Bilsteins for stock suspension - HD or digressive?

[Arne2]

I've been reading lots of threads on selecting shocks, and many talk about benefits to having stock Bilsteins re-valved digressively. But when I read for details, it seems that most people having this done have upgraded their suspension, especially with stiffer torsion bars.

I'm preparing to refresh the suspension on my '84 coupe. It came with Bilsteins (green) from the factory. They now have 165k miles on them, and I figure I'll want to refresh them when I do the bushings and such.

I'm not a hard driver. My car will probably never see any track time while I own it, I use it as a Grand Touring car. I definitely won't be upgrading my TBs, nor will I be using anything other than normal rubber bushings (Elephant OE-hardness). I don't find the ride on my car to be bad now.

So given that I'm looking at keeping the rest of the suspension totally stock, is there really going to much difference between simply replacing my Bilsteins with new HDs vs. having Elephant re-valve them digressively?

[Bill Verburg]

Originally Posted by Arne2

I've been reading lots of threads on selecting shocks, and many talk about benefits to having stock Bilsteins re-valved digressively. But when I read for details, it seems that most people having this done have upgraded their suspension, especially with stiffer torsion bars.

I'm preparing to refresh the suspension on my '84 coupe. It came with Bilsteins (green) from the factory. They now have 165k miles on them, and I figure I'll want to refresh them when I do the bushings and such.

I'm not a hard driver. My car will probably never see any track time while I own it, I use it as a Grand Touring car. I definitely won't be upgrading my TBs, nor will I be using anything other than normal rubber bushings (Elephant OE-hardness). I don't find the ride on my car to be bad now.

So given that I'm looking at keeping the rest of the suspension totally stock, is there really going to much difference between simply replacing my Bilsteins with new HDs vs. having Elephant re-valve them digressively?

Here's a general view of the difference between stock hd linear valving and digressive valving which is most common in modern racing shocks
The linear will be fine for street use but what happens is as shock speeds build shock control forces also build, you get less control at low speeds and too much at high speeds, this is fine for street use but on track on smooth pavement and in most corners the shock speeds are relatively low and you want more control but you also want the shock to handle turtles etc at speed w/o upsetting the car so the tapering of control at high speeds is a big plus, this is where the digressive setup shines

[Arne2]

Thanks, Bill. That is the best description I've seen of the difference, even after reading other posts of yours.

So it sounds like the benefits to me in my car (stock suspension, street/touring use) will be small. The decision for me then boils down to whether a rather small improvement in ride is worth having the car down for several weeks. I drive it regularly year-round, so that is not a easy call.

[Elombard]

from my limited experience - and not with porsches, the ride will be noticeably different on the street and in my opinion its much better.

You can run a much stiffer spring rate with zero (again in my opinion) degradation in ride quality. My understanding is it costs more or every shock manufacturer would do it.

That said it has not been important enough for me to pull my fairly new bilstiens off and have them re-valved by chuck or similar.

[Bill Verburg]

Originally Posted by Elombard

from my limited experience - and not with porsches, the ride will be noticeably different on the street and in my opinion its much better.

You can run a much stiffer spring rate with zero (again in my opinion) degradation in ride quality. My understanding is it costs more or every shock manufacturer would do it.

That said it has not been important enough for me to pull my fairly new bilstiens off and have them re-valved by chuck or similar.

Yes, generally you run stiffer springs w/ a digressive setup and it does work great on the streets as long as the pavement is smooth or you are going fast, even on poor pavement, the ride will deteriorate on poor pavement at slow speeds due to overdamping

[Bill Verburg]

one more thing
Many like Bilstein hd front and sport rear, that's what I use(w/ stock linear valving) on my street car, custom valved digressives on the track car, the reason is that the rear sport is a closer approximation to the digressive curves at most speeds and on most pavement seen in street use.

[javadog]

Given that, on the street, most ride discomfort from shocks is due to harsh impacts from expansion joints, pot holes, etc., wouldn't digressive valving give a better ride? Or is the valving too firm, overall?

JR

[tirwin]

I probably don't have a good "before" comparison but I absolutely love my rear setup now. I went to stiffer torsion bars and digressive curve on the Bilsteins and polybronze bushings. It very well could be that the old stuff was just worn out so I can't say how much can be attributed to the digressive curve alone. However, the work has completely transformed the way the car feels. "Feel" is subjective but all I can say is - wow! Hitting potholes and crossing railroad tracks was a very jarring experience before. I'd almost wince sometimes. But now I hardly notice them. I can't wait to see the final results on a track or AX course once I complete the front end job.

[tirwin]

Below is an example of the curve plot that Chuck provides on a digressive curve setup. This was done to match my torsion bars (28's).

EDIT: And here is a good article that explains how to read the shock dyno graph.
http://www.nttyres.com/downloads/guide_to_dyno_graphs.pdf

[Driven97]

What's missing from Bill V's graphs is what the axes are.

The y-axis is shock force. On top is compression (bump), on the bottom is extension (rebound.)
The x-axis is shaft velocity.

The left side of the charts is low shaft velocity. This means transitional stuff. Getting on the gas, getting on the brakes, turning the wheel. Weight shifting in the car. Dive, squat, and roll. Note shocks don't control the total amount of these things, they only control the rate at which these occur.

The middle of the chart is small or "soft" bumps - pavement cracks, expansion joints, or maybe track curbing.

At the right, high shaft speeds, is larger or "sharp" bumps. Poor pavement, large cracks, frost heaves, potholes, etc.

Digressive valved shocks build a lot of force in the low shaft speeds. Since they're "firm" in these conditions, acting as if the car had bigger springs. As stated above, this gives better body control in maneuvers. They then have valves that "blow open" at higher shaft speeds, which lowers the slope of the line / decreases the gain. This can let the shock be actually softer in large bumps than a linear shock, controlling the spring but without overdamping like an aggressive linear shock curve. Best of both worlds.

The big caveat is best of both worlds if they're done right. Getting valving close on a linear shock is pretty much by the book. If you know a spring rate & corner weight, you can pick 60% of critical damping piston out of a chart, and end up with a pretty good driving and handling car. Setting up a digressive shock is more of a black art. Picking the right shim stack that gets the right initial forces, puts the "knee" in the right place, even setting up the digressive valve shims can greatly affect the performance of the shock, the performance of the car, and the ride.

tl;dr a linear shock is fine for most people and driving conditions. A high quality and well tuned digressive shock will be superior in both control and comfort - but the hard part is getting there.

[tirwin]

Maybe this helps...

And again this is a DIGRESSIVE, not linear, graph.

[Arne2]

Originally Posted by Bill Verburg

...the ride will deteriorate on poor pavement at slow speeds due to overdamping

So paraphrase what I'm understanding all this, with stock torsion bars for street use at semi-legal speeds and normal roads (which are not as smooth as a track), there is no benefit to digressive, and digressive may in fact cause more ride harshness due to overdamping. True?

[Driven97]

Originally Posted by Arne2

So paraphrase what I'm understanding all this, with stock torsion bars for street use at semi-legal speeds and normal roads (which are not as smooth as a track), there is no benefit to digressive, and digressive may in fact cause more ride harshness due to overdamping. True?

Unfortunately, the answer is "it depends."

Even the best picked linear valving is likely underdamped at low shaft speeds and overdamped at high shaft speeds compared to modern valving.

Digressive valving can be vastly better, but you need someone that knows what they are doing to get it right. More variables = more chances to screw up.

As a double unfortunate, there is no magic bullet. What's "best" for one person is certainly not best for another. Between the car, the road, the tires, the intended use, the setup - too much to say "this is perfect for all cars."

The silver lining is that for spirited street use, something "in the ballpark" will likely be more than sufficient. You don't need $4000+ worth of JRZ shocks, even though with time you could develop them to ride better than just about anything. It's diminishing returns. Just pick something better than Monroes and don't fret about it.

[Arne2]

Originally Posted by Driven97

Unfortunately, the answer is "it depends."

Even the best picked linear valving is likely underdamped at low shaft speeds and overdamped at high shaft speeds compared to modern valving.

Digressive valving can be vastly better, but you need someone that knows what they are doing to get it right. More variables = more chances to screw up.

As a double unfortunate, there is no magic bullet. What's "best" for one person is certainly not best for another. Between the car, the road, the tires, the intended use, the setup - too much to say "this is perfect for all cars."

The silver lining is that for spirited street use, something "in the ballpark" will likely be more than sufficient. You don't need $4000+ worth of JRZ shocks, even though with time you could develop them to ride better than just about anything. It's diminishing returns. Just pick something better than Monroes and don't fret about it.

Yup, I got that. Everything is a compromise. But since I'm not unhappy with the current ride and handling on the factory Bilstein HDs and bushings with 165k miles, replacing with a fresh set will probably yield acceptable results.

As a side benefit for my situation, new Bilsteins will have a lifetime warranty, while rebuilt/re-valved ones are probably 90 days, maybe a year?

[javadog]

Originally Posted by Driven97

Digressive valving can be vastly better, but you need someone that knows what they are doing to get it right. More variables = more chances to screw up.

So, does Bilstein even offer this service in the US, or do you have to turn to a vendor?

Who's "the guy"?

Chuck at Elephant? Someone else?

JR

[Driven97]

Originally Posted by javadog

So, does Bilstein even offer this service in the US, or do you have to turn to a vendor?

Who's "the guy"?

Chuck at Elephant? Someone else?

JR

Chuck @ Elephant can do it in house, or send them to Bilstein USA in California.

I guess if you have the right tools and know how you can do the shocks in your garage, but the struts require a pretty specialized tool to disassemble / assemble / regas.

[tirwin]

I wish there was a way to overlay the linear curve on top of the graph I posted above. I think it would help to see exactly where the differences are. I could hazard a guess at what that might look like but it would be a guess.

Apples to apples comparisons are going to be hard to come by in terms of real world feel. You would have to find two cars that were very similarly set up with the lone exception being the digressive curving and put them through the paces under the same circumstances. Not very likely. Too many variables to realistically account for.

I already had the Bilsteins and they were in need of replacement. I don't remember the numbers now but I think the take-away is that if you're at the point of needing to replace your shocks, there is minimal cost difference (maybe even no difference?) in doing a re-valve vs buying new. And if you're going to re-valve why not go ahead and do the re-valve on the digressive curve? (assuming it is done properly of course) The number I have in my head is the price difference was about $50 each but don't hold me to that.

[Driven97]

Chuck charges $275/pair for either shocks or struts for a revalve to his specs. ($550 all 4)

Bilstein charges $127.50 each for shock, $160 ea for strut revalve & rebuild. ($575)
Rebuild only is $95/$127.50 ($445) - a good inexpensive option if you just want to restore your car to original.

I chose Elephant. My OE 130k mile Bilsteins needed new shafts, which Chuck installed for the same price Bilstein charges. ($65/ea iirc?)

[Bill Verburg]

I agree w/ driven, there is no one best solution, it depends on the knowledge and skill of the re-builder in matching the needs of the end user.

The best modern race shocks are going to be digressive but they will also have high/med and low adjustments that are separate from each other and rebound /compression settings that are separate from each other. Or they will have a tractor trailer full of shock parts and a dyno to dial in what they need ala NASCAR.

You won't be unhappy w/ Billstein hd all around, I like hd/sport a little better and would probably like a digressive revalve on the street car even more.

The thing that amazes me is that my 993 w/ full RSR race suspension and digressive revalve on the Bilstein Cup shocks rides every bit as well as the street car w/ bone stock suspension and Bilstein hd/sport and handles worlds better to boot.

Steve Weiner mentioned a while back the Bilstein San Diego is no longer doing 911 revalves only 964 and newer motor sport shocks, I don't know the truth of that as it doesn't make sense to me. I do know that the oe Bilstein 911 shocks are not supposed to be rebuildable.

[emcon5]

Originally Posted by Bill Verburg

Steve Weiner mentioned a while back the Bilstein San Diego is no longer doing 911 revalves only 964 and newer motor sport shocks, I don't know the truth of that as it doesn't make sense to me. I do know that the oe Bilstein 911 shocks are not supposed to be rebuildable.

Huh, I thought all Blisteins were rebuildable. I had them redo some sport shocks for a 1983 GTI a few years ago, I would think if they could rebuild those, they could rebuild the ones from a 911.

The HD/Sport is a good combo, it tunes out some of the built-in understeer nicely.