Frame crack

  • Thread starter Thread starter Davo
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I saw a few Laverdas racing at Knockhill in the late 70s/early 80s and more than one had front mudguards hitting the centre downpipe. Likely bent forks and/or frame issues. And these were relatively new bikes at the time.
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Hmm - Think that may have been more likely where the infamous "frame chop" had taken place  ;) ;)


....interesting; so how did the 900ss do in the Avon series? Can't recall hearing much about them...which were the top 3? Sorry for thread hijack!!
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The 900ss's did Ok - Dave Railton was one of the best.  - Regards - Phil
 
nick, PK Davies had his racing career ended when his frame broke on the IOM which i doubt had done too many miles.
andy
 
Jeez, a frame break doesn't  bare  thinking about....anyone for 4 pot modern calipers to help things along?😃mind you, I recall that Marvic wheel collapse on the IOM  on a Honda....so not just old stuff or Laverda.
 
thats why i don't put GXR or any other up grade on my triples, the forks don't take it and if you upgrade the forks then its the frame and then the rear and so on, and at the end of it could anyone still ride one like the old proddy racers . me thinks not.
but i do like specials and what people do to them.
andy
 
That's why the whole bike needs to be uprated when one change is made.
I went through the checklist before uprating my project bike POP.



 
we spent two seasons racing Rosie with a standard frame, braced at the headstock and swingarm, with GSXR rims, race compound tyres, 150 rear, 110 front
310mm rotors on the front, 4 pot brakes, 120 hp at the rear wheel, 100kg rider
didn't have any issues with the frame cracking up and we would be throwing way more load onto the frame racing than any street rider will ever do

really don't see the issue truthfully

 
Key point is 'braced at the headstock'.
Seems to solve the main problem.
Everything else I did just improved the bike over standard.
 
::) If I've said it once, I've said it a thousand times:


Re: Mudguard hitting middle downpipe (3CL)
? Reply #36 on: August 24, 2016, 23:54 ?
QuoteModifyRemove
Quote from: Various on August 24, 2016, 21:07
or is it a good idea to improve the headstock strength on the frame anyway?

Yes, because those flimsy pogo sticks up the front flex back and forth at a rate of knots (do not look down the line of the forks when riding at 100klms, you'll never ride the thing again. And if you do, you'll never look down again) and weaken the headstock until it cracks. That's my theory anyway.

I like Lothars diagram, but the occasional braking force won't have that much effect, if that were the case the cracks would appear at the top joint at the headstock. It sort of makes sencs that the place where it cracks is from force being pushed forward at the bottom triple clamp. See above comment in bold, add to that, the weight the bottom mounts are holding up at a angle. But if parked for 40 years, they'll never crack there.

Upside down forks don't flex back and forth like the original pogo sticks these old girls had fitted. My Kawasaki with USD forks are rock solid at all speeds (that I'm game enough to look down the forks line anyway).
 
Grant said:
I know of some Jap bikes that had frame cracks back then too. Frame design was still loosely based on bicycle frames back then.
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My old Kwak Z1B cracked right through both downtubes and gussets just below the headstock. It was 3/4 of the way on a trip around Australia. Managed to get across the Nullabor, up the WA coast, across to Darwin, down to three-ways, across to Townsville and down the East Coast. Frame finally came apart around Grafton in NSW. Bike started handling like it had flat tyres and there was a clunking noise as the cracks in the downtubes were opening and closing as we went over bumps. The only thing holding the headstock on were the 3 top frame tubes. I still don't know how the top tubes didn't fold and let the bike's belly collapse onto the ground, especially as we were travelling two-up with luggage.
 
Chris, for me it is clear that under breaking (but also under riding), most of the force goes into the area where the crack happens. the stearing head-tube's lowes point where it is connected to the rest of the frame is the fulcrum; however, as this point is relatively strong, the forces rund down to the weakest point, the force gets transfered exactly to the point where the downtubes crack.

this happens a) by normal riding due to the stearing-head angle and the wheel beeing pushed up vertically causing the fork to flex and b) under breaking, when the fork is compressed and the negative g-forces push the fork backward.

Grant's longer and better supported headstock distributes these forces to a wider area, which improves the whole thing I feel. Also, due to the lower triple clamp going down a bit, the unsupported area of the upper fork tube is shorter and therefore the flexing is reduced, which leads to a more controlable front-end (but leads more force into the improved headstock area again, as it can't be compensated by the flexing fork anymore - as it does not flex that much anymore).

My Motoplast Frame f.e. has also a way longer headstock than the original frame.
 
laverdas said:
nick, PK Davies had his racing career ended when his frame broke on the IOM which i doubt had done too many miles.
andy
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  The frame wasn't a Laverda frame - it was Uk made from a very thin wall tubing. At the time myself, and others, doubted the wisdom of such light tubing being used on a road circuit that isn't as smooth as a closed track. As to whether the frame broke and was the cause of the accident or the exhaust broke away and grounded - causing the frame to snap is still debated. - Regards - Phil
 
I think (dodgy memory?) that early onset frame cracking in Commandos was found to be partly due to them dropping the bike's crate, at height, off the back off the ship/truck when disembarking in USA.
 
laverdas said:
thats why i don't put GXR or any other up grade on my triples, the forks don't take it and if you upgrade the forks then its the frame and then the rear and so on, and at the end of it could anyone still ride one like the old proddy racers . me thinks not.
but i do like specials and what people do to them.
andy
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The frames will crack in bog-standard form, just depends on how much strain you put them through.

Being a late and heavy braker doesn't help matters, the leverage afforded by the long forks and the short head stock result in ernormous stresses going into the frame.  That the top tube and front down tubes do not cross before they join the head stock is imho the main flaw in the design (see Norton featherbed, BMW /5-/7 series).  In that case, the fore-aft forces would have been a lot easier to manage.  The triangulation helps a bit, just not enough...

The huge unsupported length between the rear upper engine mounts to the front engine mounts could also be a cause of the cracking, the join to the down tubes is the weakest point imo.  Steering and braking forces have huge leverage and are countered by engine stiffness a long way away.  I've been thinking a about a sturdy head-steady for years, just haven't worked out something that won't greatly hinder any top-end maintenance work.

piet
 
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