Chassis Building 101. Ride Height

[RobNoLimit]

OK, you opened the door. This is something I have wanted to discuss for a while. I generally put 'weight' into three categories. 1) Sprung weight. All of the weight that is help 'up' with, or moves to compress, the springs. This is most of it. 2) Un-Sprung Weight. This is all of the weight that is NOT held up by the springs. 3) Rotating Weight. Some of this is "Sprung" and some "Un-Sprung". For now, lets just look at "Sprung Weight" - SW - and "Un-Sprung Weight" - USW. While you think about total weight, and 'lightening' the car/truck for performance, thinking about USW would be smarter.

When the suspension reacts to a change in the road/load/corner... there are two main goals, 1) to keep the contact patch of the tire on the ground as much as possible 2) to get the vehicle/suspension back to it's static RH as quickly - and smoothly - as possible. So, the 'load' compresses the spring with a #force, and the spring wants to go back to it's normal position with an = force. BUT, the ghost in the room is USW. Imagine you are driving down a road and you hit a 2x4 with the left front tire. 1) the RF suspension compresses some, loading the spring more. 2) that additional spring load caused the RF of the truck to 'lift' up some. 3) ** some of the energy from the bump hit the tire, and caused that mass to change direction ** OK, Laws of Physics. .....A force in motion will remain in motion until some other force acts upon it..... So, for a moment, the direction of the wheel/tire (USW) was changed from 'forward', to 'up'. Then the spring/shock have to use some force to control and change that back to 'down' and then 'forward'. The more the wheel/tire (and the rest of the USW) weight, the harder it is for the spring/shock to control. Also, and another negative, is that the higher the percentage of USW, the more the USW can move the rest of the car/truck - SW - around. - a bumpy ride.

This is why there is a term "rides like a Caddy". Cadillacs are really heavy, but, because of all of the SW, the %percentage of USW is relatively low. So, when a Caddy hits a 2x4, the USW doesn't have as much leverage over the SW. But, not too many people ever said "handles like a Caddy" - (current CTS-V owners excepted)

To calculate USW, lets say of the RF of a C10 (OK, I'm going to take LOT of assumptions here) you would need to weight some components separately, and some half supported. I'll go through the list.
Wheel and tire = 70 lbs.
Rotor, bearings, studs, seal = 18 lbs
Caliper, pads, hardware = 12 lbs
Spindle, hardware = 19 lbs
Now, we have to look at what part of the next group is actually moving with the suspension
LCA, - support the pivot end and weigh the ball joint end = 8 lbs
UCA - support the pivot end and weigh the ball joint end = 6 lbs
TRE - support the inner TRE and weigh the tie rod and outer TRE = 4 lbs
Sway bar - support the middle bar and weight the 'arm' and end links - this will weigh both sides, use 1/2 of the weight per side. = 4 lbs
Spring/shock - for now, use 1/2 of the total weight = 5 lbs (coil-over)
So, for the average guess C10 we have 146 lbs of USW. - (this hurts to say) So, if you have the average C10, at 3500 lbs and it has 56% front weight there is 1960 lbs on the front. Or, 980 per side. Now, at first you may think, 980 vs. 146 isn't so bad, but that's not how it is. 980 is the Total weight. To get a % of SW vs. USW you need to take the total weight - 980 and subtract the USW - 146 to get the SW of 834 lbs. now you have a comparison of 146/834, or 17.5%. Now, as a rule of thumb 15% is the good/bad break, and 12% is the good/great line.

So, keep this in mind. Light weight wheels/tires/suspension/brakes are a big help. Also, if you are looking to really compete in some track events, 'truck' based suspension is just too heavy. Now, if you cut some USW, and then you lighten up the front, and move the motor back and change the ft/rr %, then you also reduce the front SW, and the % goes back up. It's a fight. When you get down to 800 total on the RF, you will need to be down to about 85 lbs USW to get a 12% ratio.

This little understood ratio of SW vs. USW is why shock valving is such a big topic in the USCA and Auto-X circles right now. Cars are getting lighter, and the % of USW is going up, making it harder for the shocks to control the forces in motion.

Here is a sketch to help imagine all that is moving around. Have a Happy and great Handling New Years!