That Cracked Aluminium frame

[Shebeen]

Maybe a silly question. Why paint strip the whole frame?

 

Surely you want to try repair the crack, so treat that area and if it is successful, then redo the rest. Just seems like a lot of work for something that may or may not work out.

 

I know you find more cracks, but that wasn't really expected was it?

[Omicrom]

Maybe a silly question. Why paint strip the whole frame?

 

Surely you want to try repair the crack, so treat that area and if it is successful, then redo the rest. Just seems like a lot of work for something that may or may not work out.

 

I know you find more cracks, but that wasn't really expected was it?

It's as fair question, 

Stripping that part of the frame only will require a colour match and I am no paint expert, so I feel it would have left me with a silly looking patch if I've done that myself.

If I strip the whole frame I can just go ahead and paint it one colour and it will look alright.

I suspected there might be another crack due to the amount of time he rode the bike like that.

Stripping the whole frame mean I could try another avenue of repair.

I hope it makes sense

[rvdm1]

40MPa with the riding I intend to do.

Curious to know how you defined your dynamic loads? Have been toying with the idea of doing an FEA on my mtb frame for some time.

[RiverInTheRoad]

Share some pics of your model and explain loadcases please. Did you incorporate all the tube tapers and special bends?

[Omicrom]

Curious to know how you defined your dynamic loads? Have been toying with the idea of doing an FEA on my mtb frame for some time.

Download an accelerometer app on cellphone, strap it to the bike securely so no movement takes place. Ride around for a set time , say 2 minutes.

Have a look at the peaks and try to sort them in 3 categories, low med and high at even intervals.

manually count the peaks of each interval.

ect.

0-2G 90 times

2-4G 40 times

4-6G 10 times

Set up a static simulation in your favorite CAD or simulation program with a model of your bike. calculating the applied force from

F=ma

=(weight of you and bike)[(top interval in G)*9.81]

 

decide where you want to apply the total force, amount at top of seat post and amount at BB (doesn't have a big influence because each member in a bike frame is essentially a two force member and the both act in 'almost' linearly to seat tube)

apply horizontal forces at BB

 

Use the results and a probe to measure stress at given points.

Repeat for each of the G force intervals you chose.

 

calculating fatigue 

Get the SN formula for the given material and use the relations in which they occur and calculate the fatigue as a %. use 100%  for point of failure. and solve x which is the amount of fatigue cycles. 

[Omicrom]

Share some pics of your model and explain loadcases please. Did you incorporate all the tube tapers and special bends?

No I did't incorporate all the tube bends and profiles, I just used the appropriate diameters and wall thicknesses making sure my cross sectional area is correct.

Will do

[Omicrom]

Attached is 2 screenshots of the simulation done , It is a track frame I designed .I just tweaked the tube profiles for the simulation and added two tabs at the BB to apply additional vertical forces. I used a safety factor of 2.

[V18]

This thread made me moist.

[BaGearA]

Download an accelerometer app on cellphone, strap it to the bike securely so no movement takes place. Ride around for a set time , say 2 minutes.

Have a look at the peaks and try to sort them in 3 categories, low med and high at even intervals.

manually count the peaks of each interval.

ect.

0-2G 90 times

2-4G 40 times

4-6G 10 times

Set up a static simulation in your favorite CAD or simulation program with a model of your bike. calculating the applied force from

F=ma

=(weight of you and bike)[(top interval in G)*9.81]

 

decide where you want to apply the total force, amount at top of seat post and amount at BB (doesn't have a big influence because each member in a bike frame is essentially a two force member and the both act in 'almost' linearly to seat tube)

apply horizontal forces at BB

 

Use the results and a probe to measure stress at given points.

Repeat for each of the G force intervals you chose.

 

calculating fatigue 

Get the SN formula for the given material and use the relations in which they occur and calculate the fatigue as a %. use 100%  for point of failure. and solve x which is the amount of fatigue cycles. 

* Science gasm 

[dave303e]

Considering I have that exact frame(and it has over 20000km on it) and I love welding and engineering, you now have my full attention, excited to see this.

[Brett]

Keen to see how this pans out, as I had an alloy frame repaired a few years back. A few rides after the repair and noticed it had cracked next to the new welds. Hopefully yours doesn't do the same thing!

[Omicrom]

Hi Hubbers so today was the big repair day.

After cleaning all the weld areas again, back purging the frame with argon and doing a few “warm-up welds” I felt ready.

On initial arc I shook so much I had to stop take a few breaths and go at it again.

The rest of the welding didn't go any better; between struggling to reach spots with the torch and filling up the holes I drilled I had my hands full.

I am by no means pleased or proud of the welds, and I have done much better welds before on thinner material. 

I wanted to have as little possible torch time on the frame and keep the heat affected zone as small as possible. Unfortunately due to the struggle that was not possible and I ended up with big fillets that I just smooved out with the torch. 

The results are not as hoped but I feel the hope in the repair is not lost since I managed to salvage it.

Being aluminium it will eventually fail again, it is not like steel that has a fatigue limit, no matter how small the applied load it will eventually fail due to fatigue. The frame held approx 3-4 years out of the factory so if these repairs last half of that time I will be pleased, and I will probably weld it again since the frame will stay naked.

 

Lessons learnt today,

Use a gas lens, you can have much longer tungsten stick out and reach tighter fillets that way.

Use a stubby kit so the torch can have a better angle inside the frame.

If it doesn't work stop and have a go at it again.

 

[Fox3]

Well done, at least now you know

[Odinson]

Hi Hubbers so today was the big repair day.

After cleaning all the weld areas again, back purging the frame with argon and doing a few “warm-up welds” I felt ready.

On initial arc I shook so much I had to stop take a few breaths and go at it again.

The rest of the welding didn't go any better; between struggling to reach spots with the torch and filling up the holes I drilled I had my hands full.

I am by no means pleased or proud of the welds, and I have done much better welds before on thinner material. 

I wanted to have as little possible torch time on the frame and keep the heat affected zone as small as possible. Unfortunately due to the struggle that was not possible and I ended up with big fillets that I just smooved out with the torch. 

The results are not as hoped but I feel the hope in the repair is not lost since I managed to salvage it.

Being aluminium it will eventually fail again, it is not like steel that has a fatigue limit, no matter how small the applied load it will eventually fail due to fatigue. The frame held approx 3-4 years out of the factory so if these repairs last half of that time I will be pleased, and I will probably weld it again since the frame will stay naked.

 

Lessons learnt today,

Use a gas lens, you can have much longer tungsten stick out and reach tighter fillets that way.

Use a stubby kit so the torch can have a better angle inside the frame.

If it doesn't work stop and have a go at it again.

 

Huck to flat the biggest drop you can find and then we'll get an idea of your craftsmanship. 

[Omicrom]

Huck to flat the biggest drop you can find and then we'll get an idea of your craftsmanship.

Unfortunately I think I will get hurt if it fails or not.   I am by no means a professional enduro rider 

[RiverInTheRoad]

 Charge your GoPro and take a lekka jump with it.