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PostPosted: Sun Apr 01, 2012 9:28 am 
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So, chainstay bridge added.
Image
Image
I was also thinking about maybe instead of having the pivot attached directly to seattube, I could use a clamp instead. This would allow extra adjustments.
I did make it in the model but my model laptop crashed :evil:
And no this isn't a 'windup' :lol:


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PostPosted: Sun Apr 01, 2012 10:36 am 
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it would be interesting to make the pivots adjustable ,tho that would
probably entil a 4 bar link type arrangement and some heavier tubes
but as a development frame it might be worth while
tho if things are adjustable it might lack rigidity


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PostPosted: Sun Apr 01, 2012 7:19 pm 
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First principles would tell us that the closer the pivot is to the BB ; the less effect chain tension will have on the suspension. It's been done a few times in the motorcycling world :

http://www.flickr.com/photos/breganze981/2386406669/

for example ; but I don't recall seeing any bicycles designed with concentric pivots ( among the problems are excessive frame width as that Laverda shows). An engineer friend and I discussed this ; and the only possible solution we found was to use the BB shell itself as the inner part of the pivot ; then to wrap the outer part around it. Good Luck with the project!


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PostPosted: Sun Apr 01, 2012 7:35 pm 
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Sod the frame design! A Spondon Laverda! Image


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PostPosted: Sun Apr 01, 2012 7:52 pm 
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With the clamp on mount instead of welded, it would allow longer use of the frame in terms of possible breakages. If the mount was to fail, the clamp could be replaced instead of having a potentially useless frame.

kampos wrote:
An engineer friend and I discussed this ; and the only possible solution we found was to use the BB shell itself as the inner part of the pivot ; then to wrap the outer part around it. Good Luck with the project!


A while ago I was trying to think up a full suspension fixed gear, and because of the movement between the axle and bb, I thought of a roller bearing around each end of the bb sheel, then an outer shell on the bearings attaching the rear triangle to front. The only issue I could think of was, the downtube and seattube would have to be really thin to accommodate the extra parts, which could result in complete failure of the join.


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PostPosted: Sun Apr 01, 2012 8:19 pm 
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^ Yup ; that's pretty much what we decided -- too much gubbins and too little space for it. Ultimately ; any design will be a compromise: You can make a frame to this design stiff enough by adding material ; so up goes the weight etc.etc. ; I'm pretty sure this is why most frames now are single pivot "Marin/Whyte" - ish frames ; it's the best overall compromise. As an aside ; ever seen one of these?
http://www.pinkbike.com/photo/7703577/

Virtual Pivot Point ; interesting idea.


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PostPosted: Sun Apr 01, 2012 8:35 pm 
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kampos wrote:
^ Yup ; that's pretty much what we decided -- too much gubbins and too little space for it. Ultimately ; any design will be a compromise: You can make a frame to this design stiff enough by adding material ; so up goes the weight etc.etc. ; I'm pretty sure this is why most frames now are single pivot "Marin/Whyte" - ish frames ; it's the best overall compromise. As an aside ; ever seen one of these?
http://www.pinkbike.com/photo/7703577/

Virtual Pivot Point ; interesting idea.


I just decided to leave that to the professionals :lol:

That Intense is a beauty!! :shock: It seems to have an extra pivot on the bb that looks like it would just make it all flimsy.


Here's a clamp for the rear pivot. It will be double bolted to the seattube.
Image
Image


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PostPosted: Sun Apr 01, 2012 8:40 pm 
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mikee wrote:

it all depends what your trying to achieve here
a uni engineering project or just dicking around on solidworks ?


I await correction but that doesn't look like solidworks to me.

Titch, I don't know you're experience and these days I am concerned to state my own on here. but you need to consider the forces being transfered through the pivot system you are looking at.

Not just the direct compression but the rotational forces passed on to the main seat stay pivot. At the "moment" the back end will be very flexible and the pivot isn't going to last.

I don't know what package you are using to draw it but if it has any FEA functionality try running your model through it.


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PostPosted: Sun Apr 01, 2012 9:06 pm 
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Not Solid Works, it's 3DS Max. Not sure if I can get a simulation plug-in for it but I'll have a look around soon.
I was thinking that with the chainstays as they are now, it will give alot of rigidity in lower rear. The clamp on pivot is something which needs a bit of work, but it should be ok to use just to find an optimum position.


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PostPosted: Mon Apr 02, 2012 11:10 am 
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Quote:
I don't recall seeing any bicycles designed with concentric pivots


Cove G-Spot:

Image

Kona A and Cowan:

Image

Rotec DH:

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