harmonic shimmy

[how]

This is just a warning to any Rad Rover step-thru owners who are choosing to add a second battery to their rear rat trap. 

I occasionally & momentarily sit upright and take my hands off the handlebar on long rides (when I deem it safe to do so) to stretch while riding.  Now that I've added a second battery in a lockbox on the stock rad rear carrier I have found that a harmonic oscillation can randomly arise that could take a rider down if they did not get their hands back on their handlebars within 3 -5 seconds. It appears to be connected to the location in height and weight of my lockbox(containing 2nd battery, battery rack, stock tool kit, electric air pump & parallel battery controller all totaling about 12-13 lbs) over the rear tire that can cause a compounding shimmy in the front handlebars/ tire when a rider's weight is not leaning forward in the usual bicycle position to countermand it.
So if you're trying to decide whether to put the second battery on the front of the step-thru frame or the rear rack, this might be important.
I have decided to keep it in the rear because of the security and the weatherproofing that a rear lock box can offer.
Cheers all.

[Ddaybc]

That's the same issue motorcycles have with the rear trunk type of "saddlebags". One needs to keep the hands on the bars or the front wheel begins to shake. On some bikes the design is such that it doesn't happen but I don't know the how or why of it.
On your bicycle try sitting upright and keep one hand on the bar, stretch and then switch hands. That's what I used to do on my motorcycles.

[Veggyhed]

How strange. I wonder if it would happen if the weight was lower. As another post around share that's similar to this in the person was carrying a kid if I remember correctly.

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[Altema]

I have the same problem when I use my rear battery. The rear weight bias make the front end light, and you get a harmonic oscillation. If I take off my rear battery and just use the middle battery, I can ride for miles with no hands. I did find that mounting my heavy folding lock to the front of the bike improves the situation, and allows me to take my hands off the bars long enough to quickly adjust my gloves or helmet. Using a lighter battery in back (9 pounds versus 15 pounds) also improved things.

[Radio Runner]

I like what you did to the stem. How much steer tube is left out the top of the headset to mount the adapter?

[rjp]

I have the same bike and when I put my spare battery in my luggage bag on my rear rack I also get the same wobble. Take the battery out of the bag and wobble goes away. I'm glad to here I'm not the only one experiencing  this issue. I also have a stem riser like that one only longer. There's plenty of stem sticking up to install the riser to.   

[how]

One limitation to stem rise on my bike was that my handle bar electrical and brake lines became more stressed the higher I raised it.

[Tony]

Your problem is the extra weight you carry is too high up and too far back.