Tuesday 4 August 2020

The Crankshaft

There is no “off the shelf” crankshaft and connecting-rod assembly available that could be simply built into the engine. A decision was therefore made to use Alpha Bearings in Dudley to build the complete assembly. Over the previous years, I had developed a good relationship with Max Nightingale, who ran Alpha Bearings, and Alpha had already built 2 complete crankshafts and con-rods for Mk 1 OHC Velocettes and reconditioned numerous crankshafts from other bikes for me; I therefore had every confidence that they would do a top job on the AJcette crankshaft. 

By this stage of the engine build most of the important dimensions were known for the crankshaft except for the connecting-rod length. There is one design requirement that dictates one of the most important dimensions of the engine, namely the cam-chain tension and this determines the distance between centres of the camshaft and the camshaft drive spindle that is driven by the crankshaft. Small changes in the distance between centres has a huge effect on chain tension and the Weller chain tensioner is exactly what the name suggests, namely a chain tensioner, and it is definitely NOT an adjuster that is there to take up large changes in chain length.

The following sequence of design steps was therefore adopted:

     1)    Set up the chain length (number of links) as close a possible to the correct tension but to be slightly slack.

     2)    Set up the correct chain tension by adding a spacer under the cylinder barrel. This could also be done by raising the cambox further from the cylinder head but this would have the detrimental effect of changing the original rocker-to-valve geometry that Velocette had determined. The distance required between the camshaft and camshaft drive spindle centres was 11.75"

     3)    Determine the position of the piston in the cylinder barrel to give the desired compression ratio. This must be done by putting the rings on the piston and filling the combustion chamber space with oil from a burette, shown below, to measure the volume and then making a mathematical determination of the piston position for the chosen compression ratio. I chose a compression ratio of 7.5:1.

    4)    Knowing the stroke (81mm) it is then possible to calculate the connecting-rod length. This turned out to be 6.42"

There is, in fact, another way in which this problem could be solved and that is to fix the connecting rod length to a value that is as close as possible to  a “round number” of a suitable connecting-rod from another engine, such as 6 7/16”,  and to then determine the stroke. Clearly this would change the swept volume of the engine but has the advantage that an “off-the-shelf” connecting rod could be used. This approach was used in the V-twin version of this engine ….to be described in due course.

In my own workshop, I do not usually work by making detailed drawings but tend to make “back of the envelope” sketches with all the main dimensions for each component as the engine evolves. This is obviously not possible when a component is outsourced and so a detailed drawing was made of the complete crankshaft.

A couple of months went by and then I received a message from Alpha Bearings that the crankshaft was progressing well....

Shortly after that the crankshaft and connecting-rod was finished and I duly picked it from Alpha in January 2017. To an engineer, this a stunningly beautiful piece of work.

Unfortunately Max Nightingale died in June 2018 at the young age of 59 and I don’t believe this level of engineering is now available. Thank You Max for all the work you have done for me over the years. RIP.

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