Oddity In Rear End!

No, this wasn't after waking up drunk that night at EOM, but actually an oddity in the rear end of my Feej.

***Professional Rider on Closed Course****

Here's the strange thing which I noticed. When approaching 110 on the speedo I get a very strange "thumping" sound/feeling coming from the rear end. This "thumping" begins to lessen and then dissipate entirely at around 120 and then on up it is once again rock solid.

Up until that point the bike runs as you would expect....perfect. This happened with the original Z6 tires and now with the Avon Storms which I just put on at EOM. It happened on the freew...er..closed course coming back from EOM with loaded bags and screen up or down. However the first time it happened bags were on but empty and again screen up or down.

Prior to hitting that mark the bike is smooth as silk as we've come to expect. Even after 120 it again is like buttah.

Any suggestions as to what to look at? Or what could cause that? Luckily it's only in that range which I'm not normally in but still.

I was going to convince Renegade (evil grin) to help me pull the tire and lube the splines one of these days but haven't done it yet.

Does the cadence of the thump sync with the RPM's of the wheel? Faster, slower, not at all?

To add to TWN's question: any shifting of gears, ie rpm drops, in the equation?

I noticed a similar feeling/sound. Does seem to vary with speed/RPM. But, only happens at that 105+speed range (at the track)

I just did a quick calc :blink:

rear wheel is about 25"OD

circumference 78.5"

110mph=580800fph

=9680fpm

=161fps

161fps/78.5"=24rps for the rear wheel

The cadence of the sound isn't that frequent on my bike-

might be 12 or 6 per second.

mmmmm..........

back to work <_<

Go Far said:

I noticed a similar feeling/sound. Does seem to vary with speed/RPM. But, only happens at that 105+speed range (at the track)I just did a quick calc :blink:

rear wheel is about 25"OD

circumference 78.5"

110mph=580800fph

=9680fpm

=161fps

161fps/78.5"=24rps for the rear wheel

The cadence of the sound isn't that frequent on my bike-

might be 12 or 6 per second.

mmmmm..........

back to work <_<

Click to expand...

So, what rotates slower than the tire back there and is it a consistently timed thump that would equate to something cylindrical, round or otherwise rotating? Is this happening with the top box on?

Ya know. One time I was hangin with odot, I had a rear end oddity...... :blush2:

[SIZE=8pt]don't let him show ya his Gerbils[/SIZE] :nono:

:jester:

I've never had a thump like that, but I get a thumping on my chest right around 130 whenever barb's on the back....

AdrianNKY said:

No, this wasn't after waking up drunk that night at EOM, but actually an when I was sober.

Click to expand...

Fixed it for ya.

I have no clue (what's new there right?)

You don't need Renegade to lube your splines (That don't sound right either).

Its an easy job. I coulda done it for ya at EOM ifn I had known.

So, what rotates slower than the tire back there and is it a consistently timed thump that would equate to something cylindrical, round or otherwise rotating?

Click to expand...

Shooting in the dark here.

I'll take the ABS sensor ring for $100, Alex.

Ya know. One time I was hangin with odot, I had a rear end oddity......

don't let him show ya his Gerbils

Click to expand...

Joker,

I'm almost at a loss for words here.

You know its the military policy on the Forum of Don't Ask/Don'T Tell right?

Richard Geere (sp) wasn't there with a vacuum was he? :blink:

FJRFencer said:

Shooting in the dark here.
I'll take the ABS sensor ring for $100, Alex.

Click to expand...

Hmmmm, come to think of it Juan, the speed would be slower, but the time it takes to rotate is the same... I'm gonna vote for a rotational imbalance of the drive shaft as everything is rotating in time with regard to the wheel and that's the only other rotating thing in the rear end - or the top box dynamics if one is involved...

Note to self: Stop trying to think that hard. Synapses are shutting down en masse. :blink:

FJRFencer said:

Joker, I'm almost at a loss for words here.

You know its the military policy on the Forum of Don't Ask/Don'T Tell right?

Richard Geere (sp) wasn't there with a vacuum was he? :blink:

Click to expand...

Shit!!!! Didn't know about the "policy" But, I have a sneaking suspicion on who posted the vid on useless junk.

Yer sick dood :fuck:

:blum:

:jester:

Nothing rotates slower (in angular velocity) than the tire. The drive shaft is faster. It's a resonance in something, since it happens at a specific speed and goes away. Sounds like a balance problem.

If the experiment is interesting (and the "course" accessible), change the damping settings back there, see what happens, if it changes the speed at which it occurs.

wfooshee said:

Nothing rotates slower (in angular velocity) than the tire. The drive shaft is faster. It's a resonance in something, since it happens at a specific speed and goes away. Sounds like a balance problem.
If the experiment is interesting (and the "course" accessible), change the damping settings back there, see what happens, if it changes the speed at which it occurs.

Click to expand...

Two different bikes and three different sets of tires? Balance has a slim chance of being the culprit, IMO. Especially when the riders claim to hear the thump, too. That's why I keep tossing in the top box theory... or maybe even a side case that picks up an oscillation at that certain speed. Dunno.

I wonder how much flex there is in the bag lids? A little bit of aero popping?

Centerstand it and run it up to speed.

That'll get rid of the top box /aero theory.

You can even put you ear right up next to it and listen. Just be careful or the tire may pull a Mike Tyson on you.

The feeling/sound reminds me of the old hard skateboard wheels hitting the cracks in the sidewalk. Only it is 1% of the strength....

Yep, I have a top box.

twowheelnut said:

So, what rotates slower than the tire back there and is it a consistently timed thump that would equate to something cylindrical, round or otherwise rotating? Is this happening with the top box on?

Click to expand...

I wonder if the ABS could be up to something, kind of like what it does on a low traction (gravel) road.

Just thinkin' out loud here :blink:

No top box for me. Just side bags. Admittedly I haven't tried it with the bags off as I normally always run with them on but I'm going to see if it happens with them off also.

If I recall the cadence did change with the RPM even though the intensity diminished (make sense?) but I'll have to confirm that one. Like mentioned above it almost feels like when you go over two joined sections of pavement with one side slightly raised although not as significant.

I had considered the sensor ring and even thought about changing the preload/compression of the rear shock to see if that changed it also. Will hopefully experiment on my local closed course this weekend and see what's up.

Thanks for the input.

Fencer, it's aight. I know you wanted to get your hands on my rear end at EOM. S'all good.

Go Far said:

I wonder if the ABS could be up to something, kind of like what it does on a low traction (gravel) road.Just thinkin' out loud here :blink:

Click to expand...

Why just at that speed range? I would think that a faulty ABS... wait... Slightly warped rotor that makes itself known at that speed range?

AdrianNKY said:

Fencer, it's aight. I know you wanted to get your hands on my rear end at EOM. S'all good.

Click to expand...

Coulda done ya right after I got done make'n Tom's rear tighter. It brought a crowd from Jersey Ed. We discussed his rear as well