Please keep all posts relevant to RX-7s, we don't have 4wd for example so let's not worry about things that pertain to that. Any question about any of the terms anyone has posted should be kept in this thread so everyone can find them in the same place.

Please add to this list, but be sure your information is correct!

 

Camber: Camber is the angle of the wheel, measured in degrees, when viewed from the front or rear of the vehicle. If the top of the wheel is leaning out from the center of the car, then the camber is positive, if it's leaning in, then the camber is negative.



Caster: Caster is the axis around which the steering pivots when viewed from the side and is measured in degrees. If the top of the pivot is leaning toward the rear of the car, then the caster is positive, if it is leaning toward the front, it is negative.



Toe: The toe measurement is the difference in the distance between the front of the tires and the back of the tires when viewed from above or below and is measured in inches. Toe in has the front of the tires closer (pigeon toed) and toe out has the rear of the tires closer.



Steering Axis Inclination (SAI): SAI is the measurement in degrees of the steering pivot line when viewed from the front of the vehicle. This angle (along with camber) causes the vehicle to lift slightly when you turn the wheel away from a straight ahead position and causes weight to transfer across the car even at rest.



Scrub Radius: Scrub radius is the distance between where the SAI intersects the ground and the center of the tire contact patch. The distance between these two causes the tire to "scrub" around the SAI when the wheel it turned. Cars with improperly offset wheels sticking out the fenders have huge scrub radiuses.



Thrust Angle: Thrust angle is the direction that the rear wheels are pointing in relation to the center line of the vehicle.

 

Contact Patch: The surface of the tire in contact with the road. The purpose of any performance suspension is to maximize the performance of the tires' contact patches.

Spring Rate: A measurement of the force it takes to compress a spring a certain amount. Springs with higher rates are stiffer. (Not to be confused with wheel rate!)

Wheel Rate: A measurement of the force at the wheel it takes to compress the springs when installed on the suspension. Wheel rate is determined by the geometry of the suspension and the spring mounting. The rate could be linear with suspension movement, or have a varying ratio.

Anti Roll bar (sway bar): A linkage between the right and left suspension that deters body roll by increasing the wheel rate on the outside tire.

 

Motion ratio: the amount of leverage a spring has when installed in a particular suspension geometry.

Wheel rate = Spring rate * Motion ratio

Motion ratio affects both spring and shock rates and induces effects on the anti-roll bar. The lower the motion ratio is of the spring in use, the lower the wheel rate.

 

LINEAR
A linear spring has a given rate of resistance that is constant, usually given in lbs/in compression. If a spring has a rate of 400 lbs per inch and when the RX-7 is sitting still or at equilibrium, that spring is compressed 2.25".

Now you enter a corner ... one side compresses and one side extends .... A linear spring will allow the vehicle to roll "evenly" since one spring is compressing, say one inch, and the other spring is extending one inch ... one spring loads with 200 lbs and one spring unloads with 200 lbs. The roll of the vehicle is balanced and controlled.

Now let's say you encounter a bump during the corner, and the deflection is an additional two inches ... again, the suspension loads and unloads evenly, and again, roll is controlled, the vehicle maintains a constant attitude.

PROGRESSIVE
A progressive spring has two or more rates on one spring ... let's assume that for the first 3 inches or three coils, the rates is 300 lbs per inch and that for the next two inches, the rate is 400 lbs per inch. Therefore, when at rest or equilibrium, the RX-7 will be fully compressed on the 300 lbs spring section, and the next inch of movement will be in the 400 lb per inch range. And the RX-7 will now rest at approximately 3". This is unfortunately not the most accurate description, but it gives you a general idea of what's happening.

Now let's use the same scenario .... a corner is entered and the outside spring is compressed one inch and the inside spring is extended 0.75 inch ... remember the two different rates.

Now you encounter a bump in the corner ... and since you are currently using the outside spring at a rate of 400 lbs per inch and the inside spring rate of 300 lb per inch ... and the suspension is compressed the additional two inches .... what happens to the spring rates? Since they differ, the chassis rotates around the roll axis... and the vehicle attitude changes/rotates differently than when in steady state cornering. This motion becomes slightly skewed since, to offset the bump deflection, each spring must work through a different spring rate. If you were to then hit another bump, the attitude would change again...and so forth.


... okay, I'm done. Who else wants to share?

 

so say i'm setting up a suspension for my FD
for street use and autocrossing in some weekends

i'd want something like a 12k spring rate? is that too soft or too hard?

i'm very new to this suspension game so any advice would be much appreciated!

 

o and how does coilovers play into this then because those are also adjustable...

 

^^Read this: https://www.rx7club.com/suspension-wheels-tires-brakes-20/howard-colemans-fd-chassis-setup-723617/

Then read it again.

 

DamonB

This is a little confusing. lol

"Anti Roll bar (sway bar): A linkage between the right and left suspension that deters body roll by increasing the wheel rate on the outside tire."

I was going to say that the swaybar increases the rate of the inside wheel, which reduces roll. But thinking about it, it also increases the rate of the outside tire (by utilizing the resistance of the inside wheel).

So I guess I'll just scratch my head and wander into some other thread now...