260DET Posted October 20, 2003 Share Posted October 20, 2003 My understanding of the Z steering setup is that if you move the outer tie rod end pivot points forward you will get more Ackerman. Or, to illustrate it another way, you would get more Ackerman if you moved the steering rack back. It appears that the Z originally has 'normal' Ackerman ie the rack is in line with the tie rods. Looking at it from above. That so? So, has anyone done any experimenting in changing Ackerman or got any thoughts on it? More or less Ackerman? Is there anything really to be gained here? Quote Link to comment Share on other sites More sharing options...
74_5.0L_Z Posted October 21, 2003 Share Posted October 21, 2003 I have looked at the Ackerman on the Z. The steering rack is slightly(~1/2") closer to the ball joint than the connection of the steering arm on the strut. Having the rack closer to the wheel center than the steering arm pivot increases Ackerman. So, yes, moving the rack back would increase ackerman. I have toyed with the idea of making a crossmember that allows fore and aft adjustment of the rack. Quote Link to comment Share on other sites More sharing options...
Drax240z Posted October 21, 2003 Share Posted October 21, 2003 The other option of course is to make your steering knuckles adjustable in and out. (granted, probably not quite as easy) But you could quicken steering at the same time as adjusting ackerman if it was desired. Quote Link to comment Share on other sites More sharing options...
Guest Aaron Posted October 21, 2003 Share Posted October 21, 2003 At the risk of sounding stupid, I have heard of Ackerman (angle?) before, but I have no idea what it is, or how it affects the steering/handling of a car. Could you please explain? Quote Link to comment Share on other sites More sharing options...
johnc Posted October 21, 2003 Share Posted October 21, 2003 The following are just uneducated guesses on my part... There is a little bit of Ackermann built into the steering of an early Z as 7450LZ mentioned. Its probably there to counter some of the bump steer issues in the front steering. When autocrossing an early Z I've run as much as 7/16" total toe out to get the front to turn in. It helps but you end up with a push mid-corner that can be overcome with power. Being able to run that much toe out tells me that the toe out gained with the Ackermann in the Z is very small. Increasing the toe out gain via Ackermann would allow someone to run less initial toe out and get the same results. But you would also have to look at what happens to bump steer and the built in camber gain. It would take a lot of of measureing and trial and error to come up with something that works. Quote Link to comment Share on other sites More sharing options...
johnc Posted October 21, 2003 Share Posted October 21, 2003 Ackermann is the non-parallel movement of the front wheels as they are steered. Most people think that the front tires stay parallel (when looked at from above) when the steering wheel is turned. In fact, almost all cars have the steering designed to turn the inside wheel a bit more then the outside wheel when turning. The amount of difference between the angle of the inside and the outside wheel is Ackermann. Ackermann is used to make the car's turn into a corner more responsive. The front end bites better. But, too much Ackermann angle and the front tires start working against each other and you get understeer. A tiny bit of Ackermann also reduces the loads on the steering system and reduces tire wear on the front tires. Quote Link to comment Share on other sites More sharing options...
gramercyjam Posted October 21, 2003 Share Posted October 21, 2003 I've done a little research on Ackerman as related to improved steering performance in an FP autox car. What I have read is that professional chassis designers feel Ackerman is most effective when used on a car that makes mostly low speed, low G (tight) corners, because of the larger difference in the turn angles required by the tires to follow the radius of a tight turn compared to a wide turn and the lower tire slip angles. For cars that are setup for high speed, high G, low steer angle turns, some designers feel that the difference in the angle between the inside/outside tires is small enough as to be negligible and ackerman can even be detrimental to handling. Why? Partly because of the low loading/low slip angle of the inside tire. On the other hand, short track oval racers say ackerman is extremely important in minimizing tire wear. State of the art chassis design and suspension setup is really a black art that requires a lot of testing on the track to try and get it right. Suspension and steering and very complex issues because of the number of variables involved and the interactions between them. This stuff ain't easy. The top racing teams and tire manufacturers know the state of the art, but they aren't telling. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 21, 2003 Share Posted October 21, 2003 So Gramercyjam, what did you decide to do on your car? Jon Quote Link to comment Share on other sites More sharing options...
gramercyjam Posted October 21, 2003 Share Posted October 21, 2003 Here's the plan. I have decided to apply a systematic approach and to deal with the things that should make the biggest differences. Step 1, acquire necessary tools to make the measurements. This includes good caster/camber guage, toe plates, and drive-on measurement platform that allows easily repeatable measurements in a 3-d coordinate system. It also allows chassis adjustments to be made without jacking the car up and unloading the suspension. Although hard to justify, owning a set of good corner weight scales would be nice, but not necessary. Right now I rent for $50/day when I need them. Step 1 is 90% complete. Step 2 - take measurements and baseline current setup. This is where I am at now. Initial measurements indicate things like caster and camber, rear toe and squareness are far enough off that corrective action is required. This is step 3. Step 3 - adjustable heim-jointed suspension end links and ball joints to correct baseline measurements and allow for setup experimentation. Front: Caster, camber, toe and bump steer, ride height, adjustable track width; Rear: camber, toe, ride height, adjustable track width. Heim jointed sway bar end links allowing for pre-load adjustment and stiffness adjustment. 30% done? Step 3a - Fabrication of new steering linkage and mounting providing for high quality steering U-joints, elimination of flexible steering coupler and better position for relocated driving position. Since this will require a dash bar for mounting, it will be done in conjunction with the removal of the stock pedal box and the addition of chassing stiffening (spars and ribs - something different - everybody has a cage). Relocate fuel cell. (BAH! - New SCCA rules outlaw my current install) Media blast and paint chassis. Step 5 - Depending on the results of the above, which should keep me busy for quite some time, evaluation of rack and pinion, aftermarket replacements, steering ratios, and power options. I've been itching to put in a fresh L24 motor too with a new racer brown cam and freshly modded E31 head that I have, so that will need to be done before next spring. Quote Link to comment Share on other sites More sharing options...
260DET Posted October 22, 2003 Author Share Posted October 22, 2003 Thanks all for your useful input. Yes, moving the rack/crossmember forwards/back seems to be the most practical way to change Ackermann on a Z. That would not require a full wheel alignment each time a change is made. Only toe? To do it methodically it looks like you would have to know the various turned front wheel angles (in plan, from above) in relation to the point where they intersect with an imaginary line (in plan) running from the center of the rear 'axle' to the center of the front 'axle'. That is if you have decided what Ackermann variation you want. Alternatively one adjustment could be made at a time and the car tested each time until you get the best result. For doing laps on a sealed surface track I'd guess that having the inside wheel turn just a bit sharper (a couple of degrees?) than the outside would give the best result. The site below has some useful info on Ackermann and other suspension adjustables. http://stockcarproducts.com/techindx.htm Quote Link to comment Share on other sites More sharing options...
johnc Posted October 22, 2003 Share Posted October 22, 2003 FYI... I recall that every 1 degree of Ackermann is the same as 1/2" of total toe-out. I don't know where that 1 degree is measured (at full lock, halfway, etc.) but it just shows that a little bit can be way too much. Quote Link to comment Share on other sites More sharing options...
Drax240z Posted October 22, 2003 Share Posted October 22, 2003 Ackermann was origionally developed so that you wouldn't tear the crap out of your gravel driveway. Go figure. On cars with high front tire slip angles, anti-ackermann is run. I think I have a writeup from racecar engineering on ackermann somewhere here... Quote Link to comment Share on other sites More sharing options...
Guest Aaron Posted October 22, 2003 Share Posted October 22, 2003 Ackermann was origionally developed so that you wouldn't tear the crap out of your gravel driveway. Go figure. I need more Ackermann on my Kubota Tractor so I don't destroy my yard. Quote Link to comment Share on other sites More sharing options...
gramercyjam Posted October 22, 2003 Share Posted October 22, 2003 Drax, Great post. Do you have the anti-ackerman argument article? Quote Link to comment Share on other sites More sharing options...
johnc Posted October 22, 2003 Share Posted October 22, 2003 Anti-ackermann is generally referred to as Reverse Ackermann here in the US. Quote Link to comment Share on other sites More sharing options...
Drax240z Posted October 22, 2003 Share Posted October 22, 2003 Hmmm, yes I do. It's in the July 2001 issue. I don't have a scanner at the moment, so I can't really get it online right now. Maybe someone else out there has the issue and will post it. Quote Link to comment Share on other sites More sharing options...
260DET Posted October 23, 2003 Author Share Posted October 23, 2003 Plenty of forum discussion at this site. http://www.eng-tips.com/supportus.cfm Search in particular 'Ackerman Geometry'. A very controversial subject apparently. But the statement which made the most sense to me was that in high speed/g corners anti (reverse) Ackerman produced the best result because the heavier loaded outside tyre could operate at a higher slip angle than the lighter loaded inside tyre. Quote Link to comment Share on other sites More sharing options...
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