Figuring out correct Spring Rates

We are in the infancy of building a new MX-3 autocross car and would like it to be competitive locally in Street Modified. It already has a 2.5L KL-ZE in it and we got it for a song.

Is there some guide to picking the correct sping rates?

The STS2 CRXs weigh just under 1900lbs and run 450+lb springs. We can buy Springs thru Mazda Motorsports and have been looking at the 600# versions.

The MX-3 GS weighs around 2500Lbs.

Any help & Guidance would be much appreciated!

300 front, 500 rear

We already have 375# in the front and they are too soft.

If you're developing a car, I'd absolutely get coil-overs. You can get 2.5" springs on Honda-Tech and trade them for different rates for only the cost of shipping.

Don't compare rates to other cars without knowing the motion ratio! If a car has a motion ratio of 0.5 (spring is halfway between pivot and wheel, like some trailing arms), it will need 4x the spring rate of a car with a motion ratio of 1 to get the same effective rate at the tire. It's squared - (0.5)^2=0.25, 1^2 = 1, 1/.25 = 4.

Generally, strut cars are .85 to .95, and trailing arms and double-wishbones are 0.5 to .75. To figure out a good rate, you'll need this number as well as the weight of the car.

autoxfil wrote: If you're developing a car, I'd absolutely get coil-overs. You can get 2.5" springs on Honda-Tech and trade them for different rates for only the cost of shipping. Don't compare rates to other cars without knowing the motion ratio! If a car has a motion ratio of 0.5 (spring is halfway between pivot and wheel, like some trailing arms), it will need 4x the spring rate of a car with a motion ratio of 1 to get the same effective rate at the tire. It's squared - (0.5)^2=0.25, 1^2 = 1, 1/.25 = 4. Generally, strut cars are .85 to .95, and trailing arms and double-wishbones are 0.5 to .75. To figure out a good rate, you'll need this number as well as the weight of the car.

Right on the money. You are most interested in the wheel rate, which is controlled by the motion ratio. My latest project is starting with wheel rates about 390 inch pounds at the front (550 inch pound springs with a .71 motion ratio) and 225 at the rear (225 inch pound springs mounted directly on the axle, 1.0 motion ratio), it's a ~2000 pound rear wheel drive live axle car. That's only a starting point and YMMV.

I'm guessing the motion ratio on your car to be somewhere around .85, so your 375 inch pound springs become 318 inch pounds at the wheel and for a nose heavy FWD car that weighs 2500 pounds yeah that would be too soft.

For comparison, Stinky (my old RX7) used 375 inch pound springs in the front and was somewhere around .95 MR for about 355 inch pound wheel rate, 200 inch pound in the rear with 1:1 MR and weighed around 2100 pounds with a more even weight distribution than a front driver.

I have no idea what any of that means... but it sure sounds good.

We have Ground Control coilovers, but will be getting new replacement coilover springs from Mazdaspeed. You are saying the honda Tech may be a better deal though? Thru Mazdaspeed they are $60ish each I think.

The MX-3 is A Strut car BTW.

We are also looking at koni Inserts since there are no REAL performance struts for the MX-3.

atlantamx3 wrote: I have no idea what any of that means... but it sure sounds good. We have Ground Control coilovers, but will be getting new replacement coilover springs from Mazdaspeed. You are saying the honda Tech may be a better deal though? Thru Mazdaspeed they are $60ish each I think.

they are talking about the fact that when the spring is acting further in on the control arm, rather then at the very end where the wheel is your getting extra leverage on the spring...

Its like if you mounted the spring 1 inch away from the inner pivot point of the control arm, you could use a really stiff spring and it wont offer much resistance... If you mount the same spring so that its as far out as possible its gonna offer a lot more resistance, so rather then looking at just the rate of the spring its important to look at the resistance at the wheels, the spring rate of an imaginary spring acting directly upon the wheel itself. Then look at the control arm as a lever and figure out what spring rate would be nessisary to get an equivilant resistance.

I'd say going with autoxfil's suggestion of coilovers and trading/testing spring rates on forums would be your best bet... otherwise you are gonna have to know wheel loads, roll rates, roll axis to CG height, buncha random stuff like that to calculate how much it rolls under load and how much you want to let it...

RedS13Coupe wrote: I'd say going with autoxfil's suggestion of coilovers and trading/testing spring rates on forums would be your best bet... otherwise you are gonna have to know wheel loads, roll rates, roll axis to CG height, buncha random stuff like that to calculate how much it rolls under load and how much you want to let it...

How do I figure all of this stuff out? I didnt think it was going to be this difficult. (eek!)

atlantamx3 wrote:

RedS13Coupe wrote: I'd say going with autoxfil's suggestion of coilovers and trading/testing spring rates on forums would be your best bet... otherwise you are gonna have to know wheel loads, roll rates, roll axis to CG height, buncha random stuff like that to calculate how much it rolls under load and how much you want to let it...

How do I figure all of this stuff out? I didnt think it was going to be this difficult. (eek!)

lol, it doesn't really have to be if you take a more trial and error/experiance approach... but if you want to sit down and figure it out (or at least close) with out trying one spring after another its a bit more complex...really not too sure of finding the CG height... the rest of the stuff is measuring out all the suspension component mounting points and doing some geometry...

You honestly would probably be best to just scour the internet for other people who have used the car and see what they did, or look for vehicles with similar weight/distribution then do the math to find out their wheel rate, then apply that to your car and doing coilovers with a common sized spring as mentioned before.

Motion ratio is easy. Let's say the control arm is 10 inches measured from the center of the ball joint to the center of the inner bushing. Now measure from the center of the shock mount on the C/A to the center of the inner bushing, let's say that's 7 inches. Divide the second (smaller) measurement by the first (larger) measurement, 7 divided by 10 = .7, that's a 70% motion ratio. This is your 'constant' for figuring wheel rates. Like this: say your spring used in the above example is 500 inch pounds, multiply that x .7, 500 x.7 = 350, so your wheel rate is 350 inch pounds.

The angle at which the spring is mounted will have a bearing on wheel rate as well. Anything under 10 degrees from vertical will have a negligible impact on wheel rate. Over 10 degrees, that's a different story. You take the wheel rate that you get from the motion ratio calculator and then multiply it by the correction factor for spring angle to come up with the effective wheel rate. The Abomination's shocks are at a 25 degree angle and that makes the springs effectively softer. IIRC, the multiplication factor was .88. So, if the spring is 500 inch punds and is mounted at 25 degrees, 500 x .7 = 350, x .88 = 308 inch pounds. There's plenty of tables on teh int3erweb showing the correction factors.

The wheel rate you want is determined by how much weight is on that particular corner and that's where knowing the total car weight and weight on a given wheel are good things to know.

Using my car for an example, each corner will weigh roughly 500 pounds and I will be using ~390 inch pound wheel rate in the front. Subtract the 'unsprung weight' (wheel, etc) leaves roughly 450 pounds per corner in the front, meaning the car will settle approx. 1 inch when the suspension is loaded, assuming the spring is not 'preloaded'. I'm going with softer rates in the rear (225 inch pounds) to keep the car from being too loose.

Remember, the rates I am using will have no bearing on your car at all!

http://farnorthracing.com/autocross_secrets.html

Honda-Tech is good because they're all people trading used springs - get the Eibach or Hypercoils and then you can sell and buy, or just trade. The going rate is $50-60/pair, last I checked.

Note you must SQUARE the motion ratio when calculating wheel rate. In Jensenmans example, it should be: 500 x .7 x .7 = 245 lb/in wheel rate (This is because the .7 works against both the lbs AND the in in the equation. :) )

Nathan (military suspension designer)

Oh, my. I left out an important step. njansenv is exactly correct. That's what I get for trying to do 40 things at once.

Where do you guys learn this stuff? Seriously. Is there a book or something?

Thanks.

Several. Carrol Smith, Fred Puhn, Herb Adams and other have all written excellent books about this stuff.

Check out:

http://www.ssapubl.com/

and break out the credit card.

Herb Adam's 'Chassis Engineering', Fred Puhn's 'How to Make Your Car Handle' and Don Alexander's 'Performance Suspensions' all have good basic info. There's others like Milliken's 'Race Car Vehicle Dynamics' (I gotta get that one) and some other SAE publications as well. You start getting into big bux with the SAE pubs, though.

The trick with a fwd car is to go real stiff on the rear. Ford Racing at one time had a kit for the ZX2 that had 300 front and 500 rear with adjustable sway bar. They were coil overs. Unfortunately it is no longer available.

check out Pro-shock web site they used to have a calculater but you need to use motion ratio one 1st and you'll need corner weights too.

44

All the talk of motion ratio is great. The thing to remember is the MX3 is nose heavy and inheritly understeers. Go with stiffer front springs without stiffer rear and it will understeer more. Put the stiffer springs in the rear to reduce the understeer. A bigger rear sway bar really helps.

One of the local hotshoes has a FSP Rabbit which understeers terribly in stock form. He is currently running the 255 width A6's on 13x9 rims on the front and 245's on 7 inch wide rears. I have driven it; it will actually rotate. He won't divulge any setup info (if he tells you he will have to kill you!), but from observation it's super stiff in the rear and sits nose high at rest. I'd say get the coilovers.

iceracer wrote: All the talk of motion ratio is great. The thing to remember is the MX3 is nose heavy and inheritly understeers. Go with stiffer front springs without stiffer rear and it will understeer more. Put the stiffer springs in the rear to reduce the understeer. A bigger rear sway bar really helps.

Actually, stiffening the front is priority number one for any strut car. The rear must follow suit to really balance the car, but big front springs on a fwd car and big front bar on a rwd car are magic. In terms of lap time, not subjective handling, I'll take front roll stiffness over rear on almost any strut car.

The reason is that struts have lousy camber curves. As the car rolls, the heavily-laden outside tire gets positive camber, dramatically decreasing grip. Stiffening it up a lot will prevent the car from rolling as much, and keep that tire flatter. Mucho negative camber is good too - generally 3-4º for an autocross car, sometimes 5+!

As the saying goes, "any suspension will work if you don't let it."

It's not terribly difficult to get a nose-heavy FWD car to oversteer. Stiffer (than usual) rear springs and a stiffer sway bar in the rear is all it takes...running mismatched tires is a cheap way to get the car to rotate, but I imagine the transitional cornering would be awful. Check out the setup on the 2005 Civic hatch and the AE92 SR5. I'd also tell you to check out an old Mini but I don't know how much useful info you could get on its suspension

Yeah, mismatched tires are the cheap way out but ya can't argue with results. Transition is fine in that car, its only flaw is horrible torque steer. He had to put a power rack and pump on it after the tire swap.