why you shouldn't be buying a coilover with 8/6 spring rates

[95lstegman]

well, maybe you should, if you've stripped the rear of your car down to sheet metal (including spare) and drive around with no gas in the tank. then the 8/6 combo works well. otherwise, read on.

SUMARY IF YOU DON'T LIKE READING/LEARNING: run 8/7 instead, as a good guess, if you have stock-like chassis and stock-like suspension geometry (usually including aftermarket arms). less if you're driving on bumpy tracks (or roads!), more if you're driving on something that resembles a billiard table's smoothness.

this is one of the many issues i've been wanting to contribute for a while, but i was just too lazy. sorry if you've bought coilovers and feel kinda crunchy. rest assured, 8/6 coilovers will net you better lap times than stock junk. this is just for those looking to eek out every last thousandth of a second out of their race car.

two words for you: natural frequency. it has to do with a lot of things, the geometry of the suspensions and how different parts relate to the tires' contact patches and to the chassis itself, the mass of the chassis (including driver, fuel, etc.), the mass of the suspension parts, the mass of the parts held by the suspension (wheel/tire, brake rotor/caliper, knuckle, hub), yada yada. in any case, the point is that the spring and damper's job is to keep the tire in contact with the pavement (or dirt, ice, or whathaveyou), not to keep the chassis from experiencing roll. limiting roll is the job of the ARB's, or anti-roll bars, as we call them. limiting yaw is the job of suspension geometry, and the 240SX (all) already have nice anti-squat geometry, which can be accentuated or diminished as required. note that anti-squat geometry does not only affect the amount of squat, but also the rate, and some other minute things, but esp. a slow rate of squat/unsquat can unsettle you as a driver.

anyways, if you want to keep the tire in contact with the pavement, you're going to be concerned with how big your bumps and undulations on the pavement are, and how frequently you hit them. hence, we discuss desired spring rates for a vehicle as a ride frequency, measured in Hertz (Hz). i won't go much into selecting the frequency that you want to start with, as that is a whole other complex issue, but i will suffice to say that stock is 1.15Hz front and 1.09Hz rear. i am not using high-precision measurements for angles and such, just protractor, and ruler, and drafting T, etc. this is pretty good, as a low frequency will give a fairly compliant ride, and allows the tires to keep traction on rather bumpy roads (not potholes and such, but much larger imperfections than a super smooth racetrack). by comparison, our SAE Formula team seeks out 3.0Hz front, 2.8Hz rear, hoping for very smooth pavement. 3.0Hz is for smooth tracks with Hoosiers on a 390lb [dry wt] formula car, so please don't bother choosing that as a starting point if you ever drive on the streets. pebbles will cause your tires to skip over the pavement. note that in both cases, we have a slightly higher frequency in front than in rear. since this frequency also plays an important role in how fast the suspension reacts to driver inputs, you don't want them too far apart, but if they are too close then [as i understand it] the movements/vibrations of the chassis resulting from bumps on one end will input into the other end's suspensions, rather than the chassis isolating one end from the other [so that each can absorb bumps independently from the other, i.e. faster and more smoothly].

so, back to the old 8kg/mm front, 6kg/mm rear springs. for a stock 240SX, with stock 53F/47R weight distribution, and no really crazy staggering of tires, and brakes of stock-like weight (size is almost completely irrelevant, only actual weight matters here, so heavier rotors + aluminum calipers i.e. Z32 is probably about the same as stock), and stock-like weight of 2900lb (with driver and fuel and whatever else is in there), we get 2.07Hz front, and 1.78Hz rear. not that bad, i guess, but they could use to be quite a bit closer. with this setup, the rear suspension will react slower than the front by 16.3%, rather than the 5-10% usually preferred. but just change the rear rate to 7kg/mm, and you get 2.07F/1.89R, a much better match. 8/8 seems to be too much, and yields 2.07/2.00, which might be a tad too close. 7/7 comes out to 1.97/1.89, marginally better, but maybe enough better to warrant a try.

of course, if you're planning on running more than 1-2kg/mm different than your coilovers ship with, you should be having them revalved. choosing your valving curves is nice, but probably won't be possible, and would require a heck of a lot of brainpower and processing power on your part.

personally, i don't have stock weight distribution, so this is very different for me, but i figured while i was busting out the excel goodness, and since i have mostly stock suspension geometry, it wouldn't be hard to put out some good info for people with stock chassis. FWIW, i have 48/52 wt distribution, among other things and intend to run approx. 8F/10R springs (2.29/2.15 ride frequencies), and i'll just stay away from tracks like Sebring Raceway.

i hope you enjoyed this, i hope i made sense, and i hope i was correct in the explanation of target ride frequencies. most my education in the automotive engineering field comes just from buddies at SAE, so i didn't have a textbook to refer to or anything.

[crzycav86]

Your post was all over the place. It was too long and went into detail where it didn't need elaboration(e.g: weight of z32 calipers vs stock..)

Basically, you're saying that the front and rear natural frequencies need to be within 5-10% for the best performance. That's fair enough.

First, you didn't explain what a natural frequency is. Second, you seemed to pull numbers out of a hat without explaining. Third, your post could have been this long to say exactly the same thing.

[E7-S14]

i sorta got itwith my limited knowledge on the subject

but still kinda confusing

[95lstegman]

sorry. i intended the short summary at the top to be the succinct version. i didn't want to go in depth too much, but obviously i went a little off here and there. i tend to do that i guess. i'll get into the natural frequency more when i'm not in class, i.e. won't get in trouble for being on the computer.

[Nismo_Freak]

sorry. i intended the short summary at the top to be the succinct version. i didn't want to go in depth too much, but obviously i went a little off here and there. i tend to do that i guess. i'll get into the natural frequency more when i'm not in class, i.e. won't get in trouble for being on the computer.

What are you using for motion ratios?

Sounds like alot of arbitrary numbers that need to become empirical.

[95lstegman]

not arbitrary, i just didn't go into it. i don't have the tables in front of me (they're on my laptop, which is at home), but the front motion ratio was something like 0.91 i think and the rear came out to a hair over 0.70 (this includes angle and leverage ratio). i simply used the inside end of the lower control arm as the base. the actual center of rotation of the contact patch is close enough to the same distance for these purposes. if you were a race team, maybe not, but then you wouldn't be looking for off-the-shelf coilovers.

there are a lot of other numbers that go into it, and these are just two of them, but ever since i looked at the angles and leverage of each end, and how much lower the rear motion ratio is, it really made me wonder why the heck anyone sells coilovers with softer rear springs than fronts. with same spring front and rear, the rears still turn out quite a bit softer anyways.

BTW, i used the center of the contact patch as my relation, and my rear track is i think 10mm wider than front, but 5mm won't make it so that 8/6 is acceptable, or a good idea. sure, they're stiffer than stock, so you'll get less body roll, and the noob to car modification will be very pleased. but the goal isn't to reduce body roll; it's to keep the tires against the pavement, making frictional forces available more of the time.

[Nismo_Freak]

On a FR car you typically want the front harmonic to be higher than the rear. When you move to something like an FSAE car you want the harmonics to be similar. It has alot to do with the dynamics of the car.

The core reasoning behind the higher front rate is simply to control travel. It's the same story in BMW's, in Porsche's, and other cars that run McStruts.

Another aspect is with RWD cars you generally tend to make the rear a concession point. It's common sense because it's slip angle at the tire (the derivative of much suspension tuning) is easily modified by throttle manipulation. Not only that, but the rear must follow the front. If you can't get the front turned in, then all the perfect suspension in the world is not going to help you in the rear.

What ends up being the problem on 240SXs, as well as 300ZXs, is that the car is very unstable on corner exit. It's very hard to put down the power.

[redsx13]

This is an old post but i feel i need to elaborate a little. Lets just say I have a pretty good connection with a lot of Formula D car setups on s chassis cars. I can tell you that rookie of the year in 2011 was running a 10k front, 5k rear setup. I know it's drifting, but I feel that 90% of 240 owners who track their cars are using them for this purpose. Just a real world example for everyone...p.s the driver also owns and runs a very reputable suspension shop. I would assume that given those two criteria he knows what he is talking about...

Sorry to destroy your natural frequency theory

[Hijacker]

I wouldn't say you "destroyed" what teg was talking about. He was using knowledge gained from working with a formula SAE team to discuss spring rates. Comparing a drifter to the theory and application behind an open wheel road race car setup is apples to oranges.

Alan (Nismo_Freak) worked for SPL Parts in Texas. He knows a thing or two about car setup, and he offered a good counter-point to teg's harmonic theory.

[flohtingPoint]

This is an old post but i feel i need to elaborate a little. Lets just say I have a pretty good connection with a lot of Formula D car setups on s chassis cars. I can tell you that rookie of the year in 2011 was running a 10k front, 5k rear setup. I know it's drifting, but I feel that 90% of 240 owners who track their cars are using them for this purpose. Just a real world example for everyone...p.s the driver also owns and runs a very reputable suspension shop. I would assume that given those two criteria he knows what he is talking about...

Sorry to destroy your natural frequency theory

Drift cars != good engineering. You cant compare a drift vehicle to anything engineered for something other than jackassary. The vehicles for that hobby are setup to do things that vehicles in the sport of racing want to avoid, getting massive slide angles and trashing tires doesn't require science.