Susupension Thread

[old greg]

Originally Posted by Dimman

Is there a formula for determining a spring's 'fitted' rate? (The one based on the mounting angle, not wheel rate which is the leverage thing.)

Umm... Could you explain what exactly it is that you're trying to figure out? I think there's a bit of a language barrier as it were.

If you're asking what I think you're asking it's just trigonometry and it's included wheel rate. If your spring is 10deg from vertical, the effective spring rate at the lower attachment point would be k*cos(10deg). The same thing would apply to damper forces as well.

[Dimman]

Originally Posted by old greg

Umm... Could you explain what exactly it is that you're trying to figure out? I think there's a bit of a language barrier as it were.

If you're asking what I think you're asking it's just trigonometry and it's included wheel rate. If your spring is 10deg from vertical, the effective spring rate at the lower attachment point would be k*cos(10deg). The same thing would apply to damper forces as well.

I think this is what I'm asking.

I've been reading a bit and some of the frequency formulas deal with the following:

You start with 'coil rate', the basic spring. Then this is adjusted based on how it is mounted/difference from 90 degrees. Then this 'fitted rate' is adjusted based on leverage for the 'wheel rate'.

As for the damping forces, if the setup is a coil-over damper then doesn't the damper just 'see' the coil rate since it is in-line with the spring? Like you use the wheel rate for your frequency, but determine your damping based on the coil rate?

[old greg]

Originally Posted by Dimman

You start with 'coil rate', the basic spring. Then this is adjusted based on how it is mounted/difference from 90 degrees. Then this 'fitted rate' is adjusted based on leverage for the 'wheel rate'.

Ok, yeah. Usually that's dealt with in terms of motion ratio, which includes both of those things.

Originally Posted by Dimman

As for the damping forces, if the setup is a coil-over damper then doesn't the damper just 'see' the coil rate since it is in-line with the spring? Like you use the wheel rate for your frequency, but determine your damping based on the coil rate?

Yes but the damping ratio is also based on the sprung mass, which (from the damper's perspective) will either be amplified or reduced if the motion ratio is not 1:1.

[Dimman]

Argh, again with the learning... Thanks, though.

Edit: May be giving up on trying to understand damping...

Went through Old Greg's post and don't think anything sunk in. Also been re-reading the chapter on damping in a suspension book I have. Book also contains two handy things.

First is Penske's flow-chart for damping adjustments.

Second is Koni's process for shock setup.

So damping solution for me: Leave it stock for a while, save up for some good Koni ($$$$$) double adjustable rear dampers/front strut inserts, give Koni all info needed to recommend base valving, make strut housing to fit, install, follow Koni's setup advice, when at track follow Penske's flowchart for handling issue resolution.

So the wallet will be hurting, not the brain...

[Dimman]

Originally Posted by serialk11r

If I understand correctly, springs can only resist so much transfer. When under lateral acceleration, you have weight transfer due to the "torque" acting on the car around the center of mass (which creates body roll) and then you have weight transfer due to body roll itself, which actually moves the center of mass outward. Stiffer springs resist the movement of the center of mass outward, but I imagine the more important role they play is to resist unfavorable camber changes which kill grip more...not sure on this though.

With a lower center of gravity, you experience less "torque" on the body because the angle at which the force acts is smaller. Since the center of mass is closer to the ground at any given angle of body roll it's moving a smaller distance horizontally as well, and you should be able to use softer springs because they don't need to counter as great of a force for the same weight.

I think of it in terms of leverage, which may be incorrect from a physics point of view, but whatever... The length of the 'lever' is from the roll center (intersection of points based on tire contact patch and angles of certain control arms. Or something... Can be above or below ground.) to the COG. So lower COG = less leverage to roll the car side to side, or pitch it front to back. Other way to resist roll/pitch is through wider track/longer wheelbase. Then we have anti-roll bars that are torsion bar springs that resist roll but not in pitch. Lastly (well, may be more, Old Greg is the guy to ask, I think) we have 'anti' suspension geometries that angle the motion of the control arms to act as a 'ramp' and naturally there are centers and the COG plays a role as well with them. COG can affect the % of anti-dive (forward pitch under braking) or anti-squat (rearward pitch under acceleration).

As for the camber change, that depends on the geometry. With the struts in the front, this can lose negative camber as it travels, so resisting roll can help. But with the double-wishbone (really mutli-link with an upper wishbone) rears depending on the lengths of the upper and lower arcs the car can gain negative camber as it travels.

I'm sure complicated tuning is required to optimize the roll rate to available grip and there may be tradeoffs between loading a tire (which increase its grip, up to a point) and optimizing its contact patch. As well as balancing load and contact patch side to side, front to rear, in steady state and transient movements, changing mass (passengers, decreasing fuel load). deflections of bushings, control arms (Old Greg's educating me about E there...), tires, chassis etc... Ouch... Partly why suspension has the 'black art' bit associated with its tuning.

Though I get the impression, Serial, that once you've looked at a few diagrams and done some calculations you'll have a vastly better understanding than myself. Stupid physics...

Are you still considering Formula SAE?

[serialk11r]

If you have the same track but lower center of gravity yea the "leverage" does decrease because you are slightly reducing the distance from the center of mass to the tires, but with typical cars I imagine what has a bigger effect is the angle at which the lateral forces are acting on the "lever".

Haha yea, I haven't gotten around to checking out FSAE but I will sometime hopefully.

[Dimman]

Originally Posted by serialk11r

If you have the same track but lower center of gravity yea the "leverage" does decrease because you are slightly reducing the distance from the center of mass to the tires, but with typical cars I imagine what has a bigger effect is the angle at which the lateral forces are acting on the "lever".

Haha yea, I haven't gotten around to checking out FSAE but I will sometime hopefully.

Have you looked at any roll-center diagrams? Track affects them, too. It's just so much damn, this-affects-this-which-affects-this, etc...




Edit:
F1 High-Nose Anti-dive diagram

[Racecomp Engineering]

Details coming first on facebook, big new thread coming soon here:



- drew

[Dimman]

^ Envy...

[7thgear]

with regards to using adjustable dampers,

the reason you want adjustable dampers is if you are going to change your balance (weight removal, weight addition, weight shifting (such as battery relocation) etc.

because during movement the front of your car will oscillate at a different rate than the rear of your car. Tuning this difference between front and rear oscillation is the nut of the problem.

Street cars are tuned for comfort so that even though the front wheels hit a bump before the rear wheels at regular speeds (40-80km/h), the whole chassis oscillates in unison.

When you modify your car by changing chassis weight and/or weight distribution and slap on after-market springs, you are modifying your balance. While "suspension kit" manufacturers (hopefully) do the background work for you, they tailor to some middle level feeling of "good." If this is not enough for you, then suggest looking into adjustable dampers.

If you are bad at math and prefer visual ques, you can and purchase some 3 way (2 way could also work) accelerometers and put one in your trunk, and one somewhere under your hood. then drive around and look at the outputs to see how damper tuning affects your car.

if you're hardcore enough (i've never done it) is to attach these accelerometers right on to your control arms at all 4 wheels and record data that way, but that's money and headache.

[Dimman]

^ My understanding is that the adjusters can be used to alter the speed of weight transfer (but not ultimate weight transfer). And thus how a car reacts at corner entry and exit can be modified with shock changes.

I'm checking google for Penske's flow chart on double adjusters but not having much luck...

(no personal experience, though...)

Edit: My god, technical shock info overload:

Start with this:

http://www.penskeshocks.com/files/Adjustable_Manual.pdf

Tons more here:

http://www.penskeshocks.com/Downloads.php

(I was just looking for a damn flowchart...)

[Want.FR-S]

Originally Posted by Dimman

I think of it in terms of leverage, which may be incorrect from a physics point of view, but whatever... The length of the 'lever' is from the roll center (intersection of points based on tire contact patch and angles of certain control arms. Or something... Can be above or below ground.) to the COG. So lower COG = less leverage to roll the car side to side, or pitch it front to back. Other way to resist roll/pitch is through wider track/longer wheelbase. Then we have anti-roll bars that are torsion bar springs that resist roll but not in pitch. Lastly (well, may be more, Old Greg is the guy to ask, I think) we have 'anti' suspension geometries that angle the motion of the control arms to act as a 'ramp' and naturally there are centers and the COG plays a role as well with them. COG can affect the % of anti-dive (forward pitch under braking) or anti-squat (rearward pitch under acceleration).

Sorry I am late in the game, but as I posted in another thread,
http://www.ft86club.com/forums/showp...&postcount=263, lowering COG has some effect to the weight transfer. Not really much. Using the BRZ/FR-S as an example, roughly speaking, if raising the COG by 50 mm (about 2 inch), the increase in weight transfer under 1.00 G braking (to the front) is about 54 lb out of 388.2 lb, compared with 334.5 lb with the current 450 mm COG. This is considering the whole car, not per wheel.

I am not sure the typical spring rate is for any given car. 500lb/in? This is the number I got from Wiki. Can someone enlighten me on this?

The formula is described in many places. But the thing is, the weight transfer is proportional to the height of COG, and weight, but inverse proportional to the wheelbase. If you want to reduce weight transfer: lower weight, lower COG or increase wheelbase and you can achieve the goal. However, it all depends on where you did to see how much effect it can be.

[fatoni]

Originally Posted by Want.FR-S

Sorry I am late in the game, but as I posted in another thread,
http://www.ft86club.com/forums/showp...&postcount=263, lowering COG has some effect to the weight transfer. Not really much. Using the BRZ/FR-S as an example, roughly speaking, if raising the COG by 50 mm (about 2 inch), the increase in weight transfer under 1.00 G braking (to the front) is about 54 lb out of 388.2 lb, compared with 334.5 lb with the current 450 mm COG. This is considering the whole car, not per wheel.

I am not sure the typical spring rate is for any given car. 500lb/in? This is the number I got from Wiki. Can someone enlighten me on this?

The formula is described in many places. But the thing is, the weight transfer is proportional to the height of COG, and weight, but inverse proportional to the wheelbase. If you want to reduce weight transfer: lower weight, lower COG or increase wheelbase and you can achieve the goal. However, it all depends on where you did to see how much effect it can be.

depends on the type of suspension and how inboard the spring is. then there are some cars that kind of cheat in order to be cheap. many cars are designed to ride on the bumpstops while hard cornering. that way the rates can be soft enough to be comfortable but then the car can dig in once things have settled in the turn

[old greg]

Originally Posted by 7thgear

because during movement the front of your car will oscillate at a different rate than the rear of your car. Tuning this difference between front and rear oscillation is the nut of the problem.

This is a function of spring rates, not damper rates.

Originally Posted by Dimman

^ My understanding is that the adjusters can be used to alter the speed of weight transfer (but not ultimate weight transfer). And thus how a car reacts at corner entry and exit can be modified with shock changes.

This. Low speed damping controls transient weight transfer distribution.


If you want to talk about weight transfer in terms of torque, it's the inertial force (mass x acceleration) acting on the CG with the pivot at ground level. Total steady-state weight transfer is completely independent of the suspension geometry and spring/damper rates. If you replaced the springs/dampers with solid steel bars you'd get the same amount of weight transfer (CG movement due to body roll happens, but it's mostly negligible in anything other than trucks/SUVs/etc).

Roll Stiffness is a function of roll moment (acceleration x mass x distance between cg and roll axis) and roll rate ((wheel rate+ARB) x track width). It's usually defined as Roll Gradient (degrees/g). "Anti" geometry comes into it by reducing the distance between the CG and roll axis by raising the roll axis rather than lowering the CG.

The more important aspect of roll center height is it's effect on roll inertia (roll inertia about the CG + sprung mass * distance between CG and the roll axis squared). With a higher roll center, even with identical roll gradient, you get a higher roll frequency which means sharper response (which is better, up to a point). There are a couple of downsides to a higher roll center. It puts more load into the suspensions members, causes increased jacking and reduces the response of the suspension to imperfections in the road (reduces grip). The exact same stuff applies longitudinally as well, but with pitch axis height instead.

As for camber change, you lose as many degrees as the chassis rolls by, plus whatever you gain or lose through suspension travel. Higher roll stiffness will help in this regard, but only up to a point. Sooner or later you will reach a point where the grip you gain by further reducing camber loss is outweighed by the loss of grip caused by stiffer springs/bars. Where that point is is a function of sprung/unsprung weight ratio and how crappy the road surface is.

Originally Posted by fatoni

depends on the type of suspension and how inboard the spring is.

Not to mention the most important factor... How much the vehicle weighs.

Originally Posted by serialk11r

Haha yea, I haven't gotten around to checking out FSAE but I will sometime hopefully.

Out of curiosity, what school do you go to?