Geo

OK, lots of good and bad answers here.

First of all some simple corrections.

1. The books you want to read are by Carroll Smith, not Carroll Shelby.

2. Body lean has zero impact on load transfer (read, the inside wheels will have the exact same load).

As you can imagine from the answers here, the answer is rather complex. It can also be something of a holy war depending upon how passionate each side is in their position (which usually is being jaded by certain personally pre-conceived parameters in the discussion).

I'd go with optimized as an answer, but there really is no such thing. Some people go with light springs and heavy bars and others go with heavy bars and lighter springs. Heavier bars and heavier springs both improve handling by speeding up load transfer. This makes the car feel less like it's wallowing in transitions. This is more important in autocross than road racing and why I always recommend heavy bars on an autocross car - it helps greatly with transitions which is totally what autocross is about.

It used to be the whole equation was keeping the tire planted on the road. But, there is more going on typically and that can alter the equation. The lastest thinking in Speed Touring Cars is to use very stiff springs (ungodly stiff) to absolutely minimize changes in camber. They trade off compliance with bumps for avoiding nasty parts of the camber curve. This is especially important on strut suspension cars as struts get weird at exteremes of the travel and these cars are set up at the extreme while stationary.

I personally am a stiff spring/softer swaybar guy. To me, the swaybars are just a tuning device and not for altering stiffness of the chassis. I believe in letting each component do it's job. I also believe in controlling the camber change. This is a new one for me, but I can really see the benefits and I'm going to set my car stiffer than I would have.

In the past, in theory I would shoot for the stiffest spring that would keep the tire planted. This meant softening up enough to do so. With camber control being a bigger concern, the spring choice may not always keep the tire planted.

In the end, there is no single correct answer.

Which is better or faster? That depends upon who is doing the driving and what they prefer I'd say. Most people do tend to gravitate towards stiffer because they don't like the feel of the body moving around. But it's not always fastest.

iloveporsches

Engineering would be so much easier if there were....

Good call on Smith/ Shelby, I always mix the names up when I reference the "To Win" series.

That's true, but you set your design goals and engineer around them. Pick appropriate wheel rates based on what you want the car to do, and design your suspension to meet that. Obviously, every car owner isn't going to be calculating exact wheel rates and everything, but I get sick of seeing the "Stiffer=better=faster" being thrown around when it simply isn't true.

Yep, load transfer is simply a statics problem. You can have an infinitely rigid suspension (ie no roll) and you'll still get the same load transfer as a very lightly sprung car. This is covered in Milliken, maybe Smith too.

Manning

I'm still confused from the argument I had a month ago about this and these two comments don't help much. If body lean has zero impact on load transfer what difference does it make then if you have stiffer springs/bars or not. Aren't both of those things controlling body lean/sway/roll?

Mighty Shilling

and if the inside wheels have the exact same load, how come when going around a corner really fast while there was a photographer near by, the picture showed me lifting a wheel?

Serge944

There is nothing definite about suspension tuning and nearly everything has exceptions.

The guy does have a point - you don't want too stiff of a suspension on your average city road. I really have a hard time believing your konis set on full hard will do as well on an imperfect road as compared to a softer setting. Especially since konis on full hard are valved incredibly high.

Also, body lean does have an affect on load transfer... When you enter a corner, the center of gravity shifts to the outside of the turn.

The shorter the distance is from th center of gravity (at ground level) to the tires, the more weight is downloaded. Therefore, body lean, does affect load.

Why do you think wider is better?

Mighty Shilling

they do a pretty decent job of road upkeep on the highways of Colorado

iloveporsches

Weight transfer has nothing to do with spring rates thought.

(delta)W=(WAyh)/t, where W is the initial wheel load, Ay is lateral acceleration/32.2 ("g's"), h is height of CG, and t is track width.

So yes, a wider track width results in less weight transfer, but weight transfer is not affected by body roll. Its simply a function of static measurements and the centrifugal force, WAy. Milliken goes over this much better than I can.

Serge944

But doesn't H increase with lean? Wouldn't that increase the weight transfer in the above equation?

iloveporsches

If anything it would decrease slightly, because H would drop a bit. But we're talking extremely small amounts, since the CM is usually located right near the roll axis. Enough where the change is considered zero, at least by Milliken and my vehicle dynamics professor. There's so many other "fudge factors" with vehicle dynamics that tiny, tiny changes in H due to body roll, if any actually occurs, can be ignored.

Edit: For instance, if our CM was located 2' above the roll axis (these are purely guesses, I have no idea where the CM or roll axis for a 944 is), and the car rolled 5*, which is a pretty good amount, the change in height would only be .0076'. If the CM is even closer to the roll axis, which it probably is, it's even less.

Mighty Shilling

Yeah, Like I said before, the Colorado Highways are generally kept up really well... and I do the majority of my driving on the highway.

Also, I do a great deal of track driving, DE's, track days, etc, and it would be tiring to go underneath the car all the time to swap out the rear shocks... so I leave em full until winter, then tone em back down to soft.

I don't see that as being that hard to beleive...

Geo

Pretty crazy concepts, huh guys? You ask a simple question, you get anything but a simple answer.

To the question of what do stiff springs and swaybars actually do, they speed up the load transfer. You can alter the cornering attitude by speeding up each end at a different rate.

GlenL

Yeah, that's the deal, George.

I've been mulling this over recently in terms of dynamic response. Should get the old differential equations book out, but I sold it.

Looking at each corner independently it's a fairly simple system with two stages of spring-mass-damper in a row.

One way to view it is by looking for the harmonic frequency and the associated time constant. The softer the spring, the lower the frequency and the longer the time constant. Thi smooths the ride. A stiffer spring and there's higher frequncy oscillation and a lower time constant. This is why potholes bash the stiff car, because the position change of the wheel is transfered quickly to the body.

Tracksters like a stiff suspension because the vehicle reacts quickly to the steering inputs and the body doesn't flop around. When the wheels turn, the body doesn't roll over before the CG starts moving in an arc. The other part is that the CG doesn't wobble back and forth, or at least it does it really fast.

I've been poking around the Web looking for some simple step-response simulators. That'd show the effects of changing damping and springs.

As to sways versus springs, it is a balance and choice. I'm going with stiffer sways and mild springs as I think that will give good handling response while avoiding a too rough ride. Still streeting the 944.

I'm sure someone with MatLab or such could put the whole thing together. One thing I'd like is to has explicit measures of damping rates.

iloveporsches

Another note:

Cranking up your Koni's to full stiff isn't the greatest either. You're going to over-damp your suspension. Its better than under-damping, in that it will settle down reasonably quickly without a bunch of oscilations, but it damps out the spring slower than a critically damped spring. Look at the graphs about half way down the following website. The first one is critically damped, the second is over-damped, and the third is under-damped. Notice that the over-damped system has a longer response time.

http://tutorial.math.lamar.edu/AllBr...Vibrations.asp

My professor made a simulation about a month ago, I think using matlab, to simulate damping responses. I'll ask him about it, and see if he can send it to me. Not sure what to do with it from their though, to give you guys something to see.

Geo

Ah yes....

A shock dyno.

http://www.roehrigengineering.com/

One of my best friends does their software.

Danno

"Also, body lean does have an affect on load transfer... When you enter a corner, the center of gravity shifts to the outside of the turn.

The shorter the distance is from th center of gravity (at ground level) to the tires, the more weight is downloaded. Therefore, body lean, does affect load."

Yes, there is more than one factor that contributes to total load-transfer:

1. load-transfer due to cornering-G
2. load-transfer due to C.O.G. shift
3. load-transfer due to suspension-geometry (instantaneous roll-centers, C.O.G height, etc.)

Out of all of these, by far the greatest contributor is 1. load-transfer due to cornering-G. Check out the "Physics of Racing" series... available on my 951 RacerX website. The 2nd factor, load-transfer due to C.O.G. shift accounts for less than 10% of the total lateral weight-transfer (COG moves maybe 2", take SIN(f) of lean-angle and figure out that transfer amount).

I'm not going to re-write the excellent books that are out there on vehicle dynamics. These are the minimum recommended for your library (especially the last one):

How to Make Your Car Handle - Fred Puhn
Performance Handling - Don Alexander
Tune to Win - Carroll Smith
Race Car Vehicle Dynamics - Miliken

A summary of lateral weight-transfer presented here:Rear Sway bar helps a lot!

The basic reason why stiffer suspension on our cars work better than stock is because the geometry was a severe compromise of cost vs. performance. ALL high-performance cars with minimal concessions to cost have double-wishbone suspensions. These are designed and calibrated with the springs and dampers so that as the body leans, the camber-curve on the suspension cancels out the lean and the tyre stays flat on the ground.

Cars like Ferraris, Lamborghinis, Corvettes, SupraTT, BMW M3 & M5 have double-wishbone suspensions and are able to keep their tyres flat on the ground even with softer spring rates than us. They retain more of the maximum potential grip on the tyres when they lean as well. On our economy-car suspensions, as the body-leans, the tyres get more and more positive camber and the outer edge is overwhelmed while the inner edge lifts off the ground.

The only function of stiffening up the suspension is to keep the tyre flat on the ground (it also introduces some negative effects as well). Since we can't do it by revising suspension components and geometry into a double-wishbone set-up, we can only keep the tyres flat by increasing spring-rates to keep the body-flat. Although the Japanese team campaigning a 968 Turbo in the JGCT series did build their own double-wishbone suspension.

Here's an example of what good suspension design and geometry can get you:



The BMW E46 with its 30% higher weight than me and 40% more weight than the E30 M3 easily outcorners both of us by a wide margin. I'm sliding through the corners as he's pulling away. Note also that the E46 has the MOST body-lean out of all of us. But due to the great suspension, the tyre stays flat on the ground regardless of lean angle. It also has much nicer ride on a 300-mile trip as well... In this last year's OTC, there was an STI pulling 1.55 Gs in the corners on 140-treadwear street tyres !!!