[Dzvarich]

Hey Guys,

It's been a while. I've been looking for a bike for about a year now and I stumbled onto a 1981 Honda cb750K for ultra cheap. I started building it into what I wanted in a cafe racer.

My current dilemma. I have a 2012 R6 fork for it and want to put the R6 wheel on it. I also want a R6 wheel in the back for the brake and aesthetics of the bike. The front is easy to switch. I'm using the original triple trees and ordering a cognito moto stem. The back, I don't really know where to begin. I've done motorcycle restorations before but I've never replaced the swing arm.

Let me know what you guys think.

 

[woodsman]

The original R6 triples?

Does it run?

 

[Dzvarich]

The original R6 triples?

Does it run?

Yeah, I'm using the original R6 triple trees with a different stem.

The bike runs. I did pull the engine to replace the piston rings due to compression issues.

 

[Dzvarich]

Do you guys know if a 1991-1993 Honda CBR600 F2 Swing arm will fit on a 1981 Honda cb750?

 

[Miniman2]

Do you guys know if a 1991-1993 Honda CBR600 F2 Swing arm will fit on a 1981 Honda cb750?

It won't fit.... well not without a lot of surgery to the frame or the swingarm.

Think that just because you can get it to fit doesn't mean it will be functional.

Your trying to put a swingarm that usually operates with a rising rate linkage and monoshock to a frame designed to run tubular swingarm with twin shicks.
Chalk and cheese.

What are your original suspention geometry numbers? I see you have dropped the standard forks in the triples... why what are the geometry numbers now?

What are the numbers with the new forks?
I will wager that it will be in the realms of being unsafe with frighteningly little rake and trail.

Stop before you go any further and actually plan this using the correct mathematics and engineering principles. This is the literal difference between life and death if not done right.

 

[Dzvarich]

It won't fit.... well not without a lot of surgery to the frame or the swingarm.

Think that just because you can get it to fit doesn't mean it will be functional.

Your trying to put a swingarm that usually operates with a rising rate linkage and monoshock to a frame designed to run tubular swingarm with twin shicks.
Chalk and cheese.

What are your original suspention geometry numbers? I see you have dropped the standard forks in the triples... why what are the geometry numbers now?

What are the numbers with the new forks?
I will wager that it will be in the realms of being unsafe with frighteningly little rake and trail.

Stop before you go any further and actually plan this using the correct mathematics and engineering principles. This is the literal difference between life and death if not done right.

That's very fair in your thought. I appreciate your response and it definitely has me thinking about the proper functioning swing arm.

I actually did not drop the forks. Yes, in the picture it is dropped but I have new forks from a 2012 Yamaha R6. So, the geometry will change but I won't be dropping the forks on the R6 conversion.

I will be writing everything out and looking at the mathematics.

Again, thank you.

So, last question. Is there any swing arm in your guys opinion that would best fit the 1981 Honda cb750k for a bigger back tire?

 

[woodsman]

The pictures didn't come up my first stop here.

You're not likely to get to much input on this here. If you wanted to make the bike perform better you'd likely get lots of feedback from guys that know them. None of them are going to support what are doing, it just doesn't make sense to them or me. It won't be worth riding and it's not go to be something desired in the man caves, so what's the point..

 

[Farmer_John]

The stock stock swing arm is designed for a much narrower rim and tire. That also means the chain line is set for that combo.

You should plan on major surgery to fit a modernish rear wheel, including the probable significant moving of the engine to compensate at worse, or an offset pinion gear at best.

Easy answer to this dilema? Run stock wheels...

 

[oldhondacafe]

I pretty much tried this with my '78 cb750 a few years ago and I know from that, what Miniman2 is saying is true. even small changes make the bike handle differently. I did end up using a "90 CBR1000 swingarm, but I swapped the entire rear section to make it work. including the single shock set up. Manufactures spend $$$ and time and research in design and just eyeballing it is not going to give you good results. With my first attempt, I couldn't ride the bike without the rear wheel hitting the fender. I finally got a whole CBR1000 and set the 2 bikes side by side to see how different they were. when I swapped in the CBR rear section, I lined it up to match the CBR geometry and that's the only way the bike works. It rides nice and has better brakes but it's not a CBR. If you tried a CBR swingarm with 2 shocks I don't know how well it will work because the swingarm is 4" longer than the 750 swingarm, it would probably take a lot of trial and error to get it to ride ok. Also this will never handle like a R6 so getting it to ride ok is maybe the best you can hope for. Honestly, I mostly did this for looks and personal preference. I also ran the CB750 wheels and tires cause like he said, the wider wheels don't fit. since these tires limit the handling, having a smooth ride is as good as I could do.