Cause and Effect!!!

hi all, just a couple of pics and comments to explain cause and effect!!!
Race bike i started racing last year has been so good and reliable and no signs of issues.
Doing track days and race meets getting more confident with the bike.
Last weekend , the Hamilton MC held its first round of their winter series at Hampton Downs Raceway., for you northern hemisphere bods, it is our winter!!!
I entered 2 x classes, so that gave me 2 x practice sessions, nearing the end of the second session, after heading over the top of the hill and the fastest part of the track , changed down from 5th to 4th and 3rd under brakes, for turn one which is a blind down hill right hander, bike backfired so badly that the guys following closely behind thought they were going to get scorched with the flames coming out of the pipes, I immediately knew something was seriously amiss, so clutch in, pulled the engine "kill' cord , right hand up to indicate me stopping , coasted to a stop on the grass verge, looked down to see a crack in the DS case!!!
Well for the cause and effect.
During my checks on the day before race day, I missed the bottom gear box bolt, which came out completely during practice, allowed the belt drive to tighten up so much it put enough weight on the crank to close the 0.014" gap between the rotor and the stator of my crank mounted ignition, which made the stator rotate enough to put the timing out of phase to cause the backfire and the split case !!!
Anyway no other major damage done except to my pride (yes still have some) and in a round about way uncovered an issue with temperature and oil that could have been a bigger disaster.
JSM has posted a coupl of close ups of the pistons, here are a few more pics.
Enjoy, regards Mike
Pics arent great, phone pics only

Ignition rotated in its mounts


Split case and inner primary that is there only to mount the ignition

No valve damage and burning clean enough

Some slight wear on the cam followers, hot oil problem I suspect

From want of a nail the shoe was lost...
Expensive views of your priceless experience. If piston decent lags a tad behind flywheel mass sling, the crank ends tends to deflect-bow most towards the 7 oclockish area. I guess there's still no sense talking ya out of trying it again.

Bugger.... You were posting 1m28 times too.
You passed me going up the hill, and I though I'll see if I have the speed to keep up, was looking promising till you were 1/2 way thru turn one, looked down , I passed wide then heard 'noises'.
I suppose " that's racing".
Will you have it back together for July or a more measured later in the year approach?

Oh my gawd at least you are optimistic headed. :roll:

Thanks John, yes will be up and running soon, SBR cases and billet will have to wait a little longer, this motor will run again very soon, just need a couple of cases and to sort the oil problem, working on that now.
Glad it was a good day for you and Adrian, thanks for the pic, I didnt get any at all. Did Paul manage to keep the Triumph upright for the rest of the day?
Regards Mike

You have a direct cause and effect, but in time maybe those cases were going to be a problem anyway....wth the kind of spec you have, and riding it hard....though I understand the logic that says those rods and pistone will improve longevity....

SBR cases are overkill for what you need, of course its a 'local' product for you, but you would be amazed how much thicker Maney cases are all over...I suspect a set would have survived this incident......and they are a bit more affordable than SBR, you would have enough left over for a crank and a FullAuto head

Good luck with getting it out quick, but check over the crank well

Bad luck Mike. But that's racing!

You weren't that unlucky really though, you're not hurt and you managed to trace the problem right back to a root cause. Often the cause and effect is lost in an expensive pile of shrapnel!

Its nerdy I know, but I used to get quite a kick out of trying to get to the root cause of each issue when I was racing. Then I had a rule of never just fixing the symptom, but always improving. But that's why I had to stop, it was becoming a full time obsession!

What were you revving to Mike? It's difficult to see in your picture to be sure, but that cam follower wear looks like the
kind of wear one might expect to see from valve bounce.

I respectfully disagree with the failure mode hypothesis, the loose trans would have caused belt failure, but not broken case. Nor would backfiring break the case.
The forces applied on the crank/bearing/case from torque during operation exceed the pull of a tight belt.
I expect a cracked case was existing, a trans bolt had been loose quite a while, and finally the cracked case allowed misalignment of the ignition enough to cause backfiring, and the slowdow/stoppage.
Just my most humble opinion. :mrgreen:
Glad you didn't go for a slide :shock:

Thanks guys, fast eddie, have the rev limiter set at 7500 rpm and taking it regularly to 7000
Yes was warned about the 40 year old cases, but I had to start somewhere and went with what I had in hand.
The reason behind the SBR cases and Full Auto head was more the fact of getting great enjoyment out of the build, yes being local also helps, using lots of researched expertise and just knowing I can go and have some reliable fun. I am not a real quick rider, probably be no faster than I ever thought I was , also there is no greater thrill than riding a beefed up Commando motor with heaps of torque in a standard frame , drum rear brake, single standard disc with kitted master cylinder , all on sticky treaded tyres and having lots of fun in the wet and dry!
Our classic racing scene waxes and wanes, especially in the Open Post Classics, so the more the merrier!
Gives another perspective to the forum, as there are many who are not able to be in the same position.
Regards Mike

Concours, have had it mentioned my hypothesis may be incorrect, the weekend before this I was at a trackday and the gearbox bottom bolt was tight and in place, after this incident the bolt was completely gone, not just loose, so I have to wonder the chicken or the egg?
Still , other than a drive side case , everything is all OK and we will have it running soon.
Just the oil problem to solve now.
Regards Mike

Ouchie!

Hey Brooking 850, glad it did not grenade on you.

I would crack test the crankshaft.

Well Mike, it proves you dont "Pussy foot around" i would have expected the crack to be vertical if the clucth had pulled back..more "Crank flex :?: " .

Sure is possible to spend lots of money on motorcycle racing.
Thanks to my able machinist Nick my 1000cc engine has run to over $700 including the two donor engines.
Look forward to meeting on the grid in July.
I'm comfortable doing 1m 26's around HD, Adrian was doing 1m22 chasing the CB 350 twins...... I never knew till Sunday he smoked ...lots. :shock:

Hi John, yes Adrian does and so did his bike!!!
Crankshaft on its way to be tested.
Maybe I should pussy foot around and not break anything!!!
All in the pursuit of good classic bike fun
Regards Mike

Awesome thread Brooking 850, and I must say, that is a really impressive crack you've made there :wink: thanks for posting the photos. Cj

The racer cracked cases that didn't blow apart were cracked more horizontally from center of crank hole rather that from 7 oclock to 2 olcock diagonal. To force crank cheeks into lower front case implies something prevented the pair of pistons jerking down freely so flywheel bowed upward backward while cheeks and crank ends bowed down forward. Could just be a casting fluke let go so no clue at all on what failed first. Might consider the old school recovery and re-enforcement mods that with stood Peels horrendous no load stuck throttle event.

Steve, he mentioned a back fire.. i would think a huge timing advance a full revs would put the crank under heavy load?

hobot said:

The racer cracked cases that didn't blow apart were cracked more horizontally from center of crank hole rather that from 7 oclock to 2 olcock diagonal. To force crank cheeks into lower front case implies something prevented the pair of pistons jerking down freely so flywheel bowed upward backward while cheeks and crank ends bowed down forward. Could just be a casting fluke let go so no clue at all on what failed first. Might consider the old school recovery and re-enforcement mods that with stood Peels horrendous no load stuck throttle event.

Click to expand...

Note the opposite moving cyclic masses deflecting... I'm guessing about...

Even if a casting fault in the cases, which I doubt after decades testing.[*] something extra happened here and we may never really know. We've a few reports of the piston bind evidence in lower bore from that cooler area not expanded as much as bore and piston near TDC. This would cause resistance-delay of piston rising which would let flywheel mass bow upward most and may explain the horizontal case fratures that aim to pilots groin. Something that delays piston descent would-has blown out in the forward downward diagonal as this instant case and my 3 Combat instructive events. Unstable ignition was what did in Combats the most at first so certainly a blow up cause on its own but mainly would be destructive if spark was too soon, it would mimic stuck down pistons causing crank center deflection to rise and the ends to descend, ie: busting out the top forces. Lost of tanny bolt over tension on DS crank end would tend to pull it rear ward causing more forward crank center deflection. So adding above together paints a picture to that lack of main shaft out rigger allowed it to candy cane twist and spring which loosened the gearbox restrainer which threw off ignition to over advance which over heated top more than bottom of bores and also allowed more forward downward crank deflection which also caused out of line journal bind in the rod big ends which added to piston resistance to change direction most from TDC so hung up enough to stop the show. Ken Canaga was seizing big block engines till he opened bore to .005" clearance. So will ya just put back together w/o alterting anything influencing this?

Our kissing cousins Harley also break up

If the big bang stops / reverses the piston , the rod stops the crank at the tangent of the trow , the rotateing mass is arrested at the big end ,
THROWING the mainshafts DOWN ish . Oh Dear .

Id be checking the center crank area rather thoroughly & the mianshas to webs , as the load reversals gone through there & them .