Cafe Racer Forum banner

Custom frame design comments/concernces/constructive criticism

9852 Views 66 Replies 10 Participants Last post by  sebwiers
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.

See less See more
1 - 16 of 67 Posts
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.
See less See more
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?
"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!
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.
See less See more
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.
See less See more
Here, guaranteed you will find this interesting, start here and watch all of this 3 part series of videos, this will take you to part 1:
I know you're kidding but I still wanted to mention that I say fluid dynamics because one of my end design goals is less than a certain drag coefficient. So I'll see how that works in the (fairly distant) future...
Not sure how to interpret that "less than a certain drag coefficient", is your goal to make it wind slippery to the point of having less than a 1.0 wind drag coefficient? If yes, pretty sure you are going to need access to a wind tunnel test facility, that is how BMW and Moto Guzzi did it.

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.
View attachment 82562

Check out the monoshock bracing on this one. Nice.
Must be for one of those motors where the countershaft sprocket comes off the top of the transmission ;)
What has changed in the last 20 years really depends on the motorcycle brand that is being considered, BMW for example is doing really well in racing right now but their current hot bike is worlds away from the bikes BMW built in 1989 up to ~2004. The Foale book series was most recently updated in 2006 so I would expect that most recent version to potentially contain considerably more current info.
lol Ya we are getting old and the older you get the faster time fly's, the give away should be "book" printed paper is near obsolete because information changes faster then it can be printed and bound into books. UniHalf needs to be designing innovative motorcycles for the future for his efforts to be worth anything, otherwise he is just re-inventing the wheel and anybody can do that.

... not to suggest that lots of people aren't getting rich by re-inventing old technology or copying other peoples iPhone, but it's the innovators that are in demand and remembered for their contributions to advancement.
See less See more
Oh by the way UniHalf, can you even ride a motorcycle and are you any good at it?
If you set out to design a better mouse trap it is a good idea to start by having some mouse killing experience ;)
Cool! I have a 1986 K100RS that I've owned since new. Heavy bike but not a hard bike to control at normal speed. Mine won me a lot of slow races and it's a freight train on high speed super slabs. The RS is one of the bikes that was designed in a wind tunnel, the K1 version had the best in class wind coefficient ever. Screw the Ninja, you should work on an upgrade to the K bike frame or a racing sidecar frame for that engine, that would be way more interesting!
... racing sidecars are 100% one off design concept hand built motorcycles, everything you do would be breaking new ground and absolutely nothing could be considered wrong if it can wins races.
I SO hope you're kidding...
Only slightly.
Starting with the frame design he originally posted and putting a Ninjette motor in that doesn't impress me as
:/ well actually it doesn't impress me in the least.
So if this thread is all about a design exercise to pass a test or graduate from somewhere, I give it 35% grade at most and that is only if it runs and rides like a motorcycle.

I do like what you are saying though, I'd give your post 85%
  • Like
Reactions: 1
Velcro! :D that was a very cool innovation and I wish the heck I had come up with it. I give that one 100%
... even though the design credits really should go to a burr bush.
Good man! My comments are not intended to discourage but rather to encourage you to strive for greatness.
  • Like
Reactions: 1
... moncoque construction everything goes inside of.
... from PLYWOOD - high quality wood is actually not to far off good quality composites..

actually wood fibre is one of the strongest lightest material known to man, but you gotta be really good to work with it.
See less See more
  • Like
Reactions: 1
1 - 16 of 67 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.