Any thoughts on why some like to use very little nitrogen, and companies like AST use 175psi+ on teh nitrogen charge?

What I mean by old school is that none of shock guys that I've worked with tune using compression any more. I've worked with two NASCAR championship teams and we always would use a rebound adjuster, never compression. Compression tuning is more to "band-aid" an issue than to fine tune handling.

Back in the early 90's it was real common to tune the suspension with compression, but as car set-ups starting leaning towards softer springs rebound became increasingly more important. As I posted earlier, I want the least amount of compression needed to control the unsprung weight. Why would I need to tune this if it doesn't change?

So what "changes" to warrant adjusting rebound? I was a little let down that my AST's only had rebounding adjustment, but after running with them I appreciate the simplicity and the results from "one click" that made a real difference. My buddy has 3-way adjustables going in on his car and I'm happy to avoid that impeding doom...however he will probably get some help from AST to get them up and running.

A lot of shock companies build their shocks for worst case scenarios. They also dyno their shocks at ridiculous speeds. Some companies using speeds as high as 20 ips. At these speeds you need nitrogen pressure that high to control cavitation. But in the real world, we know these cars will never see 20 ips unless you drive off the road, into a field, just plowed, at 100 mph. So, in my opinion, it is unnecessary.

This question also ties into your previous one concerning compression tuning. A lot of the standards that shock companies go off of today were set in the early days of tuning. This was the era of higher compression valvings where the higher nitrogen benefited the shock. The reason some still do this today is that, for some, change is bad.

If you only have one adjustment, you will have a bigger window of adjustment with rebound.

Until you get into shocks with high-speed adjusters, the adjustment that you are using controls an adjustable orfice. Some use a needle style valve while others use a slide valve to meter flow, but the results are the same. You are metering the amount of oil that can flow past the piston and not the shims.

Since most shocks have more rebound than compression, the metered oil has a greater affect on rebound giving you a bigger range of adjustment per "click" of adjustment.

Bernie, in a serious autocross car, one that is a little "lazy" in transition like mine, would you suggest an increase in compression damping, an increase in spring rate, or both to match?

I would do an increase in spring rate and lower the ride height.

Thanks for answering all these questions, I'm fascinated by suspension.

There is the opinion that too much rebound will cause jacking down, and if I understand correctly, it is preferable to maintain a ratio of compression to rebound that isn't too excessive in rebound. What's your take on this?

Gotta say that this thread is very much appreciated.

You are correct. Too much rebound can cause the suspension to jack down. However, I would not put a relation between the compression and rebound.

Compression and rebound are independent. I've never been in a situation where I had too much rebound and needed to add compression to solve the problem. If you have too much rebound to where the car is jacking down, the only two things can fix that are a stiffer spring, or less rebound.

too much rebound or rebound damping? What does "jacking down" feel like?

Thanks for starting this, I at least am learning a lot.

I have two questions:
1) My current double adjustable are in about the middle of their compression range but seem to be happiest with the rebound maxed out or almost there. Would it be advisable to have them revalved so that the rebound adjustment is "best" near the middle setting to allow more adjustment for varying surfaces and conditions? My car is used exclusively for autocross.

2) Do you have a simple guideline for adjusting rebound? Something like (just and example and I hope it makes sense)
Weight skipping front to rear - add more rear rebound
Fronts skipping in constant radius thanks in advance
--Chris

Thank you for the thoughts, you pretty much nailed the root of my question. Cheers.

That'd be rebound damping.

it feels like all bumpstop all the time

This jives with what I was told by a multi-championship winning tuner in regards to increasing shock compression.

What do you feel are the best springrates for NA Bilstein HD's? Is getting a set of revalves going to dramatically improve on what Bilstein offers out of the box?

I'm curious as to why there is no dual rate rebound adjustment on automotive suspension. Beginning and endind stroke. A slower rate for body roll and a higher rate for preventing jackdown. Bicycles have dual rate rebound and the effective adjustment range is huge.

Its obvious that a track specific car would just need to be properly set up with springrate and rebound adjustment, but for a daily it would be an adjustment I would appreciate. Either way, how could having two adjustments be bad?

The only thing I can see being an issue (and a common one) is that people generally have no idea how to setup their suspension, and thus more adjustments would generally feel worse.

Even if this is the case though, just make both adjustments the same and it will still feel like there is only one rebound adjustment.

There is dual rate rebound adjustment for automotive suspension, but it is mainly on the high end shocks such as Penske. Here is a link to one: http://www.penskeshocks.com/files/60...H%20MANUAL.pdf

As you can see, it is very complicated and expensive. It is much harder to do a high speed rebound adjustment than a high speed compression.

The rebound adjuster that is readily available from most brands does adjust the low speed and high speed, but not independent from each other.

FWIW I am of this opinion.

Big rebound to bump damping ratio is good for top heavy cars on soft springs with lots of bump travel.

I was of the same mind as you, Jason, but I'm beginning to see that upward travel and control of the wheel is it's own independent problem.

You want to optimize this parameter just like you want to optimize downward motion. Given a spring rate, the unsprung weight of the suspension system, and the road properties you'll arrive at a compression damping that controls the bump of the wheel as best suits your goals. Travel in the opposite direction is an independent problem.

There might be a range of compression to rebound damping ratios that work because they do share variables (spring rate, vehicle weight, unsprung weight), but they're not so connected that you'd design one based on the other. Properly designed you might find the ratios are similar between similar cars, but the ratios more valuable as a sanity check than a design tool.

For example, on an extremely smooth road you might want little compression damping and little rebound damping; the ratio might be 1:1. On a very rough road you'll need more compression damping and more rebound damping, but the ratios may begin to vary from 1:1 to 1:1.5 or 1:2. I don't know, I'm just trying to reason this through. Thoughts?