new front tire, fork seals: Low speed front tire "hop," very smooth at other speeds

[gtem]

new front tire, fork seals: Low speed front tire "hop," very smooth at other speeds

Hi All,

I have a bit of a new one (for me): I mounted up a fresh Shinko 712 front tire on my '81 GS1100E last week, did a static balance on it, everything seemed to go fine. Out on the road at speed this thing is dead smooth to 80+ mph (speedo tops out at 85). I was happy with all this, then I noticed something odd: at low speeds, specifically at 30-35mph, there is a very notable front wheel "hop" that you feel in the handlebar and can observe in real time looking at the tire from the side. I'm convinced that this is likely an issue with the new tire (seated unevenly?), but the other variable that changed at the same time was I replaced the fork seals and in the course of doing so set the front fork to [NODE="1"]Home[/NODE] preload and [NODE="2"]Forum[/NODE] on damping (had been set stiffer in both regards previously). I actually have a spare '82 front rim set up with a known good/balanced older Avon Venom that I will swap over to try and isolate the issue, and will throw the current wheel/tire on my static balancer to spin and observe closer, but even easier than that I will try upping the variable damping. My understanding is that I might be(?) hitting the spring resonance frequency at that low speed where a minor ripple in the road sets it off.

So I already have a plan for diagnosing this but I was just curious whether anyone else has had this experience of a tire being dead smooth outside of a small window at low speed.

 

[93Bandit]

If you search the forum, a common cause is tension in the forks. However, it sounds like you may have already addressed that.

I have the same hop at 35mph on my bike and have been unable to get rid of it, by balancing or relieving fork tension.

 

[gtem]

If you search the forum, a common cause is tension in the forks. However, it sounds like you may have already addressed that.

I have the same hop at 35mph on my bike and have been unable to get rid of it, by balancing or relieving fork tension.

Hmm, "tension" in the forks, what exactly does that mean? Preload in the fork springs set wrong or something else?

 

[Gorminrider]

I'm convinced that this is likely an issue with the new tire (seated unevenly?)

I think your first thought-poor tire mount-is the most likely.

Tires often (always?) have small "bead line" on the tire very near the rim-look for a dip in this. You can also spot bad tire seating on the bead by spinning the tire in the forks versus a stick/ruler/ clamp/whatever held in place on the forks to spot a wobble...to fix,you'll have to A) apply a lot more airpressure or B) break that bead, lubricate and try again to reseat it properly.

 

[93Bandit]

My suspension knowledge is incredibly rudimentary but my understanding is tension in the forks means the fork tubes aren't "relaxed" relative to each other. An example situation that can cause this would be removing the axle and wheel (to change a tire for example) and then reinstalling them, torqueing the axle nut. By doing that, you potentially put a twisting tension between the forks. Because you loosened one end of the fork but did not loosend the other end (triple clamps, fender). Anytime you do front end work, you're supposed to follow a procedure that "settles" the front forks, releasing any tension. The service manual for your bike should have the procedure. I believe you brake all then fasteners loose, then tighten the top triple clamp, bounce the front end several times, then snug them all up moving from top to bottom. I could be wrong/missing some steps so be sure to check your service manual.

Again, and am a complete suspension noob so do some more research first. If you search front end hop on the forum, there are several threads on the topic.

 

[gtem]

I think your first thought-poor tire mount-is the most likely.

Tires often (always?) have small "bead line" on the tire very near the rim-look for a dip in this. You can also spot bad tire seating on the bead by spinning the tire in the forks versus a stick/ruler/ clamp/whatever held in place on the forks to spot a wobble...to fix,you'll have to A) apply a lot more airpressure or B) break that bead, lubricate and try again to reseat it properly.

I strongly suspect that this is what happened. I will say messing with tire mounting is one of my LEAST favorite DIY jobs on motorcycles! Off hand, do you know how high a tube type tire can be inflated to properly seat a bead? I've always shied away from taking it very high on tubed setups.

 

[chuck hahn]

Did you set the forks in the triple tree exactly the same? Doesnt take much to throw the geometry off. I always put the top of the tubes right at the top of the top bridge. I use a utility knife blade as my guide. Lay it on the surface and when it goes into the crease between the top of the tube and the fork cap is my spot.

 

[Gorminrider]

I strongly suspect that this is what happened. I will say messing with tire mounting is one of my LEAST favorite DIY jobs on motorcycles! Off hand, do you know how high a tube type tire can be inflated to properly seat a bead? I've always shied away from taking it very high on tubed setups.

Tubes or not, my cheapie gauge on the cheapie compressor can say 60# but we were all cringing in fear... so I can't swear that's accurate
More than the tire's "max pressure" or you're supposed to need anyways. I don't worry about the tube-it's constrained by the tire.

 

[KiwiAlfa156]

The first question is, is the hop rhythmic and vary with the rotational speed of the wheel? If so, it could be -as others have suggested- incorrectly seated, so check the bead relative to the rim on both sides of the tire. Did you balance the wheel on an axle stand? If so you can use a pointer to check for rim trueness and for the tire being out of round. There is always a little variation on the tread but it should be minimal. Definitely not enough to cause hop.

The other thing that might be occurring is stiction in the front forks. 'Stiction' is the term used to describe the combined internal friction of fork seals, bushing and dust covers that resist the fork telescoping. At low speeds the the bumps and irregularities in the road surface may not subject the un-sprung front fork and wheel assembly with shock loads large enough to over-come the stiction in the fork. The fork essential acts more or less like a solid strut and the whole front of the motorcycle goes up and down or hops.

A crawling speed the up and down movement is too slow to be noticed and at higher speeds the higher shock loads from the bumps work the forks.

Another possibility is caster wobble. Although this is a side to side shake through the bars rather than an up and down hop. This occurs in virtually all motorcycles, almost always at around 30 mph (50 kph). In most cases never noticed unless you close the throttle and take your hands off the bars.

 

[gtem]

Welp, I was definitely overthinking this one. Just not seated right. I was able to deflate, put some tire soap solution around the bead, inflated to about 50psi to reseat it evenly. Will test ride to confirm the fix. I feel silly to have even made a thread about this, though I'm surprised that such an obvious unevenness could only be felt at such a narrow range of speed and that it was so smooth at all other speeds!

 

[Gorminrider]

Good oh! I can bet you got it.... I've had exactly the same thing...from experience, "silly" is transitory compared to the feelgood of "fixed"!.

 

[93Bandit]

Glad you found the issue! Wish I could solve mine.

 

[gtem]

Just chiming in to confirm that my tire hope woes are solved with that reseating. Went ahead and mounted a new rear Shinko 712 as well, made sure to pay attention to seating on that as well. Dead smooth from 0-90+ mph.

 

[Grimly]

Just chiming in to confirm that my tire hope woes are solved with that reseating. Went ahead and mounted a new rear Shinko 712 as well, made sure to pay attention to seating on that as well. Dead smooth from 0-90+ mph.

Good stuff.
It's so easy to be caught like that even if the tyre seems to have popped on easily.
DAMHIK.