74_5.0L_Z

Those will be way too tall. Look at the Hoosier 23.0x9.5-15 or the Avon 23.0x10.5-15. About the tallest tire you can hope to fit on a non-lower 240Z is about 24.5 to 25 inches tall.

I haven't shown my rear end in a while, so here you go:

What size wheels do you need to be able to run those?

Cool. I didn't have dial indicators laying around, and I stumbled upon my bump steer set-up on ebay for $75.00. It is essentially two dial indicators, an adjustable base, and a graduated plate that mounts on the hub.

I am not sure how you would measure bump steer with just two dial indicators. What are they attached to for a stable reference, and how do you track wheel height displacement from nominal ride height?

I have the InterComp Bump Steer Gage. I had to buy the four hole plate separately. When you bump steer the car, you need to do a few things to get accurate repeatable results. 1. Get the car on a nice level surface. 2. Center the steering. For this I have made two equal length (2.39") spacers that fit over the rack shaft under the boots that force the rack to its center position. 3. Find ride height on the strut shaft. For this I push the bump stop against the strut housing, lower the car, roll it back and forth, and then raise the car and measure the displacement of the bump stop. 4. Remove the springs and disconnect the sway bay. 5. Follow the instructions in the bump steer gage. Take measurements at 1/2" increments of bump and droop starting from ride height. You want to perform the bump steer measurement on both sides of the car. If the measurements from the two sides are significantly different, then the cross member may be shifted left or right, or the rack may not be square with the car. Attached below are two different rounds of bump steer measurement for my car. The top set was a baseline after I raised by LCA attach points 1/2". For that initial measurement, I used a stack of spacers to raise the rack .39 inches. From the curves, I had not raised the rack enough because bump was causing toe in. So, I made thicker spacers that raised the rack an additional .050". The second set of data shows that I shifted the bump steer curve slightly to the right and up. So, for at least the first 1/2" of bump, the tires bump out. My car has 500 lbf/in front springs, so I do not expect to get very much deflection over typical road bumps. If I raised the rack further, I would shift the curve farther to the right and farther up. As an aside, I am not really happy with these results. My next action will be to verify that a shorter rack will help the bump steer. My calculations indicate that a rack about 2.25" shorter center to center will virtually eliminate bump steer. To test this I am going to steer the current rack 1.125 inches and lock it in place. Then I will make a longer tie rod assembly to return the toe to its original setting (on one side). Then I will repeat the measurements above to verify that the shorter rack will fix the bump steer. Figure 10 of the following link explains how to interpret bump steer curves. http://www.woodwardsteering.com/PDF/tech%20section%20guide.pdf

Everyone with an S30 has bumpsteer. Some bump understeer is good, but bump oversteer can be very bad. Some people notice bump understeer more than others. Everyone will notice bump oversteer because it makes the car difficult or even dangerous to drive. From the factory, the S30 has bump understeer. The front wheels toe out during bump and toe in during droop. This is desirable for a street car because it is stable on the highway. The factory bump steer is desirable because the following happens when you hit a bump with the factory bump understeer: Tire hits bump, suspension on that side compresses, tire toes out steering toward side which hits bump, car leans away from side which hits bump causing suspension to droop and toe to return toward original heading. Depending on severity of bump, the car may overshoot the original heading, but the steering will want to return to center. Now, lets say that you have lowered the car, raised the LCA attach points, added roll center spacers, and added caster without changing the rack height. It is highly likely that you will end up with bump oversteer. Bump oversteer happens when the tires toe in during bump and toe out during droop. This is a highly undesirable condition because of the following: Tire hits bump, suspension on that side compresses, tire toes in steering away from side which hits bump, car leans toward side which hits bump causing suspension to compress further and toe in further. This condition requires the driver to compensate by steering the wheel to keep the car pointed straight. Unfortunately, there is a critical speed above which the driver cannot react fast enough to correct. Bump oversteer is VERY bad for cars that are driven at high speed. For a race car, bump steer should be minimized (It is impossible to completely eliminate). The bump steer that can't be eliminated should be in the form of bump understeer.

For me the S30 is just a blank slate for whatever I want to try. The only impediments to my imagination are time and money.

I don't want to redo the bodywork to fit taller tires.

I have seriously been considering some sort of SLA front suspension. The C6 is a logical choice for a donor. One of the things that concerns me about the C6 set-up will be the necessity of using 18 inch wheels. I would love to be able to use 15 or 16 inch wheels with whatever SLA set-up that I go with.

Swap the fully threaded bolt for the appropriate shoulder bolt and you're in business. I like the idea of an adjustable mono-ball in place of the lower ball joint. Unfortunately, I'm not sure that I could package those inside my 15 x 10 front wheels.

As far as the Woodward racks go, I think you can change the ratio just by getting a different pinion. I just went from the stock 260Z rack which had a gain of 1.59 in / rev to the stock 240Z rack which has 1.81 in / rev. The effort increase wasn't terrible, and the increased rack speed was noticible. Next I was planning to get the Woodward 2.09 in / rev (22 inches center to center) rack and install it without power steering. Then as funds permit I am going to add an electric power assist column. With a rack that is too long, all you can do is raise/ lower the rack or ends to get yourself on the right side of the bump steer curve. If you have bump steer, you want the front tires to toe out on bump and in on droop. Unfortunately, the slope of the bump steer curve is almost entirely dependent on the center to center length of the rack.

3D Kinematics analysis of the S30 suspension shows that minimizing bump steer requires a rack with a center to center distance shorter that what came from the factory. Any rack longer than 24.25 inches center to center will make bump steer worse. For what I want from my car (autocross), minimizing bump steer is a priority. While power steering would be nice, bump steer and rack speed are more important to me. I have my eye on a custom Woodward steering rack that will be about 22 inches center to center.

First, cool work on the rack. I hadn't considered modifying the length of an existing rack. I just measured up a 240Z rack for the purposes of modeling the front suspension kinematics. To do so, I disassembled a couple of spare racks that I have and measured their component parts using calipers and a tape measure. I even cut apart an inner tie rod to measure the ball diameter. Then adding up all of the measurements, I was able to determine that the center to center distance was between 24.2 and 24.3 inches. My measurements agree with yours but I can't specify the accuracy beyond one decimal point. It sounds like you have access to better measuring equipment than I do. Did you use a CMM for your measurements? Also, the stock rack has a total of 4.78" (2.390 each direction from center) of travel unless the factory limiters are removed. From my kinematic analysis of my front suspension, if I were to replace the stock rack with something different, I would buy or make one that was between 22 and 23 inches center to center.

I have a friend who owns the company that makes the Brunton Stalkers. The Brunton M-Spec cars come with LS Engines and have side pipe exhaust. The sound of his cars is very nice, and well below the SCCA dB limit. He uses catalytic converters in line with the mufflers to keep the exhaust quiet. If you give him a call (Scott Minehart), he can tell you which mufflers and cats he uses. http://stalkercars.com/ Here is his car on track.

Cary, I did run limiters on the front for a while just to keep the springs from getting loose when I jacked it up. I've since raised the front just enough to no longer need them. Clark, I am not having either an understeer or oversteer problem. The car pretty much goes where I point it. It just takes more slip angle to make it change direction. I am still not used to the Bias ply slicks. The car always feels at the limit of traction (even when its not). I am slower on the bias ply than on the radials. I may get used to them and get faster after a bit more time, but I plan to go back to radials. So, I probably won't spend much effort trying to perfect this set-up until I am back on radials.

The car could use more droop in the rear. Unfortunately, way back when I sectioned my rear struts, I shortened them too much. I have a couple of options to get some of that droop travel back. 1. Lower the rear. I can lower it about 1/2 inch before the spring loses all pre-load at full droop. Unfortunately, that gets me really close to rubbing the inside if the wheel well at full bump. 2. Add an aluminum spacer between the rear camber plate and strut tower to lower the camber plate mounting relative to the body. Then also perform option 1 but lower the spring farther. 3. Get some new rear strut housings and start over.

I started with the tires in the 18 to 20 psi range and the tire temperatures seemed to indicate that was the correct pressure. Unfortunately, the car felt like it was falling over all the time and the tires were rolling under pretty badly. So, contrary to the tire temps, I started adding pressure until the car felt more stable. I've got the car down to 2580 full of fuel with me in it. I have 610 lbs on the right front and 638 lbs on the left front.

Yes, I am experiencing the same problem that I spoke of earlier. This was a really tight course where you had to brake pretty hard and then turn pretty tight. I am trying to decide what I want to do to settle the car down. Currently, I have 500 lb/in front springs, 425 lb/in rear springs, 20mm front sway bar, no rear sway bar, and Koni 8610-1437race struts all around. The front suspension has the following static settings: camber: -1.8 caster: 6.3 toe: 1/8" out front roll center on the ground or maybe slightly below The rear suspension has the following settings: camber: -1.2 toe: 1/8" in The rear roll center is about 2.5 inches above ground. Tires: AVON 10.5/23.0-15 Cross Ply Slick (A11 compound) all around 22 psi rear / 27 psi front Wheels 15x10 with 5.25" back space mounted with no spacers. This gives me about 2 inches of scrub radius. I have no droop limiters on the car at the moment (except the struts themselves). The car really pitches around more on the bias ply tires than it did on the Hoosier A6 tires I was running. The tire itself seems to deform much more. When driving, I do not notice the tires lifting (front or rear). When these tires are done, I plan to go back to a radial. I am thinking about lowering the rear roll center, raising the front roll center, softening the rear springs, or adding a stiffer front sway bar.

If you don't want the 6061-T6 to crack when you bend it you have to ensure that the inside bend radius is at least 2.5 times the thickness of the sheet. So, if that is .125 thick sheet, you need a .375 radius. If you anneal the material in the region of the bend before you bend it then you can get away with a tighter bend (approximately 1.5 time the thickness).

I like how you blended in a rear spoiler to the quarter panels. What spoiler is that?

I have the Subtle Z kit (modified) on my car and I have plenty of space for 275/35-15 Hoosier A6 and Avon 23x10.5-15 Tires on 15x10 wheels with 5.25 inch backspace.

Just an aside: The Ford engine family that included 221, 260, 289, 302 (5.0L), and 351W all have 90 degree angles between their banks. It would be more appropriate and correct to refer to these as Windsor engines and not 60 degree engines.

Mike, Midwest Control sells adjusters like the ones that you made: http://www.midwestcontrol.com/series.php?id=194

Caster has some really good benefits on our cars and also causes some problems. First the good: With our front struts we have a large Steering Axis inclination (SAI). With factory settings, the cars have very little caster. So, with the factory settings the outside front wheel first tends slightly toward negative camber when the wheels are barely turned and then swings toward positive camber as the wheel is turned further. Similarly, the inside wheel will slightly gain positive camber as the wheel is turned. Ideally, the outside wheel would gain a lot of negative camber and the inside wheel would gain a lot of positive camber as the wheel is turned. Adding caster helps the camber gain a bunch. On my car I have 6.4 degrees of caster and -1.8 degrees of static camber. When I steer the front wheels 5.7 degrees, my outside front wheel gain negative camber so that it has -2.35 degrees, and the inside front wheel gains positive camber to -1.1 degrees. This change in caster is before the outside wheel compresses and the outside wheel droops. The changes in caster from strut compression / extension will add to that from turning. The change gets really significant for tight turns. If I steer my front wheels by 30 degrees(essentially full lock) the outside wheel goes to -2.9 degrees camber and the inside wheel goes to +3.9 degrees of camber. It has been shown that having caster about half of the SAI give pretty good caster gains with turn of the wheels. So, for our cars 6-7 degrees is the rule of thumb. Now the bad: Adding a bunch of caster causes problems. First, it causes the weight jacking / de-wedging problem that I talked about earlier. The combination of caster and SAI combine to cause the front spindle to swing on an arc. Adding caster tilts the steering axis back toward the fire wall. The tilt of the axis from caster causes the spindle to rise on the outside tire and fall on the inside tire. So, the SAI and Caster effects are additive (but in a non-linear kind of way). The large SAI and caster magnify the rise and fall of the spindle pin. If we could get an SAI of lets say 5 degrees and run a caster of 2.5 degrees, we would get the same level of caster gain as we do with our struts and 6-7 degrees of caster. However, the low SAI spindle would have much less weight jacking associated. Unfortunately, strut cars need large SAI for wheel clearance. The only way to get low SAI like I propose is to convert to a SLA set-up. On a strut car the weight jacking can be minimized by running the smallest scrub radius possible. Theoretically, a zero scrub radius car would not have any weight jacking from the steering effects discussed here. Next, adding static caster increases caster gain. If the strut had no caster and the lower control arm were parallel to the ground, the strut would maintain zero caster for all of its travel. As you add caster by tilting the strut back toward the firewall, bump travel will cause the caster to increase. The more it is tilted back, the faster the caster will increase for a given bump motion. On my car, I have raised the TC rod 0.5 inches and run 6.4 degrees of caster at ride height. One inch of bump increases my caster to 6.9 degrees. The problem with the caster gain is bump steer. The caster gain causes the outer tie rod to rise relative to the steering rack. If the rack is in the stock position, then this rise of the outer tie rod end causes the virtual length of the tie rod to shorten. This causes toe in on bump, and we all know that toe in on bump is a very bad thing. The problem of bump steer can be mitigated a little by raising the rack or by adding the bump steer extension to the outer tie rod ends. But that is a Band-Aid and not the best solution. I have found that I can almost completely eliminate the bump steer in the front end (for my current set-up) by going with a much shorter rack (~22 inches center to center of the inner rod ends).