270 degree crank

Just got this beauty in today. Shortstroke 270 deg nitrided crank going into a fully kitted racer down under.

What class of racing is that intended for ? Seems a lot of expense for period 4 historic racing. It is a very progressive piece of kit - I don't think I would ever go there because there are too many unknowns. At least with the standard crank, the ground is well-trodden. Are you allowed to divulge your customer's name ?

That looks like a nice bit of turning work Jim. Is it from billet or a forging. Do you get cranks made to order?
Martyn.

Hi Jim
That's a nice looking crank, what size engine is it going in 750?
Nitrided what grade of steel, was there any stress relieving carried out during the machining?
270 degree what sort of engine mounts?
Are they using your long rods and pistons?
Just a few thoughts, I would like to have the time and money to play around like this but, I seem to spend my time doing prototype work for others who I do not see once the prototype is finished :?: !
Best Regards
Burgs

acotrel said:

What class of racing is that intended for ? Seems a lot of expense for period 4 historic racing. It is a very progressive piece of kit - I don't think I would ever go there because there are too many unknowns. At least with the standard crank, the ground is well-trodden. Are you allowed to divulge your customer's name ?

Click to expand...

In truth the 270 (or 90!) crank is pretty well trodden ground these days!

I am not sure what unknowns are left, modified cam, modified ignition, separate exhausts!

All racing is expensive, and every entrant spends more than it is 'worth'!

Burgs said:

Hi Jim
That's a nice looking crank, what size engine is it going in 750?
Nitrided what grade of steel, was there any stress relieving carried out during the machining?
270 degree what sort of engine mounts?
Are they using your long rods and pistons?
Just a few thoughts, I would like to have the time and money to play around like this but, I seem to spend my time doing prototype work for others who I do not see once the prototype is finished :?: !
Best Regards
Burgs

Click to expand...

Its cut from 4340 billet for a 750 short stroke (80.4mm) with JS long rods and lightweight pistons with approx 2:1 rod stroke ratio, JS3 270 deg cam and beehive valve springs. For a high RPM full race Commando with isolastics. Recommended balance factor for the 270 crank is around 50%

Burgs - if you don't see your customers after the prototype is finished - then they're probably happy.

SteveA said:

acotrel said:

What class of racing is that intended for ? Seems a lot of expense for period 4 historic racing. It is a very progressive piece of kit - I don't think I would ever go there because there are too many unknowns. At least with the standard crank, the ground is well-trodden. Are you allowed to divulge your customer's name ?

Click to expand...

In truth the 270 (or 90!) crank is pretty well trodden ground these days!

I am not sure what unknowns are left, modified cam, modified ignition, separate exhausts!

All racing is expensive, and every entrant spends more than it is 'worth'!

Click to expand...

The remaining unknown (for me at least) is how to balance it and at what factor?

Is this also well 'sorted' nowadays in a Norton context?

Is that a crankshaft by Moldex?

What was the driver behind selecting a 270 degree crank? Has the homework been done on the cam loading and consequences?

Dances with Shrapnel said:

Is that a crankshaft by Moldex?

What was the driver behind selecting a 270 degree crank? Has the homework been done on the cam loading and consequences?

Click to expand...

Yes - I warned the owner about the strain the 270 deg crank setup puts on the cam chain and insisted he use a Jwis chain (Andy chain). I know several racers who use the 270 deg crank and apparently they think the lowered crank stress and vibration is worth risking cam chain failure. I get occasional requests for the 270 deg cams because I’m one of the few who make them available.

One racer came to me after a Reynolds cam chain failed (not changed soon enough). His solution was to use beehive springs with the tension lowered to street tension specs to make things easier on the cam chain.

Going to a JS2 (PW3 profile) with smoother ramps does a lot to reduce strain on the chain (jerk) and eliminate valve bounce as well.

jseng1 said:

Dances with Shrapnel said:

Is that a crankshaft by Moldex?

What was the driver behind selecting a 270 degree crank? Has the homework been done on the cam loading and consequences?

Click to expand...

Yes - I warned the owner about the strain the 270 deg crank setup puts on the cam chain and insisted he use a Jwis chain (Andy chain). I know several racers who use the 270 deg crank and apparently they think the lowered crank stress and vibration is worth risking cam chain failure. I get occasional requests for the 270 deg cams because I’m one of the few who make them available.

One racer came to me after a Reynolds cam chain failed (not changed soon enough). His solution was to use beehive springs with the tension lowered to street tension specs to make things easier on the cam chain.

Going to a JS2 (PW3 profile) with smoother ramps does a lot to reduce strain on the chain (jerk) and eliminate valve bounce as well.

Click to expand...

Not just chain but cam, adjuster and drive pinion. An 80.4mm stroke really does not rev that high so maybe the problems we saw will not manifest with the 80.4mm stroke.

I would not even call racing a standard configuration Commando 'well-trodden ground'. There are still people romancing about short stroke motors and light cranks - trying to make a Commando motor into a poor copy of a Triumph motor ? Changing the head on a Triumph motor might be a lot easier ? The biggest difficulty with the Commando motor is that the inlet and exhaust cams cannot easily be moved independently. So how can you get it right ? How many cams do you have to buy ? and then you rely on somebody else's development work. When you do that, you always know there is somebody faster than yourself. Regrinding the cam to change the lobe centres is not really conducive to development. My thinking is often in terms of two-stroke development. How many people get port timings really up the creek ? I suggest it is MANY.
One thing I have noticed on this forum, is that some people seem to believe that if you buy all the good bits and assemble them, you will have a fast motor - an additive effect. I don't think fast bikes happen that way. When you put all the bits together, that is the beginning of development - making the best of what you've got. Over the years most of what I raced were Triumph twins. I spent hours progressively modifying, adjusting and testing to get something which was somewhere near reasonable. I have never just installed a cam to the dots, bolted on an exhaust and gone racing.

acotrel said:

I would not even call racing a standard configuration Commando 'well-trodden ground'.........

Click to expand...

So lets ignore the going on 50 years of Commando racing because you did something else!

jseng1 said:

Just got this beauty in today. Shortstroke 270 deg nitrided crank going into a fully kitted racer down under.

Click to expand...

What I would be interested to know is if the crank, or the cam configuration in fact defines 270 vs 90 in a crank of this type?

Or does it simply relate to which cylinder leads? i.e. timing side big end (and rod/piston) is in advance of drive side, or vice versa?

SteveA said:

jseng1 said:

Just got this beauty in today. Shortstroke 270 deg nitrided crank going into a fully kitted racer down under.

Click to expand...

What I would be interested to know is if the crank, or the cam configuration in fact defines 270 vs 90 in a crank of this type?

Or does it simply relate to which cylinder leads? i.e. timing side big end (and rod/piston) is in advance of drive side, or vice versa?

Click to expand...

The timing side journal stays where it is as on a stock crank. This keeps the cam keyway correct for the timing side. The drive side crank journal is retarded 90 degrees/advanced 270 degrees (rotated 90 deg counterclockwise when viewed from the timing side). But the drive side cam lobes are rotated 45 deg clockwise because the cam turns 50% slower than the crank and in the opposite direction (cam turns counter clockwise when viewed from timing side). This means that the drive side fires 270 crank degrees after the timing side.

I've called it a 90 deg crank but some insist that 270 is correct so now I'm going with that.

Below is a smooth ramp JS2 cam (good for over 9000 RPM). Note the drive side lobes are rotated 45 degrees clockwise compared to a stock cam.

The crank looks to have the standard alternator rotor mount on the drive side. Is this because the customer does not want a total loss battery type ignition system (for every last bit of energy to be used to turn gearbox) or just to give more options as to how to build motor.

With a standard configuration crank, there must now be a large choice of cam readily available. Is the same choice available for the staggered cranks ? Seems like a nightmare to me. How many guys do you think ever try to get the most out of the standard cam by advancing and retarding it , then testing for performance changes ? Bearing in mind , that if you have not got a dyno, you don't know if you have improved torque unless you raise the overall gearing.

toppy said:

The crank looks to have the standard alternator rotor mount on the drive side. Is this because the customer does not want a total loss battery type ignition system (for every last bit of energy to be used to turn gearbox) or just to give more options as to how to build motor.

Click to expand...

When I built my first race motor in '75/'76 I had the crank turned off at the end of the taper and bored and threaded for an allen bolt. Worked fine and reduced the potential to deck the end of the crank!

Others did similar but bored a bigger hole and fitted a G50 flapper valve as a breather.

No one seems to do that sort of thing these days, so on my current motor, I haven't either!

It leaves you options for ignition systems.

The customer didn't request anything special ignition wise so I gave him the regular alternator setup. One piece cranks seem to be more difficult to get now. And Maney is simplifying his options for his bolt up cranks.

Steve Perlinski has been using a 270° crank for quite a few years now with considerable success: he won the French Championship (VMA) and was very close to win the British Championship as well...

Just saying... :mrgreen: