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?