Topic: CB750 Super Sport Swing Arm on Non-Super Sport Issues...

[zeech]

I've got a 1978 CB750 I'm working on.

I got a great deal on a set of wheels and swingarm from a 1979 CB750 Super Sport with the rear disc brake and thought, "great, I can just swap this out on my non-super sport bike!" 

Not so fast apparently... It looks like the SS swing arm is 1/2" wider than the stock CB750 arm. I would never have thought of this and apparently that means the SS frames are not built the same as regular ones?

The question is, can I simply trim the extra 1/4" off each side of the swing arm axle tube since it's there, is there another option, or am I #$%* out of luck? Would a 1978 swingarm from a SS work?

Thanks.

 

[andrewk]

Main problem there is that you've got a swingarm from a dual cam bike.  78 is the end of the model run, 79 is a new frame.

I'm sure the arm from the 79 can be made to fit, but a 77 or 78 F (super sport) arm would fit right up.  The caliper hanger and caliper should interchange between the two, though.

If you cut the arm you've got, mock it up so you can ensure that your wheel and chain lines are inline with the rest of the bike.

[zeech]

Excellent. That helps a ton. Thanks Andrew, really appreciate it. I'll try and track down a 78 swingarm first and go from there if I need to chop.

What I really liked about this swingarm is that it added 2" extra wheel base length, does the 78 version have that as well or only the 79? The 79 swingarm is also tube steel and seems to be much more heavy duty.

[70CB750]

It was done, I mean 79 arm in older K frame but as Andrew says, get SS swingarm and live happy.  At the bottom of this page:

http://forums.sohc4.net/index.php?topic=115305.0

I have exactly that, late SS arm in K3 frame.

Edit:  I believe the additional 2" are feature of the 79.  I can compare K0 and SS swing arm tonight if I remember.

[Geeto67]

the DOHC is asymetrical at the pivot. I believe you can just lop off the excess on the side that is longer and call it good.

The DOHC swingarm is longer you know. That means you need longer shocks with a different spring rate to accomidate for the change in shock angle.

[Xnavylfr]

The DOHC swingarm WILL work on the SOHC!!!  You have to machine off or cut off the extra width EQUALLY on each side!! You ALSO have to REAM the pivot bolt holes to size for the DOHC bolt. This can be done by step reaming a little amount of metal out at a time.   Reamers are EXPENSIVE and if you don't KNOW an Aircraft Sheetmetal mechanic or a machinist that will LOAN you the PROPER reamers, you can RUIN the frame at the pivot point!!!
NOT a HARD job to do  if you had reamed as many holes as I had in 43 years.

My avatar bike has a DOHC swing arm with Goldwing shocks!!

Xnavylfr(CHUCK)

[Geeto67]

I don't think that's right.

you can see in this pic that the pivot is not symetrical (not my pic got it from another's build thread - 1980 cb750F arm):


Find the measured center of both arms - then line them up by the indexed center mark and the SOHC pivot will tell you how much to take off the DOHC pivot.

[zeech]

After reading 4 or so threads, it seems like it's a pretty easy job to get the 79 to fit in my 78. I overlaid the 79 on my 78 and all that needs to be done is ~1/2" needs to be cut off the right hand side. Everything else appears to line up right and that side is indeed just longer.

Will report back with photos etc. Thanks for the help.

[Geeto67]

Find the center of the rear axle (while installed in the arm but NOT tightened down to distort the arm). From there pull a string at a 90 degree angle and it should tell you where the center of the arm is (make sure the string is at a 90 degree angle to the pivot). Mark this at the pivot. Do this for both arms. Then line the arms up with that index mark. Ideally this should reveal to you the center of the tire provided it sits in the center of the arm. I don't think the wheel is offset in the arm. 

[zeech]

The 79 is indeed off center on comparison to the 78, I will post a photo when I get home. The left side of the axle and the wheel curve match up perfectly, while the right side sticks out longer and is what needs to be trimmed. That's my observation at least.

[Xnavylfr]

This is how I cut my DOHC swing arm to fit the SOHC. It fits centered and I didn't have any problems getting the wheel centered  and at the same time the cush drive and chain aligns.  I just know what I did worked!!!



Xnavylfr(CHUCK)