Long term suspension project

[ShatterPoints]

A few people expressed interest in my suspension project based on some initial info I have posted previously. I will document my project here for those who may like to follow along, or use it as inspiration for their own tastes.

As a note I do not want to bash any aftermarket product, or anyone's decision to choose what they choose. If you are part of stance nation, cool, if you like stiff rides, cool, if you disagree with me cool. I am not here to tell you how you need to do things.

With that out of the way I called to get info about the s550 chassis and the gt350 specifically:
Steeda, BMR, and Cortex racing all gave me lots of their time and I would encourage people to start with these vendors for good info and a good experience. The only negative experience I had was talking with vorshlag, I will leave it at that.

As far as springs which fit the stock location, only the steeda spring comes close with their 800/1280 dual rate. If I go coilover then swift 24k springs will be what I have to go with in the rear. I have asked the guys at vorshlag and they will sell a magnaride delete kit for the gt350. Its like $400 but ¯\_(ツ)_/¯

The above post was when I was looking at a spring change while maintaining the factory spring position. I was able to find a vendor (cortex racing) who sells aftermarket rear control arms which modify the mounting point for the shock, allowing us to fit conventional round end/ hole equipped shocks. They also sell magnaride delete kits.

Back with an update for the route I am going. I found a vendor who sells magneride delete kits as well as they will custom valve a set of shocks for me.

I will be relocating the rear spring for a true coilover, this will mean the spring and shock motion ratio will be the same. The front does not change since they are struts to begin with. The rear shock MR is 0.77, so I will be using 450lbs springs front and 750lbs springs rear. I can make a separate thread discussing the valving I will be targeting for damping if anyone is interested.

Motion ratios for struts are very close to 1:1 (typical is 0.98:1) Our cars have front struts. The spring is coiled over the strut and so its MR for the front is the same. This simplifies any sort of calculations needed to be performed. **To that end, I fully expect to have to pay to revise, or perform the revision myself with appropriate tooling as nailing something like this on the first try... well... doesn't happen.

The factory rear spring MR is 0.50:1
The factory rear shock MR is 0.70:1

With the cortex control arms the MR is increased to 0.77:1 - with coilovers the spring MR becomes the same.

I have spoken about flat ride in the past, when I first heard about it I thought it was BS. After lots of reading it turned out to be a valid configuration pioneered by GM. Lots of cars employ flatride today including the GT350 in its stock form that is why we run a stiffer spring in the rear, the very low rear spring MR of 0.50:1 allows us to run the 914lbs rear spring and not spin like a top. In short the idea is that as the car hits a bump there is a phase delay between the front and rear of the car. In order to address this delay the rear of the car must oscillate at a higher rate than the front. The tire, spring, shock, and linkage all contribute to the total ride rate for either end. This is expressed by undampened and dampened frequencies. So like a OEM, I chose to focus on a ride frequency to target front and rear. This number is ultimately meaningless. However it is the way to quantify the changes made to the car. (https://www.drtuned.com/tech-ramblings/2017/10/2/spring-rates-suspension-frequencies)

" For rough road operation spring rates are very important. It is necessary to take advantage of all of the suspension travel to keep the wheels on the ground as much as possible. If a flat ride (that is, the car lands flat after crossing a bump) is desired the spring rates must be adjusted to give slightly higher undamped natural frequency on the rear than on the front, i.e., approximately 10% stiffer on the rear. This often is not possible because of the high front spring rates required for front roll stiffness. " -Milliken RCVD

The OEM frequencies for the gt350 (non R) is ~1.35hz front and 1.55hz rear

Based on past experience I decided I'd like to increase overall stiffness expressed by the increased frequencies to a target of 2.0hz front and 2.2hz rear. This roughly maintains the factory split and I am hoping the overall balance to the car. I am also looking to maintain OE ride height as I do not have aerodynamic grip, I can afford to let the chassis move a bit more freely in order to gain mechanical grip.

I highly recommend this entire site be read a few times over. Lots of good info here:
http://farnorthracing.com/autocross_secrets6.html

So with the springs chosen for the desired frequencies I need to obtain shocks that are able to dampen the bump/rebound with said springs. Using resources like milliken & milliken among the info posted above I plotted out a damper curve that I will result from custom valving performed on 2 way adjustable dampers.

These are my current plots, there is some small revision I will be making for a more flat blow off for a more digressive curve. I have not decided if I am happy with what I have right now.

I have calculated:
66lbs/in = critical damping for the front
72lbs/in = critical damping for the rear

I have settled on a low speed damping rate of 0.62 or 62% of critical for the front you can do the quick maths like so:

182/.025= 7280 (convert from si to imperial) 7280*.2214*.0254 = 41.49
41.49lbs/66lbs = .62

For the rear I have about 11% less damping so a low speed rate of 54% This is to accompany the higher spring rate and to control the weight transfer to the front under braking or other maneuvers. LESS damping = a faster damper = a tighter car (within reason of course)

I have chosen 3inches per second as my bypass or knee / transition point. The reasoning is a large majority of city driving and even tarmac racing for that matter the shock experiences 2-4 inches per second. A digressive damper is desirable so that you can have sufficient low speed damping but then an amount of blow off for pot holes, kerbs, or bumps. The blow off to less damping saves on the shock's lifespan while allowing for the chassis to not become out of shape over a moderate bump.

Like I said the jury is still out on the high speed section... I will update as I make up my mind where I want to start.

**I mistakenly posted the incomplete thread**
I'll roll with it and continue updating it

You will notice that I have a force matched damper curve. That means my bump and rebound are identical. It is perfectly fine to do this for a street car, and as I am learning takes some additional understanding to employ on a track car. The general rule is that cars with heavier sprung masses you want 2:1 or even 3:1 rebound to bump ratios. Rebound controls sprung mass, bump controls unsprung mass. This will result in a harsher ride when COMBINED with lowering your car from OE. This is because ANY bias in the R:B ratio will cause jacking. For rebound biased setups the end of the car will jack down onto the bump stops which act as an infinite spring rate. The opposite for bump biased setups, you will jack the end UP or cause it to launch over bumps. This of course happens over a series of bumps, OR a single large impulse.

With softer springs you can close the rebound to bump ratios as the bump setting is basically an inline spring with the rest of the system. Drivers also usually prefer the feeling of an over-dampened roll mode and so to combat that I will possibly be adding a larger front sway bar to balance that out. I have heard from a couple engineers that the 350 has terminal understeer and there is not a lot that can be done about that. It is an inherent "feature" of the chassis.

After having driven a car with force matched dampers I am inclined to believe that a slight rebound bias would be best for how I like the car to feel. I am not even sure what it would translate to around a track. I will find out in December when I get out to COTA. I will data log the OEM setup, repeat this a few times and then switch to my custom setup and data log some more.


--more to come.

Sponsored

 

[ShatterPoints]

I wanted to add onto this post for context, in both my own conclusions as well as information made available in a multitude of resources.

Here are some notable quotes from Milliken & Milliken Race Car Vehicle Dynamics:

"A brief discussion of choice of damper characteristics is given in Ref. 51, Chapter 3 (by Richard Hodkin). He points out that in practical dampers some friction exists, due to piston movement and seal-also inertia and hysteresis in the valves. Thus dampers seldom develop forces strictly proportional to velocity. He notes that the work done by the damper per cycle is measured by the area under the force-velocity curve. The bump setting of the damper is generally low to minimize the forces put into the body, whereas the rebound setting is larger and accounts for more of the damper work. This is the case when optimum ride is sought but he suggests that for road-holding, bump and rebound settings may be closer together. "

" Up to this point we have simplified the system by assuming the automobile has only one main spring and one wheel. As soon as we consider the more realistic model of an automobile with a wheel at each end, we should consider the dynamic behavior of one end relative to the other. In general a lightly damped vehicle will exhibit two modes of vibration: a "pitchy" bounce and a "bouncy" pitch, each with its own center of oscillation and frequency. The "pitchy" bounce has its center of oscillation outside the wheelbase and the "bouncy" pitch has its center of oscillation within the wheelbase.

Taking the example shown in the upper part of Figure 22.12, the front wheel will hit the bump first and begin moving at 84 cpm whereas the rear wheel will start moving a little later and move at 68 cpm. The comparative behavior is shown in Figure 22.l3 and will tend to give a pitchy ride. These frequency and mode positions are controlled by the static deflection (mass/stiffness) at each end. The upper part of Figure 22.12 shows a static deflection (referenced to the unloaded height) of 5 in. at the front and 7.5 in. at the rear. By softening the front springs and stiffening the rear springs we can give a front static deflection of 7.5 in. and a rear one of 5 in.

Hence the rear of the automobile will move faster than the front and tend to catch up, with the relative behavior as shown in Figure 22.14. This gives the "flat" ride. The dynamics of a vehicle can be modeled on a computer and run on a typical road where the input to the rear wheel is delayed according to the speed.

The third graph in Figure 22.15 shows how the soft-frontlstiff-rear car exhibits less than half the pitch angle displacement, giving the flat ride. To obtain the flat ride in a lightly damped vehicle it is important to have the static deflection at the front greater than the static deflection at the rear. With heavily damped vehicles, where the main ride resonance is well suppressed, this requirement is less important (race cars, for example)."

^^This basically translates to: A stiffer damper slows the frequency response == More static deflection. Paired with soft springs in the front and stiff springs in the rear causes the phase delay for flat ride to occur. (Flat ride confers comfort to the driver and passengers).


Some take-aways from this is, we know that our cars stock are still an OE passenger car. As such the car has terminal understeer (mainly for safety reasons). But you can also do the math to find that our cars are most likely lightly to moderately dampened as with the spring rates we have ~1.35hz front and ~1.55hz rear ride frequencies; or about 15% flat ride which is significant as roughly 20% is the delta between each end of the vehicle to provide the correct "flat ride".

Ultimately this tells you that the less aero and the more you run street tires the more you should target flat ride as resonances will affect both the comfort and performance of the vehicle.

As you load the car with Aero AND racing tires you will be adding damping (stiffness) everywhere. Both for road holding and to control aero pitch/ balance. So you should target flat ride less.

 

[ShatterPoints]

I wanted to give an update to this post. I have the shock dyno and I have been tinkering with how I want to achieve my goal. Some of my initial numbers were off by a small amount and I needed to revise how I was calculating the changes to the damper curve. My main hangup right now is getting a strut body that I can easily place a take apart shock insert into. Pictures, updates, progress, videos, etc to come.

Teaser:

 

[shogun32]

how much for the dyno? What is the size (dia) of the piston you're using?
what software is the screen-grabs from?

 

[ShatterPoints]

how much for the dyno? What is the size (dia) of the piston you're using?
what software is the screen-grabs from?

Software I use to develop chassis configuration(s):
https://www.chassissim.com/


Dyno info:
https://www.performancetrends.com/Shock-Dyno.htm

• 2.0 HP, 110 VAC motor
• Stroke is adjustable for 1 and 2 inch strokes.
• Up to 14 inch/second shock velocity.
• 2000 lb load cell.
• With standard 36 inch masts and 2 inch stroke and the load cell adjusted to maximum height, the eye to eye distance of the shock will cycle between 23 inches to 25 inches.

Sponsored