I already know my corner weights, unsprung weight, motion ratio starting at 1" below and continuing to 1" above ride height in .1" increments. what do I do now?
Calculating spring rates
I already know my corner weights, unsprung weight, motion ratio starting at 1" below and continuing to 1" above ride height in .1" increments. what do I do now?
What are you trying to figure out? What's on the car or what's ideal? To calculate spring rate you also need to have an idea where you want to end up for ride frequency, wheel rate, and roll rate. Then using motion ratio to calculate what you need.Originally Posted by dgoetz
It depends what you are trying to calculate. If you are trying to calculate the spring rate because you don't know what springs are in your car it would be more accurate to take them out and see if the rate is engraved on the end or take them to someone with a spring tester.
Wheel rate = spring rate X motion ratio squared.
If you want to calculate the spring rate from that you still don't have enough information. You need to measure your wheel rate. You can do that by discoonecting the sway bar and puttinga jack and scale pad under the wheel. Move it a know distance and measure the weight difference. Calculate your wheel rate in lbs/in and then plug into teh formula above. This won't be nearly as accurate as using a spring tester.
Crossle 32F, Piper DF5 Honda
Basically what you now have is the way to tell what the wheel rate is based on the spring rate you install in the car. The hard part is to know what wheel rate you want to have. You've done the science bit, now the art part starts...
What I do is either use the same spring rates as someone with the same car who is quick, or something else. What something else is depends on what kind of car you have and what kind of racing you will be doing. If you share that, more ideas will pour forth...
Brian
wut
You'd have to fix (prevent from moving) the chassis somehow.
How are you going to get the jack under there?
You put the car on chassis stands to get the jack and scale pad under the wheel. You can't use this method on very stiffly sprung cars like cars with aero becuse you will lift the car off the stands. Sometimes you can get a little measurement before it moves. Sometimes having someone sit on the corner of the car helps. That's why I said it's not the best method. Better to know or measure the spring rate and calculate the wheel rate. The OP really didn't make what he was asking clear or give any pertinent information about his car.
Crossle 32F, Piper DF5 Honda
i strapped my car to the stand, and then actually stood on it to add more weight to keep it down and the corner on the scale. toward the upper limits it still started to lift, but by then - i had a pretty good (consistent) curve. so not perfect, but way better than nothing.
BT29-24 Swift DB1 Matra M530
ditto
just what are you trying to learn?
Good Thread on determining Spring Rates
Between PF and Richard and others here is a good thread on suspension, motion ratios, wheel rates and spring rates:
http://www.apexspeed.com/forums/show...t=richard+pare
No jack, cut up two by fours (three high will do it) and make some ramps 45 degree cuts on end and sequential rise with enough room for wheels to sit on 3rd one, roll car up and use regular $39 store jack, put anything underneath and start your examination.......gonna have to raise the car to swap to rains anyway LOL ............car in air measure spring travel vs wheel travel to form motion ratio (perhaps get Smiths books,
above from mousies irony, sounds like you have mesurements, .....do below
actually use Brian's idea, find a hot shoe that runs your car and ask what they use or used when they first got the car, if he is using rock hard springs, you may have to come back a hair,.....rock hard springs can roll your eyeballs around if you are a little pudgy, ruff on wrists too, but fast
Last edited by Modo; 01.08.15 at 6:31 PM.
While I don't advocate the above procedure for measuring wheel rate it is pretty good for measuring what your actual sway bar rate is. With sway bars, blades on a 45 degree angle, and linkage, etc. it becomes difficult to calculate what your sway bar rate is. Take out both shocks, leave the sway bar connected, and put a strut in place of the shock on the side you're not measuring and you can use the scale pad and jack method to measure what your actual sway bar rate is. It has the same limitation. At some point you start to lift the car off the stands.
Crossle 32F, Piper DF5 Honda
At the risk of sounding like a smart/dumb ass, ultimately the stopwatch will tell you what springs you need to run. That number is subject to change of course.
Jim Gustafson
LF RF LR RR
449 457 522 463 C/W w/driver 1891lbs
61 59 83 82 unsprung weight
388 398 439 381 sprung weight car without driver 1685lbs
spring travel 3&1/4in.
Custom built Car has too much under steer with the org 200# frt and 350# rear springs, tried softer fronts and have concluded needs stiffer rears. have calculated 3 different rates and trying too decide where to start, probably 460#
Car used only for USCCA Emod autocross
what are your tire pressures?
Frt 22lbs G/Y 9.5x20x13 R116
Rear 18-20lbs G/Y 13x23x13 R110
Without going through all you math, I can make a suggestion.
Try choosing springs that give wheel rates that are then related to the weight on that wheel. So the wheel rate for the front would be 400 lbs. per inch and the rear would be 450. This makes the ratio of wheel rate to weight on the wheels of about 0.9. For a race car, this is not a bad place to start. It is on the soft side for race cars.
Do you know what the spring rates of your tires are? That is a spring rate for a given tire pressure. When you test, you can change tire pressures by maybe 2 psi and effectively increase or decrease the effective spring rate by 50 lbs. per inch or more. This will give you a good hint if more or less spring will work.
When you change springs, you always want to adjust you ride height by an amount that keeps the dynamic ride height constant.
D2640 (20.0x9.5-13)
0 Deg Camber
1228 lbf/in @17 PSI
1444 lbf/in @21 PSI
2 Deg Camber
1229 lbf/in @17 PSI
1493 lbf/in @21 PSI
D2453 (23.0x13.0-13)
0 Deg Camber
1411 lbf/in @17PSI
1650 lbf/in @21 PSI
2 Deg Camber
1445 lbf/in @17PSI
1678 lbf/in @21 PSI
What is dynamic ride height?
Dynamic ride height is a ride height that results in the car just clearing the ground as it laps a track. Or it is the ride height at which the car works best. With a winged car, it is quite different than the static ride height.
Once you establish a workable ride height for a given track, you want to maintain that ride height as you change springs.
Wheel Rate
"Try choosing springs that give wheel rates that are then related to the weight on that wheel. So the wheel rate for the front would be 400 lbs. per inch and the rear would be 450. This makes the ratio of wheel rate to weight on the wheels of about 0.9. For a race car, this is not a bad place to start. It is on the soft side for race cars."
It's always made sense to me to compare wheel rate to sprung corner weights. Is that your intent? With my old DSR, I started out at about a 0.8 wheel to sprung ratio and ended up about double that as we gradually built in download.
Marty
Last edited by Marty Nygard; 01.11.15 at 1:51 PM. Reason: Corrected unsprung to sprung
That is what I have found works. I have been as high as double the corner weight. But the number will vary with tires and the type of car. But as I develop a car, I might vary a lot form the front and rear wheel rates in the same ratio to weight on the wheel. But I only do this when I find a reason to do it.
Marty, did you mean sprung or un-sprung? Wheel rate = unsprung weight means wheel rates around 50 lb/in or less.
Dave Weitzenhof
Thanks Dave
Good catch. Yes, I meant sprung.
For my design, I decided that cheap was better than light when it came to the uprights. I used standard size CS tube machined internally to fit VW bearings, laser cut CS brackets and std VW hubs--at least twice as heavy as CNC or cast aluminum but only about $75 each. For a light weight car like a DSR, that resulted in a fairly large proportion of unsprung weight but once we began to develop reasonable aero and upgraded to bigger springs, it didn't seem to matter. Of course, this was on a car with no expectation of RunOffs podiums.
Marty
It's sprung weight, usually. Unsprung weight is just the wheels and tires and corner stuff, and what you are holding up in the air is the car, which is the sprung weight. Or what I do is relate the wheel rate to the total corner weight, since I am lazy and that tends to be equally valid for a given car. For example, for a vintage style formula ford on treaded tires I start with wheel rate equal to half the corner weight. This tends to give a compliant, easy to drive car that is easy to tune but probably not ultimately quick. But it gives you something to take to the track that won't be silly.
Brian
I want to thank you all for the input that you have given me. I know I don't belong on this site and my inquiry's are about a vehicle that doesn't belong here. I have an Emod auto crosser and that doesn't fit but you have responded with more input and information than any place else on line. The real title of this thread should of been " How to compute Wheel rates and what is the ideal wheel rates for Emod in Autocross"
I have concluded that the ideal or at least starting point rate for autocross is somewhere between .8 - 1.2 wheel rate to corner weight ratio. What was never brought up or mentioned what does a sway bar or in my case lack of any effect this formula? I would think that without a front sway bar that the front ratio would be higher than the rear and if so how much? I am thinking of starting with a 1 to1 rate on the front and a .8 rate on the rear. Thanks again
Now, this is just the way I think of it. ARB's, front or rear, are used to be an easily tunable addition to the designed in roll stiffness. Things that affect roll stiffness are spring rates, ride height, roll center height, virtual swing arm length. They all affect the roll stiffness, which itself affects weight transfer. More roll stiffness creates greater weight transfer to the outside tire, which affects it's traction, usually more weight transfer makes that end of the car looser. So having an ARB is completely optional, it's just an easily tunable thing that does something that you can get from a lot of other places.
At the end of the day, having absolutely no idea how your front suspension is designed I would be foolish to say what you should do. But I thought this might give you some idea on how I would think about the problem. Same with spring rates - no idea if what you are suggesting is anything like what I would do. What I would do is this - figure out what you can do to tune the car, put on what you think is right in the middle of the range for what you think is the best, drive the car, and start fixing the end that has the least traction. And making it turn in with gusto!
Brian
Since it is for autocross, don't get too stiff. In my experience, non-winged autocross modified category cars should be down around .6 to .8 wheel rate versus corner weights.
Jim
I wish I understood everything I know.
go with what you got it with or ask another guy with ur car ....... don't re-invent unless u like 'WILD GOOSE CHASES' ( Websters: same place 5 years later!!!) honked at a few Canadian Geese walking along the Potomac by the airport a few hours ago, short 50 minute walk not the longer one .... I know, get a life
Last edited by Modo; 03.24.15 at 5:12 PM.
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