BSA B50 -500 APG Build

[Koncretekid]

Don't worry Bak, I would never consider you to be a "bad guy" for offering constructive criticism.  When I decided to build my own bike (when I didn't know you), I relied heavily on what I read, and I think it was John Bradley that said "if you're using anything heavier than 1" 17 gauge, you're building a tractor and not a motorcycle."  Of course, he was using Reynolds 531 tubing.  But I reasoned that, dang, how could a little 40 hp BSA motor in a straightliner ever put as much stress on a frame as a 100 hp motor in a road race bike?  After all, the records in my class were something like 111 mph (open class) and 114 mph (partially streamlined.)  And I still think the design is good and the forward part of my frame is sufficient.  As far as the lower motor mount goes, it is done in the typical British fashion of the bolt thru the frame.  I can't lower the motor because I wouldn't be able to remove the side covers, and I don't want to raise the motor, so it goes.  However, there are ways to reinforce the frame around those holes, and that is what I'll do.  I probably won't change to 1-1/4" tubing because I don't have the bending dies for 1-1/4", but I think if I re-design the rear sub-frame I can prevent the same kind of failure in the future. I might even spring for a newer cast aluminum rear wheel in the hopes that it might actually be round!

Oh yes, about the top head steady.  Yes I have one and it is substantial. All the crossmembers are 1" by 1/8" wall, so I believe they will survive.
Tom

[bak189]

One thing to keep in mind...you have there a "dry sump" engine that can be tipped and/or tilted to a certain degree in the frame with no problem.....Unlike most of todays "wet sump" engines......Also most of the good handling and strong British frame used larger than 1 inch dia. tubing....Rickman....Norton feather bed.....Oh Yes,
they were also Bronze Welded....Congrat, to you are in order for making that "Old"
Beezer go that very fast speed......Good Aero............

[saltwheels262]

Tom,
build a new frame like it's for a Mack truck.

I would reconsider the steering stem on the new 1.

bf

hope to see you in limestone.

[55chevr]

I absolutely love the steering neck ... That bike has a lot of thought in it

[saltwheels262]

yes, the see through neck is neat.

I was thinking .5" or .625" thread { minimum } ; grade 5 or 8 , 6 or 8 inches long.~~~  stem .

bill

[55chevr]

The first stem was all thread, I thought that was way cool.

[wobblywalrus]

Tom, in the past I have had similar problems.  My experience is that I would reinforce something to resist vibration and something next to it would crack.  I was constantly chasing cracks with gussets and repairs.  One time a made a strong engine mount and the motor broke itself up at the cases where it attached to the mount.   

What I learned was to kill vibration at its source.  This would be having the engine balanced, and if that did not work, using a different engine configuration.

Another thing I learned by observing power transmission from farm tractor power take offs to farm equipment.  Vibration is a form of power loss like heat or friction.  A smooth engine puts more power onto the ground.         

[Koncretekid]

Bo,
The motor has been dynamically balanced to 65%.  You simply can't balance a single cylinder (or a 360* twin) without a balance shaft.  I questioned the 65% factor, but that was what was recommended. And as for using a different engine configuration, bring on the options for the pushrod class.  There are some, namely Honda CX-500 & 650, Moto Guzzi, and BMW, but they present other problems (converting to chain drive, for example.)  Now there are some other cranks available for Triumphs - -180* and 270* - - which might be smoother.  And of course there is the 750cc Triumph/BSA triple, but I think Tom Mellor has that class quite well covered.  Norton simply isolated the motor and drive train from the frame with rubber dampers.  That would make an interesting build.  When this one blows, I'll check into them.  So many ideas, so little time!

However, I think it is significant that most of the cracks are behind the motor cradle, which makes me think the cause of the offending vibration is the rotating wheel along with the not-so-smooth surface.

Tom

[JimL]

Might be good to have a prime number of teeth on one of the sprockets.  I remember hard tail conversion choppers breaking tubes, and fixed by playing the sprocket tooth counts to get the chain settled down.

If you werent breaking until you added 10-20 mph, what else really changed?

I worked for a real smart guy, when I was young....wish I'd listened (and believed) more.

[bak189]

JimL, is correct.....the way the rear chain runs`will effect the pull and some vibrations.....also the relationship of the front spocket to the rear sprocket.
Back many years ago when we did some Dirt Track frames we ran the bikes on a rear wheel Dyno using a strobe light on the rear chain....it is very scary what you see.....by changing the number of teeth on the rear sprocket in relation to the front we were able to settle down the chain somewhat.....we then made up for the gear ratio needed by changing (BSA A-65) clutch to a other number teeth pre-unit Triumph hub (single row Pri. chain)........we also picked up 1 1/2 hp.....when you are working with a maybe 50hp. ....1 1/2 is a BIG THING.................................
The Dyno runs also showed that the twin engine (BSA A-65) was a lot easier on on the rear chain than the single (BSA Gold Star)......In those day we had no 4 cyl. engines to check the chain runs.......Also keep in mind that a long chain will give you a lot of "Whip"...Our LSR sidecar has a long chain, with a roller sprocket guide on the lower run to keep the whip down.....................................................

[DND]

My flat track buddy ran their Triumph 750 on the Axtel dyno, and using the strobe you could see where the barrel was trying to come off of the crackcase and how the frame was moving in the engine mounting area.

Racing is pretty interesting seeing all the moving & shaking going on, and how we cope with it !!

Don

[Koncretekid]

Last year I ran 18T on the front with 45 and 44 on the rear.  This year I ran 19 on the front (which happens to be a prime number) with 44 and finally 42 on the rear 'cause I didn't have a 43.  I have a 42" chain run with an idler sprocket about a foot behind on motor on the lower run.  In regards to the dyno, I believe it is harder on the bike and the frame with the rigid rear than are the runs on the salt.  The first break in the rear vertical strut occurred after a dyno run, and I didn't notice any cracks except at the lower motor mount (which only showed up after I reassembled the bike after rebuilding the motor) until after another dyno run.  Coincidence?  Maybe I should have reduced the air pressure for the dyno run.
Tom

P.S. Just realized I upped the front sprocket to 20 for the most recent dyno run (after Bonneville).  Not exactly a prime number.

[Koncretekid]

With the help of Steve, my welder, I've made the decision to rebuild all of my frame except the headstock and upper forward sections.  It will require about 40' of new tube and will cross at least 30 tube intersections.  Steve advises to just replace one tube at time to preserve the geometry, and re-use the crossmembers (which are 1/8" wall) which will save my motor mounts.  I've ordered 1" x .058" wall 4130 chrome moly aircraft tubing (normalized) today, and was pleasantly surprised by the price at $2.94 per foot. It is more than twice as strong as the tubing I used.  Of course, I have to pay the freight from Toronto, but it's cheaper than I expected.  This will be very time consuming.

The first two photos are of attachments I made for my frame jig to make sure I get the seat and gas tank mounts in the same place, as well as the rear tube which supports the tail section.  I am redesigning the rear subframe with triangulation to support the tank and tail.  The third photo shows  jigs to support the motor mount crossmembers when I cut them loose from the siderail.  The last photo shows two of the tubes I cut out of the rear subframe which were 90% broken. 

[wobblywalrus]

Resonant vibes might be breaking up that square shaped area on the tail.  Some trianglation can help.

[bones]

Hey Koncrete
   I think it's the engine vibration that's killing the frame.
 You could change the balance factor to suite a different rpm.
  The weslake is at about 30deg in the frame, about the same as a modern speedway engine. It was balanced at about 60% and vibrated so much there was a fountain of fuel coming out of the floatbowl.
I rebalanced it to 35% which helped heaps.
Maybe a new balance factor could help at the sustained high engine revs.
Another help might be to make the add-on brackets(chain guard) with a large radius to help spread the load and not have a stress concentration at a sharp edge
Going to a thicker wall tube-2mm- would not add too much weight and help with fatigue.
I took the easy way out and put my old tz350 in my chassis for now.
   cheers    Bones