The amount of spring stiffness applied at the wheel centerline (or at the ball joint on an a-arm vehicle) is called the wheel rate. This figure is used to understand the balance of the race vehicle. The average amount of wheel rate on the front wheels of an a-arm vehicle is about 70% to 80% of the total. On a straight axle vehicle it should be about 50%. The more front wheel rate you have, the vehicle will tend to be tighter or push in the corner. Normally the less you have, the vehicle will be tend to be loose.
The wheel rate for a wheel on an a-arm vehicle is computed by taking the spring mount length and dividing it by the a-arm length, square that number and then multiply it by the spring rate. For example, AL = a-arm length and SpL = spring mount to frame pivot length. The spring mount length (SpL) is 8", the a-arm length (AL) is 16", and the spring rate (SR) is 1250 lbs/in. The spring motion ratio (SpMR) is calculated by taking the spring mount length (SpL) and divide it by the a-arm length (AL) then square the answer. Multiply that number by the spring rate (SR) and that equals the wheel rate for the spring. Do this again for the sway bar if you run one.
To determine the proper wheel rate, the formula is:
(SpL ÷ AL)2 x SR = wheel rate
8(SpL) ÷ 16(AL) = .5 .5 x .5 = .25(SpMR)
.25(SpMR) x 1250#(SR) = 313# wheel rate.
Now do this for the other side and the sway bar if you run one, add them all up and that's your total wheel rate for that end of the vehicle.
The wheel rate for a straight axle is easier. Take the spring mount width (the distance from the center of the spring mount on the left side of the axle to the center of the spring mount on the right side of the axle) and divide that by the track width (the distance from the center of the left tire to the center of the right tire). Multiply it by each one of the springs, add them together and that is the total wheel rate for that end of the vehicle. Or:
Spring mount width = 40 Track width = 65 Spring rate = 150
40 ÷ 65 = .6154(SpMR)
.6154 (SpMR) x 150#(SR) = 92# wheel rate. (If the springs are different rates, make sure you use the other spring rate and recalculate the other side)
So let's see what we have on this vehicle.
On the a-arm suspension (the front in our example with 1250# springs on both sides), we have a 313# wheel rate times 2 which equals 626# of front wheel rate.
On the straight axle suspension (the rear is our example with 150# springs on both sides), we have a 92# wheel rate times 2 which equals 184# of rear wheel rate.
626# (total front wheel rate) + 184# (total rear wheel rate) = 810# (total vehicle wheel rate)
626# (total front wheel rate) ÷ 810# (total vehicle wheel rate) = 77.3% (front wheel rate percent)
184# (total rear wheel rate) ÷ 810# (total vehicle wheel rate) = 22.7% (rear wheel rate percent)
Our example vehicle may be a little pushy or tight. It also depends on the front roll center, moment arm, stagger, cross weight, etc. - if any of these settings are out of range, they are just a few of the things which can cause a vehicle to be ill-handling.
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