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"Ideal" Bumpsteer/akermann questions

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  • plusONETEN
    started a topic "Ideal" Bumpsteer/akermann questions

    "Ideal" Bumpsteer/akermann questions

    Hello guys, allow me to introduce myself. I have been lurking around here for awhile, and I finally decided to join.

    I have a 1992 240sx coupe that i am swapping a vq35HR and a few other 350z parts into... i'll probably start a thread here on that some time, but until then, heres a link: http://my350z.com/forum/engine-and-d...s13-240sx.html


    anyways, I always hear that when you lower a 240sx, you need to correct the bumpsteer. ive seen the outer tie rod ends that allow you to space the outer pivot downwards, and i get how that works, but i was wondering what we are actually looking for on our cars as far as bumpsteer.


    say i have 1/16th" of toe-in up front.... when the wheels hit a bump that compresses them 3 inches, what would we want the toe to be then? what about when the wheels droop an inch or two? Im looking for a theoretical answer, more than say, "you should drop the outer end one inch..."

    i realize that there is no magical answer, but you may assume that i have a slightly lowered car, stiff springs, and i like to go fast.



    on a somewhat related note, how do you guys feel about the akermann in our cars? if you could hypothetically change it, would you increase it, decrease it, leave it? why?


    the reason i ask is that i moved my steering rack down 2 inches to fit my engine in, and i am planning to compensate by moving the outer pivot points on knuckles... with either long studs or welded extensions. i need to know what to aim for as far as geometry goes.


    thanks

  • Def
    replied
    Dropping the tie rod end without moving the LCA bj makes the arcs of the arms
    not match up. That's the main source of bumpsteer when you start messing
    with the front suspension and steering.

    Leave a comment:


  • plusONETEN
    replied
    yea, i dont think it will be that bad...


    in theory, if the outer tie rod end gets dropped down 2 inches and the rack drops down two inches, then, when the steering wheel is straight, my tie rod should always be parallel to where the stock tie rod would be, so my bumpsteer should be the same...


    infact, as i was making the crossmember, i wasnt concerned because i figured i could match the stock specs exactly...
    the only reason that this doesnt hold up in practice, and my tie rod wont be parrallel to a phantom stock one is because you cant actually just drop the outer end 2 inches, you have to make a 2 inch downward extension on the spindle, which is only actually pointed directly 'down' when the wheel is straight.

    I did a few quick models in 3d so I could get everything sorted out in my head... the length of the drop on the spindle is exagerated to make things more obvious and because i dont have actual dimensions....

    I was going to try and model the whole thing accurately in 3d, but i dont know... i might just try and make a bumpsteer guage and set it up in the real world...


    as you can may be able to tell, it does have an affect on akermann as well... it appears to increase the akermann effect somewhat. I could negate this my moving the outer tie rod pivots outward (in addition to downward) but its obviously never going to be 'perfect'


    I feel like 1 degree of bumpsteer while the wheel is at full lock isnt a big deal compared to 1 degree of bumpsteer when the wheel is nearly centered...
    Attached Files

    Leave a comment:


  • Matt93SE
    replied
    2 deg is quite a bit- you're going to have some pretty significant tire wear, as your suspension is almost never at static ride height while driving.
    Car will still be driveable, but it will be pretty darty or vague, depending on the direction of the toe change and the rest of your setup. It's definitely not ideal. keep trying to minimize if you can.

    Leave a comment:


  • plusONETEN
    replied
    Originally posted by RBbugBITme View Post
    A few already said, none at all is a good place to start. This can be somewhat of a philosophical decision though for the suspension designer depending on what kind of racing you're doing. For any road car though I'd say shoot for 0.
    im not going to be able to get zero, though, even if i aim for it.

    if it toes out 2 degrees at 3 inches of travel, am i going to die!?!

    s15 seems to have less than that... i wish i had specs like you guys have on the s15 for a few more cars... i'll have to search around and see what is 'resonable'

    Leave a comment:


  • plusONETEN
    replied
    i was aware that the pivot points were what mattered, but i must have underestimated how tall the ball joint actually is, or how much travel our cars have...


    either way, nothing a little static camber cant fix...

    Leave a comment:


  • RBbugBITme
    replied
    Originally posted by plusONETEN View Post
    I still dont know what degree of bumpsteer is acceptable, though...
    A few already said, none at all is a good place to start. This can be somewhat of a philosophical decision though for the suspension designer depending on what kind of racing you're doing. For any road car though I'd say shoot for 0.

    Leave a comment:


  • Def
    replied
    Originally posted by plusONETEN View Post
    negative camber relative to the chassis should increase with suspension travel until the point where the LCA is at a right angle to the strut. until then, it should gain negative camber.... if you think about how much the struts lean inwards on a 240sx, youd be out of suspension travel before that happened. however, even within the range of reasonable travel, the closer you get to 90 degrees, the more subtle the increase in negative camber.

    also, relative to the chassis, youre going to keep getting more positive camber as the suspension droops. i know when i had the engine out of the car, the front end came way up and it had obvious positive camber.

    of course, thats relative to the chassis... if you take into account body roll, you need even more camber change than that, which is more than youre going to get from a strut setup, which is of course why we see all these 240s with wicked static negative camber.
    These cars start gaining positive camber
    way before they run out if wheel travel. As I mentioned before, look at the line from the balljoint pivot to the inner pivot, notthe downslopping top of the arm. If the top of your arm is at 90 to the strut(a little more upward than level with the ground) you are actually well into the pos camber realm. The actual line that matters(pivot to pivot) is very different than the line the top of the LCA makes.

    Leave a comment:


  • plusONETEN
    replied
    Originally posted by Matt93SE View Post
    With the way our cars are set up, the inside tire will toe out and the outside tire will toe in under cornering forces as the chassis leans. So as you enter a corner and the car begins to take a set, the chassis causes itself to turn in even more- making for more body lean and more turn-in. which causes MORE alignemnt change.
    hmm. i actually forget how exactly the arms are all aligned in their stock positions... i havent had the s13 on its wheels for awhile...

    but, you make a good point. I have read that OEMs sometimes try to make cars toe out under compression and toe in under droop, and that makes sense, because its the exact opposite of the dartiness you were talking about here.

    since im kinda on my own here, i might as well just shoot for as ideal rather than aim for stock.... I cant acheive either, but as we all know, stock isnt always best.




    Anyways, i did a quick representation of my suspension in paint. my inner tie rod pivot point is outside of the LCA pivot point, just like stock. however, mine is also below it, while stock ones are at similar heights. Notice that 'neutral' is actually at an angle... as pointed out earlier, while the LCA may look horizional, the outer pivot point is up in the ball joint, which is the point that actually matters.

    Basically, ive realized that my car will infact toe out under compression and it will toe in under droop.
    Directionally speaking, its going the 'right way' which is good, especially considering that, stock, it goes the 'wrong way'
    However, the magnitude is what concerns me. because if it goes the 'wrong way' stock by .1 degree(close to no bumpsteer), thats way better than it going the 'right way' by 5 degrees (extreme bumpsteer)


    I still dont know what degree of bumpsteer is acceptable, though...
    Attached Files

    Leave a comment:


  • plusONETEN
    replied
    negative camber relative to the chassis should increase with suspension travel until the point where the LCA is at a right angle to the strut. until then, it should gain negative camber.... if you think about how much the struts lean inwards on a 240sx, youd be out of suspension travel before that happened. however, even within the range of reasonable travel, the closer you get to 90 degrees, the more subtle the increase in negative camber.

    also, relative to the chassis, youre going to keep getting more positive camber as the suspension droops. i know when i had the engine out of the car, the front end came way up and it had obvious positive camber.

    of course, thats relative to the chassis... if you take into account body roll, you need even more camber change than that, which is more than youre going to get from a strut setup, which is of course why we see all these 240s with wicked static negative camber.

    Leave a comment:


  • SoSideways
    replied
    That data is really hard to read, and the graphs are kinda tough to follow as well...

    I have no clue what is going on there.

    Leave a comment:


  • Def
    replied
    It shouldn't lose camber with bump at stock height, the LCA is pointed upward.

    Leave a comment:


  • hai1206vn
    replied
    if you look at the s15 camber data posted above, you're already losing camber starting at factory height. I guess there's no good stock geometry to keep here, just slam it and run static camber.

    Leave a comment:


  • SoSideways
    replied
    Originally posted by hai1206vn View Post
    yea, the tie rod is actually a better indicator of the effective FLCA angle (ball joint to inner pivot). 1" of lowering already makes it point up.
    Sigh...

    Guess Nissan intended the S-chassis to forever have a 4x4 ride height lol

    Leave a comment:


  • hai1206vn
    replied
    Originally posted by Def View Post
    FWIW, the S chassis FLCA are deceptive by looking at them. The upper surface goes from about the ball joint pivot to a little over 1" ABOVE the inner pivot. So your arm appearing to be parallel actually has the outer pivot canted up by quite a bit.
    yea, the tie rod is actually a better indicator of the effective FLCA angle (ball joint to inner pivot). 1" of lowering already makes it point up.

    Leave a comment:

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