Rising rate and motion ratios

[Christopher Crowe]

Is there any consensus on a logical starting point for the motion-ratio as applied to a Penske 8760 damper? Will the Penske's happily tolerate 1:1?

Also, on a high-nose car with some downforce: Any thoughts on a good (staring point, estimate-guestimate) for a rising spring-rate curve? How many quarter inches of travel before the spring rate should begin to increase? How progressive should the increase be? What percentage of spring-rate-increase maximum, when nearing the end of usable suspension travel?

Clearly these are ball-park kinds of questions (unless some kind soul has a plotted chart for a chassis that's been proven to really work!). I'm just looking for some educated opinions on the basic, ball-park starting points in this area.

Thanks much ---

Christopher Crowe

[S Lathrop]

Yes to ratio and forget the rising rate.

[Christopher Crowe]

Thank you, Steve. And I believe you.

Chris Crowe

[R. Pare]

Steve is correct - forget the rising rate. If you need a rising rate, stick with using bump rubbers to get what you want - if you really think you need it once you have the car sorted.

When using any sort of rising rate, you need to match it at both ends of the car, which can get quite tricky unless you are already very versed with exactly what the car does with a linear rate, and even then it can bite you in the butt - even the big boys can have issues keeping the car manageable and with characteristics that the drivers can handle (especially in emergency maneuvers).

[Christopher Crowe]

It's amazing the kind of mental horsepower you can harness on this site -- and for free.

Frankly, I'm glad we can move aread without entering this "black-hole" area of design. I knew it'd be incredibly complex... frankly, the idea of just diving into it gave me that funky, "I'm-not-sure-about-this-at-all" feeling.

Thanks to you too, Richard.

Chris

[R. Pare]

I must have missed something somewhere - just what are you planning on building? Experience?

[R. Pare]

From your PM's, i can see that you are doing it correctly. Good luck on your dream project - you'll never regret doing it, no matter what the exact outcome!

[na94]

Out of curiosity, is it common to run a 3rd spring setup in the rear of FB cars?

If anyone feels like sharing, what is the front/read static weight distribution that seems to work in these cars? With the bike powerplant, what is an achievable CGz? And lastly, how far rear of the cg should the net aero load be? A couple inches, or even more?

[S Lathrop]

Out of curiosity, is it common to run a 3rd spring setup in the rear of FB cars?

If anyone feels like sharing, what is the front/read static weight distribution that seems to work in these cars? With the bike powerplant, what is an achievable CGz? And lastly, how far rear of the cg should the net aero load be? A couple inches, or even more?

Third springs were developed in cars that had aero loadings that were 2 or more times the static weight of the car. It was a way of balancing the spring rates necessary for good mechanical grip and spring rates necessary to keep the aero platform stable and the car off the ground. FB does not seem to need that level of down force.

For weight distribution, I know what my car is but it varies a lot with driver weight and the placement of the balast. Our car can make weight with a 240 lb driver so we might have a lot of balast in most cars. The range is between 50 and 60 percent on the rear with most cars about in the middle. I don't think there is a magical number. The height is largely determined by the driver. We are using steel skid plates under the car as ballast so the CG for a light driver will be quite low. The Honda and Yamaha engines are better than the Suzuki for CG height but the difference is not enough to get excited about.

I have not tried to calculate the aero loads but my guess is that some cars might make the car weight at 150 mph. I don't think we have the power to use that much down force. The down side of this is that we may have to have several aero packages for the different tracks we run. The same is true for wheels and tires.

In short FB is a class that rewards good driving and engineering.

[na94]

Thanks for the info.

I've notice that a few FB chassis (like the speads) are optimized to have very stable instant centers, but very poor camber compensation, whereas the majority of chassis actually have decent compensation at the cost of what looks like a bit of roll center migration. Is this observation correct?

Also, is it common to run any kind of anti-dive/squat with these chassis? My understanding is that anti-forces are not really a necessity with open-wheeled cars, but I have noticed that my old swift has a little bit dialed in. Any comments on this?

Thanks.

[S Lathrop]

Thanks for the info.

I've notice that a few FB chassis (like the speads) are optimized to have very stable instant centers, but very poor camber compensation, whereas the majority of chassis actually have decent compensation at the cost of what looks like a bit of roll center migration. Is this observation correct?

Also, is it common to run any kind of anti-dive/squat with these chassis? My understanding is that anti-forces are not really a necessity with open-wheeled cars, but I have noticed that my old swift has a little bit dialed in. Any comments on this?

Thanks.

There is so little chassis movement during cornering that a lot of the issues we worried about years ago are no longer issues. What is important about any geometry today is how the tires are loaded during a transition event -- cornering, braking, or acceleration. This is also true for anti dive/squat. There is as much roll from tire deflection as there is from chassis movement. It is the combination of all the suspension variables that really counts when you develop a car setup.