Stiffening up Swingarm?

[TopCatGr58]

Question for the senior/experienced 11 owners, or structural engineers?

Since I have my niftly little wire feed welder, I've been thinking about using it to apply some more pieces of metal to the swing arm "H" in order to perhaps stiffen it up a bit, to reduce any flexing that might be occcuring under heavy throttle and turning?

I know the newer style of swing arms are the square tubes and such which I'm assuming is for providing a more stiffer design to reduce flex. I'm not worried about the little extra weight that it will add, but just wondering if it might help? Is there really much flexing that occurs in the swingarm that could contribute to the "hinge" feeling the XS11 has under strong throttle and cornering and uneven roads?? I'm also thinking of adding some bracing to the frame around the steering joint, have some pictures from a guy IIRC in OZ, a structural engineer added the braces to help stiffen the frame!?
T.C.

[uk1.1s]

Bracing

Hi Top Cat

Not a Structural Engineer, (my bag is Materials Engineering). I work in an engineering consultancy and so have a lot of experience of structural / mechanical problems from a lot of time exposed to that kind of thing... with "real" engineers who help when I don't know the answers!

After about 13 yrs XS11 ownership I feel like I'm one of the "greybeards" here too - at least in terms of a bit of experience of the breed.... the beard still has some red left in it!

Frame Bracing

My 'gut feel' is that bracing in your pictures won't really help too much. The strengthening plates are connected to a short stiff existing brace - almost impossible to flex (i.e. the horrizontal tube). The vertical member (front of the engine loop) is already stiffened by the engine bolts a bit lower down and the attached stiffening plate does not extend down enough to contribute significantly if there was a problem on this part of the frame loop.

I think the original design of the plates was to assist the stiffening of the horrizontal member at this location, but it doesn't really need it (IMHO).

Swinging Arm

The square section thicker walled aluminium arms of newer bikes are designed to either be stiffer (reduce any flex) when riding a 10/10ths - or at least as stiff as tubed sections... but lighter.

With the old early Jap frames (famously the Z1 - KZ900 series) a number of reasons have been bandied around to account for the "hinge effect". To be honest, with any older bike a lot of the flexing is likely to be from the connection point due to some bearing / bush wear.

For the XS11 particular, as a shafty, the swinging arm with "shaft tunnel" has a lot of meat to it and I would be surprised if it flexed like a chain drive bike from the same era.

So.....

IMHO I believe that the main problem with the XS is too much mass. The engine is huge and HEAVY (having removed it a couple of times alone I know believe me!!). To combat this the frame is also hefty and braced.

Where the whole thing struggles (from a handling point of view) is the long frame, load on suspension components and (with age) wear in fork bushes, fork springs, swinging arm bearings, shocks and worn tyres - the latter which can make a large difference. (oh and lack of a Tkat fork brace ).

Once all this is sorted you still have a HEAVY bike which will need to be ridden in a certain way.... like not backing off the throttle in a fast corner . A sports bike it is not... even less so 20+ yrs on.

Sorry for arguing against a new project for your welder - what a kill-joy

[Pat Kelly]

My amateur opinion is the "hinge effect" is from the shaft drive.
As the bike accellerates the pinion gear (the back end of the shaft) tries to 'climb' the ring gear (connected to the rear wheel). This causes the back end of the bike to try to raise up. Decellerating or engine braking does the opposite, causes the back of the bike to squat.
This pogo effect causes the steering angle to change (slightly) making the front end not track true (notice the difference in handleing just by sliding the fork tubes 1" up in the clamps?)
This is made worse by the fork lowers (sliders) moving independant of each other (slightly) held in sync by the front fender That's why a forkbrace helps, it ties the sliders together and the front tire dosen't try to change it's angle in relation to the bike.
Brake for a corner, forks comperess. Off the brakes (decompress) then into the corner. Accellerate through or at the end of the turn and the rear rises (shaftie) and the rake/trail decrease like tapping the brake again. Let off the throttle a bit because the bike feels 'hinged' and the steering angle changes again. Hopefully this makes enough sense that you see what I mean.
Just the nature of the beast. It's not a sportbike. It's an Olympic power-lifter trying to do gymnastics.

[TopCatGr58]

Hey Graham and Pat,

Thanks for the lengthy reply and thoughts. I've got fairly new decent rear shocks, and also new front upper tubes and seals, but old springs! I also have really changed the geometry of my front end by putting those 4" over length upper tubes on there! I have the TKAT brace, but with longer tubes I know I have introduced some more room for flex. I remember the rear end rocking horse activity years ago when I first got my XS11, but with these newer shocks, I don't experience that at all. I have new tires. When I had the bike apart 4 years ago for my topend rebuild, with the rear wheel off I checked the swingarm, didn't notice any looseness. Same for the front wheel and bearings. Tires are balanced.

As a few others can attest, I haven't had any trouble running the twisties, it actually steers quite nimbly, almost flickable!! I've had it up to 110mph, but that's when it seemed like it was a little squirrelly, with or without a small handlebar windscreen. I'm not sure if it might be because of the longer tubes, however the Special has the leading axle, but I think I remember reading about the triple tree angles the forks closer to the frame on the Special, hence the leading axle design, whereas the Standard's triple tree angles the forks out further, but with the bottom tube axle design, it's supposed to be the same rake/trail!?

I've been told that if I were to put the Standard's Triple tree on, that it would increase the rake angle, would that increase the stability of the front wheel, sorta like a long forked chopper and how they have the tree angled further out vs. a standard bike!?

I've run some mild sweeping turns along some highway interchanges near home, and over some modest bumps at 80mph the bike started some interesting harmonic type wobbles, I didn't let it degenerate into a tank slapper, but felt like it might have if I hadn't let off of the throttle!

I don't need to go over 100mph, but would be nice to be sure that it would be stable if I had the chance, like at the local drag strip later this summer! I guess I could try lowering the triple tree down onto the tubes a few inches to see if that increases the stability, if so, then I will have found the problem!
T.C.

[John]

TC. Do you have risers on the handlebars? If you don't then the top of the bars will hit the bars after about 1/4 inch. That was the whole purpose of my quest for some barbacks. I lucked into a set the other day. A member, old bartender, had some that just removed them from his bike after he repoaced the bars. He made me an offer I couldn't refuse.

[Pat Kelly]

Longer forks (more rake and trail) makes for more stability at higher speeds.
Sliding the fork tubes up in the triple trees decreases the rake and trail making the bike less stable but easier to manuver in tighter turns.
Look at the forks on the top fuel drag bikes, kicked out and flatter to the ground.
Road race bikes have forks that are more straight up and down.
I have noticed that the MN Special requires for force to corner than my Standard. As for high speed stability I have yet to open-up the MN Special (bad tires). My E gets a wobble around 120 MPH, the LG hasn't been over 80 MPH.
Run a line through the steering head tube of the frame to the ground, this is the steering rake. Run a plumb line from the center of the front axle down to the ground. The distance from this line to where the rake line intersects the ground is "trail"
Both bikes (E & LG) show rake of 29 deg and 5.12" of trail (I wonder why the LG takes more 'oompf'?)

[uk1.1s]

Rake & trail... turning.... wobbles....

For the era of the bike the mild harmonic wobbles in an 80mph curve over bumpy roads that you describe is normal. Don’t think there is too much you can do without engineering a frame with lighter / radically different geometry. For an interesting time on that same road you should have a thrash on my KZ1000! Put it down to character.

If it really worries you try a steering damper to stop any tendency for the head to shake. If that doesn’t work then it’s the tail that’s shaking due to worn bushes / bearings or poor attachment design. They say (whoever “they” are) that the early ‘70’s Kawasaki Z1’s suffered from a weak frame at the swinging arm attachment points which allowed flexing when pushed hard… so however new your bearings you just couldn’t win!.. Later KZ1000’s have bracing around the frame here.

The rear jacking that Pat describes is indeed a shaft response effect. Much more fun on my T3 850 Guzzi where the in-line (with the frame) crank pulls the bike to one side if you rev it up giving interesting assistance / hindrance in corners - (right handers it'll dip down into)! At least the XS1100 (with an across frame crank) doesn’t suffer from that too!

Really fast corners on the XS are challenging sometimes. As with most heavy 1970’s-‘80’s bikes backing off the throttle quickly or worse, a dab of front brake, can make the whole thing sit up and try and take you straight on. I think it has a lot to do with mass, inertia and such rather than anything that can be cured swiftly by tinkering with the existing frame. The forks however are slim and tend towards stiction when loaded up, so any flexing and sudden unsticking of the forks (braking force) gives us something more to adjust for. The Dutch XS1100 site has an answer for this with a number of bikes shown with modern fork (and rear wheel) replacements…. Love to have a go on one of those.

As for forks themselves, I fitted Progressive Springs (had air assisted forks on original springs) a couple of years ago at fork seal change time and found the difference startling.

What is surprising is that with 4” “overs” you haven’t found an enormous difference in the handling. I agree with Pat above, longer forks equal more rake and trail giving you high speed stability at the expense of faster response to turns (flickability). That’s why Harleys (or any other custom) are not the best for high speed canyon bashing. You can haul them about, but really not best suited for the job.

Funnily enough the subject of rake and trail came up on a sidecar forum I am a member of recently and the guru (Hal Kendall) posted some useful stuff. The relevant information for solos I think I’ve extracted below…

**************************

“You can, to some extent, make a partial correction for a low lateral stability by adding a well designed set of fork braces so that many of the problems caused by the small diameter forks can be overcome. It is very important that any bike equipped with a fork brace be carefully adjusted so that the forks are in fact truly parallel else fork stiction will be felt.”

For rake “.... High speed solo - figure 4 to 5 inches”

“Another point - wobbles - all articulated vehicles from shopping carts to solo bikes to bikes with sidecars to trucks to supersonic aircraft when on the ground are subject to wobbles. Some will wobble, some will not.

The greater the trail the more the self centering force and the greater tendency there is to overcome any effect of a wobble.

Conversely, the smaller the trail the lower the self centering force and the less force there is available to overcome a wobble situation.

Finally, if a rig has a tendency to wobble, first check all obvious signs – such as out of round wheels, loose connections, out of balance wheels or tires, loose bearings in the wheels, swing arm, steering head, and so on.

After you have corrected for this, if there are any wobbles left, carefully fit a well designed steering head friction damper or a sensitive adjustable hydraulic damper to dampen at the steering head. Use as little damping as possible, that is, if you must use any at all.”

[TopCatGr58]

Pat and Graham,

Thanks again for all of this info, it's very enlightening! First, Pat, what's the spread of your handlebar grips on your LG? Are you still running the buckhorns, vs. a different style on your "E"? I think the buckhorns are much closer together grip to grip distance, which reduces your effective turning leverage due to a smaller length of "lever". I have Goldwing bars on mine SH, and it's very easy to turn the bars, the triple tree bearings were repacked 4 years ago, and they are plenty snug, but still very agile!

I just put my bike up on the center stand. I checked the rake, however with the rear wheel up and no weight on it, the rake comes out to about only 25 degrees. However, at that angle measuring the trail actually lenghthened to ~7" vs. the 5.12" it's supposed to have STOCK with 29 degrees. This seems oxymoronic, but with the leading axle design of the SH, decreasing the rake actually pulls the steering head tube center line further away from the plumb line down from the axle!?!?

Now, I realize that with me sitting on the seat, compressing the rear end and shocks, would tilt the frame and steering head further putting it closer to the stock 29 degrees, but the way the geometry of the front end seems to work, that actually reduces the trail? I'm going to draw an example and post it to prove this process!
T.C.

[TopCatGr58]

Okay, I got the drawing done. I used MS Paint and drew a few lines, one to simulate the fork tube, the axle placement, and one for the steering head center line. I drew it first with both lines vertical. Then I rotated them to appx. 45 degrees and 25 degrees. I had copied the first diagram to keep dimensions the same! I then drew the plumb lines vertically from the center of the axles to the rake line. Then after relearning from Ken's post, I then drew a horizontal "Green" line from the plumb line forward/left to the point where it intersects with the rake line! THIS is the trail distance!



Confusion clarified, this post has been editted to reflect my new enlightenment!

So to increase the trail I think I would still need to place Standard Triple Trees which places the forks further forward from the center steering shaft. But I will investigate all of the other trouble areas as suggested in Graham's earlier post! And then IF I need to add a stabilizing arm, I can then get out my trusty welder to affix mounting points to the triple tree and frame!
T.C.

[Ken Talbot]

Hey, TC, here's a fairly detailed article on steering geometry Dunno if this makes it any clearer or not....

[pggg]

Just saw my old pics. When I got that XS11, I took one look and thought nah, that braced steering head would be just a waste of time and wouldn't wouldn't make any noticable handling difference at all. A ride over a winding and bumpy saddle road changed my mind because my other unbraced XS11 felt like a wobbler in comparison. Also, narrow and low handlebars make the handling on an XS11 better on twisty roads.

[uk1.1s]

Doh

Is it time to eat humble pie yet for my long 1st post above then!!

Agree on the bars issue, wouldn't trade my low adjustable bars on the 1.1 Sport for anything

Graham

[Dennyz]

my opinion on the hinge

I found the rear swing arm to be very flexible. I have bent two. With my XS strapped to a hoist and lifted 5' in the air, I can deflect the top of my rear tire 1/4" by grabbing it and twisting it. There is no strength in the swing arm to prevent this and riding loads are much greater than my arm strength.

So, imagine some force acting on the rear tire (like cornering) that cause the swing arm to twist. In a corner the swing arm is not horizontal, but leaning at some angle. The force on the bottom of the rear tire would have significant leverage over the swing arm, causing it to twist. The stored energy of the twist would be waiting for the opportunity to rebound, setting off an oscillation from one side of the swing arm to the other. If the XSs shocks are worn out, the springs could also contribute. This oscillation would be changing the effective lean angle of the rear tire, inducing a kind of bounce steer.

I believe this is one of the major advantages to a mono-shock design. Back in my early dirt bike days I rode hard and fast in the desert outside L.A. We always had problems with high speed wobble. One of our favorite maneuvers was to get going 40-50 mph on the sand and lay the bike over into a wide fast power slide. There where some speeds that we could not hold it together and eat it big time. When Yamaha came out with the first mono-shock, one thing we noticed is that it had much better high spend stability in the desert. We could do our power slides at any speed. Of course, some of that was due to better geometry and stiffer front fork. I have always felt that the XSs hinge wobble was related to a two shock swing arm design.

I started a bolt on stiffener sometime back, but never completed it. It turned out to be a bit more complicated that it looks at a first glance. So many projects and so little time.

[pggg]

It ain't rocket science and we ain't the undisputable world leading authorities on the advanced physics of geometrical processes - but the combination of stiff fork brace, progressive forks, Koni shocks and beefed up steering head gave my bike a 'tauter' feel.

Of course she's still a bad-tempered, grumpy, heavy old oil-tanker though! Bull wrestling must be a similar experience to hustling an XS11 round bends. Can a rocket scientist describe the word 'character' ?

[TopCatGr58]

Ken, thanks for the link, I had misread from Pat's post on how and where to measure the trail...DOH , I have since editted that post, the photo, to reflect where Trail is actually measured with our leading axle design so as not to confuse others!

Pggg, thanks for letting me show the pix, and I'm thinking I'll still try that bracing, plus perhaps more on the frame where the shocks attach??

DennyZ, thanks for your input, confirms what I suspected about the swingarm, and so I thought with some 1/2" angle iron I was going to run a few strips of it along the bottom center of the swingarm leg, across the center, and down the other leg to see how it might provide some stiffness to the whole assembly. Do you know about how thick the steel is that comprises the swingarm? I would like to be able to weld to it without melting/cutting thru it and possibly destroying it, I don't have a spare!! I also don't have a "TIG" machine, just a simple MIG, with 4 power settings, I've gotten decent with it, would like to get good penetration, but don't want to warp/mess up the OEM swingarm! Will let you all know how it behaves once I get it all done!
T.C.