When you watch downhill, the world cup’s in particular where the courses tend to be the most technical and hard on the bikes, you often see guys losing a chain, having it jam or even break completely. It makes you wonder why they even run gears at all and don’t just go for a single-speed set-up.
The riders could try different gear ratios in practice and settle on one for the race. There don’t tend to be many pedalling sections in most courses anyway so you probably won’t lose much time even if you slightly miscalculate your gear. Plus, who has time to change gears when you’re hurtling down a mountainside trying desperately just to keep hold of the handlebars for 3 or so minutes?
By the time you actually find the right gear on your cassette you’re likely out of the pedal section and back into the technical parts or flat out downhill where your best bet is to tuck in tight and hold on.
This is true of FMB riders as well. It’s a gravity based sport so even though there are some pedalling sections in some courses, generally you don’t have to do much of it if you stay on course. I get why some tracks work better for short travel full suspension bikes instead of the generally preferred hard-tails but I can’t really ever see a situation where you would benefit from running gears. One brake is also probably enough for most FMB courses, but of course, not in downhill where you need two of the best brakes money can buy.
But to summarise, I want to see single-speeds in downhill racing. The bikes would be simpler, more durable and more beautiful too. If it’s not allowed in the technical rules currently, change it UCI.