Bagger wobble exposed!
Seasoned HDF Member
Joined: Dec 2005
Posts: 19,830
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From: poway
So the FXR has a different frame stiffness to the baggers near tbe rear mount. The centre crossmember helps tie in the bottom frame tubes with a boxed section not just a thin flat plate with single line of weld as at the baggers have at the rear.
One way to check the frame stiffness in this area is ondo the bolts that hold the rear mount plates in place untill you have lost the preload. Take an accurate measurement of accross the frame then bolt the plates up again to see if it has spread the frame at all when the preload comes back on again.
One way to check the frame stiffness in this area is ondo the bolts that hold the rear mount plates in place untill you have lost the preload. Take an accurate measurement of accross the frame then bolt the plates up again to see if it has spread the frame at all when the preload comes back on again.
I've not check this but have checked deflection with dial indicators. Use a pry bar between the swingarm and frame. Not much difference when the preload is the same..
Remember the cross brace works under tension. the box on a FXR is thinner material than bagger flat plate..
Tourer
Joined: Apr 2014
Posts: 288
Likes: 3
From: UK
I think you find the amount of flex depends on the preload of the donut more than frame stiffness.. I would appreciate the data you have on this..
I've not check this but have checked deflection with dial indicators. Use a pry bar between the swingarm and frame. Not much difference when the preload is the same..
Remember the cross brace works under tension. the box on a FXR is thinner material than bagger flat plate..
I've not check this but have checked deflection with dial indicators. Use a pry bar between the swingarm and frame. Not much difference when the preload is the same..
Remember the cross brace works under tension. the box on a FXR is thinner material than bagger flat plate..
My hypothesis on the centre crossmember is that spreading load on the mount puts the rear crossmember into tension and acts like i pivot and the resulting in a force trying to put a centre crossmember into compression.
Seasoned HDF Member
Joined: Dec 2005
Posts: 19,830
Likes: 7,968
From: poway
I have measured the rubber donut in compression and its nearly 10,000 lbs per inch. These are mounts i have in the tool box, not sure if they are genuin HD but they are in good condition. Working together makes 5,000 lbs per inch as the are in series. So you can work out the spreading force based on preload you measure. Because of this high stiffness, any variation in preload due to frame tollerences could have a significant effect on performance.
My hypothesis on the centre crossmember is that spreading load on the mount puts the rear crossmember into tension and acts like i pivot and the resulting in a force trying to put a centre crossmember into compression.
My hypothesis on the centre crossmember is that spreading load on the mount puts the rear crossmember into tension and acts like i pivot and the resulting in a force trying to put a centre crossmember into compression.
Tourer
Joined: Apr 2014
Posts: 288
Likes: 3
From: UK
Not measured the frame, I don't have access to one at the moment. It would be interesting to get an accurate measurement of the frame width at the mounts with and without the mounts preloaded. With the stiffness of the mount and the deflection of the frame (if any) its possible to work out the stiffness of the frame at that point.
Banned
Joined: Apr 2014
Posts: 678
Likes: 1
From: LA
Funnily enough, I was doing that just last night considering the idea of putting a bold in struct between the top two swing arm mount bolts. Unfortunately, their holes are not thread all the way through but it strikes me it should be easy to do.
You're making me thing it would be best to check whether preloading the frame/swing arm compresses it any.
As an unqualified amateur, it strikes me resonance is our enemy here and what all the bandaid approaches do is alter that so it no longer happens at working speeds. What that resonance is happening in the frame or just the rubber swingarm mount, I don't know.
I've read of others theorising that the frame stresses "pivot" around the bottom crossmember as well ... but what of the entire 'belly' of the frame?
Does it swing like a pregnant sow, or is the problem merely the chassis hinging at the swing arm mount?
I measured the tolerances of my swing arm pivot and it's at about 4 and a bit thou, I reckon. This allows for about a 10 thou back and forward movement. The STA-BO guy says he has seen them wallowed out to about 30+ thou but that 10 is the recommended limit.
What would be an idea tolerance? Not H-D acceptable but real world best?
Should it be a sliding fit or would heat seize it up?
Who is the guy who sleeves the transmission hole to fit?
Last edited by Dun Roamin; Aug 19, 2014 at 09:37 PM.
Thread Starter
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Club Member
Joined: Jun 2007
Posts: 45,429
Likes: 2,897
From: Bedford UK
Allow me to point out that all Buells and the rubber-mount Sportsters use the same principle, except they have a third rear stabiliser. Be careful what you call a 'bandaid', as you are at risk of losing sight of the fundamental concept of these bikes!
HDF Community Team
Joined: Nov 2012
Posts: 6,186
Likes: 2,412
From: USA
Working together makes 5,000 lbs per inch as the are in series.
Tourer
Joined: Apr 2014
Posts: 288
Likes: 3
From: UK
I would expect the stiffness ratio on a rubber profile this shape to be about 10 to 1 compression to shear (radial).
The interleaf plates increase the compression (axial) stiffness without increasing the shear (radial) stiffness. Its a cleaver design.
Last edited by 4_stroke; Aug 20, 2014 at 05:15 AM. Reason: added info
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