Spring rates and ride rates

[tarun garg]

I have calculated our spring rate as per the formula given by the optimum G: Ks=4*3.14^2*f^2*Ms* Mr^2

Where Ks is the spring rate, f is the ride frequency, Ms is the sprung mass per corner, Mr is the motion ratio, defined as wheel travel/spring travel.

By taking
f=2.5Hz
Ms=51Kg
Mr= 1.25

Ks= 19.67 N/mm=112 lbs/in.
This value is coming out to be very less in comparison to the spring rates used commonly.
We are not using any anti roll bar in our car. So please correct me if i m wrong in calculating the spring rates.
Secondly, we are assuming the ride frequency and calculating the corresponding ride rates from that frequency.
Is the above mentioned procedure appropriate or is there a better alternative?

[tarun garg]

I have calculated our spring rate as per the formula given by the optimum G: Ks=4*3.14^2*f^2*Ms* Mr^2

Where Ks is the spring rate, f is the ride frequency, Ms is the sprung mass per corner, Mr is the motion ratio, defined as wheel travel/spring travel.

By taking
f=2.5Hz
Ms=51Kg
Mr= 1.25

Ks= 19.67 N/mm=112 lbs/in.
This value is coming out to be very less in comparison to the spring rates used commonly.
We are not using any anti roll bar in our car. So please correct me if i m wrong in calculating the spring rates.
Secondly, we are assuming the ride frequency and calculating the corresponding ride rates from that frequency.
Is the above mentioned procedure appropriate or is there a better alternative?

[exFSAE]

I wouldn't pick wheel rate from ride frequency (initially).

But don't do something because Claude says so, or because I say so. They're just examples.

YOU figure out what wheel rate you need and WHY.

[Mike Macie]

Also be careful with your CG height and mass estimates. Our team recently tried to not run arb's and after testing, our cg height and vehicle mass estimates used in our suspension design were off. The car rolled a lot more then designed and basically all we could do with the time we had was to add camber. I would just advise to have a couple different set of springs for testing.

[gt_biker13]

That's close to what we will be using, however we have antiroll bars front and back. If you don't have ARBs then that spring rate looks a little low.

[BilletB]

Why don't you have arb's?
Have you done other analysis other than an equation you found? If you're justifying no arb's you should have lots more wheel rate analysis at this point. You should be able to develop your own equations and should be checking all the influencing behaviors of your chosen springs.

Technically ride frequency would also include the spring rate of the tire and I see you've shown that nowhere. What you're using as 'f' is suspension frequency.

You've got a lot more to investigate.

[Anvit Garg]

Speaking of springs, are they custom made or is there a seller that offers a variety of sets that fit the Cane Creek damper?

I was under the impression that the springs for the CC dampers were inexpensive. So far ive found them to be 200+ (and even more for Ti).

We also are performing estimates of our ride frequency/picking a motion ratio but arent sure what spring rates are available.

[Jersey Tom]

Last I bought CC springs they were $35 I think.

[McMasteRacer]

Have you tried Motorsports Spares International:

they sell the CC spring kit

http://www.motorsportsspares.com/fsae.html

[Mike Cook]

The motorsports spares stuff is pretty good. We get our stuff custom made by road magnet. They will make anything you want at a reasonable rate and good turn around times.

[Mike Cook]

Ride freq are basically meaningless.

If it were me, i'd be more interested in how stiff your car needs to be to keep it from bouncing on the ground and then I'd think about what springs you need front/back to get the balance correct (which is not easily calculable-takes testing).

Oh, and sway bars adds lots of build time and complexity to the car. If I were a new team or tight for time I wouldn't worry about having them, i'd just make my bellcranks adjustable. Our team has pivots for swaybars but we wont be running any for VIR.

[murpia]

Picking wheel rates based on ride frequencies, yet stating you don't plan to use any form of anti-roll bar means you probably won't have the correct roll moment distribution for your car.

A rough priority plan would be:
Define a roll moment distribution that suits your track widths, CoG, weight distribution and tyre characteristics.
Define a range you would like to be able to adjust that roll moment distribution over.
Decide how much wheel travel you want to use during dynamic manouvres.
Decide how much additional wheel travel you will allow for bumps.

Once you have that data then you should be able to work out upper and lower bounds for wheel rates, and whether or not you can achieve those targets with or without an anti-roll bar.

Then test to find out what roll moment distribution works best for your tyres, and whether a soft car is better than a stiff one. Usually if you go softer you get more grip and if you go stiffer you get more response. Let the stopwatch be your guide as to which is better.

Regards, Ian

[Anvit Garg]

thank you so much for that website with the spring options, that will help a lot!

[Mike Cook]

Ian, why do you need anti-roll bars to get the 'correct' roll moment distribution?

Mike

[J.R.]

Perhaps not getting "independent" tuning of ride and roll? We ran no bars last year and the car rolled like you wouldn't believe (1.63 deg/ g).

If you run springs hard enough for roll and ride, then they won't do anything on a semi bumpy surface, ie skidpad at MIS.

Not saying our setup was "optimized" but I remember pulling up the inside front on larger radius corners at the Toronto Shootout, which softer springs would have gone a long way to reducing. Bars would have allowed that to be tuned out

[Yellow Ranger]

I like the whole build your car to be super adjustable thing- cause you never know exactly what your gonna get (like forset says).

there are so many variables, like from the tire data- who knows what happens when we use our tires which are luke warm at best on skidpad and autocross, not to mention the data is for goodyear 2692s which aren't made any more (now 2696s) and the hooiser R25As are now R25Bs...

then you got to remember that all those camber curves and designed roll gradients are dependent on you getting your manufacturing tolerances dead on and then you have all the compliance in the suspension and tires...

and none of it matters if you design for a kinematic roll center to be somewhere and its actually somewhere completely different and the forced based is who knows where and your perception of what is balanced and what isn't just got kicked in the huevos...

tarun garg, that seems like a reasonable starting point- the fact that your asking people here means that your not 100% sure of what to do, so make sure that with that setup, you can change the hell out of it if needed- then test the car to see what your driver and your driver's lap times like best