Author Topic: Aps frame material size  (Read 16133 times)

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Offline Koncretekid

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Re: Aps frame material size
« Reply #30 on: February 16, 2015, 08:27:23 AM »
Top view of the frame would be instructive.  Is that a single top tube exiting the headstock at the front?  You will want to read recent posts about CP vs. CG, especially if you plan to add bodywork.
Tom
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Offline Greaseballracing

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Re: Aps frame material size
« Reply #31 on: March 04, 2015, 03:45:29 AM »

Thanks Tom I'll look it up here's a veiw from the top it has dual top tubes, I kinda figured it would be stronger than a single.

Offline Koncretekid

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Re: Aps frame material size
« Reply #32 on: March 04, 2015, 08:24:34 AM »
Couldn't see your top tubes well so I lightened it up.  Two tubes twice as good in compression as one. Lots of cross bracing to increase torsional strength as well.  Click on the photo to zoom in.  Now we can read your license plate on the '59 Chevy!
« Last Edit: March 04, 2015, 08:28:47 AM by Koncretekid »
We get too soon oldt, and too late schmart!
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Offline Greaseballracing

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Aps frame material size
« Reply #33 on: March 05, 2015, 03:50:11 AM »
Thanks Tom yeah two has to be better than one in my figuring I have tried to over build this thing as much as I can the cross bracing is two in an x brace at the first uprights and alternating diagonals at every upright there will be plates to stiffen around the neck and will be fitting more plates at various intersections for mounting things like seat and hardware. The 59 is my sensible work ute. You know run to the dump pick up engines and steel and stuff.

Offline Koncretekid

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Re: Aps frame material size
« Reply #34 on: March 05, 2015, 08:54:43 AM »
Another observation is that with that very long frame and extreme fork angle, you're going to get a lot of stress where the frame tubes meet the headstock.  I would like to see some plating on the sides, using the same technique that is used to gusset roll cages.  Lots of bikes use gussets at this point so I assume that this is a common place for frame breakage.  That is, leave a little gap at the tube-to-headstock joint so that any crack that starts (that you may not see) is less likely to propagate, and weld the gusset to the headstock and both upper and lower tubes.  Also, inspect this section often to ensure no visible cracks appear. I lost my drafting program when I upgraded to newer computer (ain't that great!) so I can't illustrate what I mean.
Tom
We get too soon oldt, and too late schmart!
Life's uncertain - eat dessert first!

Offline Greaseballracing

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Re: Aps frame material size
« Reply #35 on: March 05, 2015, 07:11:42 PM »
Haha good thinking Tom I actually started making the gussets for that area last night at work I have 6mm mild steel plates on top bottom and both sides the idea is to gain more weld on the neck itself and as you said spread the load evenly into the frame they extend 200mm from the neck once installed I'll post some pictures, I had planned on welding them as per roll cage gussets with an inch of weld then an inch of gap as you said. I really would not like to encounter any cracking in that area especially.

Offline Speed Limit 1000

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Re: Aps frame material size
« Reply #36 on: March 05, 2015, 08:36:50 PM »
It is common for drag bikes to use the frame as an air tank. Simple and you know if a crack starts to develop.
John Gowetski, red hat @ 221.183 MPH MSA Lakester, Bockscar #1000 60 ci normally aspirated w/N20

Offline maj

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Re: Aps frame material size
« Reply #37 on: March 05, 2015, 11:48:17 PM »
Hey GB are we going to see you in a couple of weeks at the big white dyno ? , not expecting the bike just yet

Offline wobblywalrus

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Re: Aps frame material size
« Reply #38 on: March 06, 2015, 11:10:57 PM »
There are many stresses in structural design as listed from biggest to smallest.  Tensile stress is the greatest and that is when things break.  Yield stress is when they start to permanently deform after loading rather than springing back to original shape.  Fatigue stress is when they start to crack under repeated loading.  The stress creating maximum allowable deflection is less than all of these.  For example, the typical floor beam cannot deflect more than 1/360 of its length under the maximum design load  This prevents plaster from cracking on the ceiling of the room below and it does not feel bouncy when people walk, dance, etc in the room.

Elastic deflection in a bike frame is really scary and we instinctively design them so it is minimized.  The stresses from loading are typically quite low within these beefy frames when we do this and they are far below the elastic limit.  The metal's modulus of elasticity governs performance in this realm.  The elastic moduli of carbon steel and chrome moly are similar and neither offers a big benefit in low stress design.

We are not enclosed in the bike frame.  The frame is history after a big crash no matter what it is made from.  A roll cage or car frame is different.  There, we want strength and protection.  Elastic deflection under typical driving loads is not the governing concern.  Resistance to permanent deflection is.  The stronger more exotic steels can provide benefits in this application.

The stronger mild steels are plenty good for a bike frame.

 

Offline Koncretekid

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Re: Aps frame material size
« Reply #39 on: March 09, 2015, 09:41:09 AM »
There are many stresses in structural design as listed from biggest to smallest.  Tensile stress is the greatest and that is when things break.  Yield stress is when they start to permanently deform after loading rather than springing back to original shape.  Fatigue stress is when they start to crack under repeated loading.  The stress creating maximum allowable deflection is less than all of these.  For example, the typical floor beam cannot deflect more than 1/360 of its length under the maximum design load  This prevents plaster from cracking on the ceiling of the room below and it does not feel bouncy when people walk, dance, etc in the room.

Elastic deflection in a bike frame is really scary and we instinctively design them so it is minimized.  The stresses from loading are typically quite low within these beefy frames when we do this and they are far below the elastic limit.  The metal's modulus of elasticity governs performance in this realm.  The elastic moduli of carbon steel and chrome moly are similar and neither offers a big benefit in low stress design.

We are not enclosed in the bike frame.  The frame is history after a big crash no matter what it is made from.  A roll cage or car frame is different.  There, we want strength and protection.  Elastic deflection under typical driving loads is not the governing concern.  Resistance to permanent deflection is.  The stronger more exotic steels can provide benefits in this application.

The stronger mild steels are plenty good for a bike frame.

 
Bo,
Although this is generally true, fatigue failures are the most common type of failure for motorcycle frames, and Chromoly (normalized is the only form you want to use) is almost twice as resistant to fatigue failure as mild steel for a given section size.  Also, stresses that result in fatigue failures are not due to static forces but dynamic forces caused by impact loads that can be many times higher than static forces.  Structural elements, especially those adjacent to motor mounts and other parts that may vibrate, are subjected to literally millions of cycles of these impact forces which will cause frame tubes to fracture, even when the computed forces seem low.  I recommend the use of thicker sections or stronger steels for these areas and by all means, keep them clean and accessible and visually inspect suspect sections.  Following are a few real examples of frame failures after just 2 years of land speed racing!  The frame was 1" OD by .060" wall mild steel.  It has been replaced with normalized chromoly of the same size and has at least survived 2 more years at higher speeds with no visible cracks........yet!  I continue to monitor it.
Tom
We get too soon oldt, and too late schmart!
Life's uncertain - eat dessert first!

Offline wobblywalrus

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Re: Aps frame material size
« Reply #40 on: March 10, 2015, 11:15:17 PM »
A rough estimate of the endurance limit for steels is 50% of the tensile strength, plus or minus 10%.  Normalized chrome moly has a tensile strength between 95 and 100 ksi, as I remember.  This is stronger than a lot of mild steels and the endurance limit is also greater.  This provides more fatigue resistance, as Tom says.

The problem with the problems I dealt with was as the builder, often a well respected manufacturer, used much thinner wall tubing in the chrome moly applications.  This is typical.  Stress is force divided by area, so the stresses from the cyclic loads in the thinner wall tubes were as close, or closer, to the endurance limit as with the typical thicker wall mild steel frame.  The dang things broke.  Also, they flexed a lot under use and the handling was spooky at best.

Builders, and especially me, figured out that the welds needed to be normalized to prevent cracking.  This sorta worked.  Sometimes we normalized the weld area, other instances we annealed it, or we did not get it hot enough to do any good.  One thing we learned is the thinner wall chrome moly worked best for trellis like space frames.  Bending loads are minimized and replaced by tensile and compression loads in these structures.  Why this frame type works better, I have no clue.  There is a book by Carroll Shelby, "Engineer to Win," that has some hints on using this metal.

Offline Peter Jack

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Re: Aps frame material size
« Reply #41 on: March 11, 2015, 12:05:04 AM »
The book is by Carroll Smith. He wrote a whole series of books on race car construction, preparation and tuning. They're all worth reading if a guy's serious about being competitive.

Pete

Offline wobblywalrus

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Re: Aps frame material size
« Reply #42 on: March 12, 2015, 10:35:46 PM »
Hard to find info.