Cause of sagging/twitchy rear?

[don_matese]

Can anyone hazard a guess as to why my bike feels a bit twitchy at the back? or if one of these is the problem after my rebuild. I don't know if its just me noticing it now or if it always has been like that, also it does feel like it sags a bit more than it used to when I sit on it. I haven't gained much weight in the three years I've had the bike (4kg) and not changed any of my gear so that hasnt added any other weight.

1. I seem to have sorted the rear brake a fair bit so it feels like using it wants to make the back end step out.
2. I don't know if this is possible but it does feel like the rear suspension is a bit softer than it used to be, cracking open the throttle feels a bit like its tipping the bike back on its rear suspension
3. how much exposed thread should there be on a stock 1995 m600 on the rose joints? I think mine is a bit too low.

Would adjusting the preload do anything to the softness/twitchyness? Should I adjust the rebound a bit? ideas welcome.

[Dukedesmo]

When you say rebuild, what exactly did you rebuild? if the swingarm has been off, did you refit it with the correct shims between engine casing and inside of swingarm - because although this wouldn't affect the height it might twitch if there's too much gap...

[don_matese]

Dude...erm the full thing was rebuilt following a slightly almost catastrophic leak from the cylinder bases. I took the opportunity to strip and repaint the engine, frame, did the front suspension, cleaned and repainted the hoop etc. so the bike looks almost new.

I'm pretty sure I put all the shims back in the right place as there were no spares - by twitchy I mean it follows the imperfections in the road - I've gotten better with it the past week so maybe its more that its the way it should be now.

The saggy back end is more of a concern - i'll have a look at the ride height this weekend in the light to see what I can do about it.

[Flip]

Raising the rear will definitely help with turn and it holding a better line through corners at the expense of a little stability.

First check the obvious, tyre pressures (assuming the tyres haven't squared off too much) and wheel alignment. Then I would definitely go through everything that has been taken apart and make sure there is no movement in anything that there shouldn't be and double check that you haven't changed anything from before the rebuild i.e. forks in a different position through the Yoke? Different weight/oil quantity in them? Have you moved the position of the hoop rose joints?

Then think about how and where you are noticing the feeling you describe- a particular piece of road you ride regularly or only on right (or left) hand bends.

One thing that is almost certainly past it's best would be the rear shock though- they were never good when new so after twenty years I'd say it's pooped.

[jerry]

Weird thing is that my M750 has brand new M factory shock set up for 175lbs rider
,new hoops bearings and rod ends , new swing arm bearings and all the correct shims and raised ride height 20mm and it still feels a bit saggy at times ??????

my S4 does not feel that way

[don_matese]

I think I shall start saving for a new rear shock.

[Mr Gazza]

What they all said, and also...have you looked into getting your shock re-gassed?

Look for a shraeder looking type valve at the top of the shock.
If it is that type of shock you might be able to get it re-gassed for not too much dough (about twenty quid I think).

Most off roady moto cross type places can do this...There must be one near you.

[utopia]

Did you adjust the spring preload rings during the rebuild ?
Spring free length (unfitted) should be 185mm, with recommended preload being 20mm.
Therefore the fitted length of the spring should be 165mm (at full shock extension).
Figures are from my Ducati workshop manual for 2001 400/600/750 models.

[Dukedesmo]

Sounds like the shock's done to me, as said above regassing may help but ultimately a new one would be a better option as the original shock was a budget jobbie when new.

Have you altered the suspension geometry in any other way, e.g. raised the front etc.?

[don_matese]

No suspension changes from before the rebuild in terms of height - although the rear may be a bit lower as I cant remember how high up the rose joints were.

I whacked the rebound right up this morning and it seemed to feel better (that might be psychological though), maybe the whole thing is just a bit tired.

I'll raise the rose joints this weekend to 1CM thread (including the lock-nut).

[Dukedesmo]

Many fit ride height increasers, such as these; http://www.ebay.co.uk/itm/LOUIGI-MOTO-DUCATI-RACING-REAR-HOOP-SUSPENSION-JACK-UP-BOLT-851-888-MONSTER-/161755463568?hash=item25a9607f90:g:dpIAAMXQgwNSKcV 4
I built mine with them as I thought it would be too low otherwise, although to be fair I've never ridden it without.

You can't get much from the joints, although 25mm is a fair increase and may not be to everyone's taste?...

[jerry]

my shock is set at 185nm i have upped the damping from 18 to 24 clicks out of 48 I think I set mine too low

[Mr Gazza]

I'm not really sure what raising the ride height is going to do for your problem.

It will quicken the steering a little which could have the effect of making it feel more twitchy.

My own experience of raising the ride height was that it made more sag....So the opposite of what you want.

I did find that increasing the preload a little put that right but I had to also put in a couple of clicks of rebound damping.

I was on the verge of suggesting a couple more clicks of rebound, but you have already done it and reported an improvement....I reckon that may well have done the trick.

I would further suggest that if you have compression damping adjustment on the front forks, to try a couple more clicks on that....But not before really giving the new rear settings a good try out.

Re sag and ride height...This really baffled me so I made a model of the rear suspension links to try and see why I got more sag from the raise....Still none the wiser I'm afraid...!!:dizzy:

[Dirty]

Last time I had a twitchy rear it was down to a dodgy biryani!

[don_matese]

I'll check the preload this weekend, its too dark to do after work outside.

I think I will just do a whole load of checks this weekend based on the geometry to try and sort it all out.