MM Road & Track Springs

[Houstonnw]

...coil springs work by reducing weight transfer the stiffer they get. The stiffer your springs are side to side the less body roll right?...

That's not quite correct. If you increase the spring rate at one end of the car you will increase the weight transfer at that end and reduce the grip at that end, hopefully adding grip at the other end. You can balance a car with spring rate just like you can with sway bars. Body roll does not have a lot to do with weight transfer. It is mostly due to track width and CG height.

Or did I misunderstand what you meant?

 

[sheizasosay]

That's not quite correct. If you increase the spring rate at one end of the car you will increase the weight transfer at that end and reduce the grip at that end, hopefully adding grip at the other end. You can balance a car with spring rate just like you can with sway bars. Body roll does not have a lot to do with weight transfer. It is mostly due to track width and CG height.

Or did I misunderstand what you meant?

You got it.

I was wondering if anyone was going to notice the link in post #57. Whiskey was having trouble remembering the proper term to use. That's why I left the link. "A softer spring transfers less weight and promotes more movement of the chassis, at that corner, in all four modes of movement. We are only re-distributing the static weight, so there must be another wheel pair to take up extra weight transfer. "

 

[kcbrown]

You got it.

I was wondering if anyone was going to notice the link in post #52. Whiskey was having trouble remembering the proper term to use. That's why I left the link. "A softer spring transfers less weight and promotes more movement of the chassis, at that corner, in all four modes of movement. We are only re-distributing the static weight, so there must be another wheel pair to take up extra weight transfer. "

Correct me if I'm wrong here, but ... that may be true of the instantaneous transfer, but not of the steady state transfer.

In the steady state, the softer spring on the outside will be compressed more when the car is going through a given corner at a given speed, but the reason it's compressed more is that it is carrying the same load but with a lower rate, so it has to be compressed more in order to absorb the same steady state force. Similarly, the spring on the inside will extend, and the softer one will extend further because it requires more spring movement to relieve the same amount of force. Hence, cars with softer springs tend to lean more in the turns, all else (sway bars, especially) being equal.

Because the stiffer spring requires less movement in order to achieve the same amount of force change, it thus requires less time to reach its steady state during a transition, so the car with stiffer springs will feel more responsive compared with the car with softer springs, unless the dampers on the car with softer springs make up the difference. Dampers can theoretically make a big difference even in the transfer rate, I expect.


So: the springs always transfer the same amount of weight, but they differ in how much movement, and thus time, they take for that transfer to happen. That's because static weight transfer is purely a function of the forces being experienced by the car, which the springs will always counter completely (as they must, as a force which isn't countered completely causes acceleration in the direction of the force) and those forces are independent of the springs being used.

 

[sheizasosay]

I need to get off this iPhone and on a computer to break this up for response, but right off the bat since you've put the scenario to steady-state:

In steady state dampers do mostly nothing. When I say "mostly nothing" I mean "nothing."

Body roll is a result of weight transfer. Chicken and egg thing. I'll get back to it tommorow.

 

[NoTicket]

People need to stop thinking there is such a thing as steady state cornering in any but the most purely theoretical scenario.

Assuming you are cornering on a machined disc with 100% perfect throttle input to maintain exactly the same speed, while not only do your hands stay put in the exact proper position, but your suspension also either does not deflect at all or stays in the exact same deflected position, etc. is not a good basis for thinking about suspension.

That doesn't mean kcbrown didn't have valid points.

Additionally... A pressurized shock will affect wheel rate. Higher nitrogen pressure in the shock will reduce over all body roll given all other 10mil variables stay fixed.

Unless I am wrong. Then everything I said is wrong.

I should make that my signature.

 

[kcbrown]

People need to stop thinking there is such a thing as steady state cornering in any but the most purely theoretical scenario.

Admittedly, "steady state" cornering is a theoretical thing more than anything else, but I would argue that a very large portion of the time you spend cornering is spent near the "steady state", at or near the limits of grip.

However, that doesn't diminish the importance of the transitions, since I expect it's how the suspension handles the transitions that determines the controllability of the car.

Unless I am wrong. Then everything I said is wrong.

I should make that my signature.

Me too.

 

[Norm Peterson]

Steady state for your control lnputs is not the same thing as steady state where instantaneous wheel loading is concerned. Even pavement unevenness and roughness constitute changes in suspension loading, and a softer spring will vary its loading less when that happens. What may feel like steady state from the driver seat is really more of an overall-average-approximation of steady state.


Norm

 

[SoundGuyDave]

Norm, as usual, got it in one... "steady state" is a great model to use to examine suspension motion, but it isn't terribly applicable to the "real world" we find on track. In addition to pavement variation mentioned, take a hard look at the dynamics of a typical late-apex line... Or throttle-steering on a carousel... Factor in trail-braking into the dynamic model...

 

[2008 V6]

Steady state for your control lnputs is not the same thing as steady state where instantaneous wheel loading is concerned. Even pavement unevenness and roughness constitute changes in suspension loading, and a softer spring will vary its loading less when that happens. What may feel like steady state from the driver seat is really more of an overall-average-approximation of steady state.


Norm

++1

 

[2008 V6]

Norm, as usual, got it in one... "steady state" is a great model to use to examine suspension motion, but it isn't terribly applicable to the "real world" we find on track. In addition to pavement variation mentioned, take a hard look at the dynamics of a typical late-apex line... Or throttle-steering on a carousel... Factor in trail-braking into the dynamic model...

+++1

 

[sheizasosay]

Norm, as usual, got it in one... "steady state" is a great model to use to examine suspension motion, but it isn't terribly applicable to the "real world" we find on track. In addition to pavement variation mentioned, take a hard look at the dynamics of a typical late-apex line... Or throttle-steering on a carousel... Factor in trail-braking into the dynamic model...

Steady state for your control lnputs is not the same thing as steady state where instantaneous wheel loading is concerned. Even pavement unevenness and roughness constitute changes in suspension loading, and a softer spring will vary its loading less when that happens. What may feel like steady state from the driver seat is really more of an overall-average-approximation of steady state.


Norm

I'm pretty sure KCbrown has been told that before. I believe "sticks for springs" comes to mind. I believe he was using it to try and remove a lot of factors. Semantics. I got what he was saying when he used the term.

 

[kcbrown]

I'm pretty sure KCbrown has been told that before. I believe "sticks for springs" comes to mind. I believe he was using it to try and remove a lot of factors. Semantics. I got what he was saying when he used the term.

Yep.

Of course there's going to be suspension movement even when you as the driver feel like the car's in a "steady state". That's not really the point. The point was simply that the actual amount of weight transfer won't vary massively due to a spring rate change except perhaps in fast transitions, but that spring change will certainly affect the rate of transfer as well as a bunch of other things (ability of the suspension to remain in contact with the pavement despite imperfections there, speed with which the suspension reacts to changes in inputs, etc.).

In a dynamic situation, the rate of weight transfer will matter a lot, for a number of reasons.

The only reason I responded is that in the engineering world, the precision with which a problem is described matters a lot. Sometimes it makes the difference in whether or not the solution to the problem reveals itself at all. A precise description of the problem can reveal insights that otherwise would remain obscure.

 

[sheizasosay]

In the steady state, the softer spring on the outside will be compressed more when the car is going through a given corner at a given speed, but the reason it's compressed more is that it is carrying the same load but with a lower rate, so it has to be compressed more in order to absorb the same steady state force.
I'm not sure I agree with that. The softer spring on the outside will be compressed according to the forces acted on it through the roll center which varies by it's length or distance from the CG. "softer spring on the outside" sounds to me like "softer of the two outside springs." Just because a spring has a lower rate does not mean it will be compressed more. The CG height is gonna have a play on that. Roll couple and ground track vs centrifugal force I think. It's not like the front and rear roll centers have to be or are the same heigth. You adjust a roll center then you change how much force is going to go through the roll center and act on the CG or the body/sprung weight.



So: the springs always transfer the same amount of weight, but they differ in how much movement, and thus time, they take for that transfer to happen. .

I'm not sure about that last sentence KC. There are three weight transfers (according to Puhn) that happen: weight transfer due to body roll, due to roll center heights and then weight transfer of unsprung weight. I don't know if the springs always transfer the same amount.

 

[kcbrown]

I'm not sure about that last sentence KC. There are three weight transfers (according to Puhn) that happen: weight transfer due to body roll, due to roll center heights and then weight transfer of unsprung weight. I don't know if the springs always transfer the same amount.

There will be some difference due to the change in the geometry of the situation (e.g., the amount of body roll), of course, but those are second order effects. They will be small compared with the amount of weight transferred as a result of the primary forces.

I'd wager that lowering the center of gravity of the sprung weight will have more effect on how much weight is transferred than anything else.

Thing is, though, that generally speaking, stiffer springs are used in conjunction with lowering the car, which means you lower the CG of the unsprung weight, and that will affect how much force the springs have to counteract when the car is in a turn. But, again, the question must be "by how much?". And that is going to depend on how much of a percentage change there is in the vertical distance between the sprung weight CG and (I'm guessing, and possibly oversimplifying) the wheel's rotational axis.

 

[Houstonnw]

Steady state for your control lnputs is not the same thing as steady state where instantaneous wheel loading is concerned. Even pavement unevenness and roughness constitute changes in suspension loading, and a softer spring will vary its loading less when that happens. What may feel like steady state from the driver seat is really more of an overall-average-approximation of steady state.


Norm

Nice, I don't know what that means, but nice.

So this is what I know. If an end of the car is stiffer, whether from springs or shocks, it will have less grip relative to the other end of the car.

This is true for transient or steady state conditions.

Am I wrong?

I respect Norm for his knowledge, but I also have an engineering degree, and I have also raced CMC for 3 years.

I also think that I am very good at understanding the question.

Let's talk.

Edit to ad: Wasn't this a question about spring rates? You want to go further, make another thread, seriously!

 

[kcbrown]

Nice, I don't know what that means, but nice.

So this is what I know. If an end of the car is stiffer, whether from springs or shocks, it will have less grip relative to the other end of the car.

This is true for transient or steady state conditions.

Am I wrong?

Hmm...I'm not sure, but it seems logical to me.

If you have one end of the car that's stiffer than the other, then when the car takes a corner, the softer end will try to rotate about the longitudinal axis more than the stiffer end, which means that you wind up with additional lifting force on the inside on the stiff end (transmitted through the body) and additional compression force on the outside on the stiff end, so the total grip at the stiffer end winds up being less, right?

 

[Whiskey11]

Nice, I don't know what that means, but nice.

So this is what I know. If an end of the car is stiffer, whether from springs or shocks, it will have less grip relative to the other end of the car.

This is true for transient or steady state conditions.

Am I wrong?

I respect Norm for his knowledge, but I also have an engineering degree, and I have also raced CMC for 3 years.

I also think that I am very good at understanding the question.

Let's talk.

Edit to ad: Wasn't this a question about spring rates? You want to go further, make another thread, seriously!

That assumption works if the car is properly sprung as a starting point. Stock they aren't, obviously, otherwise we would never upgrade the springs! The reason being that it is possible to stiffen an end of the car and improve grip at that end. This is particularly true with strut suspension outside their "happy" part of the camber curve for the tires. A perfect example is my car. At 440lbs/in front springs and the Strano 35mm front bar jammed full stiff (642lbs/in) and -3.0º of camber was still destroying the outside shoulder of the tire because the car is lowered enough to need more negative camber at full load. Now with 550lbs/in front springs, I'm hoping I'm in the right stiffness range to keep that under control.

With a car that is properly sprung to begin with (somewhere north of 350lbs/in up front), I would agree that adding more spring rate is going to decrease grip which is why we increase rear end spring rate to compensate for the slight loss in front grip.


Also, I was mistaken with what I typed on the previous page. It took a little mind mashing to wrap my head around the concept, but I think I get it.

 

[Norm Peterson]

Originally Posted by Norm Peterson
Steady state for your control lnputs is not the same thing as steady state where instantaneous wheel loading is concerned. Even pavement unevenness and roughness constitute changes in suspension loading, and a softer spring will vary its loading less when that happens. What may feel like steady state from the driver seat is really more of an overall-average-approximation of steady state.


Norm

Nice, I don't know what that means, but nice.

Mostly that was in response to the "steady state" part of the discussion. Basically what you may think is steady state because you aren't doing anything with your hands and feet really isn't as far as the contact patches, springs, and suspension are concerned. And because pavement-contour-caused spring (and tire) load variation happens on a tens to maybe hundreds of millisecond time scale it's not something you're likely to feel under most conditions.

kc had mentioned that during transients the rate of change in spring load won't be the same for a soft spring as for a stiff spring, which may have been what brought steady state into the discussion. This applies to both inertia loading (from lateral and longitudinal accelerations that are hopefully caused only by you) and displacement loading (from pavement unevenness that's just there). Meaning that a true steady state at the contact patches is unlikely, and that you're working with some sort of average.

Maybe a wet sweeper when you're doing as close to nothing as humanly possible and the car is still dancing around a little as the four tire loadings constantly vary is the best chance you'll have of (indirectly) feeling this sort of load variation (and perhaps gain an appreciation for why wet setups are softer!). Five half hour sessions with every single lap in the rain last Saturday has left a few things about suspension loading and tire grip still fresh in my mind.

Gratuitous screenshot pulled from a video of one of the medium-wet laps. That's NJMP/Lightning's long T9 sweeper ahead, which you're in for quite a few seconds and long enough for you to get to steady state with your hands and feet. I caught the wipers down and you can see the water drops (look along the top of the wall; the car directly in front of me has just entered T9 and the lead car is about 1/3 of the way through it).

Norm

 

[sheizasosay]

KC,

Two things:

1- The weight transfer you lose from the inside tires is not an exact proportion to the weight gained on the outside tires.

2- *IF* you did transfer all the weight (from the inside tires to outside), one more pound of weight transfer (in to out) and your inside tires are in the air.

It's not a 1:1 transfer. That's why weight transfer is undesirable. That's why it's common to hear "enough spring rate to stay off the stops". More spring = more weight transfer.

KC I think you would really like How to Make Your Car Handle. I got parts I would like to reference, but there is a ton of other info in there that I'm not gonna quote(the whole section) which you could read if you had it. In any case, this quote is in regards to the whole "springs always transfer the same amount of weight" bit, but I didn't have a reference for my statements previously. Pg 44 :

"The maximum limit to total weight transfer of the car is the total static weight carried by both inside tires. If the total weight transfer exceeds this amount both inside tires lift off the road the car tips over."Puhn


This is why I said in post 73 "I'm not sure about that last sentence" and that sentence I'm talking about is " the springs always transfer the same amount of weight...."

My apologies if I'm addressing something you already know.

 

[claudermilk]

I really need to pull Puhn's book back out & give it a good read. I've had it for years & only read bits of it. Time for a cover-to-cover read just to I can keep up with you guys.