Yaw Response from Handbrake application

Topics related to collision & Trajectory analysis formerly on our 'Registrants only' area however which we get asked about frequently so believe shoud be in the open forum too
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MSI
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Yaw Response from Handbrake application

Post by MSI » Wed Oct 16, 2019 9:40 pm

Q: Is anyone aware of any scientific or conference papers on an inadvertent yaw resulting from handbrake application at high speed (greater than 60 mph) where no steering input was made? There is a bunch of material on handbrake turns and some grey literature but no scientific literature that I am aware of where someone has tested vehicles at reasonable speeds to assess if a vehicle yaws when traveling in a straight line and the handbrake is suddenly applied without any steering input

A: A tried and true saying which has been around for a while and which applies is:
  • The locked end will lead!
That is why pulling on a hand brake at speed will cause the vehicle to swap ends...that pesky rear end want to come around and lead!

i recall a video on LinkedIn? or possible a SAE seminar on Vehicle Dynamics by Gillespie of a simple rubber-band vehicle on an inclined surface where they run it down no lock, then front lock then rear lock to demonstrate the concept.
We have also in the past set up a scale model car (1/43 or larger( where we could insert a paperclip to lock the front or rear axle (think we used a drill to insert the clip in either front of rear). It was either for an XCar issue (on wet roads light braking could lock the rear wheels) or a proportioning value issue case (before ABS proportioning valves were used to try to keep rear from locking and it didn't always work properly)

In stunt shows (as my brother Stan is very familiar since he was the announcer at one for years back in the day!) they set the vehicle up to allow locking ONLY the front or rear wheels in order to allow a performance of the "rum runners turn" or a Boot-Leg (thanks Steve!):
  • Back up at high speed and lock front wheels as vehicle comes around then release the brakes timed so the vehicle stops rotating at 180 degrees and then the vehicle can proceed on down the road at speed!
  • NOTE: for hand-brakes they generally disable the ratchet so easily pulled on/off and do not lock on
This will happen with wheels straight or with steering included:
  • For wheels straight it still will happen BECAUSE of the slight offset from absolute perfect 0 yaw/heading that every vehicle has no matter how symmetrical you try to make them!
  • steering can speed up the process if done properly (and long enough so rear end comes around)

My father Ray McHenry also mentioned that he recalled a paper by Hans. B. Pacejka, who used to work with Ray at Calspan, which technically explaned the physics behind the phenomena.
It is probably contained in his 600+ page book Tire and Vehicle Dynamics
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