High speed decel wobble?

[Mike G in SC]

So, if you were way out in back country, good road & visibility, you know your bike is in good shape (tires, shock, steering is tight, stock windshield), and you should take it up to 110 MPH (Mom, don't read this), what would make it get a steering wobble if you let off the gas to coast back to highway speed.

 

Testing the (RSTD) bike yesterday, after a new rear shock was installed, I took it to speed. Acceleration and sustained speed 100 to 110 was fine, but when I let off to coast back to 60 it started pretty serious wobble. I corrected it by bring the engine back up to speed and decelerating more gradually. Seems, perhaps, the engine torque did not agree with something about the quick drop in speed. As it was correctable by increasing the RPM's, I don't think I can attribute it to steering (pretty new Avon tires and the steering head is tight).

 

Ideas,,, (besides don't do that)?

 

Mike G in SC

[atlm]

When you let off the throttle, you're loading the front end more. When you give it gas, you're shifting the weight back again. So, it's not all RPM related. Just something else to think about.

 

[bryan52577]

I had brand new Avons put on my o5 and had the same problem. Wobble / shimmy when letting off gas. Took it back to the shop and dealer lifted the bike up and spun the front tire. That is where it got interesting, bead was set good and true. But tire had a bow in it side to side and was not "round" tire would go in and out as we spun it. Only time I could tell was when I let off the gas.

Avon sent a new tire "free" but it didn't get here by the long weekend. So dealer put on a Dunlop E3 he had (we tested the bike with it on before telling Avon too) and I bought it. Turns out it was of the 130 size not the 150 size...... After just 100 miles on the 130 I will never put the 150 back on! WOW is all I can say. " Now I have a 150 new Avon just sitting in my garage."

Avon had less noise than the Dunlop and Dunlop or Avon don't matter to me but I am hooked on the 130 now.

Long story short have the front tire checked, (even new tires can be bad) or the steering head bolt could be loose too.

 

Bryan

[V7Goose]

So, if you were way out in back country, good road & visibility, you know your bike is in good shape (tires, shock, steering is tight, stock windshield), and you should take it up to 110 MPH (Mom, don't read this), what would make it get a steering wobble if you let off the gas to coast back to highway speed.

 

Testing the (RSTD) bike yesterday, after a new rear shock was installed, I took it to speed. Acceleration and sustained speed 100 to 110 was fine, but when I let off to coast back to 60 it started pretty serious wobble. I corrected it by bring the engine back up to speed and decelerating more gradually. Seems, perhaps, the engine torque did not agree with something about the quick drop in speed. As it was correctable by increasing the RPM's, I don't think I can attribute it to steering (pretty new Avon tires and the steering head is tight).

 

Ideas,,, (besides don't do that)?

 

Mike G in SC

Well, since you said you already KNOW that your bike is in good shape, including the tires, shock and steering head are all in good shape, then nothing could cause it (assuming you meant to include the forks in the general "shock" category).

 

So something you "know" ain't known right.

 

Most likely cause is the front tire. Check concentric bead seating and trueness. Next, check the aire in your front forks - MUST be absolutely equal - when in doubt, let all the air out. Now re-check the steering head bearings. Just because they are tight, doesn't mean they are good. Perform the test shown in the owner's manual, where you raise the front wheel off the ground, grab the bottom of the forks, and push/pull to see if you can detect any slop or movement. In every bike I have tested with shot bearings, they still had this slop even when tight. The last thing I would include in likely candidates for this problem is the swing arm bearings.

 

Of course, frame damage would also cause something like this, but I asume you'd think of that if your bike had ever been hit or dumped.

Goose

[Pecker]

Mike, what shock did you put on, a stock replacement or an aftermarket? My reasoning is this, if you put on an aftermarket, you may have raised the rear end a touch and thrown the steering geometry a little out of whack. So when you really load the front end on a hard deceleration, your rake and trail figures will be off and cause issues like this.

If you put a stock replacement on, I would play around with the air pressures to see if the problem goes away.

Hope this helps, let me know if you would like to get together and let me take a look at it. I'd be happy to help.

Dave

[Mike G in SC]

Okay, yep, so I see, I still have a few things to check.

 

So, what I hadn't said yet:

I have not checked the front fork pressure lately.

Have not done the test for the bearing slop.

The new rear shock is Yamaha, put on my Matt in Sumter (good mechanic).

I do have the Barons rear lowering kit but had dogbones made to only drop it 3/4".

I did lower the front 1/2", which is about all you can do on a RSTD.

I do have the smaller MT90B 16 74 Avon Venom (like the 130).

(So the bike is basically back to level but down 3/4".)

 

I'm caging it out for the weekend but will do the checks during the week.

 

Thanks, Mike G in SC

[gunboat]

hi mike

you have been to "wall" as i call it. this has happened to me on my rsv. when i still had the m/c tires on it. as you i had her wide open, when as an other vr rider said the "pig slaping began". he had just installed new m/c tires front & back (only had a couple hundred miles on the set). he went back home after the ride and rebuilt the front end. you did just about everything i did to recover from this wobble. welcome back from the edge.

reguards

don c.