Re: CUT THE FRAME ? 4 heads bolt back on PICTURE only

Posted by Peterwh on Jan 20, 2012; 4:14pm
URL: http://vintage-and-classic-hondas.81.s1.nabble.com/CUT-THE-FRAME-4-heads-bolt-back-on-PICTURE-only-tp3668125p3675888.html

Rerun, I really don't mean to offend anyone and I'm sorry if i have.  I just think this is an interesting mechanical structure.  I also think I'm being misunderstood here. I'm not saying I think a bike with this mod will fail.  Im not even saying it will be anywhere close to failing.  I'm saying that, as a Mechanical Engineer, I think a frame with the mod will be weaker than a frame without the mod.  I'm saying that I wouldn't even know how to go about analysing the strength of the frame with the mod without an FEA model because it's such a complex space frame subject to very complex forces.  I'm saying that if the maker of the mod has had a go at analysing the forces and strength of the frame with the mod I would be interested to see the analysis and calculations and how he/she went about it.  

Clearly the original engineers had something in mind when they designed the frame because, to my mind, this particular part of the frame looks very much like an after-thought to address a particular issue and they designed what is a very stiff space frame in this area, particularly noting the use of the bracket that connects the main tube to the lower two tubes.  It looks to me as if the bracket was an attempt to introduce a lot of torsional stiffness.  Why did they feel they needed to do this?

Call me conservative if you wish but I personally, would be very cautious about cutting those tubes unless I understood the strength changes that would result and without understanding what was in the designer's mind.

Regarding the strength of the original materials, the frame is constructed from DOM tube.  The cold working in the DOM process imparts a lot of additional tensile strength and I think you would maybe be surprised at how strong they are compared to, say, ERW tube.  Do we know the chemical specification of the tube eg 1010 (which I doubt), 1012, 1020 (which I suspect), etc.  If someone knows that, I can easily get the tensile strength.  You have to also remember that the bolts used in the mod are in shear (unless you  can get sufficient friction between the inserts and the frame tubes) whereas the tube is in compression/tension/torsion, which makes a big difference.  I don't think you can assume the mod has very much torsional strength at all because of the bolted design.