Digressive vs Linear vs Progressive Piston & Valving Article (and video)

There are a lot of choices when it comes to what type of damping you want in your shocks so we put together an article which explains the different types, their benefits, and how they work.


Article: The Pro's and Con's of Digressive, Linear, & Progressive valving, and how each one works.

When it comes to Tacoma shocks Fox and King have linear pistons with fairly progressive damping curves while Icon and Bilstein use digressive pistons and digressive damping curves.

This article is fairly generic and I'm happy to answer any specific questions you may have.

 

I just added a review of the Icon 2.0 shocks with Digressive Rebound.

See how the shocks work and how they've managed the digressive rebound.

 

Interesting, I learned something new.

 

I'm sure you have forgotten more about shocks and shock tuning than I will ever know, but you state on your link that, " digressive valving will provide better handling than progressive valving due to the higher force at 2 in/sec."

It seems to me that digressive / progressive does not necessarily mean that one will be stiffer than the other. Progressive / digressive only describes the shape of the force as a function of velocity curve, F(v) . Progressive is concave up or F''(v) is positive, whereas for digressive F''(v) is negative, and linear would be F''(v)=0. I see no reason why a progressive shock could not have a Force curve that remained greater than a digressive force curve for the entire range of v. That's not to say that any manufacturers are doing this but I found the information a bit confusing.

 

I would have gotten a much better grade in Calc 1 if all the test questions were phrased to reference suspension components

 

Thanks for the feedback. I should clarify that the assumption was that they were being tuned for the same vehicle with similar use. So overall the net area under the curve would be similar, but digressive has more force at the low speed, and progressive has more force at high speed. Handling (cornering) happens at ~2 in/sec so the digressive shock will have better handling. Is that a better explanation?

 

2 things:

It's been a while since calculus so I might have screwed this up but for the integral of F(v) I get units of mass/time. What would that represent physically? That is a mass flow rate but doesn't really represent anything to me in this context.

Also, in order to have similiar areas, the progressive curve would need to be integrated over a longer interval; what would that mean?

 

You have hit the nail on the head here. It describes the shape of the curve and is not enough information to determine whether one shock is stiffer than another. It is safe to assume that digressive will be stiffer in the low speed range than progressive/linear, but you cannot assume that one will be stiffer than the other in high speed ranges.

 

We are only describing the shape of the curve, however the trade offs still exist and impact how the shock performs.

Tuning a digressive piston to match the high speed compression performance of a linear piston is very difficult. The way they're designed results in the forces blowing off, and you'd end up with so much low speed compression it would be unbearable. The piston is a good indicator of how the shock will feel, especially on compression.

 

Please elaborate on what you mean by "forces blowing off".

 

This is the simplest explanation i can think of but it gives a good overview of the systems. It is not a perfect explanation but if you arent familar with valving it should do.

Digressive curves rely on high flow pistons and preloaded shim stacks in order to get high low speed forces (preloaded stacks) but reasonable high speed forces. The pistons flow enough oil such that the shims dominate the valving profile and at high shaft speeds oil flow through the piston is not choked. This provides moderate force ramp up from about 20in/s and up.

Linear/progressive curves rely on low flow pistons and non-preloaded shims. This provides small low speed forces but large high speed forces. At high shaft speeds , >20in/s , oil flow through the piston is choked (low flow piston) which provides greater force ramp up than a high flow system.

 

We've got some good discussion here!

This is round two with more details on each piston design and how to work with each:

https://www.youtube.com/watch?v=E_XOW09Qqf4

 

It is my understanding that fox is linear and Icon is digressive, as mentioned above.
I am interested in Fox DSC coilovers up front, but am intrigued by Icons RXT and more specifically Omega bypass rear shocks.
The off-roading I do makes me lean towards linear valving, as I’m going faster over terrain. But still like the idea of being able to run a bypass shock.

Would it be a bad idea to mix the two from front to rear?

 

All bypass shocks are progressive due to the extreme amount of free flowing oil at slow speeds, so the piston design isn't the dictating factor.

I'd pair them up.

 

Hey Ryan. Great articles and videos on your site. Easy to understand and it demystified a lot.

I've seen a few questions mentioned on this site asking if it's possible to add a linear/progressive profile to ICON 2.5 coilovers. You stating it's difficult to valve a digressive piston to linear/progressive characteristics. Are there linear pistons out there compatible with Icon ID shock bodies and their corresponding shaft flanges?

 

It's not likely, tolerances on those items are between 0.001 and 0.003", so it's a very precise fit.

 

Thanks for these videos and explanations. It's been the most helpful thing I've seen with regards to picking suspensions yet, and I'll be ordering the Stage 5 kit from you as soon as I take delivery of my Taco in 2 weeks!

 

I too found the info on the Accutune Off-Road website extremely helpful. I spent probably 45 minutes on the phone with one of their reps (Andrew) and he really helped me figure out what would be best for my use. Pulled the trigger on a Fox 2.5 setup and can't wait for it to arrive!