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# Spring rate calculation

### Tech Articles

Hi!

Anyone know where to start, when calculating spring rates for my project build?

I have 2 suspension setups, but will probably stay with the 2way Öhlins over the 4way Intrax. This car will be used 70/30 for ice racing vs tarmac, and the suspension travel of the Öhlins is favoured over the adjustability of the Intrax's

But back to the question: I need some insight of how to calculate spring rates.

Let's say the 1300kg car has a 52/48 (front/rear) weight ratio.

You're giving zero information about the actual vehicle.

Probably the first thing will be to establish the wheel rate through the suspension movement - the ratio - for the front and rear, so you have an idea what you're working with. Perhaps the next thing is to establish the spring lengths you can work with - fully compressed (you should leave clearance between the coils even then, to prevent impact damage to the seats/chassis) and fully extended to see when/if helper springs may be required.

You haven't mentioned the anti-roll bar(s), so perhaps you should intially work on the pitch and 'ride'in a straight line, for braking and acc'n, before looking at the roll control and corner balance?

Rather than re-invent the mousetrap, I'd suggest starting with spring, or more accurately wheel rates, similar to what others in similar vehicles are using and work from there.

Calculating springs rates can be really challenging, and we need more information regarding the car and goals you're trying to accomplish. It's often easiest to target something other than the spring rate, and calculate into it. A ride frequency, or degrees of body roll per g of lateral force are often good targets to set.

First and foremost, your suspension is a complicated lever. It looks like those dampers are for double wishbone or multilink, so theres a huge range of possible motions ratios. If you put the dampers on with no spring, raise the wheel 1 inch, how much does the damper compress? If we go through its whole range, is the first inch notably different than the last?

Next up, we need the CG height, or atleast, an approximation of it. National highway and traffic safety association has a huge database of tested cars with this information, atleast when the car was stock. Measuring it involves putting your car on scales, lifting one end to ~30 degrees, and a bit of trig. CG of the unsprung is also useful, but a close estimate is 1/2 your tire diameter.

Roll center height - Basically, force acts on your center of gravity, the distance from your center of gravity and your roll center is a lever, and defines how much of the force travels through your control arms and how much through the springs and dampers.

And finally some simpler stuff, Track width, roll bar stiffness, roll bar leverage. Ideally we'd want to use unsprung and sprung mass to get a better picture.

Anyway, once you gather all that information it becomes possible to target and calculate what springs you'll want to use. Your primary interest should be in the weight on the outside tires towards the bottom. But this can only output loading at 1g. I threw the corrections in at the bottom should you need it.

This is the spreadsheet for the simplest method I know of. I think I labeled everything well enough to follow it however it is in imperial units, so convert before you change the values already in there. If you dont have a roll bar front or rear simply set the leverage at .00001, them to 0 ends up dividing by 0 and messing things up.

There should be a picture of roll center, and a more hardcore version of the same calculations, but can account for more complex issues, motion ratio curves, roll center migration, tire stiffness, road camber and target a specific roll stiffness. Edit, attachments don't seem to be working.

Ok!

Thank you for your effort and thorough elaboration of what I'm facing!

I understand that this is a lot more complex to figure out than I could imagine, so for now, I'll stick with values that others with similar setup/weight are using, and get a feel from there.

When you... crowdsource your spring rates make sure to pay attention to peoples use for the car, and primarily ride height. Roll center often moves a large amount when lowered, so they tend to run stiffer springs to avoid bottoming out, and then stiffer again to keep body roll under control.

The car in the spreadsheet is mine, 275 lb/in front springs with oem sway bar, 220 lb/in no bar in the rear.

Nationals level autocross of the same car, 700 lb/in front no bar, 500 lb/in rear.(this is as stiff as some f1 cars)

Obviously this is a wild difference, and a autocross only vs a street car, but both setups are setup for neutral steer(neither over nor under) and have datalogged results to back it up. There's alot of correct setups, but you need to find the one thats correct for your use.

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