Wheel rates and ride frequencies, suggestions?

[DLinson]

I’ve got a question for you suspension gurus out there. I’m trying to figure out the spring rates for my car and was wandering what a good rule of thumb is for ride frequencies front and rear as well as what wheel rates to shoot for.

I looked up past posts on the subject but still have not found the answers. On a previous post stated that Herb Adams suggested that the wheel rate should be equal to about 2g of loading over the entire travel. If the corner sprung weight is 800 lbs then the you would take

1600lbs / 6in = 266 lbs/in wheel rate.

I was figuring on a 1g load for the corner for the bump travel from ride height. Is this too much for a street car?

A friend send me some data on wheel rates for the C-5 and the Viper which are:

C-5 = 105 lbs/in
Viper = 149 lbs/in

A coworker used to work for Moog and they would design the springs for most passenger cars with about a 100 lbs/in wheel rate in order to get the comfortable ride. I assume the Vette makes up for the softer springs with sway bars and the fact that it has a low CG. Is that 2g corner load target from Herb Adams really for race cars and not street cars? Would a good compromise between the two be about were the Viper is at 150lbs/in? Another suggested shooting for the Viper wheel rates as it was much better at the higher speeds.

I have a spreadsheet for calculating the frequencies, spring rates, and wheel rates. If I shoot for the 1g load through bump travel, I end up getting about a 260lbs/in front wheel rate with a ride frequency of 1.81hz. The rear would end up being about 225 lbs/in with a frequency of 1.9hz. These frequencies seem really high to me but I’m just starting to learn this stuff. Is the .1 higher ride frequency enough of a split or should I be shooting for a little higher frequency on the rear.

I ran the softer wheel rates through the spreadsheet like 190lbs/in front and 170lbs/in rear which results in ride frequencies of 1.55hz front and 1.70hz rear. Is this closer to what I should be shooting for? I know my car won’t see any track time at least for a few years, it will live it’s life on the highway driving to shows and just having fun.

This was pretty long but if someone were to have suggestions on what to shoot for, I’m sure I’m not the only one who would appreciate the advice.

Thanks,
Dennis

[Jagarang]

Holy crap Batman...thats a zinger!

[David Pozzi]

Dennis, did you read this one?
https://www.pro-touring.com/forum/showthread.php?t=5132&highlight=frequency
I'd lean towards the softer version: 190/170, but I've not done any cars from scratch like Katz has, so I defer to him.
David

[DLinson]

Thanks Dave. I forgot all about that thread and I started it. I must be slippin.

In Katz's example he had the rear ride frequencies lower than the front. This is opposite of everything that I have read so far. I'd be curious to know if the car pitched for and aft over bumps.

It looks like from Smithees-racetech that average passenger car frequencies are about 1.33 to 1.67 and the race cars are between 2.0 to 2.13.

Looks like I'll try to shoot for the lighter springs and the lower frequencies. I'm still curious what the front/rear ride frequency split should be.

Thanks again.

Dennis

[David Pozzi]

Race Car Vehicle Dynamics (The Millikan and Millikan book) shows recomendations for non-aero (non-downforce) racing sedans a ft freq of 1.6 to 2.0 hz, and a note that the front should be higher hz than the rear. This is was meant for race cars, so street cars would be lower, I think the 1.3 or 1.4 range would be good.

I had some freq notes saved on my computer, but had a crash a while back and lost them so I'm flying blind except for some books...

I'd figure a front WR of 170/180 a pretty stiff rate, 100 a stocker rate, so maybe something in the 140 range would be a good compromise.

[David Pozzi]

I found this data on the McLaren F1:

Keep in mind this is a mid-engined car and the rear cpm is probably higher because of that.
Wheel travel front and rear was set at a generous 90mm (3.5in) in bump and 80mm (3.1in) in rebound and the target unladen bounce frequencies at 86 cycles per minute (1.43Hz) at the front, 108cpm (1.80Hz) at the rear. With the finalised car slightly over target weight, the actual ride frequencies have fallen slightly to 84.5 and 105cpm. Although these frequencies are higher than those of everyday road cars, they are still low for a sports car of this performance potential.

It was the wheel rates and wheel travel which determined the downforce generated by the underbody. Too much downforce would simply have squashed the car on to its bump stops, making the handling dangerously unpredictable at high speeds.

Describing the suspension as double wishbone sells it ludicrously short. Its cleverness lies in how longitudinal wheel compliance has been engineered in without loss of wheel control. It is this compliance which allows the wheel to move backwards when it hits a bump, endowing the F1 with its remarkable ride.

Murray didn’t know how much longitudinal wheel compliance to provide. In racing cars every effort is made to eliminate compliance to maximise wheel control. So McLaren bought a Honda NSX and put it on the electro-hydraulic kinematics and compliance rig at Anthony Best Dynamics. A Porsche 928S and Jaguar XL16 were also measured.

Different methods of achieving the required compliance are used front and rear in the F1 because the suspension pickup points, the forces acting on the wheels and the required geometrical constraints are different at either end of the car.

At the front wheels the priority was to prevent castor wind-off under braking, which compromises stability. Here, where braking and cornering forces are reacted through the tyre contact patch, a solution was adopted which McLaren calls Ground Plane Shear Centre. Subframes on either side carry the wishbones on rigid plane bearings but are mounted to the body by four compliant bushes, each 25 times stiffer radially than axially. These are aligned at tangents to circles which have the middle of the tyre contact patch as their centre.

The castor control of this arrangement is outstanding. Castor wind-off has been measured at 1.02 degrees per g of braking deceleration, whereas the NSX, 928 S and XJ6 measured 2.91, 3.60 and 4.30 deg/g. Toe change under braking and camber change under lateral force are also very small.

[Mean 69]

Hey, nice stuff.

A couple VERY important things to keep in mind is that there really is no "general" case, other than the approximate freq' recommendations from the literature, etc. The main reason is that there are a ton of other variables, mainly in the execution of the suspension system overall. For many cars, the springing medium plays a very important role in providing roll stiffness, and therefore the cars would look "relatively" tightly sprung (high frequency). Cars like the later model Mustang suffer from horrible camber issues due to the Mac-Strut design, so they typically run pretty tight.

In my opinion, and certainly my design approach follows this, it is best to leave the springs as light as you can while never bottoming out the car on a particular course (or street in general). This would translate to lower wheel frequencies, in general. The reason is that the wheels would have a better tendancy to stay on the ground over irregularites, which is a really good thing. Compliment the wheel rate with appropriate roll resistance (stabilzer bars), and be prepared to tune.

When in doubt, consider the layout of your particular car in terms of center of gravity, sprung weight, tire selection, wheelbase, etc, and model after a similar car that works well.

And also, keep in mind that the open whel type cars, and certainl the Can Am-ers (how sweet are they) will get away with higher frequencies for a number of reasons, most specifically the tire selection. Contemporary F1 cars have incredibly taught suspension, but the tires have an enourmous effect on the "combined" suspension systems. Just watch the slo-mo's of Kimi whacking through the chicanes, the tires really move around a bunch. Not so on a street car (hopefully!!!).

Good luck, keep us posted.
Mark

[David Pozzi]

F1 uses the tire for a spring as much as anything, tire deflection, especially vertical height loss when cornering is a big deal on F1 and Indy type cars, - a whole different world. I had a crew chief tell me he ran a car at Indy that had .25" suspension movement! The tires provided another .25".

Another thing is a pro racetrack is very smooth and you can run a stiff setup with little downside, but on a rougher course a stiff car is tough to hang on to.

Probably one thing to figure out early is how much available suspension travel you will have, when braking, then spring the front to keep you off the bump stops. You will certainly hit some bigger bumps when braking and may get into the bumpstops a little at that point.

[Norm Peterson]

. . . the rear ride frequencies lower than the front. This is opposite of everything that I have read so far. I'd be curious to know if the car pitched for and aft over bumps.Relative ride rates sort of depend on the intended usage, and also on the front and rear roll center heights. More competition-oriented cars usually want less lateral load transfer occurring at the drive wheel end, which tends to push the spring compromise toward a higher front frequency in the case of a RWD car.

But for something that you must live with on a daily basis, you want the rear ride frequency to be higher so that after going over a bump or heave in the road the car comes back evenly after one full cycle of suspension motion. You can make up at least some of the difference in lateral load transfer distribution relative to the competition car with different sta-bars without penalizing the ride too much. The ride quality with the rear frequency being higher also tends to remain better over time as the shocks age and lose some of their damping.

Actual ride frequencies to select would depend on what the roads look like in your area and on your own preference for firmness and tolerance for NVH. Just as a data point, my '79 Malibu is something like 1.25 Hz front and 1.45 Hz rear, which works out to about a 58 mph damped flat ride speed with its 108.1" wheelbase. It has a slightly firm but not uncomfortable street ride and can be driven anywhere without second-guessing (it's still too soft for auto-x, though). Damped vibration analysis can also be done in a spreadsheet and a number of results plotted to give some idea about more than just the two frequency values themselves.

Norm