Spring rates for asymmetric weight distribution

[datstang]

I have a question regarding spring rates for an AutoX / track day car with left side percentage greater than 50%.

Should spring rates be chosen such that the frequencies of the front wheels match each other and the frequencies of the rear wheels match each other?

The corner weights of the car are as follows:

--------Left-------Right-----%
Front----669--------626----48.4
Rear-----714--------668----51.6
%-------51.7%-------48.3%

The Sprung Weight is

--------Left-------Right-----%
Front----586--------543----48.49
Rear-----613--------567----51.1
%-------51.9%-------48.1%


In working through chapter 16 of RCVD and Smithee's WTW, I came up with spring rates to set the front suspension frequency at 2.49 Hz and the rear suspension at 2.39 Hz. These frequencies were based on the average of the left and right side percentages and assume left/right symmetry and an infinitely stiff tire.

After I finished this (and ordering springs), I realized that the WTW assumes 50% left/right distribution. Applying the spring rates to the actual wheel rates rather than the averages, I end up with the following frequencies at the four corners:

--------Left-------Right
Front----2.44--------2.54
Rear-----2.34--------2.44

If I use lighter springs on the right side of the car, I can get a closer match for the frequencies of the front and rear wheel pair. It seems to me that the pitch motion of the car will load the tires more equally in longitudinal acceleration if the frequencies at the front match each other and the frequencies at the rear match each other.

 

[evelrod]

Conventional wisdom says that on a FWD you set the front spring rates equal and the rear at whatever you get. On RWD the job is a bit more difficult. Too many variables come into play to make a blanket statement.

My Mini Cooper is 490----490 1658 lbs wet with 220 lb. driver and 8 gal. fuel.
RHD and FWD 308----372

My Lotus Cortina 500----500 2020 lbs. wet with 220 lb. driver and 12 gal. fuel.
LHD and RWD 540----480

Setups may vary, track to track, but stay pretty close.

For what it's worth, I would think that sticking to a basic setup and concentrating on driving the car and figure out what the handling is like before you get all crazy with the technical stuff. Anti roll bars (sway bars), wheel/tire, ride height, roll centers, etc. etc. There is a LOT more to making a car handle to a standard than just spring rates. To get accurate data you must calculate from "wheel rate" and not concentrate solely on "spring rate". I've been at this a long time and I still find myself confused most of the time.

Rod

 

[datstang]

Thanks for the reply.

My car is a 1974 260Z with a 5.0L mustang engine. I've been autocrossing this car for about 5 years. Previously it was a dual purpose car (street/Autocross), but I've reached the point where it will no longer see street duty. I want to concentrate on autocross and track days.

As part of changing the car's mission, I decided to get serious about the suspension. I have measured the motion ratios, geometric roll centers, wheel loads, and the unsprung weight.

As part of all of this, I have been reading RCVD and pouring through Mark Ortiz's newsletters and articles in RCE. As I look deeper into suspension analysis, I find myself getting more and more interested. My car is the perfect place to experiment.

 

[GregLocock]

If you don't have any better data then assume a tire rate of 250 N/mm

As I understood it, for non-aero circuit cars for a first cut design, you basically want the softest possible total spring rate for a given circuit, to maximise grip, and then the softest possible on the driven wheels, using the other end to stabilise the car in roll. This makes me think that a frequency based analysis is unlikely to be much help.

Can I also recommend you wander over to

http://forums.autosport.com/forumdisplay.php?s=&forumid=8

and ask the same question.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[datstang]

Thanks for the reply. I was going to ask about a ballpark figure for tire stiffness. I am currently using Hoosier A3S04 DOT Radials (245/45/16) on all four corners. Is the ballpark stiffness that you provided applicable? I e-mailed Hoosier about stiffness and received no reply.

As far as the frequency based design, I am keeping my options open on this one. Several people that I know swear by the higher frequencies(~2.5 Hz) for an autocross car. If I find that I cannot put the power down at the rear wheels, I plan to install the lighter springs that I removed from the rear (250 lb/in).

With the lighter springs in the rear, the natural frequency will be ~1.8 Hz. The front will still be on the order of 2.4 Hz. To balance the car in that configuration, I will either add a light rear sway bar (I currently use none), or a smaller front sway bar.

 

[GregLocock]

I don't know if that rate suits that tire, but it is a typical figure.

High frequency suspensions are typically used where aero is important, or where the fundamental suspension architecture has glaring faults, particularly in camber.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[datstang]

Using the tire stiffness that you suggest (rather than assuming an infinitely stiff tire) reduces my suspension frequency to 2.2 Hz.

Still, because my car is left side heavy, my left and right frequencies won't match unless I install different springs on the left than on the right. The springs that I have to work with at the moment are (2) 450 lb/in and (2) 425 lb/in. Originally I had planned to install the 450 lb/in on the front and the 425 lb/in on the rear. Then I realized that would result in a fairly large difference between the left and right frequencies. So, now the plan is to install both 450 lb/in springs on the left and install the 425 lb/in springs on the right. This brings the left to right natural frequencies much closer together, but they will still be a little high on the right compared to the left. Here are the frequencies that I get using a tire stiffness of 250 N/mm:

---------Left-------Right
Front----2.18--------2.22
Rear-----2.16--------2.19

In order to get the left/ right frequencies to match exactly, I would need to install 410 lb/in springs on the right with the 450 lb/in springs on the left. Since I don't know where I'd acquire springs rated at other than 25# increments, I'll call it good and see if I can reduce some weight on the left side of the car (not going to be easy).

I guess I'm still wondering: Is there a benefit to matching the left/right frequencies by using unequal spring rates?

In any event, I'll be running the car in this configuration in two weeks. So, I guess I'll see then how it works using different springs on the left than on the right.

Thanks again for your input, and I'll try not to belabor the point any further except to post my results.

 

[NormPeterson]

I think I'd be more concerned with making the left vs right cornering behaviors as symmetrical as possible and let the sta-bars and dampers deal with the asymmetric pitch issues. It's not going to naturally "flat-ride" at any sane speed anyway, nor will you be using ride-oriented damping. If you were circle-track racing, you'd get into all sorts of chassis asymmetries by virtue(?) of not having to turn both ways at speed and it would be a different story (probably still wouldn't be frequency-controlled).


Norm

 

[evelrod]

Go ahead and give'er a try. Nothing ventured nothing gained, but for my money your on the wrong track for a road race/autocross car. At least the first high speed corner will be entertaining for the spectators.

Rod

 

[datstang]

I am always entertaining to watch when the car is hard to drive. When all else fails, I steer with the gas pedal.

I drove the car for the first time with the higher rate springs two weeks ago. At that time, I had the 450 lb/in springs in the rear, the 425 lb/in spring up front and a 1" front sway bar. Besides pushing a bit, the car seemed to pitch unevenly. While accelerating and decelerating, the car seemed to yaw a little to the left. I am uncertain of the cause of the yawing because I had not aligned or corner balanced the car after the spring swap. I will have that done before the next event.

Though the car was a little unruly, I still was the fastest of the four Emod cars present.

 

[Echoshill]

I am with Rod on this.

Though I am primarily a lurker here, I have worked as an engineer in North Americas predominate sports car racing series and in production vehicle dynamics for over 15 years. With this, I can say that if I had $10 for every time I put a setup on the car that fit the simulation, only to be made to look the fool because of track or whether conditions, I could buy everyone on this forum a beer. You can go through all the academics, we have all done it, but the fact is the car will tell the driver what it wants. PERIOD!

In my opinion, and you can take it or ignore it, for true road races (not on a Roval) I use only enough spring to get the ride height and static distribution I think I need. You can have a perfect didactically produced setup on the car and you will still chase your behind for hours because the track has the rubber washed off, or the ambient temp is cooler than it was supposed to be so you are making more power, or the mechanic set the bump steer wrong, or the shock program gave you the wrong presets, or the diff is going bad.…etc…etc. The road to hell is paved with the good intentions of engineers who thought that had the car figured out when they unloaded. And believe me, we have a whole darn hauler full of springs, shocks, ARBS (you name it) and we still tend to use much the same package at VIR that we would MidOh or WG.

So before this becomes a book, I am not saying that you should not think about what you are putting on your car, good on ya for that, but don’t get in to analysis paralysis. If it is SoloII, doesn’t the track change every time? And aren’t you running in maybe one or two gears? If memory serves me, once your heat starts, you barely have time to change air pressure so if you get it wrong, you are flat wrong the whole event. Lastly, if you have the money to go to track days, and just want to sit in the garage and change parts, I want to do what you are doing.

The driver is the biggest part of road racing and the easiest and least expensive to change. If you are certain it is not you, then I would buy the stickiest tires and biggest brakes I could keep heat in, and go nuts. The rest will sort itself out over time.

 

[evelrod]

Echoshill:
I like that! When I was an SCCA instructor back in the 80's I would always start by telling my students that one of the most important parts of a race car and the easiest, cheapest part to fix was the "spacer". Of course they asked!

Definition: Spacer---The nut that fits between the steering wheel and the head rest. ;o)

datstang:
One more that I heard Smokey Yunick tell John Timanus at a TransAm in Sept., 1968---"There's a thousand parts to check in a race car and if you got 99% right, ya still missed ten"! With respect to Smokey, I've made it MY mantra!

Rod