Mik - Without actually seenig Crumb's setup, there is another possible cause that is (relatively) common (for cars in general, not specifically datsuns) or better stated it is common if and when such situations (darting left or right over a bump).

The other thing is scrub radius. I'll try and describe this in everyday terms (and I'm not suggesting I am an engineer, so it's more like I'm trying to relay my understanding of it, to whatever extent that I do actually understand it, but it's not 'the last word') . Basically scrub radius is a sort of imaginary line drawn through the top of the suspension (or not quite that high) and though the centreline of teh turning 'centre' of the strut or the stub axle, and down on to the tyre tread/ground. If you have the wheel offset 'right' then if you 'hit' the wheel (like you ran into a big bump) then it will remain pointing in the same direction, and be relatively stable.

But if you go to wider and wider wheels, on a setup where you'd hit some part of the suspension or the strut or whatever, well, to go to wider wheels you might have to go with different offset wheels or spacers to situate the existing wheels outward a little wider. Now as you move this tyre centreline outward, the forces (if you hit a bump) would no longer be centred, instead the forces are outward further. And when a wheel hits a bump it will try and move backwards/twist. SO if you hit the right hand tyre, it'll dart to the right, and the left, it would dart to the left.

It doesn't even have to be a hard hit, on some setups it can be so severe that just normal road undulations are enough to have it pulling left and right. This effect did show up on minis with wide enough wheels (not that there's too many options there any more). Although it did increase the track and make the car (theoretically) better handling, the loss of stability over bumps could rule it all out.

Interestingly that imaginary line is on a slight angle (goes out wider as it goes lwoer and lower toward the road surface) what this means is that if you ran taller tyres, the line would now go out wider at the road surface contact point, and would LESSEN the effects. The problem of course is that there's no such thing as a free lunch and taller tyres do make (when it is originally not so good due to wheel offset) things better as far as scrub radius goes but they also raise the car, and bring in their own potential to compromise other ok suspension geometry/factors.

I can't say I've seen it on datsuns but I haven't run massively wide rims on one, but in theory it could happen eventually. I'd certainly at least look into that as well if the car is darting this way and that over bumps.

Both issues can be present at the same time. As a further tiny addition - they can also (scrub radius issues more likely to make more severe handling issues than bump steer, but either or both is possible) - cause the car to be very unstable under heavy braking, sometimes even on a pretty flat road surface, but worse still if you are braking heavily AND hit bumps or potholes or whatever.

I have no doubt at all that there's far clearer explanations and diagrams on the next explaining and demonstrating bump steer and scrub radius. If nobody else manages to find them, I'll see if I can when I get more free time.

I would have thought that the engineer would test the bump steer on the actual car in question , there are so many combinations of parts that it would be unreliable to rely on rough measurements of the arms alone ,the ball joint height, spacer height between arm and strut leg , height of the tierod end , all have a effect on the bump steer profile

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he hasn't asked to drive it yet, but honestly, how many of you have actually had a problem with stanza struts in a Y, it's been so long I've forgotten what a stock car drives like.

that sounds like caster problems?

edit: I'd be checking caster bar bushes, the condition of the bars, the inner control arm bushes as well

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what struts top are you using?

if they are pillow ball type camber plates it is likely that they will be dragging.

I have had the exact same problem when I converted to coil overs with camber plates.

Quote:

Stanza strut conversion doesn't create a bump steer problem. We had a thorough look at my old BRE painted 1200 coupe with Stanza struts at LEDA suspension.

As McGee said, I think your engineer is a bit too focused on just one aspect that affects bump steer. He should really be measuring it to see if it's within specs if he's that concerned. One dimension on the steering arm is pretty meaningless when the conversion affects at least 4 factors affecting bump steer, 3 of which are on the steering arm - length, height and offset. The other being LCA length - Stanza is longer which reduces bump steer, all else being equal, due to less angularity change for a given suspension movement.

Good point Harry, with the extra 10mm or whatever the stanza ball joint pushes out the steering arm it would fall into a negligable margin in regards to effects on bump steer. I just want this plate done so I can drive with some assurance that govco won't start raping me for choosing a car that is not a Camry

I'd be looking at the strut tops too - how the top of the spring and it's retainer/cap contacts under the bottom of the strut top - they would perfectly explain why it pulls to the side you last turned toward.