The Suspension Setup Thread

[ke70dave]

<div class='quotetop'>QUOTE (sr_rolla @ Aug 14 2008, 08:42 PM) </div>this is interesting to read....

i have S13 front end in my car (s13 stocko LCA's),8kg springs, and am running 15x6.5 + 23? wheels, with 195/50/R15 tyres and with completely untouched ke70 guards, -2 degrees camber (set with camber tops), and castor set to zero. The tyres foul on the guards but only when you hit really really big bumps (so essentially clear guards, just need to give them a bit of a roll). and car is around 130-140mm off ground at chassis rails.

anyway....

i do not experience any bump steer at all with this setup (my definition of bump steer as explained in the following)

the way i have tested this is as so, i drive down the bumpiest road i can find (not hard in backstreets of wynnum), and get the car going straight, and let the wheel go between my hands, so let it do what it wants. i do not find that the steering wheel moves at all in relation to when the wheels meet the bumps.

i have looked at my LCA and steering arm, and they appear to be very close to parallel at the static position as shown in the pic i just took (not the best, but its too cold to stay for too long out there..)



now unfortunately i have not had a chance to experience "bump steer" in the sense of lowering my car with stocko stuff withouth adding RCA's....as i went straight from stocko height to this setup.

i also have not had the opportunity to drive "proper" ae86 coilover/lowered spring with RCA setup to compare with, so i guess I'm kinda in the dark a bit when it comes to that...

so basically I'm wondering if any of you goo-roo's of suspension can either point me in the direction on how to find this apparnetly bump steer that is present with s13 gear, or address me in any of my possible wrong thinking?

:2thumbs:

ps: one thing i would like to try, is to take the springs out of my coilovers and put it all back, jack up the car, and move the front suspension through its complete suspension travel, and see if i can see any of this bump steer...but taking springs off is to much hard work....

[sr_rolla]

I am so glad you put that picture up in here

Basically, bump steer, is not steering caused by bumps, that is a scrub radius problem (refer to 1st post).

Bump steer (refer to 1st post) is when the toe angles change under suspension compression/extension. This is commonly caused by different lengths of the tierod end to rack end vs ball joint to inner bush, or the LCA and tie rod being in different points of there arc of movement

Now on to said pic, and I'm gonna be brutally honest here.

Firstly, the tie rod end is all cocked over at a bad angle, that has the posibility of pulling out the ball of the tierod.

secondly, the steering can, under the right circumstances, invert, as in the tie rod could pass across the LCA and end up in front of it.

Third, the arms arent parallel, well, the arms are but there pivot points arent. u need 2 draw a line between the rack end and the tierod end and thru the center of the ball joint (where it pivots) and the center of the LCA inner bush. If you do that you see that the arms arent level at all.

fourth, u need 2 measure the length from the rack end to the tie rod end and the measurement from the balljoint pivot point and inner bush pivot, if these are way different then you will get bump steer.

My basic point is that while i think this setup could be made to work, it is much easier to make the standard design work. All u really need to do for a basic setup that will do what 90% of people here want, drift or grip drivers included, is get some nice bushes, some corona front arms, some RCA's and a rosejointed castor arm arangement. The major issue in the front of ae/ke's is that the castor rod bush is a shitty design and moves around alot, giving a vague feeling, if you rosejoint it or change the design to something similar to a silvia castor arangement then it will handle very predictably and wont have any issues with geometry.

[sr_rolla]

on a side note, i know the pics in the 1st post don't work. I'll b puting them back up by next week, i just got to find them again.

[ke70dave]

thanks for that SR-rolla

tomorrow i shall get out the measuring tape and see what i can come up with :2thumbs:

[Simon-AE86]

<div class='quotetop'>QUOTE (sr_rolla @ Aug 15 2008, 03:19 AM) </div>this is the reason in my choice of using the nissan crossmember and rack, all pivot points should be the way that nissan designed them, in theory anyway.

KE70dave: interesting to see you used stock length arms, you didnt find them too long and cause massive camber? Or did you slot the top bolt hole on the coilover strut to reduce camber?

[ke70dave]

<div class='quotetop'>QUOTE (Simon-AE86 @ Aug 15 2008, 10:44 AM) </div>nah seems to be ok, but I'm using pretty "crap" offset wheels (in ke70 and ae86 terms) as stated they are 15x6.5 +23 with 195/50's

this is with the camber top on full negative (-3 deg), and i havent modded the top holes at all, standard placement.

i don't have a pic of it now (with -2 degrees) but its pretty much flush with guard (those stocko ke70 guards in the picture are essentially untouched)

i would like to get shorter ones, but better things to spend money on/worry about for the moment

[ke70dave]

ok i have results

at the crossmember end it was found that where the LCA bolt and where the steering arm joints the steering rack (right at the ball joint) is essentially in EXACTLY aligned.

i did this by putting a screwdriver in line with the LCA bolt and lined it up with the ball joint in the rack end, and the screwdriver was essentially parallel.

also i figured this end wasnt as important because its the same setup that is used with ae86 stuff anyway (Same LCA bolt position, and same steering rack position)

now onto the other end.....

here is a diagram i drew up in solid edge, its 1:1 scale so its all exact. also note i took these measurements with weight on the front wheel, i put some bricks under the control arm and let down the jack to get as close as i could to static position.

also the measurements of the ball joint were taken at the pivot point of the ball joint (or close too..since you can't actually see it)


now as can be seen, the steering arm is 30mm shorter than the LCA, and the steering arm sits up 10mm higher than the LCA at the brake disc end.

now i have nothing to compare to, but how bad is this in terms of "bump steer"??

just thought id post my results anyway...

[Simon-AE86]

the more crossed up those two arms get the more pronounced the bump steer effect, when i fitted my susp up, i had a horrible angle on the tie rod, since the S14 rack is a lot lower then KE, so the start of the tie rod was a lot lower then the ball joint end where it bolts to the steering arm, I temporarily fixed mine by using bump steer adjusters on the toe arm, dropping the arm about 50-60mm lower from the steering arm.

[ke70dave]

ok i have my own answer

after doin a bit of thinking, and mucking around in this program, i have come up with the following results

considering 2 scenarios

the first scenario is with a suspension compression of 50mm (a fair way with 8kg springs, but certainly obtainable) it sias 49.5mm but close enough

here is the geometry:



now as can be seen, as the control arm is moved up 50mm, the pivot point at the ball joint moves 3.52mm, where as the steering arm moves in 4.64mm, resulting in a toe out
change of 1.12mm (considering only one front wheel, assuming the other wheel does not get effected by this suspension compression)

now the next scenario, is a worst case scenario, where the suspension is compressed 100mm (this would be a pretty psycho bump, and woudl result in worse things than bump steer...namely bottoming out...but neverthe less...)

here is the geometry for a 100mm compression:



as can be seen from this diagram, for a suspension compression of 100mm, the control arm moves towards chassis by 14.59mm, where as the steering arm moves in by an amount of 16.08mm. resulting in a toe change (toe out) of 1.49mm with 100mm of suspension travel

now obviosly this is a definite answer to the fact that there IS bump steer in s13 in ke70 suspension (bugger eh), so my question is...how bad is this? compared to what is acheivable with ae86 stuff? anyone keen to get out the measuring tape on their cars?

comments are most welcome :2thumbs:

[Simon-AE86]

pretty decent findings mate,

in your first senario... 1.1mm toe change is SFA really... so theres your answer right there there IS bump steer, but not heaps