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Thread: AE86/XE7X with RA40/60 struts bump steer 3D explanation

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    Warning! This might take a minute or 2 to load.

    I got bored and did a 3D model to explain what words have great difficulty to express.

    This comes up so often it needs to be explained properly. Steering arms are meant to act on the same plane as the steering rack.

    The further you are from the plane of the steering rack, the more bump steer you get as the steering arm moves in and out as the strut compresses and rebounds because the pin for the tie rod is not perpendicular with the rack.

    Don't try and look at the whole thing, just watch the link pin for the tie rod end.

    Not 100% to scale, but it clearly shows what's going on.

    Corolla Strut Not too bad in a lowered position, but the steering arm is not on the same plane as the steering rack.


    RA40 Strut Fitted to corolla with longer LCA's and/or camber tops ... Steering arm is not acting on the same plane at all.


    Now lets have a look with some Roll center adjusters added:

    COROLLA RCA Notice that the steering arm now nearly operates on the same plane as the steering rack



    RA40 RCA There is an improvement of the angle, but it still isn't as good as a factory strut.



    To some people this kinda thing won't worry you, but it's all about how hard you drive your car, or if you're planning to race competitively. Things like this to the eye don't look like much, but when it comes to shaving a few seconds a lap on the track to mow down a racer with more power it can mean all the difference

    The steering arm references aren't spot on, but I hope you can visualise what I mean

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    Senior Member Anthony's Avatar
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    Nice pics but I don't understand what an RCA or a change of strut can possibly do to bump steer. These items are dumb, bolted on passengers, subservient to the arcs and geometry already dictated by the rack, knuckle and LCA.

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    Incorrect, not all struts are the same. The stub axle angle (Kingpin angle) is wrong on a RA strut, it induces positive camber. Fitting them without serious mods does the same to your geometry as cutting around 20mm of length from your standard LCA in an AE86, sounds like a sweet mod :blink:

    Bump steer is caused by the tie rod end not being perpendicular to the steering arm. As the strut compresses the pin of the tie rod goes from being at 75 - 80 deg to 90 - 95 deg and the wheel toes in or out under its leverage, the more lock you've dialed in the worse the effect.

    Roll center adjusters are a must to reduce bump steer on vehicles that have been dropped on there guts.

    <div class='quotetop'>QUOTE (Anthony @ Sep 1 2008, 07:27 AM) </div>

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    Senior Member Anthony's Avatar
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    Yeah no offense but you seem to have a few terms mixed up. Kingpin angle problems are a static problem from mixing parts from totally different cars. bump steer is a dynamic problem.

    Roll centre adjusters adjust roll centre (hard to believe I know) not bump steer. They are totally different things.

    It's a great idea to try and demonstrate the problems with these mods before they acquire too much gravitas, but it's pretty important to be spot-on when doing so. Otherwise you risk adding to the disinformation, not dispelling it.

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    ok, so what youre saying is that to regain a "normal" camber setting with a celica strut, you need run a longer lca(true for sure), which causes the steering knuckle to be cocked over, causing bump steer?

    we are talking about a 3* difference though? as thats roughly the difference in KPI, as the steering arm mounting surface is at a right angle to the strut tube. i guess once you talk about then trying to add another 3* (as most people do) to get the camber setting you want that might compound the problem, but i don't think its that simple, the celica steering knuckle youre forced (well mostly likely) to use due to the difference in bolt spacing has significant differences in angle comparatively as far as tie rod to ball joint/mounting plane alignment. this is obviously to compensate for the different rack position within the vehicle.. this position also means the steering arms are alot straighter on a celica, reducing the Ackerman effect when used in the corolla.. which may or may not be the more pertinent issue.

    i guess without an actual suspension analyzing program its going to be hard to define which of the many variables is contributing the most. but i am leaning towards what youre saying as being the main factor no being the case.

    still, certainly we are in agreement, using celica struts in a corolla is probably not the best idea. the saving you make on the strut casing/easier brake upgrade probably isnt worth the step back in steering geometry.
    78's

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    <div class='quotetop'>QUOTE (Anthony @ Sep 1 2008, 08:29 AM) </div>Kingpin Definition:

    "The kingpin is the main pivot in the steering mechanism of a car or other vehicle. Originally this was literally a steel pin on which the moveable, steerable wheel was mounted to the suspension. In more modern designs, it may not physically exist as an actual pin, but the axis around which the steered wheels pivot is still referred to as the kingpin."

    No offense, but you're agreeing with me that fitting struts from another vehicle with an incorrect kingpin creates problems, but trying to be argumentative that I'm wrong in the same sentence, get no sleep? By changing a static property of your suspension, you alter the dynamic nature of how it behaves, in this case creating bump steer. Not rocket science.


    <div class='quotetop'>QUOTE </div>You do realise that many people call "roll center adjusters" , "bump steer spacers". Possibly to do with cause and effect? Take yours out of your car go get a nice wheel alignment and tell me you're car doesn't bump steer more than before.

    If you want to dispel my information as disinformation, perhaps it would be better to contribute you're deep understanding of subject matter.


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    Senior Member Anthony's Avatar
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    I got heaps of sleep thanks, it's possible to agree and disagree in the same sentence.

    Yes, adding the RAxx struts causes problems but, no, bump steer isn't one of them. See, simple.

    I'm just trying to stop bump steer getting a bad name, like Ackerman's could from recent discussions. They're both deliberately tuned into race cars and street cars alike.

    By the way, since the "kingpin" in a macsrut car is the imaginary line between the centre of the ball joint and the centre of the strut top, putting another strut in the car actually doesn't change the king pin angle at all, so you're kinda wrong there too. The problem is what happens to the KPI when you dial out the camber. This is why when you crash a car, it may align up OK statically, but still drive like shit (much as an RAxx strutted 86 does even when the alignment numbers are great).
    This shit driving feel is usually described back to the mechanic as "bump steer" but unfortunately there is a lot more to it than that.

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    <div class='quotetop'>QUOTE (Anthony @ Sep 1 2008, 12:00 PM) </div>Go on, I'm interested to hear why exactly the setup feels different/weird/shit compared to an 86 strut having never used anything but 86 struts myself. What is it specifically within the geometry that changes make it feel like it does?

    I'm confused where you say bumpsteer is tuned into cars and should not be given a bad name? How could anyone possibly want bumpsteer?
    RT142 Estate.

    AJPS.

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    Well good bump steer is symmetrical bump steer, and is usually there for safer heavy braking. It is also usually very minor so that even in a car rolls a lot, the difference from left to right can hardly be felt. Bad bump steer is usually asymmetric, and it usually comes about when someone has a run in with a gutter. Just trying to say that it is possible to have "good" bump steer, just as its possible to have good ackermans, good positive camber good low caster etc etc. Just depends on the application.

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    couldnt agree more,

    there are many misconceptions, with a set up car that is being driven on a track, a setup can be good for one corner even and not others.

    rca's have their place, but your not moving the control arm down when you fit them, your moving the strut up.

    moving the rack is the answer.
    wheelfriends.wordpress.com

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