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My dad has a 2008 Anniversary Edition Ultra Classic. It has a 21" front wheel (not sure what size the rear is, but it matches the front). It is a laced wheel, but I don't know if it was "stock" for the anniversary edition or if the dealer added it. I can only guess that it came from the factory with these wheels, as the spokes are color matched to the anniversary paint.
Anyway, the bike has a wobble at any speed above 40-ish. I checked and adjusted the steering head bearing. It was already ok, but I tightened it so it would be on the tight side of spec. The bike also has a True-Track on it, which was installed at time of purchase.
I inspected both front and rear wheels for any "play" in them, but everything checks out ok. I looked over the frame for any cracks, but again everything looks ok.
So that brings me to my question... Is this wobble maybe just a character of a large wheel? I've never ridden a bike with 21" wheel, so I'm not sure.
If a wobble was part of the 21 package, they would not sell many of them. Plenty are sold as are 23 and 26 to the extreme. You said they were spoke wheels. I would check to see if the spokes are tight as they can loosen over time. Next I would check for broken motor mounts and next check for play in the swingarm cleve blocks. Sounds like you have a problem somewhere and may be a safety issue to ride.
as mentioned, check the spokes and check wheel balance. Personally I dont like 21s on a bagger for looks and Im willing to bet a bagger with 2 up is exceeding the load rating of the front axle. The bike is too heavy to have something that light on the front end. JMPO
A larger wheel will exaggerate any imperfection (like an out of round situation) in the wheel. They're also generally heavier, which means more unsprung weight, which means poorer acceleration, braking, and handling.
A wobble isn't a characteristic of a larger wheel, but a larger wheel will negatively affect ride and handling with less wrong with it than a smaller wheel.
I used to have a bit of shimmy in the front...had the wheel checked...out of balance and a handful of spokes a hair looser than they should have been. Took care of those two issues for complete elimination of wobble.
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