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Into the dreaded 'no self centring' ballgame
Slimy38 - 29/9/25 at 03:37 PM

My car is 99% finished, with only a few bits of trim to add. Everything looks and feels ok... apart from steering. The car will go in a straight line, and it will turn... and keep on turning.

Bearing in mind I can only get to just above walking speed in the little cul de sac I live in, but turning the wheel slightly off centre turns the car and it stays in that direction when I release the wheel. If I turn it too far it'll actually go to full lock and stay there. Even typing that out loud I know I don't want to be going any kind of speed and have that to deal with.

The first thing I think I should do is actually measure caster. How is it measured, what two points are used and what is the reference? I'm assuming it's the floor rather than the chassis? I see anywhere between 6 and 10 degrees, is that about right?

I've checked the wishbone orientation, and toe in is slightly in from zero (I got it to zero then gave the track rods a 'tweak' so not very scientific). The other consideration is that they're built with MX5 hubs with the Saturn wishbone design, but without the lower wishbone adjustment.

adithorp - 29/9/25 at 05:39 PM

It's measured from vertical. 6-10⁰ should be ok.

First off; eyeball it.
Is the centre of the top ball joint behind the lower one? With the track rod ends disconnected, do the hubs turn easily? Similarly does the rack turn freely?

Slimy38 - 29/9/25 at 07:10 PM

From vertical it's measuring 6 degrees, so it's at the bottom of the range. I went back through my blog and I can also see I calculated it to be 6 degrees then, so I've built it to spec. It just drives horribly.

Everything is loose and easily moved, I can control the steering from a road wheel with very little effort.

adithorp - 29/9/25 at 07:42 PM

Then it should self centre to some degree.

Toe can affect it. How much was a "tweek"? A nudge of toe-out can help but shouldn't be necessary. Likewise a bit more tyre pressure. What's your camber like?

I've got 6deg castor (from memory... might be 8) and if I put a 1/4 to 1/2 turn of lock on and push the car it trys the self centre.

JC - 30/9/25 at 06:12 AM

Double check everything!
For example, I discovered my (cortina) uprights were warped!

Measure vertically up from the centre of the bottom ball joint.
Measure how far back the top ball joint centre is from the vertical line.
Divide the distance the ball join is behind by the vertical distance between the ball joints and hit INV TAN on your calculator.

As a rule of thumb, if the vertical distance between the ball joints was 20cm, then the top ball joint needs to be 2cm behind the bottom one for 6 degrees of caster.

Sorry if I’m teaching you to suck eggs - whenever I encountered an inexplicable problem, I had usually made a really obvious error so I’m judging you by my own low standards lol

Slimy38 - 30/9/25 at 06:49 AM

quote:
Originally posted by adithorp
Then it should self centre to some degree.

Toe can affect it. How much was a "tweek"? A nudge of toe-out can help but shouldn't be necessary. Likewise a bit more tyre pressure. What's your camber like?

I've got 6deg castor (from memory... might be 8) and if I put a 1/4 to 1/2 turn of lock on and push the car it trys the self centre.


For toe I went for roughly one revolution of each track rod, but I wound them in not out. I thought I was aiming for toe-in so I need to sort that. Tyre pressures are 25 if I remember rightly.

Camber could be an interesting one, what should it be? Along similar lines I do need to look at ride height, at the time I was more interested in the front wheels avoiding the arches rather than driving well.

Slimy38 - 30/9/25 at 07:09 AM

quote:
Originally posted by JC
Double check everything!
For example, I discovered my (cortina) uprights were warped!

Measure vertically up from the centre of the bottom ball joint.
Measure how far back the top ball joint centre is from the vertical line.
Divide the distance the ball join is behind by the vertical distance between the ball joints and hit INV TAN on your calculator.

As a rule of thumb, if the vertical distance between the ball joints was 20cm, then the top ball joint needs to be 2cm behind the bottom one for 6 degrees of caster.

Sorry if I’m teaching you to suck eggs - whenever I encountered an inexplicable problem, I had usually made a really obvious error so I’m judging you by my own low standards lol


Keep the ideas coming, at this stage I need all the egg sucking that's available! In fact, just typing this response I've realised I've made a mistake in measuring. I was measuring off the bit that is fixed to the wishbones, so the 'cap' that is on the back of the ball joint. There's no guarantee that is actually in line with the pivot point, just thinking about it the bottom one definitely isn't because it's horizontal. I need to measure from the parts that are fixed to the upright, IE the ball part. I think I'm going to have to try again.

gremlin1234 - 30/9/25 at 09:02 AM

what tyre pressures are you using?

edit:
ah, I see you have answered that already

[Edited on 30/9/25 by gremlin1234]

Slimy38 - 30/9/25 at 10:55 AM

quote:
Originally posted by gremlin1234
what tyre pressures are you using?

edit:
ah, I see you have answered that already

[Edited on 30/9/25 by gremlin1234]


Aye, is 25 about right for the car? It's a bit tubby for an average locost, if I understand my corner weighting method correctly I'm heading for around 800kg.

steve m - 30/9/25 at 03:32 PM

My Locost, was 17-18psi on all 4 wheels, and was perfect, any lower pressure the tyre would creap on the rims, anything above 22psi (ish) it was like driving on ball bearings

To help with self centering, i believe folk would put 30+ psi in the front only, and just to satisfy the SVA/iva self centering rule, and once passed, reduce to 20

adithorp - 30/9/25 at 04:45 PM

[For toe I went for roughly one revolution of each track rod, but I wound them in not out. I thought I was aiming for toe-in so I need to sort that. Tyre pressures are 25 if I remember rightly.

Camber could be an interesting one, what should it be? Along similar lines I do need to look at ride height, at the time I was more interested in the front wheels avoiding the arches rather than driving well.



A full turn each side will be giving you about 1 to1.5deg toe-in. 0 to 0.5 would be more like it and even a touch of toe-out can give better turn in though to me makes it a bit twitchy.

Camber probably 0 to 1 deg negative.

jacko - 1/10/25 at 03:32 PM

Just as a matter of interest do f1 cars have self cantering
Jacko

coyoteboy - 1/10/25 at 10:12 PM

quote:
Originally posted by jacko
Just as a matter of interest do f1 cars have self cantering
Jacko


Driver61 says yes. I can't imagine a car being pleasant without.

Slimy38 - 3/10/25 at 09:09 AM

quote:
Originally posted by coyoteboy
quote:
Originally posted by jacko
Just as a matter of interest do f1 cars have self cantering
Jacko


Driver61 says yes. I can't imagine a car being pleasant without.


I can certainly attest to my car being quite unpleasant at the moment!

JC - 5/10/25 at 06:09 AM

Maybe post some pictures of your front suspension?
Someone may spot something you missed?

jacko - 5/10/25 at 02:08 PM

Who made the. Chassis did it come as a kit chassis body etc could the front suspension part be wrong ?
Is the top suspension arms on the wrong side pushing the top forward to much ?
Just thinking out aloud
G

Slimy38 - 5/10/25 at 05:33 PM

quote:
Originally posted by jacko
Who made the. Chassis did it come as a kit chassis body etc could the front suspension part be wrong ?
Is the top suspension arms on the wrong side pushing the top forward to much ?
Just thinking out aloud
G


Haynes Roadster self build, and yes the suspension being wrong was my first thought. It's not the wrong way round but I do wonder whether I've done something else wrong. Unfortunately the weather has limited what I've been able to try this weekend, I'll have to get back in there during the week.

jacko - 6/10/25 at 06:06 PM

It’s not the Healy then ?

Slimy38 - 6/10/25 at 07:48 PM

quote:
Originally posted by jacko
It’s not the Healy then ?


Yes but thats just a body on top of the haynes roadster.

jacko - 7/10/25 at 07:27 PM

Very nice , a mate of mine had a Serbring with a Nissan straight 6 engine

G