Steering Setups

[gramercyjam]

Regarding setting up chassis and steering on an prepared autox 240Z with massive lateral grip - beyond the typical discussions on springs and shocks, toe-in and bump steer, I have a serious need to know about ackerman, scrub radius and king pin inclination. I think there are a couple of different camps on these items - both pro and anti, theoretical static models VS real world dynamic effects. Any opinions or knowledge on the subject here as it relates to grip, handling, steering effort of the Z?

 

--John B

[JMortensen]

My experience with Ackerman has been positive, although it was not in a Z. I have a friend with a 510 that has modified steer knuckles for more Ackerman. You just can't get his car to push and it turns in REALLY hard. Of course he also has 7 degrees of caster and 2 degrees neg camber, and cantilevered Hoosiers.

 

I'm wondering how you would get it in a Z, though, since the steer knuckle would have to be bent out instead of in like the 510. I seem to remember the rule of thumb is you want the theoretical lines from the tie rod to the ball joint to cross at the middle right where the diff is (not an expert here, so don't quote me on that). That would require a pretty crazy offset on the front wheels to clear the tie rods.

 

Also, would that be legal for Prepared???

 

Jon

[gramercyjam]

Glad to hear of a positive experience with Ackerman. I'm not sure how it would be accomplished - some modeling would need to be done. It is legal for prepared. You can move suspension mounting points, hubs and uprights are totally open. You can change cars with struts to double A arms but you can't mount shocks inboard.

 

--John B

[JMortensen]

Switch to rear steer?????

 

Jon

[gramercyjam]

You are radical dude! It might be easier to to just learn to drive drive backwards!

[260DET]

IIRC scrub radius and king pin inclination are related. One problem with a Z with 'outset' front wheels ie wide wheels which are mounted further out from the hub than stock, seems to be the condition of steering kick back when you hit a large bump when turning. Not bump steer which is similar but which can occur when driving straight. Steering kick back seems to be related to a change in scrub radius, whereas bump steer is related to an out of sync front suspension/steering arms situation.

 

Now I'm not clear on any of the above so comment invited.

[johnc]

Its only been in the last year that I've been able to race my Z in a class that lets me make significant enough changes in the front suspension and steering to start thinking about the above items. Unfortunately, it will probably be towards the end of 2004 before I start making changes. I do have access to some modeling software so, maybe when I get a few models done I'll post the results.

[gramercyjam]

I have 16X10 wheels, and the inside edge of the rim is as close to the coilover springs as possible. Maybe too close (1mm clearance)? I'll need to make some scrub measurements to see.

 

I thought about the 280ZX possibly having better geometry than the 240 and did a visual comparison last night between the front 280ZX strut and a 240. There are definitely some major differences. Although the overall length is identical on both, the 240 spindle is much further away from the strut tube, there is a larger angle between the spindle and the strut tube, and the spindle is placed further up on the tube than the 280ZX. The 280ZX strut may have inherently less scrub, but with the spacers required to push the wheel out far enough to clear the springs, I don't see any advantages to using a 280ZX strut.

 

Kick back is definitely a major issue with me. Not so much with bumps, but when the tires grab in transition during a low speed (40 mph) manuver such as when sliding on some gravel in a turn and then hitting a clean surface, and it started showing up when in search of more grip, I try lowering tire pressures to the 21-23 psi range, down from the 28 psi I was running. There may be no ideal solution, but I'd like to improve the situation if possible.

[JMortensen]

While you guys are researching this, I want to ask a question that's been nagging me for some time, because I simply don't know if this was worthwhile or not.

 

My friend with the 510 and the steer knuckles modded for Ackerman also did something else that was interesting. He moved the TC pivot up the same amount as he moved the control arms. He said this would counter dive under braking and restore the geometry of the original design. He already had rod ends on the TC's, so he made a new bracket with 1/8" plate, boxed it all in, cut the original TC mounts off the frame, and welded his new ones on. He then made a hole to mount the rod end at the original height, and a new one that was higher by the same amount that he had raised the control arm pivots (which was about 1.5 inches I seem to recall.

 

Any thoughts on that???

 

Jon

[gramercyjam]

I am not familar with 510's, but it sounds like a reasonable thing to do if the inner suspension joint arm was raised. As the control arm travels through an arc, if the T/C rod didn't travel through the similar arc starting at the same angle as the control arm, it would pull on the control arm.

 

I can visualize how raising the control arms could increase the veritcal component of the forces at the inner control arm pivot during braking that could help counter act dive. I can't say how well it would work in practice.

[260DET]

Using the stock first gen strut assy, it appears that the only way to reduce the scrub radius is to use larger diameter wheels/tyres, and/or, relocate the wheel inwards.

 

Yes, the 280ZX strut seems to be better designed in this regard, particularly as it is possible to locate the top of the wheel/tyre under the lower spring perch and so closer to the strut tube. Depending on the diameter of the wheel/tyre of course.

[gramercyjam]

My wheels/tires are already under the top spring perch with a 240Z strut. I have about 1mm clearance between the rim and strut tube. I'm limited on tire diameter by rules (16"). So there doesn't seem to be a way to reduce scrub by either moving tires inward or going to larger tires. Increasing the angle between the spindle and the strut tube may be one possible way to do it.

[260DET]

From memory, the book is out on loan, Fred Pughn's book How to Make Your Car Handle says that many cars are designed to have a significant scrub radius but he doesn't say why. Perhaps some scrub radius gives better steering feel?

[gramercyjam]

According to this site, http://www.auto-ware.com/shoptalk/0_scrub.htm scrub radius helps provide better feedback to the driver. I found a nice link that describes everything very nicely here http://www.rqriley.com/suspensn.html

[JMortensen]

If the problem is kickback, would power steering help? I know how hard it is to steer my Z on slicks, especially on a tight autox course. I've found that I don't like anything bigger than a 14 inch steering wheel either. In fact I prefer a 13, otherwise I seem to hit my hand on the door too much.

 

Jon

[gramercyjam]

Kickback is a problem, and power steering is definitely an option. This would look very cool http://www.woodwardsteering.com/hot.htm.

 

My first thought was to just get everything up to par and making it as solid as possible by using 3/4" tubing and heim joints for a steering shaft, elminating the steering coupler and putting in some new high quality u-joints and a high quality steering rack to tighten everything up. That is still part of the plan. I am just digging into the why of things to trying to gain a full understanding. Maybe even find a competitive advantage there.