My Bugbear - Self Centering

[Guest Taffy Rob]

Hiya (again)

still looking at improving my self centering prior to retest. Someone (apologies, I cant remember who) posted a link to an excellent website that explained suspension settings very well, and states that for best self centering, I should have positive castor (like a bike fork), however the sva center, and Dave Andrews' article on Build Tipz, are of the opinion that negative castor (like a shopping trolley) are in order.

 

So, who's right? I have made some alterations, such as straightening the action of the steering shaft, increasing the camber, setting the lower bush to a measured perpendicular state, and greasing it throughout, which has all helped a little, but Im not confident it will be enough.

 

Also, any suggestions for altering the castor will be welcomed, as I fail to see how it is really possible to any degree (wishbone 2b), unless its acceptable to shave down the nylaspa bushes???

 

Cheers

Rob

[Guest Ant]

Was this the link you wanted:

 

http://www.rqriley.com/suspensn.html

 

Here's _my_ understanding of caster angles:

 

The caster action is just like a caster on a supermarket trolley. Push the trolley and the wheel falls into line behind the point which is pushing it. This means that front and rear wheel drive cars are different:

 

Front wheel drive:

 

The centre of the hub should be in front of the rotation point for the wheel. This is because the force of the engine is delivered through the wheel and it is the wheel which pulls the car along, hence this is the opposite of a shopping trolley situation. Here the car trails behind the wheel, hence the wheel is pulled ot the straight ahead position by the load of the car.

 

Rear wheel drive:

 

Here the front wheels are being pulled along by the car (rather than the car being pulled by the wheels). Hence the centre of the wheel should be behind the steering rotation point. This is identical to the shopping trolley. The car goes forwards, and the friction of the wheel on the road holds the wheel back until it is dead straight behind the point pulling it.

 

I know that other factors like camber angle also affect self centering, but I believe that this is the most significant of them when the camber angle is close to zero.

 

Ant

[Guest piddy]

Hi there.

Instead of shaving down the bushe's to increase adjustment would it be possible to shave down the wishbone eye tubes. Carefull measurement though dont take to much off.

Piddy.

[geoff shep]

Ant - can that be correct about fwd? When you coast, with power off, there is still self centring on fwd cars.

[Guest Ant]

I reckon that it's true for the majority of the self-centering effect. It makes sense anyway.

 

When the car is coasting then other factors come into play and become more dominant. When the steering is turned the car rises slightly at the front. This means that gravety helps the self centering too. The rising effect comes from the the angle between the greatest slope up the wheel and the line up the centre of the steering rotation axis, viewed from the front or rear of the car horizontally. Have a look at that article I linked to above - it's in there with a nice diagram and a much better explanation.

 

Camber also comes into play to a lesser extent though. The camber angle moves the contact point relative to the steering rotation axis. That's in the article too.

 

BTW - I'm no expert, I'm just trying to look at it logically.

 

Ant

[Guest Greg]

Clarification of shopping trolley wheels.....

 

With mechanical trail the tyre follows behind the steering axis. More trail means the tyre side force has a larger moment arm to act on the kingpin axis, this gives more self centering and is the PRIMARY source of self centering moment about the kingpin axis at speed.

So it follows more trail will give higher steering force.

 

HTH

Greg

Kingpin_axis.doc

[Longboarder]

Taffy, Dave Andrews article describes setting positive castor but he calls it negative. Increasing King Pin Inclination will give more self-centering effect so try setting as much negative camber as you dare for the test that doesn't make the front of the car look too wierd. Try 5 degrees. That will help but give tyre wear problems long term. If you still can't get enough self centering effect take the car to someone who can measure castor and see what the figure is. If its only 1-2 degrees positive or worse negative then you are looking at having new top wishbones made. But that would be pretty unusual. I don't suppose its possible to fit the top wishbones either way up which might alter the castor?

 

Nigel

[Guest Taffy Rob]

Cheers everyone for the replies!

So to sum it up, the camber thing is correct and should help. The wheels should be set with positive castor so they lean backward at the top (like a bike fork)-apologies but this whole positive / negative thing is too confuddling for me!

I don't believe Ive put the wishbones on the wrong way, though of course i could have. They have a straight edge and an angled edge, I assumed that the angled edge would be leading, and thinking about it this must be right because otherwise that would bring the top forward and decrease the castor further (by a lot). So if someone could just confirm this for me?

Also, in the belief that it would help, Ive increased the front tyre pressures by a sizeable amount (35psi!!!), but now thinking about it maybe they should be lower to create more drag?

I think shaving the wishbones may be a good option if all else fails, I can't see it will hurt as long as long as it is snug within the chassis with all them washers - and obviously I can only reduce the tube not the wishbone arm bits!

Cheers again everyone, further comments are of course very welcome!!!

Rob

[Guest Ant]

Top wishbones are straight at the rear, angled at the front.

 

Personally I'd be reluctant to shave the wishbones, but if you need to change the caster angle then I can see no alternative to this.

 

In support of the discussions above, I had a play in my wife's polo on an empty car park on the weekend. If the car rolls forwards there is hardly any self centering effect. As soon as I engage the clutch and hit the pedal it self centres immediately. I also tried it in reverse. Both with and without power the car doesn't self centre in reverse at all. Remember this is front wheel drive.

 

Ant

[Les Welsher]

Rob,

you do have the wishbones in the correct way, the coilover would never fit if you had the angled arm at the back.

As far as shaving some metal off the front mounting tubes on the wishbone to alter the castor, if yours are like mine, there is no excess tube protruding to take off without cutting into the weld where the wishbone tube is attached. Plenty of room for adjustment on the rear mounting (lots of washers)

I took my car for the SVA test with about 30 psi in the tyres as at lower pressures the steering seemed fairly heavy. Forgot to reduce the pressures after the test and even drove to Stoneleigh and back with 30 psi in them, I have dropped them down to 24 psi now to see how it goes.

 

Have you booked your retest yet ?

 

Les

[Guest Taffy Rob]

Hi Les

yes, retest is on the 20th, and its GOT to pass this time as there won't be time for a retest between then and the wedding...

Everything is done except the steering, I put my new sump on over the weekend (bit of a job actually), the guy shortened it for me for 25 quid - a bit of a bargain really.

Everyone except me seems to have loads of adjustment on the rear mountings! The top rear is fixed, and the bottom front is fixed, so Its going to be hard to pack out to obtain the right settings, and would involve bending the wishbones slightly anyway. It would have been better I think to either have all four mountings adjustable, or failing that have four fixed mountings in the right place to start off with.

Why can't they just watch while Im demonstrating self-centering action - I think a bit of fancy knee work on the sly would see me right! Look, no hands, etc

Rob

p.s. IF it passes do you fancy a run out on bank holiday weekend? And anyone else in our area???

[Les Welsher]

Rob,

on mine the front top and bottom mountings are fixed where they go into the square sections that are welded to the chassis across the car, the rear ones are angled plate that you drill and fit in the correct position then fill in the remaining gap with washers.

 

If it passes I wouldn't mind going out for a spin, I think Pauls SVA is near the end of this month so there could be three of us on the road in this area soon.

 

Les