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PeterAllen

Wandering Steering?

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When I first drove the car on freeways and generally good roads I noticed a tendency for it to wander. I'd sometimes take in the scenery or take longish looks at the gauges, etc,  only to discover the car wandering out of the lane. It took full-on, two-handed concentration to drive the thing. I just put that down to 60s suspension geometry and the lack of power steering.

A 4,500 km around Tassie last year alerted me to the issue but didn't overly concern me.

However, on a recent trip from Adelaide to Sydney along the Sturt Hwy the car bottomed badly a few times so I tried to drive with the near-side wheels on the un-depressed centre of the lane and the off-side wheel close to the un-depressed dividing line. If it moved from the crests to the worn depressions (for on-coming trucks, etc) it was something of a handful to maintain a steady trajectory. Overtaking required full-on concentration as the car rode the crests and worn depressions on both sides of the road. I wasn't always sure whether it was air turbulence from the trucks or just the road contour and suspension geometry, or both, creating difficulties.

Is it just so long since I've driven a 60s car under these conditions or have I got a real issue to be addressed? Thanks.

The 240Z is running 16" rims (6" F & 7" rear) with high aspect ratio tyres giving 25" dia F and 28" dia R.

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Maybe you need to test drive a fellow Sydney siders Z to compare ?  Sorry I would but I’m 3 1/2 hours away.

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During the build I had the steering rack rebuilt, replaced the bushes and rod ends and then had an alignment. I don’t think it’s a wear issue. The castor is reduced rather than anything as I relocated the lower arms outwards about 10 mm and the tension rod is subsequently too short - I have the Falcon rods to install but haven’t got around to it yet. 

It’ll be a difficult issue to assess without running at higher speeds on a road with such bad wheel track depressions as the Sturt Hwy.

I'm hoping some of the Redbacks might recall their experiences on the Sturt.

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Peter 

If you have moved the lower arm pivot points and the tension rod is too short its probable that the toe angle is changing as the suspension travels (bump steer) more than it would normally Also your caster is not be correct as you said. All of this is making the car very steering sensitive. A steep strut angle (as you now have) (less caster) will make the steering very lively where as a slack strut angle (more caster) will make the steering less active.

Jeff 

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Jeff. I was in no real hurry to fit longer tension rods as I favoured the supposed lighter steering but I think I’ll have to reassess the priorities.

 I’ll wind the coil-over off and measure the toe as the suspension moves. I modified the tie rods to compensate for a lower ride height but never measured the actual outcome.

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Posted (edited)

They are Pirelli Scorpion STR and I bought them very early in the piece so I sort out the profile of the guards - probably around 10 years ago. I notice they are now discontinued.

They were marketed as SUV suitable so I was forced to compromise a bit. They were just about the only tyre which had the  high aspect ratio and narrow width I was looking for to give a 60s look but also had a high speed rating ('V' if I recall) which I thought was required for engineering approval of the car.

F = 205/65/16

R = 215/70/16

Looking at them now they do appear to have a square-ish shoulder like cross plys but I'd guess cross plys were the go in the mid-sixties when the 240Z suspension was designed.

https://postimg.cc/ph1qJ2yT

 

Edited by PeterAllen

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Ten years is fairly old for tyres and most likely not legal anymore. May have gone hard in that time. I had Kumo tyres on a car once and got rid of them for the same reason. Couldn't hold the car on the road and downright dangerous. Other thing to think about is you set the car up with no load then fill her up with two people and lots of weight in the back including fuel and that is going to change the way it behaves on the road. The LS is also lighter than a L24. I am no expert on suspension set up just chucking a few ideas out there.

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The LS is only about 30kg lighter so it really shouldn't matter.  This weight is more that made up for by the T56 also.

I support the reduction in caster angle is making it a bit 'flighty'.  It will respond to input (both internal and external) more quickly.  You might see significant improvement if you increase the caster a little and consider adding in the steering spacers from TTT and others to reduce bump steer angles.

Roll Centre Adjusters

We all enjoy a 'spirited' drive, but when the undies get a stain it is time to fix the problem.

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Posted (edited)
On 5/17/2019 at 8:56 PM, MaygZ said:

... You might see significant improvement if you increase the caster a little and consider adding in the steering spacers from TTT and others to reduce bump steer angles.

Roll Centre Adjusters

MaygZ - I avoided fitting the spacers by heating and straighten the threaded portion of the forged steel rod ends to have the steering rods parallel to the LCA.

I have copied the diagram below from a 260Z manual and I've assumed it is identical to a 240Z (?).

The red line represents the Steering Axis Inclination (aka King Pin Inclination) of a OEM set up. The blue line represents the altered SAI which resulted when I raised the pivot point of the LCA on the crossmember and also moved it further outward (ie. lowered the car and widened the track). Red 'X' marks the new location of the ball joint. The red line indicates a large POSITIVE scrub radius as standard, something that is now not considered 'the go' for Macpherson struts and may be why OEM cars have relatively heavy steering (?) All wheels on modern cars have significant positive offsets and I wonder if that is anything to do with achieving NEGATIVE scrub radius (?).

Anyway, the blue line suggest the SAI on my car is now pretty close to neutral - although my 16" tyres have 22mm more radius that OEM 14" tyres and could produce slight positive SAI. The information out there tends to suggest a neutral SAI makes the steering less stable so that MIGHT be the issue I have? Although it's a little flighty at speed on uneven surfaces it's still quite heavy at very low speeds.

Any thoughts?

 

260Z front Suspension 3.jpg

Edited by PeterAllen

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Hi Peter, not sure if I understand what you're trying to articulate above but relocating the inboard LCA pickup point (or even swapping to a longer aftermarket LCA) won't affect the SAI as the relationship between the top strut mount and the lower ball joint doesn't change, it's fixed.

The only way to alter the SAI on a strut setup is with different offset wheels or to change the angle of the strut tube to the hub carrier.

Your changes above will certainly change the track, the bump steer characteristics, the roll centre and also camber gain rates but not the SAI.

Roger

 

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You're absolutely correct Roger, crikey, what a senior moment I had there.

I remember (now) that I was going to re-bore and sleeve the strut location to minimise camber change on a widened track but ultimately decided against it.

I can't recall the off-set of OEM wheels but the wheels I've fitted have 0 off-set however the larger dia tyres should reduce the scrub radius but not that much.

Any thoughts on why it was such a handful on the Sturt Hwy? I'm assuming you've travelled it a few times. It could just be the air turbulence around the trucks I was overtaking. The torque of the LS1 means you start off about 110 km/h to overtake but you're doing 150-160 km/h by the time you get to pass the truck cabin. It was exhilarating but a handful at the same time.

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As others have already said directional stability is mostly affected by castor or toe. Higher positive castor (ie lengthen the compression rod) creates a useful self centring effect but makes the steering heavier.  Toe out might be good on a race car as it promotes good feel on initial turn in but this also makes the car nervous, so running zero or even a small amount of toe in at the front helps reduce this and is what you should run on a street car.   Also don't count out the rear of the car, make sure there is a small amount of toe in across the rear.

Plus all the bits are 50 years old and there is bound to be play in everything.  My road car has mostly new bushes and ball joints etc but is still a little wander prone and just the slightest play in the steering rack means chasing the car all the time. Our rally car on the other hand has a rebuilt rack with no play (and electric power steer) and is actually nicer to steer.

If you are bottoming out on the highway then I suspect you have the car set very low indeed, our tarmac rally car rarely touches the ground (exhaust) and that's after it has used all the bump stop travel first.  If you have the car that low then it's possible you are already very close to the bump stops which apart from making the ride very harsh will certainly introduce weird handling if you end up running on the stops.

Our rally car runs only 3deg camber on the front and comfortably tracks at 200kmh on some stages and while you do have to hang on it's certainly not fighting you in a straight line or I wouldn't be driving it!

I'd be looking to put a bit more castor in first (lengthen the compression rods) and set the front toe to zero or 1-2mm in and go from there.

Roger

 

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25 minutes ago, RLY240 said:

As others have already said directional stability is mostly affected by castor or toe. Higher positive castor (ie lengthen the compression rod) creates a useful self centring effect but makes the steering heavier.  Toe out might be good on a race car as it promotes good feel on initial turn in but this also makes the car nervous, so running zero or even a small amount of toe in at the front helps reduce this and is what you should run on a street car.   Also don't count out the rear of the car, make sure there is a small amount of toe in across the rear.

Plus all the bits are 50 years old and there is bound to be play in everything.  My road car has mostly new bushes and ball joints etc but is still a little wander prone and just the slightest play in the steering rack means chasing the car all the time. Our rally car on the other hand has a rebuilt rack with no play (and electric power steer) and is actually nicer to steer.

If you are bottoming out on the highway then I suspect you have the car set very low indeed, our tarmac rally car rarely touches the ground (exhaust) and that's after it has used all the bump stop travel first.  If you have the car that low then it's possible you are already very close to the bump stops which apart from making the ride very harsh will certainly introduce weird handling if you end up running on the stops.

Our rally car runs only 3deg camber on the front and comfortably tracks at 200kmh on some stages and while you do have to hang on it's certainly not fighting you in a straight line or I wouldn't be driving it!

I'd be looking to put a bit more castor in first (lengthen the compression rods) and set the front toe to zero or 1-2mm in and go from there.

 

 

Agree with Roger on all points.

Check toe first as its easy and the most likely culprit.

My car has all new stock rubber bushes and standard arms and geometry (no adjustable camber or caster) and at Sandown last weekend it was fine at 200+kmh. On the road it doesn't exhibit any significant bump steer and is very relaxing to drive. I have hydraulic power steer and the car is very low  however MCA coil overs means I still have good travel and while it does use the bump stops it has to be a pretty decent bump to do so and it doesn't upset the car.

David

 

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Posted (edited)
On ‎6‎/‎13‎/‎2019 at 9:53 AM, RLY240 said:

..... relocating the inboard LCA pickup point (or even swapping to a longer aftermarket LCA) won't affect the SAI as the relationship between the top strut mount and the lower ball joint doesn't change, it's fixed.

 

 

Roger,

I'm not sure I agree with the above. 

I hope my memory is surviving, but I remember the KPI as being the angle between a vertical line from the lower ball joint and a line between the upper and lower pivot points (The strut mount and the lower ball joint).

Moving the LCA pickup up, down, in or out (and the LCA length remaining constant or changing the length of he LCA) will move the lower ball joint in or out relative to the strut mount.  This movement has to affect the KPI, as the strut mount stays fixed any change of the lower ball joint position has to either increase or decease the KPI.

If the diagram above is accurate, then having the KPI line through the centre of the tyre means there is no 'scrub angle'.  Effectively reducing the 'auto-centre' of the steering to zero and therefore full steering input is required to straighten the car.  This also has the effect of meaning that any change in geometry due to suspension travel will require steering input to rectify.  The higher KPI will also produce more negative camber and more toe-in on compression (because our steering arm is forward of the spindle?) - which will produce a little understeer and may result in a 'feel' of wandering as it will require more steering input to correct.

The upside is that the steering should be quite light.  :)

Again, if I recall correctly, much of this can be improved by increasing caster angle - pushing the wheel forward - which also reduces the KPI and increases toe-in.  This will make the steering heavier but increase the auto-centring and 'feel' of the steering.

It may be a good idea to jack up the car, remove the spring (and the shocker?) and push/pull the suspension through its full travel.  Measuring the changes through travel will show you exactly what is happening.

Something to remember is that many of the negative issues here can also be the result of low tyre pressure (simply allowing the car to 'roll about' on balloons) - so that would be my first check as it is the cheapest.  ::)

I hope this helps and I hope I've remembered my trade school suspension classes correctly.

I've never even held one of the TTT bump steer spacers, but again if my brain is working, it seems to me that these would bring the LCA angle back closer to stock with only very small changes to KPI, caster and camber as they seem to me that they don't alter the lower outer pivot point they just bring the LCA back into the right arc.  At least that's what they look like they would do to me.

Edited by MaygZ

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Maygz, I don’t disagree with you, my comments about the SAI not changing related to Peter’s diagram where he was expecting a change to the intersection of the SAI and the contact patch which doesn’t change.

Sure, pushing the lower ball joint outboard increase the actual KPI angle and also camber angle but doesn’t affect the relationship between the strut and the tyre.

Roger.

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Roger,

yes, that is something I didn't see when I looked at Peter's diagram.  Obviously the drawn position of the tyre needs to be moved to stay in the correct relationship with the now moved stub axle - as you are quite right that the relationship with the stub axle (and therefore the wheel and tyre) with the strut tube is unchanged by changing the KPI.  Peoples, please ignore my comments in the above post regarding 'scrub angle' as this doesn't really change (with the exception of possible reduced tyre/road contact due to increased negative camber)

The most significant change made by the move would be increase of negative camber

It would be interesting to see how much the KPI (and therefore camber) changes through suspension travel even with what I suspect is only a move of the LCA inner mount by about an inch.

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