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Author Topic: Custom Racer with Aluminium Frame and Sportster Engine  (Read 6020 times)

Offline teazer

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Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #20 on: Jan 17, 2018, 18:47:42 »
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?


Offline DM_Andreas

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Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #21 on: Jan 18, 2018, 01:39:43 »
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.

Offline DM_Andreas

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Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #22 on: Jan 18, 2018, 01:41:42 »
Are you going to polish the frame and swingarm or keep it machined? I like the machines look myself up, looking great so far


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Offline der_nanno

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Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #23 on: Jan 18, 2018, 06:23:54 »
I am with Teazer on this one, that is some lovely CAD and CNC-work. Why did you notch the tube and not simply design the steering head, so you'd have the necessary room between engine and frame backbone?

I shall follow this thread with interest.

Cheers,
Greg
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Offline DM_Andreas

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Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #24 on: Jan 18, 2018, 07:08:08 »
I am with Teazer on this one, that is some lovely CAD and CNC-work. Why did you notch the tube and not simply design the steering head, so you'd have the necessary room between engine and frame backbone?

I shall follow this thread with interest.

Cheers,
Greg

Hi Greg,

We notched the tube due to a range of reasons.
- The diameter of the upper tube is dictated by the required stiffness/strength

- The position of the swingarm pivot point is determined, as that engine gives you already the rear engine mount for this.
(While it would be also an option to cut off that mount and fabricate something new, we wanted to keep it for sake of simplicity.

- The swingarm and angle and length are also pre-determined, by our required specs for the rear suspension.

- Up front we're also limited by choice of fork tubes, as we want to be able to use these new Showa BPF which only come in a few length.

Making the frame 20mm higher would allow us to keep the tube intact, without a notch.
But that would require a compromise somewhere else

Basically we had the choice between:

- Longer fork tubes, fork extensions or triple trees which have an downwards step/offset.

- Lower swingarm angle and/or a significant shorter swingarm

- A notch in the bottom of the tube

We went with the notch, as we feel it's the best option.
It'll be welded shut with a aluminium plate, once the rest of the frame is ready.

best regards
Andreas

Offline SF

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Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #25 on: Jan 18, 2018, 08:31:54 »
Itís going to be one very cool looking, engineer, badass Proto type..... after you finish this one do something with a triple like H1 or H2!


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Offline DesmoDog

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Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #26 on: Jan 18, 2018, 12:57:20 »
- 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.

I'm not sure how much of a difference your overlap will make - the maximum stress will occur at the joint/change in area between the tube and the steering head. That weld shape/area will be critical and as you say, modeling that isn't trivial. No matter how "strong" you make it now, aluminum has a finite fatigue life, that frame will have a use by date. How do you determine what that date is? Typical values for fatigue limits are based on unnotched specimans, the aformentioned weld most certainly adds a notch factor to the equation.

Offline 1fasgsxr

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Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #27 on: Jan 18, 2018, 13:20:52 »
This thing has some serious cool potential
http://www.dotheton.com/forum/index.php?topic=41083.0

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Offline Sav0r

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Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #28 on: Jan 18, 2018, 13:27:36 »
As mentioned earlier, fantastic CAD/CAM work.

Also, I enjoy the spiciness of this this engineering debate. I look forward to the machine operating.
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Offline jpmobius

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Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #29 on: Jan 18, 2018, 14:13:22 »
I'm not sure how much of a difference your overlap will make - the maximum stress will occur at the joint/change in area between the tube and the steering head. That weld shape/area will be critical and as you say, modeling that isn't trivial. No matter how "strong" you make it now, aluminum has a finite fatigue life, that frame will have a use by date. How do you determine what that date is? Typical values for fatigue limits are based on unnotched specimans, the aformentioned weld most certainly adds a notch factor to the equation.

Indeed.  Despite having no endurance limit, aluminum can survive a right long time even in motorcycle chassis though it is tricky.  We only see it in street chassis due to the creation of propriety alloys that simply didn't exist back in the 70's.  The trick with conventional wrought alloys is making the elements burly enough (and assembled cleverly enough) to have a reasonable service life without ending up being heavier than what you could do in steel.  You are making some really lovely parts and they are looking plenty stout, and I don't doubt the chassis will perform well with the thought and effort you are putting into it.  And it is entirely possible that you might never put enough hours on it to see any fatigue failure.  However, since there is no practical way for you to predict how many hours that might be, it would be a good idea to keep a close watch on it for problems which certainly will eventually occur.  It is only a matter of time (and frequency, amplitude blah, blah).  Making the design stronger, more clever, or out of improved alloys only increases the time.  As long as that time is beyond the foreseeable service life you are good!  It seems a shame to cover up such nice parts and fab work with paint, at least from my own aesthetic sensibilities, but I'd certainly wish to keep it bare for inspection purposes.  In my experience, fatigue cracking, once it gets started to where you can see it, advances very rapidly so you would like to notice it as early as possible!  Keep up the great work, this looks like a great project!
Mobius


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