Ti Monster Frame*Completed Pics*Headed for DucStock

[Triple J]

As long as the operating stresses are below the limit for the number of stress cycles, failure is unlikely.

This is key. As long as the stresses are well within the elastic range for the frame material, fatigue failure should be unlikely.

Plenty of things are made from aluminium and titanium (i.e aircraft wing structures) that have very long service lives under cyclic loading.

[Randy@StradaFab]

  Here's a pic of the bike on the scale. It did have oil in it. After I took the bike off I loaded up everything that goes back on the bike. I think it's going to end up 350-360 dry.

[Punx Clever]

As long as the stresses are well within the elastic range for the frame material, fatigue failure should be unlikely.

Not true.  Even with steel, the "fatigue strength", if you will, is about half the yield strength.  Then you factor in scratches, squared edges, and any number of surface imperfections...

You just pulled '20 years' out of thin air.....
I'm ten feet away from a 20 year old Honda, that has 20k miles on it, and it has an aluminum frame.
It's not broken.
As long as the operating stresses are below the limit for the number of stress cycles, failure is unlikely.

I'd be really surprised if the average lifespan of a bike was more than 30k miles.
Most die due to neglect or a crash before that.

Does anyone here have FHE with an aluminum frame cracking in normal service?
Or does anybody here know someone who did?

All of this is true.  I wouldn't be surprised if the design on the aluminum frames was such that some other component (probably engine / transmission) was designed to fail earlier.  But, ideally (ie, I was building a frame, not normal service) I would want to build it to last 100k miles, and be light.  5k a year isn't too much mileage is it?

[/off topic]

Back on topic...

Thats a fine looking machine you are building there.  Keep up the good work!

[Triple J]

Not true.  Even with steel, the "fatigue strength", if you will, is about half the yield strength.  Then you factor in scratches, squared edges, and any number of surface imperfections...

Sure it's true. Keep the stresses "well within" the elastic range and the material should be fine. I never defined if it was 1/2, 1/3, 1/4, etc. of the yield strength because I don't know and it depends on the specific material.  The point is, keep the stresses low enough (i.e. proper design for the material and its intended use and desired lifespan) and fatigue failure should be unlikely.

Commercial aircraft wings are a great example of an aluminum frame structure which experiences cyclic loadig over a long life span without failure. There is no reason the same cannot be done for a motorcycle frame (Aluminum, Titanium, whatever). Whether that frame would be lighter than steel is a different issue.

[Randy@StradaFab]

  I'm gonna go out on a limb here and say that maybe there is not as much stress on a frame as what we think. There is very little penetration on those Mig welds on a Monster frame. You ever look at a Harley frame? Crap! My brother had a 74 Harley and I went over to his house one day when he had it apart. Those welds looked like something Macgyver welded with a car battery, a set of jumper cables and a piece of coat hanger for filler rod.
  I wish I could find the article interviewing one of Ducati's engineers. He was talking about how light they could make a frame on one of the earlier SBK's(correct me if I'm wrong). He said they would cut tubes out of the frame and send the test rider out on the bike. When the rider could feel the frame flex they welded one set of tubes back in and called it good. Interesting....

[Drunken Monkey]

BTW: You mentioned you used a waterjet to cut the frame tubes or was that just done on the gussets?

If it was used to cut the tubes, was there any specific software you used to program the cuts?

[Randy@StradaFab]

   The waterjet was just on the gussetts.

[DRKWNG]

This thread made me remember how nice it was to ride my Ti frame, so I took it out for a quick spin after work today...

[Syscrush]

Are those carbon seat stays?  I assume that the fork and the seat post are carbon, too?

[Syscrush]

Commercial aircraft wings are a great example of an aluminum frame structure which experiences cyclic loadig over a long life span without failure.

Agreed 100%.  Although, the little bit of materials science I know makes me feel a bit freaked when I look out the window at the Al wing of a 20+ year old plane and see it flexing like that.

[Raux]

Agreed 100%.  Although, the little bit of materials science I know makes me feel a bit freaked when I look out the window at the Al wing of a 20+ year old plane and see it flexing like that.

actually they are inpected OFTEN and parts replaced as soon as sign of cracks show.

I don't think we would check our frames every few road hours for signs of wear and replace if any signs show.

[Triple J]

actually they are inpected OFTEN and parts replaced as soon as sign of cracks show.

I don't think we would check our frames every few road hours for signs of wear and replace if any signs show.

As far as I know the wing structures of commercial planes aren't inspected often. I believe planes are torn down every 3 years, otherwise it's a superficial check of the skin, not the structural frame beneath.

Even if the inspetction is yearly, I wouldn't call that often given the number of oscillations a wing is exposed to over that time period...compared to what a moto frame would be exposed to.

[DRKWNG]

Are those carbon seat stays?  I assume that the fork and the seat post are carbon, too?

Carbon for the forks, and both the seat stays and tube.  It's still got the alloy seat post that came with the frame set at the moment.

[sbrguy]

geez you guys have a lots of money nice seven cycles there guys..

[DRKWNG]

I don't have any money; mainly because I spent it on nice toys.