76 degree offset crank

Classic Norton Commando Motorcycles.

76 degree offset crank

Postby RoadScholar » Fri Aug 31, 2012 4:42 pm

I am in the purchasing phase for a British parallel twin engine do-over, not a Norton, but could be. My goal is to be reliable in the 7000 to 8000- RPM engine; whether I use that part of the tach remains a question. The vendor that I spoke with talked to me about having a forged crank made where the pins were 76 degrees apart; he said that the idea was to have one piston at TDC while the other was at maximum velocity, he added that the engine would benefit from longer rods than stock , very similar to JS Engineering strategy with his long rod offering that I have implemented in a '72 Combat that feels silky past 3200 RPM; he also told me that the rocking couple vibration would virtually disappear; this motorcycle has a solid mounted engine/transmission. The crank will be complemented with forged rods, a nikasil billet jug and forged pistons. I am looking for cylinder head services, I will be running 32 mm Amal Mk II carbs, std diameter headers and glass packed pea shooters. I am not trying to unseat the crotch-rockets, just to spend my money before the grim reaper gives it to my heirs. :-)

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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Re: 76 degree offset crank

Postby Dances with Shrapnel » Fri Aug 31, 2012 5:04 pm

RoadScholar wrote:I am in the purchasing phase for a British parallel twin engine do-over, not a Norton, but could be. My goal is to be reliable in the 7000 to 8000- RPM engine; whether I use that part of the tach remains a question. The vendor that I spoke with talked to me about having a forged crank made where the pins were 76 degrees apart; he said that the idea was to have one piston at TDC while the other was at maximum velocity, he added that the engine would benefit from longer rods than stock , very similar to JS Engineering strategy with his long rod offering that I have implemented in a '72 Combat that feels silky past 3200 RPM; he also told me that the rocking couple vibration would virtually disappear; this motorcycle has a solid mounted engine/transmission. The crank will be complemented with forged rods, a nikasil billet jug and forged pistons. I am looking for cylinder head services, I will be running 32 mm Amal Mk II carbs, std diameter headers and glass packed pea shooters. I am not trying to unseat the crotch-rockets, just to spend my money before the grim reaper gives it to my heirs. :-)

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

RS


Properly balanced, a 360 degree parallel twin has no rocking couple. A 76 degree crank will have some rocking couple; how much. I do not know.

If it were me I would spend my money with a stock crankshaft (if not stock then 360 crank) as that is what most of the cams are set up for.

Money is better spent on the cylinder head, suspension and weight reduction of the unsprung mass and brakes if you are inclined to use them. This is not to make a rocket but to make an enjoyable and relaible ride.
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Re: 76 degree offset crank

Postby Rohan » Fri Aug 31, 2012 6:42 pm

RoadScholar wrote: My goal is to be reliable in the 7000 to 8000- RPM engine; whether I use that part of the tach remains a question.

RS


Sounds like a complete redesign required - Mr Garner eat your heart out ?

Throwing a bucket of money at something that is not going to be used anyway sounds like fitting a RR engine and wings to a redesigned mini minor - if you are not going to fly it.

But maybe we have missed the point of the exercise ?

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Re: 76 degree offset crank

Postby Dances with Shrapnel » Fri Aug 31, 2012 7:59 pm

Further to the bit about reliable in the 7,000 to 8,000rpm range, properly designed billet cranks can do that all day long. My expereince with Nortons is that the crankcases will vigorously protest as will the valve train. Race duty (Steve Maney) Cases are up to the task and careful componenet design and selection of the valve train is most critical.

In the end you are better off buying or building a Steve Maney race engine; might as well go with the 1,007cc for reliable brute power or better still, break loose for a Nourish eight valve twin. I believe Dave is making a 920cc or 980cc.

If you are serious about it I suggest you do not burn time and resources reinventing the wheel. Been there, done that.
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Re: 76 degree offset crank

Postby hobot » Fri Aug 31, 2012 8:33 pm

Lookie here for examples of dream machine crankshafts and reviews. Only live once so get going so once is enough. Maney runs his 89mm stroke 360' crank, race 920's to 7200. Let us know how it feels compared to your silky Combat.

commando-crankshaft-porn-t8365.html
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Re: 76 degree offset crank

Postby Rohan » Fri Aug 31, 2012 8:59 pm

By the time you have that much engine, and then get a gearbox, clutch, brakes, wheels, frame, suspension and styling to keep up with it, Mr Garner will be envious....

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Re: 76 degree offset crank

Postby Matt Spencer » Fri Aug 31, 2012 9:11 pm

Knactually , would pay to have one piston at Max Velocity when the other was at max Resistance , perhaps .

And if its a crankey olde pushrod motor , youre liable to find yourself stuttering and leapfrogging about ,
if youre not going to ride it spiritably .

the Commandos probably better in this respect than others though .

Forged pistons weigh more , so youll need the fancy rods .

If you Just got a decent stock one , low time , in the first place .and spent your efforts on polishing / balanceing / ligtening & fitting at optimum clearances ,
Youd probaly find it is manadgeable , and with mild carb improvement and a decent exhaust , outperforms the dysfuctional opposition . Even on a track .
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Re: 76 degree offset crank

Postby Mark F » Fri Aug 31, 2012 10:30 pm

I have heard of 76 degree Triumph offset cranks and 90 degree A65 BSA's but never any Norton's. Not sure why. If you offset the crank you will also need to alter the cam and the ignition. Pazon do a 90 degree electronic ignition system so that would be fairly easy to get over. Just need someone to reprofile the cam.
Have a squiz at this to get an idea of how a 90 degree can sound. Awesome!

http://www.youtube.com/watch?v=oBD2g_bBog4
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Re: 76 degree offset crank

Postby Nortiboy » Sat Sep 01, 2012 12:05 am

RoadScholar wrote:I am in the purchasing phase for a British parallel twin engine do-over, not a Norton, but could be. My goal is to be reliable in the 7000 to 8000- RPM engine


Gooday Roadscholar.

Road or Track ?
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Re: 76 degree offset crank

Postby john robert bould » Sat Sep 01, 2012 3:53 am

There is a guy in Canada who will do this..1000 bucks,you provide the two outer webs. This idea is/was the brain child of Phil Erving, Thriumph Boss Edward Turner dismissed it...Well not all speed twin owner's wanted 8000 rpm :?:

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Re: 76 degree offset crank

Postby RoadScholar » Sat Sep 01, 2012 7:02 am

Nortiboy wrote:
RoadScholar wrote:I am in the purchasing phase for a British parallel twin engine do-over, not a Norton, but could be. My goal is to be reliable in the 7000 to 8000- RPM engine


Gooday Roadscholar.

Road or Track ?


This will be a road bike ridden in a sprirted fashion, the reoriented cams are part of the deal, as is the Pazon digital ignition. Falicon will be making the crank, the rods will be Carillo, the pistons are from Arais, the cylinder head will get some time on a flow bench and the fuel will be mixed by 32mm Amal Mk 2s; the cases will get their turn.

I appreciate the opinions, some good stuff here, thanks.

RS
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Re: 76 degree offset crank

Postby Whitworth Ranch » Sat Sep 01, 2012 7:58 am

There is a lot to consider, more than I have knowledge to process. For sure, maximum piston velocity with a 2:1 rod ratio is achieved at about 76 ATDC. This in fact is the point at which the crank arm and rod are at right angles to each other. This would essentially smooth out torque delivered to the rear wheel (as in Yamaha's 90-degree-throw inline four MotoGP bike).

This is precisely the kind of tech talk I wish I had better command of.

Perhaps the cases might hold up better at 7000 to 8000 rpm by separating the piston/rod masses and running them 76 apart (but I think you'll want a balance shaft, ultimately!), but unless you shorten the stroke and do some fancy biz with the valve gear that rpm would seem to bit a bit of a stretch.

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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Re: 76 degree offset crank

Postby MikeG » Sat Sep 01, 2012 9:19 am

Well there's this:
http://www.offsetcrank.com

I was planning on having this done to a BSA A10 crank someday...then I bought a Norton instead. I know of a few folks running 76 and 90 degree cranks in Triumphs and they seem real happy with them.
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Re: 76 degree offset crank

Postby acotrel » Sat Sep 01, 2012 1:34 pm

If you are going to do this stuff, start with the Nourish engine, you will be buying the results of years of Dave Nourish's customers' pain. The staggered crank has been done a few times here in Australia. Some of our guys love heros, and Phil Irving was one of them. The staggered crank works, but y ou instantly have the problem of losing the interchangeability of camshafts when you are developing the motor to go faster . Balance of the crank is never a problem, you just have to choose what revs you intend to pull when you build the motor, and understand that if it is going to develop a lot of mid to top end, you will have to tolerate the low revs shakes. My own bike vibrates like hell while I'm waiting behind the pit gate, but once on the circuit - up an going, it's an absolute delight. The balance factor is only 72% because that is all I could get it up to after tapping and plugging the hole in the bob weight, and judiciously drilling holes in the other side.
About the Phil Irving thing. One of our guys has completely redesigned the Vincent engine, built a bike with modern suspension with the 1400cc engine, and calls it the 'Irving Vincent' - it is a world beater. Also in 'Tuning for Speed' Phil Irving says the if an alcohol fuelled engine is rich it still makes good horsepower, so the guys have interpreted this as 'Phil Irving says you should run alcohol fuelled engine rich'. They miss the point that alcohol hides up tuning errors, and is just as difficult to get right as petrol. In fact years ago when people like Jack Ahearn and Ken Kavanaugh were racing in Europe they'd had experience with fast racing in Australia because of the alcohol fuels. But the sad fact was that the Poms could always get their manx nortons going as fast as ours just using petrol.
When I built the Seeley, I bought two 34mm MK2 Amals with the alcohol kit. I tapered the ports similar to an aermacchi 350. The needles and jets supplied from the UK were really stupid. I simply fitted the leanest mikuni needles ( I think 6DP6), and made my own needle jets from brass hex. I selected a number drill which gave about 0.117 inch hole (Triumph experience) . Riding the bike around the twisty stuff on Winton I dropped the needles until the motor coughed on gear changes around the twisty stuff then raised them one notch. I'm using the old Triumph main jets of about 650 amal size. Around our 3Km circuit , the bike is excellent, it is still a bit rich on the mains, but you can cop that with methanol. It goes like buggery ! The leaner you can get the carburettor settings without destruction, the better. The motor will go fastest just before it blows up.

We use this stuff on our Kawasaki two stroke sidecars my brother races, and it is a real hoot. We've almost come to blows arguing about jetting for alcohol. I will link you to a video of him racing, but he won't allow me to upload the one where the 750 two stroke jumped into the back of another sidecar outfit and flipped three times.
http://www.youtube.com/watch?v=HYnXEQ0Aw1I
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Re: 76 degree offset crank

Postby Nortiboy » Sat Sep 01, 2012 3:38 pm

RoadScholar wrote:
Nortiboy wrote:
RoadScholar wrote:I am in the purchasing phase for a British parallel twin engine do-over, not a Norton, but could be. My goal is to be reliable in the 7000 to 8000- RPM engine


Gooday Roadscholar.

Road or Track ?


This will be a road bike ridden in a sprirted fashion


I think it may be cheaper to supercharge if you want to go nuts. I am in the process of supercharging a 750 shortstroke. It wont have to rev past 7000 to make power.

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