Spring Rates

[Dave Freitas DFR]

This is a dumb question I know, but that should come to no surprise for anyone who knows me.

Spring Rates, how does one determine a good starting point? I know we need to stiffen up the suspension, but by how much? I have no idea what's on the car now, no markings on the springs and I haven't had them checked as of yet. I hope to get that done this week.

Is there a formula to find a good starting point? Or do we just start swapping springs till we get it right? The car in question is Lola 342 with a 150lb driver.

I told you it was a dumb question.

Also if anyone happens to have a steering shaft
u-joint (9/16" 36 spline to a 3/4" weld/bolt on)
please let me know. Thanks.

[Scott]

Mr. Pare has some specific advice, but for a 5th spring Lola it might not apply. ;)

If I understand correctly, he feels that the wheel rate in pounds/in should be equal to, or on the order of, the corner weight in pounds. Note you have to convert from wheel rate to spring rate to get the right spring number.

Some folks feel that really high spring rates are not appropriate for older chassis, however, as it really beats on the suspension parts and chassis. The reason you might end up with a high rate is that the wheel/spring motion ratio will not be anywhere near 1:1 and the wheel-rate-to-spring-rate multiplier can be 4 or more.

That being said, I have a stiffened 340, and I use a moderate 350# on the front and 300# on the rear and I'm happy with it. If the springs you have are old and rusty, toss 'em and get some Hypercoils.

Also remember that a higher spring rate will no doubt change the length of the spring at static load, so measure the current at-rest length, the mid point of the perch adjustment, the current spring rate and free length, and choose the new spring length accordingly to put you at mid-adjustment at the desired ride height. Or do like I do and buy the wrong ones first. [img]smile.gif[/img]

(Carroll Smith has a valid point that you don't want the spring to uncouple at full droop, or to collapse completely at full bump either.)

tim

[size="1"][ January 14, 2003, 12:53 PM: Message edited by: timos ][/size]

[PRSCHMN]

I redid a 1981 Crossle 45f recently.

Car had original 200 lb springs.
Neil Porter (Pacific Crossle 209-722-7373) recommended 400 front & 350 rear. This supports rate near cornerweight theory above, but backwards. More weight is on the rear. Total spring weight is 1500 lbs vs 1109 lbs actual with driver & fuel at end of race, a 400 lb discrepancy. Probably heavier on front to prevent excessive nose dive under hard braking.
My lap times are middle-of-the-pack.

Give Neil a call to check things out.

[Chuck Gillis]

John: The Kid is too darn fast as it is. Good luck getting any more data outta me!!! You are welcome to try any of the springs in my box, you know the springs that AREN'T in the car. Ok, call me and we'll talk about bribes, Annuals and such. gm

[Dennis Cleary]

As the third member of this "team", you are welcome to my collection of springs. Give me a call...I will trade you for belting me in this year. The wife is never going to get the arm strength!

I would compare the wheel rate you have now to the recommended 1 to 1 setup and go halfway between to start. BTW - what's your preload? any?

Greg, we gotta change the rules to outlaw drivers under a 36 inch waist and 200 pounds; 150 pounds is just not fair! ;) (Remember, he was faster than us with loose lug nuts)

[Jeff Smith]

remember wheel rate is MRsquared x spring rate, not just the rate of the spring. First determine the motion rate (MR) then work backwards to determine the proper spring rate for the wheel rate to equal wheel weight.

On my RP31 I went what i believe is to stiff. meaning changes in spring rates were not being felt like i thought they should. I started backing the rates down until a spring change made a difference. I believe that the rates had gone past the stiffness of the chassis. This maybe more of a problem with your car as i have heard that they were once called the flexi-flyers.

Good luck and if you need more info consider one of Richard Pare's seminars. It is well worth the time and money invested.

John

[Ravemotorsports]

FYI: According to Carrol Smith's Tune to Win, wheel rate = spring rate DIVIDED by the square of the motion ratio, NOT spring rate X MR squared. OK, having said that here's my question, based on the many threads, including this one, which says to start with a spring setup in which the wheel rate in lbs/in should equal corner weight.

For my 79 Tiga, Front corner weight is 232lbs. Motion ratio (squared)= 4. Rear corner weight is 337 lbs; motion ratio (squared)= 3. This works out to a spring rate for the fronts of 928 lbs/inch and 1,014 lbs/inch for the rears. What's wrong with this picture!

[Jeff Smith]

Will, You are correct, but it depends on how you determine the motion ratio. If d1 =shock travel and d2 =wheel travel then with the ratio of d1/d2 then WR=MRxMRxspring rate. And vice-versa if the MR is d2/d1 then WR=spring/MRxMR.

So does you Tiga have a front MR of 2:1 (d2:d1) and a rear MR of 1.73:1 ? Better get them springs short because you will only depress the fronts .25" and not much more on the rears...no body roll ...lol

My Rp31 has 1.5:1 front and .96:1 rear

to figure what springs you need use MR as d2/d1 then MRxMRxWR. That should give you the proper springs for WR and WW to be equal.

These formulas are courtesy of the ICP seminar workbook page 53

John

[size="1"][ January 15, 2003, 07:32 PM: Message edited by: John Robinson ][/size]

[Chuck Gillis]

Will: I think you MR is more like .5 not 2. Shocks at roughly 45 degrees are going to move at about 1/2 the wheel movement. Haven't looked at Dave's car for a bit but my flawless memory says it's in that range, about the same as the Lola.

The rest of you quit pickin' on Lolas. The real 5th spring chassis was the 440, due to the spacer and engine abortion. A properly stiffened T340/342 is pretty stout. Heavy as a consequence as well.

Morgan, me 'n Tim are gonna force-feed that scrawny kid till he weighs enough to take the 30 pounds of ballast out, then we're gonna sneak it back in at night when you're both sleepin'.

gm

[Ferris11]

OK,

Good thread, .. but I got one, .. how do you figgure the wheel rate, or spring rate for a mono shock car???

Curtis
vector FF

[Ravemotorsports]

John, your analysis is correct. My initial calculations for MR used d2/d1, where d1=shock travel & d2=wheel travel. This gave MR(squared)= 2 (front) and 1.73 (rear. I then used wheel rate = spring rate = 232lbs/in (front) and 337lbs/in (rear) in the equation WR=spring/MRxMR to get the spring values of 928 (front) and 1014 (rear). Greg's MR values of 0.5 and 0.58 are correct for MR=d1/d2. When you solve for spring rate using WR=spring rate/(d1/d2)[squared], as expected you get essentially the same values for the springs: 928 fronts & 996 rears. So, it appears that, in this case, the theory doesn't track and I'll have to get to the track if and when I can.

Thanks to everyone for input.

[timmy]

Curtis,
You should fly down to the ICP seminar in Atlanta. Before I go, my guess is that you just calculate the number as if the two front wheels were one. So if you wanted a WR to match the two corner weights, you would be shooting for a WR around 520 (est). Nah... it couldn't be that easy. When I get back from the class maybe I'll have a better answer. [img]smile.gif[/img]
Dr. Will,
I have to believe you are measuring the wheel rate wrong. Not knowing your make chassis... I still gotta believe you are running a wheel rate closer to 104 with those 232 springs.

[Ferris11]

Frog,

Right now I'm running a 200 lb. front with 3/4" pre load and 550 lb. rears, ...

I tried adding more pre load to the front just to see what happened, .. the car responed better at turn in but the front didn't settle & grip.

I think if I added rear pre load or better yet a higher spring rate all around the car would be more resonsive.

To me it feels kinda "mushy" right now, .. too slow to take a set at / after turn in.

Frog, .. thanks for the help, .. I WISH I could get to Atlanta!!

Curtis

[timmy]

Curtis,
You really really should call Smith down at 10/tenths.....

[Rick Eskola]

I had the same trouble with a car I just bought I had 2 of the original springs (I lucked out one front and one rear) and wanted to know what the current spring weights were.

I did not know the corner weights so I used a formula that I found in Fred Puhn's book "How To Make Your Car Handle” the formula is:

spring rate = (coil wire diameter to the 4th power * 12,000,000)/(8 * (number of active coils +.5) *((outside Diameter of the coil – coil wire diameter) to the 3rd power)

or if your math is rusty you can use the spring rate calculator I made at [url="http://www.webdesign.zoneit.com/racecar/calc.html#spring"]www.webdesign.zoneit.com/racecar/calc.html[/url]

I used this to order my starting point springs.

I am going to add some more calculators to this site, wheel rate will be the next.

If anyone wants any others please let me know.
If you have the formula send it to me, I have a few books with a lot of formulas, so I might have what you are looking for. Hope this help someone.

Eddie

[Al]

Curtis,

on 2L VD cars, we used 4 digit spring rates up front. Keep in mind when you add pre-load or additional spring rate, that you need to compensate with shock adjustment in most cases. If you added preload to the front with no additional rebound dampening, it would not feel settled mid turn because it is now "springing" the car back from bump faster. It probably felt better at turn in since it didn't roll as far in the front.

Regards,

Bill Vogeley
10/Tenths

[Ferris11]

Bill,

Thank you, ..

Ummm, .. duhhh, .. I added the preload and never tried anything with the shock.

These cars are complicated, .. but I'm starting to "get it", ..

Now I can't wait for the next test day to try some things.

Curtis

[Dave Freitas DFR]

Boy Oh Boy, and I thought this was a simple question. You guys are great, thanks. I guess I'm doing some number crunchin this weekend.

Greg, don't forget I sleep with one eye open.

[Lebowski]

All this got me wondering so I went out and measured one of the front springs on my '84 Reynard. I got 0.360 wire diameter, 3.5" outer diameter and 6 coils (how exactly are we counting free coils).

Those numbers in the calculator come out to 125 lbs. Can that be? Sure would explain alot.

[Chuck Gillis]

John: One of these evenings I'll drop my box of springs in your garage. BTW, Neil has the spring rates in his setup notes, I'm going down there tomorrow to do Annuals, will pick his brain, check my setup and let you know this weekend. Can you do something about the fog please?

On that one eye open thing, does CALTRANS give classes in that or did you pick it up in the field?

gm

[Rick Eskola]

Dave are you sure the outside diameter of the springs are 3.5? I thought most race springs were either 2.5 or 2.25 inch inside Diameter.

If it is a 2.5 which would give you a outside diameter of 3.22 that is 2.5 plus .72 (.360 times 2) this gives me 166 lbs.

The active coils are counted from where you can see light between the springs on both ends.

Also make sure you get a correct measurement of the wire diameter as just .005 makes a 10 lbs change with your other numbers.

I used a Vernier Caliper what one is and how to use it can be found here:

[url="http://www.physics.smu.edu/~scalise/apparatus/caliper/"]http://www.physics.smu.edu/~scalise/apparatus/caliper/[/url]

The measurements on mine where 3.0 outside dia. .375 dia wire and 8 coils front and 7 coils rear.

Hope this helps.

Eddie

[Lebowski]

I used a caliper to measure the wire diameter and that's where the .360 came from.

I'll go out when I have a bit more time and double check all those numbers.

Thanks!

[El Guapo]

Careful trying to theoretically calulate the spring rate. Been there. It'll get you kinda close, maybe within 50lbs if you're lucky, but you really need to measure on a spring tester. Paint thickness can be around .005", so measure directly on the metal. Also carefully measure the wire for slightly oval cross section. That can really throw off the calculation. The number of coils seems to be a big factor in my experience, all might not be as it seems due to effects happening in the last (end) coils throwing off the calc. Try calculating on a spring with a known rate and you'll see what I mean!

[Scott]

Yep.

Machinery's Handbook (I have 25th ed.) goes into excruciating detail about calculating helical spring rate... like how the the number of active coils is determined, cross section of the wire, spring steel types, curvature correction, etc., etc.. All too complicated for me.

[timmy]

Dave,

I would not be surprised if your '84 Reynard had 125# - 150# front springs. If you decide to stiffen, try to do it scientifically. The first jump should probably be to get the wheel rate up to corner weight. OBTW, that probably will not be a really large jump. '84s can not handle some of the high rates the new cars run...the rockers will usually bend.

[JONKATHY]

Where can I have my spring rates checked in RI or Mass?

Also- for someone who needs an electronic spring tester:
[url="http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=33582&item=2405069691&rd=1"]eBay Auction[/url]

[Bullitt]

Mike,

I've got a guy in Warwick who can do it, if you can't find anybody closer I'll dig up his number.

Geoff

[JONKATHY]

Geoff-
Since I work in Providence, Warwick would be perfect. If you find his #, please let me know.
Thanks.
Mike

[Aiken Racing]

I guess I have a related question. On my citation I have rockers still on the front and pushrods on the rear. So the rear I have good motion rations in the rear and I run about 450lb springs in the rear – 380lb wheel rate. I use1400lb springs in the front to get good corner weights due to a bad motion ratio – 258lb wheel rate. My wheel to corner weight is about ~1.3-1.1 to 1.

The question I have is jumping up and down on the front end equal to any of the forces seen out on the track? Since most racecars are setup with Anti-dive and anti-squat it seems that the totally vertical loads I am putting on the chassis with my feet are much different then what is experienced on the track (lateral loads). In the garage if I jump up and down on the front of the chassis I can get the rub strips to touch the ground. I have the front end set at about 1" clearance and the rear at 1.75" so it's low and I think I need to jump up a little higher in the front for spring. Last year when I ran the car a few times I wasn't scraping the ground under braking BUT I was running too big of a master cylinder for the front brakes and I wasn't getting any pressure. Last month I went down to a 5/8" but I won't be able to test it till the weather gets better.

I know that the forces of jumping up and down on the front don't replicate track conditions but if I can hit the thin rub springs with a good jump should I be going to a stiffer spring? Or should I just wait till I get it on the track and see?

Thanks,
Chris

[Darrell Benner]

Chris,
All I can tell you is what Steve Lathrop told me when I talked to him about this identical subject a couple of years ago: "Quit jumpin' up and down on your car".
You need to test the car under real situations. (you also ought to get ICP to make a new front suspension for you).

Jim

[Scotty Segers]

I've used the "jump-on-the-car" technique to get a rough idea of where my dampers are set, trying to see if the response is under-damped, etc., but it doesn't work for spring rate. I can crash my car into the ground in front by jumping on it, and it may not hit bottom on the track. Remember that when you are standing on the front, you are adding ~50% more static weight than it really carries, and, as a previous post said, anti-dive is not accounted for.
BTW, I run zero anti-dive because any significant amount of front anti-dive can cause a loss of front grip.

[size="1"][ 02-27-2003, 10:07 AM: Message edited by: DaveW ][/size]

[JONKATHY]

What goes where?

Car is 1976 Crossle 30F:

I think these #'s have something to do with valving:
2 shocks marked 228-64
2 shocks marked 260-90

2 300 lb. springs
2 350 lb. springs

Can someone tell me the proper combinations and where they go on the car?

[Bullitt]

Found it:

LaRocca Engineering, Inc
86 Warwick Industrial Drive (behind the airport)
Warwick RI

401-739-4990

This guy does a lot of roundey-round work, but he's got a digital Intercomp spring test rig.


Geoff

[JONKATHY]

Thanks Geoff