Custom frame design comments/concernces/constructive criticism

9776 Views 66 Replies 10 Participants Last post by sebwiers, Jan 13, 2018

UniHalf

Discussion Starter · Dec 31, 2017

Alright, y'all. Not sure if this belongs here or in technical, but I'm assuming this is okay.

I've got an ambitious project in the works currently. I'm a Mechanical Engineering major in my junior year at Ohio State. I've done a lot with suspension and carbon fiber molding with the Formula SAE team there so I'm not entirely out of my depth here, but we'll see how it goes. I've got an internship in Pennsylvania so over the next 8 months I'm going to start designing and building a motorcycle. First step is drawing up a 3D model of the frame. I'm posting the first draft of the design here because you all have way more experience building/modifying bikes than I do. I'm curious to get your opinions.

Does my design have any glaring errors that I'm missing? What could I improve here? Be harsh if you want, I want to make my design as good as possible. I'm mostly going to be setting it up for road racing/street driving. Good handling and feel is one of my top priorities here.

Diagram Technical drawing Design Drawing Plan

This is my recent iteration.

Thoughts/comments/concerns?

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UniHalf

Discussion Starter · #2 · Dec 31, 2017

3

Some more detailed views:

Technical drawing Text Drawing Diagram Line

Line Diagram Parallel Drawing Automotive window part

Line art Drawing Diagram Technical drawing Parallel

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UniHalf

Discussion Starter · #3 · Dec 31, 2017

Stupid mistake #1 that I've just discovered going over these photos: the rear chassis brace between the rear engine mounts would block me from putting a chain on.

XB33BSA

· #4 · Dec 31, 2017 (Edited)

welcome you are doing it kinda wrong,actually you could not do it more wrong if your instructor in school said "do this the perfect wrong backwarsds doomed to fail way that you can come up with "
building a frame from scratch gives you many good advantages
but just penciling in a structure with nothing else being comsidered is like being the last man on earth with a beautiful fertal woman and then cutting off your dick and balls ,never to save the human race
but it is typical im patience that you have ,put that aside
there is a huge amount of work to do before you design the frame
mayb e you can figure out what must be done first before the frame
you really should know this i speak of

XB33BSA

· #5 · Dec 31, 2017

if building a motorcycle from scratch as you are the frame is the very last structural hard part to be designed not the first

XB33BSA

· #6 · Dec 31, 2017 (Edited)

UniHalf said:
Stupid mistake #1 that I've just discovered going over these photos: the rear chassis brace between the rear engine mounts would block me from putting a chain on.

this is a joke right ?good god you are looking at a chain as though it was a rubber band or an average drive belt, for fucks sake
this is what CAD has done to the human race, step away from the fucking cad before it completely lobotomises all creative thought and function

UniHalf

Discussion Starter · #7 · Dec 31, 2017

I'm designing the frame around the engine (Kawasaki Ninja 250R) and suspension I have in mind. So the dimensions came directly from my height and the mounting points on that engine.

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.

Criticise the CAD all you want but I get paid partially to know how to use it so it's still to my benefit.

I have engine, suspension, swingarm, steering, fuel tank, battery mounting points that I'm working with on the design. A few of those still need to be e added. What is it that I'm completely missing here that makes this a bad starting place?

woodsman

· #8 · Dec 31, 2017

How did you determine the angle of your head tube? What forks, swing arm/shocks, rim size are you using? Why?

TrialsRider

· #9 · Dec 31, 2017

WTF is a fertal woman ?:I

UniHalf, good to hear what the engine is, that is kind of an important start point. The frame you sketched out is very old school and the most obvious area being overlooked is the steering head. Your design uses a single tube backbone at the top of it and a double loop cradle, that is ancient design methodology. Think triangulation. This is a 2014 Kawasaki upgraded frame, study it, learn from it and try to improve on it, anything else is going backwards.

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TrialsRider

· #10 · Dec 31, 2017

Does the motor have fuel injection? Fuel injection means it has a fuel pump, fuel pump means you don't need gravity feed, not needing gravity fuel feed means you don't need a saddle tank full of heavy liquid fuel located high on the frame. Lose the notion that you need to locate your fuel supply high on the bike and make the frame skinny enough that a fuel tank can straddle the frame. You planned to fit it with twin rear shocks? What is this 1970 or 2018?

TrialsRider

· #11 · Dec 31, 2017

"Good handling and feel is one of my top priorities here" so make everything as light weight as imaginable but wrap it all up in a 'rigid' frame that securely connects the motor to both the front and rear suspension systems, centralize the mass of the bike as much as possible and carry the heavy stuff low as possible on the motorcycle. Keep the unsprung weight down and use suspension components that feature adjustment for spring rate, compression dampening and rebound dampening. Plan on using wheels that can be fitted with modern high performance tires and those are Not available in a large range of sizes.

... hope you have deep pockets!

XB33BSA

· #12 · Dec 31, 2017

UniHalf said:
I'm designing the frame around the engine (Kawasaki Ninja 250R) and suspension I have in mind. So the dimensions came directly from my height and the mounting points on that engine.

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.

Criticise the CAD all you want but I get paid partially to know how to use it so it's still to my benefit.

I have engine, suspension, swingarm, steering, fuel tank, battery mounting points that I'm working with on the design. A few of those still need to be e added. What is it that I'm completely missing here that makes this a bad starting place?

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

UniHalf

Discussion Starter · #13 · Dec 31, 2017

I like the handling of the KTM RC 390, so I'm modeling the front suspension after that bike, so steering head angle 65° and 100mm trail. I'll revise the spring rates because this bike should be significantly lighter.

I'll also most likely be using the wheels from the 390 so 17s on both ends. I'm using those because the wheels are relatively like to keep the unsprung weight down and because the can be bought cheaply.

The rear suspension I have to get a little more creative with, so I have some research to do before I decide that and the swing arm's exact geometry. I'm planning on dual adjustable socks and dampers from Race Tech.

TrialsRider

· #14 · Dec 31, 2017 (Edited)

UniHalf said:
I like the handling of the KTM RC 390...

So look at the frame geometry on that bike:

... NOT a single tube backbone double loop motor carrier.
Also doesn't have a linkage rear suspension so there is room for improvement in the handling department there too!
Linkage suspension is more complex leading to higher production and maintenance cost but it also handles better!

"because this bike should be significantly lighter" <- don't count on it, the 390 is a single cylinder motor last time I checked.

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woodsman

· #15 · Dec 31, 2017

Your using a honda front end aren't you? How are the triples going to play into your KTM geometry? 100mm trail seems excessive, I believe that generally it should run in the 55 to 75mm range. What you do with the rear suspension will affect the front end geometry.

UniHalf

Discussion Starter · #16 · Dec 31, 2017

It'll be fuel injected.

Not needing a saddle fuel tank is a brilliant point. I could have some fun with that, I'm sure.

Anyway the point here is to not create an exact geometry replica. I'm using the engine from a Ninja 250, but that's about all I'm using from it. If I wanted a Ninja 250, I'd go out and buy one. It'd be way less expensive and time-consuming than this. Also computers are virtually all of engineering at this point. This being a terrible design is the reason it's in CAD in the first place. I can revise and revise until it's a good design.

I had been planning 1 1/4" steel tubing. Not sure of the wall thickness yet. Should I go higher than that? I'm thinking 1 1/2" might be overkill and add unnecessary mass.

woodsman

· #17 · Dec 31, 2017

woodsman said:
100mm trail seems excessive, I believe that generally it should run in the 55 to 75mm range. What you do with the rear suspension will affect the front end geometry.

I'm confusing posts, skip the honda front end comment

TrialsRider

· #18 · Dec 31, 2017

UniHalf said:
It'll be fuel injected.
Not needing a saddle fuel tank is a brilliant point. I could have some fun with that, I'm sure ...

Good start because that thing that looks like a fuel tank on a modern GP sportbike is not a fuel tank, it's the air box.
Almost nothing related to frame members is overkill and unnecessary mass, the frame is what holds the bike together and stops it from feeling like it has hinge in the middle. But just thinking something is the right shape or size is not going to cut it, manufactures pay huge bucks to do stress analysis and destructive testing on those things during the design phase, they don't just do it by eye. At least not in this century.

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UniHalf

Discussion Starter · #19 · Dec 31, 2017

woodsman said:
Your using a honda front end aren't you? How are the triples going to play into your KTM geometry? 100mm trail seems excessive, I believe that generally it should run in the 55 to 75mm range. What you do with the rear suspension will affect the front end geometry.

You're probably right here. Especially because it won't be a super fast bike in a straight line. I could lower the trail without too much trouble.

UniHalf

Discussion Starter · #20 · Dec 31, 2017

TrialsRider said:
UniHalf said:

It'll be fuel injected.
Not needing a saddle fuel tank is a brilliant point. I could have some fun with that, I'm sure ...

Click to expand...

Good start because that thing that looks like a fuel tank on a modern GP sportbike is not a fuel tank, it's the air box.
Almost nothing related to frame members is overkill and unnecessary mass, the frame is what holds the bike together and stops it from feeling like it has hinge in the middle. But just thinking something is the right shape or size is not going to cut it, manufactures pay huge bucks to do stress analysis and destructive testing on those things during the design phase, they don't just do it by eye. At least not in this century.

Gotcha. The stress analysis is going to be important to the project too so I can show potential employers I can do that type of FEA.

I'll go back to the drawing board and check back when I have a second draft.

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