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Thanks for posting the Dick Cookson article, very interesting.
His “cut and shut” with the crank and cams made me think of Allen Millyard.

The conrod length in the C-range Triumph 500 is near as dammit 135mm, the stroke 65.5mm (ratio 2.061).
AFAIK this was the only oversquare twin Triumph produced (69X65.5mm), apart from the short lived TR65 (76X71.5mm) which has a rod/stroke ratio of 2.131.

Both these models have impressed me with their relative smoothness (for a 360 twin), compared to usual 650’s and moreso 750’s.

The instance of the TR65 I think particularly relevant to this discussion, because it used almost entirely 750 components apart from the shorter stroke crank and correspondingly shorter barrels. So this illustrates the difference between the rod/stroke ratios (TR65 2.131 vs T140/TR7 1.859).

As I understand it, the 76deg theory was developed for the geometry of an engine with conrod twice the stroke.
As NickL suggested, 76deg may lose its particular relevance when the rod/stroke ratio differs significantly from 2:1. Yours will be 1.816. Only the 750 used a ratio below 2.

The MAP crank certainly looks delicious and stronger in the middle, and I do believe the 76deg theory when applied to an engine with 2:1 rod/stroke ratio.
However I think you’re missing something if you imagine offsetting the cranks doesn’t introduce a rocking couple (whereas a dynamically balanced 360 crank will have no rocking couple, but obviously considerable primary imbalance).

In a 360 crank, all the partial balancing (say 50, 70 or 85%) of the reciprocating masses happens in one direction. The crank can be perfectly dynamically balanced, so no rocking couple, but say if the balance factor is only 50% the resultant vibration will be mainly vertical, with only small longitudinal (front-back) component from the crank imbalance at 90deg from TDC.
If the factor is 85%, then there is greater crank imbalance (to counter the reciprocating mass), the vertical vibration will be reduced, with a corresponding increase in longitudinal vibration.
Whichever factor is chosen, there is always residual imbalance, both of which are in the same direction in a 360 crank, so it doesn’t produce a rocking couple.

However, with the offset crank, these imbalances are at 76deg to each other, so say when the counter-weights are at bottom on one side, they are 76deg from bottom on the other side. The ones at bottom are exerting a downward force, the others are exerting a largely longitudinal force. This is a rocking couple, as the forces don’t coincide, and are laterally separated.

At the end of the day, as has already been said, the whole thing is a vibration compromise, as it is with the std engine, trade-offs between primary, secondary and rocking-couple vibration, and I suspect no amount of computer modelling would reveal the facts of the matter as well as suck-it-and-see technology.

All I was hoping to suggest with the earlier stuff was that straying too far below the 2:1 rod/stroke ratio may not get the best from the 76deg crank.

Just food for thought.

whatever you decide to do will be interesting.

keep us posted.

Anyone know rod length to stroke comparisons on other bikes? Like Norton and BSA?

NickL when you used the Norton crank in your BSA, was that short rods, or how did it work?

How did you manage the various different angles, I can imagine the difficulties with cams? Even harder with BSA than Triumph I would expect (1 cam as opposed to 2).

I admire you pioneers!

The thing I spotted was the folding foot rest idea. If my bike falls over in the garage the foot rest bends so I have to take it off, strip the paint off, get it straightened with heat, repaint it and put it back on. What a pain! The solid foot rests don't even seem to protect the cases because there are marks on them.

I'm not saying this happens all the time but when it does I am usually working on it and slow its fall but this doesn't seem to help.

A lot more important would be the con rods and see these have been discussed in the thread. Are steel ones better than alloy? I understand that on a road bike that is just used normally the most likely thing to make a rod break is lack of oil to the big ends.

Dave

On the first conversion i was using a Norton crank with Norton rods+ t140 pistons in a 360 deg config. It vibrated badly at higher 5k+ rpm.
I tried a couple of factors 65 and 85%. the Norton rods are shorter than the 6 inch a65 rods. I really cannot remember flywheel weights.
The second time around was a 68 deg offset with 6 inch special corrillo rods and t140 pistons, using 65 then 55% balance factors, the drive
from the motor out of corners was noticeably greater and with 55% as a factor the rpm was able to be extended to 7000 until retina detachment
or handlebars greater than 6 inch diameter forced gearchanges. 50% factor improved the higher end, it was like that for a while.
When i bought the long type srm barrel i used a 76 deg crank with 79mm B44 pistons this is where i used it most with a 50% factor and tried
lighter and heavier flywheels, heavier was better. That engine was run at 80.5mm bore with a 91mm stroke at one time and was very good,
it would rev easily to over 7.5k but better lap times were achieved with a 6800 rpm gearchange. The old crate would pull a house down.
There's a few japper owners here that went home with their tails between their legs after meetings. Beezers ain't supposed to go that fast,
was a comment i heard a few times. They didn't know the hours that went into it though.

After all the messing about though, i still think it would have been cheaper and easier to have just bought a 900 Weslake motor to start with!
I won't mention gearbox problems......... or cylinder head heating problems.....or valve guide wear etc etc.
We were lucky in as much as we knew Ivan Tighe well, he was a camshaft guru from way back and worked with the Repco team (Brabham)
He would build us up cams and was a keen supporter, sadly he's brown bread now. His son still runs the firm but not so interested.

Happy racing days, i still miss it.

When my brother had the shop here he did several offset conversions on various bikes including a 500 Notron twin which was bloody fast.
A few 76deg t140's with Norton cranks etc etc. The 180 was a motor i didn't use much as i was not that happy with the feel of it but after
putting some extra weights on the cheeks it was much better, that was how it was when sold. Classic racing up here in Queensland died
a death in around 2002.
Using a Norton Commando crank mean't that by making up different flywheels you could set the offset, Pete (my brother) had a lathe and
a later a mill and by making backing plates we could turn them up, he was the real mechanical engineer, i was just an apprentice but i made
good tea and chivied him up. He was a toolmaker and worked in the R+D division at Glaciers in the UK. So was a cluey old bugger.
The offset of 68 degs was arrived at based on piston speed at crank angle as well as the couple effect. Believe me, the offset motor was
a lot smoother at higher rpm and the later ones we did were not dynamically balanced either, just a pair of knife edges.

Very interesting build thread.

Yes interesting.....I stil read alout about Chevy V8's modified for racing. Because more of these engines have been used for racing than another engine it's a vast sum of experience still be gathered after 60 years.
It's said a longer rod engine makes more power at higher rpm, shorter rod at lower and midrange. The cam timing needs to be changed to get best advantage because rod length affects piston dwell time at tdc...
This may also hold true for vintage bike engines....

I'll be consulting with MAP as to the best cams to use for the combination.

Well, I decided...

I sent a deposit off to MAP for a 76°, 89mm, "billet" crank.

I'll also be buying their "billet" barrels, for an 840cc engine.

I'll consult with Marino as to the best cam for such an engine.

Marino says it will be January or so before the cranks come in.

As an aside, that 500 Norton twin pete did, ran as a twingle IE slightly offset single.
rather than a staggered twin. He used around 68 degs on that from memory. On
it's first outing it was beating the club champion before he broke the primary chain.

There has been much work going on in the garage. I still have to finish paneling and run a couple wires (through the ceiling), insulate the ceiling, and install the ceiling tiles. But the bike is on the work stand!