Wobbling front end

[FJRocha]

Howdy all...

After nine months of contemplating the idea, I finally took the weekend and fabricated a mounting plate to install my Yamaha Top box. I made it of cold rolled steel since I don't have the resources to weld aluminum. Any whom, it ended up being a good project, and I am happy with the results. I installed the top box and called it a day.

However...

Yesterday, on my 20+ miles commute (with the top box installed for the first time) I felt that the front end of the bike started to "wobble". At highway speeds, it feels like the front wheel is leaning from side to side; I checked it and it is not physically leaning or moving sideways. I get the impression that the front end is lighter than normal and it feels like it is sliding sideways on slippery ice. It is strong enough to kick up the pucker factor a few notches. BTW, temps are above 70ºF and it hasn't rained for more than 100 days... this is AZ.

Now, this was my first trip with rear trunk installed, and I am wondering if the trunk's aerodynamics have anything to do with the front end feeling lighter than normal. The loaded top box including the plate should not be more than 25 pounds and I don't think this would weight the back too much.

Has anyone experienced this after installing the trunk?

I am bamboozled.

 

[JimLor]

I have the Givi V46 on the back and yes, I do notice some movement when I have the box mounted. In fact, rode to work with it yesterday 40 miles up Rt 95 - that means at least 5 minutes of 70+mph (sarcastic comment meant to highlight the absolutely joy-of- life sucking reality of living and commuting in N. VA) :angry2: riding. I have always noticed a slight side-to-side kind of wobble that's present when the Givi is onboard. That said, I notice it and then don't worry about it - doesn't seem to pose any safety threat, just something to be aware of and act accordingly. I've noticed it even with the sidebags on and/or off.

 

[OrangevaleFJR]

I think it is the aerodynamics of the top box. I notice a discernable difference in the bikes attitude at 70+ mph. I have the Yamaha top box

The reason I think it is aerodynamics is that when my wife is on board, the lightness of of the front end isn't present. It is only when the top box is on with no passenger.

I have experienced this with an empty top box. I just don't think the weight is a factor as compared to the aerodynamics.

Because of this experience, I stopped riding with the top box when I am riding solo.

I must say that I didn't feel a wobble as you describe, I simply notice a lighter front end than usual.

 

[FJRocha]

Thank you for your comments, they easy my worries.

Come to think of it, I don't believe there is a physical wobble, but it is more like the feeling of it. I have the tendency to be extra sensitive to any strange noise or variation on the bike's behavior. It is possible that this extra sensitivity is amplifying the effects of the front end lightness and I end up thinking that the bike is going to disintegrate on reentry.

I agree that the wobble has a lot to do with the aerodynamics of the box since I notice it more when there are trucks or large vehicles in front or around me than otherwise. Also, it only happens at atmospheric reentry speeds (65+mph) h34r:

 

[donaldb]

FJRocha, mine w/E460 Givi wobbles at slowing speed, I don't take my hands off the handlebars w/Givi on, makes a noticable difference w/ and w/out

 

[radman]

Well known and documented phenomena when trunk is mounted. Try bringing the tubes up a bit in the trees-move some weight forward.

 

[FJRPierre]

I have the Givi 46 mounted (and full ) all the time and I have never noticed a wobble in 60+k km. Maybe cause I'm pretty heavy, it off sets the rear wt?.

 

[FJRocha]

I have the Givi 46 mounted (and full ) all the time and I have never noticed a wobble in 60+k km. Maybe cause I'm pretty heavy, it off sets the rear wt?.

I am approaching 260lbs (~118kg) asymptotically, but that does not seem to bear any effect on the wobbling.

BTW your speed numbers are a wee-bit off:

65+mph equals approximately 104.60736+kmh, whereas 60+kmh equals approximately 37.2822715342+mph

 

[radman]

I have the Givi 46 mounted (and full ) all the time and I have never noticed a wobble in 60+k km. Maybe cause I'm pretty heavy, it off sets the rear wt?.

I am approaching 260lbs (~118kg) asymptotically, but that does not seem to bear any effect on the wobbling.

BTW your speed numbers are a wee-bit off:

65+mph equals approximately 104.60736+kmh, whereas 60+kmh equals approximately 37.2822715342+mph

Read it agin. In 60K of riding, not at 60K.

 

[gr8eyes]

I believe Orangevale makes a good point. I remember back in the fall of 02 alot of the european riders complaining about a wobbly feeling with the trunk on. Come to find out if their wives were on with them the wobble disappeared. So there is an easy experiment for you. Go try it and see. Let us know.

Glenn

 

[FJRocha]

I have the Givi 46 mounted (and full ) all the time and I have never noticed a wobble in 60+k km. Maybe cause I'm pretty heavy, it off sets the rear wt?.

I am approaching 260lbs (~118kg) asymptotically, but that does not seem to bear any effect on the wobbling.

BTW your speed numbers are a wee-bit off:

65+mph equals approximately 104.60736+kmh, whereas 60+kmh equals approximately 37.2822715342+mph

Read it agin. In 60K of riding, not at 60K.

Yep, you're right.

I got to clean my glasses, my bad. :blush2:

 

[radman]

I believe Orangevale makes a good point. I remember back in the fall of 02 alot of the european riders complaining about a wobbly feeling with the trunk on.  Come to find out if their wives were on with them the wobble disappeared. So there is an easy experiment for you. Go try it and see. Let us know.
Glenn

How the hell do you expect him to talk their wives into riding with him? Think their guys might object a little, huh? I mean, duuuuuuuuh!! For an eye guy, you don't see so guud......

 

[Shadowfax]

I haven't noticed a wobble when solo w/ the top box on if I keep it under triple digits.

Someone somewhere brought up adding the ST1300s top box "wing" for aerodynamic stability. Did anyone actually try that?

 

[odot]

The answer is simple kids.....eat more quarter pounders with cheese!!!!!!!!

:lol:

 

[sgoat]

You can use most any "bag" to fill the void between you and the top box. It works!

 

[OrangevaleFJR]

You can use most any "bag" to fill the void between you and the top box. It works!

Yep, that was my thought if I needed to ride solo with a topcase, that's what I would do

 

[Knifemaker]

This problem, (lets call it an "event") is purely aerodynamic and although it may feel like it, is not a your front end getting lighter. I would not raise the tubes up to add "more weight" to the front end, as this would just make the steering quicker and may even make matters worse.

The reason this happens is because of the empty space between the rider and the trunk. At higher speeds, a lower pressure bubble is formed, which causes an ocsilation that in simple terns, is trying to twist the case off the back of the bike.

If you have ever seen a street sign shake back and forth violently in a severe wind, you can relate this to what is going on here. Because the rear of the bike is fairly solid, the stresses caused are transmited to the one part if the bike that can actualy pivot..the steering stem. Luckily most of the torque gets lost in the length of the bike and its mass...so what you feel is a subtle shaking..almost as though you were trying to move the handlebars back and forth (left to right) as quickly as you could with the least amount of actual movement. This can easily feel to the rider as though the front tire is "floating" in some way.

The bad news is there is nothing you can do to remove the event, but things that can effect it are : riders size..and here width matters more than weight.. rider position, windscreen width, speed (of course) and pevailing wind direction.

Removing the side cases may help, adding a steering dampner , or as suggested , placing a bag between the rider and the top case that fills the void...(or a passenger of course) Stiffening the front suspension may help a bit too.

Best solution to me would to design the mounting plate to ride on a adjustable rail..so when riding solo you could "Slide" the case over the passenger section of the seat behind the rider.

I have been following the various reports of this event, and am hoping that the subtle changes to the aerodynamics of the 06 model will in some way make things better than...worse.

KM

 

[FJRocha]

Knifemaker,

Lovely avatar... has a gravitational effect on my eyes that got me distracted from your writing a couple of times.

Your explanation made a lot of sense to me. I can see how the void behind me is creating this slight movement and the consequent feeling of side to side sliding of the front.

I will place a bag between me and the box to see if the effect goes away. I am also tinkering with the idea of setting the top box in place of the pillion seat, since I ride solo 99% of the time (the wife is terrified of motorcycles and my doughter is too young). I like the "rails" idea... I'll explore that option too.

Come to find out if their wives were on with them the wobble disappeared. So there is an easy experiment for you. Go try it and see. Let us know.

Since my wife won't ride with me, I'll try one of those other guys' wifes... Lets see what comes out of it...

 

[FJRPierre]

but things that can effect it are  : riders size..and here width matters more than weight..

that explains it... (seriously I think)