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Hello.

I'd like to ask if someone knows why on 91x engines crankshaft is of press fit type with monolithic bearings/conrods (not split).

it's a very unusual solution for a 4-stroke engine, Usually in mature fields like powerplant design solution converges but in my (limited) experience nobody in automotive industry nobody use this type of crankshaft. 

It's compact and light but it seems a 2-stroke design derivative. Rotax in 1990 has a lot of 2 stroke experience….

Thanks.

  • Re: why press fit crankshaft?

    by » 4 days ago


    "Compact and light", exactly.  Not something auto engines are as concerned with.


  • Re: why press fit crankshaft?

    by » 4 days ago


    Thirdly as history has shown in the Rotax engines it works very well and is very reliable


  • Re: why press fit crankshaft?

    by » 4 days ago


    Hi Nicola

    Most motorcycles do press fit cranks and the reason is the same.  it reduces weight and makes the engine shorter (light) for best power to weight ratio.  The crankshafts are actually very reliable on this design and even the type 915 engine, with 141HP on takeoff power uses this system.  The major difference on 915 is the larger lower rod bearing and rod journal for the torsional loads.  

     

    Cheers


  • Re: why press fit crankshaft?

    by » 3 days ago


    However, at least the Norwegian guys do some welding on the crankshaft when they lift horsepower towards and above 140. At the end of the day we cannot cheat physics.


  • Re: why press fit crankshaft?

    by » 3 days ago


    There is a text book on pressed cranks with tables on sizes, tolerances and pressures. Basically each crank is designed for the power and torque that the engine produces, you can get away with 10-20% more but more than that you need to either design another crank ie as Rotax did with the 915 or spot weld it as is normal in racing and as the Norwegians did.

    So welding a crank does not mean it is a bad design or system, it just means you are using it far beyond what the designer intended.

     


    Thank you said by: RotaxOwner Admin

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