I've had some issues with my 1991 Honda ST1100 that I, nor my mechanic, can't seem to figure out.

It started with some kind of bang, probably because of a hole in the road. Then, whenever I had to brake, it had a pulsating feel to it, but it was still relatively mild. I took it to my mechanic, who said that the front wheel bearings needed to be replaced.

I picked up the bike a couple of days later, after the wheel bearings had been replaced. The mechanic warned me that the bike still had some mild issues that he can't really put his finger on. He said that he suspects the front tire to be the culprit, but this doesn't really make sense to me, because a front tire isn't suddenly going to start causing instability issues. I'm the last one to deny that tyre choice can make a great deal of difference on a motorcycle, but once a tyre is on it, the behaviour of the motorcycle won't really change (unless you've got under- or overinflated tyres, of course). If the front tire was the issue, it should have either always been there or have come very gradually. He insisted that the bike was certainly safe to ride. I have my doubts about that.

The 'mild issues' that he mentioned, were much worse than my original complaints. The bike is really unstable now; the back of the bike seems to 'float' (this started only after the repair; there was nothing like it before it), while the front of the bike is very shaky whenever I put more weight on the front tire (by using the front brake) or less weight on it (by using the rear brake, most often in slow-speed riding). This also wasn't there before the repair; the only thing that was there was a pulsating feel when braking. My passenger also complained about a mild wobble at highway speeds (70 mph) and said that she felt really unsafe. Coming to a stop is quite hard too; due to the instability and the front wheel shaking, it is really hard to come to a controlled stop. Driving slowly in traffic is almost impossible; the shakiness is really uncontrollable. Using the rear brake, normally a good thing for low-speed control, makes it worse.

It took me a while to realise that this is indeed a serious issue (I really wanted to believe that the mechanic solved my problem, rather than made it worse), but I've now decided that I want to figure out first what is wrong and how to fix it, rather than take unnecessary risks with a bike with stability issues.

If anyone has any suggestions as to what the cause of this rather mysterious problem could be, I'd be very grateful.

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    I'm not a MC guru, but it sounds as though your front forks could be tweaked. – Pᴀᴜʟsᴛᴇʀ2 Nov 11 '14 at 1:34
  • @Paulster2 that indeed sounds like a possibility. Do you mean that they should be aligned, as in this video: youtube.com/watch?v=vSunBRB6-r8? – Ben Nov 11 '14 at 11:24
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    Yes, exactly. My thinking is that when the front and rear wheels are not in alignment, the centripetal/gyroscopic forces fight each other and you have issues. I don't know if this is your case or not, but it's something you should check, I'd think. – Pᴀᴜʟsᴛᴇʀ2 Nov 11 '14 at 14:43
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    Did you check the wheel runout? Suspension issues shouldn't cause brake pulsing. Tires can cause scary vibrations if the tire manufacturer doesn't balance the tires well, but that usually shows up after a tire change, though can present itself slowly as a tire wears. I had a set of Avons that would cause vibrations and then the whole bike to wobble at freeway speeds. Even though the balancing was done via machine and then manually, it was mostly fixed by replacing the front tire, and completely fixed after I replaced the rear. – Paul Nov 12 '14 at 4:52
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    Oh, and it is possible your brake pulsing is caused only by warped rotors and is unrelated to other symptoms. You may also try checking the steering bearings for play. – Paul Nov 12 '14 at 4:56