Post Thu Oct 01, 2009 1:48 am

durability and otherwise; older vs newer designs

this came from the HDRCGB forum, but it occured to me that it might be of interest to folks over here. The original thread was about a twisted crank on an 8,000-odd mile twin-cam

Originally Posted by Kiwi
yes but are they the latest engine designs or earlier designs (read evo)?

The evo was proven on the road before it was put on the market, the twion cam however was rushed prematurely onto the market before they ironed out all the faults, however with the age of your bike it should have been sorted before it left the factory

the older engine designs ( iron engines, for me; I don't know to what extent this applies to the Evo ones ) are designed and built to a design formula which requires fitting at virtually every step, and is designed to allow for endless rebuilding. This wasn't done for the purpose of providing an indefinite service life; that's a side-effect.

It was done to accomodate the relatively low state of metallurgy, lubrication technology and quality control available to the designers, plus the relatively high wear rates from being used routinely on unsurfaced roads with crude felt air and oil filters. It was done to allow production by assembler-manufacturers using a lot of bought-out components, and using assembly-track methods with limited supplies of skilled labour and large, heavy factory equipment driven by overhead belts - which means that machines tend to stay in the same place for very long periods of time. It was done to cater for a market for whom purchase of a new unit was a rare, major event. It's expensive to do.

improvements in materials technology, road surfaces, maintenance schedules tend to mean that anyone who wants to, can now keep these old beasts in service until the main casings fail from fatigue if that's what they wish to do. Given the sheer strength of these old castings - due in large part to the relatively low working stresses and empirical design methods employed - this can be a very long time. The vehicle has long since passed beyond any rational calculation of cost-per-mile, but that's not the point.

there's a calculation engineers use, called Total Process Cycles, to estimate the working life of any given machine or assembly. This allows for the total use vs how hard you use it. For older machines, these numbers are actually quite low.

I mentioned my Uncle's 45 on another thread, how it had remained in service - largely untouched - for around 25 years. This is true, but not the whole story. I happen to know that the mileage on this bike was around 60,000 miles when it was sold. This means that the bike only did around 2,500 miles a year, or more likely did a higher mileage for a shorter period and spent long periods of its later life parked up.

those of us old enough to remember the old Brit bikes will know that it wasn't common to find machines with total mileages much above 60,000.

the Japanese realised this early on, and designed machines to be thrown away at relatively low mileages, but to be totally reliable within that design frame. This meant that in practice, a Japanese bike lasted as long as it took you to decide that you needed a new one, had saved up the money and been totally reliable while you did so, and hence wanted a new one from that manufacturer.

It also means that if you ride your Super-Duper-Ultra-Glide-with-all-the bells-and-whistles at 45mph, it will probably last for ever, or at least until it rusts away.

so, I would certainly agree that a twisted crank at 8,000-odd miles is bloody disgraceful, and modern Harleys are grossly overpriced for the performance and quality they offer, but they are ( in general terms ) nearer to the durability in service of the older ones than peopletend to give them credit for.
Shoot, a man could have a good weekend in Dallas with all that stuff...