View Full Version : big differences in roll centre...

16th February 2009, 11:24 AM
curious about something guys.

my understanding of bump steer was DIFFERENCE in the arc of the of the lca and the steering arm. as they ravel through their suspension arc, one travels on x radius and the other on y radius and the resulting change i toe is bump steer??

with that in mind, what would be te likely results of a BIG difference between these arcs?

i have s13, and i am aware of the problems associated etc etc etc. i was looking at the max steering knuckles as one of the issues i have had is the steering rack forming a solid line and tightening up. These provide 2 inch of drop on the knuckle, but i can only get a max of 35 mm rca ball joint for the lca.

i have also seen the same setup used without any rca for the lca??? what sort of effect would this have on overall handling?

so what say you oh masters of grip??



16th February 2009, 08:53 PM
angle grinder

16th February 2009, 09:21 PM
yeah besides cutting the tip off my finger what would i do to it?

16th February 2009, 11:15 PM
dont know about angle grinder. pretty sure you can remedy youre little problem with a spanner. lol.

jokes aside. have you had a good look at the relationship between the lca and tie rod, the way it is now?

i have noticed these sorts of nissan alignment products dont seem to be marketed as a system.

i cant really give an impression as to why, as i havent had a look at the s13 stuff when lowered, not in an s13 anyway.

possibly, when lowered, and more so, when a lot of caster is wound in when lowered, the steering knuckle arm angle is changed enough that a miss match in these products "heights" is required to return the alignment of the 2 arms (lca, and tie rod) to closer to their correct relationship.

should be noted, that even for ae86s, where a simple spacer (rca) can be used to manipulate both pivot points at once, there are still available after market tie rod ends for fine tuning of the relationship between the 2 arms.

ive personally never got that far into it.


thats them.

having the s13 stuff in an ae86 though, kinda puts you out there on your own somewhat. kinda uncharted territory. you might need to make friends with Adam. not an easy man to make friends with. lol. but no im sure he could give you some better insight.

17th February 2009, 09:22 AM
i read that a couple of times and im not exactly sure what you are asking.

Yes, any difference in the steering arm arc to the lca will create toe in/out on bump which is the bump steer you talk about.

Correcting it is a little tricky, and usually a case of trial and error, especially when dealing with a mish mash of componentry.

Correcting this may involve a number of approaches

moving the steering rack foward/backward, up/down
modifying the lca pivot point
adjusting the static height of the steering arm/lca

The indicator of what is going on is to build a simple gauge to show what the toe is doing through the suspension range. It is simply a hinged bit of wood (say 2 pieces 40 x 40cm) with 2 screws that point to reference areas eg the outer lip of the rim. with the front springs removed raise/lower the car in increments to identify the toe during bump/droop.

when you have done this you canbegin to rectify the problem

17th February 2009, 08:46 PM
yeah i sometimes get lost in my own awesomeness... i mean badly worded questions :)

my main queery is how gnarly an effect would a static height difference of about 2 inchs?
and yea i would rather eat arse than grovel to adam lol. ill figure it out myself befor that!


22nd February 2009, 11:42 AM

fanta what is it with you and the male ass

22nd February 2009, 04:19 PM
meh male donkeys female donkeys??? all donkeys to me:)

yeah i know about that page and have looked into most of the stuff on there, howvere most dont have anywhere near level suspension arm arcs when applied to the max setup.

i have spoken to the guys at max and i think it is something along the lines of what dimitri suggested, that the changed angle etc etc etc and a change in the positioning angle of the extension works in a less than clear fashion, but the guys do say it works :)

ps do i know you? :)

22nd February 2009, 04:54 PM
would they have suggested sticking the car on a bumpsteer guage first ?

ps i dont know any persons that are into paddlin

22nd February 2009, 07:34 PM
already been suggested abpove and done....

and your missing out :)

25th February 2009, 08:33 AM
and what did the reading say ? what was it at 2'

25th February 2009, 08:19 PM
ruff numbers but 2mm toe out from memory... basically something that wasnt a huge deal considering the gains, plus the mount changes ackerman and a few other things slightly so i figured it would be different anyway, so plan on redoing it when i get the knuckles sorted

26th February 2009, 11:54 AM
2mm toe out where? at what point of bump/droop did they measure?

2mm is alot.

any change in toe straight ahead will be compounded as the ackerman is a dynamic change in toe.

Adding RCA as slyder said will change both piviot points and (gasp) will only change the roll center, which is a good thing if you havent changed the rear.

(hope this makes sense i dont have paint on the mac)
As for the lowering thing, imagine a tangent line 10cm long attached to a circle with 4 times diameter. the point where line touches the circle is the static ride height. The top of the line is 100% bump and the bottom of the line is 100% droop. This would be they typical plot for a standard car with mcpherson strut. Assuming the car is setup like a ke70 bump would have a small amount of toe out getting worse as the travel was taken up. Conversly as the suspension moved into droop the toe would come back inwards and have a small amount of toe in.

now look at around 50% bump, this would be a lowered car around 2-3 inches. See how you would start in the place where the angle gets more pronounced and the change in toe is greater. Thats why you get bigger change in toe in a lower car. Fitting a rca to a stock car will place the angles at the original position acting like 'bump steer correcters' as they are sometimes known as.

Now because you have changed your steering knuckles, total strut height lca length etc etc youre in a world of time money and pain to get it all corrected.

the easiest solution i would think is to check out a s13. check the distance to the strut tops, the position of the rack and lower control arm mounts on both axis. Compare it with what you have and draw up diagrams.

You may be able to get away with simply changing the height of either the tie rod end or the lca. I would assume that there is a difference in rack positioning that causes all the problems in the conversion. noone can tell you exactly what to do unless they have the same setup as you and have done the research themselves.

26th February 2009, 07:21 PM
Does a s13 have a wider rack and track & wheelbase ?

26th February 2009, 10:16 PM
toe out was at the top of the storke - 100% compression.

i know the toe will be there... more curios about the theoretical effects because the nissan guys dont seem so keen on rca. However, they all seem to think that lowering the tie rod end will result in a correction of bump steer issues. But i dont see how as there is a 2 inch difference in the static position of the ball joints, and im sure this would be exzaserbated , ie an increase in toe changes as the arcs run through their different strokes. I think they are talking about the dead spot a lot of nissan guys have in their s13's at lock. i think eddie was dealing with this when he moved his rack. I dont think it has much to do with toe changes as such, more to do with the binding of the tierods. or i could be smoking the crack again :)

26th February 2009, 10:29 PM
Head to toymods, there is a good roll centre discussion thread atm.

26th February 2009, 10:44 PM
heheh i keep forgetting to join lol

Jonny Rochester
28th February 2009, 12:42 AM
Roll center discussion, on Toymods... so many reputation points... I am shaking in my boots. :worried: I dare not even look. I'm sure it goes for many pages.