Experience and sometimes just have to try it to figure it out. For new big jumps, I usually will watch someone else do it and try to gauge the speed. If I can, I also try to follow someone that is doing it in practice before hitting it.
I’ll still sometimes over or under jump a jump I’ve done before and I just use that experience for the next lap.
Most jumps require much less speed than you think. 3rd gear 1/4 throttle will be more than enough for most jumps at local amateur tracks.
Vet AM rider in Western NY
For smaller jumps of, say 20-40 meters you can kind of get sense of speed based on trial and error and experience. The suspension of the bike gives you enough margin for error.
You can also design jumps in such a way that they’re tolerant of varying errors in airtime.
For very large jumps you want to measure the exit and landing ramp angle then get someone with engineering knowledge knows how to massage the relevant physics equations and can give you the exact speed you need for landing. Believe it or not air drag starts to become an important factor in those equations faster than, say 80km/hr
Travis Pastrana, the original moto x innovator just to do high school physics, using free body diagrams for most of his stunts ack in the day. They also have courses at home that have the exact same jump with a type of air cushioned ramp to help with fall while perfecting it. When you see these guys perform it in front of people they can do it it literally 99/100 or better on the practice ramp before attempting it on a ramp without the safety guards never mind in front of people.
Same way you know whether you can you can jump a gap by standing and jumping, a walk and jump or if you need a running jump.
Same way you know that when you’re driving, going 60mph round a bend would be too fast and you wouldn’t make it, but you can at 40mph.
Same way you know how hard and high to throw a ball to someone standing at varying distances.
It’s practice which leads to instinct. Start small and slow and build up speed and distance. It’s obviously a lot more complicated and more involved than my examples but the principles there.
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