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

[beerhead]

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.
http://www.ae86drivingclub.com.au/dcimages/4/8/4/3/176054.jpg

RA40 Strut Fitted to corolla with longer LCA's and/or camber tops ... Steering arm is not acting on the same plane at all.
http://img258.imageshack.us/img258/5466/ra40strutbo3.gif

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
http://img225.imageshack.us/img225/6810/corollarcaqo6.gif


RA40 RCA There is an improvement of the angle, but it still isn't as good as a factory strut.
http://www.ae86drivingclub.com.au/dcimages/4/8/4/3/176057.jpg


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 :D

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

[Anthony]

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.

[beerhead]

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) <{POST_SNAPBACK}> (index.php?act=findpost&pid=582219)</div>
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.[/b]

[Anthony]

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.

[slydar]

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.

[beerhead]

<div class='quotetop'>QUOTE (Anthony @ Sep 1 2008, 08:29 AM) <{POST_SNAPBACK}> (index.php?act=findpost&pid=582228)</div>
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.[/b]

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>
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.[/b]

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.

[Anthony]

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.

[Konakid]

<div class='quotetop'>QUOTE (Anthony @ Sep 1 2008, 12:00 PM) <{POST_SNAPBACK}> (index.php?act=findpost&pid=582283)</div>
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.[/b]

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?

[Anthony]

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.

[driftke70]

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.

[sr_rolla]

driftke70 in a corolla/ae86 moving the rack is the hard way to do it, the easy way is to use a rose joint instead of a tierod at the steering arm and space that rosejoint away from or nearer to the steering arm (look at the front end pics of the TRD sprinter in my suspension thread, you'll see what i mean)

beerhead your part right, those drawings that you posted show that when you put the an RCA or "bump steer spacer" as you called it, you retain factory roll center characteristics and factory bump steer settings aswell. All thats being done with a RCA as driftke70 said "rca's have their place, but your not moving the control arm down when you fit them, your moving the strut up" the whole steering arm is moved away from the strut, not just the LCA or Tierod. To truly effect nothing other than bump steer you need to either raise/lower the rack or raise/ lower the tierod at the steering arm.
Basically the idea is to get the tie rod and LCA is alignment.

The angle of the stub axle to strut effects camber and dynamic caster, the steering arms tends to effect acerman geometry, usually as a by product of changing struts to celica or whatever.

[sr_rolla]

just thought of something else aswell, if you move the rack it would probably be a pain in the arse due to the column aswell, just a thought

[beerhead]

Cheers guys, that makes some sense, I like informative posts where I learn something new.

Honestly, I don't think many people have actually realised having steering arm in the wrong plane can have such an adverse effect on handling, even if you don't think it's "Bump steer". It'll cause a wheel to toe in and out, and I don't know another word more fitting to describe the effect.

I know that exactly what a kingpin is. But you don't know that the kingpin angle is exactly what I said it is. It's the angle of difference between the wheel and the strut, and that is dictated by the stub axle. 90 degree + the kingpin angle = stub axle angle.

http://www.onlinetires.com/images/tech/img_wt_5.jpg


Honestly ant, it's not what you say, its how you say it, if you want to show you're Omega 1 suspension guru master, can you actually explain things in decent manner.

<div class='quotetop'>QUOTE (Anthony @ Sep 1 2008, 12:00 PM) <{POST_SNAPBACK}> (index.php?act=findpost&pid=582283)</div>
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.[/b]

[sr_rolla]

Are you talking about the toe changing as u steer? as in, the more lock you wind in the more toe in/out you'll have?

[driftke70]

or toe out on turns, which again leads to ackermans,

the king ping angle isnt so much as relevant as the king pin inclination,

castor effects toe on turns slightly, depending on how much you run,

also it is relevant to the suspension you run, if you are speaking about a car with say 8 or 9 kg springs where the front is never really taking much a of a stroke, the bump steer effect doesn't really occur, if your car is set up properly, suspension wise, you should be able to drive through bump steer easily.

[Ksevn-T]

Theres a few little bits and pieces worth adjusting as the other guys have picked out but the drawings are Impressive!!!

[streets]

Just finished reading this thread, nice drawings by the way; and thought I'd add my 2c (in case anyone cares).
Beerhead is actually correct in suggesting that changing the control arm/tie rod angle in a corolla/sprinter affects the bumpsteer characteristics (or increased toeing during suspension movement to be specific), and the reason is this...
The rack end and contol arm are not perfecty parallel, nor do they pivot from the same position. Hence; if you have the supspension lowered away from the 'ideal' factory relationship, the toeing affect of the suspension movement is exaggerated as the components move further from parallel.
And here lies the advantage of the bolted on steering arms and control arms in the toyotas, as a simple bolt on RCA can restore the components to roughly their factory positions. The xE7 and AE8 chassis however are certainly not extreme examples, as lowering will not result in dramatic increases in bump steer (as shown in beeerheads first pic); however, fitting different struts with changes in strut length and KPI can certainly have adverse affects.

Hope that makes sense.

[sr_rolla]

yes, but all fitting an RCA does is restore the factory bump settings, if you wish to dial out only the bump, u need 2 move the tie rod in relation to the steering arm to bring them back to parallel (generally done by either moving the rack as driftke70 said or modify the tie rod ends as i desribed)

What i get from beerheads 1st post is that he is saying that puting celica struts into a ae/ke will cause bump steer, it wont, it will however, stuff up kpi and ackerman angles.

[streets]

Fair nuff.
It should be mentioned though, that fitting RCA's is still crucial on a lowered car, even if the tie rods/rack are repostioned. I say this as even in a sensibly lowered 86, the control arms will sit close to or beyond horizontal, meaning that as the outside wheel is weighted during cornering; the control arm movement results in a reduction in negative camber, possibly going past zero into postitive territory. :hammer:

[af300e]

<div class='quotetop'>QUOTE (driftke70 @ Sep 1 2008, 07:26 PM) <{POST_SNAPBACK}> (index.php?act=findpost&pid=582438)</div>
rca's have their place, but your not moving the control arm down when you fit them, your moving the strut up.[/b]

If you where moving the strut up, RCA's would change the ride height, but they don't.

They move the arm down and bring the roll centre closer to that of stock.

[slydar]

i think he meant stub axle.

[af300e]

But they don't move the stub either.

[Konakid]

True.

[slydar]

yeah not physically. but if you think about a car with a shorter strut, and an rca, compared to a stock length strut, and no rca. the overall length of the assembly should be similar, only the stub axle is higher.. or "moved up".

[af300e]

Ah, well, if he meant in that context, fair enough.

Technically, that would be moving the car down though :P

[slydar]

yes i guess you are right, the stub axle never really moves if you think about it like that.

anyway all this talk about moving the stub axles is making me think of drop spindles, and low luxs, so can we stop? please.

[sr_rolla]

slydar: agreed, getting ideas about drop spindles on the crown aswell :D

streets: also agreed, but he was talking about using RCA's to dial out bump steer, they dont, they return it to factory and thats it, hence my above comments