Author Topic: BSA B50 -500 APG Build  (Read 460878 times)

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Offline bak189

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Re: BSA B50 -500 APG Build
« Reply #390 on: November 24, 2012, 10:44:57 PM »
Roadracing sidecars get a lot stress on the chassis...in order to check for cracks in the frame, there is no paint  on any part of the chassis, it may be sprayed with clear to keep the rust down.....but it is certainly very easy to find any and all cracks.
There is also a very good reason why most frames on British bikes were bronze welded in the old days.....the frames could stand the vibrations of most British bike engines ...we all know they do shake.........Bronze welding if done with a in-line-fluxer and the right rods make for a outstanding vibration proof frame....However , it should be noted that bronze welding these days is a lost art, and not easy to do.
I learned from the old boys many moons ago in Europe......

PS. Never powder coat a racing frame.....you will never see the cracks until it breaks
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Offline wobblywalrus

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Re: BSA B50 -500 APG Build
« Reply #391 on: November 25, 2012, 12:50:12 AM »
Most of my early years were running British twins, mainly BSA.  The smoothest was the unit construction A-50.  It used pretty much the same engine as the A-65 650 twin with smaller pistons.  It was a strong engine cause most of it was made for the more powerful 650.

   

Offline Koncretekid

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Re: BSA B50 -500 APG Build
« Reply #392 on: November 25, 2012, 05:45:07 PM »
Thanks for all the replies and suggestions, all of which will be taken into consideration.

Fatigue failure in motorcycle frames

Sitting in my sunroom today looking at whitecaps on the lake, with ice pellets blowing horizontally and small birds flying backwards, I was not encouraged to finish disassembling my frame for sandblasting in the shop, so I wrote this probably rather boring analysis of my frame failures.

The frame I built for my bike is made from 1" x .060" wall 1018 CREW steel tubing.  The yield strength of this material is approximately 34,200 psi and the ultimate strength is about 51,300 psi.  It's elongation may be as low as 10%. The area of the steel tube is  .177 square inches.  Two years of landspeed racing, or less than 6 hours of wide open throttle run time has resulted in 5 cracked or broken frame members.

One of the tubes has now failed twice, and it should be fairly easy to analyze the forces acting on it.

Analyzing this tube would show that the force required to cause it to fail in compression or tension would be approximately 6053 lbs.  The endurance limit of this tube, if it were polished and had no other physical defects could be as high as 4590 lbs. which is 50% of its ultimate strength.  That is, if a load of less than 4590 lbs is applied, the steel tubing should last indefinitely.  There are many other factors that reduce the endurance limit of steel including size, temperature, differential loading, reliability, and most importantly, surface condition.  Polished gives the 50% endurance limit; corroded, notched, welded, or otherwise imperfect surface conditions can reduce the endurance limit.  The lowest I've found is about 20% of its ultimate strength.  For the above tubing, that would result in a minimum force of 1816 lbs to induce early fatigue failure.

The static forces acting on this tube are only that of about four feet of tubing at .6 lbs per foot, one quarter the weight of the gas tank equal to about 4 lbs, 1/2 the weight of the fender which equals 3 lbs, and 1/2 the weight of the tailpiece, or about 10 lbs.  Total weight is less than 20 lbs.  There would be no force induced by the acceleration of the bike, as that is transferred to the axle which is supported in the well trussed main frame members below.  Not even the force of the air friction should be transferred to this member, because the tailpiece simply rests on the tail end and is not fastened at this location. (It's fastened thru the use of Dzus fasteners to a crossmember attached to the mainframe, and to the forward bodywork.  There is one Dzus fastener into the right side of this sub-frame, but none on the left side.)  There is also a chainguard connection as shown, which is one of three such connections to the frame.  A fourth chainguard connection is located further back by a simple link to the sub-frame above.  I can fathom no other loads on this tube except those that may have been induced by the welding of the frame together into a rigid unit.  So how does a 20 lb load become magnified to a force of over 1800 lbs required to initiate a fatigue failure?

I believe the answer lies in the load factor known as impact. Most impact factors used in construction rarely exceed two, which is obviously not high enough.

I found an interesting study on the internet on the failure of a three wheeled motorcycle frame here: http://products.asminternational.org/fach/data/fullDisplay.do?database=faco&record=1410&trim=false
(It was a 500cc motorcycle built in 1982, so it couldn't have been a BSA!)
It made use of a finite element analysis, but I have no such software.

So my theory is that the moving mass in my case is the above mentioned 20 lbs, and its velocity is related to the up and down movement caused by the rotation of the wheel, which is not exactly round (.070" out of round as measured at the outside of the tire.)  I also believe that the .070" of actual movement probably results in much greater movement of the parts because the deflection of the tire.  Hence the rotating wheel, at about 1900 rpm at 135 mph, creates a vertical high frequency vibration.  The tube in question and the weight above it is moving up and down at the same frequency, which means that it cycles at about 32 times per second.  Because this mass changes direction 32 times per second, it must be accelerated and de-accelerated at the same rate.  I believe that since force is equal to mass times acceleration,  if the acceleration can be determined,  the force can probably be determined.

Can anybody come up with a suitable formula to determine the appropriate impact factor to help determine the actual forces generated?  This would help greatly in the re-building of this frame.

Tom
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Life's uncertain - eat dessert first!

Offline WhizzbangK.C.

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Re: BSA B50 -500 APG Build
« Reply #393 on: November 25, 2012, 06:55:34 PM »
I'm no engineer, not even a formally trained welder/fabricator, so this is just my own opinion based on my own experience and observations, as well as "tribal knowledge" that has been handed down from those who've shared with me over the years. (end mandatory disclaimer) :-D

I think you need to factor in the roughness of the racing surface in any stress calculations that you're making, as it's been far from smooth on the salt the last few years, and the amplitude of the impacts is much greater than that induced by the runout of your wheel. Also don't rule out engine vibration as a contributing factor, as Brit singles have been known to shake a little bit.  8-)

I've looked at all the pics of your frame failures, blown up as large as I can get them on my screen, and they all seem to share one common feature. Every one of the cracks originates at the edge of a weld bead. I think that the inherent stress concentration at the weld bead, combined with the operational stresses, is the primary cause of the failures that you are seeing.

 Personally, I won't build with less than 1" .090  1018. I feel that the thicker wall is more resistant to weld induced stresses. I prefer DOM as well since it lacks the weld bead running the length of the tube. If I must use CREW I make every effort to have the bead on the center line of any bends. I also won't build a frame from CroMo, which I feel is way too brittle for a frame.

I know that currently accepted standard welding practice holds that stress relief of mild steel weldments is not necessary, and in the vast majority of cases I agree. In your case though, with a rigid frame built of relatively thin wall tubing, I believe that stress relief should be done at all welded joints, especially in light of the failures. I would do it with a rose bud torch, working from weld to weld in the same order as you welded the frame to keep distortion to a minimum, after proper repairs are made to the frame, of course.

To test this, you could make up a simple coped T weldment, and then cut into the top of the T tangent to one side of the vertical leg, and see how far the kerf pulls apart as you approach the weld bead. Make another identical test piece, stress relieve it with a torch, and cut it the same way, note the difference. :cheers:
Ah, this is obviously some strange usage of the word 'safe' that I wasn't previously aware of.  Douglas Adams

Offline Interested Observer

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Re: BSA B50 -500 APG Build
« Reply #394 on: November 26, 2012, 09:18:33 AM »
Tom,
To draw together some of the points mentioned earlier, and perhaps add a couple considerations:
Whizzbang is correct in that the cracks were probably initiated at the toe of weld beads.  The relatively thin 16 gauge wall tubing would be rather unforgiving, especially if the elongation is down in the 10% range.  That is a relatively brittle material condition and although the heat from the weld may have tempered some of the cold work, the quick quenching may have made it worse.  The frame was probably born with cracks at the welds.  Counting on the strength numbers of the cold worked raw material is probably not justified if it is subsequently welded.  So, you likely had a frame with cracks in the tube wall with residual thermal stresses acting in that location, and possibly other residual stresses due to distortions of the overall frame as a result of fabrication.
Given that initial condition, you are probably correct in the results of your analysis that high frequency vibration of various masses drove the subsequent crack propogation.  It is hard to say what the nature of the cracking was without looking and the fracture surfaces, but from your photos there seems to be very little ductile behavior.  This would indicate a brittle and/or fatigue related crack.  While you have investigated the fatigue life of material test specimens with various surface conditions, and found that they can substantially reduce the fatigue life, there is another aspect to be considered.  The geometrical stress concentrations at the tip of a crack running in a material can be very large (see “Fracture Mechanics”).  As a result, even relatively small load variations can be sufficient to cause incremental growth of the crack.  With the small cross-section of the 16 gauge material, this process would proceed relatively rapidly. 
What to do about it?  Heavier wall and more ductile material would seem indicated.  Also, pre-heat might be useful.  And, as mentioned in earlier replies, stress relief and inspection for cracks after welding would be essential.  Also, joint preparation and matching the relative sizes of the wall and weld beads may be beneficial. 

This failure mode probably explains the success found in the technique of brazing socket connections in frames used historically.  The whole area is heated relatively uniformly and cooled slowly, thermal shrinkage stresses are reduced and sharp changes in cross section are not large.  Cracks were not built into the structure.

Offline Old Scrambler

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Re: BSA B50 -500 APG Build
« Reply #395 on: November 26, 2012, 12:47:40 PM »
Tom, Your a fortunate guy 8-)

Your willingness to share your experiences is generating some very helpful advice.....to ALL OF US :-) :-)

I only wish to add to your effort to reduce the overall vibration and load factors.....

Since you experienced the clutch failure, have you checked the shaft and chainwheel to see if they are true?

I would eliminate any holes in the frame tubes and use lugged-clamps to go around the tubes to fasten things like your footpegs.

I would try to measure the 'sag' in the frame when you incrementally add weight.....motor & tranny......RIDER.....and then attempt to calculate the flex-factor for every bump you hit along your highway to speed :wink: :wink:

Personally, I like rear suspension and rely on it to solve most of the downward-force problems......including keeping the tire in constant contact with the earth :-) :-)

I know you will solve your collective problems.......and......just maybe.......the end result will help to keep you away from the flag-poles :-D
2011 AMA Record - 250cc M-PG TRIUMPH Tiger Cub - 82.5 mph
2013 AMA Record - 250cc MPS-PG TRIUMPH Tiger Cub - 88.7 mph
2018 AMA Record - 750cc M-CG HONDA CB750 sohc - 136.6 mph
2018 AMA Record - 750cc MPS-CG HONDA CB750 sohc - 143.005 mph
2018 AMA Record - 750cc M-CF HONDA CB750 sohc - 139.85 mph
2018 AMA Record - 750cc MPS-CF HONDA CB750 sohc - 144.2025 mph

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Offline wobblywalrus

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Re: BSA B50 -500 APG Build
« Reply #396 on: November 26, 2012, 09:43:29 PM »
The chassis Briz built looks plenty strong and low.  One like it should hold up well with the BSA if the big Westlake could not break it.

   

 

Offline WhizzbangK.C.

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Re: BSA B50 -500 APG Build
« Reply #397 on: November 27, 2012, 08:58:29 AM »
I had a moment of clarity last night at work and thought I saw another similarity between the cracks, came home this morning and looked again to verify. For what it's worth it looks to me like all of the cracks originate at the toe of a weld in or adjacent to a non-triangulated part of the structure. If you want to continue to run this thin wall frame I'd bet you can get away with it by adding more triangulation to the seat/tail subframe and beefing up the lower motor mount area with some thicker tube sleeved over the existing tubes.  :cheers:
Ah, this is obviously some strange usage of the word 'safe' that I wasn't previously aware of.  Douglas Adams

Offline Koncretekid

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Re: BSA B50 -500 APG Build
« Reply #398 on: November 27, 2012, 09:50:09 AM »
I had a moment of clarity last night at work and thought I saw another similarity between the cracks, came home this morning and looked again to verify. For what it's worth it looks to me like all of the cracks originate at the toe of a weld in or adjacent to a non-triangulated part of the structure. If you want to continue to run this thin wall frame I'd bet you can get away with it by adding more triangulation to the seat/tail subframe and beefing up the lower motor mount area with some thicker tube sleeved over the existing tubes.  :cheers:
Ed,
Your observations confirm my faith in the ingenuity and intelligence which I have observed in acquaintances and employees over the years who have never had the privilege of advanced education!  You claim not to be an engineer, but you have made the same conclusion that I (and I am an engineer) have made, that the sub-frame/tail section was not properly triangulated.  It is more of a parallelogram with the nodes welded to form moment connections. The various forces generated by surfaces roughness of the salt/rotating out-of-round wheel/vibrating motor, etc., etc. have caused this parallelogram to flex at the nodes introducing high stress concentrations adjacent to the nodes.  Once a crack was initiated, it would have grown due to "fracture mechanics" as indicated by Interested Observer.  What I can't figure out, is why the location of the first cracked/broken member occurred at the attachment point of the chain guard, and not at one of the corners. The first time this happened (at Loring in July), I poo-pooed it as being a fluke, probably caused by the fact that I had welded that tab to the frame and had probably burned a hole in the tube. But the repair job was done by complete replacement of the tube and welding by a certified aircraft welder.  And the resulting broken tube is identical to the first.  I think the important thing here is not to add attachment points for accessories except at the juncture of tubes so as not to introduce possible stress concentrations at other points in the frame.

The sad part is that I have now found two more cracks on the opposite side of the frame, including the beginning of another one in the main frame.  This makes a total of seven known fractures involving more that 10 intersecting tubes. This seems to indicate a complete re-build of the entire rear part of the frame, if not the entire frame.

The chassis Briz built looks plenty strong and low.  One like it should hold up well with the BSA if the big Westlake could not break it.

 

Bo,
Briz's frame is a copy of the famous Norton Featherbed frame still being used today in vintage racing circles.  I studied that frame design, and the forward part of my frame has many of the same attributes (double perimeter top tubes, double loop downtubes).  I diverged from this design at the headstock because I did not like the crossover design of the Norton, which has a weakness at this point, and I wanted to make use of the headstock as my "window."  Of course, if I had used a sprung rear end, my frame may have never failed, but I was trying to make it simple.  The other design problem that comes up is that of creating a subframe at the rear to support the long overhanging tail section on my bike.  Not that I did a very good job in this area, as it didn't seem to be a major structure at that time.

Dennis and Bill,
You both have indicated that I am indeed a fortunate guy, having dodged a few bullets.  At Loring in the spring of this year, I blew the primary chain out thru the side case at about 135 mph, which could have seized up the mainshaft and hence the rear wheel (been there, done that), at Bonneville I got blown slightly off-course and it ingested one of the flag poles at 137 mph, and now I see that I seem to have gotten off this bike at about the right time, before it just went bang.  I'd just say lucky, but there are those that would use the term "death wish!"  No! It's a calculated risk!

And for those that contributed technical advice, it was very informative.  I think this failure demonstrates how important the careful inspection of the frame is after each and every event, and even careful observation during an event.

Again, thanks to all.

Tom
We get too soon oldt, and too late schmart!
Life's uncertain - eat dessert first!

Offline thefrenchowl

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Re: BSA B50 -500 APG Build
« Reply #399 on: November 27, 2012, 10:22:17 AM »
Koncretekid, Can you put arrows on this photo of yours to see where all these cracks are?



7 cracks seems a lot considering the size of the engine -small- and the size of the frame -biggish compared to my 900cc frame-



Patrick
« Last Edit: November 27, 2012, 10:30:21 AM by thefrenchowl »
Flat Head Forever

...What exactly are we trying to do here?...

Offline Koncretekid

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Re: BSA B50 -500 APG Build
« Reply #400 on: November 27, 2012, 10:37:14 AM »
Patrick,
I haven't yet figured out how to put arrows on my photographs, but all of the fractures occurred in the rear sub-frame area, and at the juncture of the seat hoop with the main frame, except the one at the lower motor mount.

I have just now made an observation as to why the first failure occurred at the vertical strut over the rear axle at the attachment point with the chain guard, which was added after I took that photo.  If you look at the previously posted photos, you will see the one I'm talking about.  If the rear subframe vibrated fore and aft, as we know that the motor does, and was constrained at the attachment point, a bending stress would have been induced at the attachment.  Along with the stress concentration created by the welded tab, the endurance limit of the tube was probably exceeded causing a crack to form at this location, and the rest of the damage followed.

Tom
« Last Edit: November 27, 2012, 10:40:32 AM by Koncretekid »
We get too soon oldt, and too late schmart!
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Offline bak189

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Re: BSA B50 -500 APG Build
« Reply #401 on: November 27, 2012, 11:10:21 AM »
Hey, to much engineering talk......Never build a M/C and/or sidecar frame out of tubing less than 1 1/4 dia. .....nothing wrong with .060......and bronze weld the joints.....as it appears your cracks are next to the weld...................................
Our LSR sidecar when raced is under a lot more strain (with 2 people aboard) than a solo bike...our engine is a lot heavier....we are rigid at the back end and sidecar wheel....................We build the outfit in 2002 and have run the salt every year till 2011 .....not one crack or chassis failure..........................................................

PS. Yes, I also have a engineering degree....but I keep it a secret.... I don't want the verbal abuse...........
PS.PS. Does your BSA single have a head to frame mount?...if not make one....will fix some of the vibration....................................................................................
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Offline bak189

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Re: BSA B50 -500 APG Build
« Reply #402 on: November 27, 2012, 11:30:24 AM »
As a follow up to my last post......looking at your frame....cut off the back end of the frame at the rear motor mounts and start all over.....run a 1 1/4 dia. tube from the rear axcle plate to the tube that has the engine rear motor mounts, on both sides....than build the rest off those tubes......Ever look at 1/4 mile m/c drag frame?
Not engineering talk........over 60 years of BRAZING frames and chassis..............
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Offline Peter Jack

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Re: BSA B50 -500 APG Build
« Reply #403 on: November 27, 2012, 11:36:21 AM »
Tom, PM sent.

Pete

Offline bak189

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Re: BSA B50 -500 APG Build
« Reply #404 on: November 27, 2012, 03:01:52 PM »
I keep hearing .060 called "thin wall tubing".......039 CroMo, now that is thin wall
that I have used for roadracing 250c.c. frames..and yes, the CroMo was Tig welded...you don't bronze weld CroMo.....Also I noted on the frame picture the motor mounts......spread the load out on the mounts with a plate....makes for less vibrations....also less load on the tubing.......I know some of you may find fault
in my posts "he is not being very nice".....Hey, we are trying to help and keep the man from being hurt................"Nice guys finish last"......................................
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