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Author Topic: Custom Racer with Aluminium Frame and Sportster Engine  (Read 5150 times)

Offline teazer

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Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #30 on: Jan 18, 2018, 15:34:19 »


- 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.
....


Not exactly.  For sure on a bike that has steeply downdraft intakes, a perimeter frame is at the very least, desirable, but that's not the whole story.  In this case, the tall motor would fit much easier behind those lower forks and steering head without a top tube.

But the reason that perimeter frames are so much better than top tube frames relates to their relative rigidity in the way that the steering head and swingarm pivot are tied together is what is much closer to a straight line.  In addition, the three dimensionality of the steering head arrangement adds significantly to torsional rigidity.   

One could certainly argue that big V twins handle "adequately" with single top tube frames and that large diameter tubes are inherently stiffer than smaller diameter tubes, but if you have the ability to design and machine those parts, why not kick it up a notch (pun intended) and make something even more amazing. 

Admittedly, Buell did that with their frames and so did Over racing from Japan, so few things in life are really new and original, but it's a shame in many ways that this project isn't more innovative in its design.  But it's your project not mine and you are having fun with it and that counts.
 

Offline The Limey

  • Posts: 379
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Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #31 on: Jan 18, 2018, 18:52:25 »
Will milled alloy be strong enough for a headstock?  It may well be, but I've never seen such a thing so it makes he nervous.  Either way, it certainly looks fantabulous, and chicks dig danger, so all the boxes are ticked.

I'm in.
I was born a rocker.  I'll die a rocker.  And I'm proud of it.

Offline farmer92

  • Posts: 627
Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #32 on: Jan 18, 2018, 18:59:22 »
Will milled alloy be strong enough for a headstock?  It may well be, but I've never seen such a thing so it makes he nervous.  Either way, it certainly looks fantabulous, and chicks dig danger, so all the boxes are ticked.

I'm in.

Old Greeves Motorcycles used cast alloy down tube and headstock.

They actually still sell the frame kits this way
http://www.greevesmotorcyclesltd.com/frame.html

Offline Sav0r

  • Posts: 533
    • ChrisLivengood.net
Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #33 on: Jan 18, 2018, 19:06:06 »
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!

Sage advice here. Paint isn't a great idea. This is in part why my shock bearing all aluminum sub frame on my RD is sans paint.

Not exactly.  For sure on a bike that has steeply downdraft intakes, a perimeter frame is at the very least, desirable, but that's not the whole story.  In this case, the tall motor would fit much easier behind those lower forks and steering head without a top tube.

But the reason that perimeter frames are so much better than top tube frames relates to their relative rigidity in the way that the steering head and swingarm pivot are tied together is what is much closer to a straight line.  In addition, the three dimensionality of the steering head arrangement adds significantly to torsional rigidity.   

One could certainly argue that big V twins handle "adequately" with single top tube frames and that large diameter tubes are inherently stiffer than smaller diameter tubes, but if you have the ability to design and machine those parts, why not kick it up a notch (pun intended) and make something even more amazing. 

Admittedly, Buell did that with their frames and so did Over racing from Japan, so few things in life are really new and original, but it's a shame in many ways that this project isn't more innovative in its design.  But it's your project not mine and you are having fun with it and that counts.
 

Inertia (polar moment), small tubes forming a large diameter will be stiffer than a single tube of a much smaller diameter. There's a reason why monotube/mono-spine frames simply haven't remained successful.

That said OP, we all know that HD unit is hardly a performer. Sure it will shake, but it's not going to wind out like a modern unit (frequency, amplitude blah, blah, this has been said?), I like what you're up to.
Sav0r the adventure.

Visit www.chrislivengood.net to get more information on my RD350 dubbed Mia Wallace as well as my other projects and snafus.

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Offline The Limey

  • Posts: 379
  • Evil English Villain
Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #34 on: Jan 18, 2018, 19:34:54 »
Old Greeves Motorcycles used cast alloy down tube and headstock.

They actually still sell the frame kits this way
http://www.greevesmotorcyclesltd.com/frame.html

Is that headstock not forged, or forged and then machined?  (I know little of these things)
I was born a rocker.  I'll die a rocker.  And I'm proud of it.

Offline Popeye SXM

  • Posts: 95
  • Also used for MX
Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #35 on: Jan 18, 2018, 19:41:29 »
I love this project. Great work, but a HD unit rigidly mounted in a aluminum frame ???? Not good engineering for me. Don't get me wrong I hope it works for you. HD motors are not good on power to weight and fatigue with all the vibration?? Thanks for documenting and I hope YOU are happy with the finished bike

Offline DM_Andreas

  • Posts: 25
Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #36 on: Jan 19, 2018, 00:10:16 »
Not exactly.  For sure on a bike that has steeply downdraft intakes, a perimeter frame is at the very least, desirable, but that's not the whole story.  In this case, the tall motor would fit much easier behind those lower forks and steering head without a top tube.

But the reason that perimeter frames are so much better than top tube frames relates to their relative rigidity in the way that the steering head and swingarm pivot are tied together is what is much closer to a straight line.  In addition, the three dimensionality of the steering head arrangement adds significantly to torsional rigidity.   

One could certainly argue that big V twins handle "adequately" with single top tube frames and that large diameter tubes are inherently stiffer than smaller diameter tubes, but if you have the ability to design and machine those parts, why not kick it up a notch (pun intended) and make something even more amazing. 

Admittedly, Buell did that with their frames and so did Over racing from Japan, so few things in life are really new and original, but it's a shame in many ways that this project isn't more innovative in its design.  But it's your project not mine and you are having fun with it and that counts.

Your points in regards the stiffness are certainly valid, no questions there.
Also the connection steering-head to swingarm is of course important...But there you can see several manufacturers already going a different route, with separate "frame" sections for these and using the engine as stressed member in the middle (Think Ducati Panigale)

It's mainly a question of conservative evolution from a proven design, one small step at a time.

But to be honest...I'm certainly strongly biased in my design....The way I'm building the frame is a result of many factors:

- Given the existing engine and suspension components, which design can give us sufficient performance?
- Which design can I properly calculate?
- What can we manufacture here in our shop with the given tools/machines?
- Which look/Style are we going for?

All in all, I'm sure the resulting bike will perform nicely and the engine won't bring the chassis to it's limits.

Offline DM_Andreas

  • Posts: 25
Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #37 on: Jan 19, 2018, 00:11:59 »
Will milled alloy be strong enough for a headstock?  It may well be, but I've never seen such a thing so it makes he nervous.  Either way, it certainly looks fantabulous, and chicks dig danger, so all the boxes are ticked.

I'm in.

The machined section is certainly stronger than a welded assembly or cast piece of the same dimensions.

Offline DM_Andreas

  • Posts: 25
Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #38 on: Jan 19, 2018, 01:03:53 »
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!

That's a very good point.

We're using 6061T6, which is a well defined alloy.

You're totally right that Aluminum does not have a clear fatigue limit like for example steel.

But the fatigue limit can be described in a curve, through which you can estimate the cycle life of a certain component.

If the design is "aggressive" the part are more light weight, but the cycle life can be quite short.

As long as you're on the lower end of the curve, the cycle life goes towards infinity and that's the area you want to be in, when you build a motorcycle frame.

What's not factored in right now are the long term effects of the engine vibrations and the heat expansion from the engine.

We do not have the possibility to simulate this on our end and that's something we simply need to test, once the prototype is ready.

Offline SF

  • Posts: 57
Re: Custom Racer with Aluminium Frame and Sportster Engine
« Reply #39 on: Jan 19, 2018, 01:21:13 »
Look at the 2004 ZX10R frame...cast head and bottom mounts welded to boxed sheet in between.


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