Talk to me about rack and pinion steering damping?

I've owned several BMWs over the last few years, and one of the things that keeps me coming back is the way the steering feels in the car.  I think that everyone likes a car with good steering feel, though in the last decade or so, EPS systems with zero steering feel in fast cars have proven that it doesn't really matter much to lap times whether you can feel the track surface or not.  And I think its undeniable that EPS systems free up more power and drag from the motor, making the vehicle faster and more efficient.  A good steering car is still a wonderful thing though.  I've found that on long road trips when I'm a bit fatigued, or in bad weather, a nicely steering car is a nice thing to have.  With that said, HPS vs EPS is not what I wanted to talk about so much in this thread, but I do think that its an example of an extreme.  

The BMW's I've owned were mostly E9x generation vehicles, and I also had an E83 X3.  The shop I used to take my cars to in Raleigh gave out high mileage, well maintained E39s and E46s, so I have a slightly embarrassing amount of seat time in a certain black E39 and grey E46.  Anyways, the E46/E39/E83 had different steering feel from the E90 and E91s that I owned, the main differences between them being in steering damping. 

The rack and pinions in a car seem to have a similar "spring/dashpot system" to a wheel suspension, with the dashpot moderating and damping the force of the "spring".  The spring force in a steering system appears to be composed of the tire flex as it relates to the KPI and caster angles.  The confusing part for me though, is that I don't really know what components constitute the "damper" of the steering system.  To my knowledge, there's no shim stack in a rack and pinion steering gear.   Damping seems to reduce the amount of backtalk from the wheels, and increase the drag on the wheels as the operator turns it.  In the AWD E91s I owned, I found that there was a significant amount of damping, while the E46's had the least.  Until I owned the E83, I thought that the damping was due to the AWD system somehow, but the E83's AWD system is very similar to the E91's, but the steering is better.  

So I guess the question is, what are the reasons for wanting to add steering damping, and what controls it?  On steering box cars, there's an actual damper attached to the linkage.  On EPS cars, the motor itself is the damper due to motor friction.  Is there an intentionally restricted orifice in a hydraulic steering rack that provides a damping effect?  Is it possible to "port" a steering rack to get better feel out if it?

Most steering racks do have a shimmed spring damper on the rack at the pinion. Also there are some rack cars (Lancia 037) that do have hydraulic dampers as well. If it trembles or shimmys hard adding a hydraulic damper where there was none is a possibility too. Note that I am not familiar with any BMW newer than E36.

Generally reducing the hydraulic pressure increases the "feel" at the expense of effort. This is easily accomplished with a pressure regulator, and can even be made easily adjustable, high pressure for long distance cruise, lower for track day.

Caster also plays into this a great deal.

Steering feel is a combination of effort, feedback and accuracy.  There are a ton of factors that affect each of those categories.  In the NA/NB Miata world the factory power rack has great feedback and accuracy and relatively low effort.  I've never driven a factory manual rack car but my understanding is that they have a higher effort than the power rack but that the feedeback is actually reduced.  Spec. Miata's often use a de-powered power rack which is heavier than ideal but still has the excellent feedback and precision.  The big advantage is that you're not using the limited power to run the pump.

I once reduced the pump pressure in my '68 Coronet in an attempt to do something about the factories Novocaine numb steering.  All that really did was make the car harder to parallel park.  Cranking in all the castor, running zero toe and replacing all the rubber with Delrin bushings did more to improve the feedback and precision.

In my Camaro it took getting rid of the subframe bushings, adding subframe braces that run to the firewall, a quicker ratio box, a lower pressure pump, new upper and lower control arms with better geometry and solid bushings to get good steering feel.

Adding a damper will increase steering effort and can help with un-controlled steering oscillations but it's likely to hurt feedback since you're adding effort to the steering that's completely unrelated to what the front tires are doing.

EPS doesn't have to mean poor steering feel. The Porsche GT3 is widely acknowledged as having excellent steering feel (it's the best of any production car I've ever driven) and had EPS.

The point of all of that is that you need to really determine what component of feel you're missing and try and target you changes to address that.  I guess the second point is that effort is probably not really the issue.

I'm not 100% sure what the OP is asking but I'll add a datapoint. I've had a bunch of older Benzes over the years: R107; W124; and W201. They all have recirculating ball steering boxes with an external hydraulic damper. I removed the damper on my W201 16v racecar and found slightly better steering feel without any noticable downside.

LanEvo said:

I'm not 100% sure what the OP is asking but I'll add a datapoint. I've had a bunch of older Benzes over the years: R107; W124; and W201. They all have recirculating ball steering boxes with an external hydraulic damper. I removed the damper on my W201 16v racecar and found slightly better steering feel without any noticable downside.

True. When I got my W201 the damper was worn out. When I replaced it with a new one, I was expecting better steering but I actually got worse feedback!

And I modified a W201 damper to replace the failed original on the 037. It had acquired an air bubble which amplified the shimmy. Common aircooled VW failure point. 

Building damping into a steering box is definitely possible.  See BMW.  My E38 definitely has some damping in the steering (more than the Jeep box does if I pull the damper on the Jeep), but it doesn't have an external damper of any kind. 

TurnerX19 said:

Most steering racks do have a shimmed spring damper on the rack at the pinion.

Interesting! I didn't know that.  I'll have to buy a junkyard rack and take it apart to see if I can find it on an E90 rack.  

Well dammit, I had a big post typed out and I lost it.   

LanEvo said:

I'm not 100% sure what the OP is asking but I'll add a datapoint. I've had a bunch of older Benzes over the years: R107; W124; and W201. They all have recirculating ball steering boxes with an external hydraulic damper. I removed the damper on my W201 16v racecar and found slightly better steering feel without any noticable downside.

I think I'm confusing everyone here by comparing the steering to the wheel suspension, so I'll try to do a better job.

On a wheel suspension, you've got two major components, the spring and the damper.  Yes, I know there's more to it than that, but bear with me.  The spring provides the force to stick the wheel to the ground, and the damper controls that force.  To simplify, the spring controls how hard the wheels are pressed into the ground, while the damper controls how fast the wheels move.  I know there are bushings and tires to consider, both of which contribute to spring force and damping, but we're trying to keep it simple here.  The combination of spring and damper work together to keep the wheel on the ground without disturbing the chassis.  

So lets apply that to a steering gear.  Your "spring" is the force that you're turning the wheel against, and these forces are developed by kingpin angle and caster.  The kingpin angle dictates the leverage rate at which the car is lifted as the steering angle is turned, while the caster angle dictates how much the tire is flexed as the wheel is turned.  At least for me, its fairly well understood how that "spring rate" is achieved.  However, its a bit less clear about the damping for me.    It seems like the goal of a steering system's spring/damper is the opposite of a wheel suspension, rather than isolating the chassis, you want to know what's going on at the other end of the system.  So why add damping? I've driven cars, like an xDrive E90, that didn't have strong self centering, but the wheel was still hard to turn, ie, it was heavily damped, but had a fairly low spring rate.

The point of this thread is to figure out: Why would someone add damping, and how do I get rid of it?  It seems like Mercedes cars from back in the day had dampers that could be removed for better steering feel, but I'm not aware of an equivalent for a steering rack and pinion.  

rothwem said:

The point of this thread is to figure out: Why would someone add damping, and how do I get rid of it?  It seems like Mercedes cars from back in the day had dampers that could be removed for better steering feel, but I'm not aware of an equivalent for a steering rack and pinion.  

Adding damping is done to control steering behavior.  Either to reduce excessive feedback (relative to the designer's intent) or to stabilize the steering if the design is prone to oscillation under some conditions. 

APEowner said:

Steering feel is a combination of effort, feedback and accuracy.  There are a ton of factors that affect each of those categories.  In the NA/NB Miata world the factory power rack has great feedback and accuracy and relatively low effort.  I've never driven a factory manual rack car but my understanding is that they have a higher effort than the power rack but that the feedeback is actually reduced.  Spec. Miata's often use a de-powered power rack which is heavier than ideal but still has the excellent feedback and precision.  The big advantage is that you're not using the limited power to run the pump.

I think that "effort"= spring rate, "feedback"=damping.  Accuracy isn't something I've really considered much since all of the BMW's I've owned have had fairly stiff bushings or direct connections in the steering system so slop isn't that much of an issue.  

I'm not totally sure how power assist plays into all of this.  I'm assuming that it effectively reduces the spring rate, but I'm not certain.  

Adding a damper will increase steering effort and can help with un-controlled steering oscillations but it's likely to hurt feedback since you're adding effort to the steering that's completely unrelated to what the front tires are doing.

Yep, last thing I want to do is ADD damping.  I think, at least.  Part of me is worried there's a reason for the factory damping besides "non-car enthusiasts are stupid".

EPS doesn't have to mean poor steering feel. The Porsche GT3 is widely acknowledged as having excellent steering feel (it's the best of any production car I've ever driven) and had EPS.

I don't fully disagree.  The motor itself causes a decent amount of drag (damping) I think though.  It makes sense that Porsche can afford to put a lower friction motor in their cars.  

It has been alluded to here but needs amplification: The reason external dampers are added is to eliminate oscillation (shimmy) that is inherent in certain  suspension designs. This is particularly present if the bump steer is bad and is always present with a non independent suspension. Also MacPherson struts are prone to it, especially with low caster settings. It can also mask wheel imbalance to a small degree.

TurnerX19 said:

It has been alluded to here but needs amplification: The reason external dampers are added is to eliminate oscillation (shimmy) that is inherent in certain  suspension designs. This is particularly present if the bump steer is bad and is always present with a non independent suspension. Also MacPherson struts are prone to it, especially with low caster settings. It can also mask wheel imbalance to a small degree.

Ah, that makes a lot of sense.  I didn't realize that certain suspension designs were inherent shimmiers, but it makes sense if you think about it.  

EPS has come a long way in a very short time.  The Power Steering in my Abarth is electric and it is nearly transparent.  Higher boosted when in regular mode and less boosted in "sport" (where the car lives 99.99% of the time) and I really love that I can leave the wheel cranked over when shutting the car off and it doesn't try to hydraulically break my arms when it loses pressure.

In reply to TurnerX19 :

I experienced a damper failure with my XJ. Dependent suspension with low caster. Scary stuff when I hit a pothole at 75.

Is the hydraulic rack damped by the hydraulic system itself? Any feedback from the front wheels goes to the rack which has to push hydraulic fluid, a bit like a damper.

In reply to buzzboy :

I like how you call your live axle "dependent suspension" I think the presence of positive pressure as soon as the spool valve opens is the opposite of damping. Some of the de-powered Spec Miata racks I have tried (with the looped lines) did have a light damper sensation. 

mad_machine (Forum Supporter) said:

EPS has come a long way in a very short time.  The Power Steering in my Abarth is electric and it is nearly transparent.  Higher boosted when in regular mode and less boosted in "sport" (where the car lives 99.99% of the time) and I really love that I can leave the wheel cranked over when shutting the car off and it doesn't try to hydraulically break my arms when it loses pressure.

I agree with the first part, though I think the second part of your post is confusing weight (spring rate) with feel (damping).  My wife's old 2005 Trailblazer had really light steering, but the feel was surprisingly good once you adjusted to the lightness, it was hilarious to drift in a snowy parking lot.  The Nissan Rogue we got after the Trailblazer actually had pretty nice steering weight, but it was completely numb with absolutely zero steering feel through the wheel.  It was terrifying to drive in the snow.  

Also, adjustable steering weight is something that can be done with hydraulic power steering--ZF makes/made a system called "servotronic" that is just an electronic pressure regulator that can vary the rack pressure with vehicle speed.  The E39 M5 and the E38's had it on their steering boxes, and it was an option on the AWD BMWs from the E53 X5 onward until they switched to electric power steering in the F-series BMWs.

But anyways, it appears that EPS can be tuned decently.  I didn't realize that the last generation Mazda3 had EPS, I've driven one and the steering feel wasn't bad at all.  We've got a 2017 MDX with EPS, and its not bad at all.  I think a lot of it does come down to damping though, and it makes sense that it would have to be added to the steering in some cases because of other compromises made in the suspension.