Soft Suspensions

[hardingfv32-1]

We do have a high unsprung weight to sprung weight ratio. There could be something to this, but I have no idea how.

Expand on why Vees need a soft suspension because of high unsprung weight.

Brian

[robert]

How stiff does the front end of a Vee become in a corner when the anti-roll bar is working?

It must get quite stiff in order to resist 100% of the roll force.

So if it does indeed get quite stiff, why not start out stiffer, and us a softer roll bar?

Why suffer the toe changes and increased ride height needed for a car that is soft primarily on the straights?

I'm learning from Brian H. . . . . ask questions more, and reveal less of my opinion . . . .

[robert]

"Robert, the roll axis is completely mechanical and not a predicted thing. Yes, it can move around, but perfectly predictable

When I say mechanical roll center, I am talking about something like a watts link that mechanically locks the roll center at a specific point on a solid axle. For example, a panhard bar may attempt to constrain the RC, it does not do so mechanically. With a panhard bar, the car can actually roll about almost point you choose . . . throw a dart. I don't believe a kinematic RC is mechanical nor necessarily the actual point of roll.

Another thing to consider . . . build a simple 2 dimensional card board model of a swing axle set up. Use some thumb tacks to locate pivot points. You can get the chassis to roll about any point you choose . . . until you pin the tire's contact patch, at which point the kinematic RC becomes constrained to a point. In fact, if you pin both contact patches, no suspension movement is possible. If you layout a "normal" independent suspension for a kinematic RC above a realistic CG, that suspension will have a ton of scrub that will resist suspension movement, and probably have a horrible ride even with soft springs.

My simple card stock models suggest to me that with any independent suspension design, if the contact patch is not "stuck", the RC is not defined. That suggests to me that the only valid roll center is force based, (no force at the contact patch, no roll) although it is far beyond me to determine where a force based RC might be at any single moment in time.

The following quotes from Wm. C. Mitchell state what I am not capable of expressing, and I believe them factual.

FAP is force application point. I selected these comments from 6 pages of text . . . . although I don't believe them out of context, they seem to stand on their own.

The FAPs and the Force-Based Roll Center are important. The Kinematic Roll Center is of little value.

Myth #3: The distance from the kinematic roll center to the CG is the moment arm.

If you believe in the kinematic roll center it is easy to consider the distance to the CG as a moment arm. A long distance from KRC to CG produces lots of chassis roll. The problem is that this line is not perpendicular to the lateral force. The FAPs, defined as being under the CG, yield a moment arm perpendicular to the lateral force. The distance from FAP to CG is a valid moment arm.

Myth #4 - The Chassis Rolls about the Roll Axis

The chassis moves in reaction to a lateral force: it does not roll about a point or axis. The movement includes chassis roll as well as vertical movement. The kinematic roll center concept clearly describes the roll yet neglects jacking force, which might be small for symmetric cases where both tires contribute equal lateral force. But for racing cars the majority of the lateral force comes from the outside tire. In some applications the inside tire may even be off the ground

The Roll Center is important but you have to separate reality from myth. Kinematics are easy to visualize and that aids understanding. But forces move the race car.


If you look at the other two sites listed, you will find two conflicting ways to predict the ( kinematic) roll center with swing axles. So build a simple model, check it out first hand.

I don't advocate solid suspension, but on a Vee it would permit tires to be run at as optimal (fixed) camber as possible, with no scrub due to suspension travel, the car could be run very low, the weight saved could be replaced with belly pan height ballast, no toe changes, no camber changes, and zero roll would attempt to maintain corner weights, permitting the four tires to follow the track. Unlike a non zero roll car with solid suspension that would suffer extreme changes to load distribution due to uneven track surface . . . one wheel would likely be always lightly loaded or off the track, even in a straight line.

I have complete faith in Mr. Mitchell's comments, although it is highly possible I don't understand what he states. I choose to basically ignore kinematic roll centers . . . . you can't change them on a Vee, and they may have little value for predicting the axis about which the car truly rolls.

I've obviously been a bit bored the last few days, but I'll be away next week so you will be spared. Excuse the errors of my posts, they will no doubt be corrected.

Chris, thanks for your links. When I went to save Mitchell's article I discovered I had saved it before, back in 2007.
.

[hardingfv32-1]

I ask questions because I'm looking for answers. I'm looking for other peoples thoughts that might challenge my assumptions.

Brian

[jpetillo]

Robert, I'll check out the Mitchell paper. Whether we agree with any one theory or not, at least the paper will provide a basis for discussion. I did see that the first of Chris' links showed the conventional definition, the second was a crap shoot, and then Mithcell's paper that I'll read later.

John

[jpetillo]

We do have a high unsprung weight to sprung weight ratio. There could be something to this, but I have no idea how.

Expand on why Vees need a soft suspension because of high unsprung weight.

Brian

Brian, yes, we need to get back onto topic with this thread. Brian M. has me thinking with his comments. This is an excellent question, but I need to let it rattle around in my head until it gels. John

[robert]

Brian M. I agree with asking questions . . . I screw up by trying to explain myself. I suspect questions get more responses as well.

A car with high unsprung weight has less sprung weight, and requires less spring to support it?

Maybe the inertia of that unsprung stuff needs more damping?

Perhaps higher unsprung weight needs stiffer springs and stiffer shocks to prevent (reduce the affect of) the inertia of all that junk in motion from impeding compliance.

Is compliance maintaining vertical load to the highest degree possible?

[brian]

Brian H. my feeling is that with all that unsprung weight our cars tend to be insensitive to track surface irregularities. If combined with a stiff suspension or high spring rate, the car tends to skip over stuff rather than track.

Robert, I have little concern regarding toe change since I have designed virtually all toe change out of my car. Between geometery and droop stops, my car has none.

When looking at my design issues, I try not to isolate any one event and concentrate generally on the overall grip and comfort of the car. I am more concerned with how a car feels on turn in and not so much once I'm in a turn so frankly, I have never thought about spring rates coming from a twisted roll bar.

Given the limitations of our cars and rules, I'm not too concerned about theory since I can't really change much about the car but I can refine it's behavior.

Maybe it's an age issue, but I prefer to control the rate at which things happen rather than limit or restrain them.

[jpetillo]

Brian M, I think that larger unsprung weight makes the car more sensitive to track irregularities. Once that tire starts to go up from hitting an irregularity, then it doesn't want to stop due to its inertia. I think the natural tendency is to increase the spring rate to keep the tire from moving too far. When you stiffen that spring rate the rest of the car will be affected more - it will move in response to keeping the tire down.

Can you tell me about what rear suspensions you're using that stops toe changes? I know that leading links like the Caracals and Mysterians have can do it. Are you running a leading link or trailing?

I agree with you that turn-in is more important than the steady-state part of the corner. I believe this is what Brian H started with.

About theory, your experience and intuition can get you very far, but a little theory may help explain what your intuition is telling you and give you a basis for new ideas.

John

[jpetillo]

Robert, with regard to your e-mail on roll centers, good points. I think you hit the nail on the head. You do have to define any instantaneous suspension movement in terms of it's current configuration and forces, including their location. This is at the heart of both the kinematic or the forced-based approach - you can't do either without defining the forces. You're right that some people don't think of it that way, and some don't think of it past the instantaneous roll center idea from the center of both tires. But people should include forces with either approach and you'll get the same answer if you do it right. With the cardboard models, it's almost impossible to apply similar forces to see how the suspension would move. In your example, once you pinned the tires and the suspension doesn't move, I think you'll find that most suspensions will not move, except perhaps leading arm (like our fronts) or a solid rear axle housing. If you solve the problem of a suspension being allowed to move when you pin down the tires, then I believe that you'll also have solved the jacking problem - there will be none. That our swing axle suspension do this scrubbing more than the better suspensions do, without thinking about it, I'd find that hard to disagree with. Thanks for bringing up the scrubbing issue - it's a good concept for us to keep in mind.

I read through Mitchell's writeup. Thanks to you and Chris for the tip. However, this is nothing new. He's just describing what Chris and I mentioned a few times - that what is now termed the Kinematic Roll Center will move and not stay in line with the CG. This is what Mitchell is going after with the forced-based approach. Mitchell agreed in several places in his writeup that with no roll that the two (forced-based & kinematic) are the same. In our case, when the roll is very small, then we're pretty much talking about the same thing and the centers are pretty close to one another. Perhaps close enough for this discussion that Brian H started.

Anyway, it seems that Mitchell seems to know what he's talking about. His summary note was short and lacking details, but I could read between the lines. Do you know of a more detailed writeup? I'd like to read it. Have a good trip. John

[brian]

John, I am using the common rear, forward facing trailing arms but after talking to some very smart folks, I spent a lot of time locating the rear pickup points. Virtually every car I look at has issues with the rear pickup point. I started out trying to reduce dive under braking but learned much more. I placed my car on the ground with a string surrounding all 4 wheels. After measuring the static state, I jacked the car around to simulate roll, dive and squat remeasuring all the toe changes. By moving the pickup points up and down and in and out, I eliminated roll steer and virtually all the ride height toe changes.

I only wish I could understand the technical writing and theories better. It coulda saved me a lot of time. But I deal with my limitations any way I can. Asking questions in person really helps, just ask Bruce and Brian H, I wear them out when we visit. Reading these discusssion helps a bit too. In college, I read aloud into a tape recorder, remember those?, then listened to the tapes. It still took me 4 trys to pass Calculus.

[jpetillo]

Brian M., that's excellent! The relative ease of locating/moving the pickup points is clearly an advantage to the leading arm suspension. Do you know if anyone has tried the same with the trailing arm suspension? I know it's difficult, but not impossible, it seems.

About theory vs. experience, I wish I had the level of experience you and several other folks here have. Um, yes I remember tape recorders! John

[robert]

John,

It seems to me that the nice thing about force based thinking is that one need not consider instant centers out in left field. A lateral force at the contact patch is transmitted to the frame though any path available . . . springs, push rods, pull rods,, shocks, suspension links, even tie rods.

Only the suspension linkage locating the wheels are considered for kinematic roll centers, which leaves out other paths available to oppose the lateral force at the contact patch. In my mind that means the kinematic study is too simple an approach, and can be far from the truth.

My comments about pinning the contact patch on a cardboard model was to suggest that a lateral force there was needed before a kinematic roll center could form. Pinning was my simplistic effort to represent a lateral force.

There is a really good book, but pricey, An introduction to Race Car Engineering, by Warren J. Rowley. Rowely and Mitchell ( who writes WinGeo software) seem to collaborate.

Bruce,

What was your Dad's conclusion about the reverse roll car? It seems to me that in order for an independent suspension to obtain a high enough roll axis to place it above the CG, would require a design that would produce tons of scrub. My take is that the high degree of scrub would make for a harsh ride. I'm also thinking that reverse roll will actually increase load transfer, not reduce it as the designers claimed.
.

[rphillips]

I have been thinking about this discussion and I'm wondering what is the best way to tell (handling wise) if a rear spring is too soft. Obviously if it is way too soft then it won't support the car but what are the tell tale handling characteristics if it is just a little too soft?

Ray

[brian]

If I sense that the car is moving up and down during the turn, or waffling, I think it's too soft. If the car is taking a set then loosing grip and then regaining grip then repeating the cycle, it may be too soft. I start with stiffening the compression setting on the shock to see if I can stop the hunting. If that doesn't work, I'll go up a step on the spring. You can try increasing the pressure on the shock as well. Some may think this won't have an effect, but it's an easy thing to do and every car is different. My experience tells me it takes about a 15# change in a spring to feel the difference in a vee. Many of us have extra springs so if you can borrow a spring to test, you'll save yourself some money.

[remmers]

here's a question. what is it about a softer suspension that makes it better in the rain? even on a very smooth racetrack?

[DanRemmers]

here's a question. what is it about a softer suspension that makes it better in the rain? even on a very smooth racetrack?

I think it has to do with the smoothness of the lateral forces. If you look at the lateral g-forces in a turn, any bump can cause a spike. A spike in lateral g's can push the tire over the limit of cohesion. In the rain, the margin of error at the limit of cohesion (static to dynamic friction) is very small. A softer suspension will dampen the spikes in lateral g's, as will a driver who is smooth with the throttle, brakes, and steering. And that allows a car to be faster.

[hardingfv32-1]

So why not run "rain" soft all the time? The advantages you state are valid in the dry too.

Brian

[tiagosantos]

Pros and cons, right? To my inexperienced self, it sort of goes back to something that was discussed earlier in this thread - soft is easier to drive, stiff is faster if you can handle it (please let's not argue this, I'm just over simplifying to make a point ). In the rain, you need all the help you can get to take the car around the track safely. The margins are a lot smaller, mistakes end up in fun but possibly expensive results. So making it softer might just be a way to slow everything down a touch, increase the margins, etc.

If we sort of agree that a stiffer setup is faster if the driver can react quickly enough to the cars feedback, then maybe we can agree that it would also work in the rain, but no one can react as quickly as you'd need in the rain hehe.

[hardingfv32-1]

Yes, we need all the help we can get in the rain, but why not use these same helpful attributes in the dry? I assume that most of us need almost as much help in the dry?

Brian

[remmers]

which is kinda what i was getting at, as far as i'm aware, the car's handling characteristics are the same regardless of the level of grip, it just happens to occur either at faster or slower speeds, so what holds true in the rain should theoretically hold true in the dry as well, yes?

[tiagosantos]

Well Brian, I thought you had some good points earlier in the thread as to why a soft(er) suspension isn't ideal in the dry - higher ride height necessary, more time in transition and less time in steady(ish) state, etc..

This may sound silly, but this is a big deal in racing simulators as well. On something like iRacing, you'll see the so called "aliens" driving setups so stiff and so far from reality that hardly anyone can handle them. But they can, somehow, and they're incredibly fast. Watching replays of how they drive and how fast they react to incredibly small changes is very impressive. The reward for being able to handle those setups is lap times 2-3 seconds faster than anyone else. Those of us who are just as good as everyone else, drive softer, more realistic setups that mean we can finish every lap instead of taking a gamble on every turn.

So with that said (and I have no scientific proof of any of this, it's just my opinion!), I think the softer rain setups are merely a compromise. Softer = easier to drive, which you need in the rain. In the dry, you can get away with a faster, more responsive setup.. The softer setups would probably still be easier to drive, just not as fast in the end. Or that's how it works in my mind!

[DanRemmers]

So why not run "rain" soft all the time? The advantages you state are valid in the dry too.

Brian

I think the main reason is speed.

The faster you go, the stiffer the suspension needs to be. (A stiffer spring has a higher frequency, so it reacts faster to an input.) In the rain, lap times are slower, so a softer suspension is indicated. If a wet lap is 20% slower than a dry lap, then 20% softer springs may be a good starting point. Of course, you would have to look at individual corners, exit speeds, road surface, etc. to get a better idea.

[jpetillo]

I think the main reason is speed.

The faster you go, the stiffer the suspension needs to be. (A stiffer spring has a higher frequency, so it reacts faster to an input.) In the rain, lap times are slower, so a softer suspension is indicated. If a wet lap is 20% slower than a dry lap, then 20% softer springs may be a good starting point. Of course, you would have to look at individual corners, exit speeds, road surface, etc. to get a better idea.

Interesting point. You can also decrease the frequency by increasing the damping. But, we see people decreasing damping in the rain. What are your thoughts on that? John

[CitationFV21]

I have a 5K in the morning so here is a quick reply:

In the rain all forces are less - less acceleration, less braking, less cornering. In a proper race car, the suspension geometry is set up for maximum cornering in the dry. Since there are less forces, you soften the suspension to get the same geometry as you would in the dry, just as you adjust your brake to get the proper balance front to rear. Now, on a proper race car, you may have a completely different alignment based on what works for your car. On a Vee, you don't have the time or adjustments to do this.

So what can you do on a Vee? Since we do not have adjustable anti-roll bars, the quickest thing is to soften the rear shock (rebound and bump - if we can.) The front end is pretty soft as is and unless we run in a perpetual monsoon season, we are not likely to change the anti-roll bar.

What does the softer suspension do? It allow the tires to keep closer connection to the racing surface. More roll might allow more weight transfer to the outside tires resulting in more cornering, (Remember lower cornering force) It also gives the driver more feedback, allowing him to keep the car closer to the limit. The main thing is to decrease oversteer and make the car more neutral or even give a little push (NASCAR talk for understeer)

In the rain you could probably stand the rear tires up straighter. This would give you a larger patch for braking and accelerating. Again, the fronts are not that far off (even at -1.5 degree - compared to the rears).

I will go back to my mantra - as soft as possible, as stiff as necessary. Even F1 guys can drive a dry setup in the rain, and many a time I have been out on slicks in the pouring rain. A stiff setup is not necessarily slower, just more on the edge and more the chance of a mistake.

ChrisZ