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This is not a how to,It is what I have done.

 

Mesuring the 240 rack I came up with 24.230" from center to center of ball scokets (iner tie rod pivot point)

and a travel of 5.125". The pinion is 7.5" from the center of the housing.

 

The Miata rack's I reviewed are from a 92 and 97. The 92 has a vavle body cast with the gear case and is held togeather with C-clips. This makes it harder to modify. The 97 has a vavle housing bolted to the gear case and the end cap screws into the housing. This is what I will cover here.

 

The Miata rack is 26.040"center to center of ball scokets with a travel of 4.704". The pinion is 8.75" from the center of the housing. Ball scoket threads are 17x1.0MM RH. Center of ball scoket to mating face of rack is (including lockwasher) .740".The travel is limited by ball scokets contact with the housing. Removing the ball scokets allows more travel that is now limited by the distance between the fluid lines less the with of the piston inside 5.354"-.287"=5.067" max travel for this rack. Not as much as the 240's 5.125",But with power steering I will go with shorter steering arms to make up for it.

 

To get Center to center of 24.230" I removed 1.812" off the right end of the rack remachined the end with the exception of the lenght of the small OD witch was shortened to .25". This aera and the vent hole must be polished to prevent seal damage.

 

The housing was cut down 1.582" on the right side and remachined. The thread is 40x1.0MM RH, I used 1.575"x24TPI RH (My lathe is english)

 

A new shorter end cap was made alowing an extra .385" travel. It stops the piston befor it blocks the fluid port on the left. Using a face pin spaner insted of a hex alowes more of the housing to be left in place for the rubber mount. This made up for cutting less off the housing than the rack.

 

On the left end of the housing a counter bore 1.5" DIA x.150"deep alowing the ball scoket to travel inside the housing,and the piston to travel to the edge of the right fluid port.

 

The tie rod's were cut down 2.25" and threaded 12x1.25MM RH. Miata outer tie rod's will be used.

 

I now have a rack with the same center to center as the 240z, and a travel of 5.067".

 

Other problems to address will be the pinion is now 6.562" from the center of the housing (15/16" closer to the motor), steering shaft and mounting made.

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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.

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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. 

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Agree that rack speed is a huge advantage, although you can get most of the way there by putting a 1.5 quickener in line. My Woodward is about 1.75x faster than stock. I had to decide between the 2.66 and 3.14 per turn racks, ended up going with the 2.66. Now that I've driven it at an autox, I know can get my wide car through slaloms without hand over hand, but I'd probably go 3.14 if I had to do it again. On bumpsteer, I think that you can tune it out to a point where there are better things to concern yourself with. Mine is 23.75 CTC for what that is worth, and I after minimizing bumpsteer through shimming I didn't notice any issues with it on track.

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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.

Edited by 74_5.0L_Z
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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.

 

I wouldn't do the electric assist personally.  The woodward rack is a real racing rack and doesn't have all the damping that normal street car parts do.  You can change the t-spring in the spool valve for different levels of assist.  You can fine tune how it feels and it will have better feedback than most of the electric columns you can get.  I have one of the older woodward racks that you can swap out the pinions.  It looks like they may have dropped this on their newer racks.  I picked up my rack off ebay for $400.  It's 19 inch center to center measurement and while Woodward doesn't like it Sweet and others make rack spacers to change the length.  You could go this route to get your 22 inch width for a lot less than the new price.  Just a thought.

 

Cary

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I wouldn't do the electric assist personally.  The woodward rack is a real racing rack and doesn't have all the damping that normal street car parts do.  You can change the t-spring in the spool valve for different levels of assist.  You can fine tune how it feels and it will have better feedback than most of the electric columns you can get.  I have one of the older woodward racks that you can swap out the pinions.  It looks like they may have dropped this on their newer racks.  I picked up my rack off ebay for $400.  It's 19 inch center to center measurement and while Woodward doesn't like it Sweet and others make rack spacers to change the length.  You could go this route to get your 22 inch width for a lot less than the new price.  Just a thought.

You're correct on the Woodward gear changes. Tony over there told me they weren't doing that anymore, I guess it was too much hassle to set up correctly. For a new one, you buy a rack and get the gear ratio you want up front.

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Lurcher,

     Most of the Z's I have driven had a lose feel in the center, some cars had a shimmy at speed. Adjusting the rack only made for hard steering. Looking at the racks the gear teath are worn beond service at the center.The fix was to shift the rack to one side.

 

74_5.0L_Z,

     Measument was taken with a caliper ±.001. The production tolorance of the rack is unkown as are production line changes. A 25' tape measuer has a tolorance of ±1/32 over all.I have checked tapes using a master scale and found that a good tape is hard to find. No matter the name on it. The 12' and shorter tapes are printed with a wheel, longer ones are printed with a belt. The belt is prone to strech, slip is a problem for all. Worst one I found was off by1/16" at 24".

      The rack I measured had no stops between the locknut and housing. do not know if this was stock or not. I went and checked a rack from a 72 with stops It is 4.744", Looks like I have some extra travel, maybe this will help me when drifting at the Auto-X, or maybe shorten the center to center some more.I have a 71 I bought from the orignal owner, I will check this one as I know it to be orignal.

      Looking at the 240 rack if it were cut off at the stops driled and taped for new style tie rod's of 3/4" center to face. Center to center will be about 22.5"no lss of travel no housing machining.

 

JMortensen,

       Just copying what was there. Right now I am working with the cross member/engine swap and needed the rack for mockup.

 

MikeMileski,

       Same center to center, The 92 rack has a ball bearing on the end of the pinion (97 is a needle bearing) the bushing is metal and much longer(97 is plastic) Moving the racks by hand the 97 has more drag.

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