Chassis Cracks and Broken Brackets

[Ji Gantor]

When dealing with old bikes you will eventually come across a crack in the chassis or a broken bracket.
There has been a lot of words written about this topic on this forum but bike owners and restorers are still confused.
All agree that a repair is required.
I have seen lots of repairs and some frankly make me embarrassed  to be part of the VMX crowd.
I am not saying for one instant that I am an expert, in fact I consider myself a white belt on the subject but some people should not do work they have no idea about, after all some repairs are just dangerous for anyone that rides that bike. So with that said lets explore how it can be done, keeping the structural integrity of the frame in tack while maintaining the original look.

Attached is a typical seat bracket that over 30 years has become cracked in a few places. The metal would be mild steel fixed to a chrome molly tube frame.

Ji

[Ji Gantor]

Start by wire brushing all the loose paint and rust off.
This will give a better idea of what is wrong and clean the surface ready for welding.
Take a small hammer and a dolly and tap the bracket back into its original shape.
Take your time with this as it will pay off down the track.
Once done fit the seat and see if everything lines up.

Ji

[Ji Gantor]

TIG welding is the best method in my opinion.
Use an appropriate filler rod and weld the cracks up.
Seeing how this metal is mild steel and the repair is so far away from the chrome molly tubes there is no need to post heat. If you feel that post heat should be done to remove stresses then that is great, it will not harm the frame if done right.
After the welding is finished sand not grind the bead flat and make the affected area look like new.
Take your time with this stage and the repair will hardly be seen.

Ji

[Ji Gantor]

You will have to sand both sides some times but it will be worth it in the end.
TIG welding allows you to lay the filler rod right where it is needed and gives you the time to get the job done right. It evenly heats the plates even though we are just welding not much more than a tack length, while still penetrating the metal.
This will look fantastic once it is bead blasted and powder coated not to mention strong.

Ji

[Ji Gantor]

Another typical bracket that gets cracked or torn off completely is the side number plate bracket.
These get treated pretty mean when the bike is crashed.
Our subject is cracked in two places and will need to be welded up.
This metal would be mild steel but this time the HAZ (heat affected zone) is close to the chrome molly tube.
The HAZ extends past the weld about 25 to 50mm and can be easely seen after welding.
If the HAZ extends to the tube post heating will be required.

Again wire brush the area and tap the bracket back into alignment.

Ji

[Ji Gantor]

TIG weld the crack and make sure there is plenty of filler at the end so the crack does not start again next time the bike is dropped.
This extra filler makes sure there is not a V left behind.

Ji

[Ji Gantor]

After the welding is done we can sand down the bead and make it all look great again.

Some resorters feel that a 30 year old frame is dangerous and should not be used and so they pay $10k or so for a replica frame and swingarm.
This is fine, each to their own, honestly I can not see how you can call yourself a restorer when you have started with all new parts. What did they restore? They should call themselves Bike Assemblers. If you are conserned that your frame may have unseen faults you can have it ultrasounded. This will reveal any potential problems for a fraction of the cost of a replica frame. If you feel your welding is not up to par get a boiler maker to weld up any known cracks, this to will not cost as much as you think and you will have peace of mind and a really good looking frame.

Ji

[huskibul]

   Hi  Ji  ,Do you think it be ok to mig weld a crack ,half way threw on front tube just above where both downtubes meet below side panel on bultaco frame? dont worry i wont hold you too it just your opinion would be helpful cheers

[Ji Gantor]

Hi Huskibul,
Can you please upload an image of the affected area.

Is the person that is going to do the repair a competent MIG welder?
If so he will know what to do.
If it is a mate that has a MIG that only uses it now and then I would say don't.
Wire brush or sand blast the damaged area and do all the panel beating as required, then take the frame to a welding works and get a quote. It will only cost $50.00 and ask them to TIG weld it.
You will be surprised at the result.
Again what price would you pay to make sure when you land your bike over a big jump that you won't end up in the dirt because of a weld failure. We pay $200.00 for chest protectors and $700.00 for a Leatt why not shell out $50.00 for piece of mind.

Ji

[Ji Gantor]

Just in case bike owners and restorers think that only these old chrome molly tube frames crack, think again.
The modern cast aluminium frames crack more often and are harder to repair. I am talking about bikes that are only a few years old. The disadvantage with tube frame is that it flexes, this deflection of the tubes ads say 5% to the shock absorption, while the cast alloy frames are very stiff. This stiffness allows the suspension and steering to do their jobs consistently, but think of a MX track like an earth quake. To survive the frame needs to be flexible or it will crack.
Some modern road bike frames crack the first time it jumps a kerb and the repair costs so much the owner is advised to sell the bike.

Ji

[huskibul]

   Thanks Ji ,he's a ex fitter come boiler and he's already seen it, i will just get it done and keep my eye on it,thanks for the concern .as for the picture i haven't advanced(computer wise) that far yet, it,s a very fine crack running half way threw inside of downtube, should be fine ,i suppose it would be a worry if it was front downtube;)

[Ji Gantor]

There are a few problems when MIG welding up chrome molly tubes.
First and most important is the filler rod. If you have a mate with a MIG he will most likely have mild steel wire on the spool. A spool of MIG wire costs about $100.00 and takes 10 minutes to change out. The mild steel wire will make the weld but is not the right metal for the job. A chrome molly TIG filler rod on the other hand costs $0.50 .

On small cracks that are only as long as a tack weld MIG in my experience is to quick and in the hands of the non-expert produce cold rope like weld beads. With TIG you increase the current or heat slowly via the foot pedal until the puddle is formed and then you add filler. This gives your eyes enough time to adjust to the arc and position the torch and filler.

Chrome molly is effected by heat, whether brazing or welding there will be a HAZ.
This HAZ can reduce the strength to only 40% of the tubes original load carrying capacity.
The HAZ needs to be post heat treated to retain strength and avoid brittleness.
In a frame making factory the frames after welding are placed into an oven and taken up to a set temp and then the heat is reduced over a long time.
Ji
 

 

[Freakshow]

how dpo you post heat after welding now ?

[Ji Gantor]

Gas

Ji

[huskibul]

    Ji thats getting a little over my head but maybe i,d be better chasing the chrome moly rod/tig job now you put it that way?,i dont know about the post heating would any bloke with a tig know that ???or what would i tell him?  :-\cheers