Thread: Rough Ride ( Suspension Help )

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I talked to the tech he ask for all of that info I called him back to make sure the info was put in correct that's what he said I needed. When I put them it seemed odd I was having to crank it up so high. I called back told him what I had after the install he said it didn't matter keep cranking if I needed to.

Originally Posted by 39 Chevy

I talked to the tech he ask for all of that info I called him back to make sure the info was put in correct that's what he said I needed. When I put them it seemed odd I was having to crank it up so high. I called back told him what I had after the install he said it didn't matter keep cranking if I needed to.

Sounds like a typical response. If they put the right numbers into the spring rate calculator, it includes an approximate preload value that's usually not much more than an inch. If the spring is much less than 7 inches long at ride height, that would seem odd, but the most critical thing is the shock length.

The way it is now at ride height, spring is 5.25 long

Originally Posted by 39 Chevy

The way it is now at ride height, spring is 5.25 long

That's means it's a soft spring IF that's what it takes to get the shock up to 11.2-11.6 inches. Seems like there would be a good chance of coil-bind under compression.

edit:

Upon more thinking though, the shock is supposed to compress from it's fully extended length of 13 inches to at least 11.6 which is 1.4 inches. Since the spring has compressed by 2.75 inches, that means the preload is about 1.35, if the shock is at 11.6. That's quite a bit, but some of the softer spring suggestions for my car have just over 1 inch of preload.

Looking at the photos first and fourth pics-it seems that you have some issues with the left rear-upper link heim is almost all the way in and lower rear heim is screw ed way out, plus front one is way more screwed in. aren't the liks right and left hand threads????


Looking at your tire size/air pressure/ wheel size I can see where some of the ride harseness comes from .


Wher do you have the schock valve stings set???


By the way-nice clear pics that show the detail very nice and clear.

Here's an article that discusses spring rate and motion ratio. One of the downsides to the 8" spring length coil-overs is an increased spring rate to keep the shock from bottoming out, since the shock has so little travel. On my car, choosing an 8" spring instead of the 10" spring that I now have, increases the suggested spring rate from 350 to 450 (same as I have now). Since my car rides so harsh in the front, most likely, I'll be moving my upper shock mount up to get the proper A-arm angles, while keeping the longer springs.

Choosing the Ideal Spring Rates and Spring Tech

The threads are R & L hand should they be adjusted till they are about half & half? The valve setting is at the light as they will go. I had already figured the tire size was some of the problem. What should the A-arm angle be ?

Yes, most bars have right and left hand threads, so the bars can be adjusted without unbolting one of the ends. I've got triangulated 4-bars with rubber bushings that should yield a better ride, but only one end is adjustable, so it's a real pain to make any changes. I'm thinking of changing that too, but not to heim joints, maybe polyurethane bushings.

The A-arms are supposed to be approximately horizontal and parallel at ride height. When that's achieved, you've got the "ride height" that's built into the suspension. In my case, it's about an inch lower than the height that I had been using, since my shocks are too long. I set mine up by the shock length. The lower height will require some slow driving when turning the wheels and going over a ramped road into a parking lot. The tires can hit the wheel opening, easily. When going straight, there's lots of room for tire travel.

The tech guy at Ride Tech suggested that I start with shock adjustment at mid-point. I had always had mine set mine very light (knob screwed out most of the way). By that time, my car was partially torn down and out of service (until it's painted and reassembled).

From looking at the Spring Tech article I also read the sway bar article. From reading that it looks like that could be part of the problem also.

My A-arms are about 1" out of parallel setting high at the inside of the wheel.

If the A-arms are angled upward, at the ball joint end, that would mean that the car is sitting lower than intended. If the shock length is proper - in the 11.2-11.6 inch range, it means that the A-arms pivots were welded to the frame to work at a greater ride height and the shock length is wrong. Ideally, the shock length should be set to allow 50-60% of the travel in compression and the A-arms should be horizontal and parallel, all at the same time.

At least with my car, sway bars are not an issue because I have none. I detected very little body roll during my 1200 miles of test driving last year, but I have thought of trying to adapt a Heidt's sway bar to my front suspension, while I've got it torn apart.

In the pics of both rear springs it appears that the top mounting bolt isn't centered in the shock upper end and that the springs are not centered around the shock body-is this because the springs have a larger id than the mounting components are designed for???( wrong spring ID )

Hi, I understand this is an older thread, but I wanted to drop in and give my two cents since someone referenced one of my articles above (sways and the spring article).

My Recommendations

The front springs are under-rate for what I perceive to be the motion ratio. (~78%)

Be aware that increasing the front rate (and ride height) will cause additional static and dynamic weight on the front. Adjust accordingly.

Running a fixed height shock for this application is a really mediocre solution. Adjusting the ride height with an adjustable base mount would give you full travel without having to do janky stuff to get the geometry right. This would let you reduce the spring rates considerably.

Tie a small piece of string to the base of the shock shaft, drive around, and then see where the string is now located. If it's at the top, you are beating the shock to death. Increase dampening considerably.

The golden ticket is adjusting the front and rear harmonics to be near each other. Milliken & Milliken found a 20% lower frequency in the front worked well.

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Shock travel is not always limited by the stroke length of the tube itself. There is barely 5" of compression travel in a 8" long spring, nor is there enough compressive force at that motion ratio to even remotely achieve that without trying your hand at the Duke's of Hazard driving school. You have to consider a few things:

1. Spring Rate & Spring Compression Travel
2. Shock Compression & Rebound Dampening
3. Shock Gas Pressure & Design
4. Force Input Speed & Duration
5. Unsprung Mass
6. Tire Dynamics

Tires

When you hit a bump in the road the tire is compressed. The amount the tire is compressed is due to the resistance (damper / spring) and the inertia (corner weight / unsprung mass) acting to deform the tire. How rigid the sidewall is and the load bearing capacity of the tire also have an effect, a rather large one. A taller sidewall, larger load rating, non-reinforced sidewall (or lower plys), and lower pressure all allow for greater tire deformation, which acts to dampen the blow. A stiffer tire will deliver more force and with a sharper duration ("impact" loading).

The force accelerates the unsprung mass (wheels, tires, brakes, etc.) upwards. There are a couple of very vital components at this point, the speed of the unsprung mass and the weight of the unsprung mass. That's because velocity x mass = inertia.

Suspension

So up comes the wheel into the wheel well. The second the force starts to move the wheel the suspension is acting to counter the force. This counter force comes from several sources:

1. Shock Compression Dampening
2. Heim Joint Bearing Friction
3. Spring Force
4. Sway Bar Spring Force
5. Suspension Geometry

Initially the upward movement creates a very fast piston travel speed within the damper. This causes the compression dampening to be considerably higher (stiffer) which creates a large resistive force to the unsprung mass traveling upwards. This has a second effect of low shock compression and delivering the load to the chassis. This causes the front to pitch upwards, and ultimately downwards. It is this upward / downward sine curve that the body perceived as the harshness. (It should be noted that rear frequencies are perceived as harsher than frontal frequencies).

Concurrently the coil spring is compressing from the same movement. The linear Hyperco spring above will produce, say 350 lbs. / in. of travel. Meaning, the first inch absorbs 350 lbs of force, and two inches absorbs 700 lbs of force. This force is transmitted to the chassis, but also acts to accelerate the unsprung mass on it's downward rebound travel with the same force, minus losses. Hence why we run shocks. During the rebound moment the piston travels back through the fluid which causes different flows through the valve stack, creating rebound dampening. In this case, the high speed rebound dampening region, which is a high amount of force.

This force travels back and forth several times until the shock (and other losses) overcome the force. This creates a decaying sine wave of body movement (vibration). Both overdampening and underdampening can cause excessive body movement which will be perceived as harshness.

In NVH studies, harshness is measured in Hz and kept to a specific range. Some people may experience discomfort at different thresholds depending on their relative tolerance to it. This is why I don't care much about running 600# springs on my car while my dad enjoys spring rates in the 300# range.

It should be of note here that a "5-inch travel shock" is a very limiting descriptor. If Shock A has very low force dampening, it will cover that 5" very quickly. If Shock B has very high force dampening, then it may not ever use it's 5" of travel.

Furthermore, a 8.00" coil spring with 6 - 0.500" coils only has 5.00" of travel before it is completely closed. At that point, any additional compressive load is transferred to the mountings as an impact and the bending stresses on the suspension immediately ramp up. It should be noted that packers, bump stops, and spring solid height should ALWAYS be used to limit suspension travel to avoid damage to the shock and/or control arms.

Sway bars only really add forces when one side moves in a different direction. So if you hit a pothole, for example, on the left side, it will add spring rate to that corner. The amount of rate depends on the sway bar setup (thickness, material, lever arm length, etc.) It should be noted that offset hits to both axle wheels creates a lot of torsional / weird movements that are perceived as very rough (because they are annoying). A sway bar makes this even worse because rate is quickly shared.

Lastly, you need to heavily consider the angle of the shock as it has an effect on the motion ratio. The further away from 90 degs. it gets the higher the necessary rate.

Suspension geometry affects shock piston speeds and effective spring rates, as well as other dynamic concepts.

Originally Posted by Rennmeister

Hi, I understand this is an older thread, but I wanted to drop in and give my two cents since someone referenced one of my articles above (sways and the spring article).........Suspension geometry affects shock piston speeds and effective spring rates, as well as other dynamic concepts.

Welcome to the forum, and thanks for taking time to post. I hope that you'll stick around and help us with suspension issues, as questions arise.

Ditto Fascinating read!