Digressive or progressive shocks

Digressive or Progressive shocks.

  • Digressive

    Votes: 0 0.0%

  • Total voters
    6

cr500taco

Adventurer
Which do you guys use? I have the Bilstein 5100/5125 front/rear and just got the Bilstein 6112. Both are digressive. But, got to thinking about progressive shocks and wondered which one people prefer and why.
 

Buliwyf

Viking with a Hammer
Linear. Sorry. King and Fox 2.5 are nearly linear. You can add shim preload to make them more digressive

Digressive shocks are a bandaid for lack of suspension and shock travel. So budget shocks like Bilstein 4600 and 5100 are perfect for such. This is where the ''firm ride'' complaints stem from. But without more shock travel or much larger more expensive shocks.......what can you do? The same Bilstein shock design is perfect for Mustangs and sport tuned Lexus's, for example.

Progressive works when you have plenty of travel with expensive suspension kits or race trucks. And hopefully bypass shocks. Tuning a 2.5 to be a bit more progressive with a flutter stack is tedious, but possible.

 
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cr500taco

Adventurer
So, I guess the Bilstein shocks are perfect for me. My travel is limited and they are what I can afford. Actually, that's the reason why I bought them instead of Icons.

I thought King and Fox are progressive?
 

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Shovel

Explorer
I think a lot of it is subjective outsidebthe context of racing. With a nearly stock street legal passenger vehicle a digressive shock gives you tight handling on curvy highways and some relief on surprise bumps, but then it's a really good idea to have Timbrens or some other kind of improved bump stops to help soften the inevitable slam. Linear is better when you have a performance suspension.
 

Metcalf

Expedition Leader
I think a lot of it is subjective outsidebthe context of racing. With a nearly stock street legal passenger vehicle a digressive shock gives you tight handling on curvy highways and some relief on surprise bumps, but then it's a really good idea to have Timbrens or some other kind of improved bump stops to help soften the inevitable slam. Linear is better when you have a performance suspension.
The bumpstop typically adds spring rate which has to be dealt with in the rebound valving. It is a balance.
 

Metcalf

Expedition Leader
So, I guess the Bilstein shocks are perfect for me. My travel is limited and they are what I can afford. Actually, that's the reason why I bought them instead of Icons.

I thought King and Fox are progressive?
It think King and Fox is more linear than anything. That is just the nature of most non-position sensitive shocks that are not being tuned to be 'firm' at slow speeds like the digressive pistons in Bilstein and some Icon stuff.
 

b dkw1

Observer
King and Fox 2.5 are nearly linear. You can add shim preload to make them more digressive
To make them digressive you would need a dished piston like Bilstien uses to pre-load the shim pack. There is no extra pre-load to be had on Fox and Kings, the nut is tight. You can plug bleed holes for more slow speed damping.

Digressive is for road cars, it will make the ripples in fire roads chatter you right off the road.
 

Buliwyf

Viking with a Hammer
You can dish the piston and use a preload spacer. I wouldn't, but there's all kinds of weird stuff you can do, if you have a big enough bucket of shock oil.
 

1stDeuce

Explorer
I have Bilstein 5100's on my Jeep and 4600's on my truck. The feel like they are valved way to stiff for washboard or potholed roads. The Jeep actually drives pretty well without the front sway bar, which says to me that the low speed damping is way too high... They do work well for really bombing through stuff though. On my truck, washboard and expansion joint hits are pretty stiff when empty, but with the camper in the back, the ride is fairly well controlled.

I would have preferred a more linear or even progressive valving for Jeep, and linear valving for the truck, but there's no good place to get good valving without spending a lot of money, and often sacrificing travel. I found that Fox shocks have ~1" less droop travel than comparable Bilstein shocks for a given application. I need all the travel I can get with my more or less stock suspension...

Somewhere on here there's a thread about trying to find shocks for my Jeep. Turns out they're all too stiff unless you have 1000lbs of offroady crap bolted onto your Jeep... I think the trend to make shock damping so stiff that it's noticeable when you replaced perfectly good shocks is ruining the ride on many otherwise good vehicles. :(

Additionally, one of my employers does a good bit of shock sales. The Bilstein shocks seem to hold up. The Fox shocks almost always blow out the seals fairly quickly, even on mildly driven trucks and Jeeps. While I like the idea of spending my money with Fox, I can't justify it with their terribly track record and limited travel.
 

luthj

Engineer In Residence
What Fox units are you using? There are several series with different costs and strengths. Depending on the shaft material, some fox units need a boot to protect the shaft, otherwise rock chips kill the seals quick.

Most any good shock tuner can tune a fox/king/icon shock for your application. They will need the weight, sprung/unsprung, usage, and spring rate.

With too much damping or too much speed, you will overheat the shock, and the seals will fail. The higher end units use higher strength/temp seals. Shocks with external reservoirs have better cooling, as do larger shocks. Running at too high of tire pressures on the washboard will increase the shocks heat load though.

If a vehicle has too stiff of springs, no amount of shock will solve the problem. Progressive shocks can be paired with progressive springs. They are useful for suspensions with somewhat limited travel (IFS for example) where a progressive style bumpstop (rubber spring) is used to prevent bottoming. This allows for maximum travel and wheel speed in the normal range, but prevents harsh bottoming or loss of control.

A whole other discussion is variable rate damping. Many shocks will have different damping rates depending on the shocks travel speed. Usually only available in compression for design reasons. This can provide excellent body roll control with soft springs, which still allowing for a smooth ride at speed.

The point is, that its not a simple matter or progressive, digressive, etc. The shock needs to be selected and tuned for the application. Both weight, spring rate, and road surface/speed. Becuase all of these change with your typical 4x4, you are not going to find a one-size-fits-all approach. Any decent shock tuner can walk you through the options, and tune for the application thankfully.
 

Metcalf

Expedition Leader
If a vehicle has too stiff of springs, no amount of shock will solve the problem. Progressive shocks can be paired with progressive springs. They are useful for suspensions with somewhat limited travel (IFS for example) where a progressive style bumpstop (rubber spring) is used to prevent bottoming. This allows for maximum travel and wheel speed in the normal range, but prevents harsh bottoming or loss of control.

A whole other discussion is variable rate damping. Many shocks will have different damping rates depending on the shocks travel speed. Usually only available in compression for design reasons. This can provide excellent body roll control with soft springs, which still allowing for a smooth ride at speed.

The point is, that its not a simple matter or progressive, digressive, etc. The shock needs to be selected and tuned for the application. Both weight, spring rate, and road surface/speed. Becuase all of these change with your typical 4x4, you are not going to find a one-size-fits-all approach. Any decent shock tuner can walk you through the options, and tune for the application thankfully.
Stiff springs are the root of all evil generally, especially when talking about off road stuff. I find that you only want enough spring rate to hold the vehicle up to the proper position. Having some preload at full droop is good. Zero, of negative, spring rate at full droop is bad. Roll control should be mitigated with suspension geometry and weight location when possible. If you can't make the vehicle happy with that, add a sway bar vs adding stiffer ( usually digressive ) valving.

Do you have any examples of a 'variable rate' damping shock?

Any 'normal' shock with a piston and valve stack is going to be 'speed' sensitive? The curve of that change is what we are talking about with things being digressive, linear, or progressive. It doesn't really matter if it is compression or rebound. Now, we can get into position sensitive valving which is basically the next level of 'progressive' valving available in shocks. This is typically done with an internal or external bypass system. A few places like Icon are doing some 'bump zone' progressive valving where all the oil flow, once past a certain point in the body, has to go through the piston ( not ported to the reservoir via shaft displacement ).

One issue to watch for with 'progressive' springs and bumpstops is that all that extra stored energy that is created by increasing spring rate on compression has to be absorbed in the rebound valving in the shocks. If you don't, the extra bottoming resistance basically just creates 'kick' issues on the other side.
 

luthj

Engineer In Residence
This is a nice graph. A linear shock has the same damping rate of change for its operating area. Progressive/digressive shocks change the rate depending on the velocity. Several shock builders offer external adjustable compression rates. Some have a single adjuster for low speed, some have two adjusts for high and low speed. Which allows fine tuning the compression curve.

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As you mention with progressive springs/bumpstops, the reboind damping needs to be proportional. Especially at high velocities (and with high unsprung weights).
 

Metcalf

Expedition Leader
This is a nice graph. A linear shock has the same damping rate of change for its operating area. Progressive/digressive shocks change the rate depending on the velocity. Several shock builders offer external adjustable compression rates. Some have a single adjuster for low speed, some have two adjusts for high and low speed. Which allows fine tuning the compression curve.

As you mention with progressive springs/bumpstops, the reboind damping needs to be proportional. Especially at high velocities (and with high unsprung weights).
Right....basically all shocks are 'variable rate'. All shocks are speed sensitive, it is just the slope of the line that changes.....Digressive, Linear, or Progressive.
Even the Linear shock still changes damping rate vs speed though.
If the line was completely horizontal through all the shaft speeds....that would be a non-variable shock ( and that would be pretty bad )

Yes. You an add some external compression adjustment...some better or worse ( Fox DSC being one of the best right now). All those adjusters are only typically only working on the flow of oil from shaft displacement that is going to the reservoir. On a small shock with a big shaft this is a larger flow ( 2.0 with a 7/8 shaft as an example ), but as the shock diameter increases the total effect external adjusters can have on overall damping drops quickly.
 

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luthj

Engineer In Residence
This article does a good job breaking down the difference.

 

Buliwyf

Viking with a Hammer
I have Bilstein 5100's on my Jeep and 4600's on my truck. The feel like they are valved way to stiff for washboard or potholed roads. The Jeep actually drives pretty well without the front sway bar, which says to me that the low speed damping is way too high... They do work well for really bombing through stuff though. On my truck, washboard and expansion joint hits are pretty stiff when empty, but with the camper in the back, the ride is fairly well controlled.

I would have preferred a more linear or even progressive valving for Jeep, and linear valving for the truck, but there's no good place to get good valving without spending a lot of money, and often sacrificing travel. I found that Fox shocks have ~1" less droop travel than comparable Bilstein shocks for a given application. I need all the travel I can get with my more or less stock suspension...

Somewhere on here there's a thread about trying to find shocks for my Jeep. Turns out they're all too stiff unless you have 1000lbs of offroady crap bolted onto your Jeep... I think the trend to make shock damping so stiff that it's noticeable when you replaced perfectly good shocks is ruining the ride on many otherwise good vehicles. :(

Additionally, one of my employers does a good bit of shock sales. The Bilstein shocks seem to hold up. The Fox shocks almost always blow out the seals fairly quickly, even on mildly driven trucks and Jeeps. While I like the idea of spending my money with Fox, I can't justify it with their terribly track record and limited travel.
If your shocks are good and firm. Try some softer springs. Coils are cheap. Not sure if I'd bother with the trucks leaves.

Any in line shock is a huge risk of blowing out. Another thread has Bilsteins blowing out everywhere, and Fox's lasting longer.

An inline shock has a very small airspace for shock rod displacement. You might have 200psi in that shock, but when it compresses, you'll have as much as 800psi! Resi shocks have much larger air chambers, so less pressure build up.
 
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