welding I love welding overhead.
We did try and weld one before fitting to the vehicle, oh dear it bent like a roller coaster track!

Trevorleach · 2026-03-22T10:06:02+0000

Agroshield said:
Would any kind of heat shrinking work?

AIUI, heat locally and quickly and then cool quickly. The surrounding metal staying cool stops the longitudinal expansion of the heated bit, so it can only expand to get thicker. Then you cool it quick and that creates a longitudinal pull, reducing the arch slightly.

Would welding on the (underside of the) top flange of the channel cause it to shrink (in the same way we use to remove a bearing race)?

The left side did get better after I had welded the chequer plate on to the side and cross raves, I hoped this would happen due to weld shrinkage, but it had little effect on the right hand side.