R100GSPD Better for Gas Tank to be Full or Not

The big 8.5 gallon tanks on these bikes are plastic. For normal storage periods, 2-3 weeks, are the tanks better to be kept full or not?? Does a full tank present more risk of plastic failure????

I know a partially filled tank can induce more internal condensation since more moisture laden air is present inside the tank but I'm wondering about the stability of the plastic material of these tanks.

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I'm not sure it really matters how full the tank is if the bike is only sitting 2 or 3 weeks at a time. I don't think gasoline sitting in the tank is a issue. I believe the problem might be if the fuel in the tank detrtioates or varnishes to the point that it makes it necessary to clean the inside of the tank. At that point I'm not sure that whatever cleaning solution you might use could damage the inside of the tank.
If it puts your mind to rest, I've owned three GSPD's in Phoenix, AZ where temps get ridiculous in the summer. Some of those bikes sat for months without ever showing any sort of tank problem. I never took any precautions. Diffrent story with a couple of bike I owned with steel tanks.

If it were me personally, in warm or hot weather storage I'd keep the tank low and refill with good fresh gas just before using.

In cold storage, I'd keep it full to keep out moisture.

I expect the real issue is whether ethanol affects the plastic in these tanks. If non-ethanol gas is not available, a gas treatment that claims to counteract ethanol couldn't hurt.

ccolwell said:

I expect the real issue is whether ethanol affects the plastic in these tanks. If non-ethanol gas is not available, a gas treatment that claims to counteract ethanol couldn't hurt.

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That is a big concern for me. I think I have all non-ethanol in the tank if I can believe the pumps from which I filled it.

So many plastic tank bikes that I am familiar with have issues with non-ethanol. I have seen them swell and manifest other problems. I have not yet seen many accounts of issues with these GSPD tanks but they are somewhat from the past and not great in number.

Did BMW coat these tanks on the inside with any sort of liner??

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I do recall a generation of red and white plastic R100GS tanks that allowed fuel vapors to migrate through the plastic. This caused the paint on the tank to blister. There was a recall to replace these tanks with steel tanks, I think. I do not remember any of the details however. I would be pretty sure Snowbum does.

Snowbum does have a blurb on alcohol in plastic tanks. See Section 7 here:

https://bmwmotorcycletech.info/fuelmiscl.htm

No effect of you're talking that short of time. If you ride mostly street, half a tank lowers the center of gravity...more pronounced if you have the late model nylon tank since the gas is down lower.

I modify the nylon tanks so they look asthetically great on the R80G/S...then you can get almost 400 miles if you keep it around 65.

Some of the early tanks has issues with gassing through and over time there'd be blisters showing. A good painter who knows the right primer can solve that problem. It's like working with Teflon..so almost nothing sticks.