"Ideal" Bumpsteer/akermann questions

[plusONETEN]

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

[Matt93SE]

Part of the problem with a Mcstrut setup is that you can't move just the tie rod end to get a decent steering geometry after you lower the car. You must also lower the ball joint (or raise the inner pivot point) in order to get decent geometry. If you move the tie rod ends, it's only going to make the bumpsteer horribly worse.. You may be able to get it centered when the car is at normal ride height, but it will ALWAYS toe in under compression and out under droop unless you move the ball joint. That's just a matter of the camber curve and the angles of the control arm and steering linkages in relation to each other.

that said, the steering angles are set up pretty well from the factory. Thus if you lower the car 2" (or move the rack 2"), then you should move everything else 2" to compensate. That will give you just about zero bumpsteer- which is what you want at the end of the day.

[plusONETEN]

so then, you would say the goal is to have slight (minimum) toe in under compression and minimum toe out under droop?

i read a few articles, and i got the impression that cars should have toe out under compression and toe in under droop... does the 240sx suspension geometry not allow this?

hmm... i think id better stare at my suspension a little more...

[Matt93SE]

It depends on the camber curve and the location of the steering rack. If the rack is in front of the control arms, then it will be as you mentioned. If the rack is behind the control arms, then it will be opposite.

It also works relative to the camber curve. Once you lower a 240SX excessively, the camber on our cars goes more positive the lower you go. so when you hit a bump, the bottom of the wheel pulls in, relatively lengthening the steering rack and making the car toe in.. When you crest the bump and the suspension begins to droop, the camber goes negative, shortening the steering rack and causing the wheels to toe out.

The goal is to have zero bumpsteer. your wheels should always point forward at the same angle no matter where they are in the travel range. that way your alignment doesn't change as you're going around a corner. 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.
see where that goes. you touch the wheel just a tad and the car darts that direction. scary at high speeds. I found it very difficult to stay in my lane even on the highway cruising around corners at posted speeds.
On the track, I could throw the car into a corner and make it take a set pretty quick and then drive through it. but on the street, it was flat out dangerous because you can't throw the car into a corner and drive through it half-sideways.

The way I fixed it was to go back to OEM geometry, raise the car to about 1" lower than stock, and just deal with the limitations.
The other option is to do custom roll-center corrected control arms and tie rod end spacers and whatnot.

For the amount of work you've done to the front subframe and whatnot, then I don't see that being beyond your skill level- you just gotta know what to do (which is obviously the point of this post).

The problem I see is that you lowered the steering rack 2"- you went the wrong way. ideally you should have raised the rack by 2"... but that would of course put it in the middle of the engine, which isn't exactly desired.

Thus, you need to find a way to drastically modify the steering mounts on your control arms to correspond to the lowering of the rack.
Honestly that's above my pay grade, and I would personally enlist a qualified race fabrication or hot rod shop to help get all of that set up, since they deal with excessively lowered cars and setting up custom steering racks on a fairly regular basis.

Hope that helps.. good luck with th eproject!

[jmauld]

Is there anyway to get that engine in there without dropping the steering rack? That really compounds the issue of bumpsteer since it adds more misalignment to your already lowered car.

[plusONETEN]

it might have been possible, but it would have required leaning the engine such that the transmission sat extra low and the engine was higher, otherwise the bellhousing wont clear the tunnel. the pinion housing on the steering rack wants to occupy the same space as the starter motor, thats the main clearance issue. as for the rack to the upper oil pan clearance, theres probably almost 3/4 inch, but the pinion housing to the starter motor is just enough for me to get my fingers behind. the engine in my car is actually offset towards the passenger side by a half inch as well, to get more clearance for the steering.

i may have been overly cautious with the clearances i left for engine vibration though, so im not going to say you can or cant fit it this way or that way... only that everything is very tight.

my initial plan was to just match the 2 inch drop of the steering rack by entending the knuckles 2 inches downward, plus a bit extra because i do plan to lower the car somewhat. that would keep bumpsteer fairly close to stock... although in cases where the wheels are severly turned, there would still be some effects due to the fact that the steering axis isnt vertical, not sure that it would matter much though.

[plusONETEN]

ah yes, and i dont plan on lowering my car as much as the 'usual' 240sx. just a bit. i want to run 275-40R17s which are a bit too big for 240sx's by most standards, but i have a different powerband and final drive, and the tires end up being 1 inch shorter than the 350z's stock setup.

[Umai Kakudo]

This thread hurts my brain without pics

Bumpsteer in the front in any amount is bad so it is always desirable to eliminate it as much as possible. Feeling the steering wheel turn itself under compression is not desirable when your wheel hits a bump in the middle of a high speed, high commitment corner!



http://www.modified.com/tech/0508_sc..._geometry.html

It's fairly easy to design a suspension system that doesn't have bump steer. To do so on a double-wishbone-type suspension, the steering tie rods must lie between two vertical lines drawn between the upper and lower control arm's pivots while pointing at the suspension's instant center.

In order for a MacPherson strut suspension to have no bump steer, the tie rods must lie in line with the lower contol arm with the inner tie rod end in plane with the inner pivot of the control arm (see illustration on this page).

However, most production cars have the steering rack placement compromised by packaging constraints so steering tie rod location is often less than optimal. Lower the car and the problem gets worse.

What can you do to reduce bump steer? Many cars have aftermarket parts available to relocate the tie rod ends of the steering linkage. Tie rod ends with spherical bearings and spacers can be tuned to reduce bump steer by placing the tie rods at a more favorable angle.

That should help to visualize how you want to keep the inner pivots of the tie rod and LCA in the same vertical plane and then locate the outer piviots to follow the same arc so toe changes as little as possible through the main length of travel.

Ikea Formula also has come good descriptions and visuals that are now in English

http://www.ikeya-f.co.jp/en/index.html



Additionally - here is another guide on geometry with lots of good graphics for RC car setup of all things. It has one of the best descriptions of Ackerman I've come across.

http://www.rctek.com/technical/handling/index.php











In terms of the best Ackerman setting that depends on what you are doing and how you like the feel of the car. Most people don't mess with it as it requires modification of the front uprights so it's what the factory blessed us with.

Extremes in Ackerman (more pronounce the more the wheels are turned) would cause additional scrub and induce additional slip angle forces on the tire contact patch of the inside tire.

Unless you are doing something that involves lots of steering lock (drift, tight gymkhana or autox) there is not much to be gained in changing it.

P.S. - I've only lowered my S13 about 2" where the front LCA is parallel with the ground and there is almost no bumpsteer (at least perceptible through the steering wheel) with all stock components. If you lower it any more than that bumpsteer starts getting worse the lower you go.

[Def]

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.

I knew of this before, but it leads to a big optical illusion even when you try to compensate for it. I just put on some "semi-custom" roll center adjusting FLCAs(write up coming at some point), and started off with a 1.5" total spacing from the bottom of the spindle to the top of the spherical bearing(about 1.8" from bottom of spindle to pivot). I was surprised to find out that the line from pivot to pivot is just barely canted upward with this much spacing. The stock ball joint maybe has 0.5-0.6" from the pivot to the bottom of the spindle once everything is torqued down.

I plan on trying to get a few rough measurements to do some roll center height calculations, but the rear is a bit harder to tackle with a tape measure(to put it mildly), so not in a big rush for that.

[hai1206vn]

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.

[SoSideways]

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

[hai1206vn]

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.

[Def]

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

[SoSideways]

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.

[plusONETEN]

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.