locoboy
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| posted on 22/8/07 at 07:56 PM |
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struggling self centering
My GTS Panther failed SVA on a few minors all of which (fingers crossed) are fixed bar the self centering.
My re test is on Friday morning 
The self centering test was carried out as per the manual
'driving in excess of 10 mph and a degree of self centering should be evident'
basically drive slightly over 10mph and weave a little left and right whilst letting go and looking for a degree of self centering.
I know LOTS of you had your self centering tested in a different way but this is the way mine will be tested.
here is what i have done so far.
set the rack to centre and lined the wheels up by eye as a datum point.
i have moved the wheels over a few stages to reach toe out thats too bad to drive with - i can hear the tyres scrubbing and squeeling and you cant
push the car for love nor money.
I have then wound it off back to my datum point.
then over a couple of stages i have wound it to toe in untill again i have reached the point where the tyres screech and scrub.
still not attempt at self centring.
all this is with soft tyres at approx 15psi
My column is free from snags, you can move the wheels by just gripping the column where it exits the footwell and turning so no excessive friction
there.
My car is using sierra uprights and has no adjustment in the tophats to allow for castor adjustment.
Im struggling for what to try next and time is ticking.....................................................
any ideas?
ATB
Locoboy
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esn163
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| posted on 22/8/07 at 08:04 PM |
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Hi,
we thought we'd struggle with this but in the end we didn't. All we needed with the indy was a couple of turns toe out and highish
(~30psi) tyre pressures. You could try increasing the pressures since this seemed to help us.
HTH and good luck with the retest
Ed
Indy Build Photos
** Build pages **
Photo Bucket Pics
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blakep82
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| posted on 22/8/07 at 08:12 PM |
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springs in the steering rack? just to get past SVA, then sort it after?
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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David Jenkins
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| posted on 22/8/07 at 08:13 PM |
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That's bad news - but I'm going to sound like a miserable sod here, and say that you should try and fix it so that you have decent
self-centering WITHOUT any bodges or dodgy work-arounds.
The reason they look for self-centering is that it is A Good Thing that should be found in every car - it makes the car safer, easier to drive and
could save your life (or someone else's). Dodgy fixes that get you through the SVA are not the proper answer - increasing the castor angle is
probably the only real fix.
Sorry - but that's my view. I really hope that you get through the SVA, but you'll be happier if it's a proper pass.
David
(Good Grief - all of a sudden I'm getting serious... I need a drink to get over it!)
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nitram38
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| posted on 22/8/07 at 08:15 PM |
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Sorry that I am going to say this, but remake your top bones with the correct castor.
It is the safest thing to do and once you start driving your car for real, you will appreciate how easy your steering will feel, especially when
powering out of corners/bends.
Do a search, it has been discussed alot!
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blakep82
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| posted on 22/8/07 at 08:19 PM |
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i deffo agree with these two ^ i was only saying springs because the retest is on friday. he's only got tomorrow to add more castor, whether it
be by redoing wishbones, or brackets
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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locoboy
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| posted on 22/8/07 at 08:20 PM |
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Nitram,
They are GTS bones on a GTS chassis, and besides i dont have the time to make the bones before friday.
David,
I see what your saying and it would be somthing i would address post SVA when i have time to ponder different solutions etc.
Blakep82, there is no point putting springs in the rack as at no point during the test is full lock achieved.
ATB
Locoboy
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David Jenkins
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| posted on 22/8/07 at 08:24 PM |
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It's worth saying that many of the SVA testers are aware of the tricks people use to achieve some form of self-centering - anything obvious and
they'll just send you off with a flea in your ear, and tell you not to come back until you've done a proper job.
So, if you try any 'dodgy tricks', make sure they're subtle! 
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blakep82
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| posted on 22/8/07 at 08:25 PM |
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good point...
looking at you pictures in your archive, could you trim off some of the bushes in the wishbone tubes, and replace with washers (as spacers) to push
the bottom wishbone forward, and the top wishbone back? don't think it would be enough of an adjustment though, but that combined with higher
tyre pressures, as mentioned before?
i guess higher pressures reduces the contact on the road * and causes less friction, making what little effect there may already be, more noticable
* not a great idea to have less tyre contact witht the road though
just suggestions to help with the re-test on friday before sorting it properly after
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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nitram38
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| posted on 22/8/07 at 08:29 PM |
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I don't know much about gts bones etc, but I remade my top bones in 2hrs.
All I used was a sheet of mdf with some holes in to set the bushes up and I recut new material.
I don't know your set up or skills etc.
Are the top bones on the right way around?
What I will say is that the sva man might notice one of these get you through mods as they are now getting wise to this sort of thing.
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David Jenkins
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| posted on 22/8/07 at 08:31 PM |
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As said above ^^^
Try to set the top wishbone back as far as you can, and the bottom one as far forward as you can, will help a bit. It might tip the balance for the
tester.
Personally, I'd postpone the test and get it fixed - but that's your call and no-one else's...
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locoboy
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| posted on 22/8/07 at 08:50 PM |
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There is NO adjustment in either top or bottom bone aside deom samber on the top one.
Looking from above the top ball joint is further towards the rear than the borrom one thus giving some deree of correct castor.
Possibly a set of adjustable mushroom adaptors would suffice if i can scrounge some before friday!
I wouldnt feel confident in measuring and making my own wishbones, especially in a rush!
ATB
Locoboy
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jos
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| posted on 22/8/07 at 08:51 PM |
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col
15psi is way too low for self centering sva test
double it when you get to the sva station but dont let them see you do it
set a little bit of toe out before you go - youll scrub the tyres a little (unless you use the trailer again) on your way over but it using the toe
out & high tyre pressures it should make an effort & get the pass you need
.: Motorsport / motor racing circuit / track wall art Apex Traxs :.
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Macbeast
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| posted on 22/8/07 at 08:52 PM |
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I haven't got as far as you, but I found it quite difficult to decide which way round the top wishbone should go - suggest you check.
It's quite easy to bend the top and bottom wishbone mounting brackets so you could try bending the top ones to the rear and the bottom ones to
the front to give a little more castor.
In addition to that, as suggested above,
you could try skimming a bit off the polybushes on one side and packing the other side out with washers to bring the top wishbones back and the bottom
ones forward.
Good luck, and let us know what happens.
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lotustwincam
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| posted on 22/8/07 at 08:55 PM |
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We had this same problem some years back with using the Cortina uprights and book spec wisbones.
The top wishbones were remade with loads of castor (can't remember the actual figure) and it made not one jot of difference.
In the end applying a touch of Toe Out was all that was necessary.
IIRC part of the cause of the problem is due to the King Pin Inclination that is built into the cortina uprights (and not adjustable) that was
designed around the weight of a Cortina rather than a 7 type car.
I know nothing about the Sierra set up, but wonder if it could be a similar issue that may be exacerbated if you are using a different offset of
wheels than where used on the RWD Sierra?
The wheels in your photo archive look very FWD ish.
Drew
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locoboy
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| posted on 22/8/07 at 09:00 PM |
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I was under the impression that it was better to lower the psi?
I will put it back up and try again with a bit of toe out.
I will be trailering it again, got my emissions within limits and i dont want to do ANYTHING that might mess it up!
Mac.
I dont really want to go trimming the bush (so to speak!) and i would advise against bending any of the brakets
I dont know if any other GTS'ers have had this problem, i suspect not as most of the other centres appear to conduct the test in a differnt way
to how the book states.
ATB
Locoboy
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David Jenkins
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| posted on 22/8/07 at 09:04 PM |
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I was fortunate that I had heard about this problem before I made my top bones, and was able to put in a decent amount of camber when I made them. I
use Cortina uprights, and have more than sufficient self-centering...
As an afterthought - are you sure that EVERYTHING in the steering moves freely? All the ball joints, linkages, etc all moving easily? It's
worth checking, to be sure that any tendency to self-centre isn't lost in stickiness somewhere.
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locoboy
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| posted on 22/8/07 at 09:06 PM |
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Pretty sure everything is ok David
quote:
Originally posted by locoboy
My column is free from snags, you can move the wheels by just gripping the column where it exits the footwell and turning so no excessive friction
there.
ATB
Locoboy
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David Jenkins
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| posted on 22/8/07 at 09:12 PM |
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Jack up the front (both wheels together) and then try pushing one of the wheels left and right - you'll get some idea of what the castor
geometry has to overcome! This is not too scientific, as doing this messes up the front geometry, but it is educational.
I'm puzzled about one thing - I've got a suspension book that says that having some toe-in makes the car more stable and thus it's
easier to steer a steady course. It then says that having some toe-out aids cornering, but makes straight-line driving harder and more
'edgy'. But everyone recommends toe-out to help with self-centering - I'm confuddled...
[Edited on 22/8/07 by David Jenkins]
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tks
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| posted on 22/8/07 at 09:26 PM |
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Yeah
basicly
what i think you need is TOE-IN.
and a bit of caster.
sow in fact the front wheels are looking from the front and from above: / and \
i think that as long as just both wheels try to win from each other the situ is perfect. problem could be that you need to set it up right for both
the wheels.
every degree wich is not the same will result in unbalance. (one wheel winning from the other) and that will result in the car change ing from
direction. also i think that you cant apply to much else the force needed to go back to its situ will be huge!! Sow basicly just give it a bit and
look where it pulls to should be an easy job.
Tks
p.s. in my opinion i would give one wheel some degree wich i think is right! and then just correct it with the other giveing it more or less.
also the tyre pressure influences allot! on my daily tintop had a 0,8bar loss the car pulled to the right inmensily (it was the front right wheel)
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Hellfire
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| posted on 22/8/07 at 11:05 PM |
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On our Indy:
As the castor is pretty much fixed at this moment the answer is toe-out and higher pressures. On full lock the wheels tend to pull further into the
turn using slight toe-in.
The SVA only tests the steering from FULL LOCK - self centring will be almost negligeable in the real world for most of the time.
IMHO - Get the pass and sort it after, if you're 'that' bothered!
Steve
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nitram38
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| posted on 22/8/07 at 11:17 PM |
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quote:
Originally posted by Hellfire
The SVA only tests the steering from FULL LOCK - self centring will be almost negligeable in the real world for most of the time.
Steve
Negligible?
It is in use all of the time!
It keeps you from drifting about on the road and helps you unwind the wheel out of corners without having to feed the wheel.
I have tried my car with and without castor and it is a total pita without.
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lotustwincam
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| posted on 22/8/07 at 11:41 PM |
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See HERE
for an explanation of how your wheel offset MAY affect the self centering action.
I think everyone under estimates the importance of King Pin Inclination (Or steering pin inclination) in the self centering action.
Drew
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Hellfire
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| posted on 23/8/07 at 01:25 AM |
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quote:
Originally posted by nitram38
quote:
Originally posted by Hellfire
The SVA only tests the steering from FULL LOCK - self centring will be almost negligeable in the real world for most of the time.
Steve
Negligible?
It is in use all of the time!
It keeps you from drifting about on the road and helps you unwind the wheel out of corners without having to feed the wheel.
I have tried my car with and without castor and it is a total pita without.
Hang on Nitram38 - you have misread and exaggerated my point... what I am stating is that at SVA they test from FULL LOCK^^^^^ (see above) How often
do you use FULL LOCK, all the time?
Driving properly means feeding the wheel at all times, therefore reducing the need for Self Centring from FULL LOCK.
Indeed Self Centring (on a Se7en type car) is only slight and may only just show signs of a return of the front wheels to zero (it could be a positive
disadvantage if over castored) but MR SVA is looking for a positive return from Full Lock which is what I said initially.
ATB - Steve
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nitram38
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| posted on 23/8/07 at 03:42 AM |
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Hellfire, if you have the correct amount of castor for your car, you will get self centring across the entire turning circle, not just at full
lock.
You are saying that it is negligible because lots of 7's only centre AT full lock? If so, this is wrong from a road driving point of view.
You will be amazed at how the correct castor will transform your steering.
It would seem a 7 trait that has not been corrected and keeps getting copied with each generation, otherwise this question would not keep on
surfacing.
I think the new roadster book has dealt with this, but I am not sure.
Race cars have about 3 degrees of castor because drivers can put up with light "wandering" steering in short races as a trade off for
"faster" steering.
Road cars have 5-7 degrees for relaxed, safer driving.
I have 7 degrees (rear engined though) and my car centres right across the entire steering range.
When I ran 3 degrees, the car would not attempt to centre.
lotustwincam, KPI does affect centring to some degree, but this is due to tyre scrub, where the KPI does not intersect the tread in the centre of the
tire tread patch.
This can only be sorted by custom made uprights as the cortina/sierra setups have KPI for a McPherson strut.
Some of this is reduced by the correct wheel offset, bringing KPI closer to the centre of the tyre tread. (you can get positive and negative KPI
too).
[Edited on 23/8/2007 by nitram38]
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