MK7
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| posted on 29/7/03 at 09:56 AM |
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Castor angle / self centring
I'm no expert on this, hence the question...
I understand that self centring of the steering wheel will be facilitated, to a large extent, by the castor angle of the front wheels.
I also understand that many MK Indy builders have had to resort to applying extensive toe out to achieve the SVA required self centring capability
(and then subsequently reset the geometry to save excessive tyre wear).
I have trial fitted my front upper wishbones in accordance with the directions provided by MK, namely that the top ball joint is pointing upwards and
that the slightly longer arm of the wishbone is to the front.
To my eye, this arrangement seems to contribute to a reduction in positive castor and may even produce negative castor.
Has anyone fitted the wishbones the other way around with the shorter length to the front, if so how is self centring?
It would seem that with the shorter length to the front this will move the top ball joint forward with respect to the lower ball joint and therefore
improve castor.
Am I missing something?
Russell
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GO
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| posted on 29/7/03 at 11:47 AM |
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IIRC You want the top balljoint behind the bottom ball joint for correct castor (positive I beleive)
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MK7
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| posted on 29/7/03 at 12:44 PM |
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Go,
You appear to be right. I found this web site
http://www.lancerregister.com/faq/T01/t01.html
which gives a nice explanation of steering geometry.
I have to say though, this seems counter intuitive (at least in my case  ).
Thanks though for the feedback
Russell
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Rorty
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| posted on 31/7/03 at 02:14 AM |
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MK7, I haven't looked at that site you mention yet, but if it's of any help, my own site fully describes all the main suspension/steering
functions in plain english, and with diagrams.
As GO says, the top ball joint must lie behind the bottom one, when viewed from both the side and above.
The angle is normally between 4 and 8 degrees, though I seem to remember on another thread, the ideal angle for the book Locost is 5.5 degrees. Anyone
support this?
Just had a quick look at that site. It describes a method of accertaining the correct amount of toe by measuring off the rims.
That method can prove inaccurate. I prefer to measure off the (scribed) tyre centre line, as ultimately, that is the area that's in touch with
the road. Using the tyre centreline also allows for any rim damage/tyre defects/inaccuracies.
[Edited on 31/7/03 by Rorty]
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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MK7
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| posted on 31/7/03 at 08:03 AM |
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Thanks Rorty,
But why then do so many locosters (or is it just MKers) have problems with self centring?
I've not had chance to check the angle on my MK but from a visual check there is positive castor and yet so many people have to resort to
excessive toe out to get through SVA?
Russell
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zetec
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| posted on 10/8/03 at 06:43 AM |
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I had zero self centreing action when I first finished, and as you say had to go for max toe out to get thru SVA. Now I am back to almost zero toe
out I am finding that as the suspension starts to loosen up things are getting better, not a lot, but better. A mate of mine had a long chat with
Martin K at a recent show about the issue and he says that more castor than is available with standard wishbone has little effect. I would also like
to know if W**tfield or Cateringvans have the same problem.
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Peteff
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| posted on 10/8/03 at 08:20 AM |
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I know of one westy that has the same thing I have with my locost. I don't see it as a problem when driving. My own theory is that it is due to
weight (lack of) and distribution. Do what you have to to SVA it then change it after to what you like.
yours, Pete.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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8smokingbarrels
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| posted on 10/8/03 at 06:34 PM |
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This thread raises a question that Ive been puzzling about.
As is stated here- castor angle is created in the upright by having the upper balljoint slightly further back than the lower.
My question is- does the shape of the upper wishbone have an effect on castor? Both the locost and Tiger Avon books show upper wishbones with one side
longer than the other (obviously to take account of castor).
What puzzles me is - why not make the upper wishbone symetrical (both sides same length) and move the upper brackets (by moving FU1/2)rearwards.
Can anyone tell me if this would affect castor (or something else) in some way.
thx
chris
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Stu16v
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| posted on 10/8/03 at 08:45 PM |
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quote:
What puzzles me is - why not make the upper wishbone symetrical (both sides same length) and move the upper brackets (by moving FU1/2)rearwards.
There is no problem doing that, except that the wishbone brackets are unlikely to line up with the chassis, or at least not enough to support them
sufficiently, hence the reason they are built offset. FU1 and FU2 would need to be leaned rather than moved rearwards, otherwise they would end up
fouling the bottom wishbone rear mounting bracket.
You could get over that problem by making the bottom wishbone assymetrical...
[Edited on 10/8/03 by Stu16v]
Dont just build it.....make it!
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suparuss
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| posted on 10/8/03 at 09:32 PM |
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howdy, i believe the standard sierra castor angle is 7 degrees if that is any help, the more castor angle you have the easier the steering will self
right, but im not sure of the implications of having too much.
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Rorty
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| posted on 11/8/03 at 05:03 AM |
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I know of one builder who swears by his 12 degrees of pos castor (350 kg race car), but he has zero scrub radius.
If he had any scrub at all, the steering, with that amount of castor, would start to get really heavy. The car definitely self-centres though!
I've never driven his car, but I'm sure I wouldn't like a car with no scrub radius.
MK7:quote:
But why then do so many locosters (or is it just MKers) have problems with self centring?
There could be a couple of factors.
- Light-ish front end (unlikely).
- Incorrect castor (Zetec's post would confirm this. Search for Mark Allanson's thread on castor. Apparently the book is incorrect. No!
Surely not?).
8smokingbarrels:quote:
...does the shape of the upper wishbone have an effect on castor?
No. The wishbone could be mounted as far back as the driver's elbow, and also attached to the front bumper, as long as it's
pivots are in the same vertical and horizontal planes as the book wishbone.
Sometimes upper wishbones are made assymetrical to accomodate the shocks, which are usually mounted centrally on the lower wishbone.
In a perfect world, the wishbones could be made symetrical, and the inner mounts shifted to suit.
Has Jim McSorely sorted this out?
suparuss: quote:
...the more castor angle you have the easier the steering will self right, but im not sure of the implications of having too much.
Too much castor will make the steering heavy, as the steering effort has to lift the tyres from one lock to dead ahead, and down again to the
other lock.
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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Surrey Dave
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| posted on 11/8/03 at 10:25 AM |
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Caster / Self Centering
My car was built with 5 degrees ,which as Hicost says the top balljoint is about 20mm behind the bottom one.
However although the steering weights up as you put on lock and tends to self centre, it could be better as there is kind of a dead spot in the
straight ahead position, which wanders a bit and likes to follow the road undulations.
So I will probably try to add some caster, if I was building again I would put more than 5-5.5 degrees.......
see car at :
http://www.groovy42.freeserve.co.uk
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stephen_gusterson
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| posted on 11/8/03 at 02:33 PM |
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I have read several times that a 'book' car needs about 7 degrees - I think that was on TOL.
The weight of the car apparently serves to force the steering to the ahead position, using the angle to provide the effort. The greater the angle, the
more the front of the car rises on lock, and the more downward, centering force is created. Too much angle = too much force = stiff steering.
I plan to run 7 degrees or so on my car, but its got power steering.
All the above is what I have read, not what I can relate as personal experience.
atb
steve
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8smokingbarrels
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| posted on 22/8/03 at 05:35 PM |
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A few people on this thread having mentioned altering their castor angle.
Just wondering how you go about doing this without modifying the upright- or is this what u are intending to do?
Thx
chris
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Rorty
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| posted on 23/8/03 at 02:35 AM |
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chris, don't modify the upright, just recalculate the three points of the triangle that is the upper wishbone.
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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8smokingbarrels
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| posted on 23/8/03 at 02:56 PM |
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Thx guys- and I hope I havent started any arguements between you two!
chris
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8smokingbarrels
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| posted on 27/8/03 at 06:59 PM |
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Thx Syd
Just one thing- from what u are saying increasing the castor angle will involve tilting the upright towards the back of the car.
Does this mean that the steering knuckle on the upright will be slightly lower than before? If so - does the steering rack have to be moved downwards
slightly as well to avoid bump steer?
Thx
chris
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Stu16v
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| posted on 27/8/03 at 10:56 PM |
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If your steering rack is forward of the wheels (i.e. conventional locost), tilting the uprights backwards will actually raise the steering arms
slightly.
And yes, it could affect bump steer, but it *may* make it better (unless it is perfect already....).
HTH Stu.
Dont just build it.....make it!
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Spyderman
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| posted on 29/8/03 at 12:13 PM |
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It is a good point that chris has brought up.
If you top ball joint is moved backward to increase caster then it will also move down slightly.
If your rack is mounted in front then it will need to be moved up and backwards. If rear mounted then it wil need to go down and backwards.
As Syd says the movement necessary for the rack would be very small, but it would be interesting to see tha actual figures plotted! 
Terry
Spyderman
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Dick Axtell
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| posted on 29/8/03 at 05:08 PM |
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Castor Angle
This thread is of particular interest. IIRC soembuilders proposed using extra-wide upper wishbone mounting brkts. Fine adjustment could then be
achieved by varying the spacing washers fore & aft of the wishbone pivots.
So who has tried this method? (Arrrgh!! Just read Syd Bridges message!!)
Here's a diag of castor angle. The distance at the road/tyre interface is known as "trail". This dimension is the main determinant of
self-centring. Modern tyres also produce an effect called "pneumatic trail". The combination of mechanical & pneu trail should result in
self-centring, if the dimensions are correct.
[Edited on 29/8/03 by Dick Axtell]
 
Rescued attachment CastorAng.gif
Work-in-Progress: Changed to Zetec + T9. Still trying!!
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craig1410
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| posted on 29/8/03 at 10:46 PM |
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Dick,
I have used this method by using 32mm diameter heavy penny washers with an M10 hole to suit my suspension bolts.
These washers coincidentally are exactly the same outside diameter as my Nylon bushes and thus they look like they are meant to be there. I have used
two washers either side and set my castor to be 5 degrees in the centre position so I think I can get +/- 2 degrees with the thickness of the washers
used and thus 3 to 7 degrees should be achievable. I have used washers on both upper and lower wishbones so I can select castor in 8 increments
between 3 and 7 degrees. This should be handy for matching the two sides of the car once I get it on the road and settled down.
My suspension brackets are 50mm between the inside faces and I think the washers were about 1.7mm thick (from Screwfix direct). I did the maths at the
time I built it but can't remember exactly now. I can dig out the details if anyone is interested. I am using "book" wishbones from
Lolocost (yes they are crap quality but I have sorted that with extensive fettling)
IMHO, this is a very easy and quite elegant way of implementing adjustable castor. Someone described it as "tatty" before but to me it is
tidy and effective.
Cheers,
Craig.
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jcduroc
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| posted on 30/8/03 at 09:50 PM |
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quote:
Originally posted by Rorty
8smokingbarrels:quote:
...does the shape of the upper wishbone have an effect on castor?
quote:
No. The wishbone could be mounted as far back as the driver's elbow, and also attached to the front bumper, as long as it's pivots
are in the same vertical and horizontal planes as the book wishbone.
Sometimes upper wishbones are made assymetrical to accomodate the shocks, which are usually mounted centrally on the lower wishbone.
In a perfect world, the wishbones could be made symetrical, and the inner mounts shifted to suit.
Has Jim McSorely sorted this out?
I guess that both the upper and lower wishbones could me made symetrical, ie, built in a single (one for upper one for lower) jig. Of course this
means you have to recalculate the position of the mounting brackets.
The lower wisbone could be made horizontally symetrical, ie, the inner mounting points are at the same distance from the centre plane of the balljoint
(is this clear?).
The upper wishbone mounts a Transit rod end in a threaded tube so you can turn it 180 degrees around the inner joints axis and it will fit in the
other side of the car; the axis of this rod end just has to be some x mm rearwards of the CL of the lower balljoint to give the desired castor
angle.
Am I wrong?
Cheers
João Matoso
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Spyderman
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| posted on 30/8/03 at 11:36 PM |
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Don't forget your coil-overs need to come up through the top wishbones. These need to be vertically mounted, not angled back with your upright
(assuming lower wishbone is horizontal).
This will make your upper wishbones unsymetrical in order to clear them. 
Spyderman
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8smokingbarrels
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| posted on 31/8/03 at 06:13 PM |
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Thx Spyder and Joao
Since asking that question about upper wishbone shape- I've since realised I have a driveshaft to worry about as well (at the back) and thus
coilover will be off centre- so upper wishbone will DEFINITELY be rather assymetrical!
Such is the long learning curve!
chris
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jcduroc
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| posted on 1/9/03 at 08:44 PM |
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Symetrical wishbones
quote:
Originally posted by Spyderman
Don't forget your coil-overs need to come up through the top wishbones. These need to be vertically mounted, not angled back with your upright
(assuming lower wishbone is horizontal).
This will make your upper wishbones unsymetrical in order to clear them.
Terry
I don't actually follow what you mean by "vertically mounted, not angled back with your upright"; usually coilovers are mounted at a
(+/-) 45 to 65º relative to the ground plane.
Of course the wishbones will have different front and rear tubes but I meant symetrical relative to its own plane (Put the LHS bone on a board, now
pick it by the apex, rotate it 180º clockwise until it rests again on the board: "voila", you have the RHS wisbone).
João
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