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Old 11-26-2019, 06:58 AM   #1 (permalink)
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Coil over shock set up technique

I’ve been tuning my truck for about 8 years now and I’ve hit a road block that I’m sure many of you have also seen. I’m curious what some of you do to address this tuning hurdle.

This situation pertains to a single coil over shock (non bypass) and dual spring rates, focusing on rebound.

Following the standard spring rate setup technique, it’s simply not possible to get your rebound set correctly to handle both spring rates.

I.E. if you get it dialed for the initial rate, things get scary when the main spring come into play. Or, you compromise and your initial ride suffers substantially causing packing on all the small stuff.

It has made me realize that this is one of the biggest reasons for bypass shocks.

But, there has to be a better set up for single shocks. Less than a 200% step up? No step up?
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Old 11-26-2019, 08:12 AM   #2 (permalink)
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I leave this to the professionals... send your shocks into AccuTune Offroad and be done.
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Old 11-26-2019, 08:21 AM   #3 (permalink)
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I don't know much, but experience and research taught me that 200% step up on the spring rates is way too much. Mine felt like I was hitting the bumpstops. Dropping to a lower step up should help.

Everything is a compromise. I don't know if you can increase free bleed in the piston and hope it chokes off at higher speeds when your most likely using both spring rates? Flutter stack on rebound? Unfortunately springs are position driven and conventional shock valving is velocity driven.

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Old 11-26-2019, 08:55 AM   #4 (permalink)
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We need more information. At least spring rates,lengths,travel and position. My guess would be something is missing from the initial setup. But also like mentioned above Ryan at Accutune is the man.
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Old 11-26-2019, 10:25 AM   #5 (permalink)
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Ryan is the man for sure. Unfortunately he stopped tuning help over the phone, and I totally understand why.

I’m using Shim Restackor. The response calculations are much better than my butt dyno. I’ve tried everything from bleed, to flutter stacks, to rate plates but simply can not get a zeta function of 0.7-1 for both rates.

This is more of a question on how do you control two different spring rates with a single velocity sensitive damper.
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Old 11-26-2019, 03:44 PM   #6 (permalink)
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If you want to make it that simple, the answer is that you compromise.
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Old 11-26-2019, 04:36 PM   #7 (permalink)
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If you want to make it that simple, the answer is that you compromise.
There is this, and IMO try for the littlest change in rates you can get. The only way to make a big step up change work Is to use the top spring as a tender that collapses completely at ride height.
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Old 11-26-2019, 10:56 PM   #8 (permalink)
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If you want to make it that simple, the answer is that you compromise.
There is this, and IMO try for the littlest change in rates you can get. The only way to make a big step up change work Is to use the top spring as a tender that collapses completely at ride height.
Thank you. I kind of came to these conclusions but was unsure if others did as well. The reassurance really helps.

I’ll post some screen shots of what shim restackor shows
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Old 11-28-2019, 12:28 PM   #9 (permalink)
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What about eliminating the step up rate completely? Keep things consistent rate. What issue is the using the step up rate helping you with?
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Old 11-28-2019, 01:04 PM   #10 (permalink)
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What about eliminating the step up rate completely? Keep things consistent rate. What issue is the using the step up rate helping you with?
That would allow you to get the best rebound response through out the stroke with a non bypass coil over.

The common trend has been to use a step up rate to help control bottoming.... mainly at slow shaft speeds where dampening rates are not significant, such as G-outs (2-5 in/sec).
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Old 11-28-2019, 04:00 PM   #11 (permalink)
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That would allow you to get the best rebound response through out the stroke with a non bypass coil over.

The common trend has been to use a step up rate to help control bottoming.... mainly at slow shaft speeds where dampening rates are not significant, such as G-outs (2-5 in/sec).
My next question would be how much up-travel are you dealing with?
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Old 11-30-2019, 07:32 PM   #12 (permalink)
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About five inches. 200/300 lb rates. 1250lb sprung corner weight.
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Old 12-07-2019, 03:57 PM   #13 (permalink)
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About five inches. 200/300 lb rates. 1250lb sprung corner weight.
The 100# split might be getting close to blocking the coil if the transfer nut is set too far down?

Have you tried narrowing the split? That should make the step up rate change more manageable.
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Old 12-09-2019, 08:08 PM   #14 (permalink)
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The stop is currently 1” below the slider
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Old 12-10-2019, 09:14 AM   #15 (permalink)
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I would raise the secondary rings so you have more travel in the initial rate zone and tune for that, let your last 1.5"-2" take that heavier rate to keep you from bottoming out. You might also want to look at some 2" hydraulic bumps if bottoming out is an issue.

Whats your use for the vehicle?
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Old 12-11-2019, 08:05 PM   #16 (permalink)
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You obviously misunderstood the point of this post. This is Is what I’m talking about. The method you are suggesting does not work well for a single velocity sensitive coil over.

As a few people who are smarter than myself suggested dropping the step up rate, or setting the stop so drop travel is the only thing that sees the initial rate.

I’ll have to post some visual graphs. It might explain it better than I can. I’ll run the numbers, 200% step up might be able to work...

I just dug into a WP PDS shock on the rear of my KTM. Interestingly, the progressive rate hits its transition at rider sag or ride height. The soft rate is for drop only. Soft rates are low falling rates.
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Old 12-11-2019, 09:41 PM   #17 (permalink)
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I just dug into a WP PDS shock on the rear of my KTM. Interestingly, the progressive rate hits its transition at rider sag or ride height. The soft rate is for drop only. Soft rates are low falling rates.
One thing that's counter intuitive, but can help when thinking about spring how some cars react to spring rate changes.

For a given ride height, a softer spring rate will have more spring force between full extension and ride height than a higher spring rate. This can help the with tracking, forcing the wheels to stay in contact with the ground.

I haven't looked closely at a WP PDS shcok, does it have position sensitive damping in rebound, or just compression? Where does the spring rate transition compare to the damping force transition?
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Old 12-12-2019, 06:33 AM   #18 (permalink)
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You are correct for the most part. The total force at ride height will be the same no matter what the rate is. In example, if your corner weight is 1,000lbs, that’s how much spring force is needed to hold the vehicle at ride height. It does not mater what the rate or the preload is. It’s above and below ride height that changes with spring rate. Stiff springs = heigh rising and falling rates
Soft springs = low raising and falling rates

The PDS shock is a bypass shock but uses two pistons and a needle. It’s similar to the king internal bypass coil overs.

The secondary piston does not come into play until just above the bump stop. It’s not much more than a hydraulic bump stop. The spring rate transition is at ride height.

Looking at the PS6 spring, the adverted rate is 7.1-9 kg/mm (measuring closer to 7.75 - 10.5 kg/mm). That is only a 135% step up rate!

Last edited by weswes01; 12-12-2019 at 06:37 AM.
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Old Today, 12:13 PM   #19 (permalink)
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visual

I took two screen shots of the response spread sheet. This is a 200/300 lb spring with the stop set 1" above the slider. 12" travel, 5" bump. All I change was the spring rate and adjusted "ride height" for the main spring.

The graph on the bottom and second from far right (zeta vs wheel position) is a good one to look at.

A zeta function of 1 is considered "perfect" rebound dampening, chassis returns exactly to ride height.

A zeta function of .7 is a lighter dampening but will give you the best traction because it will keep the wheels on the ground more. The chassis will raise slightly above ride height when it returns from the bottom of the stroke.

The graph on the bottom left shows the what shaft speed it take to bottom out and what shaft speed you will see as it returns.

For the record, I am not claiming to know the answers. I just think the way our community has been setting up single coil over suspensions should be re evaluated. There are definitely improvements to be had.

Beat95yj pretty much nailed what I am seeing. I wonder how a extremely light combined rate would perform as the axles fall away from the vehicle?
The only other thing I am considering is flipping the springs to achieve a step up ratio of under 200%



[IMG]Screenshot%208_zpstrgkg61b.[/IMG]


Last edited by weswes01; Today at 01:08 PM.
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Old Today, 01:06 PM   #20 (permalink)
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OK someone help a brotha here. How do I post photos these days?
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