81 xj750 seca swingarm gaps / rear tyre alignment

hey guys,

i have replaced the swingarm bearings and wheel bearings but not to sure of the gaps i have in the swingarm and the rear rim seems offset to the left.
left side is approx 2.5mm between swingarm and frame
right side is 7mm between swing arm and frame
The tyre seems to be about 5mm to the left of the centre line of frame
i have read the procedure for torquing the swingarm bolts and believe i have followed it as well as i believe i have all the right spacers in the right places. but this doesn't look right to me.

 

I just went through this with my Johnny Cash bike, I had trouble getting left side to seat . What I did was back adjuster out ,and held the arm up and slowly tightened adjuster . Moved to right side and did the same , you have to move the arm slightly to get these to seat and torque adjusters as I recall 77ft lbs ( don't quote me I have slept since I did this) the manual is kinda vague on this reassembly.

 

The rear tire is not going to be centered with the frame center. The driveshaft tube on the swingarm has to be centered with the output yoke on the engine.
You have it set up correctly, gaps and all.

 

Whoa there. Why would this be? Surely Yamaha would have managed to line the engine up with the rear wheel, and said wheel up with the frame middle?
How much out of centre does it sit? The driveshaft has some movement allowed (or at least is not restricted) either side of the centreline.

 

Why? I don't know.
The same is true on most (if not all) shaft driven Moto Guzzis.
A lot of Belt-driven motorcycles also have the rear wheel offset from the centerline of the frame.
The machine does not care if the wheels run slightly out of line or not.
I've never measured the offset on an XJ. I just know that it's there, whether the swingarm has ever been taken off or not.

 

Of course the machine cares if the wheel is not in line. If it's directionally correct, but offset to one side the bike will crab, causing the handlebars to be cranked over while going in a staraight line. The only thing that would correct this is to steer the rear wheel slightly to compensate - what does that do to cornering ability and stability? All this is relative of course, I wouldn't expect it to be very much either way. But really Yamaha, why would you?
I'm working on getting the xj900 diversion 17 inch wheel to fit in my 900f - I guarantee you I will try my best to get it to sit in the middle and point in the same direction as the frame is pointing. I've considered the shaft to engine alignment, and may end up with a small offset here, as well as moving the engine over slightly in order to achive centrallity (new word)?

 

I'm not so sure about that

re-think that please
if the wheels are aligned they go the same direction regardless of the offset. handlebars have nothing to do with it, they change the alignment, that changes direction, no offset involved

 

If it were a chain drive machine and the adjusters were not the same the wheel would be slightly off which would cause it to crab. But with shaft drive there are no adjusters so it shouldn't.

 

Well I had to think a while about this, it's not easy to get your head around. But if you sit in the middle, and the wheel centre is offset, your weight will tilt the bike to one side, which will require a steering correction. If its a small,offset probably unnoticeable. But small or not, why do it?

 

You weight has very little affect on the machine. Go for a ride and sit offset to one side (without inducng counter steer). The bike will track true. The gyroscopic effect of the engine alone is enough to keep the bike upright, add the wheels in and the only way to get the machine to turn or lean is with countersteering (yes, countersteering does happen even when you initiate a trun with your feet, because your upper body naturally moves to balance the forces applied).

As to the why do it: Packaging concerns. There is a limit to how compact a gearbox can be. Since it's a transvese engine, that means the position of the output shaft has to be where it is. Shifitng the engine to the right would make it both look wonky, and feel wonly because your right ankle would have to sit further to the right.
Offsetting the wheel to the left eliminates that problem without affecting handling at all.
Yamaha is not the only manufacturer to offset the rear wheel for packaging concerns.

I worked at the municipal airport for one summer, and was allowed to run my machine on the new runway during construction. It tracked straight and true with my hands off of the bars.

 

when i put my seca together, i checked the alignment with strings and don't remember having any offset at all

 

"Offsetting the wheel to the left eliminates that problem without affecting handling at all."
I don't know k-moe, I've seen some weird out there statements from you to justify your opinion, but this one takes the biscuit. So, design and build a motorbike, including the engine from scratch, and put the rear wheel off to one side. You often quote that the settings for these bikes should be "in spec", well what spec is this one then?
And btw, of course it will affect the handling - tipping into corners will be different on either side for one.
All my xj's will have the rear wheel in the middle, provided the shaft doesn't have undue load on it, if it does, I will tweek the engine mounts.

 

1. Please don't resort to ad-hominim attacks.

2. I tell people what I know, nothing more. What I know is based either on my own experience, or on research. My professional and personal life depends on me not just making things up.

3. On my Seca the locking tabs for the swingarm nuts had never been reused, and the original telltale paint was in place and undisturbed.
The rear wheel was offset to the left before I removed the swingarm. I followed the factory procedure for reinstallation, and made sure to keep the original paint marks in mind.
Upon reinstillation the rear wheel was offset to the left.

4. The other XJs that I have worked on all had rear wheels that were offset to the left. This may be on purpose, or it may have been an assembly error, but it is not inconsistant with what happens when the factory procedure for reinstalling the swingarm is followed.

5. It's entirely possible that I'm wrong, and when evidence is presented as such I will freely admit it (and always have).

6. You are free to disagree with me, but don't insult me.

 

I came across mention of some bikes having as much as 20mm rear wheel offset and it not being noticeable in riding them.
This came as a surprise to the owners, as there're no visual clues, since the rear mudguard centre line is offset in the factory

 

Just to add some info not sure if it means much, but my 1982 750 Seca has never had the swingarm removed and measures:

Left swingarm to frame - 4.8mm
Right swingarm to frame - 5.3mm
Left tire to swingarm - 12.5mm
Right tire to swingarm - 38.5mm

Tire Metzler 120/90-18

And to clarify the assembly the pivot shaft (adjuster right side) is only torqued to 43-52 in-lbs, and then the locking nut is torqued to 72 ft-lbs while ensuring the pivot shaft does not rotate.