I had some time to kill in between seeing patients and so I did some practical math
to keep me awake..

It is said that you can use the natural frequency of a corner to determine the desired spring rate for your car.

In touring car, when the surface is perfectly smooth and not bumpy, the natural frequency is around 3.2. Once you get into autocrossing and bumpy touring car courses, the natural frequency is said to vary from 1.9 to 2.2.

Natural Frequency = 3.13 SQRT (wheel rate/corner sprung weight)

If we take a typical ITR cornerweight (let's use somebody's here like Flux's:
https://honda-tech.com/zerothread?id=466075 ) , remove the driver weight, and assume the unsprung weight to a corner is 48 lb (40-57 lb was reported by Shawn Church over at Temple of VTEC):

Front Left 700 Front Right 770

Rear Left 491 Rear Right 419

Let's err on the conservative side and assume the road is pretty bumpy and we want the lower natural frequency:

A. Front

1.9 = 3.13 SQRT (WR/ 770)

(1.9/3.13) ^ 2 = WR /770

WR = 0.368 x 770 = 284 lb./in.

RR98ITR has kindly provided us with the ideal motion ratio or suspension leverage effect for an ITR at 1.5 for the fronts.

So, if we use the classic equation:

WR = Spring rate / Motion Ratio ^2

SR = WR x MR^2

SR = 284 x 2.25 = 638 lb/in.

The driving wheels should get the stiffer frequency for better exiting out of the corners and some may argue that the separation between frequencies should not be more than 10%.

B. Rears

So....similarly for the rears, the Motion Ratio is 1.35 and the desired Frequency is 1.7.

1.7 = 3.13 SQRT (WR/491)

WR = 145 lb/in.

SR = 145 x 1.8225 = 264 lb/in.

So according to this method, a 640 fr/260 rear is ideal...LOL

Now where did I go wrong?

remember this is just for fun...do not try this at home....professional driver in closed course....LOL

 

damn you are a smart ****..

i feel special cuz i know what this all means (kinda) . lol

 

Ow. My eyes!!!
It hurrts. Make itt sstop.

Scott, who prefers the old method of picking up the phone, dialing the number of somebody doing well in your chassis, and saying "Hey, what rates are you running?

Friends don't let friends do math.

 

Where you went wrong is you're using corner sprung weight in the equation... and that no equation can really pinpoint spring rate transitions, bumps, braking, lateral grip, acceleration, etc.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Catch 22 &raquo;</TD></TR><TR><TD CLASS="quote">

Scott, who prefers the old method of picking up the phone, dialing the number of somebody doing well in your chassis, and saying "Hey, what rates are you running?

</TD></TR></TABLE>

That's like asking: " hey, what do you prefer: blonde, brunnette, or red head?" or "hey, thai, chinese, italian, or ethiopian for take-out tonight?"

it's all pretty subjective as to what works for you and what you are comfortable with may not be what I am comfortable with as the car rotates into a corner.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Michael Delaney &raquo;</TD></TR><TR><TD CLASS="quote">It is said that you can use the natural frequency of a corner to determine the desired spring rate for your car.

</TD></TR></TABLE>

I'm no mechanical engineer, but I am pretty sure that the rule-of-thumb you stated is typical for deterimining rates for passenger comfort, not optimal cornering.

Last time I read Puhn, racing spring rates are selected to optimize contact patch due to camber changes during roll, while maintaining contact over rough surfaces (the Trade Off). Should have little to do with natural frequency.

Hmmm. Interesting.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by .RJ &raquo;</TD></TR><TR><TD CLASS="quote">Where you went wrong is you're using corner sprung weight in the equation... and that no equation can really pinpoint spring rate transitions, bumps, braking, lateral grip, acceleration, etc.</TD></TR></TABLE>

I got that from page 139 from the suspension bible: Fred Puhn's How To make Your Car Handle. You might want to tell Fred....

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by whitney &raquo;</TD></TR><TR><TD CLASS="quote">

I'm no mechanical engineer, but I am pretty sure that the rule-of-thumb you stated is typical for deterimining rates for passenger comfort, not optimal cornering.

Last time I read Puhn, racing spring rates are selected to optimize contact patch due to camber changes during roll, while maintaining contact over rough surfaces (the Trade Off). Should have little to do with natural frequency.

Hmmm. Interesting.</TD></TR></TABLE>

actually, the method is used for BTCC touring cars. Saloon cars aren't exactly Cadillacs. Why Jim Clark ever raced in one that fateful day instead of just sticking to F1 Lotuses, I'll never know....

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Michael Delaney &raquo;</TD></TR><TR><TD CLASS="quote">

That's like asking: " hey, what do you prefer: blonde, brunnette, or red head?" or "hey, thai, chinese, italian, or ethiopian for take-out tonight?"

it's all pretty subjective as to what works for you and what you are comfortable with may not be what I am comfortable with as the car rotates into a corner.

</TD></TR></TABLE>

I don't think its really that subjective. Maybe the last 50 lbs./inch or the shock settings are but for the most part for a given chassis per a given type of racing, there are probably 2 or 3 distinct types of setups any of which would work. Where you set the tire pressures/shocks/sway bars maybe - that's the personal comfort IMO. Then again, I ain't the type to do all that math in the first place and I am just smart enough to ask folks and just dumb enough to listen to them.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by phat-S &raquo;</TD></TR><TR><TD CLASS="quote">

I don't think its really that subjective. Maybe the last 50 lbs./inch or the shock settings are but for the most part for a given chassis per a given type of racing, there are probably 2 or 3 distinct types of setups any of which would work. Where you set the tire pressures/shocks/sway bars maybe - that's the personal comfort IMO. Then again, I ain't the type to do all that math in the first place and I am just smart enough to ask folks and just dumb enough to listen to them.</TD></TR></TABLE>

with your CRX's wheelbase difference compared to mine, I don't know if I should ask...

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Michael Delaney &raquo;</TD></TR><TR><TD CLASS="quote">

with your CRX's wheelbase difference compared to mine, I don't know if I should ask...</TD></TR></TABLE>

whaddat mean?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Michael Delaney &raquo;</TD></TR><TR><TD CLASS="quote">

actually, the method is used for BTCC touring cars. Saloon cars aren't exactly Cadillacs. Why Jim Clark ever raced in one that fateful day instead of just sticking to F1 Lotuses, I'll never know.... </TD></TR></TABLE>

I stand corrected, then, sir. I learn something new every day. Even if I read it once already . Thanks!

Mike &lt;-- heading for the bookshelf

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by phat-S &raquo;</TD></TR><TR><TD CLASS="quote">

whaddat mean?</TD></TR></TABLE>

dat meant: an Integra has a wheelbase of 101 in. I believe the CRX has a shorter wheelbase. Should I ask what your rate is and apply it to my car?

 

No-no! I thought I mentioned for a given chassis there'd be a specific or a couple specific setup that would work. And I would probably take it a step further and consider the whole (well, more than wheelbase) package - motor/diff/chassis as the power it puts down is rather important to what kind of rear rate you can run - but *I don't know*, this is just how I'd approach finding "that" setup or at least the right zip code.

 

well if we work backwards and use the same equations, corner sprung weights, and motion ratio to achieve that 10% difference in frequency between front & rear and a goal of 1.9-2.2 cycles per sec. as the natural frequency target range for bumpy race courses:

The "best" setup ranges from a 650 front/410 rear to a 850 front/365 rear.

(use 2.0 and 2.2 as your frequency and 770 lb. front sprung weight, 490 lb. rear sprung weight , front motion ratio 1.5, rear motion ratio 1.35

Frequency = 3.133 SQRT (WR/Sprung Weight)

WR = Spring rate/MR^2 )


Now Don Alexander's handling handbook says you should have the driven wheels sprung stiffer than the non-driving wheels for 2 reasons:

1. as I said before, traction out of the corner exit upon acceleration

2. allowing you to run a smaller swaybar to get the same coupling balance and stiffness, since swaybars tend to lift the inside wheel the bigger they get.

Now I'm not going to resolve the age old front stiffer than rear setup vs. rear stiffer than front setup here....but

of note, if you want a rear-stiffer setup with these corner sprung weights and natural frequencies you're going to have to use 710 front/440 rear.

Notice that in none of these are the rear spring rates greater than the front.

We've had discussions around wheel rates determining rear-stiffer setups but the difference here is the front-rear weight distribution and natural frequency factoring in the discussion.

Just thought I'd add that tidbit to the table...

 

perhaps this is why most Japanese spring kits are front stiffer bias?

if you look at the newer Integra Mugen N Zero kit the front rate starts at 16 kg f/mm and goes to 22 kg f/mm in 2 kg f/mm increments. The rears start at 6 and go to 14 kg f/mm.

You cannot get a stiffer rear spring rate here.

So you'll have to go back to the older Mugen N1 kit if you want that or of course, custom springs.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Michael Delaney &raquo;</TD></TR><TR><TD CLASS="quote">Notice that in none of these are the rear spring rates greater than the front.</TD></TR></TABLE>

Noticed! So are you drawing a conclusion for real world applications or are you just calculating a hypothetical scenario by taking into account just certain (ideal?) variables. Regardless, there is no way I can contradict or support what you are saying with more equations.

However, in the real world there may be more factors that go into picking spring rates than just equations alone. There has to be a reason why WC touring fwd cars run a f/r spring rate ratio opposite of what you describe and why I haven't seen any fwd IT car around not doing the same. I wonder if what you are calculating takes into account ideal conditions where all 4 wheels are optimized on delivering maximum grip and zero slip. Is this the goal here? If so, this might not apply directly to everything, as a lot of fwd cars are setup so that the rear is not always planted. How can rear end slip (rotation) be factored into an equation (or is it)? If so, how are such things that do not always following the rules of ideal 100% grip conditions accounted for numerically? Ok I’ll stop now since what I just said probably won’t make any sense to me when I read it again in the morning, just as it does not for anyone else reading it! I also don't personally know any mathematicians racing either. Who knows, maybe they do take the limit of the inverse laplace transform of it all until the perfect setup hits them in the face, while someone like me just has to stick with what people tell him that seem to work best.

edit: saw last post
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Michael Delaney &raquo;</TD></TR><TR><TD CLASS="quote">perhaps this is why most Japanese spring kits are front stiffer bias?</TD></TR></TABLE>

For what kind of applications?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Michael Delaney &raquo;</TD></TR><TR><TD CLASS="quote">It is said that you can use the natural frequency of a corner to determine the desired spring rate for your car.

</TD></TR></TABLE>

who said? i know natural frequencies, but all those equations you listed are for steady state phyiscis 101 equations. unfortunately, life isnt steady state, simple equations. you cannot number punch and crunch real world numbers. you have to try empirically, thats it. doing anything otherwise is simply going to mislead you. try to figure out the spring constant for every bushing, it throws everything off if you try. to ignore that is to over simplify the system. makes for great midterm questions to simplify the system, but not in real life. so, basically, youre barking up the wrong tree by throwing up some numbers with a few square roots here and there.

show me something empiricial and ill be impressed. in otherwords, drive the car, dont calculate the car.

 

Hello Michael,

You can under take every fragment of mathematic calculations in the universe in an attempt to prove your thoughts and ideas but if you can not visualize why the two driving wheels of a four wheeled rigid frame vehicle must be softer sprung than the opposite end supporting wheels to provide traction, the math will serve little purpose. Get out your toy cars and take another look!


Modified by DB1-R81 at 2:38 AM 7/11/2003

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Michael Delaney &raquo;</TD></TR><TR><TD CLASS="quote">perhaps this is why most Japanese spring kits are front stiffer bias?
</TD></TR></TABLE>

That doenst go all the way toward explaining the rest of the preferred JDM-land setup.... tire stagger, camber settings, etc, etc.

 

I think the rear stiff concept is based on comprimising to maximize front performance. I do not think the rear stiff concept intends to have the "ideal" or best suspension performance in the back, but rather to do your best to have it in the front. The rear is stiffened to reduce front lateral weight transfer-- sure this sacrifices the grip at the rear, but on a car prone to understeer, people might consider rear grip to not be the thing to strive to maximize.

Likewise, people don't strive to maximize their rear braking power when they are having problems with the front brakes overheating.

I think this discussion is a little bit interesting though.. I wonder what issues I might be ocerlooking by replying so quickly. I'm sure it will get pointed out..

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">Quote, originally posted by Catch 22 »

Scott, who prefers the old method of picking up the phone, dialing the number of somebody doing well in your chassis, and saying "Hey, what rates are you running?</TD></TR></TABLE><TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">Originally by Michael Delaney:
That's like asking: " hey, what do you prefer: blonde, brunnette, or red head?"</TD></TR></TABLE>
hmm.. see if I can nest quotes.. edit: can't!

hehehe.. anyway, just a chuckle.. this seems to imply that you will soon be posting the math showing redheads are, in fact, demonstrably superior.

As for "seeing what works".. I work in R&D.. There is nearly _ALWAYS_ a clash between theory/model and data/results/real world. My opinion has become: theory is _great_ to guess at the _what_ of something until you have data... if the data and the theory don't agree, you gotta trust the data... the new role for theory is to help you understand the _why_ of that thing.

Example: I helped design and build a trebuchet for the World Championship Punkin Chunkin. My model predicted a certain performance. When the thing was built, it performed differently. I worked with the real world thing, gathering data until I could form a reasonable understanding of why my model didn't work (what wrong assumptions I had made, and what I hadn't considered). With that information, I had a fresh new model. With that new model, I recommended some modifications to the machine. We performed them and saw a 33% increase in performance (distance thrown). That performance increase agreed with my model's predictions (within the accuracy I expected from my model).

dang.. long tangent.

 

Do you take into account the addition to spring rates of the sway bars?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by MechE00 &raquo;</TD></TR><TR><TD CLASS="quote">dang.. long tangent.</TD></TR></TABLE>

But that's a great pun for Punkin' chunkn' Trebuchet... you are going for the longenst tangent right!

I really don't like math too much - Calculus I and II kicked my but and changed my mind about becoming a Mechanical Engineer. I do like to test things out and see how they work - trial and error while applying basic physics principles works for me.

 

Tuan,

There's alot of valuable insight to be gained from the authors you're reading. And there's alot of valuable insight that you WON'T get out of them too. None of the classic texts will help you understand how to make a FWD car work - unless you start from the very basics.

Go read my basic FWD summary: https://honda-tech.com/zerothread?id=285747

That's an example of how you really should try to work out the basics - non-numerically.

But, Spring Rates themselves are numeric - how do you select? I've said it so many times: you go lower and stiffer till you don't go any faster or the car becomes undriveable. That's costs alot of time and money. If you're lucky - and you are with your Integra - your car has been highly developed over a decade and it's solutions are well known (to all those who don't refuse to recognize it).

In a recent thread started by someone asking about the same basic thing I quoted Carroll Smith as saying that we don't need to concern ourselves with Natural Frequency - and it's true we don't. It's alot like believing that the speed of sound is some immutable natural barrier that man cannot fly faster than.

Scott, who understands the desire to understand...it just can't be built on the texts of those who've never successfully thought about FWD...