MM Road & Track Springs

[kcbrown]

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.

Well, that can't be the case, at least on average, because weight and force are really the same thing here, and a force imbalance means that something is accelerating.

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.

Right. What I meant is that if you're taking the same corner at the same speed in the same car (with the same sprung CG height) with the same tires, and assuming that the contact patches on the tires are the same for both instance (something that is clearly not the case for the Mustangs when comparing a softly-sprung setup with a stiffly-sprung one), then the centripetal acceleration the body is undergoing is the same for both, which means that the tires are going to be under the same loading, which means that the force they're pushing back on the suspension with is the same, which means that the suspension is also under the same load, which means that the springs are also under the same load. The only difference to be found there is how much the springs have to move relative to their "unloaded" (car is going in a straight line) condition, and that difference is precisely because the spring rates are different.

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.

But "more spring" has to account not just for the difference in spring rate, but also the difference in available travel.

Which is to say you can stay off the stops using a combination of two things: increased spring rate, and increased available spring travel.

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

Right. That wasn't the circumstance I was thinking of, though. I was thinking of the more typical circumstance where the suspension had not hit the limits of its travel.

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.

No worries. I'm always interested in learning more! That book sounds excellent.

But as regards even the case where the situation has reached the point where the inside wheels start to lift, all that means is that the torque about the longitudinal line drawn through the contact patches at the outside tires of the car is enough to overcome the weight of the car, and the inside tires lift. It means that the outside tires have sufficient grip on their own to prevent the car from sliding under those circumstances. That is, I expect, the point of maximum cornering capability of the car. If the outside tires don't slip in the event that you try to take a corner harder than that, you'll flip the car instead. The corner would have to be banked for you to be able to take it any faster.

And that means that having the inside tires remain in contact with the ground won't help you in terms of making additional grip possible. You'd have to lower the car's sprung CG, so that the generated torque on the body is less for the same amount of horizontal force counteracted by the tires, to do any better.


Or so it seems to me. Hopefully my description above gives some sense of what I'm trying to convey here...

 

[sheizasosay]

Well, that can't be the case, at least on average, because weight and force are really the same thing here, and a force imbalance means that something is accelerating.

Weight transfer and weight re-distribution are not the same. I have already said all I can offer and my reference for technical discussion is Puhn. That's what I got for weight transfer KC. Maybe I read it wrong.

 

[Norm Peterson]

It's easier to keep this stuff straight if you think in terms of load transfer (either lateral or longitudinal) rather than "weight transfer".

Except for things like fluid slosh, and loose cargo and stray passengers getting tossed about in their respective containers, no weight actually moves during acceleration, braking, or cornering. Not relative to the general concept of the single sprung mass that makes rough calculations easier, anyway.


Norm

 

[kcbrown]

It's easier to keep this stuff straight if you think in terms of load transfer (either lateral or longitudinal) rather than "weight transfer".

Except for things like fluid slosh, and loose cargo and stray passengers getting tossed about in their respective containers, no weight actually moves during acceleration, braking, or cornering. Not relative to the general concept of the single sprung mass that makes rough calculations easier, anyway.

Well, yes, but the thing is that weight/force is a zero-sum game here. It has to be, because any part of a force that isn't counteracted results in acceleration (well, relative to the measurement reference frame, at any rate. In this case, we're using the reference frame of the car, which is actually under acceleration relative to the outside reference frame).

The amount of horizontal force the suspension needs to counteract in order to keep the car in the same arc at the same speed is determined solely by the mass of the car, the characteristics of the arc, and the speed (this all presumes a flat surface -- a banked surface will, of course, add bank angle into the equation).

Thus, any horizontal counteracting force provided by one part of the suspension which is subsequently reduced must be taken up by another part of the suspension, because the sum of all the horizontal forces must remain zero, and this is an invariant in both the static and dynamic contexts.

That leaves the vertical component, and that changes with variations in the surface, but it must average to zero as well, else the car will be undergoing a net vertical acceleration.

Suspension geometry and other things will affect the distribution of those forces, as well as how quickly force redistribution occurs when there is a change in the loading, but as long as the car is staying on the same arc at the same speed, the horizontal component remains the same, and as long as the car remains on the surface, the vertical component must average to zero as well.


Of course, all bets are off if "this nigga bout to fly off a mountain yo".

 

[kcbrown]

Weight transfer and weight re-distribution are not the same. I have already said all I can offer and my reference for technical discussion is Puhn. That's what I got for weight transfer KC. Maybe I read it wrong.

I just ordered that book. Can't wait to read it to see what it has to say!

 

[Norm Peterson]

Puhn's book is arguably the best softcover introduction to vehicle dynamics. Enough math to actually be able to do something with the topic, without being so overwhelming that few would read it or so expensive that few would buy it. "Race Car Vehicle Dynamics", which is a college-level hardcover available through the SAE bookstore, currently sells for about $100.

http://books.sae.org/r-146/


I'll leave you with the thought that any road disturbance that you feel (or can be measured) does in fact mean that at least one corner of the car is being accelerated either upward or downward, and that the sprung mass is therefore not in equilibrium at such times . . .


Norm

 

[kcbrown]

Puhn's book is arguably the best softcover introduction to vehicle dynamics. Enough math to actually be able to do something with the topic, without being so overwhelming that few would read it or so expensive that few would buy it. "Race Car Vehicle Dynamics", which is a college-level hardcover available through the SAE bookstore, currently sells for about $100.

http://books.sae.org/r-146/

I'll go through Puhn's book first, but if I find it wanting, then the above is going to go on my list (it might anyway just because it does look fascinating, and I'm curious to know things like how they model dampers).

I'll leave you with the thought that any road disturbance that you feel (or can be measured) does in fact mean that at least one corner of the car is being accelerated either upward or downward, and that the sprung mass is therefore not in equilibrium at such times . . .

That is absolutely true. However, all of that must average to zero, else the car would not remain on the ground...

 

[sheizasosay]

Good deal. I think that's great KC! That other book Norm referenced I have always wanted to check out. I have seen that book for well over $100. Can't remember the price, but $279 comes to mind. Norm referenced the book one or two times and I went to look at buying it and basically said to myself "self, you're not paying that much for a book."

For my "mental capacity" I think Puhn is amazing.

 

[Norm Peterson]

That is absolutely true. However, all of that must average to zero

Averaged over time, yes. Instantaneously, I don't think so. Crudely at any given instant,

ΣF = Weight + Σ ( [corner weight](i) * [az](i) )

(i) is subscript-i because I don't know how to do subscripts in HTML. Lower case intended but auto-correct keeps trying to uppercase it
az is acceleration in the vertical direction by SAE definition, which followed aeronautical convention (see RCVD).


All of this gets complicated by the fact that suspension geometry does not remain symmetrical once any roll has developed. Basically that means that the motion ratios for the springs, dampers, and sta-bar ends are no longer identical side to side, and what you perceive as pure roll is really some combination of roll plus heave. This heave involves a net upward or downward acceleration of at least one end of the car (not just a single corner). Usually it's a minor effect, but there is at least one fairly simple A-B test that might make this obvious enough to notice. It may be easier to see this analytically if you work with front view instant centers directly rather than the geometric roll center.


Norm

 

[kcbrown]

Averaged over time, yes.

Yeah, that's what I meant. I should have made that explicit.

Clearly it can't be true at any given instant, else the sprung mass would never move at all. And as observation trumps all else...

Instantaneously, I don't think so. Crudely at any given instant,

ΣF = Weight + Σ ( [corner weight](i) * [az](i) )

(i) is subscript-i because I don't know how to do subscripts in HTML. Lower case intended but auto-correct keeps trying to uppercase it
az is acceleration in the vertical direction by SAE definition, which followed aeronautical convention (see RCVD).

Yep, exactly.

However, it seems to me that the reason you get that at all is because of surface imperfections. Which is to say, if you were driving around a corner on a perfectly smooth surface, the suspension would be stable as long as the control inputs were stable.

In the real world, you have surface imperfections, and a job of the suspension is to handle those imperfections without generally destabilizing the car as it moves over them.

All of this gets complicated by the fact that suspension geometry does not remain symmetrical once any roll has developed. Basically that means that the motion ratios for the springs, dampers, and sta-bar ends are no longer identical side to side, and what you perceive as pure roll is really some combination of roll plus heave. This heave involves a net upward or downward acceleration of at least one end of the car (not just a single corner). Usually it's a minor effect, but there is at least one fairly simple A-B test that might make this obvious enough to notice. It may be easier to see this analytically if you work with front view instant centers directly rather than the geometric roll center.

Some suspensions have the instant roll center change as the suspension moves as well, right? I can't remember if that's a characteristic of the typical S197 suspension or not...

 

[sheizasosay]

Some suspensions have the instant roll center change as the suspension moves as well, right? I can't remember if that's a characteristic of the typical S197 suspension or not...

They all move unless they have sticks for springs.

 

[Ivan 5.0]

Update

I moved my front swaybar to the "full stiff" position and the car immediately felt less over-steerish. I took a few back road corners that I'm familiar with and the handling was a lot more balanced than before. Dampers were still at "full soft."

Once I get to the track in two weeks I'll firm up the dampening and hope it turns out well. Gonna try "full stiff" in the front, and 3/4 "stiff" in the rear on my Koni Yellows. Will provide an update then... but as of right now I'm happy I switched from the Steeda Sports to the MM Road & Track Springs.

 

[11socal50]

Update

I moved my front swaybar to the "full stiff" position and the car immediately felt less over-steerish. I took a few back road corners that I'm familiar with and the handling was a lot more balanced than before. Dampers were still at "full soft."

Once I get to the track in two weeks I'll firm up the dampening and hope it turns out well. Gonna try "full stiff" in the front, and 3/4 "stiff" in the rear on my Koni Yellows. Will provide an update then... but as of right now I'm happy I switched from the Steeda Sports to the MM Road & Track Springs.

Very glad that I read this since I'm thinking about swapping over to these springs and having already upgraded my front and rear sway bars thinking that I was done with my suspension.
did you install a bumpsteer kit?

Sent from my EVO using Tapatalk

 

[sheizasosay]

KC,

So you're comment about about the mass must be counter-acted or it will result in an acceleration...it is true. I didn't leave a good response though, I don't think. Which is why Norm probably made his late comments. Which also lends credit to the whole "steady-state doesn't really exist except hypothetical discussion". Well the point I made in the other thread that credits towards your point KC is: the sprung mass will redistribute everything in steady-state. Yes, the hypothetical steady-state. It is the unsprung mass that "loses" itself and can become unequally transferred.

You put forth a lot of effort and thoughtfulness and it deserves credit. I have to read and re-read some of the suspension books. Sometimes one(me) has to humble one's self and admit you don't have it figured out like you thought you did.

 

[Norm Peterson]

Update

I moved my front swaybar to the "full stiff" position and the car immediately felt less over-steerish. I took a few back road corners that I'm familiar with and the handling was a lot more balanced than before. Dampers were still at "full soft."

Not too surprising. If you went directly to full stiff from full soft, what do you think an intermediate setting would give you?

Once I get to the track in two weeks I'll firm up the dampening and hope it turns out well. Gonna try "full stiff" in the front, and 3/4 "stiff" in the rear on my Koni Yellows. Will provide an update then... but as of right now I'm happy I switched from the Steeda Sports to the MM Road & Track Springs.

Let me suggest some thing closer to the middle of the adjustment ranges for the first session and moving up in quarter turn increments. Worst case, if it proves to be unacceptably soft within the first 4 or 5 laps, you might still have time to pit, crank in a little more damping, and get back out for 4 or 5 more before the session ends.

FWIW, I've found my car to be quite good at +1.5 front and +1.25 rear with stock springs and Sam's sta-bars . . . on decent summer performance tires that are not quite into the wear bars. At the track on the new and grippier tires I suspect I'll want to go a little stiffer, but I won't really know until I get there. If your springs are stiffer, you might consider starting eighth or at most quarter turns higher.


I'm not a big fan of going straight to max settings from full soft, as you might jump right over what works best.


Norm

 

[Sky Render]

...And keep some sort of log of your settings! Otherwise you might forget what settings work best, and then you're back to square one.

 

[Norm Peterson]

It is the unsprung mass that "loses" itself and can become unequally transferred.

It's probably good enough to assume that the distribution of lateral load transfer from the unsprung masses remains in the same ratio as the front to rear unsprung mass magnitudes. The only exception I can think of is if you can transfer enough weight off any single corner to drop that contact patch's load below the weight of that corner's unsprung weight (aka incipient tricycle), and even that is going to depend on relationships between various chassis stiffnesses.

You put forth a lot of effort and thoughtfulness and it deserves credit. I have to read and re-read some of the suspension books. Sometimes one(me) has to humble one's self and admit you don't have it figured out like you thought you did.

Agreed, and even when you've more or less been where he's heading with the analytical side it's good to be forced to stop and think.


Norm

 

[2013DIBGT]

Update

I moved my front swaybar to the "full stiff" position and the car immediately felt less over-steerish. I took a few back road corners that I'm familiar with and the handling was a lot more balanced than before. Dampers were still at "full soft."

Once I get to the track in two weeks I'll firm up the dampening and hope it turns out well. Gonna try "full stiff" in the front, and 3/4 "stiff" in the rear on my Koni Yellows. Will provide an update then... but as of right now I'm happy I switched from the Steeda Sports to the MM Road & Track Springs.

In my experience treating the end of the car that has the issue seems to work pretty well. In your case, it may be worth your time to try removing the rear bar as previously suggested by others and put the front bar back where it was.

I don't run a rear bar on my car and think it feels far more planted out back without it and my rear spring rate is mild compared to yours. I'm using an adjustable Eibach FSB up front on the middle setting. Not sure how different the Eibach is to the MM bar but I suspect their bar is from H&R.

 

[kcbrown]

KC,

You put forth a lot of effort and thoughtfulness and it deserves credit. I have to read and re-read some of the suspension books. Sometimes one(me) has to humble one's self and admit you don't have it figured out like you thought you did.

Thanks. No worries at all. There's always something new to learn and new ways to think about things. I know I certainly have a lot more to learn myself!

I'm at NASA at Sonoma again so unfortunately I can't reply in depth. I will certainly do so once I'm home.

Carry on sir!