Why are we so oversprung?

Can someone help me explain why we are running spring rates in the 600-1000 lb range for a car with such low wheel rates?

I was shopping around for some new dampers and sprung a conversation with one of the suspension guys. I was targeting 600F/800R rates on my Stohr FB. His initial reaction is that it's oversprung by at least 3 fold. He's thinking at those rates the damper isn't providing much mechanical grip and is behaving more like a roll bar if anything. To the point that changing dampers won't do much as the spring is way to stiff.

So I started searching around and found the formula which correlates spring rate, wheel rate and motion ratio. Based on what that is suggesting, I should be running in the 200-300 lb range.

Can someone set me straight? :confused:

DOWN FORCE!!!


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Arax, JLind is correct. Please find a suspension guy to talk to with some actual experience with the Stohr FB, because this guy sounds like he never progressed beyond vintage, no-downforce cars. As it is, your 600/800 is at the lower end of what people tell me they are running.

Cheers, Stan

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Originally Posted by 99sh

Can someone help me explain why we are running spring rates in the 600-1000 lb range for a car with such low wheel rates?

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I think the question is targeted to learn why the huge difference between the spring rate and wheel rate. If the targeted wheel rate is x, why not run 1:1 motion ration with x pound spring?

With really high motion ratios you get very little displacement at the damper and spring. Maybe a suspension guru can explain the advantages and disadvantages. I have my theories, but they could be all wrong....

MRs of 1:1 or higher ( where the shock moves more than the wheel) allows the shock to do its work easier ( more fluid displacement) than low ratios ( .5:1 as an example) like cars had decades ago.

The high wheel rates are necessary for aero-dependant cars ( especially underbody aero) to keep their downforce fairly constant..

Over the last several years, I have changed motion ratios from .7 to 1 to 1.1 to 1. That is 1 inch of wheel movement to shock movement ranging from .7 to 1.1 inches.

Bottom line, there is an optimum velocity range for a shock. And you need to have a motion ratio that moves the shock in that optimum range. With the introduction of radial tires, I think that shock valving is becoming more critical and getting the motion ratios optimized is becoming a real issue.

As pointed out earlier, The wheel rate for a given spring is what is critical. Looking at the spring alone is not sufficient. As an example, a 1000 front spring on a Zink Z10 sounds like a ton of spring. But that is only 250 pounds per inch at the wheel. And because the tire has a spring rate of something around 700 to 1300 pounds per inch, that 250 pound spring is even lower at the contact patch. Now with a 1:1 motion ratio, a 250 pound spring will give the same wheel rate as a 1000 pound spring on a Z10. But the shock movement for a given displacement of the wheel is 1/2 for the Z10.

A good FB will easily produce its own weight in down force at 150 mph. And a good FB will go 150 mph on many tracks. The question you need to answer is how low to the ground do you want to be at say 125 mph? A 250 pound wheel rate at the front is not a lot for a car that will produce 250 pounds down force on that corner at 150 mph.

Does that explanation muddy the waters enough?

550 / 750 springs with Hoosier bias ply tires on Steve's car was a nice compromise for me.

Stiffness and underbody aero:

Go way back and look up the term "porpoising" on the first F1 ground effects cars - as the cars got up to speed, the suction on the underside would pull them down to the ground, and at some point too close to the ground, that airflow was choked off, causing a loss of that suction, causing the car to rise rapidly, where the airflow on the underside started again, causing the car to get sucked down to the ground, and on, and on, and on. Scared the crap out of the drivers.

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Originally Posted by R. Pare

MRs of 1:1 or higher ( where the shock moves more than the wheel) allows the shock to do its work easier ( more fluid displacement) than low ratios ( .5:1 as an example) like cars had decades ago.

The high wheel rates are necessary for aero-dependant cars ( especially underbody aero) to keep their downforce fairly constant..

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I learned just the opposite, high motion ratios are where you have more wheel travel than shock travel.

I understand why it is desirable to have the shock travel more than the wheel (in types of cars and racing where the shock is designed to handle those velocities).

Since I learned the MR formula with the opposite numerator/denominator than you are suggesting, I also learned that wheel rate = spring rate/MR^2.

Guess it all depends on where we did our learnin' of such things...and making certain the formula we are using when calculating the wheel rate is either multiplying or dividing by the MR^2.

I'm trying to follow, but without seeing real numbers it's somewhat muddy.

I can understand for a really high aero car you'll want enough spring to keep the body at it's ideal height for max effect. How much downforce do we produce say at average track speeds when you're not going flat-out on a back straight?

The math doesn't quite close...for me at least.

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Originally Posted by Daryl DeArman

I learned just the opposite, high motion ratios are where you have more wheel travel than shock travel.

Since I learned the MR formula with the opposite numerator/denominator than you are suggesting, I also learned that wheel rate = spring rate/MR^2.

Guess it all depends on where we did our learnin' of such things...and making certain the formula we are using when calculating the wheel rate is either multiplying or dividing by the MR^2.

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One is velocity ratio and the other is motion ratio. I use motion ratio as wheel movement to shock movement. That is wheel movement to shock movement. The velocity ratio is the opposite. When I rear literature, I don't think there is really a standard for which is which.

"Higher" in MRs refers to the shock travel being "higher" relative to the wheel motion in every arena that I have played in from F1 on down. .5/1 means the shock moves 1/2 inch for every 1 inch of wheel travel ; .75/1 mean 3/4th inch of shock travel to 1 inch of wheel travel; 1/1 mean one for one, etc. Yes, it is sort of a semantics issue, but when using the more commonly used designation, the "higher" the number, the greater the shock travels when compared to the wheel travel.

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Originally Posted by 99sh

I'm trying to follow, but without seeing real numbers it's somewhat muddy.

I can understand for a really high aero car you'll want enough spring to keep the body at it's ideal height for max effect. How much downforce do we produce say at average track speeds when you're not going flat-out on a back straight?

The math doesn't quite close...for me at least.

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I'd suggest starting with some light reading. Maybe Gillepsie's Fundamentals of Vehicle Dynamics and following up with Miliken's RCVD. All the pertinent math is in both books. Secondly, before knocking the spring rates I'd suggest taking some measurements of your motion ratios, figuring out where the car should be starting for ride frequencies (there's common bandwidths for cars based on aero content) and calculate about where your car should land.

Even if your Stohr has a MR of 1 I bet that 200-300 lb springs are still too light. Based on my experiences with a few different chassis a wheel rate of 200-300 is a tad weak to keep the car off the ground. Now, what the shocks are up to is another discussion, but will be highly dependent on what your spring and wheel rates are followed by the velocity ranges experienced by the car.

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Originally Posted by S Lathrop

I use motion ratio as wheel movement to shock movement. That is wheel movement to shock movement.

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I'm with you. Larger wheel movement to shock movement produces a MR greater than 1.

But then:

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Originally Posted by R. Pare

MRs of 1:1 or higher ( where the shock moves more than the wheel)

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Carroll Smith's books and Kent Fisk's program all teach it the same way I am referring. Many of the FSAE documents show the same way Richard is speaking to.

I'm guessing there's no right way. As long as you are consistent when applying the various formulae.

Does anyone have any contacts for someone who's intimately familiar with the Stohrs?

The two shops I reached out to so far can figure out what I have by dynoing my current shocks, but would rather not reverse engineer and have a valving setup in mind up front.

Get a hold of Stan at Stohr or Kevin MItz. They will get you pointed in the right direction.

Something else to consider is that the STOHR F1000 (FB) does not have any type of anti roll bar front or rear. Going to a lower spring rate is going to change the roll rate considerably.

Quote:

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Originally Posted by R. Pare

Stiffness and underbody aero:

Go way back and look up the term "porpoising" on the first F1 ground effects cars - as the cars got up to speed, the suction on the underside would pull them down to the ground, and at some point too close to the ground, that airflow was choked off, causing a loss of that suction, causing the car to rise rapidly, where the airflow on the underside started again, causing the car to get sucked down to the ground, and on, and on, and on. Scared the crap out of the drivers.

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Can you give me a rough high and low ride height range for maintaining the aero on an flat bottomed sports racer with a rear defuser?

I always set ride height as track dependent. I set it so I just nicked the rub strips at the hardest braking zone with the tires up to temp. I kept records for set-ups for all tracks. Preset the baseline set-up in the shop before loading in the trailer. Then fine-tuned during practice.
The out-lap was always alarming, as things rubbed until the tires warmed up. :) Of course I was always using springs based on the philosophy that "stiffer was faster". :)

Maintaining the proper rake is important. Shocks can help with that.

It can get complicated in a hurry. Some cars have suspensions that get ugly outside a certain range of motion. Drop some older cars too low and the suspension gets all wonky.

Quote:

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Originally Posted by R. Pare

Stiffness and underbody aero:

Go way back and look up the term "porpoising" on the first F1 ground effects cars - as the cars got up to speed, the suction on the underside would pull them down to the ground, and at some point too close to the ground, that airflow was choked off, causing a loss of that suction, causing the car to rise rapidly, where the airflow on the underside started again, causing the car to get sucked down to the ground, and on, and on, and on. Scared the crap out of the drivers.

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Archaic thread bump, but a timely post for 2022.

Biggest message from this thread is don't listen to arrogant outsiders. ;)