Trying to figure out my spring rate for a pair of coil-overs on the back of my T. Being an "eyeball engineer", I'm not familiar with the methods to get it right. I do know this; Rear body/frame weight: 420 fuel weight, (approx.): 75 human weight, (200X2): 400 (20" forward of shock mounting point) I estimated human weight at 300, due to it being forward of the mount point. That gives me 795, divided by two, for about a 400 pound spring rate? I gotta be missing something big here, as Speedway has their T coilovers rated at 175-200. Any info I'm missing that's needed to correctly cipher what I actually need for coilovers? Thanks.....Roger
http://www.ridetech.com/info/spring-rate-calculator/ Personally, I'd put 150's on it, depending on the installation angle.
Assuming the weight of the "rear body" is not including the rear end or wheels [otherwise this needs to be deducted to get sprung weight ] I would start at 90lb/in springs if the springs are mounted vertical. You are better off with softer springs that require a spring compressor to install them.[softer springs are usually taller] Laying them over changes the motion ratio. Don't over spring it. For comparison: Stock 4 leaf Eaton springs for a 67 Camaro are only 126lbs/in
They are mounted vertical, and the weight given is just the rear body/frame weight. Third member was sitting on the floor, attached only by the haipins and drive shaft. And you guys got me to thinking along a differetn tangent. If I use a 100# spring, then it's 100# to compress it one inch, it's another 100# to compress it the second inch, etc., etc. So mysuspension travel limits play into it, too. Thanks, guys....Two (or more) brains are better than one. Roger
Can you put 400 pounds of water in the seat and get an actual measurement? No math or guessing involved. Is the engine and transmission installed? Some of that weight will rest on the back springs if it sits behind the front axle.
I went thru this a month ago, I was trying to compare leaf and coil spring rates. There seems to not be a definate answer. I did use 110 lb coil overs on a T bucket once and it rode great.
The engine and trans are installed. The bucket is titled, and I've been driving it this season, but the rear ride is damn harsh. The "kit" components that we got have the rear suspension set up with coils behind the rear axle, with a shock mounted up through the center. I have no info regarding the coils that came with it. I'm thinking that if I get in the ballpark, so to speak, and get a pair of reasonably adjustable coilovers, then I should be able to tune it to a livable ride. Roger
Anywhere between 110 and 140 will get you a decent handling bucket. I just always buy 140's and call it good. Don
Coil over springs with the coil attached to the spring as everyone who replied assumes? Or a separate coil spring in spring buckets with a regular shock running up though the middle of it? That would be very much like the Corvair front coils that I ran on the rear of my T bucket in the early 70's but that car rode pretty good. That would be similar to the Bobby Fields built chassis in a photo I snagged off the net.
Hi Roger, As for your method of measurement, I don't think you're too far off. Just remember that the spring is already compressed some distance at static ride height. So if it's a 140lb spring, and there is 400lbs of curb weight on it, it has already compressed around 3" at rest. At that point, it takes another 140lbs of force to compress another inch, and that can come from passengers, or luggage, or fuel, or forces transferred to the spring while driving (what you are saying is 'harsh'). So what you'll really need to do is choose a spring rate (for comfort), and then figure out how tall the spring needs to be to give the correct final ride height. Or you could call up your kit manufacturer (or someone that knows the specifications) and ask about the rate of your current springs and then select a spring with a lower rate. Good luck!
A spring rate is for one spring. If 100# rate times 2 springs= 200# to move rear 1 inch. 795# or say 800# divided by 200 would mean the car would compress the springs 4". This is a good number.
The piece of the puzzle you are missing is the inches the spring compresses onto the shock. Using your example of 795lbs/2 = 400lbs total, if your shocks compress 3" (total stroke is 6") then 400lbs/3" = 133lbs per inch, that is the rate. It'll collapse the spring 3" to support 400lbs with the shock in the middle of its' travel.
With all springs [I'll use linear rated as an example] they have a point of load equilibrium. A 90lb/in spring with 450 pounds on it will equalise at 5" of load height. 5" is actually a good load height for a comfortable [but firm] ride. If you use too stiff springs the frequency of the suspension [cycles /sec ] would be like a tuning fork. This harshness will disappear at high speeds because the car hits the bumps at higher cycles/sec Another downside to have to stiff springs in the rear is the weight transfer across the rear during bodyroll will cause oversteer [ or the rear is loose] You are better off with a car that understeers slightly [pushes] and the driver can throttle oversteer [more fun to drive] The 90lb /in I suggested earlier is on the high side of stiffness recommended . I myself would go even lower 70-75lb/in [but that would be too controversial here] A 66 mustang 6cyl has 85lb/in rear leaf springs with a max load rating of 750lbs ,so they have 8.8 inches of arch before they flatten out [approx. 6inches of that load is the weight of the vehicle] Go soft , you won't regret it.
Mr48Chev posted a pic of the exact setup I have. Apparently, someone was attempting to copy that, but failed with the coil choice. OJ came up with the final piece. I was thinking about it today, and realized that I would probably have to get something a little taller than what I already have, but I feel our "bench building session" is getting me headed in the right direction now. So, it looks like I need to get a pair of coilovers with "soft" spring rates, about 3" taller than the coil/shock setup that 's currently under it. If this thread goes no further, I'll throw out a hearty "Thank You!" to all those that replied. Roger
At the risk of muddying the waters after all the sound practical advice you got, one important consideration in choosing spring rate is the ride quality expressed as a natural frequency in Hz. The softer a spring, the lower the natural frequency of the system it's installed in. But also: the greater the load on a spring, the lower the natural frequency of the system it's installed in. That is, you can make a spring feel softer by putting a heavier load on it. Conventional wisdom is that the aim is to get as close as possible to 1Hz while avoiding the possibility of dipping into the "seasickness zone" around 0.2-0.8Hz. The necessity of making provision for variations in payload means that most luxury cars have a rear wheel-rate corresponding to a natural frequency around 1.3-1.4Hz in normal everyday use, so that it might go down to 1Hz or so when heavily loaded. An unloaded truck or a sports car will have a higher natural frequency and a correspondingly harsher ride. I'd guess that my Eibach-sprung VW Golf Mk1 daily driver is around 2.5-3Hz: not quite bone-jarring, but not the ideal tool for transporting delicate antiques over potholes, either (ask my wife.) As you'd expect, there is a formula for this: F = 3.133√(R/M)where: F = natural frequency of the suspension system R = wheel rate (= spring rate for live axles in bump mode) M = unsprung mass Taking your weights gives 210 + 37.5 + 160 = 407.5lbs per side (I've distributed 80% of the passenger weight to the rear, and assumed that the 420lbs is unsprung mass.) Kerry's suggestion of a 90lb/in (vertical) spring would then give you a natural frequency of 1.47Hz, which is right in the "comfortable" ballpark.
.866 x vert value for relative strength of spring. The load on spring would be the inverse so load/.886. The relative rate would be rate x.866x.866
Body roll stiffness is dependent on the spring mounting width on the frame. I think most all coilover rear suspensions need rear anti-roll bars to correct understeer.
With the motion ratio squared formula a 120lb/in spring laying over at 60° would have a "Wheel rate" of 90lb/1n. It would have exactly the same stiffness [including roll stiffness] as a 90lb/in spring standing vertical. You can have all sorts of levers, rockers, bell-cranks and arms controlling suspension, but in the end all the car cares about is "wheel rate" Example: My old BB Corvette racecar had 650lb front springs and a 0.66 [2/3rds] motion ratio so the wheel rate was 283lb/in My brothers Boss Mustang racecar had 1100lb front springs and a 0.5 [1/2] motion ratio so the wheel rate was 275lb/in So here is an example of a car with softer springs but having stiffer suspension.
The Jalopy Journal
