Is our suspension setup correctly by aftermarket strut MFR's? Is the bias correct?

[green-lantern]

Hahahaha, well, maybe... It's from my car. Taken a few second from this pic...

lol I'd be afraid to see my car. might be like 3000, 25, 300, 10. lol

[anyonebutme]

just a rough measurement from my spare subframe. The actual control arms are for locating and camber curves, and have no effect on the motion ratio of our multi-link trailing arm rear suspension. The above quick measurement assumes straight vertical shock, which is really isn't but it's close enough for making the point.

Circle far left is the pivot, large circle is the hub/wheel center, right is lower shock mount bolt.

[J. Fast]

just a rough measurement from my spare subframe. The actual control arms are for locating and camber curves, and have no effect on the motion ratio of our multi-link trailing arm rear suspension. The above quick measurement assumes straight vertical shock, which is really isn't but it's close enough for making the point.

Circle far left is the pivot, large circle is the hub/wheel center, right is lower shock mount bolt.

From what I've been told by Andrew we should measure the actual on the vehicle and not use the assumed lever method becuse this will give us actual vs. assumed measurements and estimated angles.

J,

Measure the motion ratio at the tire contact patch. This is what the tire actually sees. This is how you calc it.

1. Take a measurement from the center of the rear axle to the fender at resting ride height.

2. Jack up rear end and unload the springs by loosening the ride height collars so you can adjust the suspension thru tension and compression freely.

3. Place jack under tire and jack tire to resting ride height and measure the distance between the fender and the center of the tire patch.

4. Adjust jack under tire and lower tire to 1" below resting ride height and measure the distance between the spring perches. This distance is referenced as "Distance Below" or D(b).

5. Adjust jack and reset ride height to 1" above resting ride height (jack up tire 2" from D(b)) and measure distance between the spring perches. This distance is referenced as "Distance Above" or D(a).

Now calcualte the installation ratio which is: ( D(b) - D(a) ) / Distance between tire contact of D(b) and D(a)


Square the resultant of the installation ratio and your answer is the Motion Ratio.

[anyonebutme]

Yes that's more accurate, but the point is it's still going to be >1

[J. Fast]

Yes that's more accurate, but the point is it's still going to be >1

So the motion ratio is licked, excellent! Now we just need to know the exact motion ratio number so we can calc the actual spring rates at the wheel. Anyone have a jackstand, jack, some coilovers mounted, a spanner wrench, and a tape measure handy?

[anyonebutme]

Give me a day or two to switch cars in the garage if nobody bites.

[GTOJOE]

A lot of this is over my head but I did have a conversation here with a guy that races his GTO very competitively and one of the things he eluded to was that the rear spring rate needs to be stiffer than the front. He has modified almost every part of his car including modifying the aluminium front subframe from an awd Magna here in Australia and fitting it to the GTO. He also said the lower control arm mounting points on our cars are too low so when the car is lowered the arm sits almost horizontal when it should still be sitting with a more downward angle towards the wheel/hub. He drilled new mounting points for the control arms about an inch above the factory one on the rear and mounted them there. He then had to get the adjustable 3sx camber arms bent to the right shape so they don't hit the body. The front mounting points was resolved with the new sub frame he installed. To top it off he has spent crazy money on fully adjustable coilovers with external reservoirs from Japan. I can't recall the brand though. MichaelMR2 was there also so maybe if he reads this he will remember the brand.

[J. Fast]

A lot of this is over my head but I did have a conversation here with a guy that races his GTO very competitively and one of the things he eluded to was that the rear spring rate needs to be stiffer than the front. He has modified almost every part of his car including modifying the aluminium front subframe from an awd Magna here in Australia and fitting it to the GTO. He also said the lower control arm mounting points on our cars are too low so when the car is lowered the arm sits almost horizontal when it should still be sitting with a more downward angle towards the wheel/hub. He drilled new mounting points for the control arms about an inch above the factory one on the rear and mounted them there. He then had to get the adjustable 3sx camber arms bent to the right shape so they don't hit the body. The front mounting points was resolved with the new sub frame he installed. To top it off he has spent crazy money on fully adjustable coilovers with external reservoirs from Japan. I can't recall the brand though. MichaelMR2 was there also so maybe if he reads this he will remember the brand.

This stuff may sound complicated but it's actually really easy once we sifted thru all the garbage. The easy part comes when we take actual measurements off a TT 3/S GTO and determine the motion ratio. Once we know what that is we'll simply multiply our spring rates by that number and it will spit out the wheel rates. We'll look at the corner weights which tell us the front requires springs which couple eachother and at a minimum can redistribute 1000lbs dead and whatever dynamic bias comes from the front to rear and diagnals. Something like a 650lbs spring is good for this.

As for the rear, we see a coupled weight of 650 at the corners but a hidden weight from rear wing aero which in my case at speed equates to roughly 500lbs. If that's the case then my rears need to handle a coupled rate of 900lbs. A wheel rate in this case might be 600 for the rear. But depending on what the motion ratio is the actual spring rate might be 800lbs or could be as low 500lbs depending on the motion ratio value in the rear being more or less than 1. Less than 1= more spring. More than 1=less spring.


If we know our ride frequency is say 2.0Hz and we want 6XXlbs of sprung weight at all corners we just fit springs that index that amount when multiplied by motion ratio and ensure the valving adjustment is setup for the freq/oscilation and so forth.


A little hint here of things to come, the additional considerations are based off additional weight added to the chassi based on aero and lateral/front/rear bias and lateral G forces required to keep the tires planted and maintain tire contact. Fast = tires staying in contact with road surface and when you come under tension or compression you come back to neutral with minumum attenuation/vibration. When the road surface changes we don't want to bounce or bottom out or hop. We want to stick, like when a Mickey Thompson Truck goes over whoop-de-doos. The tires stay in contact with the road surface and the body stays quiet and doesn't move. When they jump they land, attenuate, and stick. They don't bounce at all.

The secret sauce in this stuff does not require a ton of spring. It requires maximizing shock travel. The more room you give the shock/springs to absorb road surface imperfections and obstacles the less bounce and rebound we experience.

In your frineds case, sometimes you have to move pickup points to give the suspension more room to travel and correct tire contact.

[Erron Spalsbury]

Right, now couple that with the compression of acceleration and braking loads and you might be some where in the ball park. Doing the calculations will get you about 60% of the way there. The best way is with a potentiometer or the old school method of a zip tie.

[DocWalt]

Old school? The ziptie method is still the name of the game for everyone except top tier teams in motorcycles or mountain biking