Does Steering Angle and Front End Grip Outweigh Minimizing Scrub Radius?

Thread in 'Technical Questions' started by Hotpocket, Jan 8, 2017.

  1. Hotpocket

    Hotpocket New Member

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    So what's with the huge scrub radius found on many drift cars?

    Iv'e been drifting for less than a season in my 1JZ RX-7 FC, i'm seeing vehicles with absolutely excessive scrub radius, in both the amateur circuit, and the professional. Companies like wisefab even offers kits there the pivot axis of steering is literally offset to where it's outside of the wheel.

    Does excessive steering angle and the need for wide front tires just outweigh reducing the scrub radius? Huge scrub radius means the wheels are literally being dragged around the axis of pivot, which, as far as I know, negates responsiveness.

    Is this because we are generally loading up at high speeds into large corners, not having to transition often? I realize Formula Drift has very strict suspension rules so teams have to get by with what they can.

    Scrub radius an effect of your KPI, and is something I have been taught to minimize in racing because of the obvious negative effects iv'e mentioned above; that is why this question arises.

    Granted, the only car set up to drift that Iv'e driven is my own, and it gets along very well, but I would like to know the theory so that I can make smart choices as I progress.
    Thank you.
     
    #1 Hotpocket, Jan 8, 2017
    Last edited: Jan 8, 2017
  2. BenRice

    BenRice Well-Known Member

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    Ok, the scrub radius you are seeing is a result of the need for clearance caused by forklift steering kits

    Wisefab pushes the top and bottom pivots further outboard in order to get the necessary clearance from the chassis rail for the front and rear of the wheel to get to full lock

    The scrub radius increases are necessary to get clearance from the caster arm for the trailing wheel on S-chassis cars, or it's becaase idiots have bolted on 40mm spacers to help small wheels fill big guards

    Not a lot of people have considered the effect these changes have had on the cars handling, the general consensus is that more lock is better, no matter the impact it has on other elements of the handling.

    Eliminating ackerman has helped reduce the wheel drag, but also reduces the snappyness of the steering

    It all results in boring driving tbh

    IMO - the best setups i've seen on Silvia's run modified knuckles with the shortest distance possible between tie rod pickup and LCA pivot to speed up steering responsiveness, keep a little ackerman to help with snappyness and max negative camber at the hub for better contact patch / feel on the leading wheel. They don't have the biggest angle, but an extra 20mm on the LCA's and quality 215/40s seem to sort clearance issues without compromising on the important bits
     
  3. Adam-MGTF

    Adam-MGTF Member

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    This made me laugh a bit lol. I've seen more than a few cars at practice days with a billion mm of spacers. they look like FD cars at first glance then rub like a twat when you have 2deg of lock... oh wait. my practice car is like that* :smash:

    Correct me if im wrong but with extended arms like wisefab, tdp, GKTech or even cut and shut arms you move the rotation point outwards from the steering rack and lower control arm attachment point (talking nissan here)

    as a result the scrub radius isn't increased at all? because your pivoting in a small arc still?

    I'm probably wrong but I'm trying to learn this stuff.

    different kettle of karp all together compared to a stock set of arms with 45mm spacers and 0 offset 12js?

    Thats how I understand it?




    *not quite, it's just offsensive
     
  4. Jites

    Jites Active Member

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    Scrub radius is the arc your wheel makes when you turn, running 40mm of spacers increases it massively and your wheel makes a huge arc when compared to moving the pickup points on your LCA out say 25/30mm which is reducing your scrub and you wouldn't need those 40mm spacers to fill your arches anymore or that's how I understood it anyway.

    if you move the pickup point you decrease the scrub, if you add spacers you increase it
     
  5. Sea Squirrel

    Sea Squirrel Jeff Mills Is Watching!

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    As Jites said the scrub radius on these modern big angle lock kits isn't actually that massive. Don't forget that for the most part these kits come with a top mount which moves the coilover out, the extended lower arm moving out a corresponding amount keeping the KPI roughly equal to stock, the actual scrub radius of something like Wisefab isn't very great. Obviously this is affected heavily by people choosing to run giant spacers or massive offset wheels but, for the most part, you'll see the big lock stuff runs fairly small wheels at the front to assist with the last part of the clearance so as to avoid increasing the scrub radius massively. Frederic Aasbo's GT86 is a lovely example of this as that runs (I believe) an 8J 17" front with a 215 tyre so as to prevent excessive scrub without sacrificing clearance.
     
  6. BenRice

    BenRice Well-Known Member

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    Correct. You tend not to see big spacers on forklift setups due to the fact they have custom big-clearance caster arm linkages.

    The reason people still may add spacers with forklift setups is when they've ditched their 9.5" +12's for 8.5" +35 steer wheels in order to not go full mexi, but now foul on the LCA and are too lazy to change their steer wheels a third time.

    Now they foul on the inner tubs with the bigger scrub radius so cut them out, then while they've got the angle grinder & stick welder out they tube frame the front, and so the horrific cycle of ruining another perfectly good drift car starts, all because competition drifting told him he needs a minimum of 70+ degrees of lock or he spun 3 times in a row at a practice day and was too lazy to actually apply himself

    tldr: forklift steering is killing society faster than isis
     
  7. Bu11et

    Bu11et Well-Known Member

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    hahaha :cry::smash:
     

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