Why do bike wiggle when hard on the brakes?

[Steviebee]

OK experts help me figure this out.

Why do some bikes wiggle under hard braking and some do not ?

And what would you change to reduce it ? more/less fork damping ?   fork oil level change ?

[Woofentino Pugrossi]

Probably just depends on whos riding.

[grasshopper]

Alot of the guys that are wagging their tails do so because the rear end gets light from being hard on the brakes. I'm sure it's a combo of alot of different characteristics. Front and rear suspension set-up, tires, RIDER, track. Most likely the rider. Watching the pro guys, they are doing alot of backing the bike into the corner, pretty amazing stuff. Flat Track style of riding. You can really see it plain as day watching super moto as well.

[Steviebee]

Theres a difference between "backing it in" and having the rear "unstable" .

Watching the SBK race the Duc was wagging alot, wereas the hondas seemed alot more stable on the brakes.

So back to my question,  what part of the setup effects this.

Im fishing for the so called experts to explain this so all you newbs can learn.

[Jeff]

when  the front forks get at or near bottom, it lifts the rear end.  Imperfections in the tire/pavement/position of the rider will affect the front end and this will transition through the bike and wiggle the back end a touch.

[tzracer]

Never had a bike do it.

From watching, it would seem to be dependant on weight bias and how engine braking (slipper clutch?)/rider handles downshifting and braking. Might be a very mild form of wheel hop.

[roadracer162]

The only experience with the rear end being unstable during braking is when my FZR had the stock set up for the forks.  Once I installed stiffer more appropriate springs the whole bike became more stable on the brakes.  Using my very technical zip tie on the front I found that the forks would just about bottom out under severe braking and wouldn't with the newer stiffer springs.  No other modifications were tried but just the springs.

[Super Dave]

Steve, it happens because of various issues.  WSB aside, etc...that's because the whole bike is light.

In club racing, it's usually a set up issue.  Might not be enough preload on the forks.  Might be other issues too.  

What did you do to try and change it?  If you did nothing, well, nothing will change.  The bike can certainly go faster, it's just a matter of set up.