Just here for the technical aspects.

[fetor56]

G'day man and welcome to MBc.
My general rule of thumb is 36cm(14") measured from the INSIDE of the triangular frame from the crank to the seat.You can go slightly smaller but it's really not recommended cos there's always routine maintenance to do.You can also get a smaller sparkplug if necessary(Champion CJ7Y)
You can also grind the engine down and have aluminium brackets welded to it but that's another story.

 

[Nerobro]

Well, that's some useful information. Thank you. Actually, I think both of my bikes would cover that.

 

[drimpact]

Welcome,

The smallest frame I have used was a 16.5" Trek Navigator (03 model) It was a super tight fit.

For my personal bike I use a 19.5" Diamond back Outlook.

Fetor's info above is 100% correct! When I am measuring a frame for use I am looking for 14 or more inches measured from the top of the bottom bracket to the bottom of the top tube.

Other frame geometry comes into play, but at least 14" is where I start.

 

[Nerobro]

*grins* I'm glad you know what frame sizes are. Thanks, that measurement helps, a lot.

 

[Esteban]

I am sure many of us do know what a frame size means on a bike. Most of the answers of 26" bikes, that you received , are still correct . Many of the bikes, you will see modified with a motor , are standard cruiser type bikes sold in department stores. They are NOT sold by their frame sizes, only by their wheel sizes, like bikes were sold years ago.

 

[friedlbug]

Wheel size is still an important component, too. I realize you asked about frame size, but you'll also be interested in wheel size when you're ready to start considering torque and speed. Also, for the money, you may find that a department store bike gives you the best value for a motorized platform when steel construction is a benefit and a few pounds one way or another doesn't matter.

 

[Nerobro]

I am sure many of us do know what a frame size means on a bike. Most of the answers of 26" bikes, that you received , are still correct . Many of the bikes, you will see modified with a motor , are standard cruiser type bikes sold in department stores. They are NOT sold by their frame sizes, only by their wheel sizes, like bikes were sold years ago.

But 26" isn't the frame size. I'm not sure how that makes it a correct answer to me asking what size frame people installed their motor on. Even those cruiser bikes have a frame size. Weather they're sold by them or not.

Even to that end http://sheldonbrown.com/26/index.html 26" isn't a fair answer. There's no less than five 26" tire standards.

I intend on using this bike in fairly severe duty. I wanted to start with a good bike. I'm not a tall person, so I tend to ride small frames. (as is exemplified by my 46cm Dawes) That's why I asked the specific question I did.

Wheel size is still an important component, too. I realize you asked about frame size, but you'll also be interested in wheel size when you're ready to start considering torque and speed. Also, for the money, you may find that a department store bike gives you the best value for a motorized platform when steel construction is a benefit and a few pounds one way or another doesn't matter.

Wheel size matters much less than frame size. If I can't fit the motor into the frame, I could have 6" or 35" wheels and still be in the same situation. (Sitting with a bike that I can't fit the motor into) No matter what size wheel I end up with, (Which is most likely going to be 700c) I can always alter the final drive ratio to match the expected speed range. I can not make the frame bigger though.

I've begun to ask some other questions which will address the power issue. We'll see where that gets me.

 

[Esteban]

I don't know if you want to be too difficult, too stubborn, or too technical about your motorbike ideas. In MY opinion, I would suggest you buy one already built.

 

[Scottm]

This has been pretty helpful and educational for me. I am building abike for someone who is short in the leg area, and it's been difficult helping describe what kind of bike he should get. I told him to take a tape measure and measure the fames for the 14 inch also.

 

[Nerobro]

I don't know if you want to be too difficult, too stubborn, or too technical about your motorbike ideas. In MY opinion, I would suggest you buy one already built.

If the technical information isn't out there. Someone needs to pry it out. It would make chosing a proper bike easy. Which it isn't at the moment.

I asked a very clear question to start this off. If you don't know, you don't know. I can accept that. That's why I later defined what measurement I was looking for. I don't see that as being difficult, picky, or technical. Just accurate.

Thankfully, I did get the answer I was looking for. And I appreciate it greatly. In fact, I even got a BETTER answer than I was looking for. The "bike frame" size is from the center of the BB to the top of the top tube. When really it's the space between the bottom tube, and the top tube that matters.