Poor Self Centreing

[Guest paul thompson]

As you guessed by the topic name, my car is still no self centreing at all. I could drive around in circles without touching teh wheel at all. I have measured what I can and I have 3,5 degrees of castor, and 0,5 degrees of camber, not a clue as to the toe as I can only look at that by eye. How likely is that going to be an SVA fail point, and does anyone have a clue how to fix it without remaking the top wishbones to give me more castor? The friction in the rack seems quite high but I have already greased it. I could try and slacken off the Yoke a bit but don't want to end up with knocking steering.

[Guest Ant]

If the wheels don't self centre at all then it's pretty certain to fail SVA.

 

My understanding is this - self centering at a reasonable speed is governed by caster angle. You want the centre of the wheels behind the pivot point so they line up like super market trolley wheels.

 

The main way to alter caster on a 2B is to rotate the dummy strut within the hub carriers which moves the hubs forwards and backwards relative to the ball joint pivot. Where are yours at the moment?

 

Also put a bit of camber on by winding in the top ball joint in the upper wishbone. At very low speed camber affects self centering because the whole car lifts slightly as you turn the wheels away from the straight ahead position, hence gravity helps to straighten them up.

 

Hope that helps.

 

Ant

[Guest paul thompson]

Thanks Ant,

I don't see however how rotating the dummy strut changes the Castor. The castor is set by the relative positions of the top and bottom ball joints. This line is the line around which the hub rotates. If you do not move the upper relative to the lower ball joint centres then the castor remains the same. I have measuring kit though so I'll check and see if it makes a difference. I can add some more camber no problemo. I can't get any more castor (distance between the top and bottom ball joints) without remaking the top wishbone

[Guest Ant]

Because of the taper at the top of the dummy strut rotating it moves the hub forwards and backwards slightly. This moves the horizontal axis of the centre of rotation of the wheel forwards and backwards relative to the line through the ball joints.

 

Think of a caster on a shopping trolley. The centre of rotation of the wheel is offset from the pivot point for the steering. The further back it is then the more it self centres.

 

Ant

[Guest Petemate]

Hi Ant

I'm following this with interest. I've yet to (1) inspect & grease my rack as nec and (2) finally adjust the castor/camber. (2b with pillock ) I was interested to note you advised screwing in the top ball joint to "give a bit of camber". Would this not create negative camber? Is this what it needs to "encourage" more self-centring? Off-club slightly but still on topic, my wife's Ford Ka has never fully self-centred in the 20-odd thousand miles of ownership (from new ) - around the centre 25 degrees or so of steering wheel travel it is most disconcerting after driving my "well run-in" Metro. Nothing can be found wrong with the Ka, but as it has an airbag and SWMBO wont let me, I'm reluctant to investigate. One has to constantly work at it to keep it nice and steady on a straight road. As I've not driven another, I don't know if they are all like it, but it sure would put me off owning one. :boohoo:

Pete(Oldgit)

[Guest trigger]

My 2B failed the SVA on the same thing!!!!!!

 

Have you fitted the supplied steering wheel from RH? if so check how free the hub moves on steering column. Quick way to check is to drop of the U/J and see how free it is!

[Guest Ant]

you advised screwing in the top ball joint to "give a bit of camber". Would this not create negative camber?

 

I always get confused with positive and negative in suspension set up so I avoid the terms! Try it and see is the basic advice. Caster is the main influence, though camber (and toe in) do play a smaller role I believe.

 

my wife's Ford Ka has never fully self-centred

 

That is totally different when it comes to caster. For a front wheel drive car it is the wheels which pull the car along, where as for a RWD the car drags the front wheels along. For RWD you need the wheels behind the steering pivot. For FWD you need the wheels in front of the steering pivot.

 

Just think of supermarket trolley casters. That's where the name is from. If the wheel is motorised then it goes first and the vehicle follows. If the wheel isn't motorised then it must follow on behind the vehicle.

 

Ant

[Guest boggie]

My understanding is this - self centering at a reasonable speed is governed by caster angle. You want the centre of the wheels behind the pivot point so they line up like super market trolley wheels.

 

That is not how I did mine. The centre of my wheels are in front of the top suspention mount by 5 degrees and it centers very well - Set it up like a shopping trolley and it will steer accordingly

 

For a front wheel drive car it is the wheels which pull the car along, where as for a RWD the car drags the front wheels along. For RWD you need the wheels behind the steering pivot. For FWD you need the wheels in front of the steering pivot.

 

Hmmmm maybe but what happens when you put the clutch in and start coasting - the wheels will act the same no matter where the driveshaft is connected.

 

 

As I understand it the bottom of your suspension should be in front of the top mount. This way when you turn the wheels they effectively lift the car up on the edge of the tyres and so gravity centres your steering. Put the wheel centres behind the top mounts and your car steering will be more vague than a politician on question time.

 

Now I may have this all wrong but it works on my car.

 

Boggie

[Guest paul mercer]

I know that the light blue bit on the diagram is supposed to point toward the center of the car but you can change the caster depending on whether it points forward or backward.

 

We cannot move the top and bottom pivot points on the 2b which happen to be above each other, so i've drawn them as static points. If we draw a line straight down from the yellow dots, we can see where the line hits the ground(black line).I have also drawn a line straight down from the centre of the wheel.

 

If we ignore richard about the orientation of the dummy mcferson struts (light blue) and have it pointing towards the front of the car as in fig.a we can introduce some extra caster.

If however we have it pointing backwards, as in fig.b we will reduce the caster.

 

I haven't done this yet on my car, but i've just sorted out the camber and toe, and if after driving i have any self centering problems, this is going to be the first thing i try.

 

As for the fact that the dummy mcferson strut isn't now pointing towards the center of the car, just means that the upper ball joint will be working a little more.

 

If i'm talking complete twaddle, please let me know.

 

Paul

[Guest Ant]

Excellent drawing. That's exactly what I mean. I don't think we have to ignore Richards advice completely on the dummy strut. With the angle pointing inwards towards the car you should get a neutral position. Turning it only slightly will have the desired effect with the caster angle.

 

On the front wheel drive thing, yes, I agree that this means the wheels don't self centre when coasting where as in reality they do. I'm as confused as you are about this. I'm quoting from a book called Advanced Vehicle Technology by

Heinz Heisler. It has a very good section on suspension design. I guess I need to read it again to clarify. Another point is that the SVA rules say that the car must self centre when accelerating don't they? Which supports this theory, kind of.

 

Ant

[Guest boggie]

Another point is that the SVA rules say that the car must self centre when accelerating don't they? Which supports this theory, kind of.

 

Maybe this covers the fact that a badly set up FWD car will suffer torque steer under accelleration.

 

I have just been out in the car park looking at a BMW front suspention. Of course these are rear wheel drive and the 'strut' definately points forward not backward like a shopping trolley. IE the bottom of the suspention is in front of the top mount.

 

It is all very confusing and I remember having to do a lot of research when I built mine 8 years ago but it seems to work this way (although not as much as I would like so I am going to fit adjustable tie bars to allow me to increase the castor).

[Guest chris brown]

At Norwich the tester checked mine for self centring but said he didn’t expect it to act like a tin top and if it came straight with less effort than it took to turn he would be happy. Which he was. It took about 2000 miles before it got better (or was it me who got used to it) It’s still a case of think left and it turns left think right and it turns right, very positive

[Guest Petemate]

Hi Paul

Latest on the self-centring saga, I've replied to the earlier post 'cos that's where I first put my oar in....

I know you've probably checked the rack to death, but here is a further 2 pennorth.

In my original reply I bemoaned the wife's Ka and the lack of self centring, saying I would never buy one if they were all like it. :angry: Well, the garage took it out a couple of weeks ago for the first time when the wife complained of brake noise shortly after the EXPENSIVE major service, (when I was a service manager, ALL cars through our workshops had a road test ) and lo & behold, the quality control chappie said there was a problem with the steering, and the workshop diagnosed a faulty steering column uj, apparently a regular problem on some Kas and Fiestas. Car went in under warranty, and they found the steering rack at fault. New bits fitted, and it is a joy to drive, self-centres like my old Metro. Are you absolutely sure that your rack is as free as it should be? Also the column?

HTH

Pete(Oldgit)

[Guest paul thompson]

Thanks for the Info pete,

I did check the column and am not happy with it. I am haveing a new one made and even posted an offer in the members benefits but no oe else seemd to be interested. It has now passed SVA however I upped the KPI to 22 degress and that made it better, still not happy though.

[Guest john]

Paul, I'm coming back in late on this one, and sorry to hear you are tearing your hair out. Previously I contributed that higher tyre pressure would solve it, it did for me, but then I've got sliding pillars and had the 165 donor wheels on for the SVA.

Applying a bit of problem solving logic, it seems to me that with so many cars passing SVA with presumably small differences in the geometry set up, it may not be a problem with your geometry.

I would want to look more at how much friction is in your steering.

Have you raised the front end and turned from lock to lock ? How free does it feel ? maybe your geometry IS ok, but cannot overcome the friction. One thought.......could you have over-tightened the ubolts clamping the rack ? it is possible to do that on mine, ( which is different to yours, so sorry if that is piffle.)

Have you got any excess friction in any ball joints ? or the steering column?

 

Incidentally , my steering is a lot heavier now that I have changed to wheels with wider offset and 195 Yoko's at 14psi.