britishtrident
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| posted on 16/8/12 at 07:01 AM |
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KPI is a major source of self-centering but there isn't easy to alter if you are using a standard suspension bits.
The self-centering effect of KPI is slightly different to caster and trail, as KPI always centres relative to the vehicle centre line,
caster and trail centre trail centre relative to the instantaneous direction vector.
If the KPI is increased the caster should be increased by a similar ammount due to the effect on camber angle in turns.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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emwmarine
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| posted on 16/8/12 at 08:18 AM |
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quote:
Originally posted by eddie99
As snapper said, toe out dramatically improves self centering.
That's a great little tip. Many thanks for that. The value of Locost builders!
Building a Dax Rush.
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britishtrident
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| posted on 16/8/12 at 08:28 AM |
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quote:
Originally posted by emwmarine
quote:
Originally posted by eddie99
As snapper said, toe out dramatically improves self centering.
That's a great little tip. Many thanks for that. The value of Locost builders!
Here be Dragons !
Toe-out destroys straight line stability.
No repeat No manufacturer of RWD road cars use a static toe-out setting on the front.
[Edited on 16/8/12 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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40inches
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| posted on 16/8/12 at 02:26 PM |
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quote:
Originally posted by britishtrident
quote:
Originally posted by emwmarine
quote:
Originally posted by eddie99
As snapper said, toe out dramatically improves self centering.
That's a great little tip. Many thanks for that. The value of Locost builders!
Here be Dragons !
Toe-out destroys straight line stability.
No repeat No manufacturer of RWD road cars use a static toe-out setting on the front.
[Edited on 16/8/12 by britishtrident]
Aren't we talking about simply getting through IVA/MSVA, and then back to a sensible setting? Trailering to and from obliviously 
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smart51
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| posted on 2/9/12 at 05:01 PM |
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Two steps forward one step back
Well I'm back from my hols and I've taken the angle grinder to the lower wishbones. The back arms are now 20mm longer than they were to
push the lower ball joint nearly 40mm further forward. Now the anti roll bar drop link mounts are now in the wrong place and the top ball joint locks
out due to the increase in angle it is at. No problem, I undid some of the previous mods until it was just about OK for a test run. Castor comes in
at 14.7 degrees and it self centres well!! Cutting off the droplink mounts and sticking them on again isn't a problem but tilting the top ball
joints by 10 degrees will need a bit of head scratching. Still, the end is in sight.
BTW, the wheel spacers made no difference. anyone want to buy a pair?
Edit to say, with this much castor, there is a comical amount of camber gain at full lock!
[Edited on 2-9-2012 by smart51]
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craig1410
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| posted on 2/9/12 at 06:04 PM |
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Glad you're making progress but I can't help feel that the increased castor is masking another problem. You shouldn't need that much
castor. I think it is between twice and three times as much as you would normally expect to need.
Sorry the wheel spacers didn't help. What width did you try?
Edit: Just found this article showing professionally designed suspension for a Caterham Seven race car. They start with 7.7deg Caster and reduce it to
under 5deg as part of tuning process. http://www.sjmmarsh.com/7files/caterhamsuspension.pdf
Edit2: Found this old thread where "procomp" (knowledgable Locost Racer) suggests that a genuine 5deg is fine and you can go up to 8deg
but it gets tiring to drive. Are you sure you are measuring yours correctly? http://www.locostbuilders.co.uk/viewthread.php?tid=87363
[Edited on 2/9/2012 by craig1410]
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smart51
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| posted on 2/9/12 at 08:09 PM |
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I used 5mm spacers as I didn't want to shell out for hub centric ones. Remember that my build isn't a locost and that with only 220 kg, I
need more castor than a locost does. I plan to take some castor out so that the castor isn't quite so big but that it does still self centre.
There will still be quite a lot though. There is 75mm of setback with 295mm between centres, which if I've worked it out right is 14.75
degrees.
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craig1410
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| posted on 2/9/12 at 08:48 PM |
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quote:
Originally posted by smart51
I used 5mm spacers as I didn't want to shell out for hub centric ones. Remember that my build isn't a locost and that with only 220 kg, I
need more castor than a locost does. I plan to take some castor out so that the castor isn't quite so big but that it does still self centre.
There will still be quite a lot though. There is 75mm of setback with 295mm between centres, which if I've worked it out right is 14.75
degrees.
5mm spacers are pretty small and I expect would have negligible effect. I've got 19mm spacers on mine although that is mainly to counter the
effect of having ET52.5 Fiesta wheels. I needed longer studs for those spacers.
Yeah I hear what you are saying about having only 220Kg of mass but if your steering column is turning freely then it should still self centre okay,
albeit it will be a bit lighter overall than a 400 Kg car. Apart from the static friction in the column, I would expect vehicle weight to cancel out
as it is the vehicle weight which both induces the static self-centring and resists it. Dynamic self centring may be a different story I'm not
sure.
You've maybe said this already but what uprights are you using? Is it Sierra, Cortina or something else? I'm using Cortina and I know that
it has been said many times that they don't have enough KPI (or SAI more correctly). They only have around 4.25deg of SAI when they would
ideally have around 10deg. This affects the scrub radius and certainly has an effect on self centring although it is difficult to visualise fully.
I've worked out that my steering axis intercepts the ground pretty close to the inside shoulder of the tyre. With my 195mm tyres, it is 10mm
inside the shoulder which is therefore (195/2) - 10 = 87.5mm scrub radius. From what I have read, this is on the high side of the acceptable range. It
would be interesting to work out what your scrub radius is.
Of course, all this may be academic - if it works okay in practice then great. For me, I'd want to understand it more fully but if that
wasn't possible then I'd certainly be looking to test it thoroughly to see if it has introduced any dangerous tendencies.
Cheers,
Craig.
ps. Feel free to say so if you just want to move on. I don't want to hamper progress through over analysis.
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smart51
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| posted on 2/9/12 at 09:20 PM |
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I have fiat cinquecento uprights (I'm using the fiat rack as it is close to ideal for a single seater) so just used cinquecento everything else.
There's about a 10 degree included angle between kingpin and camber angle, if I recall correctly, due to the adapter I made to convert it to
dual wishbone suspension. The scrub radius is quite small. again from memory it is about 10mm.
The steering is very, very light. probably because there is no weight. Loading up the steering is probably a positive thing. I obviously
don't want it to be dangerous in any way. I've tried everything I could and am fairly happy, though all advice is always welcome.
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