Mark Allanson
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| posted on 31/5/04 at 09:00 PM |
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Self centring, which setups are affected?
The self centring issue, is it restricted to those using sierra uprights, I have seen that some avons are having problems, perhaps attaching a set of
tailgate gas struts to the trackrods would be an unusual option?
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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Hellfire
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| posted on 31/5/04 at 09:11 PM |
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IMHO I can't see why any wishbone suspension should self centre (set up correctly for road driving) due to it's design.
But then I have been known to be wrong
[Edited on 31-5-04 by Hellfire]
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Mark Allanson
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| posted on 31/5/04 at 09:46 PM |
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I'm trying to get to the bottom of the problem, in THEORY, as you add lock (assuming you have some castor), the front of the car lifts, so its
natural position is straight ahead. The old cortina only had 1.5 degrees of castor, but had about a ton and a half of rust bearing down on it so had
good self centring.
In a purely self interested point of view, I'm trying to get info on who is having problems, I suspect BEC cars will have more problems than me
with my orbit destablalising pinto bearing down on the front suspension.
Avons have the front end of the car bent upwards (thats a very specific technical term - in case you were wondering) to get the castor angle, and
these seem to have more problems than most. (looking to be proved wrong here)
Strut converted uprights (Sierra) seem to have problems too, does the KPI have a bearing?
Can anyone who has problems with this list their setup and their castor angle, design and actual.
All this toe out and trye pressure stuff is only a dodge to get through SVA, I'm looking for something a little more permanent
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 31/5/04 at 11:39 PM |
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Pile on the castor and go for power steering??
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MustangSix
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| posted on 1/6/04 at 01:40 AM |
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Caster provides dynamic self centering. As the vehicle moves forward, the tire patch is rearward of the pivot and the wheels are pulled into the
straight ahead position by the force of the wheels pushing back. But caster does nothing until the car begins rolling. Witness a grocery cart whose
wheels may be at any angle, yet assume a forward position when the cart is pushed. However, there is another bit of geometry which provides
additional self centering, and also works whether the vehicle is at rest or in motion.
If you examine your spindles closely, you'll notice that the axle stub is not set perpendicular at 90 degrees to the line between the ball
joints. It is slanted somewhat. That angle is known as kingpin inclination. That angle might be anywhere from 5 to 30 degrees, more or less,
depending on the car.
Because the kingpin axis is inclined in relationship to the axle stub, when you turn the wheel, the axle itself rotates in an inclined plane. The
effect is to raise the car a small amount. It is actually the force of gravity pushing back down on this inclined arc that centers the wheel. You
can verify this by measuring the height of the vehicle when the wheel is straight and at full lock. You'll see that as the wheel is turned, the
car actually rises a small amount. This is also how many older cars (especially 50's-60's American cars) with negative caster still had a
self centering steering effect.
It's entirely possible that some spindle designs have insufficient kingpin inclinatation angle to provide adequate lift for centering the
steering, particularly on a very light car.
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NS Dev
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| posted on 1/6/04 at 07:36 AM |
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Mustang Six, a very nice explanation. Surely though caster does have a static effect too, if you think about it it will give lift in the same way as
kingpin inclination does, thus centring under the weight of the car?
The talk of weight of a cortina vs seven doesn't really wash with me, if you look at my picure archive, you will see that on my autograss car, I
have cortina uprights, and the front of the car is so light I can pick it up, but I have very good self centring, in fact I had to reduce the caster
to prevent it wearing me out before the race finished!!
To my mind, there is nothing wrong with the Cortina upright, it's just that most kits do not use enough caster angle.
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spunky
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| posted on 1/6/04 at 08:16 AM |
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I have cortina uprights and a front axle weight of 210Kg. I had a full 4 wheel allignment check carried out, with all wheels set neutral (no toe, no
camber) Caster angle is 5 or 6 degrees. Tyre pressure 25psi. It passed self centering test at SVA without any problems.
It returns from full lock to about 1/3 lock ok at slow speed, the last bit you have to pull back yourself.
John
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stephen_gusterson
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| posted on 1/6/04 at 09:32 AM |
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quote:
Originally posted by Viper
Pile on the castor and go for power steering??
I have 
I have started with approx 5 degrees and I think my mechanics max out at 9 degrees.
I think that may be enough 
atb
steve
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NS Dev
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| posted on 1/6/04 at 02:59 PM |
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sounds about the same as the range of adjustment on my grasser and it certainly gave scope for adequate castor.
Nat.
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