a question of weave

[gremlin]

out in the beautiful weather yesterday, following Mrs G i noticed a couple of times she got the handlebar shakes (not quite a full on tank slapper ). it only happen around the 70mph mark and luckily it didnt bother her and she managed to control it.
i know the bike geometry has changed, forks through yokes 2" and rear end lowered by the same amount. this has only happenened since we had new tyres fitted last week by a dealer.

the back wheel was out of line slightly (with the etched marks) but this is now sorted.
but what i have noticed is there is a knocking from the front end when you rock the bike back and forth with the brake on. its not coming from the headstock bearings, its more when the fork legs compress, they go down smoothly then there is this distinct knock. i reckon this could be what starts the tank slapper in motion. as it does feel like somethings rather loose, also it passed the mot 2 weeks ago and the guy that does it is rather thorough.
any ideas greatly appreciated.
Mr G

[Pedro]

Jules 600 also makes that knocking noise but it doesn't weave - I think it's maybe a trait of the forks.

Silly question - have the tyres been fitted correctly (direction of rotation)? Are they properly balanced? Also I wouldn't use the etched marks on the swing arm for alignment - try measuring from the swing arm pivot centre to the centre of the axle both sides.

What tyres have you got fitted?

With the front off the ground, are the head stock bearing sticky or do they run smooth?

The geometry should be no different if lowered the same amount each end - monsters tend to shake their heads when the back is up and the front is down when ridden hard over bumpy surfaces.

[Scotty]

the knocking isn't the disk's on the rotors is it ~:?

[gremlin]

Had BT014s put on and had fresh balancing weights and the tyre are on the right way so am guessing she was done properly. Before tyre change she was doing 80 quite happily without any probs. She is quite skittish now but I put it down to new tyres. I remember my 125 feeling like I was riding on marbles for ages until tyres wore in so wasn't unduly concerned, it was just the wobble that was a bit disconcerting, but it was windy so I don't know if a cross wind caught the bars and started it. Although I have ridden in windy conditions before and not had the problem.
She has been lowered evenly so can't see that the geometry has changed too much.
I'll just have to go out again and get up to the same speed and see if she does it. At least I know I can control it if she does. It may have just been the wheel not being straight which Clive has now adjusted.
The brake disc is okay so are the calipers.
He changed my oil and the clutch isn't slipping now so at least one thing is sorted!
The fuse went that does neutral light and indicators etc on the way home yesterday so with the wobble as well all Clive could say was F***ing Ducatis!
I was sitting in the drive going up and down the box trying to get neutral and then he realised the fuse had gone Doh!
Still love her though, Clive said he could feel me grinning all the way.:biggrin:
Considering she'd been standing for so long it's to be expected I'll have a few glitches.

[JMo]

Scotty might be right with the 'knocking' noise - mine does the same if you hold the front brake and really compress the forks - I always thought it was the play in the floating discs? Even if it was the forks bottoming out, that shouldn't happen on the move (unless Ped is riding ahaha...)

As a thought re the geometry - if you've lowered (or raised) the bike significantly, while the aspect of the bike might remain the same, I imagine the trail ratio of the fork might be slightly different? - not as much to cause a weave I shouldn't think, but you never know?

Mind you, lowering the bike should have the effect of raking the forks out, so if anything it ought to be more stable?

Is it something as simple as tyre pressures?

xxx

[gary tompkins]

Gadget (Gary N) had problems with a new set of bridgestones on his 600, not sure exactly which type but know he wasn't a happy bunny. Straight line stability went out the window, and it got so hairy he eventually swapped them after just a few 100 miles for something else... possibly diablo's?

I'm sure he'll be along to confirm details soon

May have been a dodgy tyre or bad fitting ... not sure what final verdict was?

BTW knocking sound could also come from pads floating in calipers. Mine does it bouncing front forks on the stoppers, & I'm sure a combination of discs/pad float makes the noise I get? ;)

[gremlin]

Cornering is great she's really planted and turns in beautifully and up to 60 she's fine but over that she had the hissy fit. But now wheel is straighter I'll try her again and see what happens. At least I won't be breaking any speed limits! As I said the new tyres on the 125 felt really unstable and vague until they wore in a bit so I guess putting on some miles may help. It will be a bit of a sod having to change them if it is a case of incompatible tyres, they've only done about 80 miles so far. I had her up to 80 before the last lowering session and she was fine and also up to 70 after and she was okay on the old tyres.
If it is the BTs what is a better alternative? I've got a feeling she had a Dragon and a Corsa before but with my divvy brain I could have got it wrong! I know they weren't matched.

[Nickj]

A thought when lowering a bike is to shove some a heavy duty pair of scales under the front and back. when its dropped recheck and tweak so they are the same.
It doesn't take much to shift the weight onto the front or rear which can upset the tyres a bit till you hit the right pressure.
Try up 10psi and then reduce till its happy.

[JMo]

If it is the BTs what is a better alternative?

Pirelli Diablos seem to get a pretty universal thumbs up for Monsters... it's what the Desmo Due guys use (ok, so they have to) and they certainly spend a lot of time over 80mph...

xxx

[gremlin]

Cheers, I'll see if there's any improvement with the having the wheel a bit straighter, if not it'll have to be a tyre change. I'm a bit miffed because the tyre guy insisted BTs were the best for my bike. I guess he had rather too many in stock! It seems an awful waste of money but perhaps I'll be able to sell them on Evil bay as barely worn, I'm sure someone will want them, they've still got some of the knobbly bits down the centre.

[JMo]

Do experiment with tyre pressures first though (as someone else has suggested) - as when these tyre threads pop up from time to time you get all sorts of recommendations for tyres & pressures, and what might work for some might not for others?

For example, I run 33psi front, 35psi rear in my Pirelli Diablos - others will say that might be too low or too high, but it works on my bike...

The Bridgestones are not bad tyres, it may be that they just have to be run at different pressures to what you were running in your previous hoops?

xxx

[Nickj]

My Contis felt a bit twitchy compared to the last Dunlops, Conti tech support suggested I start at 10psi over standard and reduce. Of course the fact it was winter and slippery to start with wasn't overly helpful.
I ended up a few psi over standard and I'm happier, was fine in the snow but still kicks a bit much over white lines.

You might try shifting the weave, usually a weave is just an oscillation that doesn't get damped out by the chassis and tyres. Bikes are inherently unstable so a little movement can amplify, its a bit of a resonace thing. So check for play anywhere, a good fix is to tighten down the head bearings by a tiny ammount, not enough to feel in the bars, the extra friction in the head stock should give enough extra damping to push the weave up to a higher speed.

[gremlin]

I was expecting to have to get used to new tyres and these are very skittish but they've only done a few miles. It's just a case of going out and trying to get the same conditions and see if she does it again or whether it was a one off. It may just be the fact that my wheel was slightly off, there was a cross wind and the road surface perhaps wasn't brilliant but then what road surface is these days. At least I know I can get her out of it if she does it again and we can then start altering pressures etc and see if that works. At least my licence will be safe while I can't go above 65 so there is an up side!

[jerry]

Bridgestones are horrible on Monster , i always use METZLERS Mez3 or 4