I am having my first attempt at making a full frame bag for my cyclocross bike, I have been reading online and some frame bag widths go upto 120mm which is massive IMO, so I was wondering what width should I make the frame bag, to ensure no leg rub.

I think my bag at the edges where it is next to a tube is roughly 2 inches, but when you pack it fuller, it gets thicker. I would guess that the thickest point on my bag is 3.5 inches in the middle where I crammed it fuller.

My bag is slightly smaller than full triangle, in the photo you can see I can put one water bottle in the frame. It is the large size Ibera that I got on Ebay for about $15. Also, I added straps to the head tube and downtube, I am not using the strap that came on it to the seat tube. I am guessing it is about 5 liters in volume.



You might get better luck getting answers for home made bag at this site:
DIY / Make Your Own Gear (MYOG)

I leave tomarrow for my first trip where I am using the bag, ask me in a couple weeks how it worked.

Thanks. $15 sound like a bargain, let me know how it goes if I forget to ask.

I expect you need stiffeners on both sides so it wont bulge outward requiring a Bowlegged pedaling technique.

About 2" or so... although the one I have on my Pugsley is tapered so it gets thicker as it gets closer to the head tube. I think its 4" wide there.

No.
Unless you have a freakishly narrow Q, which isn't common on typical off the shelf bikes these days, or have some weird leg geometry.
I can stuff my bag full of a hydro bladder, clothes, tools, etc... never give it a though.
Only time I brush the sides is when I stop and keep a leg over the bike clipped in and I stand at a light or to chat... otherwise, it doesn't happen with all the typical bags I've used.

what is a narrow Q?

When You are just barely having the crank arms clear the chainstays on both sides..

https://en.m.wikipedia.org/wiki/Q_factor_(bicycles)

Distance between the inside of the pedals where they meet the cranks.

Not entirely true. The Q on my Pugsley is very different than my Krampus and that is very different than my IF Club Racer. But they all have relatively small distances from the inside of the crank to the chain stay.

Trip did not go as planned, we got there and decided that the trail was too tough for us. So, we car camped in the campgrounds and day tripped on the trails. We saw three other groups trying to tour/bikepack the trail and two of the three groups had run out of water hours before they got to the first campground, the one group that did not had done a six mile detour to stock up on more water. Looking back, we made the right decision.

Now, back to the topic of frame bags, my frame bag worked great. This was the Large size Ibera bag on Ebay for a really cheap price. There are a few out there that were called "large", the one I have has two zippers. As I noted above I hand sewed on a strap to attach to headtube and to down tube, I did not use the strap on it for the seat tube. As you can see it fits in the front part of the triangle very nicely in my frame, I think I got lucky to find a cheap bag that fits so well it almost looks custom.



I almost never noticed it was even on the bike, my knees never rubbed on it or had any other problem. I am guessing that total volume was about 5 liters. I carried my spare tubes, tools, about a liter of water and some other small heavy stuff in it. One day I pulled out the water and instead carried my rain gear in it. You can see in the photos below that it was pretty narrow in the frame, I am guessing about 3.5 inches (~~ 85mm to 90mm) where it bulged out the most. You were concerned about width, I think a good rule of thumb would be any width that is as narrow as a water bottle is unlikely to be a problem and if you can figure out what part of your triangle your knees could contact it, any part of the triangle forward of your knees could allow the bag to be much wider.



To avoid having the straps rub off the paint on the frame, I cut strips of inner tube rubber and wrapped it around the frame, then wrapped velcro around that to hold it in place. (Electrical tape probably would have worked just as well, but on my frame the clear coat over the paint flaked off when I did that before with electrical tape.) Then the bag straps went over that.

OK, so the Pugsly does not have a Narrow at all Q.. why would it ? its got really wide Tires ..

the limiting factor on how narrow the theoretical Ultimate Q can be, is the frame .

Straight arm cranks Like TA traditional Cyclotourist .

are required to get the narrowest distances between the pedals .

Now most crank arms got curved to be Usable on MTBs .

Then the style became what the companies made .. less hitting your ankle on the crank arm, likely.
so some other foot on the pedal angles are usable.

as to the bag width, the OP's Knees to the bar style pedaling If that is what is comfortable .

will limit the bag width .. measure the width you need . its Your Bike .