Wierd front wheel hop, HELP!

[Bikedude987]

So, for the last two sets of tires (which shall remain un-named, two different brands/styles) the front wheel has developed a hop at around 35-40 mph. It feels like a badly out of balance wheel, but only in that range, so I'm thinking resonant frequency. Slower and it stops, faster and it stops. The wheel was balanced and tire pressure at 33psi. Suspension set up how I like (straight rate springs, compression stack worked so I have moderate low speed damping and little high speed damping, rebound to suit the springs). I've set the sag front and rear to 25% and 33% respectively. I'm kind of out of ideas other than playing with tire pressure and running something other than specified...

What could be causing my front wheel hop?

 

[yz454]

It depends on tire brand but that pressure could be very low .

 

[markjenn]

So, for the last two sets of tires (which shall remain un-named, two different brands/styles) the front wheel has developed a hop at around 35-40 mph. It feels like a badly out of balance wheel, but only in that range, so I'm thinking resonant frequency. Slower and it stops, faster and it stops. The wheel was balanced and tire pressure at 33psi. Suspension set up how I like (straight rate springs, compression stack worked so I have moderate low speed damping and little high speed damping, rebound to suit the springs). I've set the sag front and rear to 25% and 33% respectively. I'm kind of out of ideas other than playing with tire pressure and running something other than specified...

What could be causing my front wheel hop?

When you say "hop", do you actually mean you can visually see the wheel hopping as you drive at the critical speed? Or are you just using the term to refer to vibration that feels like the wheel is hopping?

If you can see the wheel is out of round (put bike on centerstand, jack up front end, and spin wheel), then you've likely whacked the rim on something and you'll see a spot on the rim where it was hit. You might have loose spokes too. The repair is to get the rim repaired/replaced and the wheel re-trued. Or get a new wheel.

If the rim is running true and has no significant lumps, then I'd suspect a defective tire, but you say you've tried a couple. Next would be balance and I'd certainly re-balance the wheel again and consider remounting the tire to a different spot on your rim. Who did the balancing? Do they know what they're doing? Obviously, you are looking for common factors between the two tires that both have the vibe and if the person and equipment doing the balancing is in common, then you've got one possible explanation.

Highly doubtful that suspension setup or steering head bearings are the culprit, but worth checking also. Wheel bearings too. But your problem sounds more significant.

Don't just assume it is the front because that's what it feels like. A problem with the rear can masquerade like it is from the front.

- Mark

 

[Bikedude987]

I do all my own work. Both front and rear balanced within 1/4oz. Both rims are torques and trued, no dents or bends. Tire pressure checked this morning at 33/36. Steering bearings set properly.

I can peer over the front while riding and watch it hop, it's not leaving the ground, but it's a distinctive motion, and obviously felt throughout the bike. I'll do a more thorough check of the entire bike this afternoon and see if I can pinpoint any issues.

Any other ideas?

 

[OldRider]

The 45mph wobble has been around since the motorcycle was invented. The very first place to check is the steering stem bearings.

 

[Bikedude987]

I guess they could be given some fresh grease and a good snugging. They are about as loose as can be with no play.

I also grew suspect of my air gage and went and got a nice digital one. Seems like I was overfilling the tires by right around 5 psi or so.

 

[Don in Lodi]

I've got one for you; have you kept the same rear tire during your front tire experimentation's? I had a front end wobble last year. Did everything that could be done to the front end to try to track it down. Time went by, miles added up, finally I replaced the rear tire because I had replaced the front already... the front end wobble went away! That replacement tire, same brand as the ride disturbance one, is now worn to that same point, say, last 10% or so... with the wobble. Thought I had a winner with the Full Bore tires, this is my third rear, first did fine...

 

[Derekj]

Have you checked your spokes ?

 

[Bikedude987]

I replaced the rear at the same time as the front.

The spokes were checked with the tire change and balance. I re-set my rebound to the middle of the clicker settings (I had it full out) and it seems drastically worse today. I don't think it's the source, but it was masking the problem when the suspension was more mobile.

I'm starting to think it's a dynamic balance issue. Tires can be in static balance and be out in dynamic. I ordered some balance beads and will remove the stick on weights in favor of those. It's also possible there was some spinning of the tire on the rim that I hadn't noticed, and not balance is off completely.
I will also play around with tire pressure both +10 and -10 psi from where they should be. I don't think I'd run that long term, but 'the more you know'...

I will have to inspect the rear more as well. I've noticed that you can feel the rear's imbalance in the front on dirtbikes before.

 

[shrekonwheels]

Did your balance weight stay on?

Honestly sounds like a Balance issue to me, pull it off and check again

 

[yz454]

Try this, dismount the tire,rotate it, try it again .

 

[Bikedude987]

Balance weight stayed on, tire didn't spin. I'm going to remove the stick on weights and give the beads a go.

I messed with pressure at lunch, bounced more at 43 psi and less at 23 psi, but still bounced at both.

Maybe this bike really likes to squat in the rear, I seem to have noticed it right around the time I upgraded the rear shock and was able to properly set sag. More stuff to try...

 

[Derekj]

Are you sure that the brake discs are true and not slightly bent ? Also, if this is a new tire , make sure you have "seated" it correctly.

 

[markjenn]

First, let's not bring "wobble" into the discussion which is a completely different issue. A steering wobble (back and forth oscillation of the handlebars at the front end's resonant frequency) is quite common at around 35 mph if you ride hands off and can be difficult to chase. But if the wobble damps out with hands on the bars, you usually can live with it. This sounds like something totally different.

No wheel tire combo should have visible "hop" due to tire pressures, steering head bearings, or suspension setup. Balance is the usual problem, but you seem sure you've got it balanced correctly and typically a balance issue at 35 mph will be worse at higher speeds. Is your balancing setup decent? It's something basic with the tire or rim that you're missing. FYI, no tire can spin on a rim if it is inflated to any reasonable street-bike value (i.e., > 20 psi).

I come back to a defective tire, but you say this problem occurs with two different tires. Was the rear in common between the two fronts? Is there any weirdness in the tire size or type? What are the tires?

- Mark

 

[autoteach]

if they are statically balanced using gravity, it should be tough to screw up. If they are done by machine, it could be out of calibration. If they are out of round you should be able to see that with a spin of the tire. You can balance a propeller but it hardly makes a wheel. past the out of balance and out of round, you can get into two different things (as I see it):
1) round tire, dynamically imbalanced, meaning weight of tires is not evenly distributed (right to left as viewed on bike). the problem with this is that it should cause a shake in the handlebars.
2) tires are not of an even density/spring rate and cause an oscillation as that "heavier spring" comes around. http://thumbsnap.com/s/lSAWfaxH.jpg

As you are reluctant to tell us the brand of tire, I would guess that it is not one of the big brands and therefore would be subject to the possibility of #2.

 

[Bikedude987]

I removed all weights and it seems to be better actually. Like I said I'm going to give the beads a try.

I could definitely see the rear bouncing in the shadow on my way to work. Whoever said rear, I think you're right.

Tires are Heidenau K60 (first set) and now Shinko E804/805. Both sets did this, so I'm not inclined to say it's a tire thing.

 

[Don in Lodi]

Guys on tha GL1800 forum swear by the beads. I have a hard time with things that don't seem logical, and the beads fall into that category for me. Worth a shot though.

LOL, I can't get my head around the physics of how the beads work either. Seems totally counter to how things should work.

 

[tomatocity]

DynaBeads demo video

 

[Bikedude987]

That's a cool demo. Looks like they certainly work, at least in waterbottles!

I'm officially stumped. All chassis bearings are good, suspension settings have been played with to no change, tire pressure has been played with (lower is worse, but I can't go high enough to eliminate it) and it's not balance.

I have to conclude that this is just a fact of life with knobby tires (and these are VERY knobby). I'm going to double the bead weight and see if that helps mask it, but at this point I'm just hoping it goes away as they wear in. The tires are good enough offroad that I will certainly use them, but I think I will give the Kenda BB's a try next time.

 

[OldRider]

Heidenau and Shinko. I'm inclined to think it's the tires.

So enlighten us on your experience with the Heidenau tire that would put it in the same category as a Shinko???