gruntguru wrote:TC touched on this in an earlier post. There is a geometric lower limit on the l/r ratio. When the piston is at BDC, the rod needs to be long enough for the piston skirt to clear the balance webs of the crankshaft.
The skirt height and the pin diameter are the other factors - pushing the piston pin further up and increasing the rod length. As bore size (and B/S ratio) increases the skirt needs to be longer and the pin diameter needs to be larger, both forcing the designer to use a larger l/r as B/S is increased.
given we have reminded ourselves that the N/A F1 had surprisingly large (ie conservative) rod ratio
and are looking for practical design reasons to explain this .....
the gudgeon pin (wrist pin) boss is structurally integral with the crown, so giving the most direct path eg for inertia loads
this direct load path chosen to enable extreme piston acceleration and rpm
so this higher pin position demands a longer rod
but the current V6 turbo engine rules enforce a conservative bore and stroke and rpm and piston acceleration and block height
the piston design will be dominated by crown heat (part-steel pistons were mentioned in the Gilles Simon article in RCE)
so the mix of design factors contributing to rod ratio is different
they might have a similarly high rod ratio, but for different reasons