Why Are Suspensions So Soft? – Wheel Rate, Motion Ratio and Frequency

by Rob Schermerhorn on May 3, 2012

“Why are Ferrari’s so “soft”?” is a question I get all the time, given my experience with the beautiful red cars…

This term is relative, IMO. Ferrari’s are no softer than most other road going cars, even other sporting automobiles. Suspension design is all about compromise with a road car. The environments change, the market is worldwide. Ferrari determined that this is the best solution, and I agree. Most Ferrari’s are comfortable, even on long drives (and I’ve driven them cross-country), and sporting enough to be better in many ways than the competition. Ferrari improves on the average sporting car with a bit more suspension damping. As an owner, overall you are satisfied. But this compromise in design opens the door for improvements if you (the owner/driver) have interests outside Ferrari’s average design parameters, like track events or actual competition on the racetrack.

Background on suspension

Most racers and many car enthusiast know the spring rates on their car, but very few know the wheel rates and that’s the number the tire cares about… The difference between wheel rate and spring rate is the wheel rate (also in lb/in) is what the driver feels and the tires deal with while driving. Wheel rate takes the geometry of the suspension into the equation (motion ratio) and makes it easy to compare different cars. A better comparison is ride frequency, but I’ll present one thing at a time.

Motion ratio is simple to find. Jack the spindle up one inch and measure how far the lower spring perch has moved relative to the upper spring perch. On a Ferrari 348 rear suspension, the spring perch will move an average of 0.85 inches for every one-inch the wheel moves over the range of full droop to full bump. So, the motion ratio for 348 rear is 0.85 (you may see equations that use the inverse of this number, Carroll Smith’s equations would come up with 1.18 for the ratio).

With this knowledge, one discovers that many Ferrari’s are designed with wheel rates as low as 100 LBf/in, which is the Testarossa rear double shock suspension and 308 series. So, your 200 LBf/in spring in the 308 drops with the motion ratio to 98 LBf/in. The equation is WR = SR(MR)2, WR is wheel rate in LBf/in, SR is the spring constant (also known as k) in LBf/in, and MR is motion ratio where MR is stated as <change in>shock position/<change in>wheel position.

Ferrari 360 ChallengeFor reference, the front wheel rate of a F355 Challenge car is 1,078 LBf/in with a 2200 LBf/in spring! This is very uncomfortable on the street, plus this system utilizes a tender spring to take up slack when the suspension goes full droop, and comes crashing down on this tender spring with every slight roadway undulation. But, the on-track race car’s vehicle dynamics are superior to the road car’s.

The F355 Challenge factory settings confirm that the factory knows increasing front roll stiffness increases grip and drivability on the racetrack with race tires.

So now we are into changing springs, and increasing spring rates involves increasing damper (shocks) forces. Here is where consulting with an experienced team or engineer will pay off by shortening or eliminating your development time. Shoot me an email via our contact page and let me know what you’re interested in, what your goals are and I’m sure I can help you!

{ 6 comments… read them below or add one }

Larry Snover April 19, 2013 at 2:39 pm

Rob, Very interesting info. Sharing more seat of the pants experiences that are, I believe, relevant. Many racers, particularly in production cars, run their cars too stiff in the belief that is the Hot Set Up. Actually its called suspension for a reason and there should be some weight transfer {controlled} and jounce/rebound other than provided by the deflection of the tire under load. Some years ago a friend asked me to travel to a race with him to try and figure out why the handling of his production class Porsche had turned diabolical. It would understeer to sudden oversteer and vice versa. He claimed to have LOOKED AT EVERYTHING. He always had a lot of pit crew {? hangers on/helpers} which actually was positive & negative. After 1st practice and his wide eyed complaints {he was pretty fast but not very oversteer/understeer knowledgable} I told the minions {WOW I never had minions…by design,} to put the car on jack stands. I slid under and started looking…and disconnected the shocks. After inspecting everything I got a Jack and started compressing the shocks. The dam things {he and his friends were running their cars like go carts} were so stiff, they would walk around each others cars {different brands} and press on the fenders and giggle about how great it was if the fender/suspension didn’t move, that the only way to compress the shock was with a Jack. THE RIGHT REAR SHOCK, rear engined car, WAS FROZEN! But the whole car was set up so stiff you could not tell. No wonder the car was diabolical. That tire’s distortion under load was the only compliance on that corner causing it to get stupid in different ways when turning in different directions. I had him go to the track announcer and ask for anybody with a regular car of the same type to come to his pit. A guy came who’s car had NEW, quality stock shocks. He was actually delighted to let us use them for practice and the race! We just used the rear ones. My friend got a second in class! Another time, I was running an SCCA Showroom Stock car {& a Formula C car} and was leading in points for the season with 4 lap records. My car had a factory standard rear anti-roll bar but the next years cars didn’t. Some people said that the reason I was fast was THAT anti-roll bar. They lobbied the SCCA and got a ruling with the season 2/3 finished that required me to remove the bar which I learned about just before 1st practice from the head of tech insp. Certainly I had to drive the car differently in certain corners. I was able to put the car on the pole, win, and break my own lap record. I AM NOT BRAGGING in either situation!!! All too often, I have learned, in racing, we over look the basics and put too much emphasis on the Hot Set Up technically as opposed to – how is it supposed to work originally…technically. And we often forget about really understanding vehicle dynamics and how we can use, in conjunction, a very solid comprehension of available driving technques to get the most out of the car. Regards, Larry Snover

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Rob Schermerhorn April 22, 2013 at 11:34 am

Thanks for your insight and sharing your experience Larry!

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Rob Schermerhorn May 14, 2013 at 1:30 pm

Had a question recently as to why the motion ratio term is squared in the wheel rate formula… It is due to the fact that motion ratio reduces both the force and the displacement of the spring… See Milliken & Milliken, Race Car Vehicle Dynamics, 1995, SAE.

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steve September 8, 2013 at 2:54 am

Hi Rob,
I was reading a chap’s workings (Rennlist) for upgrading his Porsche 914 front end to dual springs. Set up with tender spring below: McPherson strut.
Would it be possible to perform a similar set up to suit the A arms on my Datsun Roadster for better road handling?
Regards,
Steve

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Rob Schermerhorn September 8, 2013 at 7:10 am

Hi Steve, yes, your car can be fitted with with a higher spring rate plus tender spring. The tender spring (as opposed to a ‘normal’ suspension spring) only purpose is to locate the main spring when the suspension is at full droop. Tender springs are required when the main spring rate is quite high, where the sprung weight only compresses the main spring a small amount and you must install a relatively short main spring to achieve static ride height.

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Rob Schermerhorn September 10, 2013 at 11:12 am

We’re looking for motion ratio data on any vehicle you’ve measured, car, motorcycle, truck, etc. as we’re building a reference database to publish, benefiting all of us curious enough to take these measurements and turn this knowledge into something powerful, be it lower lap times, improved handling, more fun with your project or what have you! Can you help? Please submit your data via the “Contact Me” link at the top of our web site. Thank you all!!

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