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Self centering

smart51 - 28/7/12 at 03:37 PM

Enough of the car was finished this afternoon for a test drive The good news: it goes and it stops. All the electrics work, bar a slightly dodgy starter relay. Bad news: the speedo stays on zero and there's not a trace of self centering.

What do I have to do to the suspension geometry to induce self centering? Is there any reason why a 3 wheeler would be different (single rear wheel)?

[Edited on 28-7-2012 by smart51]

plutos3 - 28/7/12 at 03:49 PM

If every thing is square and alighned its probably just needs more caster angle

renetom - 28/7/12 at 04:17 PM

Hi
Our indy R1 had no self centering whatsoever
We changed the top wishbones to rose jointed ones and this allows you to set the caster angle at pretty much whatever
you want.
We set the caster at a bit over the top at 8 degrees , it now self centers a bit.
Good luck

David Jenkins - 28/7/12 at 05:49 PM

Before you muck around with castor etc. just check that all the ball joints are working nice and freely, and that there's no drag in any joint or steering gear.

new ball joints can be very tight, and can make you think that there's a geometry problem - there may be a geometry problem, but check the basic things first!

rusty nuts - 28/7/12 at 06:06 PM

I agree with David , I found my self centering vastly improved when I fitted a self aligning bearing instead of the normal Sierra steering column bush due to it binding slightly

smart51 - 28/7/12 at 06:08 PM

Thanks All. Just has a look and there's very little castor. I can only assume ai put the wishbone brackets in the wrong place. The steering is extremely light - mosly because the whole thing weighs about 220kg. There isn't any noticeable stiffness in the joints. Looks like I need to do some cutting and welding.

snapper - 29/7/12 at 06:22 AM

At a couple of IVA's we attended a touch of toe out got us the self centering g required fora pass

eddie99 - 29/7/12 at 11:57 AM

As snapper said, toe out dramatically improves self centering.

RichardK - 29/7/12 at 01:55 PM

Pumping up the tyres can help too if this is just for Iva.

Cheers

Rich

smart51 - 29/7/12 at 02:48 PM

Wishbones now cut and rewelded. Just waiting for the paint to dry. We'll see if castor makes a difference. What effect does camber have?

smart51 - 29/7/12 at 07:31 PM

aw crap! Cutting and shutting the wishbones produced zero difference. A plumb bob and ruler measure 5 degrees of castor but there's absolutely no return off the rack end stop. Damn.

Hellfire - 29/7/12 at 08:14 PM

Can you not fit rod ends to the top wishbone to make it more adjustable? That way, you can alter the castor (within range) until it feels right.........

Phil

smart51 - 3/8/12 at 05:34 PM

I've made another mod, shifting the top ball joint back be 12mm. Again it's had no affect. With just 70kg of sprung weight (laden) on each wheel, I suspect there's not enough weight on the wheel to force it it centre.

Dangle_kt - 3/8/12 at 06:04 PM

quote:
Originally posted by smart51
I've made another mod, shifting the top ball joint back be 12mm. Again it's had no affect. With just 70kg of sprung weight (laden) on each wheel, I suspect there's not enough weight on the wheel to force it it centre.

can you fit skinnier tyres?

smart51 - 3/8/12 at 06:26 PM

It already has 135s on the front. I don't think you can get less than this.

britishtrident - 3/8/12 at 08:00 PM

Tyre pressures is the first thing to look at the self-aligning torque generated by the flexing of radial tyre treads generate a lot self=centering torque, too high a tyre pressure for the weight carried by the front wheels will give very light steering but not give good self-centering

.Most bubble car style vehicles used trail (in addition to caster angle) on the front but that brings potential problems including weather cocking.

One thing that will improve self-centering is ackerman (toe-out in turns) you can get toe-out in turns by moving the steering rack rear wards in relation to the track-rod ends.

Static toe-out should be avoided at all cost it can put you into a ditch very quickly.

[Edited on 3/8/12 by britishtrident]

austin man - 3/8/12 at 09:14 PM

are you sure the top wishbones are on the right side this can cause the self centering proble as it throws the Castor out

smart51 - 3/8/12 at 09:31 PM

Good food for thought there BT, thanks.

Top wishbones push the inboard suspension. Can't go on the wrong way, but a good suggestion.

Steve Hignett - 3/8/12 at 09:36 PM

I wouldn't be at all surprised that at 200 odd kilos, you didn't need even as much as 12 or more degrees of castor...

I will try and dig out one of my chassis dynamics books that has that info in it as I'm not experienced enough to remember it off the top of my head, sorry...

smart51 - 6/8/12 at 08:11 PM

Another day, another mod. This one took the castor up to 10 degrees on one side (but only 7 on the other. I'll have to check that). On the 10 degree side, the steering moved off the end stop by about 10 degrees. So some progress. How much self centering does the man from the ministry want to see?

smart51 - 15/8/12 at 08:07 PM

Well I phoned the man from the ministry who said there does need to be evidence of self centring, despite the MSVA manual not mentioning it. So I've a carried on making adjustments. There is now some evidence of self centring in one direction but not the other. A little more should hopefully do it.

craig1410 - 15/8/12 at 08:54 PM

When trying to understand steering self centring, go and have a shot on a go-kart as they have it in spades due to extreme amounts of scrub radius. When you turn the steering on a kart, scrub causes it to "jack up" the kart on one side and this rise in height, coupled with gravity is what produces a lot of the self centring torque. Now you wouldn't want that much scrub radius on a car as it would dart all over the place when you hit a puddle or went over the crown of the road etc but you might want to have a little bit to amplify the castor/trail effect you already have.

In my opinion, wheel scrub radius is as important as castor and camber and toe all come into it as well. Your narrow tyres might actually be reducing the centring effect. If you have at least 6 degrees of castor then castor is probably not your problem so look elsewhere for answers. If your steering gear is moving freely and not binding then it may be worth systematically looking at scrub, camber, toe and tyre pressures.

I would start with sensible tyre pressures for the weight of the car (compare with a similar weight road car) and I would start with zero toe and 1 degree of negative camber. If possible, try and vary wheel offset using spacers or different wheels or even by reversing the wheels you have (if poss.) and see what effect this has. I'm assuming you are testing at manoeuvring speeds only by the way so temporary spacer measures should be safe, especially with the low weight of the car. Try measuring one thing at a time and note which measures help or hinder self centring. Then just try to optimise for maximum centring.

HTH,
Craig.

smart51 - 15/8/12 at 09:24 PM

Good suggestions. Wheel spacers might be the next step.

Doctor Derek Doctors - 15/8/12 at 09:53 PM

How much King Pin Inclanation do you have in the front uprights? KPI affect static self centering quite a lot especially when added to the effect of Caster angle.

You can see the KPI on this pic of a Go-Kart (which we know has pretty extreme self centreing)

mad4x4 - 16/8/12 at 06:33 AM

Met these guys at Stoneleigh - they know most things about suspension and could be able to assist this problem

britishtrident - 16/8/12 at 07:01 AM

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.

emwmarine - 16/8/12 at 08:18 AM

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!

britishtrident - 16/8/12 at 08:28 AM

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]

40inches - 16/8/12 at 02:26 PM

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

smart51 - 2/9/12 at 05:01 PM

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]

craig1410 - 2/9/12 at 06:04 PM

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]

smart51 - 2/9/12 at 08:09 PM

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.

craig1410 - 2/9/12 at 08:48 PM

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.

smart51 - 2/9/12 at 09:20 PM

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.