teazer said:
I must be missing something here. Machining and all that goes with it is really impressive, but why rigid mount a shaking engine in an aluminum frame that will quickly work harden and crack? And why use a notched tube across the top of the motor instead of creating a perimeter frame which would be much stiffer?
Has anyone done any stress analysis on the frame design? Nicely done, but why?
Hi Teazer,
I'm happy to address this points a bit further...
- Cracks through vibrations
That engine certainly vibrates a lot. To counter this, we're going with the machined steering head and axle mounts. These are the points where the vibrations are transmitted to the frame.
On both parts the frame tubes are pressed into the machined ends. There's a significant overlap between the parts and we machined the ends of the tubes to create a press fit between the components. That press fit transmits most of the stress.
While we certainly need to do some testing on this, when the bike is ready, we're quite confident that this is a stable and durable setup.
We also temper the parts after welding to improve the strength of the welds and to remove tensions in the material.
- Why a center tube frame instead of a perimeter frame?
It's actually the best design for this engine.
A perimeter frame is NOT really a stiffer/stronger design than such a center tube frame. That assumtion is not really correct.
Perimeter frames are "state of the art" now, because they allow you to use (nearly)straight intakes on the engine, which go basically straight from the airbox into the combustion chamber.
That requires a bit airbox and the throttles on top of the engine, which is basically only possible with a perimeter frame.
On the Sportster engine you have the intake on the side.
A perimeter frame would make a lot of sense, if we would use modified heads which have a downdraft intake....But honestly, we don't want to start messing with the engine.
Given the shape/size of the engine, our frame design offers a lot of strength/stiffness, with a good weigth distribution.
It's just all in all a good solution for this engine.
- Analysis
Of course the frame was once analysed in regards to strength and flex.
But real life is always a different matter.
Welds often have different attributes than the "book values" you're using for the calculations.
The engine also takes some of the forces from the chassis and we basically can't calculate that part, without making a complete analysis of the engine.
Due to this we'll still need to do testing after the bike is ready.
It's a prototype....
- Why?
Why not? It's a fun project...We wanted to do something special and try something new.