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Old 02-23-2013, 05:26 PM   #1
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Default Reasons for non-linear steering?

Evenin' all.
My car suffers from steering that is a bit mushy around the straight-ahead. It is also not as linear as I would like. In other words, if I move the top of the steering wheel an inch to the right, the car turns, but not very much. If I move it another inch, the car turns quite a bit. I have a feeling my car's mushy on-centre feel and it's non-linearity are connected.


Interesting evidence for the jury number 1: the car can only make 3.5 degrees of castor at around 1.5 degrees of neg camber. I understand that this is less castor than is usual for this amount of camber. Lack of castor could well explain the mushy centre feel, but would it also effect the linearity of the steering if my particular car makes less castor vs. camber than Mazda intended? This latter question is something I have nowhere near enough brain power to work out.

Evidence number 2: I was recently contacted by an MX5 owner from Sweden who reckons the Yokohama AD08s I use may be responsible for mushy centre feel. He says he found the same thing when he tried them back to back with other tyres. But could tyres alone also effect steering linearity?

Evidence number 3: I jacked the car up today so the front wheels were off the ground. I measured exactly how much the front of the rims turned for the first inch of top-of-steering-wheel rotation, and then measured how much they turned for the second inch of steering wheel rotation. The result? The same to the nearest half millimetre. I don't know what half a millimetre is in inches, but I think it's around bugger all/bugger all". So, at least when there is no load on the steering system, the wheels turn in relation to the steering input in a totally linear fashion.


What on Earth is going on?
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Old 02-23-2013, 08:26 PM   #2
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Oh man, where to start... hahaha.

Your steering rack is linear. Trust me on this one.

Go look up slip angle on wikipedia: Slip angle - Wikipedia, the free encyclopedia

Miatas are a bit dead in the middle due to the reed-valve-esque steering pinion. It's nothing that dramatic: Miata spool valve slop. on Vimeo
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Old 02-23-2013, 09:03 PM   #3
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Caster, with an "e".

Would you be able to provide alignment data along with tire pressure numbers?

It would be pretty pointless to talk about this without actual data.
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Old 02-23-2013, 09:50 PM   #4
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Yeah, get an alignment broham.
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Old 02-24-2013, 05:43 AM   #5
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GAMO, the spool valve is welded up. Regarding the slip angle, does it still apply to this extent even when the tyres are nowhere near their limit of adhesion?

Tyre pressures are generally set at 27psi at both ends because it feels like that's where they want to be set. I tried pumping the fronts up to 45psi yesterday to see if it effected the linearity of the steering - sadly not. I have a feeling it could be a combination of tyres and lack of adequate cast'e'r. Bear in mind I'm running stiff sidewall 185/55 profile tyres on only 6" wide rims.
I've lost the alignment printout from the last time it was done, but remembered being impressed at how accurately they'd hit the targets I provided them with. The targets were:
Front - 0.5deg neg camber per side, 2 minutes of toe in, and 'as much caster as they could get', which turned out to be only 3.5 deg.
Rear - 1.5deg neg camber per side, 2 minutes of toe in.
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Old 02-24-2013, 11:31 AM   #6
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Please tell me how a car turns. Not "turn the wheel", tell me the physics.
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Old 02-24-2013, 11:53 AM   #7
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I don't know the physics. I could take a stab at them, but I don't know them. Presumably you do. Please feel free to elaborate.
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Old 02-24-2013, 01:54 PM   #8
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rack bushings?
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Old 02-24-2013, 02:28 PM   #9
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When I depowered my rack a couple of years ago, I added as much negative camber as the stock cam bolts would allow front and rear and took caster quite a bit negative as well.

I'm fortunate to be able to align my own vehicles at work when we're slow, so I really took my time and made small adjustments then test drove it, then came back and dialed it in until I got it where I like it. I found that caster had the biggest effect on steering effort. Setting toe out a bit on both front wheels helped a lot with turn in.

Not sure if that helps you at all but...
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Old 02-24-2013, 03:18 PM   #10
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Quote:
rack bushings?
That's not a bad shout Dave. I think it would be worth repeating the experiment I did regarding steering angle vs wheel rim position, but with the wheels on the ground. Perhaps that would show up any flex in the system.

Flounder, that doesn't hugely help but it's certainly all interesting stuff. I may try running zero toe, and a decent amount of neg camber to allow me to get as much caster as possible next time I get the car aligned. Tyre wear be damned!

I'd LOVE for somebody to be able to tell me the dynamic effect of a car that runs less caster than it was designed to run in relation to it's camber. I can't find much info about the dynamic effects of caster online - "it makes the steering heavier," is generally it - much less the relationship it has to camber in regards how the car steers!

Someone from a UK forum has agreed to lend me some different wheels and tyres to try. Maybe that'll solve it. I mean, the car never used to have this problem in the days before its major surgery, and the only things that have changed (or SHOULD have changed…) in the steering system are the tyres, the depowered welded rack, and the geometry.
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Old 02-24-2013, 03:39 PM   #11
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Caster acts in two basic ways, on the one hand, as you turn in more steering lock, the wheels are turned more perpendicular to the caster angle, so you get an angle component similar to camber. Think of it as dynamic camber that you notice most in tight turns where you dial in lots of lock.
The second effect is weight jacking. Because of caster, the axis around which the wheel rotates is inclined forward. As you start dialing in lock the resulting rotation isn't perfectly perpendicular with the road. One wheel moves slightly up, the other down. This is the effect that causes stability at speed, as you need a force input to move the steering away from straight forward. The steeper the caster angle, the more severe the up down movement becomes.
Its a lot easier to understand if you build yourself some matchstick McPherson legs and play around with them.

I would try the tires first, but if that fails more camber might help as it pre-loads the sidewalls through something called camber thrust. An extra degree of camber shouldn't show up at all in your wear pattern. If anything it will make it more even. If you run poly bushes then switch to zero toe at the same time.
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Old 02-24-2013, 03:53 PM   #12
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Yep, that's why I'd really like more caster - to try and gain some weight and feel around the straight ahead. What I can't get my head around is how it interacts with camber from a driver's point of view. See what I mean?
Agreed about preloading the tyre sidewalls too. Ideally I'd like to try some 185 or 195 section different tyres on 8" wide rims instead of 6". I will try running zero toe in, too. Last time I reduced toe in the steering lost a little feel when it was loaded up, for some reason, but I'm certainly up for trying it again.
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