Spring Rates

How do you decide on how heavy to get, take the total weight and divide by 4?

So if you have a 2000 lb car you need 500 at each corner (assuming 50/50 balance for now)?

I understand the ratings are per inch of travel (ie: a 400lb spring is 400 at one inch of travel) and it's supposed to be linear?

I'm building a rear engined rear drive Mini so I expect the weight balance to be roughly 60% to the rear, and total weight to be around 1800 with 2 passengers and a full tank of gas.

I'm looking for a decent ride quality, not bone jarring stiff.....but I am somewhat limited on suspension travel, approx 4 inches in the rear and 3 in front.

So how do I decide on spring rate? I'm using dual adjustable (jounce and rebound) QA1 shocks.....

It's way more complicated than taking the weight and dividing by 4, first of all you need to decide how hard the suspension needs to be in general, which depends on (just looking at performance) how good your suspension geometry is (worse camber curves need harder suspension, see also the Colin Chapman theory of suspension stiffness) and how much downforce you're running (more downforce needs harder springs to keep the suspension from being pushed too low and/or bottoming out, if the car is has ultra-high-end suspension with heave springs you can at least isolate that force). Then once you've worked that out, how hard the springs need to be to make the suspension that hard is affected by the motion ratio. Macstruts tend to be around 1.1:1 wheel to spring ratio, double wishbones can be closer to 2:1. (Edit: And a bellcrank setup can have almost any motion ratio you want.)

Just from experience I can tell you that 500lb/in springs on a 2000lb car with a near 1:1 motion ratio would be very much on the skateboard side of the stiffness spectrum. To give some context, stock Toyobaru springs are progressive (springs can be linear or progressive BTW, both are available and have upsides and downsides) with a max rate well under 300lb/in with macstruts up front and double-wishbone-ish suspension in the rear, and that's a 2700lb car that nobody ever accused of having a soft ride. My AE92 with around 1.1:1 motion ratios all around and about 2300lbs of weight with 60% on the front axle has been modded with 600~650lb/in linear springs all-around, and it is a total skateboard.

Is 4" front 3" rear the allowable compression travel from static height or total travel? For compression travel that's fine, for total travel that would be a problem.

I'm thinking you might want rates somewhere around 150-250lb/in assuming a near 1:1 motion ratio and street-car-like travel. That should give decent body control without making the ride too harsh.

Total travel....

3" in front is std for a classic Mini, which this is......

As far as 1:1 ratio, no idea how you calculate that, but here's a pic of the current rear end suspension and trailing arms, the front is all stock classic Mini, s/l arm suspension.

The rear springs are currently 400lb/in according to QA1. I  do have more suspension travel available in the rear, but the current shocks only go 4", and frankly I don't think the axles/cv joints would allow much more travel as the axles are really short.

I'm concerned that it's going to be really stiff back there with 400 lb springs, and I can compensate for softer springs by stiffening up the shocks, but what would that gain me?

I suppose that since I already have them the best thing to do is just try them and see how it goes.....

Suspension ratio is the ratio of wheel movement to spring movement. So if you had a swingarm suspension where the wheel was at the end of the swingarm and the spring mounted at the midpoint between the wheel and the bushing to the chassis, it would have a 2:1 wheel:suspension motion ratio because if you compressed the suspension, the wheel would move twice the distance that the spring compresses.

So the rear suspension looks like it has a very MacStrut-like motion ratio, very close to 1:1. A classic Mini has a 1:0.9 wheel:suspension motion ratio in the front which again is very close, so in both cases your wheel rate will be very similar to your spring rate - the front will be a little softer than the spring and the rear looks like it will be about exactly the same as the spring.

The small amount of travel can be made to work but you'll have to be careful about how much travel is used for compression and extension. You'll probably want to reserve about 50% of the travel for compression from static ride height. This will also limit how soft you can make the suspension, a stock classic Mini basically just rode on long bump stops after all.

So now assuming a 2000lb car with perfect balance and 1:1 motion ratios (close enough for this guesstimate) where we want to leave around 1.5" of travel for extension in the front and 2" in the rear, each corner would have 500lbs of weight on it, if we use 250lb springs they will compress 2" to reach static ride height. That's fine for the rear but leaves us a bit short of compression travel on the front. If we use 300lb springs, they'll compress 1.6" to reach static ride height which is maybe less extension travel than ideal for the rear but almost perfect for the front.

You can trade compression travel for extension travel by using helper or tender springs. These are springs run on top of your main spring that are meant to compress fully with very little to no load (a helper spring is super-soft and can be compressed with your bare hands, a tender spring may have some real rate to it). You would adjust the spring perches to have extra room around the main spring which adds to your extension travel (at the cost of compression), and then add the helper or tender spring so that the main spring doesn't fall out. This is what I do with my AE92 which would otherwise have under 1" of extension, like this:

So I think you may want to use some of these on the rear.

Edit: Mixed up front and rear travel amounts, fixed now.

Thanks for the explanations!

I like the idea of what you did with the extra spring on top, I don't know if these shocks have enough room for that?

400lb/in seems a bit stiff to me for what will be at best an 1800lb car?

These are the ones MiniTec uses, and they use them for both their K series and V-6 engine swaps.

If you're worried about room on the coilovers, helper springs compress down to a very slim profile.

400lb/in might make sense for track use but seems excessively stiff if you care about ride quality.

That rear suspension is almost certainly over 1:1 with the damper mounted behind the axle centerline. The bad news is that means less wheel travel than your 4" shock travel. Calculating motion ratio is as simple as measuring wheel travel and damper travel and dividing. Helper springs are nice, but you can also use spring preload to get a desired amount of droop travel.

For reference I have a mid engine car with basically 1:1 motion ratios and 55% rear weight at about 3000lbs. The stock spring rates are 160/250lb/in, the current setup with ohlins coilovers (not a ton of travel, I think only 4-5" front) is 350/450 which is definitely stiffer than stock but also definitely not "full racecar". You could probably scale those numbers down (and then account for motion ratio differences) to get some rough targets. Really on a custom vehicle getting all of this stuff right requires either some high level suspension modeling and analysis, or a lot of experimentation.

I'm going to contact the shock MFR and see what they say......

You're working with some pretty unconventionally tight numbers. Is this middy swap built or bought? If built, you've backed yourself into a bit of a corner, and it's going to to be a math challenge getting out. If bought, the middy kit manufacturer should have already figured that out for you, and that's who you should probably be contacting.

dps214 said:

For reference I have a mid engine car with basically 1:1 motion ratios and 55% rear weight at about 3000lbs.
....

350/450 which is definitely stiffer than stock but also definitely not "full racecar"

That is 2.1Hz front and rear.  An equivalent in a 1800lb mini with 60% rear weight and an 80" wheelbase would be roughly 150 front 250 rear.