Kawasaki GPZ500R suspension issue

[Camelot]

Hello,

I have just finished my '85 GPZ500R restoration project.
The bike is now very nice, but not good . At lest not good enough.

On straight cruising, somewhere around 90 kmh it becomes instabile. Not realy head-shake, just all sudden it gets very-very sensitive. It is hard to keep it in straight line. The better the road, the worse the problem.

When leaned the problem is not that bad, but I think it can get better in the corners also. When braking hard, the bike stayes stabile.

What we have done:

- new wheel bearings
- new steering bearings
- new tyres
- rebuilt front shocks
- reconditioned, regreased rear suspension
- rebuilt AVDS system

What we checked so far:

- no play in the front or rear suspension
- the steering is neither tight nor loose
- the frame looks straight, no obvious damage or welding
- the front fork oil levels are right to specs
- the fork oil is new, and correct type (7.5W)
- wheels are straight and balanced, checked for runout (0.1mms)
- suspension geometry is good
- tyre pressure is ok (the lower I set, the better the problem gets, but cant eliminate it)
- pressure in the forks is in specs

And we are now out of steam... please give us a hand...

[jaffa90]

Which side of the road do you ride?
Are left hand bends easier than the right or the other way round?
Are the tyres hot or rasping after a run?
How old are tyres?

[stinkwheel]

Presume that's the same as a GPZ500s?

You got the belly pan on? They weave at high speeds without it and the belly pans are commonly either broken going over a kerb or melted on an aftermarket 2-1 exhaust.

They can get sketchy on a worn front tyre (not worn out, just worn down) with a tendancy to track along linear obstacles in the road.

What tyres do you have fitted? I'd personally only use BT45 or roadriders. I once fitted contis to my GPZ and took them off again after 500 miles, it was all over the shop.

You regreased the rear suspension and say you checked it for play but just have a look at the unitrak bushings for vertical play. With it on the mainstand and the rear wheel in the air, get a pry bar under the back wheel and see how far you can lift it before you come up against the shock spring, you might get a shock. They are plastic, mine had to be replaced every year to pass the MOT on both my GPZ500s until I got a machinist to make me new sleeves and bronze bushes for it. A tiny amount of play here translates to a significant amount of play on the swingarm.

Have you checked the sprocket carrier bearing? It can be totally fucked (like smashed into little bits like gravel) with little obvious external sign.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Camelot]

[Camelot]

[stinkwheel]

[Camelot]

[sickpup]

If your bike is the same spec as the 600r it uses 16" wheels which were a bit of an odd tyre size which is no longer used on sports bikes as they are prone to being sensitive anyway.

The one thing you haven't mentioned is the rear shock or ride heights.
A rear shock that is shot, has low high preload or the forks raised through the yokes will cause the bike to become more sensitive but then most bikes have a sensitive spot in a 10 mile range anywhere between about 40-60mph and this could just be that.

[-]

I'd have normally gone for worn steering bearings or worn bushing of bearings in the swing arm, although the OP says these are OK.

Maybe time to get it off the ground and check again for movement both ways?
____________________
TZR250 2MA road, TZR250 1KT road, TZR250 2MA race, TDR250, YZF-750R Boost colours.
Jaguar S Type 3.0 V6 Sport R, VW Transporter T5 GP LWB Shuttle 140ps DSG.

[Camelot]

[Camelot]

[Easy-X]

Total n00b observation... if it were off the deck so both wheels spin freely could you use something like a drill (with fuck knows what attachment) to spin up one wheel and see if a wobble appears?

Alternatively could the "rolling road" of a dyno help?
____________________
Royal Enfield Continental GT 535, Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[redeem ouzzer]

[stinkwheel]

It was very common back in the day to disconnect and blank off the anti dive system on the forks. May have been because people simply didn't like it but I wonder if there is more to it than that?

I dimly recall it not being as straightforward as simply removing it and covering the hole, something needed to be pushed in and held there by the blanking plate.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Camelot]

I dont think that the AVDS has anything to do with the straight line ability of the bike. By the way, I like the system. I had a GPX with an electric unit. It was also working well with my driving style. This full mechanical unit can even be better.