skinny
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 posted on 29/4/04 at 11:45 AM |
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self centering - cortina uprights
pls bear with me - my engineering knowledge isn't what it used to be...
just reading re the lack of self centering when using cortina uprights - you get more self centering when you increase caster, right? as you look at
the side of the car, isn't caster the angle away from the vertical that the upright is at, i.e. how much the upright leans forwards or
backwards?
if so, surely this can be changed without significantly upsetting anything else like KPI / camber / steering simply by increasing the longitudinal
offset between the upper and lower wishbone upright mounting points? for example just making upper wishbones so that the ball joint is just moved a
little further towards the front or rear of the car. (as you can see, i'm not sure whether positive or negative caster is desireable) i mean, we
would be talking less than 10° here i think
or am i missing something quite fundamental here... it's more than possible
if you don't fail, you aren't trying hard enough.
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flak monkey
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| posted on 29/4/04 at 12:18 PM |
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You need to shift the top ball joint towards the rear of the car to get the (suggested) 5.3 degrees castor. As you say this is easily done, and many
people on here have come up with their own solutions. My solution to the problem is shown in my photo archive.
Cheers,
David 
Sera
http://www.motosera.com
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JoelP
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| posted on 29/4/04 at 12:19 PM |
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you are spot on there fella, in fact that is how its meant to be, the upper balljoint should be around 20mm+ behind the lower one, this is achieved by
the upper wishbone being oddly shaped (helps it clear the spring) and the mounting being further back. The mushroom lark is only to fine tune it/
change it if you measured wrong!
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craig1410
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| posted on 29/4/04 at 12:23 PM |
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Skinny,
You are correct and without trying to fob you off, there are loads of articles on this site on the subject if you do some searching.
If I remember correctly, the upper wishbone has to be offset towards the rear of the car with respect to the lower wishbone by something like 22mm to
get the "desired" 5.5 degrees of castor. It is the balljoint centre's that need to be offset of course just to clarify.
What I have done on my car is I made the wishbone mountings wider than required by my wishbones and fitted nice big penny washers the same size as my
bushes between the bush and the side of the mounting. This allows me to move the washers around and thus move the wishbones backwards and forwards to
adjust my caster angle. By having 4 washers (2 each side of bush) on each mounting for both top and bottom wishbones I can adjust my caster +/- about
2 degrees. I think the washers are about 1.4mm thick IIRC.
However, to get my caster basically correct to begin with I moved the upper wishbones mountings about 10mm backwards and then doubled up the chassis
tube on tubes LA and LB where it would otherwise overhang. I didn't have a problem with FU1 and FU2 as it overhangs both ends anyway.
This subject is often talked about and tends to provoke much debate so I'd advise you to try and understand the problem from first principles
and then make up your own mind what the best solution is. Don't expect everyone to reply to your request and recommend the same thing because
some people will recommend modifying the wishbones, some will recommend tilting the front assembly (LA,LB,LC,LD) backwards, some will recommend using
Sierra upright with a mushroom insert on the top and some will do as I have done.
I didn't want to use Sierra uprights as I think they are ugly (IMHO). I didn't want to tilt the front assembly because it would not allow
me enough room for my V8 engine and I didn't want to make my own wishbones as I didnt trust my welding for critical components at the time.
I hope this helps you to decide what to do.
Cheers,
Craig.
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skinny
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| posted on 29/4/04 at 12:41 PM |
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thanks - it's just that it seems like a pretty simple solution but it sounds like quite a few people are risking sva failure over it or running
large toe angles just to get through the test and then changing the settings back after - surely you want a bit of self centering for steering feel
tho?
i will have a bit of a measure of my bits tonight  but if i don't have the required angles, do darren / mk etc sell upper arms with the caster
issue taken into account? i have never welded anything before so it's probably best not to trust my efforts with this!
[Edited on 29/4/04 by skinny]
if you don't fail, you aren't trying hard enough.
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craig1410
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| posted on 29/4/04 at 01:26 PM |
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Based on what a few have said, even with 5.5 degrees of castor, you may still need a little bit extra toe in to give a pronounced enough degree of
self centering to fully satisfy the SVA inspector. Self centering varies widely in type approved production cars and it may come down to which car the
SVA inspector drives himself as to whether he passes your car or not. Better to have plenty for the test than not quite enough in my view.
In my case my car is not ready for the road yet but when pushing it in and out of the garage with the engine in place it certainly does self correct
which can be a pain when trying to alternately push and steer on my own...
Cheers,
Craig.
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britishtrident
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| posted on 29/4/04 at 02:13 PM |
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Tyre pressures make a big difference a book Locost is a very light car wide on todays wide tyres (wide for a car of this weight is anything wider than
155x13" the tyre side walls will be very lightly loaded so to get self centring action from the "self algining torque" generated
by the distortion of the tread and side walls the pressures have got to be pretty low.
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Dusty
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| posted on 29/4/04 at 02:47 PM |
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I wish people would stop imagining that rotating the mushroom has any effect on castor. The castor line goes through the centre of top and bottom ball
joints which are held in place by the wishbones. It does change camber and steering arm angle and will change how the car drives perhaps even helping
self centering but it can't alter castor or KPI.
[Edited on 29/4/04 by Dusty]
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skinny
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| posted on 29/4/04 at 02:56 PM |
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trident, i want skinny wheels on it anyway, i've only got a 1300, and would rather have fun driving than have it look good. the fatter tyres
come later when i build my 1.8 k-series!
if you don't fail, you aren't trying hard enough.
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JoelP
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| posted on 29/4/04 at 03:16 PM |
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quote:
Originally posted by Dusty
It does change camber and steering arm angle and will change how the car drives perhaps even helping self centering but it can't alter castor
or KPI.
i think it can alter KPI, in so far as if you change the mushroom angle, to get back to the original camber angle you must screw in or out the upper
ball joint, hence KPI will be different.
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craig1410
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| posted on 29/4/04 at 04:10 PM |
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quote:
Originally posted by Dusty
I wish people would stop imagining that rotating the mushroom has any effect on castor.
Fair point Dusty and I see what you're getting at. To be honest I personally don't like the whole concept of grafting the Sierra uprights
onto the double wishbone setup using these adapters so I haven't given it much thought. I know there is nothing technically wrong with it and it
should be perfectly safe and is very popular but I just don't like it. Anyway, that's another debate entirely and I don't want to
deflect this thread.
Cheers,
Craig.
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crbrlfrost
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| posted on 29/4/04 at 04:49 PM |
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Sounds like you're on the money, skinney. Less than 10 deg, more than book. Shouldn't effect camber or kingpin, but changing castor can
effect bumpsteer and toe. Nothing a little time tuning out bumpsteer with shims (don't we al have to do that anyway) and an alignment
won't handle. Cheers!
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britishtrident
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| posted on 29/4/04 at 05:34 PM |
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I switched wheels and tyres over between 2 of my Rovers today, Driving the that car went down from 195/50x15 to 175/65x14 is now much more fun,
big tyres aren't always good news.
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Mark Allanson
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| posted on 29/4/04 at 05:34 PM |
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It's a while ago since it did it, but I think it was every additional mm of wishbone setback would give approx 1/3 degree of castor. I ended up
with 22mm to give the 5.5 (not 5.3) degrees. Shimming the wishbones can increase the castor appreciable if your brackets are wide enough.
 
Rescued attachment Castor lines.jpg
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
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