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.


RA40 Strut Fitted to corolla with longer LCA's and/or camber tops ... Steering arm is not acting on the same plane at all.


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



RA40 RCA There is an improvement of the angle, but it still isn't as good as a factory strut.



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

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

[Rodger]

Interesting modeling beerhead.

What does it look like when the lower control arm is parallel to the ground?

From factory most lower arms are like you have drawn. When we lower the front a lot and shorten the struts to do so we end up with an arm that rises from the chassis end. RCAs are then added to make the arm parallel to the ground.

Regards

Rodger

[beerhead]

Cool, not sure if they want to dead flat, I thought that was what the RCA was trying to do away with, but no two people like the same setup in a car.

I'll measure up on my mates AE71 with some coilovers and RCA's and I'll correct the angles of the tie rods and the LCA's to how he's setup when I get a chance, then remove the RCA.

There are other forms of bump steer, but I'm fairly certain the reason RA40 gear feels strange and unpredictable is related to this. If you want a good laugh read the same thread on AE86DC. I think I rattled the ego's over there by the way some people where posting, trying to say it's all nonsense. But the fact is, Toyota know heaps more about designing car's than all of us, if you're doing a strut swap you want everything to be as close as the standard geometry as possible, doesn't seem that hard to comprehend to me.

[Rodger]

There are also some subtle differences in the way Toyota set the axis down the strut tube and the off-set of the steering arm from the centreline of the strut tube.

The RA40 struts also have less camber as the strut tube is welded at a greater angle to the stub axle centreline, loosing camber compared to say TA-22, RA-23/28, but not as bad as the RA-60/65s. Not always what we think "stub axle perpendicular to the Strut tube".

Regards

Rodger

[bass_boy]

id like some more info on this.. ive got a set of ra40 struts lying around in the shed and they are easy to put fc rx7 brakes on... ive got an ae86 sitting in the shed too and want to know if this is a good idea or should i find some stock ae86 struts and go from there?

anybody got ra40 struts on an ae86?

[mumblezzz]

I originally put RA60 struts in my 86 years ago for the bigger brakes but changed back to stock struts so I could use 86 power steering arms for bigger steering angle and the improvement was massive. The car wanted to corner harder and it just felt better. I 'd use stock struts over RA4/60 struts any day.

[crostek]

Putting the AE86 power steering arms give you better lock but they also improve the angle of acronym, of the steering. This has the effect, that as the wheels are turned, it increasing the angle of the inner wheel.

[bass_boy]

I originally put RA60 struts in my 86 years ago for the bigger brakes but changed back to stock struts so I could use 86 power steering arms for bigger steering angle and the improvement was massive. The car wanted to corner harder and it just felt better. I 'd use stock struts over RA4/60 struts any day.

hmm... will the normal ae86 steering arms be ok on the ra40 struts?

[crostek]

RA60 steering arms have a wider bolt pattern that connect them to the strut then the AE86/71/KE70 and XT130 and I think RA40 too, if it helps

[JetspeedCamry]

Nice 3D work there man. Explained the principals really well to me.

[af300e]

Putting the AE86 power steering arms give you better lock but they also improve the angle of acronym, of the steering. This has the effect, that as the wheels are turned, it increasing the angle of the inner wheel.

Which acronym would that be, TFU or WTF?

I think you mean ackerman?

[crostek]

My bad, at least some one knows WTF I meant

[love ke70]

i must admit i dont quite see what those little 3d doo daas are showing or what plane you are talking about, is one of the lines drawn the plain they be following or something?
they seem to only show the movement of steering, not whats happening on compress or droop of the suspension?

i thought the bumpsteer worked more like OC explains here:

bump steer should just be a function of rack width and rack height.
if the rack is too wide or too skinny, then you need to fart ass around with the rack to try and get the arcs of movement so that they kinda match (but same offsets?) and give minimum change.
ie, if rack width is correct, the LCA and tie rods will be parallel and same length = no difference in their arcs of movement
http://i301.photobucket.com/albums/n.../bumpsteer.jpg

please, explain, i really am still trying to grasp this, but it seems your LCA and tie rod end are different lengths and not parallel so i dont see how this will stop bump steer...


my ke70 with1ggte RA60 struts and adjustable coilovers and 15 x 7 205/50 tyres handled much the same in feeling to my standard ke70 on 175s
could never wind enough neg camber in because of the stub axle angle, which was something i was planning on working on.
the standard ke70 with the 205s fitted handled sooo much better than the 1g ra60 setup it was incredible.
so if anyone can help and give some ideas on how to fix the issues with this setup i would be interested. im currently working on fitting in an RA60 power steering setup to lighten the steering and get things where everything sit properly on the balljoints, and it will also give me the chance to play around with rack positioning, so please, feel free to give asmuch advice as possible

cheers, andrew

[RobertoX]

I have to agree, I struggle to see how your animation shows bump steer when there is no 'bump' happening. Bump steer is usually measured as the change in toe with no steering input as the suspension moves through its range of travel (bump and droop).

You seem to have an RCA in the bottom two, which often give better bump steer characteristics on lowered cars. Basically because they put the geometry back to where the engineers intended it to be. You are exactly right that the Toyota engineers know a fair bit about suspension design but they do often have a bit of bump steer in the system. Suspension design is a compromise of many things and there are acceptable levels.

Please explain your diagrams further though, in case we've missed the point or something.
By the way, what program did you use to do it? I like the shading effects, they are nice and simple and give it a good look.

[gargoyleman]

does anyone here know if their is any easy way to raise my car mayby two inches as i have a TA22 and just put RA60 struts on it but it is way two low.

[beerhead]

Yes the diagrams don't show the bump actually happening, not sure how I could even do that with Sketchup. Use some imagination All they show the base setup of the steering arm geometry with and without the RCA with the stub axle sitting at the same camber point.

Look at the steering arms, if they aren't parallel to the steering rack, as the strut compresses the pin for the tie rod end becomes closer to vertical and causes the wheel to toe in. This crap can't be explained in words, but its obvious the closer you are to factory geometry the better the car will perform

All steering systems have an element of bump steer, its just life. But there is no real way to fix the RA40 strut issue in a corolla, apart from not running them in the first place.

[RobertoX]

Sorry I disagree, if you have the same height strut as original that just happens to be an RA40 one but the LCA and steering are remain the same then the bumpsteer will be the same. The problems start when you lower your car, this affect will happen regardless of the strut that you use.
Running an RCA puts all the geometry back to factory.

You don't necessarily need to have the steering arm parallel to the steering rack either. because it's all ball joints the 'plane' (I assume you are referring to the flat part of the steering arm) is irrelevant. The movement is defined by the location of the tie rod outer ball joint, the LCA ball joint and the upper support. You can get rid off all of the planes and what not in there and use wireframe geometry to show bumpsteer.


When looking at the geometry In front view the LCA, tie rod, steering arm and chassis create a simple 4 bar linkage. As the 4 bar linkage moves around (suspension bump) you want the steering arm to remain at the same angle (no toe change). This does not necessarily translate into having the steering arm parallel to the rack.

You're right in that it is difficult to describe and you also make a good point:

its obvious the closer you are to factory geometry the better the car will perform

So many people mess with their geometry without thinking about this sort of stuff, this is the best advice in regards to bumpsteer.

[RobertoX]

So I got bored and put together some short vids of what it looks like:
http://duivesteyn.net/rob/Bumpsteer/demo_0002.wmv
You can't really see the bump steer in this one but it gives you an idea of the geometry.


If you look at it in front view you can see the wheel toeing a bit.
http://duivesteyn.net/rob/Bumpsteer/Standard.wmv

Toe change is more pronounced when it is lowered:
http://duivesteyn.net/rob/Bumpsteer/...20no%20RCA.wmv

It is corrected back to the same amount as the second video with the addition of an RCA (represented by a gap between the steering arm and the strut )
http://duivesteyn.net/rob/Bumpsteer/...50mm%20RCA.wmv

It's a bit hard to see because the video is slightly poor quality and there is something going on with my stupid video driver. The toe change is stubtle so possibly hard to see, I might try to play with the geometry a bit to make it more pronounced and obvious on the video.
The geometry in this is made up generic mac strut but you get the idea I hope, I don't know the hard points for corolla etc.

[beerhead]

I couldn't make out your vids too well, but I know what you're trying to explain. Essentially Image 1 vs image 3 in my diagrams. But, I'm not talking about generic bump steer from lowered vehicles, hence the title, RA40/60 Struts bump steer explanation for corolla's.

The king pin angle is wrong on a RA strut, it induces positive camber. The stub axle points further down. Did you notice that the 2 struts have stub axles at different king pin angles in my animations? Fitting them does the same to your steering geometry as cutting around 20mm of length from your LCA in an AE86 with standard struts, sounds like a sweet mod Even adding absurdly large RCA's does not return the steering geometry to anywhere near as good as factory (clearly shown in image 1 vs image 4)

When you run some sigma arms to widen the track and repair the camber you also change the 3D arc which the wheel follows while steering, I don't know exactly what it does, but if its not the same as factory it can't be good.

RA40 struts in a corolla / AE86 should be frowned upon, this is all I want to convey. It seems like a nice cheap brake upgrade, but it's such a serious downgrade to handling that it shouldn't even be considered as an option.

[love ke70]

im sorry, but you can make anything work, and with 400 hp i wouldnt be suggesting going back to smaller brakes to get some nicer handling, let alone the ridiculous prices on ke70 power steering gear.

im gonna have a serious play with this setup and see if it can be made to work, which i strongly believe it can