76 degree offset crank

I sort of agree with Rohan in that all the trick stuff is a waste of time and money for the street.

An expertly rebuilt 750 or 850 with the small port head and a bump in compression to 9.5 or 10:1 will feel really good below 100 mph and it will be reliable below 7000rpm, but especially below 6000.

Use the stock cam and have one of the known experts port the head for velocity and not all-out flow.

You just saved several thousand dollars........

No kidding, brisk 30 to 80 is what makes even ordinary run of the mil Commandos so addictive. That's my power peaky Combat standard operation range in 4th gear when not in a thriling hurry, but dear dude you if you have a hardened heart you can stay in 1st to almost 60 and 80 in 2nd when the wild hair hits. My nothing at all special plain Jane Combat actually wakes up more after 85 in 4th. This makes it kind of hard not to zoom right past 100 as the sense of effort to do that decreases the faster ya go until runing out of horse power. A plain old Combat should be good puller to 120. Might just want to build a Combat engine and see how that works for ya before spending for something even better and can hang with moderns to 130 or more. I'd ridden with a bunch of the hot bikers and almost non of them is willing to press over 120 let alone 130 in scenic hyws. If ya really want to suck your eye balls to back of skull go out bid everyone on the current ebay Drouin. Going with the best parts and processing available expect to lay out like 10 grand or more to make reliable 80 hp 750, 90 hp 850 and 100 hp 920. I'd expect 110 out race level 1007 extra long stroke. That's rwhp.

I don't use methanol to take advantage of it's anti-knock ratio - my engine is on standard 8.5 to 1 comp. Methanol has an extremeky high latent heat of vaporisation, freezes the incomimg charge, and makes it more dense. This makes the jets sizes become larger to get the mixture right - more fuel is a dded - a physical/chemical supercharging effect. The fuel is more forgiving of tuning errors than petrol, so the bike performs better in races, also the heat build-up is reduced. Problem is you use about twice as much methanol compared with petrol. It is the only way to go road racing but a two stroke 350 on methanol is unbeatable for the first two laps until the motor heats up a bit, and the mixture leans off. Our historic and modern road race rules prohibit it's use on bikes made after 1972. I don't use very high compression ratios because I believe the pistons usually weigh too much, limiting the safe revs. If I'd known about the JWS long rods and light pistons a while back, I would have bought them. Short of cash these days.

With isolastic mounts I would not recommend an offset crank. Isolastics work best when there is no rocking couple and an offset will introduce a rocking couple that isolastics can not handle.

Solid mounts- go for it. Jim

Whitworth Ranch said:

A short stroke Norton (360) might be fun and bear some fruit. You could go 80mm x 80mm and with a long rod get a nice 2:1 rod ratio in about 800 cc. Valve train would take some work, but nothing money and titanium couldn't help with. Might need Comstock's help with that, particularly on the head. But the crank, rods and pistons would not be too tough. Getting the head to breathe at 8K while still maintaining that healthy area under the torque curve so you could ride it would be where most of the magic would need to happen.

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Hmmm, I am now in possesion of Maney cases an 80.4 crank, 6.57" JS Rods and an 81mm barrel, thats 828cc, interesting number....we will see how well I do getting either a good barrel together for 77mm or 81mm pistons from Jim? Avoid the titanium valve train and use JS should get there. 8000 would be easy enough....and no reason for it to throw itself apart.....but how often it would ever be used on a road bike I have no idea....

Like the man said, if you have nothing so far Nourish will do what you need, one UK based father and son run a 900, 90 degree crank "Weslake" and love it....don't think its much slower than the Watson 1007.....

Got to think that a road going 1007 would be a torque monster, but would shred gearboxes for breakfast....

I suggest that when building any bike, always start at the gearbox. It you have 6 speeds the loads are not as great, and the acceleration is greater because you are always using the best part of the torque range. The standard commando box is a horror in a race bike, if you are used to a CR gear set. It is impossible to be smooth when changing down, makes you too reliant on the tyres when you are clumsy. I feel that when I ordered the TTI box, I should have opted for a standard commando ratio first gear then had the extra five evenly distributed between normal second and top. That way the 'stagger' would be removed from first gear during clutch starts, and occur with second gear, which is the lowest I use when mobile. It would mean I could use the top five gears to work the bike up to a much higher top speed.
Probably isn't important on a road bike, I'd stay with the standard box. I feel I would have got better results with my old 4 speed CR box, if I'd used the standard commando first gear. - Wise in hind sight ?

RoadScholar said:

I am an admitted dreamer and Specialist, not an engineer. Anyone out there with thoughts about a 76 degree offset crank?RS

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If you were to take an engine with either a 360 deg crank or a 90 deg(ish) crank and spin it with an electric motor so that there was no combustion taking place, you would be able to measure all sorts of torque fluctuation and reactions at the engine mountings due the kinematics of the engine and the inertia of various parts. In such a test the 90 deg crank would be the smoothest.

If you were then to build engines of the same configuration using some theoretical material having zero density and run them under their own internal combustion “steam” you could measure the same torques and forces, but this time they would be due only to the pressure in the combustion chambers. Actually, in this case, I don’t think there would be any reactions at the engine mountings because combustion pressure acting “down” on the pistons/rods/crank/main bearings & crankcase would be canceled out by that acting “up” on the cylinder head. The internal forces on on all parts would still be considerable and there’d be mighty torque pulses of duration equal to about 90 degrees of crank rotation. In this test the peak forces/torques involved would be the same for both the 360 and 90 deg engines; the pulses would just be timed differently.

When the effects of inertia and combustion are combined as in a real engine I believe many of the theoretical advantages of the 90 deg crank disappear. It’s often stated to be an advantage of the 90 deg crank that both pistons do not come to a stop at the same instant, but as far as I can see it’s no bad thing since it “softens” the combustion torque pulse, resulting in less overall torque variation. Likewise the tendency for combustion pressure, applied through the piston and rod, to bend the crank is mitigated by the opposing bending moments due to piston inertia.

All this is highly dependent on the magnitudes of the forces involved and there may be combinations of engine speed and throttle opening that have convinced current manufacturers that the 90 deg crank has the edge, also current engines offer complimentary benefits in the form of 3 main bearing cranks and balance shafts, but for the original Commando and similar parallel twins I suspect that the simple, “crude”, 360 crank is the better bet.

I recently ordered a 1-piece crank from Dave Nourish and I was tempted go for the 90 deg, but after playing around with an Excel spreadsheet I couldn’t convince myself that it would be an improvement, especially when you take into account the valve train and ignition system changes required and the fact that simple geometry of the 360 crank arrangement must be the stiffest and strongest.

One advantage of the cranked crank , is said to be the superior traction / power transfer , of the uneven fireing impulses . As the whizz bangs leaned to their cost emmulateing .