The project is the P1800 Volvo with he 302 and Miata suspension.
When done, the car should end up 300lbs heavier than the stock Miata.
My current inventory is stock Miata springs and shocks in good shape.
I can certainly corner weight the car.
Any ideas or tips on how to baseline the spring rate? I will most likely use KYB adjustable to deal with shock adjustments.
Is this an area where adjustable coil overs might be the answer?
Coilovers won't affect the spring rate.
I thought this was helpful: http://www.specialstage.com/forums/showthread.php?34942-Selecting-a-spring-rate-101
the interwebz said: A rally car with 550 lbs corner weight experiencing a typical maximum vertical G force of 3 G's with 9" of available bump would require 550 lb * 3G = 1650 lbs spring load available at the wheel at this typical maximum load. With 9" travel available this means you have to support the full 1650 lb load over 9". The spring rate required to do this is 1650lb/9' = 183.33 lbs/inch at the wheel. The actual spring rate required on the strut is determined by measuring the distance from the lower arm pivot to the centre of the ball joint and also the arm pivot to the tyre centre. Divide the pivot to tyre centre distance, by the pivot to ball joint distance and this figure will give the motion ratio of the control arm to tyre. Multiply this figure (typically 1.1 approx) by the spring rate at the wheel (183.333 lb/inch and you get 183.33 * 1.1 = 201.63 lb/inch springs on the strut. There will have to be some compromise on rates determined by the available spring rates and lengths so opt for the closest rate. In this case probably 200 lb/inch springs. The spring length required has to exceed the wheel travel distance of combined bump and droop at the wheel divided by the motion ratio. EG if the above vehicle has 1" droop as well as 9" bump travel, the the spring needs to be 10"/1.1=9.01" plus an inch or two at the fully compressed length before coil bind
But I too am interested in spring rate selection. 
Need more information...
What is your intended use for the car? (weekend cruiser, daily, mixed use track/daily, track only, race car, etc)
What tires (sizes and compound) are you going to run?
What is your budget for dampers?
Also, rate your desire for comfort vs performance on a scale from 0 (max comfort / soft ride) to 10 (race car / stiff ride).
Choose rates based on frequency.
http://farnorthracing.com/autocross_secrets5.html
If you want to use frequency to pick rates with AND your going to be doing any sporting driving with the car I suggest bumping the frequency WAY up from what Dennis suggests on his farnorthracing site.I don't think I'd hesitate to go up into the 4hz range just as a starting point. I used his info to baseline my autox scratch build,way too soft starting at the upper end for his track use suggestion.I was up in the 5.5hz range(from memory so grain of salt thing) in it and it still rode better than my street car.
iceracer wrote: Coilovers won't affect the spring rate.
No but they make spring selection markedly easier since you don't have to get the length exact, just close.
My favorite method of spring rate is kinda simple... front rate is vehicle weight in pounds, divided by ten to get pounds-inch. Rear rate is that times .75 - 1.0 depending on driver feel. This is for a suspension with a relatively straight rate, like coil over solid axle or struts (which are really usually around .9:1 or so but we're making approximations anyway so why stop now?)
right...more information would be needed.
The reason for the Miata suspension is because I like the design and handling of Miatas. The car is being built as a cruiser, and I have done a few 1000 mile plus days in the Miata, so figure it qualifies for the job.
So, the goal would be to approximate the ride of a stock Miata.
Coilovers will help if the vehicle needs to go up or down for a given spring rate and the extra cost of adjustables will pay off in mot having to buy new shocks to experiment.
The car weight/10 rule is going to give me 2800/10 so front springs of 280 lbs/in. Seems a bit light. Anyone know what stock NA spring rates are?
280lb-in would be the wheel rate, Miata sticks the spring on a lever so it would probably be closer to 350-400lb-in spring rate.
350-400 would have been my gut guess. The Miata spring is not too far from the wheel,and fairly straight up and down, so would the wheel rate not be similar to the spring rate?
kevlarcorolla wrote: If you want to use frequency to pick rates with AND your going to be doing any sporting driving with the car I suggest bumping the frequency WAY up from what Dennis suggests on his farnorthracing site.I don't think I'd hesitate to go up into the 4hz range just as a starting point. I used his info to baseline my autox scratch build,way too soft starting at the upper end for his track use suggestion.I was up in the 5.5hz range(from memory so grain of salt thing) in it and it still rode better than my street car.
The ride doesn't have to be harsh for it to handle well, and there are many reasons why the ride will be different than your ~1.0 to ~1.5 Hz "street car", so that's not a very fair comparison.
What makes you think the ride was "too soft" with your starting frequency? And what frequency did you start with?
The ideal frequency also depends on what your autocross course is like. Some lots are VERY rough and bumpy. Others might as well be kart racing tracks where you don't really need suspension at all.
NOHOME wrote: right...more information would be needed. The reason for the Miata suspension is because I like the design and handling of Miatas. The car is being built as a cruiser, and I have done a few 1000 mile plus days in the Miata, so figure it qualifies for the job. So, the goal would be to approximate the ride of a stock Miata. Coilovers will help if the vehicle needs to go up or down for a given spring rate and the extra cost of adjustables will pay off in mot having to buy new shocks to experiment. The car weight/10 rule is going to give me 2800/10 so front springs of 280 lbs/in. Seems a bit light. Anyone know what stock NA spring rates are?
To approximate the ride of a stock Miata with your heavier vehicle you will need to know the following for both your car and the Miata:
1) Front and rear suspension motion ratios 2) Corner weights (with intended load; that means driver and fuel at minimum) 3) Unsprung weight per corner
And the front and rear spring rates for the Miata.
Or if you already know the Miata's actual suspension frequency then you just need the above info for your car.
NOHOME wrote: 350-400 would have been my gut guess. The Miata spring is not too far from the wheel,and fairly straight up and down, so would the wheel rate not be similar to the spring rate?
No. The wheel rate is the spring rate multiplied by the square of the motion ratio. The wheel rate is very sensitive to motion ratio, particularly on SLA suspensions where the motion ratio may be around 0.5 to 0.6, and is rarely much higher than 0.7.
For example, on my Fiero's front suspension (SLA) the motion ratio is 0.53 so a 800 lb/in spring gives a wheel rate of 224 lb/in. (0.53 * 0.53 * 800 = 224.72)
I never said it was soft and didn't handle well,I'm up in the great white north with frost heaved lots the norm. Soft meant to me not enough roll control,I used a miata as a donor for hubs but relocated the inner pivots. I used my finely calibrated butt to judge ride....same as everyone else.
What's your weight and CG height going to be relative to a Miata? Assuming you run stock subframes your roll center will be fixed by the Miata geometry. I would be most concerned about roll stiffness relative to the Miata and what bars youll need. If your going to be dramatically higher roll couples than the Miata has the ride frequency will have to be very high to overcome. If the opposite is true you may wind up with to much roll stiffness and be unhappy with the results.
You can fix the roll couple by either putting really tall springs on it to raise the ride height or putting really huge wheels on it (like donk drivers in their jacked up Impalas will say "damn dude your wheels are too big") to change the roll center/ground relationship
@nocones, very good point about the roll stiffness. Hopefully NOHOME's roll couple is close enough to that of the Miata's that he can find some bars that work.
kevlarcorolla wrote: Soft meant to me not enough roll control,I used a miata as a donor for hubs but relocated the inner pivots.
The frequency recommended by Dennis has nothing to do with roll control.. that's the job of tuning your roll couple. In Dennis' simple tuning method, you adjust the swaybars to get the desired roll stiffness once the frequency is where you want it.
I agree that going stiffer on the springs is a good idea if you don't have enough roll control -- but only to a degree. Better to use swaybars and keep the suspension frequency reasonable than to go too stiff on the springs and jump over bumps.
Thats the thing,900lb car 2" off the ground with no roll bars and high frequency still rode bumps like a champ.Seriously even tooling thru the paddock it was obviously not over sprung.
Thats my experience,always remember I have 0 training at this stuff.
Big sticky soft race tires are the first spring, the second is your chassis spring.
OK then...
I have enough new words in my vocabulary to keep me busy for a while.
It might be a while, but as I get closer to getting the car back on the ground, I am sure this topic will be revisited!
bentwrench wrote: Big sticky soft race tires are the first spring, the second is your chassis spring.
20" tall stiff sidewalled race tires aren't that soft.
kevlarcorolla wrote:bentwrench wrote: Big sticky soft race tires are the first spring, the second is your chassis spring.20" tall stiff sidewalled race tires aren't that soft.
Are you sure?
https://www.hoosiertire.com/spring.htm
Stock NB rates are 162/118, pretty soft. MSM rates are 215/158. FM rates are 318/233. This all came off the interwebz so it's all absolutely gospel and not open to discussion. I'd start with some 250 fronts and maybe 200 rears. It depends where the weight ends up and how soft you want it to be for a street car. If you get to the point where you are ready to try some non-stock rates Peter, let me know. I've got a bunch of stuff from around 250 to 700 laying around, all for use on a coil over sleeve and most of them are 8" length. SpecMiata springs are 6"/700# Fr. and 7"/325# rear.I have a brand new set of those in the garage attic.
KYB AGX are the 2nd worst shock you can put on a miata suspension, right behind TOcico blues.
If you have any NB bilstiens in that pile of shocks and springs start there. but some ebay coil over sleeves, throw the springs away, and put some proper spring rates on them, like 450/250 should be decent for your setup.
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