Topic: Bent rear frame on my K0? Dammit!

[Magpie]

Hey!
Just before I hit the rack, I think I have a bent rear frame. I wanted to make the project bike a roller so it's easier to move around. I had a spare set of wheels and put them on, then the ship hit the span. It's an unscientific method to find out but I put  a long screwdriver through the tangs at the rear of the bike and marked centre. I then dropped a specially designed plumb bob down and measured the distance between the ends of the swingarms. You can see in the pictures that the distance to centre from the left side is about 4 3/8", from the right, 5". What these means, I think, is that the entire top rear frame has been pushed over to the left somehow but there's no evidence of major trauma to the frame other than a small mark in one of the muffler mount tubes on the right side and the plate over top rear of the frame having a slight depression. You can also see that the left shock is on an angle too. All the other frame tubes are unmarked. I know the early frames and welding were not the greatest but this is beyond poor assembley.
I know the steering head was tweaked but I had this repaired.
I put my new rim in the swingarm and you can see the difference between the centre of the rim and the screwdriver. Dammit. I put an old rear wheel assembly on and it's really apparent.
S**T! I'm pretty sure this is very difficult to fix.
Cliff.

[Retro Rocket]

You need to make sure that your bike is sitting exactly level to use that method of finding centre, a couple of 16th's at the top will be a lot more at the bottom of the plumb bob..

Mick

[Hush]

I'd give the shocks a hard look at too, if one is holding up better than the other you will get a difference.
As you say, one looks like it has damage, try removing them and do some measurements.

[Terry in Australia]

G'Day Cliff, two quick dumb questions, is the centre stand straight, and are both your rear shocks the same length? K0 and K1 shocks are very similar, but one is around half an inch longer. If it's no good mate, send it to me, I need another K0 frame! Cheers, Terry.

[Retro Rocket]

Yeah, thats what i was getting at guy's,its bloody hard to find dead level where a plumb bob would be that accurate..

Mick

[eurban]

If the rest of your frame is level (left to right) up front and the rear looks like that then clearly something is amiss.  However it doesn't really appear that you have the entire frame any where close to level.  What's the story? . . .One thing to keep in mind is that in proper stock form, things are not necessarily "centered" up in the rear.  On my 78 750s frame for instance the upper mounts for the rear shocks protrude (the part that the inner portion of the shock eyes stops on) from the frame rails a different amount on the left side than the right.  I think the difference is 1/4" or so.  The swing arm forks seem to be similarly offset so that the clevis mounts line up with the upper shock mounts.  With all the proper spacers installed, the wheel is off center the opposite way in the swingarm, effectively bringing it back to the centerline of the bike.  I think this arrangement gave honda more room for chain clearance but I'm not sure what the reasoning was.  I do know that it is the same as other late bike frames that I have seen.  I don't know whether or not the earlier 530 chained bikes have a similar offset or not.  Hope this helps . . . .

[HondaMan]

It's actually not uncommon for the K0/K1/K2 bikes to have crooked frames: I got one like that from Honda on my first 750, a K1. In the extreme rush to build to meet the demand in those days, a lot of "little" things like these got overlooked.

The steering head assembly was built in a jig, then set into another jig where the cradle tubes and backbone tubes were shoved in, and the center member sections slid on. Then everything was spotwelded together and the frame removed from the jig for hand finishing. My K1 was dropped after the spotwelding process and the spots broke, then it was welded crooked. In production, they ovalled out the lower right engine mount hole and the engine's hole, so the lower bolt could go through.  The only evidence of this was a small flattened spot on the bottom of the right boss where the dropped frame hit the floor. When I got it, the wheels tracked a little over 1/2" apart! It was this flattened spot that finally won the proof with the Honda reps and they traded bikes with me for my current K2, for about $150 difference.

Many of the Showa shocks came in up to 1/4" different lengths, which can tilt the swingarm, especially if the bushings are phenolic (they squish, and are soft after years of grease). In addition, the centerstand is NOT level or even, almost none were: the rounded feet that sit on the ground are not even jig-bent.

So, in short, true up these frames with levels and plumb bobs and blocks against the bottom frame rails before attempting to measure a skewed frame. They are not easy to measure: I've always used the "are the wheels in line?" method, myself. 

[Magpie]

Thanks guys! All good advice and will start checking things out today. I did have a level on the rear part of the frame. Strangely enough too, the centre stand didn't have both feet on the ground so to speak. The right foot was but the lever part of the left side was on the ground, not the foot. I had to put a thin sheet of veneer under the right one to level it. I was just sent the "wheel in line" method of checking by a friend. I'll put some different shocks on today as well.
Tryin not to freak out too much.
Cliff.

[754]

Dont get so worried ..YET!

 Fisrt off..level frame..do this by making sure you have a straight rod thru the rear motor mount (or front too, if you want to check)

 Forget the rest of the frame, the motor and neck, and swingarm are what counts..

 Now insert swingarm, cant hurt to check that pivot bolt is level. You can block from below or just hold it up.  Insert rear axle and check with level for twist.
 Swingarms bend easy, it will bend way before your frame.

 Centreline of frame on a lot of bikes, is not rear wheel centerline. on the CB 750 I am not sure. If you have a spare lower end or cases with sprocket shaft installed, use them installed in frame. Mount wheel and pushing front sprocket inward..use a straightedge against that and aiming at the rear wheel. You can slip the wheel on the axle without the outer spacers. Now when sprockets are lined up and parrallel to the straightedge, it is in postion.. Now check your spacers.. if one wont go(from outside of axle) and other is loose.. this will show how far you are out.

 I am betting swingarm is pushed sideways, not frame..

[Magpie]

Ok, after  reading your thoughts and a poor night's sleep here's what I did today, not sure what I proved but it seems like I found the culprit.
I tried to work on it just on the centre stand which was terrible and I had a slow motion fall to the floor with it. So I put some eye hooks in the ceiling, strapped it up and leveled it. Next removed the lower engine mount. I bought two 3' x  3/8" steel rod (checked for true on a long level at the store). You'll see in the pictures that the rod through the lower engine mount, one through the swing arm mount holes, a rod through the muffler mounts, a level across the frame in the battery box area and  another across the tabs at the rear of the bike are all the same level. The bubble was just touching the right side line on all the points. I think this means there's no twist in the frame there.
I replaced the shocks with some stouter ones and put the wheel in with the same result as last night. So, out came the swing arm and in went a K2+ one. It all seemed to line up much better. The centre of the tire was closer to the centre of the bike now with the old wheel and my new rim. I then laid one of the steel rods next to the inside of the rear sprocket and the inside of the primary and it looked like then lined up fine. The rod was touched both edges of the drive sprocket and the full sideof the primary. I also put the level on the bare axle in the K2 swingarm and it was then same as the others.
Conclusion, the K0 swingarm must be buggered. I hope!
I didn't get to my centre stand issue today.
Now I need another K0 swingarm. Anybody got a good one?
I'll repeat this in my build thread so I ahve a record of it.
Pictures:

[Magpie]

One more. Thanks for all your help guys!!
Cliff.

[754]

Swingarm can probably be straightened..

[Magpie]

I'm wondering who could do that here. I'll have to call some friends. Thanks for your help Frank. You were spot on about the swing arm. I'll use the K2 one until I get this sorted out. A metal chain guard on a K0, is there some sort of penalty for that?
Cheers,
Cliff.

[Magpie]

The mystery continues. I took the "bent" swingarm and compared to another I had. I measured the distance between the ends, inside and across the whole thing, the distance between the shock mounts, inside and across the whole thing, and they are the same within 1/16". Very strange. The only difference between the one I installed and the one I took out, the best I can tell, is that the one I put in had stock bushings, reset in the holes and the K0 one has bronze bushing set flush with the end of the swingarm.
I installed the K2 swingarm with the stepped washers that go onside the arm and then the large washers over the ends. The K0 one would not accept the stepped washer so I just used the large washer.
One other difference which is kind of strange. Every swingarm I've put on the workbench will just sit flat. The K0 one will tip down at the top left corner. You can just see it in the picture.
Hondaman, I need your expertise here.
Cliff.

[seaweb11]

You can always roll it up to Freds to have it all checked out.

[Magpie]

He's the one that told me the front end was bent. He never looked at the rear of any he's checked for me.
Cliff.

[754]

What do you see if you put them axle end facing axle end, ie width of opening?

 Also put one on top of the other with spacers between, and observe what the axle ends look like to each other.. ie .. one is more to right or left..

 You can probably straighten it at home.

[Magpie]

Good questions! I put them end to end and top of one another - they seem the same, at least not enough to make such a difference in the frame.
Cliff.

[754]

Cliff they really do look close.  Keep in mind though that  a bar throught the pivot, amd the axle slot is where you should check for twist.

 can you get a straightedge against the bushing at front of arm and check  offset distance to end of arm?.. may have to go from bottom..

[Magpie]

frank, I'm not sure if this is what you meant but the measurements are the same for each side. The first 3 are from the straightedge, the last 2 from the swingarm pivot bolt I ran through the swing arm. I have to review everything I did because it appears the only difference are the swing arm bushings and the offsets to them. 0 on the K0 and factory specs on the K2 one I put in there. I did change the shocks but boths sets slipped on easily without any force fitting. Both wheels were assembled the same way for each swingarm too.
I have to run out for awhile. Talk to you later. Thanks for your ideas!
Cliff.

[754]

What I meant was the straightedge, goes where your tape was (against the bushing). Then you see if the fork part that holds your axle, is out of line to left or right.. and how much.
 Keep in mind different front width of arm at bushings changes the mesurement a bit

[Industrial Cafe]

hey cliff,
    I hope the swing arm I sent you wasn't bent.
 

[HondaMan]

frank, I'm not sure if this is what you meant but the measurements are the same for each side. The first 3 are from the straightedge, the last 2 from the swingarm pivot bolt I ran through the swing arm. I have to review everything I did because it appears the only difference are the swing arm bushings and the offsets to them. 0 on the K0 and factory specs on the K2 one I put in there. I did change the shocks but boths sets slipped on easily without any force fitting. Both wheels were assembled the same way for each swingarm too.
I have to run out for awhile. Talk to you later. Thanks for your ideas!
Cliff.

This will be hard to describe, but I'll give it a shot:

When Honda machined these arms, they bolted the axle end on a fixture and made this simulated axle match up to the chain adjuster indicators, as if a wheel were mounted. Then, the boring machine cut the finished holes for the swingarm bushings into the tube at the pivot. This method was used because the welding process for combining 2 stamped halves made for some warpage in the whole structure, and because Honda learned the hard way from their CB450 arms, where they bored the bushing holes into the tubes, then welded those arms together in a jig: those arms often have misaligned bushing holes as the result. I've had to "fix" a lot of those, hard to do...

But back to the 750 world...if you look closely at the bore holes (assuming the bushings are removed, or at least the little phenolic end caps) you will probably see that the holes are "off" in some direction when looking at them side-to-side.

The whole purpose of this long-winded explanation: install the collar and the swingarm bolt into the pivot side of the arm, then make your measurements between the bolt's forward edges and the index marks on the axle indicators. These are the "reference marks" for this swingarm, and all other bets are off. Don't try to measure to any outside, painted surfaces, as they are all cosmetic. 

[Magpie]

I have measured from the front edge of the swing arm pivot bolt to the rear index mark on each side of the K0/K1 swing arm and the best I can tell is that both sides are within about 1 mm of each other so it seems it's straight. I put that swing arm in another frame I have and it's not centred between the muffler hangers either. I need some help with this I think. I removed all the the paint from the frame and hand sanded it myself. I didn't see or feel any damage to any of the frame members. You would think if it was pushed out by a crash it would have some sort of visible damage to the frame members. Assuming I have the original seat and side covers, they do not show signs of a heavy crash either.
The only thing I noticed was a slight deformation in the plate across the frame to the rear of the battery box and a sharp edged ding in the right side muffler hanger member. Could it just be a welder with a hang over that built this thing and he was seeing too well? I'm puzzled, perplexed and pissed at the whole thing.
Cliff.

[HondaMan]

I have measured from the front edge of the swing arm pivot bolt to the rear index mark on each side of the K0/K1 swing arm and the best I can tell is that both sides are within about 1 mm of each other so it seems it's straight. I put that swing arm in another frame I have and it's not centred between the muffler hangers either. I need some help with this I think. I removed all the the paint from the frame and hand sanded it myself. I didn't see or feel any damage to any of the frame members. You would think if it was pushed out by a crash it would have some sort of visible damage to the frame members. Assuming I have the original seat and side covers, they do not show signs of a heavy crash either.
The only thing I noticed was a slight deformation in the plate across the frame to the rear of the battery box and a sharp edged ding in the right side muffler hanger member. Could it just be a welder with a hang over that built this thing and he was seeing too well? I'm puzzled, perplexed and pissed at the whole thing.
Cliff.

This is a "long shot", but...have the swingarm bushings been changed, and/or are the cup washers that go with the arm both the same thickness?

I have seen one cup washer worn when the collar was seized in the arm, which then essentially bored the cup face. This makes the arm tilt in the frame.