What is the minimum carb intake to not bonk (but also not put on weight)?
What is the minimum carb intake to not bonk (but also not put on weight)?
Simple Answer: 30-60g of carb per hour to not bonk (can't comment on the weight bit though).
*Real answer:
There is no calculable answer for these reasons:

The type of fuel you burn is dependent on the intensity of the
exercise (heart rate zones)
The amount of fuel you burn is dependent on your efficiency
The starting amount of fuel is determined by your diet and build

Fuel Type
At &quot;walking intensity&quot;, you burn fat, and because fat is so energy dense even a very lean person could walk for days without running out of fuel and bonking.
At higher intensity, you burn a mix of glycogen store from your muscle and sugars being digested from the food in your gut.
At very high intensity, all energy is directed to your muscles and so your digestion stops and you will be relying purely on glycogen stores.
Note that fitter athlete will be putting out more power for the same heartrate, so how your body responds is specific to you.
This conversation is about carbs, so you would need to know the intensity at which you'll be cycling to know what fuel types you'll use in what proportions.
Fuel Amount
Every body works differently, and many variables go into how efficiently you can convert fuel into power output. For cycling, things like how good your bike fit is, your fitness level, your years of experience on the bike, injuries and flexibility can all affect your efficiency.
For this conversation, this variable is basically unknowable. Professional athletes might have a good idea of their efficiency, but it would be pretty rough.
Starting Fuel Reserves
You start your ride with your glycogen stores and the contents of your stomach. It is easy to control the latter by making the meal before riding carb-rich. Glycogen stores can also be optimised by carbo-loading. There is a university study (that I can't find right now) that demonstrated that in the 24hr before an activity, the athlete could more than double their glycogen stores by eating ridiculous amounts of carbs.
For this conversation, your general diet and you diet in the last 24 hours would be very relevant.
Conclusion
While there is a generally accepted minimum carb consumption to avoid bonking (30-60g of carb / hr), the specific question also required not having surplus energy intake an so to avoid putting on weight. The question requires an exact calculation, which is not possible.
Relaxing the constraint on &quot;exact intake&quot; but keeping with the spirit of the question, the suggested solution is to eat lots and ride even more.