The front tire cups on the left side AND always pulls slightly to the right.
This happens on new rubber or old.

I have looked at the centreline alignment of the wheels and they are out by 12mm / (1/2”). The pull is only slight… if I (200 lbs) hang half a cheek off the left off the seat it will track straight.

I have new rubber and new head bearings still not able to get it to track straight.

Is this just a K feature to keeps your hands on the bars or am I missing something.

I believe I have read “everything” in the forums but I’m at a loss.

My 1990 K100 only has 30K kms and all original so I don’t think and accident is the Issue.

There is still a month before spring so if there is a teardown to happen I would be nice to do it now.

Thanks

If I recall correctly, the wheels are not in the same plane, but that isn't the source of the pull.

The "problem" is the rotating mass of the crankshaft.  Because the shaft is horizontal and parallel to the centerline of the bike, the centripetal forces acting on it will have an effect on the roll axis of the bike.

The forces are equal and should counteract each other, but the top of the shaft is higher than the lower part so it has a longer lever arm for the forces to act on.  So, in the end it will create a very slight lean angle that will cause the bike to pull to one side.

It's a feature, not a bug. 

I've had many K bikes and they all pull to the right a bit.

Also make sure that the directional arrow is pointing the correct direction. When I bought my K1100RS it had horrible tread wear on the front tire. The previous owner had new tires installed at Ventura BMW (a BMW dealer) and the morons there had mounted the front tire backwards.

Thank you for a plausible reason for the “K bikes do that”.

Though it’s reassuring to know that many riders have the same issue. It’s infuriating to not have a hypothesis as to why. I will defiantly sleep better knowing why this might be the norm.

However, now I need to know why the offset track on the tires. Is it to counter the rotational effects. Is that also why, I’ve read that the K bike is fussy about tire choice?

This is all starting to come into view. I’m running Avon spirit ST tires which are dual compound. Not available in 1990. Maybe time to switch to something else.

Thanks for the information and the extra sleep.

Switch to radial tires.

Here's why:
http://www.kbikeparts.com/classickbikes.com/ckb.tech/0.ckb.tech.files/radials/radials.htm

I think the rear wheel offset is due to the swing arm and drive shaft forcing the rear wheel over to get clearance.  I have never noticed any handling issues because of it.

Radials are nice, I used to run them on my RT, but they're hard to find now.  Still have them on my K100RS with the wide wheels.  The only place I notice a difference is on the rain grooves on a certain 5 mile stretch of road that I travel on 3-4 times a week.  The radials don't squirm on them like the bias ply tires do.

A certain Mr. Tony Foale, amongst other radical things, did some testing with offset track and came to the conclusion that within limits a bike handled better with the offset than with the wheels in line. So did Mr. BMW about twenty-five years ago, but he decided that this was too radical for the general motorcycle-buying public and that project was dropped.

The camber of the road should also have an effect on whether it pulls left or right or neutral. 
Regards Martin.

1) 1 had pressures 36 front and 41 rear. I followed up on the Avon website and they recommend 2.8 bar for the speed and load (40.6 lbs). Therefore, I think my front tire was under inflated. I’m up in Toronto and if the roads are clear I will ride. Tires don’t really heat up enough in 0 deg. C so that could contribute.
2) The camber of the road is a notable contributor, compounding with the low tire pressure.
3) The offset of the tires as a design feature confounds me however I’m satisfied to know it’s not in my control.
4) The angular momentum and the lever length is a satisfactory explanation for the minor pull to the right in my case.
5) The generosity of contributors has will afford me many nights of restful sleep.

Thanks all.

Like that desire to burble and fart when backing off on the throttle, that slight tug to the side is just part of the charm of the old beasts.

About the offset of wheels: BMW had to face big concerns about the frame geometry of the K100. The maker of the frame (Verlicchi if I remember) had to face quality issues and a large part of its production was bent.
So BMW solved the matter with courage, by organising a big recall...nah, just joking.
BMW made small shims to try to re-align the wheels, and made bigger tolerances of misalignment official.

I'm not sure that the camber of the road is a factor: The K bikes I've spotted in Ireland had the same left side wear. One here could tell us more about it.

Besides Point Seven Five explanation of the rotating crankshaft, I have another hypothesis: when running at a stable speed, the forces who make the rear wheel spin also permanently pull on a perpendicular plan, because of the bevel drive configuration. So this tilting moment has to be compensated permanently by the rider to keep balance. If you were able to ride 6000 km in straight line, at 100 km/h, you would see the same difference of wear on the tyre.

Spose you have to do a tight u turn, say for example at the end of a drive way. Do you prefer to go anti clockwise? A preference one way could result in more wear on one side, not sure about cupping though.

duck wrote:It's a feature, not a bug. 

I've had many K bikes and they all pull to the right a bit.

Also make sure that the directional arrow is pointing the correct direction. When I bought my K1100RS it had horrible tread wear on the front tire. The previous owner had new tires installed at Ventura BMW (a BMW dealer) and the morons there had mounted the front tire backwards.

Ventura BMW screwed up my R50/5 in 1973.  I have never forgotten that.  Left me stranded on i5 in the middle of the night.

i've posted about this in the past and studied in detail on the geometry of the frame, etc.

No bike with aligned wheels will pull or wear tires like a K bike.  No other manufacturer would knowingly put offset into the wheel alignment, shaft or chain drive.

i've modified my alignment by machining the mounting bosses and with shims and the K75 rides much better and with less tire wear when the wheels are aligned.  i've also shimmed it the other side to have a greater offset and this caused it to pull R much harder.

It's just a design defect, or blunder, in the  specifications between the engine/trans design and the frame design...but it sho' do look good

Have a read of this you will get some usefull info as to why the alignment is important https://www.dropbox.com/scl/fi/wg8ti67qptqax08pvwinf/Full_Control.pdf?rlkey=d06jjw2fmi8shf88x6bgp5db7&dl=0

Why I don't check alignment? I make do with keeping my hands on the bar grips, the front suspension lubed, the rear shock adjusted, the wheels balanced if they need it, and the tires at standard pressure specs depending on load, checked at the start of the day. If that isn't good enough, screw it. I'll ride a tuk tuk and haul llama feed to the locals. Besides, I don't have a gauge.
 

Too much knowledge....

A long time ago in a shed far far away, I had cause to check just exactly where the longitudinal centre of gravity of a K75 was. Y'know, was it at the actual centreline of the bike, where you'd think it'd be?

No, it was a whole 20mm to the right of the longitudinal centre line of the bike. This means that, if you stand the bike on it's wheels, its 'crank-heavy', if that means anything. This is accentuated by any offset-to-the-left of the rear wheel (all looking at the bike from the rear)

I patted that particular sleeping dog on its head and came away from all that thinking that BMW perhaps knew a lot more about the effect of an in-line crankshaft than they let on.

I've never ridden an old boxer but been told the inline crankshaft affect can be felt, especially during acceleration, I think.

I'm thinking I spose the problem (or hopefully most of it) is deliberately by design offset by the fact that the output shaft, clutch, etc is spinning in the opposite direction of the crankshaft. Bricks are incredibly unsymmetrical though, especially noticeable when I removed a crash bar from one, once.

I seem to recall that BMW had a few problems with K frames, so much so that the stick-the-frame-on-the-engine instructions come with a health warning to bolt up the gerabox end first and then shim the front end to fit. Yup; sounds like a perfect recipe for misalignment to me.

@daveyson: old boxers can be fun when giving it the welly, but I do NOT recommend winding an R100RT open such that maximum rise happens to coincide with a metal expansion joint on a wet bridge. It has a tendency to cause instant bodily suction. They were also a bit disconcerting in that if you pushed them hard enough (I've only ridden the aforementioned R100RT, plus a 600/7 and an 800/7) you could feel the frame flexing. Yes, even the poor underpowered 600/7.

I was attracted to BMW motos because the crank shaft was going front to back (different from most manufactures). I have never warmed up to a R bike just because of the engine effect on balance ( the motor wants to torque the bike over). Seems like the K bikes do not do that (as much). I too think the engineers at the plant didn't like that feel of R bikes and fixed it with the Ks and, gave them some stick to make them fun to ride.

Dai wrote: It has a tendency to cause instant bodily suction. . . .

The heartbreak of IBS.   I believe there's a video on YouTube that demonstrates clacking spoons together below your ear can remedy that.

Having been riding Airheads since 1976, I can say that they only really torque (to the right hand side) when from idle, you rev the engine. Once you grasp that they do that you forget about it and resume your riding. For me, that's over 800,000 miles all up on 2V boxers, and another coupla hundred thou on Oilheads. My '95 R100 Mystic does it. It's not a thing. Anyone who can feel it whilst riding at a normal or even a rapid pace may be imagining things. The waterboxers do it to some degree, too, but only mildly, and to the left. 

Now Airhead frame flex...their relatively soft and long-travel suspensions exaggerated that feeling. That was a thing. There are reliable ways to mitigate it, but it remains a thing.

On my lovely toaster tank '70 R60/5, the wheels were perfectly aligned front to rear, and rode in a straight line without pulling right or weird tire wear.

But the OP measured 12mm of offset, and he could shim it to get back down to the 5mm maximum limit if desired.  Every little bit helps in this case, both for wear and pull.

i have owned and rode bmw , yamaha , suzuki , honda , velocette , motoguzzi (the worst) all have a reaction if shaft drive , the k100 imo is the best but none like gear changes or emergency stops mid corner