Been riding most of my life, heard of a tank slapper but never saw it.
last weekend I was leaving the house, wound up pretty good in first, shifted to second and took both hands off the grips to adjust and the bars started back and forth vibrating. Tire pressure checked out at 36, bearings are not letting the rim wobble on the axle, tire tread is 4/32".

its a 19 Roadking

 

 

First: Up in these here parts, tank slapping is when your chest hits the tank going over a jump.

Second: They all do that. When you let go of the bars at too slow a speed.

 

or during steep downhill high speed braking bumps before a corner

 

1. How many miles on the bike?
2. Any modifications to the bike's front end or rear shocks (rake & trail)?
3. Check tire balance.
4. Check head stock bearings for wear if the bike has many miles on it.

NOTE:
The above information, while credible, is worth what you paid for it and should not be considered gospel.
Do due diligence, research and have a H-D Field Service Manual before working on your bike.
You will probably have many opinions and suggestions presented; make sure you research the person providing the helpful information as there are some who provide good information that is useful and some who speak out of other orifices with gibberish. You will figure out this quickly.

 

 

Tank slapper??? I thought that was only in porno flicks.

 

I stil ride a WR490 dirt bike hard. Call me when it is hitting lock to lock faster than you can count.

 

I had it happen years ago on a sport bike and it turned out to be low tire pressure on the front wheel.

 

A worn front tire can cause this.