Mix
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| posted on 5/3/04 at 04:14 PM |
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Steering self centering
Having just read J30fos thread re failing the SVA for a lack of the above I'm now confused! (again)
It would appear from the responses that self centering can be improved by both increasing and decreasing the toe of the front wheels. Is this the case
and if so is one option preferable to the other??
Mick
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ned
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| posted on 5/3/04 at 04:52 PM |
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Iwas of the understanding that toe out was better, though have no evidence to support my theory/opinion.
Remember those using sierra uprights with mushroom inserts can also adjust hte castor to gain more seld centering..
Ned.
beware, I've got yellow skin
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bob
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| posted on 5/3/04 at 05:04 PM |
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I'll be doing a test on this monday probably while on my long awaited test flight out of the garage.
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zetec
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| posted on 5/3/04 at 06:56 PM |
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MK told me to adjust castor to max (upright leaning back) and as much toe out as is needed to get some self centre action. This will get thru SVA but
is not good in normal use. When I've checked out a Dax Rush using the Sierra upright this has a few degree more castor and I suspect to give a
more normal self centre action which the Indy does lack.
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stephen_gusterson
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| posted on 5/3/04 at 09:23 PM |
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im of the understanding that centering is done by caster, and that using camber is a bodge.
just what I have read over last 4 years - no practical experience
atb
steve
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pbura
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| posted on 5/3/04 at 10:01 PM |
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No harm in more caster for converted struts?
A fellow I've had some email correspondence with came up with an interesting thought:
When you use an upright originally designed for use with a strut, you wind up with a hellacious kingpin angle, 12-16 degrees as far as I can tell.
Because of this, you get a lot of camber gain when turning; with zero caster, the camber gain for a 90 degree turn would equal KPI!
In this situation, wouldn't more caster help the problems caused by a big KPI???
Pete
Pete
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Mark Allanson
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| posted on 5/3/04 at 10:02 PM |
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With caster, as you move the steering away from centre, the geometry tries to lift the car, so the natural state is in the straight ahead position. A
Cortina only has 1.5 degrees of castor to give satisfactory self centring, because it weighs about double a locost, what I want to know, not having
driven my car, is why I am going to need 5.5 degrees of
castor to get the same effect, and why many builders still have no self centring even with 5.5 degrees and resort to toe out to synthesise the effect
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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Bob C
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| posted on 5/3/04 at 11:01 PM |
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When I calculated the castor angle from the book wishbone drawings I noted it was exactly half the 5.5degrees (or 5.3degrees!!!) the book claims. I
believe it's a book error - you can see where the mistake was made - and in view of the self centring fails at sva I've mounted my lower
wishbone 10mm forward to get nearer a true 5 degrees. Other builders have made the top wishbones even more asymmetric to get the same effect. The toe
out thing is a desperate bodge to scrape through sva & not a proper fix ;^) (OK shoot me down fellas....)
Having said that plenty of performance cars have fairly ineffective self centring, well my old lotus elan did anyway..
cheers
Bob C
PS book calc error - page 83 centre line to right edge 4.75" - to left edge it's 4". Ron has taken this 0.75 difference over the
height of the upright & said that's 5.5degrees. His mistake is that the centre line has only moved 0.375" from the true centre, the
castor is only 2.75degrees.
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Mark Allanson
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| posted on 5/3/04 at 11:10 PM |
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Bob, I found out about the book error on the site, and redesigned my wishbones to compensate, I worked out that a 22mm rearward displacement of the
upper balljoint would give 5.5 degrees. It still seems that many builders get self centering problems even with the 'required' amount of
castor, especially MK owners - any explanation?
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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Viper
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| posted on 5/3/04 at 11:34 PM |
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Why not just make your top wishbones adjustable?
Tim
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Bob C
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| posted on 5/3/04 at 11:34 PM |
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Yo Mark - re your previous post - is that 1.5degrees for real??? doesn't ound like anywhere near enough does it!
I don't actuall understand why toe- out would increase self centring - does it just make the steering lighter so what bit of castor there is has
a better chance of working? On my mx5 - extra toe out gives incredibly sharp turn- in on bends, some folk wind a bit on for track use.
Cheers
Bob
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Mix
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| posted on 6/3/04 at 08:29 AM |
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As always there appears to be no simple answer to suspension / steering questions. The reason I asked is that in the attachment posted by Terrapin
racing it clearly states increasing toe in will increase self centering. Is this a typo or do we have a choice??
Mick
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Mark Allanson
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| posted on 6/3/04 at 11:58 AM |
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Bob, yes 1.5 is correct, I looked it up at work when I was starting the suspension. Most production cars have caster not exceeding 2.5 degrees
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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britishtrident
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| posted on 6/3/04 at 03:42 PM |
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The mistunderstanding between toe-in and toe-out is due to the fact most modern cars are fwd drive. and have toe out to ensure they are somewhere
near parrallel under power.
RWD road cars should always have toe-in or zero toe angle, a rwd car wiith toe-out on the front wheels will be unstable running in a straight line.
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Mark Allanson
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| posted on 6/3/04 at 03:44 PM |
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The toe in/toe out thing is only the static measurement, all cars should be parallel on motion (thats the theory!!)
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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britishtrident
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| posted on 6/3/04 at 03:51 PM |
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Caster angles
Caster angles on small sports cars tended to be around 7 degrees or more, but when radial tyres became standard less was required this is because
radial tyres have built in self aligning action --- provided they are not over inflated.-- tyre presures should be roughly in proportion to the
weight on the wheel.
The Cortina 3-5 ran zero caster and depended on the tyres and toe-in for self-centring. Fitting Escort or Capri wheels to Cortina suspension upsets
this because they have a different offset.
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zetec
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| posted on 6/3/04 at 08:26 PM |
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Will ask Dax at Stafford what they run as it looks a lot more than my MK and see what they say. Have heard Tiger owners having same problems at SVA
test, with one guy having to fit lock stops as there was no self centre action at all on normal full lock.
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Mark Allanson
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| posted on 7/3/04 at 12:15 AM |
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"The Cortina 3-5 ran zero caster and depended on the tyres and toe-in for self-centring. Fitting Escort or Capri wheels to Cortina suspension
upsets this because they have a different offset"
The Cortina ran 1.5 degrees castor, not dependant on the the tyres, ran toe in because it was rear wheel drive and therefore the wheels would have a
natural tendancy to toe out under motion, RWD escorts and capris all had 19mm offset, same as the Cortina, except for the larger Capris which ran ?
5mm offset.
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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Spyderman
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| posted on 7/3/04 at 12:28 AM |
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The idea of using extra toe out for self centring is due to the extra drag that the outside wheel generates by being out of alignment.
Having a lot of scrub would do the same thing if it is offset as opposed to rearset (scrub is on outside of kingpin ground interception).
Terry
Spyderman
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britishtrident
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| posted on 7/3/04 at 03:05 PM |
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Cortinas mk 3 to 5 ran different wheel offsets from other Fords because the Escort and Capri required less back space because of the need to clear
strut spring pan.
Back in the 70s if I ever got a Cortina with brake pull one of the first I checked was that exactly the same type of wheels were fitted to both side
--- the wheels LOOKED identical, but fit the wrong wheel and the steering wheel would dive to the side under braking.
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