need some input RE stability

gggGary

gggGary

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Did a couple interesting experiments yesterday; had the restomod out for 60 back road miles, was messing with front stability. I shook the bars, watched for wobble/weave damping (or lack there of). Found a marked difference in damping the oscillation that was dependent on my seating position. used the two extremes nuts on the tank, and butt back near the passenger position on a stock seat. One position resulted in a weave strong enough that I stopped and did a loose or damaged parts inspection including forced deflection of the bars while while holding the front wheel between my legs, looking for a problem, nothing obvious found.
Would like a few others to experiment and report their findings.
Gary's hint/warning if you ever get a front shake/weave going; pushing firmly forward on both grips will damp out nearly any front end shake, pulling BACK will probably make it worse and may result in loss of control.
Full disclosure the resto mod is set up pretty much like this, now sporting a slightly smaller windshield

restomod 9-2017.jpg

3x18 spoke rims, stock hubs forks front fender 110/80/18 radial front and 140/70 radial rear. fresh bearings bushings etc. Will attempt and report the same thing on the 79 survivor and on period piece.
 
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On my GL1200, 90 MPH feels the same as 55 MPH, above that things get different. I'm not going to experiment with higher speeds, no reason to.

Slow speed wobble ......I can start that wobble easily by letting off the bars at the right decelerating speed. On this bike my seating position makes little to no difference. When riding two up, I don't think the wobble can be started at all.

I'll see how the smaller bikes do this week. High speed weave, I understand the remedy, but I'm not going to make that happen, I ride on the road, not on a track.

Scott
 
Gary,
Is the pushing on the grips vs. pulling on them affecting the wobble due to the forces you are applying to the bars, or because when pulling, you are moving your body weight forward, and pushing you are moving it rearward?
 
It's very complex subject well documented in aviation literature, but it's mainly a nerve, muscle timing interaction. Your body is a more effective shock absorber when pushing. I don't believe I weight shift when doing this. It's saved my life or, some danged serious road rash at least a few times.
 
bwthor said:
Gary,
Is the pushing on the grips vs. pulling on them affecting the wobble due to the forces you are applying to the bars, or because when pulling, you are moving your body weight forward, and pushing you are moving it rearward?
Click to expand...

I think it is the shifting of the weight and the damping of having two hands on the bars. I would say that the opposite is happening, or pulling and pushing on the bars is not a good way to describe this. Pulling on the bars is shifting the weight rearward and pushing on the bars is shifting the weight forward.

Scott
 
I have a theory or two on the reason for the difference with seating position, but sure would like to hear some other's real world experience trying this. PS I am not talking high speeds, probably in the 30 to 50 MPH range.
 
I never related seating position, bar width, bar height, foot position and these variable changes to the slow speed wobble until Skull brought them up on another thread. It does get complex, and some set ups are better than others.

Scott
 
I'm a bit puzzled about all this. I experienced high speed oscillation years ago on a 1956 Harley KH, on a 1968 Triumph Bonneville with a dinged rim (the friend responsible for the ding didn't bother to tell me about it, and I discovered the grief at 80 mph), and on a BMW R50, again with a dinged rim. But in 52 years of riding the only times I've experienced low speed oscillation have been on bridges with a mesh surface, and then it was just a matter of letting the front end track as it needed to do instead of trying to fight it. I can't induce a low speed wobble on any of my current rides. Gary, did fitting that 140/90/18 involved modification or change of the swingarm? Are the clamps lowered on the forks at all?

For what it's worth, the thing that's worked best for me to get out of an oscillation has been to relieve weight on the front wheel and let it straighten itself out; weight back, short, sharp burst of throttle, slow gently, repeat as needed. It's unfortunate that we don't always have enough road to apply that cure!
 
I can say with the greatest confidence that I can start the "slow speed wobble" on any bike. There does not have to be anything wrong with the bike, new tires or worn tires. I think I've already said all that I've learned about this subject from experience, anything different, I think I'd have to disagree with.:shrug:

Scott
 
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How to start a slow speed wobble, oscillation......Get the bike going at 40 MPH or just a little faster, let off the bars and let the bike decelerate without using the brakes. Didn't wobble? Do the same thing but downshift before decelerating for a quicker deceleration. Don't feel comfortable with your hands off the bars, then don't do it at all.

Warning, keep your hands close to the bars, this can happen fast and develop into a tank slapper very quickly. I just wobbled all the way in to town.

Scott
 
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Granted, Scott, shutting the throttle and taking hands off bars under engine braking will induce an oscillation. But if an oscillation occurred spontaneously under normal riding conditions (hands on bars, butt on seat, feet on pegs, and so on, and excluding from the discussion weaving caused by obvious road surface irregularities like grooved pavement, tar snakes, head shake caused by hard dips in deep corners, etc.), I'd be looking for suspension defects.
 
I have struggled with this since I've had my bike 4 years now but have reduced it now to the point that
the only wobble I get is if doing almost 100mph or if driving behind a car closely on the Hwy and I seem to get
a wind turbulence that will shake the front end back and forth until I pull back say 100 ft then its gone.
I have no windshield as that will increase it.Try pulling the windshield. Also I noticed that if my chain is not perfectly
aligned it will cause front end wobble, look down at the chain and see if its spinning smooth or shaking like a banshee.
last but not least I installed some Avon Roadrider tires and that really sealed the deal, almost perfect now.
I don't have a fork brace yet and that may just be the ticket to a nice tight tracking and wobble free xs650.
Soft or short rear shocks will add to the problems also.Just my 2 cents,CDN funds!!
 
Good stuff guys! now get out there and try both; nuts on the tank, and butt back, riding positions, report any differences.
I took period piece an 81 special with 19 x 2.15 and 18 x 2.5" spoke wheels quality Michelin bias ply tires. "better" rear shocks. It had less tendency to "dance". Then the 79 standard survivor; everything stock, on Shinko 712 tires. It matched period piece for stability.
Note all three bikes exhibited positive stability, shake the bars and they all damped out the oscillation in fairly short order. They also exhibited a difference in damping based on seating position. Resto mod is definitively the "least" stable of the three and that's to be expected, it has a low profile 18" front tire, and longer rear shocks. would have to get a protractor and measure fork angles but it's steeper than the other two. When the Michelin 110/80 18 on the rear wore out quickly I replaced it with a Shinko 140/70/18 podium radial, this tire is a bit much for a 3" rim, not stupid over wide but a bit "pinched" on that rim. It was the best "rear radial match I could find.
Both resto and PP have light weight front brake rotors.
Note "positive stability" comes from aviation and is an important part of testing, negative stability exists but is not acceptable in any but full on performance machines. The tester purposely deflects each axis control and notes how many oscillations occur before the craft damps them out.
 
I absolutely hate wobble/weave.

Doesn't happen with these.
...Steering damper ............... Raked front end ....
DamperKnobs25.jpg PanHead-51-c2.jpg

Gary, it's obvious that you know about dynamic stability and instability. Have you looked at what all is involved in tailfin flutter testing?

There's so many mass reaction zones and flexy things on a bike.
Sounds like a job for a few dozen strain gauges and data loggers.
Followed up with several gallons of coffee...
 
Small oscillation around 20-30 mph is common on bikes in general. Mine has it, but beyond that it's steady out to as fast as it will go. It seems to be worse with a lot of weight on back, like a pack or a passenger. Similar to changing seating position maybe. That is the same speed where how you steer changes, and maybe there's a connection. I only had one thing approaching a tank slapper, during an emergency stop. Really surprised me. Tried to replicate it in a parking lot and wasn't able to.
 
I had a new Triumph Bonneville ( 2007) that from the day I took it home had a bothersome wobble, especially if I took one hand off the bars, it was bad enough I was fearing a tank slapper. After two trips to the dealer and a bunch of inconclusive tests they announced there was nothing wrong. The wobble persisted. I raised the front end and placed a fixed pointer to the edge of the rim to see if the wheel was true, it was. Then I placed the fixed pointer to the sidewall of the tire to see if it was true, it wasn't. It had about an 1/8" deflection at one point. I replaced the tire and the bike was rock steady! I could take both hands off the bars at 50mph and coast down to 20mph with out any wiggle.
On my XS650 a while after I replaced my steering head bearings the front end was starting to feel a little wobbly until I snugged up the bearings a little. Now all is good. I have tried the take both hands off and coast and it's nice and steady.
 
NOT A SINGLE REPLY yet from anyone who went out and tried both, butt forward and back? Don't any of you guys ride? :geek:
Note period piece also has a windshield. About 3" taller, it was on resto-mod 'til about a week ago I swapped them for wify's comfort. Think we can rule that out. I am NOT complaining about the stability just noting a difference based on rider position. As usual 2damned many, is on the hunt.
 

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