Mave
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| posted on 13/5/06 at 11:04 AM |
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First drive, but no self centering!
Yesterday I took my car over to a place where I could drive it for the first time, and have it checked over.
During the drive it became clear that the car didn't self-center at all! I have the mushroom with the hole all the way to the front, so castor
angle is maximized.
I thought toe-in might help the self centering, so we tried that, but to no avail. Maybe I should have tried toe out also. I've just read that
some people put it on extreme toe out angles just to pass SVA, and turn it back to neutral after the test. But surely that leaves the car handling to
be desired, right?
How do I solve this? I have the "old" round tube wishbones, but does the new "flat oval" design have a different geometry?
If not; can the problem be solved with other top wishbones? To give the car more castor?
The guy also let me drive a Striker, which did self center perfectly. And I must say that that gave much more confidence going around corners
fast.
I know some use springs around the steering rack, but isn't that a bad fix to a bad design? Of course, I want to get it through SVA first, and
this might help. But in the end I do want a great handling car.
Marcel
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donut
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| posted on 13/5/06 at 11:07 AM |
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I have been told 0.5 or 1 digrees toe OUT for self centering.
Maybe people can post what they set theirs to for SVA.
Andy
When I die, I want to go peacefully like my Grandfather did, in his sleep -- not screaming, like the passengers in his car.
http://www.flickr.com/photos/andywest1/
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stevec
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| posted on 13/5/06 at 11:45 AM |
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Im confused.com now, All the stuff I have read says hole to the front.
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britishtrident
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| posted on 13/5/06 at 11:46 AM |
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Offset mushrooms don't have ANY effect on caster all that can change caster is the relative position of the upper and lower balljoints.
However offset mushrooms can be used to alter the king pin inclination, and they also have an effect on bump steeer.
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tks
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| posted on 13/5/06 at 12:19 PM |
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As far as i know
For better self centering you need to do this:
For better response you need to do the oposide. I refer to this only on the vieuw from above.
The vieuw from front of the car is more bumpsteer/car load relative...
Sow i would let it go 0 degrees (from front) and 1 from above to each other..
Then you have got self centering and not the bad tyre wear...
 
Rescued attachment SelfCentering.JPG
The above comments are always meant to be from the above persons perspective.
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Mave
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| posted on 13/5/06 at 01:19 PM |
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I think the mushroom insert change both king pin inclination and caster angle. King pin inclination angle refers to the relative positions of upper
and lower balljoint when looking from the front, right? And caster is the same, but then looking from the side, right? (please correct me if I'm
wrong).
So if you put the off-centre hole to the front, you have maximized the caster, and king pin inclination in the middle. If you put the off-set hole to
the outbd. side, you have maximized the king pin inclination and put caster in the middle.
TKS: if I understand correctly; if I give it more negative camber, that would also help?
Marcel
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nitram38
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| posted on 13/5/06 at 02:04 PM |
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READ THIS CLICK HERE
Should help you all!
With your top ball joint forward towards the front o the car you will get unwanted positive castor. For self centering the top ball joit should be
further back towards the rear of the car. This will give you negative castor and self centering.
Race cars should have 3-6 degrees and roadcars 6-12 degrees of negative castor.
[Edited on 13/5/2006 by nitram38]
[Edited on 13/5/2006 by nitram38]
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britishtrident
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| posted on 13/5/06 at 02:55 PM |
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quote:
Originally posted by Mave
I think the mushroom insert change both king pin inclination and caster angle. King pin inclination angle refers to the relative positions of upper
and lower balljoint when looking from the front, right? And caster is the same, but then looking from the side, right? (please correct me if I'm
wrong).
So if you put the off-centre hole to the front, you have maximized the caster, and king pin inclination in the middle. If you put the off-set hole to
the outbd. side, you have maximized the king pin inclination and put caster in the middle.
TKS: if I understand correctly; if I give it more negative camber, that would also help?
Marcel
No mushrooms have nothing to do with caster - to find out the reasons use the forum search it has been brought up several times. To get some
reasonable caster when viewed from above the upper ball joint should trail the lower one by 25mm.
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britishtrident
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| posted on 13/5/06 at 03:04 PM |
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quote:
Originally posted by nitram38
READ THIS CLICK HERE
Should help you all!
With your top ball joint forward towards the front o the car you will get unwanted positive castor. For self centering the top ball joit should be
further back towards the rear of the car. This will give you negative castor and self centering.
Race cars should have 3-6 degrees and roadcars 6-12 degrees of negative castor.
[Edited on 13/5/2006 by nitram38]
[Edited on 13/5/2006 by nitram38]
Love to see anybody even Schumy try to drive a RWD drive car with 12 degrees of negative caster and keep running straight.
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britishtrident
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| posted on 13/5/06 at 03:40 PM |
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Caster 101
Almost all cars have posative caster that is when viewed from the side the line joining the top and bottom ball joints slopes towards the rear of the
car.
Only a very few FWD cars for example real Minis & Metros use the opposite negative caster.
Light cars need more posative caster than heavy cars. Ammount of caster required also depends on weight distribution -- less weight on front
wheels demands more caster.
Big downside of caster is it makes the steering heavy in turns -- try throwing heavy car with a fair bit of caster for example an MGB round a tight
roundabout and see how the steering loads up.
Radial tyres produce a self-aligning torque which reduces the ammount of of caster required compared to that on a car running on crossply tyres.
However if the tyre presures are too high for the vehicle weight this self-aligning torque is no longer created.
What actually creates the self centering action due to caster is the distance the centre of tyre contact patch trails the point that the imaginary
line joining the upper and lower ball joints (called by designers the virtual kin pin) intersects the road surface.
On some cars (particularly classic era rear engined RWD cars) this trail was added to by offsetting the king pin line 12 to 25 mm or so ahead of the
of the stub axle centre line, just as on a furniture caster, but this is best avoided even if it were possible to apply it to Locost.
Some typical caster angles
Honda Accord/Rover 600 +2 degrees
Jaguar E Type & XJ6 2.25 degres
MG Midget/AH Sprite +3 degrees
Triumph Spitfire & 19 60s Lotus Elan +4 degrees.
MGB +7 degrees
Opel Manta (RWD) +3.5 degree
Porsche 911 +6.5 degree
[Edited on 13/5/06 by britishtrident]
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DIY Si
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| posted on 13/5/06 at 03:44 PM |
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So what should we be aiming at for a reasonable amount of self centreing? I'd obviuosly like enough to self centre but not so much the steering
is overly heavy and the handling suffers.
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Jasper
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| posted on 13/5/06 at 04:05 PM |
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I just put a whole load of toe out for SVA (not very scientific I know, but it was a visible amount), and even then it only self centred a bit. Then
once you're through SVA get it set up properly.
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Mave
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| posted on 13/5/06 at 05:41 PM |
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Hmm, just made a little sketch, and you're absolutely right; the mushrooms don't affect the caster angle, at all, since (looking from the
side) the angle the balljoints make relative to eachother doesn't change. They do however change the caster offset. Putting the holes to the
front increases the caster offset. That should make the car more stable, right? But might not have huge effect on self centering?
My distributor, who also races the Indy, told me today to put the holes in the mushrooms to the outside. That would increase the king pin offset (and
more negative camber). But what affect would that have? Britishtrident? (I'm seriously interested)
This is all getting really theoretical (like it though), but in the end what I need is a good, confidence inspiring car. I have never driven another
Indy, but from all the posts on this forum about the (lack of) self centering of Indy's, I would say that a redesigned top wishbone might be
just what the car needs. Am I right?
Marcel
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greggors84
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| posted on 13/5/06 at 05:55 PM |
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I think what the mushrooms inserts do is change the angle of the hubs when it rotates. But the hubs are still rotating around the vertical plane. As
the bottom and top ball joints are still inline when view from above.
What needs to be done is move the top ball joint back so that the hub is moving around an axis that is off the vertical.
I think this is why the mushrooms dont make any difference, i know BT has been saying it for ages, i have just been trying to work out why.
Am i close? Or does it not make any sense!
Chris
The Magnificent 7!
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Mave
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| posted on 13/5/06 at 06:01 PM |
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That's exactly how I see it. They need to move to the back; in other words; redesigned wishbones. But there must be a lot of Indy's on the
road as it is; how do the drivers feel about it? Don't they have self-centering, do they have it (if yes: how did they manage to get it), have
they fitted something like springs on the rack?
Marcel
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greggors84
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| posted on 13/5/06 at 06:08 PM |
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Just to add, if you want real castor the best way is new wishbones, Ace Sportcars do some with rod ends, you can then use shims or washers to mount
these further backwards. This is what i will be doing when i get the chance.
As seen in this thread.
http://www.locostbuilders.co.uk/viewthread.php?tid=42581
Have some post disappeared or is it just me?
[Edited on 13/5/2006 by greggors84]
Chris
The Magnificent 7!
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zetec
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| posted on 13/5/06 at 07:09 PM |
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For the SVA I needed loads of toe out...but a few weeks ago I reset to no toe in/out and now the rack and balljoints have freed up a little (5000
miles) the self centreing is OK and I find the car just right. Tyre presures also have an effect, I run 16-18psi.
" I only registered to look at the pictures, now I'm stuck with this username for the rest of my life!"
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nitram38
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| posted on 13/5/06 at 07:16 PM |
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quote:
Originally posted by britishtrident
Almost all cars have posative caster that is when viewed from the side the line joining the top and bottom ball joints slopes towards the rear of the
car.
Only a very few FWD cars for example real Minis & Metros use the opposite negative caster.
I think that you have got this the wrong way around. Taken from the link thingy:
Castor
Castor is quite important for steering feedback and self centring, the reason that castor affects steering so much is that castor (king pin
inclination) causes the swivel plane of the front hub to vary from the vertical as the hub rotates, this has the effect of raising the car slightly,
negative castor I.E. king pin inclined back at the top causes the car to rise upwards as the hubs are rotated from the straight ahead position. This
gives steering 'feel' and a strong tendency toward self-centring, with the weight of the car itself providing the self centring effect,
excessive castor will make the steering very heavy as it will tend to raise the car more. Positive castor has the opposite effect, self-centring is
lost, the steering becomes light and 'wandery'.
[Edited on 13/5/2006 by nitram38]
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britishtrident
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| posted on 14/5/06 at 10:21 AM |
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Caster angle not "castor" angle --- don't take anything off the web as gospel without a sanity check, the fact the main subject
is wrongly spelled should ring alarm bells.
Also note that king pin inclination and caster angle are very different although to an extent inter related.
[Edited on 14/5/06 by britishtrident]
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Peteff
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| posted on 14/5/06 at 01:38 PM |
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It says castor on the printout I got from the tyre centre.
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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JoelP
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| posted on 14/5/06 at 04:05 PM |
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the mushroom effect has been worked out several times before. Turning the insert indeed doesnt affect the castor angle, but it does move your contact
patch forward and back (by about 1 cm at the extremes) this affects the distance between your contact patch and your virtual KP line on the ground,
and hence should add slightly to selfcentering. It also changes your wheel base, your weight distribution, your centre of mass and your polar moment,
though only by ridiculously small amounts 
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Mave
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| posted on 15/5/06 at 07:24 PM |
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Ok, another thing I heard, was that sometimes "a plastic nut" in the steering rack is tightened too much (after it is shortened at MK),
taking out too much slack. As a result the rack is more difficult to move, and hence the self centering is hindered. Well, I can't reach this
plastic nut without removing my radiator (and I don't want to do that, as it is filled). And besides, I don't even have the proper tool
for this nut, because it requires a really large allen-key-shaped tool.
Anyway, I'm told that you should be able to push the arms of the rack in by hand. Is that true? I just tried it by trying to turn the front
wheels by hand, but they won't move at all. Is that correct?
Marcel
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JoelP
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| posted on 15/5/06 at 08:44 PM |
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i can turn my front wheels when the car is on the ground, with the full column attached, so it would seem that yours is a little stiff.
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DarrenW
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| posted on 16/5/06 at 11:13 AM |
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This worked for me:
Mushroom hole to front. Neutral camber. Loads of toe out (guessing 3 to 4 degs and yes handling is awful), tyre pressure between 35 and 40 psi.
I cant recommend driving with these settings - i blew the tyres up at the test and let them back down to 18 / reset the toe immedietely after for
drive home (at a car park nearby, not in front of the tester).
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