Do fat cyclists bonk? (this isn't a discussion on the attractiveness of large cyclists and likelihood of sexual congress...)

In my cycling career I have varied between 22 stone and 15 stone. At 6'2" the leanest I have ever been is still over 20% body fat.

I am not sure I've ever bonked...

My normal cycling day is an 18 mile ride into work before I have eaten or drunk anything. I have a yogurt granola thing for breakfast and salad for lunch. Some fig rolls mid afternoon then cycle 18 miles home.

I have done 80 mile training rides and really not eaten anything much.

I do sometimes feel tired and a bit weak but I thought that bonking was worse than that.

I assume that as I carry quite large energy reserves around with me that I don't bonk.
Bonking occurs when your body cannot metabolise stored fat (and muscle protein) fast enough to replace the glycogen reserves, you deplete the reserves in your muscles and liver, and eventually you run out of glycogen. Fat conversion requires high levels of oxygen, and is slow, so once those reserves are gone, your blood sugar plummets. 

I suspect your body has adapted to your regular routine of 18 miles, after an overnight fast, on an empty stomach. You will not only have large glycogen reserves, you will have also adapted to convert fat efficiently while exercising.  Therefore, your observation of what happens for you is based on how you body responds to exercise rather than the amount of fat you have in reserve.  Additionally, 80 miles is only twice your daily commute distance (presuming you ride home), so its not a big increase over your normal weekly workload. 

Even the slimmest person has more than enough fat to keep them going provided they are exercising at a level that fat conversion can keep up with. Therefore "bonking" or not is less about how much fat you carry and more about how adapted you are to endurance sport. 

Another aspect of this is how fast you are riding. If you are riding at your aerobic threshold, as people who are racing do, you will almost certain bonk on an 80 mile ride unless you consume food, however, if your 80 mile ride is a leisurely pace and you can hold a conversation (implying plenty of blood oxygenation), you probably won't Bonk.  

Don't give up hope of ever experiencing it. One day, it will happen. :)