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Whoops, BRAP!

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I was at a new track last weekend and they had a nice set of whoops. I love getting to blitz across a set of whoops and you don't get to see that much anymore in MX around where I live. I used to ride the whoops hard and could always weight the back, keep the front up, and just skip across the tops in 3rd or 4th pinned. Granted I am a little rusty from being off for a while but I had more issues than that this weekend. For some reason my front kept diving down. I would start to get moving then all of the sudden the front would just kick down and that's all she wrote. I tried different approaches, techniques, etc. I am thinking the rebound may need to be slowed down a little but I wanted to see what you all thought. I also wonder if this could be related to the rear kicking on certain braking bumps as well.

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Yes, it more than likely is related to the rear kick, but that can be a problem of compression, rebound, or both in combination.  Either a good high frame rate video that can be played back in slow motion, or some very careful observation by a third party is the best way to diagnose this without a bunch of trial and error. 

 

In that section, since the front is up, it sounds as if the rear end is pitching upward and causing the chassis to roll on its Z axis, dropping the front. 

 

  • Excessive compression damping can cause this by not absorbing enough of the impact with the face of the whoop mound, thereby setting up the forward pitch.
  • Inadequate compression damping can cause this by allowing the rear to bottom on impact with the mound face, which will abruptly transfer energy to the rear and pitch it forward.
  • Inadequate rebound (a common issue with KYB's) can cause the problem by initiating the rebound stroke too early after the initial impact, pitching the chassis up and forward.
  • Excessive rebound can cause it by "packing" the rear suspension (not allowing it to extend fast enough), which has the effect of putting the rear in a "pre-compressed" state as it hits the second or third impact, thus shortening the effective compression stroke.

These elements must be balanced with each other.  A video will really help you figure it out.

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I was leaning towards rebound, but after your comment, maybe it is compression. I felt like the rear was a little soft a week earlier at Broome Tioga. Maybe I wasn't imagining it. Would this be high speed or low speed compression?

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If you are suggesting reducing the preload on the rear spring, that does not make the rear suspension softer, because it doesn't change the spring rate, and it has no effect on the damping rates.  All that does is adjust the loaded ride height, which can affect steering by changing the angle of the steering head tube on the frame. 

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