Further to a previous thread, it looks like I'll be going down the custom frame bag route. I was just wondering what sort of width folk were using. It strikes me that anything up to piss bottle width would be fine - as long as there was some way to stop the contents lumping together (dividers etc) and that some added width at the front wouldn't get in the way.

I have a very wide custom wildcat bag which is 9cm. It's a half bag and has cutouts for the shock too. It's not very deep and so can't bulge much. It works perfectly. No knee strikes

No reason you could have a deeper bag that wide. As you say would need to have something to stop the bulge.

Depends on the type of bike I think. I have a Genesis High Latitude with an Alpkit full frame bag. I went for the middle option (5cm) and a single big compartment with a surgeon zip. I have to be careful packing it otherwise it bulges out quite a bit and rubs my knees. Probably wouldn't be and issue on a fatbike wih wider cranks. If I was ordering again I'd get the same width but have two sections with a horizontal divider to keep it more contained.

There's a pic here if you want to see the dimensions https://m.flickr.com/#/photos/ian_fitz/30868736403/

Mine is 80mm wide and has a removable horizontal divider to keep things in check, but I normally remove it. It only really bulges if it is packed out with the likes of clothing etc.

Alpkit custom on the Vir Fortis. 8cm at the seat tube end going to 10 at the headset end. It has a strap that allows you to pull it in if needed, but I have never had an issue with it. They made it specifically so a mug pack would fit in it.

Cheers folks. I was thinking 75-80mm might be OK. I can see that a fatbike with its wider Q Factor would get away with that anyway. I reckon some sort of internal "non-bagging" fixture like a divider or strap means it should all work.

Message to Beth I reckon.....

Mine has surgeon zips with a horizontal divider that velcros in. Works very well.