Custom frame design comments/concernces/constructive criticism

Couple of thoughts. Haven;t read through the thread completely yet, but...


RC390 is a great little bike and yes it handles great, but you are talking about a 300 pound bike vs something in the 400+ pounds for the ninja.
Additionally, the RC is a thumper and the top rails can be made relatively narrow and straight.

Looking at your original design, are you sure the engine can clear the top frame rails? Most of the time, that is why the rails bend around the engine.
Wondering why you even mentioned the rc390 because your bike is closer to traditional UJM frames than the triangulated modern frames.

Ideal motor for well handling lightweight bike would be narrow like a single or v twin and low in height like 2 stroke.

Kudos for you going through your exercise.

Personally, I don't think 4130 tubing adds THAT much more strength and adds some additional welding technical skills. For your first frame, you could stick with mild steel for ease. Plenty of frames out there mild steel that work great...

BTW, do you even weld, dude? (sarcasm, lol)

UniHalf said:

The CAD is necessary partially because it just is the standard for mechanical engineering at this point. It'll also let me do FEA on the tensile strength and fluid dynamics of the end product.

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Fluid dynamics? On a frame? I doubt it. ;-)

XB33BSA said:

oh so now you tell us you are in possesion of a donor
the plan then is an exact geometry replica ?to cover exact details like woodsman retorts
the frame you show is terrible ok ? for one thing only somebody who knows zero about structural tubing design would use such small diameter trame tubes \
the only practicle app for small dia tubing is in birdcage space frames trellis like deals
do not try anything radical unless you can spend a hundred grand on testing and a frame scrap pile
copy the best frames made for the type of racing you are doing
you absolutely wil;l not build anything viable on your own ,i can see that by looking at the tubes and steering neck i am embarassed for you it is so bad
you do not understand design and are relying on a machine to design for you
the computers do have usefullness in engineering the math

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What's with the attitude, Were you drunk when you wrote that? It was a sweet rant, too bad so little of it applies to what he's shown.

He's not going to build a cutting edge frame right out of the box on his first attempt. "The computers do have usefulness in engineering and math" Well no shit sherlock. It's 2018. Physical testing is now used to prove out designs, not develop them. What part of what you've seen here is "radical" BTW? And who is spending hundreds of thousands of dollars buiulding prototypes and testing them to find answers these days?

Relying on a machine to do the design for him? Do you have the first clue how CAD and FEA work?

"You do not understand design" Well yeah, I think he's already said he's learning. Not everyone was born knowing it all.

"Copy the best design" Yeah, because blindly copying the "best" design is so educational. And how exactly does he determine what the "best" design is to copy, as he does not understand design?

I know this forum takes pride in being the old school dickheads who haze the newbs but come on. You sound like a delusional old man who's pissed at the world because no one designs bikes by trial and error anymore.

And... somehow I screwed up the multi-quote stuff I was doing so even more disjointed replies now.

The Book on design that was mentioned. Absolutely fidn it and buy it. I got a copy back when I wanted to build a track bike and spent countless hours going over it. Very useful. I posted info on it here years ago, if I can find it I'll edit the post with a link.

Matrerials - someone mentioned sticking with typical steel and staying away from Chromoly or whatever. +1000. Do enough research and decide for yourself, but the benefits of using something fancy, especialy on your first frame, are highly overstated IMHO. Somewhere on here I made the argument that using chromoly only resulted in frame that was harder to dent.

I'd look for all this stuff now but, I just got an email from an FEA guy at work, he needs me to find some CAD files so he can do an analysis on exhaust hangers for a 2021 vehicle that have been flagged as being high risk. Lookie there, no one is building prototypes and testing them, it's all done on numerous computers and damn, it seems they don't actually design anything by themselves. (BTW, I'm not making that up to make a point. I literally do have to stop screwing around here to come up with CAD for an FEA engineer)

I would do this...

MotoD's SV650.

There are a lot of SV650 based custom frames that lends itself well to a trellis frame design.

Plenty of cheap parts.

TrialsRider said:

Book reading is a good idea too but keep in mind the vintage of the book you are looking at represents information based on things that came before that date. 20 year old books are written about 20+ year old technology.

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This is true to a point, but technology doesn't reinvent itself every few years. Discussions about materials and the physics behind things are still relevant. Philosophies certainly can/do though. Granted there are changes in manufacturing techniques too but as already mentioned most of that isn't going to be particularly relevant to a garage builders first frame. In any case it certainly is a better basis for this sort of thing than reading what a bunch of keyboard jockeys have to say. I don't know if there are any more recent frame design books that go into the same level of detail.

Anyway, likely moot now since the book has already been ordered but here's the link to a review of the book.
http://www.caferacer.net/forum/tips-tricks/24051-reference-material.html

Tony Foale is also a name that should be mentioned if it hasn't already. He's got a website that has some interesting stuff on it. Or did when I checked it out last. I have had a few versions of his chassis book too... the one I have now is probably from about 20 years ago. SOB I'm getting old.

https://www.tonyfoale.com


There used to be a chassis design mail list. For you young 'uns that what peolpe used to use before internet forums were a thing. I think it's on micapeak.com? When I was on it (again, 20 years ago) there were soem pretty interesting people on it, and guys who were actually building this stuff and racing it. It may still be functional, I now the bevelheads list is still going.

Once the OP gets the book I'm guessing we won't hear much from him for a while, and his frame will look different when we do.

EDIT: So this talk of 20 year old books piqued my curiosity and I grabbed both the Bradley books and Foale's to refresh my memory. Volume 1 of Bradley's book had a lot less frame-specific info in it than I remembered. A ton of general info though and most of it applicable 20 years later.

That got me thinking about what had changed between, say, my 1991 851 and my 2015 Monster 1200S. The frame itself pretty much went away so that's not all that helpful here. Suspension - Ohlins probably uses a different metering system now but the conversation on spring rates and geometry, rising/falling rates, and that sort of thing are all still valid. Brakes - still discs. In general I'd say the basics are all pretty much the same, the fine details may be different. But, no one is building their own forks or master cylinders so it doesn't really matter that you can buy active suspensions or radial master cylinders now.

The huge difference? Electronics. Engine controls, ignition, fuel injection, brakes, it's all electronic now. That doesn't really change anything here either.

Tires are better now, engines more powerful, brakes more powerful, but the basics are still relevant. Torsion is still torsion. The numbers may be higher but the math is the same.

Bradley's Volume 2 deals more with construction and materials. Another worthwhile book but last I heard used copies were being advertised for stupid amounts of money. No you can't borrow mine.

Foale's book - maybe more frame specific stuff but then it's titled :Motorcycle Handling and Chassis Design" so that should be expected. The thing that struck me flipping through that was, it reminded me one of the "preferred" methods of constructing the frame by a lot of custom builders was still bronze welding, which was the same way it was done in the '50s. Everybody loves TIG welds and why shouldn't they, but the old school methods still work too.