Help! Why is it tucking the front?

[idl1975]

Having a problem with the TRX that's developed over the last 100-200 miles. The front end is trying to tuck under on right-hand corners. Turning in on a steady throttle or slightly open, as you go past ~15 degrees of lean, the front starts to feel "loose", the bars deflect slightly left and right like the head bearings are loose. Feels a bit better if you're on the brake. But it was bad enough that it let go in a BIG way flicking it right onto the exit of a roundabout...while actually on the throttle...and very nearly lowsided at about 40mph. Left hand corners are still good.

I assumed the weird feeling might be some contamination or that it was just me reacting to the front end tucking on me, but I checked my front tyre when I got to work today, and something is definitely wrong. The left side of the tyre is worn evenly and nicely marbeled. The right is showing signs of tearing (bits of rubber the size of swarf, no big cuts or ripples), and the trailing edge of the tread blocks are cupped in (not sure about the technical terms), i.e. the block in front is worn down towards its rear edge back and the start of the block behind it is higher than you would expect with a wee lip on it. My commute is 20 miles of fast A and twisty B roads, and there are equal numbers of right and left hand corners.

Tyre is a 120/70 Sportec M1 at ~34 psi cold. Have used one of those previously with no problems. Front end is off a YZF750 (hence the tyre size), and hasn't given me any issues until now. I did recently drop the forks 10mm through the yokes - they were installed flush with the yokes originally, but the steering was too slow. But I have had a check and all the yoke and axle bolts appear to be tight. Back tyre is a Diablo and shows completely even wear. Rear tyre is aligned well enough (i.e. correct according to alignment marks). No obvious horrible noises from the front end to indicate bearings or anything loose. One of the brakes appears to be dragging slightly - the YZF calipers are a bit old and could use a rebuild.

Does anyone have any idea what could be causing this?

My only thoughts are:

1) right hand caliper dragging very badly - but the bike can be pushed around no problem and there's no other indication of brake drag.
2) something gone very wrong in one fork leg - but surely that would be obvious at all times, as the forks are bolted together?
3) bad tyre - but it's been fine until now and hasn't been over or under inflated significantly.
4) contamination - but how could it possibly last for over 200 miles?
5) geometry - it really doesn't like the forks being 10mm lower. But then why only in one direction?
6) wheel bearing or spacer failing on the right side. But I don't see, hear or feel anything.
7) head bearings - but why only one side? No clunking either. And these aren't new bearings - they're broken-in roller bearings.
8 ) pixies or gremlins - this seems the most sensible explanation, but which of them is it?

PS - I _may_ have lost a wheel weight from the front wheel. Surely this wouldn't be enough to cause such a big problem though? Or would it... Am going to pull the front wheel, put a spare 120/60 on it and have it rebalanced in the mean time.

[Mincehead]

Ah well as I ran an independant tyre and exhaust centre for about 7 years I CAN explain this.
Disproportionate `cupping` of motorcycle front tyres is predominately caused by excessive braking forces and road cambers.
What you`ll need to be aware of is that once a tyre is cupped it exascerbates wear in the same fashion!
So,perhaps it`s time to replace that front tryre irrespective of how much life it has left and take it from there.
I would say that under inflation of a front tyre can give similar problems over time and the `types` of road you`re travelling on most will also add to the equation too.
Is your daily ride to work on roads with steep cambers?
This can give accelerated and uneven wear too.
Wheel alignment can affect tyre wear also,especially if any of the above are also included in it.
Six pots are hard on tyres,as well as having a habit of warping discs too,I`ve spoken to at least two `ex`YZF owners who would regularly warp discs when they had been pushing on and using all the brakes power.It all gets expensive when a bikes full performance is getting used regularly.

[idl1975]

Thanks - so the cupping tends to reinforce itself? Didn't know that.

Could be something to do with a missing wheel weight or weights then. The roads are cambered (sunken edges common), but not that much, and I ride away from the cambered bits. There's really no reason for the right side to be experiencing that big a difference in wear just from cornering or breaking though. I wonder if the wheel bearing (it's the non-speedo side) is failing or failed as well? But it could well be a construction fault with the tyre, so I will start by swapping to my spare 120/60 Diablo and asking the shop to check the balance and check the rim for runout, as well as looking at the old tyre.

As an admitted expert (too late to back out now!), do you think I'll be OK using the 120/60 (TRX size) on the YZF rim (normally 120/70 - I don't really like the steepness of that profile myself, but it is the standard YZF size)?

Ah well as I ran an independant tyre and exhaust centre for about 7 years I CAN explain this.
Disproportionate `cupping` of motorcycle front tyres is predominately caused by excessive braking forces and road cambers.
What you`ll need to be aware of is that once a tyre is cupped it exascerbates wear in the same fashion!
So,perhaps it`s time to replace that front tryre irrespective of how much life it has left and take it from there.
I would say that under inflation of a front tyre can give similar problems over time and the `types` of road you`re travelling on most will also add to the equation too.
Is your daily ride to work on roads with steep cambers?
This can give accelerated and uneven wear too.
Wheel alignment can affect tyre wear also,especially if any of the above are also included in it.
Six pots are hard on tyres,as well as having a habit of warping discs too,I`ve spoken to at least two `ex`YZF owners who would regularly warp discs when they had been pushing on and using all the brakes power.It all gets expensive when a bikes full performance is getting used regularly.

[Mincehead]

The 70,or for a TRX `60`,is just the profile of the tyre.It denotes that the side wall is whatever figure(50,60,70 etc)percent of the width of the tyre in millimeters.
The 70 profile from the YZF wheel would have been throwing the speedo out a little too,having a higher `rolling radius` but not by too much really.I`m not certain what `J`(wheel rim width)the YZF rim is but I`m guessing it`s a 3.5 just the same as the stock TRX wheel.
I think the only difference you may notice is that you`ll quicken the steering `slightly` by going back to the 60 profile and that there will be less give in the tyre sidewall.This will in turn mean that you may `feel` road irregularities or pot holes etc more.
I`m still running my stock TRX front end,including 60 profile tyre,I`m struggling to find the time right now to devote to getting the YZF front end sorted and fitted.Just another one of those jobs that`s part of onging bike projects.
Those road cambers,corners and six pots are playing a merry dance with your tyre life.

[cobbadiggabuddyblooo]

Yer ditto on the camber wear..have seen it myself a few times..

[idl1975]

The 70,or for a TRX `60`,is just the profile of the tyre.It denotes that the side wall is whatever figure(50,60,70 etc)percent of the width of the tyre in millimeters.
The 70 profile from the YZF wheel would have been throwing the speedo out a little too,having a higher `rolling radius` but not by too much really.I`m not certain what `J`(wheel rim width)the YZF rim is but I`m guessing it`s a 3.5 just the same as the stock TRX wheel.
I think the only difference you may notice is that you`ll quicken the steering `slightly` by going back to the 60 profile and that there will be less give in the tyre sidewall.This will in turn mean that you may `feel` road irregularities or pot holes etc more.
I`m still running my stock TRX front end,including 60 profile tyre,I`m struggling to find the time right now to devote to getting the YZF front end sorted and fitted.Just another one of those jobs that`s part of onging bike projects.
Those road cambers,corners and six pots are playing a merry dance with your tyre life.

Yeah, it's a 3.5 x 17. I'll swap the tyres over and we'll see if it's still a problem...