The key difference in descending stability between bikes is how easy it is to get your butt right back over the back wheel.
At one extreme you have mountain bikes with dropper post which allow you to get the seat out of the way entirely and squat just above the back wheel. At the other extreme you have TT bikes with ISM/cobb style styles where you sit off the front of the saddle and the shape of the saddle discourages you sliding backwards.. Adding rear hydration makes the situation worse because you are physically prevented from getting over the back wheel. Road bikes sit in the middle because you sit towards the rear of a road bike saddle and can easily scoot forward and backward.
Its this saddle geometry issue which means a properly set up TT bike will never descend as well as a road bike. Others in the thread have mentioned the natural weight distribution of a bike but this is not totally relevant because stable descending requires getting your weight well back of where you normally sit for pedalling. This is just as true on a TT bike as a downhill bike even though the bikes have completely different geometries. So if you want to feel more stable just make sure you can push yourself back in the saddle.
At one extreme you have mountain bikes with dropper post which allow you to get the seat out of the way entirely and squat just above the back wheel. At the other extreme you have TT bikes with ISM/cobb style styles where you sit off the front of the saddle and the shape of the saddle discourages you sliding backwards.. Adding rear hydration makes the situation worse because you are physically prevented from getting over the back wheel. Road bikes sit in the middle because you sit towards the rear of a road bike saddle and can easily scoot forward and backward.
Its this saddle geometry issue which means a properly set up TT bike will never descend as well as a road bike. Others in the thread have mentioned the natural weight distribution of a bike but this is not totally relevant because stable descending requires getting your weight well back of where you normally sit for pedalling. This is just as true on a TT bike as a downhill bike even though the bikes have completely different geometries. So if you want to feel more stable just make sure you can push yourself back in the saddle.