Advice sought. Instability at moderate speeds

**Ixion** · 28th August 2005, 20:45

Not a sportsbike question, cos a BMW K75 can't classify as sportsbike. But there isn't a "non sports bike tuning and tweaking " forum. And I imagine that sportsbikers are the most likely to be able to assist.

The Whale seems to suffer from mild instabilty at 150kph.

Funny thing is , it's as solid as a rock at 140kph but at 150+, on any sort of moderately bad surface (bumps, wallows etc), I get a sort of rolling weave that I can't correct. At 140 I can hit any bump and it just shudders and settles down again. But take it to 150 and bumps, grooves etc set up his gentle weaving. It's not actually frightening, just thatI cant correct it. I've tried riding through it, but it doesn't clear at 170. Braking and dropping below 140 stops it, but that's not exactly a wonderful solution. Doesn't happen at all on good smooth surfaces. Havent been game to see what happens in corners, plus my experimental bit of road doesn't have any suitable 150kph bends.

Can anyody give me any thoughts on the best place to start looking? Not expecting answers, but a hint would be helpful. The front tyre is a Metzler, with a non longitudinal pattern. I'm a bit suspicious of this, the ribbed front was always reckoned good for high speed stability.

I don't understand why it is fine at 140 and not at 150. It's not just a matter of degree, it sets in quite suddenly, I think if I did enough runs, I could pin it down it within a couple of kph.

I've checked out some of the BMW boards, and it doesn't seem to be a "they all do that " issue. And nothing about any general instability issues with the K series at that speed.

I've checked the obvious, steering head bearings, wheel bearings, tried different weight oil in the front forks, different setting for the rear shock. Nothing makes it better or worse.

(Of course, all this took place on a closed road under controlled conditions. Well, I told them to close the road, anyway, and I assume that Transit did as I asked, didn't see any other traffic anyway.)

**John** · 28th August 2005, 20:51

//edit didnt read.

Sounds more like a tire thing to me, or maybe just the point where the frame is suffering from engine vibration, does it do it at all rpm's? tried a higher gear ?

do you have an aftermarket screen? or any other addons on it?

**riffer** · 28th August 2005, 20:54

Of course the obvious question is "why are you worried about something that happens at lose your licence speeds?"

Just curiosity, I imagine.

Have you checked for play in the swingarm?

Or a wheel that is a bit out of round?

Possibly wear in the back rear axle...

but my guess would be that the front tyre and the rear tyre aren't quite suited to each other.

Are they both the same type of tyre? Or at least the same make?

More info please.

edit: Have a look at this.

**onearmedbandit** · 28th August 2005, 20:58

Years ago I had an earlier GSXR750 and from 170km/h it would develop a serious weave. I knew it had this issue when I bought it so I took it to a tyre shop in town. Turned out the back tyre while visibly ok had suffered some damage, in turn making the bike behave in a similar manner to what you have described. New back tyre and a transformed bike. Like you said, might not be the answer but could be a hint.

I should also mention that I discovered that moving my weight over the back countered the weave, allowing the bike to 'open up a bit more'. That is until I hit a mild bump in the ashphalt at speed, turning in to the wildest tankslapper I've ever experienced. I remember being over the front of the bike looking at the front wheel shaking like a wet dog thinking 'this is it'. Very sobering experience...

**Motu** · 28th August 2005, 21:02

I had a similar issue with a featherbed,it had a lot of handling problems,not what you want or expect from an early Norton.I always blamed the stupid front Michelin,but the rear Michelin wore out first,and when I fitted a Roadrunner all problems disapeared.I've since found out that the rear tyre is more often the culprite with weave.

[edit] I see I was beaten there,so maybe the rear huh?

**curious george** · 28th August 2005, 21:02

My 2c = tyres.
Something dodgy and nonvisable.

**FROSTY:Me** · 28th August 2005, 21:04

dude it will be your tyres --Ill put money on it.

**WINJA** · 28th August 2005, 21:17

Originally Posted by FROSTY

dude it will be your tyres --Ill put money on it.

THIS COULD BE RIGHT , CONFLICTING FLAT SPOTS ON WORN TYRES CAN CAUSE
A WEAVE , I KNOW YOU CHECKED THE HEAD BEARINGS THEY MAY FEEL RIGHT BUT THIS PROBLEM IS VERY COMMON FOR TIGHT HEAD BEARINGS, FUNNY ENOUGH I RAISED THE REAR TO CURE THIS PROBLEM ONCE , IT WORKED CAUSE IT CHANGED THE WEIGHT BIAS TO THE FRONT , SO MAKE SURE YOUR REAR SPRING AINT FUCKED .
THE QUICK METHOD TO CHECK HEAD BEARINGS IS FORGET THE BOOK , BALANCE THE BIKE ON THE SIDE STAND AND REAR WHEEL , TURN THE STEERING HARD RIGHT LET IT FLOP TO THE LOCK STOP , IT SHOULD KISS THE LOCK STOP AND NOT BOUNCE ,DISCONECT STEERING DAMPER FIRST, IVE SEEN NEW BIKES WITH INCORECTLY ADJUSTED HEADBEARINGS , YOU THINK ITS BAD AT 150 TRY IT AT DOUBLE THAT SPEED

**Jackrat** · 28th August 2005, 21:36

I had a similar problem with a GS850G.
It turned out to be the rear shock set to hard an the front to soft.
That combined with the shaft effect made the front do strange things.
It only happened solo,never with a pillion or heavy pack on the back.
If I was you I'd consider having the suspension checked out.
Good luck.

**Reckless** · 29th August 2005, 08:41

Have to concur with the others!

My Z1 had a similar problem with a new set of tyres at 140k. Went away when I had a pilion on and didn't appear when the bike was leaned over off the top of the tyre. We discovered when I git sick of it and went to fit some new Pereli Demons (great tyre for the older bikes) the the front axle had been fitted incorrectly. So not quite sure if it was the tyres or the axle fix but if you have checked all the mecanical stuff as you said. Try the tyres.
A suggestion would be to go to a tyre place get them to chuck a set on and go for a ride to test. Even if they put a second hand set on just for the test then you could buy new with confidence it was the fix required.

**Ixion** · 29th August 2005, 10:00

Thanks guys for all the input. General consensus seems to be tyres, which agrees with my own suspicions. The rear is a Continental that Kerry recomemnded as good for BMWs (and I think i am quite happy with) , but the front is a Metzler which I have never much liked. It's almost due for replacement anyway so I think I may bring it forward and see if Kerry can source a matching Continental for the front. The Continental TKV11 has longitudinal grooving , like the old Avon Speedmaster, which was always my answer to straight line instability.

We'll see what that does.

Heaven knows why it would set in so suddenly at 150 clicks, but tyres are weird things at the best of times.

As an aside, it certainly is true that (most, anyway) bikes benefit from a bit of a hammering now and again. After yesterday's couple of hours running back and forth around the ton, and a third gear thrash through the Hunua Gorge on the way home, it's running really well, much smoother than it was before. Y'gotta give 'em some stick now and again.

**cowpoos** · 29th August 2005, 10:29

another thing no-one has mentioned...wheel alingment!!!

do your wheels line up??

if they are out of alingment at high speeds your tyres will develop harmonic vibrations...similar to what happens right before a tankslapper!!!

**Ixion** · 29th August 2005, 10:46

Originally Posted by cowpoos

another thing no-one has mentioned...wheel alingment!!!

do your wheels line up??

if they are out of alingment at high speeds your tyres will develop harmonic vibrations...similar to what happens right before a tankslapper!!!

'Tis a shaft drive BMW. If they don't line up I've got a BIG problem

**TwoSeven** · 29th August 2005, 14:15

Originally Posted by Ixion

Funny thing is , it's as solid as a rock at 140kph but at 150+, on any sort of moderately bad surface (bumps, wallows etc), I get a sort of rolling weave that I can't correct. At 140 I can hit any bump and it just shudders and settles down again. But take it to 150 and bumps, grooves etc set up his gentle weaving.

You wont be able to correct it - and I can almost guarantee you will be getting it at a speed less than 150kph but it wont be as bad. You may not be able to detect it at lower speeds.

I would say that either uou are either running soft tires, or your suspension settings are incorrect. Most likely your front is too soft for that speed.

What happens is the tire is deflecting on the bump which would normally turn into a slight headshake and be gone just as fast. But when the suspension is soft, it travels down the bike to the rear and comes back up the frame again, it amplifies and you get the oscilation weave. Some people mistakenly mess up the rear suspension which worsens the problem.

Is likely your front compression is too soft and you have too much rebound on the rear.

Just my thoughts.