jderson

By observing the pivot hole on the steering knuckle, it is not centered to the strut tube. One wheel set has the pivot point eccentric at 11 o'clock and the other eccentric set at around 1 o'clock. This makes the left and right wheels, relative to each other, simultaniously turn on different axis' for a left turn and a different axis for the right turn; The factory obviously did this for good reason. Ackermann and maybe scrub and maybe whatever else.
1. What are your thoughts on this?
2. Can this be more clearly stated and understood?
3.Is the factory setting good enough for autocrossing and roadracing?
4. Is it of value to get a longer steering knuckle, or bend the knuckle as close to the rotor as possible to get the theoritical line between the steering arm pivot and the SAI, kingpin pivot, to hit the driver?
John A.

Spudz

Hey John,
Now you are getting really deep into this suspension stuff . HAve you been reading
again ? If someone else doesn't answer this I will pass it along with your other question to Erik at EMI Racing.

Mike

tube80z

Hi John,

I'm currently trying to model all this on winGEO3 and understand it a little better. From initial observations I'm not really sure that this was done for ackerman but more for setting scrub and trail. It does appear that you would need to change this for both camber and caster changes if you had a desired value, part of what I'm trying to establish.

Is the factory setting good enough for autocrossing and roadracing?

I only autox and hillclimb but can tell you the front of the Z does work better with an increased ackerman. Using turning plates (before spiffy new software) it looks like the front is very close to parallel steering. With my 13s I couldn't lengthen the steering arm so I moved the rack back to get more anlge on the tie-rods, which helps with the tight turns for autox. Not perfect but I did seen an measurable improvement.

4. Is it of value to get a longer steering knuckle, or bend the knuckle as close to the rotor as possible to get the theoritical line between the steering arm pivot and the SAI, kingpin pivot, to hit the driver?

I'd say yes but only for low speed small radius turns. If you road race you may find that this doesn't help much at all and may make you even slower. If you find that large amounts of static tow-out helps then you probably need more ackerman.

Cary

preith

Thought I'd revive an old thread. This is mainly for Tube80Z. I was doing a little reading last night, Carrol Smith's Engineer to Win, and I was intrieged by his comments on Ackerman. This seems to me to be the answer on how to correct the chronic understeer problem the zcars have. He said if toe out is needed, then increased Ackerman is needed. One of the drawings showed moving the steering rack closer to the center line of the front axle. It would take considerable fabication, but it could be done, and in a way to make it adjustable. Any thoughts?

tube80z

I moved the rack back as much as I could and cut out some of the mounts. I also moved the lower control arms as far forward on the bolts as I could (mounted on heims). And I rebent the steering arms. All this netted me about 37 percent Ackerman. It helps but is nowhere near what I think I'd need for an autox car (100 to 125 percent).

On my referrence course this was worth about 2 tenths of a second (48 second lap). If I could go all the way it might be worth another tenth or two. But when you get down to the last little bits that's a lot.

My tube car is built with a range of adjustment of 75 to 150 percent.

Other areas that need attention on the Z are scrub and caster trail (pin lead).

preith

What kind of rack are you using? How's that car coming along?

I feel like I'm outta my league here, and correct me if I'm wrong, but from what I gather, scrub is somewhat of a negative side effect of struts. This is because the wider the tire, the more offset is needed, and the centerline is then moved. Sure one could use more kingpin, but there's a happy medium there, something like 7-9 degrees, plus this of course is not adjustable.

As for caster trail, I guess I'll have to hit the books some more!

EDIT: Also, I wanted to ask, what's the proportion of increment in rack movement to percent of Ackerman?

tube80z

It's a cut down Z rack. I only use one end to steer from as the lower control arms go all the way to the center of the car. The car is a roller and this winter the rest of it should be finished. I've just been having too much fun playing the unibody car.


Scrub should be less than 20 percent of your tire width (ten inch tire should have less than two inches of scrub). In a srtut car you can lay over the strut to minimize the scrub. But as you mention this increases the SIA (steeing inclination axis), which is generally considered bad. You can use a two piece strut to set the SIA (and roll center) and adjust camber separately, which is where I'm headed.

Trial is another component of scrub seen sideways. If you look at caster as you add more the amount of trail grows. To manage this you need to move the spindle forward on the strut (or move the lower balljoint) to get the trail back into a manageable amount (it also counts as scrub). I typically try to get this under a half inch. All this relates and I don't think there's any one right answer. It depends on tires, track width etc.

Sorry but I can't answer your ackerman question. It is too dependent on car and I have too many pieces that are not stock to quote numbers that might match. Ideally you need to move the entire rack back probably an inch or more and the steering arms out to get more ackerman. You can also move where the lower control arms attach closer to the rack to help too.

None of this is probably that good of an answer. Someday when I have a lot more time I will try and put something together about how all this relates on a Z and what you can do to try and make them go faster. Just don't hold your breath ...

Cary

preith

Thanks for your insight. Smith's book said it only takes small increments, but an inch is more than I originally thought. This is going the expensive route, but I've really been contemplating swapping out the rack with one from Coleman. It's square, and looks to be easier than the stock rack to mount and make adjustable. Plus you can run rod ends for the inner tie rods and make bump steer adjustment easier. But it sounds like you were able to modify the stock one to your needs, hmm.

tube80z

I'd believe about half of what I write. All this is very dependent on your car, your tires, and how you drive. I set my car up a lot more lile a sports racer and drive off the front end. Many people drive by the rear and would find this would make them slower.

You also need to remember that everything Carrol Smith wrote about was a real race car. He only had one chapter on sedans and didn't have many nice things to say :-) And I would agree on a car built for racing you make very small adjustments. On a 30 year old Z car they aren't always small.

If you cut down the stock rack you can drill the ends and use heims on the inner joints, which is what I have. I also mount on the tube using some machined aluminum blocks. I can adjust the rack in or out with spacers. And if you go this far you might as well replace the crossmember and make the lower control arms longer. That's another area of improvement that not many try to tackle.

Cary