New project... Woodward steering rack. Few people started a conversation on this in the AX/RR forum but haven't seen it brought to any conclusion... so i will try.

Woodward racks are well engineered, and used exclusively by NASCAR (spec'd) for probably 20+ years. Versatile, rigid, strong, tunable.

The cross member will be reinforced and modified to accept the GL type rack.

The length between LCA pivot determines the length of the rackshaft... about 24.2"

The steering arm on the upright seems to be about 4.75" from LBJ center, although it's difficult to accurately measure. This means that the Woodward 2.62 to 1 rack and pinion would produce 11.0 to 1 steering ratio.

More on quick reference for ratios:
https://sweetmanufacturing.com/wp-co...ING-RATIOS.pdf

The OEM 1986 Z51 setup is 13.0 to one.

The rack travel seems to be about 5" so I'm ordering the setup for 5.25" and can install a 1/8 spacer if that crashes parts into the frame or whatnot.

Some other thoughts... Rack location fore/aft determines Ackerman, have't done much analyses on that but will need to at some point. I don't want full Ackerman and i think OEM location is at least that, so rack may end up 1/4" or so forward of current location. It will be lower and the clevis at the end will be a little higher. Dirt track cars dial in a lot of Ackerman, and high grip asphalt cars much less and even negative for the tall sidewall soft slicks on indy cars and such, where the max grip slip angle versus load differentiates. So I don't know, no big deal. Height is a factor for bump steer as the angle produced needs to be correct, best checked and dialed in with a bump steer gauge. Lowest rack position has some advantages, but not at the cost of tall spacers.

Servo details, picking the 50-2 servo... TB210 torsion bar (thinner the bar, the more power steering assist on center) and can be dialed in, this one's right in the middle of what they offer. TB can be changed out fairly easily to tune it.

http://www.woodwardsteering.com/PDF/...20profiles.pdf

Note that I will be running light Caster settings so less steering effort than later model C4's need.

Love to get some feedback, most of my research is done, but i can always use advice... wish me luck. :-)

 

This thing is a beast.

 

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Because my car is RHD it has been fitted with an aftermarket rack (with 2 turns lock to lock) - pretty sure its in the original position.
the big issue with trying to move the rack fore or aft is everything is so tight in there, it's really tough just trying to get a fat sway bar in under there - you probably won't know how much room you've got until you drop the rack in there with everything else still in place.
Looking at the pressure cannister, that's right where the swaybar and fan shroud live.

 

Those are great points, thanks for mentioning. I have a mock up of the front of the car to check everything, motorplate, dry sump oil pump clearance, radiator mount, pluming, steering rack, steering coumn, suspension geometry, bump steer, even Ackerman… everything may be mocked up forward of the transmission.

This rack should provide a lot more room for the oil pump, with remote steering servo being out of the way. Swaybar “should” fit above the actuator.

There’s a very narrow area forward and aft, determined by the leaf spring location for aft, and probably will need to scallop the cross member for where the pinion housing dips below the mounting plane. The rack mounting needs to be much more solid than the OEM, this guy would rip the car apart and itself, like a powered hydraulic ram… looks like I will be placing a heavy 1/4” thick L bracket at top, maybe 1/8 bracket below, and a 3/4” 0.120 tube sandwiched in between for the mount bolts. Upwards location limited by the frame and the large clevis, but I will be mounting it as low as I can. Radiator mounting and shroud will be fabbed up.

Still have a lot of work to do… measured the Ackerman on a friends 1986 and it was only about 1/4” toe out at full lock relative to center zero. However, it looks like, from the upright, that parallel steering (90 degree arm to toe rod) will be rack forward a bit. Need to research tire loading and slip angles to come up with a good compromise (100% Ackerman then subtract slip angle delta at 1G Cornering on asphalt perhaps).




Marked approximate location of mounting points at furthest aft location (black lines intersections).

 

Ackerman: If anyone knows the amount of toe out produced by turning from straight to full lock, let me know (forward edge tire center to tire center at full lock, compared to straight ahead, let's assume zero toe as we're looking for degree of change or linear measurement of change).

Can't measure right now, no front suspension installed.

Overview: at slow speed and high turning angles, it's great to be at 100% Ackerman; the front tires rolling in a perfectly aligned arc, resulting in excellent parking manners, no tires squealing, and a car that's easy to push around. The toe out change for 100% Ackerman with a 96.2" C4 wheelbase and 20' turning radius, is 23 degrees measured at full lock, that's quite a bit. Also that's about 5" for a 25.6" diameter tire.

https://www.calculatoratoz.com/en/ac...tor/Calc-37088

I have no idea what the OEM Ackerman is, but from the plan view drawings of the 1984 C4, it seems to be quite high. For example, 100% is not a desirable situation for a performance application on pavement. The question would be: do we prefer the front tire squeal with the front tires fighting eachother in a parking lot, or fighting each other on a paved race track with the tires under load. I'll give up all of the parking lot manners for best grip.

The ideal dynamic number can be computed if we have traction/load/slip data for the tires... I can't find any for modern performance tires. Milliken/Milliken SAE text book says there's no place for Ackerman on a performance application, and indicates parallel or reverse ackerman as advisable (Steering Systems pg 715). Several books say the opposite, and some sources even reference that there's no difference in laptimes with ackerman being moved around (tall sidewalls on Indy car at Long Beach).

Once I set up the rack mount (welding in support and gusset plate) it will be a little difficult to change it. I want to figure out how much I can move it forward and retain a litle bit of Ackerman.

Parallel steering (zero Ackerman) the steering arms are 90 degrees to the tie rods straight forward, so the angle/vector is the same on both sides. As shown in the previous post, the spindle has quite a bit of out angle... so straight arms attached perfectly in line by the rack position, will have positive Ackerman.

Angle of the steering arm:

 

So far so good, I will have some latitude to move the rack forward and retain plenty of Ackermann… maybe even be able to increase steering angle without chopping up the upright (not that I need to)

 

To order a rack for C4 Corvette, they require you fill this for all the dimensions

http://www.woodwardsteering.com/PDF/...%20English.pdf

Surely someone here has already very carefully measured a stock 1984 - 1987 rack and we could post all that information here for anyone to use for their future order?

Would be much appreciated!

 

Did you want to replicate the OEM fitment with a much better rack? How would it be mounted?

Dimension A is important, as it needs to match the LCA pivot width. It’s 24.2” for a C4.

I’m trying to accommodate a dry sump engine and needed to move the OEM servo valve out of the way, as it was too close no matter how I positioned it. The rack I have spec’d should mount lower, with an in line remote servo, and I wanted a faster rate that a Woodward or Sweet rack provides.

This means that the rack should be solid mounted, much sturdier than OEM and fit where you prefer the fore/aft positioning. The servo can be customized to get the feel you want (there’s a thin torsion bar inside, the thicker it is the more direct the steering and less psi hydraulic assist per torque steering input.)

I’m using a 9gph pump mounted to a motor plate, really should be using an 8gph pump so we’ll see.

 

Made some progress this weekend...

 

Holy heck!
That is impressive!

 

Although the weekend was somewhat productive, I’ve started to modify K-Member like 14 months ago.

 

Exactly that is my goal. Original is so inaccurate with all the flex in the input shaft of the power steering mechanism. That is the case with most of C4 Corvettes.

I would make new mounts to it. But now I started to reconsider, maybe I will widen the car at front and make a totally custom setup for the steering also. Their rack with all the options can cost over 5k, so it is good to do it right the first time

Meanwhile, if somebody knows some better than OEM rack for C4 with C4 dimensions out of the box, let me know.

 

Gotcha, so the steering input flex you can fix up to a point, the remaining built in flex is from the slightly too light torque sensing bar in the servo. Then the rack and pinion gears loosen up over time, especially on center, so the added slop screws with attaining a desired steering feel.

My suggestion for improving steering feel (and that's a big part of the satisfaction of driving experience) is sending the rack and pump to be overhauled by Turn One, and letting them know what it it that you're wanting. They may not change the servo, but they can regulate the volume and pressure of the pump. Most likely it boils down to having more sports car and less Cadillac. I imagine this may end up being a $500-1000 ordeal but i haven't researched it.

I want those things also, goals for me were a quicker steering ratio, ability to have steady constant and linear feel with fast inputs, solid on center feel (stiffer torsion/torque-sensing bar, and how this works is that a very stiff one is basically manual steering), smooth operation, proper geometry, and clearance for the block mounted dry sump oil pump/frame/suspension, solid reliable bullet proof setup.

The thought process was "how do they do this in the upper end of motorsports" and the solution I came up with is a KRC power steering pump, Woodward GL steering rack with 2.62 ration, 5.25" travel limit, 0.210" torsion bar, 24.5" length with slotted clevis ends, 950-2 servo valve. Part Number GLC262CC-2450-950-2-210. The anti-roll bar is located forward 3/4" from original location (1984-1987 as the later ARB locations are slightly different on the frame) and I'm using an 1988-1996 style/dimension ARB. Custom steering shaft (Woodward u-joints and 0.750 steel DOM 0.120 wall tubing) I'm not ready to say that someone can duplicate it just yet, that might be a while, and I may switch servos or torsion bars or pump orifice or pump pulley size. Everything fits perfect so far but there's not much room for error. Rack and servo were $1600; KRC pump $400; linkages, bump steer kit, material to weld, pulleys/shafts/joints/hoses/hardware maybe $500-600.

http://www.woodwardsteering.com/PDF/...20HL,%20GL.pdf

CF-20S style rack may work also, that's the latest what Nascar currently uses, didn't go down this path as it didn't meet all of the objectives.

 

Little bit of media on the subject... here's a Woodward quick ratio rack in the red prototype car.


I notice custom steering racks a lot more nowadays, here's one I saw recently, looks to be about the same on a very nice car.

 

Fit the remote servo to a bracket mounted on the strut, finished up the installation minus hydraulics. Clears the dry sump pump very well. Everything has to be the perfect length, as there’s maybe 0.030” longitudinal allowance at each of the three Woodward Splined U joint set screws. Little bit more on the D shaft from the OEM steering shaft. No rag joints here.

 

Today I was able to measure the Ackerman with the Gyraline app... unfortunately I don't have any before and after comparisons, if someone else has the app perhaps they can provide data... seemed to consistently gives me about 1 degree toe out per 10 degrees of steering angle. I haven't crunched the numbers, but it sounds good... not much I can do to change it at this point.


Also, had to re-do the steering links to precisely position the power steering servo and hoses well clear of the header primaries.

 

OK, wanted to come back to this... now crunching the numbers, ideal ackermann geometry (C4 with 60" track and 96.2" wheelbase) defined for basically for rolling around in a parking lot, is about 1.7 degrees toe out per 10 degrees of steering input at the wheels. However, at a 1G turn with 10 degree steering input, the inboard tire may be for example loaded at 400lbs on the inside tire and 1100lbs on the outside tire... and for a typical 200 treadware tire the optimum slip angle versus load would have a 1 degree split such as ~8 degrees on the loaded outside tire and ~7 degrees on the lightly loaded inside tire. So, ackermann gain at 1 degree at 10 of steering is even better than the unknown higher number from the original location.




For a much tighter turn there will be some scrub slightly at 1.0 degree per 10 degrees of steering, but for fast slaloms 0.8 to 1.0 is perhaps the sweet spot.

 

For reasons I can't explain, I somehow missed this thread until finding it today. A very good discussion, and particularly interesting to me. I'm running a narrowed C4 suspension under my '69, and have had to do a bunch of geometry tweaking getting it working well, and have recently been trying to figure out if there would be front grip gains from changes to the present Ackermann/rack-placement of my setup. The data and calculations you've posted is appreciated.