Comprehensive Steering Information Thread

[Volke]

I see a lot of misinformation regarding steering modifications being passed around Facebook groups and forums almost daily, so I’ve decided to put together this information thread to correct the many misconceptions, and to compile a comprehensive database for steering rack swaps and power steering modifications. The comprehensive steering rack database would not be possible with the community’s help, so many thanks in advance to those who read on and contribute!

Contents

1. Understanding Steering Ratio
2. How To: Determining Steering Rack Rates, Total Travel, and Linearity
3. BMW Steering Rack & Knuckle Database
4. Understanding Power Steering Operation
5. DIY: Decreasing Power Steering Assist

1. Understanding Steering Ratio


What is steering ratio?

The term steering ratio refers to the ratio of the steering wheel rotation (in degrees) and the resulting wheel angle (in degrees). When written as 15.4:1 or 13.7:1, it tells you how many degrees the steering wheel needs to be turned to turn the wheels 1 degree. Smaller ratios make the steering feel more nimble and quick, and larger ratios make the steering feel more lazy and slow.


Is the manufacturer published steering ratio for another car model a good criteria for determining which steering rack to swap into my car?

NO!

Comparing the manufacturer published steering ratio for different models alone is not sufficient in determining the rate or “quickness” of the respective steering racks, and chances are it will not give the expected result. The only way to do an accurate comparison is to measure the steering racks directly. This is because the steering ratio is a function of the steering rack rate AND the steering knuckle design. In other words, swapping a steering rack from a 330 with a BMW published steering ratio of 13.7:1 into a E46 M3 will not necessarily result in quicker steering than using a steering rack from an E46 M3 ZCP with a BMW published 14.5:1 ratio. In fact, both the 330 and ZCP racks will result in a 14.5:1 steering ratio when installed on an E46 M3, M3 ZCP, or M3 CSL. This is because both the 330 and ZCP steering racks have the same rate. More details on this later.


What is steering rack rate and how does it affect the steering ratio?

The steering rack rate is the relationship between the left or right travel of the steering rack to the rotation of the input shaft or steering column. It has units of linear distance per angular displacement. For example, mm/rev or in/rev. A higher steering rack rate results in a lower(“quicker”) steering ratio. Assuming no changes are made to the steering rack other than the gear teeth, a higher steering rate also results in higher steering effort required.

It is also important to note that steering racks can be either linear or progressive. A linear rack has a constant rate across its whole range of travel while a progressive rack will have one rate for the first bit of travel to either side of the on center position and another, usually higher, rate on both ends.


How does the steering knuckle design affect the steering ratio?

The steering ratio is also affected by the steering knuckle geometry, or more specifically the distance between the tie rod pickup point and the steering axis. On the E46 MacPherson strut suspension the steering pivot axis is defined as the imaginary line passing through the center of the top strut mount and the center of the lower control arm ball joint, and the tie rod pickup point is the center of the tie rod ball joint. A shorter distance between the tie rod pickup point and steering axis results in a lower(“quicker”) steering ratio, and higher steering effort required assuming no additional suspension geometry or power steering changes. The E46 non-M knuckle has the tie rod ball joint closer to the steering axis compared to the M3 version, so it will always have quicker steering than the E46 M3 using the same steering rack.


Is there anything else I should keep in mind when choosing a steering rack?

Yes. Steering racks have a limited left and right travel.

If the total travel of the new rack is less than your current rack, your turning circle will increase, and you will lose some maximum steering angle.

If the total travel of the new rack is more than your current rack, you may be able to get more maximum steering angle, or you may need to add some rack stops to prevent rubbing on the inner wheel well liner.


Is there any other way to get quicker steering without changing the steering rack?

Yes. SLR Speed sells an adapter that attaches to your steering knuckle and relocates the tie rod ball joint closer to the steering axis making the steering 60% faster, but it requires machining your OE control arms to press in a new ball joint, or switching to aftermarket tubular control arms. The kit also provides bump steer and roll center correction. Provided you have the wheel well clearance, the kit will also allow you to go up to 60 degrees of maximum steering angle from the factory 40 degrees. More than likely, you will need to use some rack stops to prevent rubbing on the inner wheel well.


Will changing the steering rack or modifying the knuckle geometry mess up the DSC?

Yes, but what extent depends on how different the new steering ratio is. The BMW DSC uses signals from the rotation sensor on the steering column, acceleration sensor and yaw sensor to determine when to intervene. Put simply, the DSC unit calculates the direction the wheels are pointing based on the column sensor output and some programmed information about the steering ratio, and if what the car is doing doesn’t match up, it will try to correct it. Steering modifications make the wheels point in a different direction than the car calculates they are, so it is more likely to intervene. The quicker you make the steering, the more it will intervene. Milder setups may work well enough to daily, but DSC will need to be turned off for track or autocross.


Ok, I understand how steering racks and knuckles affect the steering ratio along with the downsides. So which rack will give me the quickest steering?

Unfortunately, I only have information on a few of the BMW steering racks available. I need help from the rest of the community to complete the rack database further down below.


I have a steering rack not yet in the database, and I want to help. How do I contribute?

Follow the instructions in the section below to take measurements, or send the rack to me. I will take measurements and send it back.

[Volke]

2. How To: Determining Steering Rack Rates, Total Travel, and Linearity


Measuring on Center Rate

Step 1

Center the steering rack and use a paint pen to make an alignment mark on the input shaft and housing.




Step 2

Measure the distance from a fixed point on the housing to the end of the toothed shaft or tie rod.




Step 3

Rotate the input shaft 1 full turn. Try to be as accurate as possible with the alignment mark.


Step 4

Repeat the measurement from step 2. The difference between the 2 numbers is the rack rate. Round to the nearest millimeter.





Checking for Linearity / Measuring Progressive Rate

Step 1

Turn the steering rack all the way to one side and use a paint pen to make a new alignment mark on the input shaft and housing.




Step 2

Measure the distance from a fixed point on the housing to the end of the toothed shaft or tie rod.


Step 3

Rotate the input shaft 1 full turn. Try to be as accurate as possible with the alignment mark.


Step 4

Repeat the measurement from step 2. The difference between the two numbers is the rack rate.


If this number is equal to the on center rate, the steering rack is linear.


Measuring Total Travel and Turns Lock to Lock


Step 1

Make sure the inner tie rods are installed. They act as a rack stop and limit the side to side travel. Without these, the total travel and turns lock to lock will be inaccurate.

You can also use M18 drain plugs or bolts and washers, but you will not be able to measure total travel as easily.




Step 2

Turn the steering rack all the way to one side and make an alignment mark.


Step 3

Measure the distance from a fixed point on the housing to the end of the inner tie rod.


Step 4

Turn the steering rack all the way to the other side, and repeat the measurement from step 3. The difference between the two measurements is the total rack travel.

You can also center the rack, make an alignment mark, and take measurements going to one side, then repeat for the other side. The total travel is then the sum of the two sides.


Step 5

If the rack is linear, divide the total travel by the rack rate to get an accurate number of turns lock to lock.

You can also approximate the turns lock to lock by counting the turns using your alignment mark, but it’s hard to get an accurate value unless it’s a whole number.

[Volke]

3. BMW Steering Rack & Knuckle Database


Steering Racks


E46 M3 Steering Rack - measured by me

Rack Rate = 47 mm/turn

Turns Lock to Lock = 3.2

Total Rack Travel = 150 mm

Linear


E46 M3 ZCP Steering Rack - measured by a friend

Rack Rate = 50 mm/turn

Turns Lock to Lock = 3

Total Rack Travel = 150 mm

Linear


E46 330 Steering Rack - Yellow tag or otherwise - measured by me on a BMW remanufactured 330 rack with no tag, yellow tag confirmed by Michael9218, purple tag confirmed by Radekxpl




Rack Rate = 50 mm/turn

Turns Lock to Lock = 3

Total Rack Travel = 150 mm

Linear


Z3 2.5L - measured by me, didn’t have inner tie rods handy for lock to lock or total travel measurements

Rack Rate = 50 mm/turn

Turns Lock to Lock = ???

Total Rack Travel = ???

Linear


Z3 1.9L - found on another forum, needs confirmation

Rack Rate = 53.5 mm/turn

Turns Lock to Lock = 2.7

Total Rack Travel = 144.5 mm (calculated from 53.5 mm/turn and 2.7 turns)

Linear


Z4 M Roadster - found on another forum, needs confirmation - most likely same as Z4MC rack below

Rack Rate = 51 mm/turn

Turns Lock to Lock = 2.7

Total Rack Travel = 137.7 mm (calculated based on 51 mm/turn and 2.7 turns)

Linear


Z4 M Coupe - confirmed by Michael9218



Rack Rate = 50 mm/turn

Turns Lock to Lock = 2.8

Total Rack Travel = 140 mm

Linear

Note: Pinion shaft is 70 mm tall vs 40 mm on E46 steering racks


Steering Knuckles


Section for anyone interested in knuckle swaps or knuckle modifications. Included some bonus information on camber impact. Negative number means it add negative camber. Positive number means it adds positive camber.


E46 M3

Tie Rod Distance from Steering Axis = ~130 mm


E46 M3 CSL

Tie Rod Distance from Steering Axis = ~130 mm

Camber Change from E46 M3 = -1 degree (approximate)




E46 non-M

Tie Rod Distance from Steering Axis = ~126 mm

Camber Change from E46 M3 = +2 degrees (approximate)




SLR Speed E46 M3 Race Kit Adapter

Tie Rod Distance from Steering Axis = ~90 mm



Z4 M Coupe - confirmed by Michael9218

Tie Rod Distance from Steering Axis = ~130 mm

[Volke]

4. Understanding Power Steering Operation


How does hydraulic power steering work?


Without getting into too many details, hydraulic power steering uses a positive flow, fixed displacement oil pump and pressure regulator to build pressure and help the steering rack move left or right. On the E46, the pressure regulator is built into the pump itself, and the pressure setting of the regulator is what determines how much power steering assist you get.


Whenever the steering rack is not moving, such as driving straight ahead or maintaining a constant steering angle, 100% of the pump output flow goes through the pressure regulator and recirculates back to the pump inlet side. The pump is doing zero useful work under these conditions, and is just robbing engine power and heating the power steering fluid.


When the steering rack is moving left or right, some of the flow goes to the steering rack to help it shift, and the rest goes across the pressure regulator to maintain the desired pressure. Faster steering inputs require more pump output flow to be able to maintain the power steering assist. The pump must always output enough flow to have at least a little go through the regulator, or there will be no power assist.


My steering feels too light. How do I make it heavier?


See the first part of this guide to make the steering ratio quicker or follow the instructions in the DIY below to decrease the power steering pressure. You can also remove the pump or pressure regulator and send it to me for modification.


Can’t I just install an underdrive power steering pulley?


No. Underdrive pulleys just spin the pump slower and decrease the amount of pump flow available. This just makes it more likely you will lose power steering assist all together if you steer too quickly at low RPM.


Is there any point in installing an underdrive power steering pulley?


Yes. For a car that spends a lot of time at high RPM, underdrive pulleys will help keep the power steering fluid a little cooler and free up a small amount of engine power. The slower pump speeds and cooler temperatures increase pump life. Underdrive pulleys are not recommended for drift cars due to the super quick, large, and frequent steering inputs required.


Are there any other benefits to decreasing the power steering pressure?


Yes. Lower pressure also helps prevent blown out or leaking power steering lines, keeps power steering fluid temperatures lower, and frees up a little engine power.

[Volke]

5. Decreasing Power Steering Assist



Tools and Items Required

27 mm socket or wrench

11 mm socket

A collet vise (preferred) or soft jaw vise with round grooves

Shims

22 mm socket or wrench

Drain pan

Magnetic pick-up tool

Banjo bolt washers

Power steering fluid


The ID and OD of the shims are critical. If the OD is too large, it will rub on the spring and cause it to fail, sending debris into your power steering system which could damage your pump and steering rack. I had to work with a local company to get some shims made up with the proper ID and OD. I have some left, and I can always get more if anyone is interested. I can sell them for $20 shipped with tracking for a pack of 10. I can ship internationally without tracking for the same price, but it could take up to 4-5 weeks to arrive.




Step 1


If you’re doing this on the car, you will need a 22 mm wrench or socket to remove the banjo bolt and disconnect the high pressure power steering line. Place a drain drain pan underneath for any oil that comes out. I haven’t attempted it on the car, so I’m not sure how quickly it will drain out.


Step 2

Remove the 27 mm hex plug on the pump output.


You will need to use an impact or a vise to secure the pump if you’re doing this off the car.


Step 3

Use a magnetic pick-up to remove the pressure regulator and the spring behind it.

If you’re doing this off the car, you can just turn the pump upside down and both will come out.




Step 4

Secure the pressure regulator in a collet vise and remove the 11 mm hex plug. Be careful not to lose the small ball or spring hidden behind it, or any shims that may be on the plug.

If using a normal vise, be sure to use soft jaws and be careful not to damage the regulator or you will need to buy a new power steering pump.




Step 5

Add additional shims to reach desired assist level and reinstall the plug being careful to keep the shims as centered as possible. They can rub on the spring if they get pushed too far to a side which will cause the spring to fail. Also make sure the tiny hole in the center of the hex is not plugged.


If you’re using the shims from me, each 0.5 mm shim will reduce the power steering assist/pressure by ~12.5 percent.


1 additional shim = ~12.5% reduction

2 additional shims = ~25% reduction

3 additional shims = ~37.5% reduction

4 additional shims = ~50% reduction


You can go up to 6 shims for 75% reduction, but I don’t recommend it because it will start decreasing the total thread engagement of the plug which could cause it to back out. If you must do this, I suggest cleaning the parts of oil and using some thread locker(ex. Loctite). Be sure to let it cure overnight before putting it in oil again.




Step 6


Installation is opposite of removal. Be sure to use new washer for the banjo bolt and top off your power steering fluid.


Closing Comments

Hope you all find this useful!

I'm not very good at keeping up with forums, so feel free to PM me if I'm too slow at answering any questions posted. I will have email notifications enabled for private messages.

[silaciM3]

The Z4MC rack was measured by another member at 50mm/turn.
He also posted the part nb which confirms it’s a Z4MC rack. Unfortunately that is now lost on the old forum.

[Volke]

The Z4MC rack was measured by another member at 50mm/turn.
He also posted the part nb which confirms it’s a Z4MC rack. Unfortunately that is now lost on the old forum.

Thanks, I will update it!

[duracellttu]

I've read that the CSL king pin gives a wider stance by 10mm. Can you confirm that?

[lapoune]

Good to see this thread back. I was the one who measured the Z4MC racks on the old thread, I measured a reman one direct from BMW and one that came factory from a car. Both were 50mm\rev linear. I also measured a variety of z3 racks, 1.9 and 2.5s. Again all 50mm\rev linear. I think it's safe to say that until proof of the contrary 1.9l z3s don't have a quicker ratio (read a couple times that some are actually slower, but I've never seen one myself).

Interestingly I also measured a rack that supposedly came from a prototype Z4MC in germany, but it had a Z4M roadster original part#. It also measured 50mm\rev linear.

Sorry can't remember all the part#s off the top of my head. I'll try to update later with them.

[terra]

Do we have a Z4MR rack measurement? It's quoted as being a slower rack. If it's proportional to linear displacement, I guess it'd be ~47mm/rev

[lapoune]

Do we have a Z4MR rack measurement? It's quoted as being a slower rack. If it's proportional to linear displacement, I guess it'd be ~47mm/rev

Yeah that's why I was surprised. Mine had non-reman part# from a Z4MR but BMW might have messed with it. Would be interesting as I don't see how the two cars can have different ratios with the same racks.

[terra]

I do find it funny that pretty much all of the rack upgrades ended up being 50mm/rev and linear. and in fact to date there's nothing confirmed to be faster. You'd always hear about people calling the 330 rack a modest upgrade which had a progressive feel while others would say the Z3 rack is so twitchy that it's hard to drive on the highway. Shows how much of our perception is susceptible to suggestion.

[Volke]

I've read that the CSL king pin gives a wider stance by 10mm. Can you confirm that?

It does not. That's a myth due to the CSL having 10 mm wider track width up front. 6mm comes from the wheels being 3mm lower offset. The other 4mm is from the change in control arm angle at the lower ride height.

The CSL king pins either add 1 degree static negative camber without increasing SAI by 1 degree like you get using camber plates to make the same adjustment, or they decrease SAI while keeping static camber the same. Just depends how you align your car. Either way, the result is your car loses slightly less caster when you turn the wheels off center compared to a regular M3 with the same camber specs, and that means you gain a little less positive camber when the wheels are turned. I've heard it makes turn in feel a little better, but I cannot confirm.

I'll add more about the king pins and the effects on suspension geometry to the original posts when I get a chance. Promised some guys on FB I would do that too.

Good to see this thread back. I was the one who measured the Z4MC racks on the old thread, I measured a reman one direct from BMW and one that came factory from a car. Both were 50mm\rev linear. I also measured a variety of z3 racks, 1.9 and 2.5s. Again all 50mm\rev linear. I think it's safe to say that until proof of the contrary 1.9l z3s don't have a quicker ratio (read a couple times that some are actually slower, but I've never seen one myself).

Interestingly I also measured a rack that supposedly came from a prototype Z4MC in germany, but it had a Z4M roadster original part#. It also measured 50mm\rev linear.

Sorry can't remember all the part#s off the top of my head. I'll try to update later with them.

I'll update the Z3 rack. I'm not really surprised all the Z3 racks are the same. IIRC, the last time I tried looking, realOEM listed the same steering rack part number for both 1.9L and 2.5L.

I do find it funny that pretty much all of the rack upgrades ended up being 50mm/rev and linear. and in fact to date there's nothing confirmed to be faster. You'd always hear about people calling the 330 rack a modest upgrade which had a progressive feel while others would say the Z3 rack is so twitchy that it's hard to drive on the highway. Shows how much of our perception is susceptible to suggestion.

The nice thing is all the 50mm/rev racks should play well with MK60 DSC set to ZCP steering.

[terra]

The nice thing is all the 50mm/rev racks should play well with MK60 DSC set to ZCP steering.

Yep, makes things a lot easier

[Johnvu]

_{Just got the shims installed! I should have the car up and running this week! Can't wait!}