Short Rod v Long Rod

All things K & Sportster

Short Rod v Long Rod

Postby John R » Tue Jun 10, 2014 6:26 pm

I’ve always been curious about a part H-D did not change between the K and the KH. The con-rods remained the same (and carried over the same centre-to-centre rod length into the early Sportsters and maybe beyond - maybe the length was even the same for the old 45s).

The K and XL have the same stroke of 3.8125 and a rod length of 7.45 giving a ratio of 1.95; the KH has a 4.562 stroke giving a ratio of 1.63. In the world of auto engineering they are each at opposite ends of the scale – extremely long and extremely short ratios.

So what difference does rod ratio make? In fundamental terms a long rod (up to 1:2) means the piston hangs around more at TDC and less at BDC. A short rod has the opposite characteristics, less time at TDC, more at BDC. A short rod also accelerates the piston harder once it’s really underway – this means, apart from anything else, the mechanical stresses and frictional losses are greater.

But that’s not all: these facts mean that a long rod lets pressure build-up longer on the power stroke whilst the angularity transfers the power with more mechanical advantage at the optimum part of the stroke; conversely, on the induction stroke a short rod has more time to pull mixture in as well causing a greater pressure-drop due to the faster accelerating piston for better incoming charge.

The final score is that long rods are generally better for higher top-end power with high revving motors, and short rods suit lower revving engines, the better induction helping low-end torque.

Owning both a KK and a KHK they are ‘chalk and cheese’ not what you would expect from just 130cc difference. I used to believe the KK and KHK used the same cams (you would think so from a quick glance at the parts book), but not necessarily (another topic). The KK needs to be revved hard and shift late to wring performance out of it – modest low-down power; the KHK goes better with an earlier shift to cash-in on really strong low-down power – due to long stroke and short rods.

There’s one other factor: the shorter the rod ratio, the harder it is to kick the motor over (all other things being equal). Now the standard engine sprocket is 30, which I have on the KK, but I have a later 34 one from an XL on the KHK (with a smaller gearbox sprocket to keep things overall pretty much the same). Sure there’s a slight difference in CR 6.5 and 6.8:1 but you would think the 34 sprocket should make it easier to kick-over. Even so, it takes a lot more grunt to start the KHK than the KK - I think that’s mainly due to the stroke / rod ratio of 1.63 (compared to 1.95 on the KK).

So was keeping the same rod for the KH deliberate design by H-D or just convenience, saving making the frame taller, and planning to go back to the old K stroke for the XLs? The KHK lived on the limits of mechanical stress, with peak mean piston speeds at over 4000ft/min, also reaching beyond what is generally regarded as ‘safe’ or sensible. Another good reason to shift early!

What do you think?
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John R
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Re: Short Rod v Long Rod

Postby Hammie13 » Tue Jun 10, 2014 8:13 pm

I'm not a techno type guy,and having ridden,and kicked both motors,I'll just say that the KHK motor was way better,and if you can't kick it,hang it up. I used to start my '53 K with my arm,not gonna happen with a KHK.
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Re: Short Rod v Long Rod

Postby dr dick » Wed Jun 11, 2014 11:39 pm

When applying auto design parameters to our twins something gets forgotten.
namely cylinder multiplication.
because we dont have it we are corralled into what works in a twin.

the 4000 fpm number get tossed around alot.
but it doesnt seem to be accurate for our bikes flattie or ohv.
for 50 yrs there have been strokers that have acceptable piston life at speeds far above the magic 4000.

and by your own experience you see the stroker runs better than 130cc's "should".

in the early 70's it was proven time and again that a 900 stroked to 64" wold demolish a 1000cc bike.
3 cubic inches, 50 cc,s is the displacement difference.
this goes against the auto parameters. but it doesnt go against what we have.
the hp was appx the same, but the rpm spread in whitch big torque was made gave the small bore stroker more powerband that exploited the lousy 4 gear ratio choices we have. if we had 6 speeds then we wold being able to move the power band up the rpm range and still get it to the road.

there is a reason big 2 cyl gas dragsters use long strokes and smallish bores.

this is a 5" stroke bike

it uses wheels like these

to rod to stroke ratio
short stroke engines usually need long rods just so pistons clear crank weights.
to clear equal bob weight diameters- rod need to be a min length to keep wrist pin clear.
say you shorten stroke 1/2". rod needs to be 1" shorter in a 2:1 motor.
think about the bdc geometry as this progresses.

flywheel weight is an important parameter for man driven motors.
as is the distance of mass center from crank main axis.
conservation of angular velocity my friend.
and newtons 1st & 2nd laws.

flywheels can only be so small or they lose their potential energy storage.
no one like a bike that stalls at a drop of a dime leaving a red lite.
dr dick
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