750 crank balance factors

phil37 said:
There's only a few of us that know the magic of a "twisted twin" - but I think they'll be a few more soon. - Regards - Phil
Click to expand...

I was at Phil's place the same day as Bob.

The drive home after riding his bike was full of thoughts of which direction to take my 'pile of bits' SF1 project in. Don't get me wrong, I really like the way my SF3 goes but his 90deg motor was just so much better, very smooth and responsive.

Stuart
 
read both the Phil Irving books while on a train trip from Irkutsk to Magistrany in Siberia, 36 hours on an old wooden Russian rattler going through the Siberian forests, outside temp around -45C
interesting train trip and very interesting books, 2nd book was his autobiography (think that is the right term)

so been twisting and turning all night trying to sleep tossing up 360, 90, 76, 180 and all variations in between
current plan, have a think about machining a new insert for my crank fixture to allow me to press at 76 degrees, is around 8 hours of machining time
design is already done as I made an insert to allow me to do 120 cranks in the 180 fixture, is just a matter of making same insert with different hole phase
pick up the bare crank tomorrow morning and strip and repress back together at either 76 or 90 degrees and take back to get balanced checked, by rights balance should be the same
I'll get Volker Sachse to make up new ignition boards to suit the twisted twin phase, be it 76 or 90 degrees

will worry about the camshafts when we get there, concentrate on the bottom end first
this will be interesting, getting bored building the same style triples all the time

 
The rivet counters will really struggle to spot that deviation from standard.

                                                                                          Dave
 
A guy I know here in NZ did a rephased CB350 crank - never really got it sorted out and it went with the spares when the bike was sold to the US. I believe the new owner spent the money required and it finished up winning it's AHRMA class championship.

Red - now you've had the same answers from the spinners I got...still waiting patiently for slingers...
 
i'm inclined to believe the original slingers were made either with a press or a drop hammer
either way a pair of dies with allowances for the metal thickness should be easy to turn
fully annealed raw stock will work harden by the shaping to form
any reason why aluminium slingers won't work?
 
Interesting topic.  In planning the new engine for the ICR bike I am still torn as to which phasing to use.  From every engineering sense the twisted twin is the way to go, but I really would love to maintain the proper SFC sound.  I can't experiment on my current crank because it's welded. :)

A guy down in Vic did it to his street SF back in the early 90's, don't recall the name unfortunately.  Maybe 76deg won't sound so much like a Ducati.

cheers
SteveB

 
But it won't be a Ducati. I'll be the most innovative Laverda to ever hit the race track in Australia. It's thinking outside the square where classic V twin engine designers never went and that's bloody intoxicating...........in my opinion anyway.

Cheers
Morison
 
Hey Steve, that Victorian bloke wouldn't be Quentin would it?? He's an old mate of mine going back to the 70s and I know he did some crank rephasing, not sure what degrees though. He's Tippie's younger brother so maybe Tippie can shed some light on it. Also Tippie has rephased his crank as well so he should have some comments on his experiences. I also have a SF1 motor minus crank [thanks Quentin!!] here that I plan to rebuild sometime and as Red has my brand spankers SFC centre crank maybe I should have him rephase it while he's at it, but then again the Motodd is gobbling all my dosh so probably better to put it on the back burner and finish the Motodd first.
Red, I've been out of touch lately but I will be back soon once I've got some other non laverda shit out of the way.
Haven't looked at the 1200 motor since throwing a shroud over it in discuss due to the cam binding issues.
Interesting discussion and food for thought when I do get around to my 750 donk.
 
Hey Rob,
Quentin - that was him, rode like a flippin demon so no surprise he's known to you and related to Tippie! :)  Bike had ZZR600 forks and front wheel (or something like that) among other mods.  I recall his was 88 deg, which is simply what you come out with if you rephase the original partially splined crank parts.  The Lav twin was basically made for rephasing - press together crank and two piece cams so all you have to do is sort out ignition. Simple as.

Silvo,
I wouldn't call doing something proven to work back in the 50's and done by numerous others on various types of vertical twin in the intervening decades particularly innovative, but certainly very interesting.

Cheers
SteveB
 
Steve, yep Quentin and Lee [Tippie] are demon riders for sure. Quentin raced a few Laverdas over hear in Perth as well as over east. I was overseas when that was happening so don't know much details. I caught up with Quentin several years ago when I went down for the Oz GP and stayed with him overnight. His 750 was looking sad in the corner of his shed not been ridden for a while. Also saw a few cranks on a bench in various pieces one of which was probably mine from my 750 motor, I thought was safely stored away at my mother's place in Perth while I was on my world travels. As I only have a centre section sfc crank I thought about getting a couple of ends from him to have Red machine up to suit the sfc centre. Last I inquired to Wolfgang about the availability of SFC ends he said he only had an electronica L/H side and the R/H side was no longer available. I can only dream about the 750 at present, doesn't quite make it on the priority list just yet.
Cheers
Rob
 
Guys, The theory behind the "90" degree crank has been around for years and in fact it was bandied around in the 20s but not so widely publicised until Irving.  Read the theory first and understand it fully before changing cranks to the "Nth" degree!   The reason for 90 is because it makes things easy. No other reason for going 90 and not any other "almost 90"    the exact degrees is NOT a "FIXED"  so don't just assume what Irving said for the Triumph engine will be perfect for your engine.
To simplify things I will lightly explain the degree part here.
The max piston speed is at the point that the centre of the connecting rod is exactly tangential to the crank pin centre line when the piston is exactly half way through it's stroke.
So imagine that the rod has an infinite length, the max piston speed will be exactly at 90 degrees.
The shorter the rod the further away from 90 the phasing will need to be!  76 is absolutely NOT the magic number!!
Either stick to 90 or do the maths and get it spot on!   (It only takes a very simple drawing and a protractor. or the simplest autocad)
BUT be sure that there is no reason in the world that re-phasing will deliver more power!!  It won't!  But it may allow you to run easier and indirectly see more power due to allowing more revs from a certain engine etc.  But rev for rev it will not and can not single handedly make the engine's volumetric efficiency or combustion process work better!!  Just smoother and probably easier to ride.

I am not knocking the 90 cranks and it works great as many have proved. The theory is sound and for sure if I were going for max usable power in a vertical twin then I would also be playing with re-phased cranks.

Julian
 
Steve,

Easy to assume ignorance on line but I read "Tuning for Speed" back in the seventies, I've read about all the famous examples and even saw one of them race a few times.

By innovative I mean: It's innovative if you, that's you I mean, build a re-phased race engine. Did you know, watching your twin at the Barry Sheene two years ago convinced me to sell my triple. Your twin then surprised me last year by beating the "other" twin. Ron Young's round case which is no ordinary bike. It's bristling with some tried and true Ducati Factory only parts and modifications and a lot of Ron's own hard work. Much like your bike which just popped out of the woodwork and made a lot of people take notice of both it and you. A 90 degree Laverda engine out there on the track is taking your skills and abilities to another level. Sure it's all been done before, just not here in Australia and to a Laverda. 
 
I made my strong and light crank by using a plain 35mm centre pin (same as SFC as it turned out) and the inner webs from a triple - then turned down and thinned the std SF outer webs. Keith now has the pins and has done conversions the same as these - think Julian and Red now have them too - this means that a strong light crank can be made without using original (rare) late SFC cranks.  Am very pleased to see the interest in this - if you rode one (the 90?) - you would be amazed at the transformation. - Regards - Phil
 
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