Gpx 600 handling woes

[hotrodder]

Can you expand on tankslapper? Obviously it's shaking it head badly but how does it begin? If it gets to fresh underwear time quickly then taking a brave pill followed by Pete's suggestion of getting someone to film it may help

Some links on bike instability, capsize can obviously be ignored as that's just a fancy way of saying you'll fall off if you're not paying attention but might help narrow down how the tankslapper begins and/or provide some new ideas...
http://www.dinamoto.it/dinamoto/8_on-line_papers/vibrations/vibmode.html which has some animations. Murray Walker on wobble and weave (gets going at about 4 mins in) although it's probably of limited use it's worth it for the haircuts alone... http://www.youtube.com/watch?v=fvsDIq3WwVA Again, of limited use and not particually nice viewing but the slow motion bit from 9 secs on shows how things develop into the nasty (real) tankslapper... http://www.youtube.com/watch?v=LZ1srcQMa_0

Googling 'motorcycle castor oscillation' and clicking on books will also get a bunch of hits but i suspect they'll probably just list the usual causes which has already been done.

 

[Lawnmowerboy]

I've only experienced a tank slapper on a mini moto and that's scary enough. I think it's braking too hard or on a big bike shutting the throttle down too quickly

 

[tigler]

In my experience instability of that magnitude is down to tyre, suspension or alignment problems. As you've checked all the obvious stuff, how about frame alignment ? I notice that you mention it's been like this since you got it, can you be certain it's never been dropped and bent ? Have a very close look around the headstock for signs of creasing, paint lifting, anything that may suggest distortion. If you have a manual, do some measuring to make sure the basic dimensions are correct.

 

[Chipper]

I'd start from scratch mate, don't take anything for granted except things you can't immediately check like wheel balance. Check wheel runout, s/a bearings, wheel and sprocket carrier bearings. Wheel alignment (not with factory marks). Forks true in yokes. Like Tigler says if poss find places to measure front to back on each side, on each lock, to give a clue about alignment.

Must be something out of true, a bearing, or just a rear shock worn or not adjusted for your size/weight, or to the front forks.

 

[octo0072000]

have you changed the head stock bearings they are known to cause problems on these or at least just tighten them up and try it
also can you measure the frame against another in case it has been bent head stock angle etc

 

[triumph]

;Hi many thx for the replys to date will try and redescribe the fault , the bike is stable uptill 40 mph and then as 50 mph comes up it breakes into a bad speed wobble with the front wheel having a good wobble, i have how ever found u can now power through said wobble at 60 mph it starts to smooth out at 70 it appears okish must be going in the rite direction as i could not do this before i changed the fork springs for some softer ones , as an experiment,when the bike slows the prob happens again rough surfaces do not help alot but the prob is on rough and smooth .
I have measured the forks and appear ok from fixed frame points but still not conviced front is perfect, duno just instinct .Unfortunatley when u spend hours looking at bits yer eyes can play tricks on yer not good.All the usual service and chassis checks come back ok all bearings appear ok etc,thanku confused gpx owner craig

 

[mike os]

know anyone or anyone near you race? even off road/enduro?

thay will know where or have the kit to check everything, and the people to do it.... depending on teh outfit it may just be beer money to take a look, but really sounds like a wheel issue...

the wheel and bearings and spindle are all true? ( obvious but....)

 

[Lawnmowerboy]

Does sound as if its the wheel out of balance.

 

[triumph]

yep i agree does feel like a dam wheel but all thats been done think with recent works shows the prob can be effected but not cured at mo.As mike said above have had a second and third set of people look it over and left scratching thier heads? Have also tryed other wheels and no difference looked at other owners sites and the bike is known for been a tad nervous steering wise but this devil is not rite. thx craig ps thx agin for links.

 

[Shox Dr]

Possible wheel problem. As an example I had the wheels rebuilt on my AT last yr, at 35-40mph, the front wheel oscillates on it's axis, giving a feeling of been on a horse. You would think it would get worse at speed, but as soon as 40mph comes round it's gone.

On closer inspection the rim is ever so slightly out of round.

 

[octo0072000]

yep i agree does feel like a dam wheel but all thats been done think with recent works shows the prob can be effected but not cured at mo.As mike said above have had a second and third set of people look it over and left scratching thier heads? Have also tryed other wheels and no difference looked at other owners sites and the bike is known for been a tad nervous steering wise but this devil is not rite. thx craig ps thx agin for links.

does it still wag two up or not ?? if not i would be looking at the back end

 

[triumph]

HI never tryed 2 up bike has no pillion pegs but i do understand what u mean about weight distribution and damaged wheels, realy looked closely at the front and rear wheel they appear fine this morning phoned a frame expert he was at a loss too, the prob does feel wheel related as u say it seems to build up the faster u go, then at the mo u get to 55 appears to iron out some what, i can only describe it as a wobble with the wheel feeling like it is out of balance, which it not other wheels been tryed no look there. cheers craig

 

[tigler]

List all that you have done, looked at etc, whatever remains or what you haven't checked is where the problem is going to be found.

 

[mike os]

has the wheel been checked for runout..... off axis & "roundness"...using at least a pair of vee blocks & dti?

 

[Tiggy_dorset]

Try perhaps checking the frame over more closely, looking for cracks/splits that may be hidden (particularly on the welds). perhaps the frame is flexing whilst being ridden.

Its a long shot, but have you got a missing bar end weight, or none at all? they are for reducing vibration through the handlebars, but vibrations might lead to instability.

Also, check rear shock and swinging arm bushes for wear/play.

Are the tyres the correct size as standard? Are they a matching pair of tyres? Perhaps try a different brand of tyre altogether. I have known customers in the past change there brand because they were unhappy with how the bike handled with their current tyres.

 

[triumph]

hi thats the thing with this particular bike i have gone over it in fine detail looking for the usual faults and unusual, bars fine does have a pair of alloy bar ends on all bushes ok tyres correct new balanced avons not my cup of tea but ok thx c

 

[Shox Dr]

Have you checked the bolts that hold the part of the frame that detaches for the engine to be removed

 

[triumph]

hold me hands up and say no to that one , they got a basic saftey check when i got the bike will take another look 99 per cent sure there ok.

 

[Pete.]

HI never tryed 2 up bike has no pillion pegs but i do understand what u mean about weight distribution and damaged wheels, realy looked closely at the front and rear wheel they appear fine this morning phoned a frame expert he was at a loss too, the prob does feel wheel related as u say it seems to build up the faster u go, then at the mo u get to 55 appears to iron out some what, i can only describe it as a wobble with the wheel feeling like it is out of balance, which it not other wheels been tryed no look there. cheers craig

You've swapped out both wheels and it's still doing it exactly the same?

 

[triumph]

Yep wheels changed and prob is still the same balance checked bits dti where possible.thx c