Spring Rate for Coil Overs

[DRD57]

I just about have the coupe all back together but I need to replace the rear coil over shocks. The old ones are just plain wore out.

What spring rate do you think I should use?

[Dave]

I was told 250 for my fenderless glass 32. I think 220 would work better for me.or even 200. Since you car is steel and has fenders and my thinking is everyone figures em a little heavy id go with 250 on your car.
My nickle..
Dave

[DRD57]

I was told 250 for my fenderless glass 32. I think 220 would work better for me.or even 200. Since you car is steel and has fenders and my thinking is everyone figures em a little heavy id go with 250 on your car.
My nickle..
Dave

Thanks, I'll go with the 250's.

[Bob Paulin]

I just about have the coupe all back together but I need to replace the rear coil over shocks. The old ones are just plain wore out.

What spring rate do you think I should use?

Can't you just replace the shocks and re-use the current coil springs?

Bob Paulin

[DRD57]

I would probably have done that if I were just replacing them with the same shocks and if the old springs were of known quality.

I changed to Aldan shocks. The existing springs were not the same length. For some reason one was about 3/8" longer then the other. They had about 60K miles on them since I got them and they were used then so, I figured it couldn't hurt to step up to a fresh set.

[Bob Paulin]

Okay......just a short note for the back pages of your notebook....

If you have "crushed" a spring out of its original free height, that means you have probably continuously exceeded the capacity of the spring.

Imagine taking the spring out of a ballpoint pen, and stretching it beyond its capacity. What happens? It gets distorted, and never returns to its original free length.

The same thing can happen when you crush a spring beyond its capacity, and it never returns to its original free height.

If your current springs do not have a spring rate etched on them (usually on top/bottom of the spring on the flat part of the pigtail) or if it is illegible due to 60k miles worth of exposure to the elements, take them to your local speed shop that specializes in oval track, and have them rated. It shouldn't cost more than $5 per spring.

You might even consider asking them to scale your car. You would then know exactly what your L/R and F/R weight distribution was, and how much each corner of the car weighs. That would go a long way in determining the correct spring rate for each corner.

If you were a little closer to downtown Maine, I would be happy to rate the springs and scale the car for you.

Then, I would suggest increasing the rate on the new springs a bit - maybe 50 pounds or so. A compression of three inches to installed height would give you a 150 pound increase in load-carrying capacity per spring.

I believe the Aldans have an adjustable spring seat (at least the ones I almost bought at Stafford last week did) so you should be able to set your ride to correct height - even with slightly stiffer springs.

Some oval-track shocks such as the Carrera and Koni have two or three different groove locations for the snap ring that supports the threaded tube, so you can actually run different length coilover springs on the same shock.

I don't know about the Aldans.....

Bob Paulin

[DRD57]

If you have "crushed" a spring out of its original free height, that means you have probably continuously exceeded the capacity of the spring.

This probably happened when I hauled Carps and his steamer truck full of luggage from St. Louis to Los Angeles
... take them to your local speed shop that specializes in oval track, and have them rated....

I live in SoCal. Oval track doesn't have much of a presence here.

Then, I would suggest increasing the rate on the new springs a bit - maybe 50 pounds or so. A compression of three inches to installed height would give you a 150 pound increase in load-carrying capacity per spring.

I suspect the originals were 200 or 220 lbs. I'll check to see if there are any markings.

I believe the Aldans have an adjustable spring seat (at least the ones I almost bought at Stafford last week did) so you should be able to set your ride to correct height - even with slightly stiffer springs.

Correct.

[Dave]

If you have "crushed" a spring out of its original free height, that means you have probably continuously exceeded the capacity of the spring.

This probably happened when I hauled Carps and his steamer truck full of luggage from St. Louis to Los Angeles
... take them to your local speed shop that specializes in oval track, and have them rated....

I live in SoCal. Oval track doesn't have much of a presence here.

Then, I would suggest increasing the rate on the new springs a bit - maybe 50 pounds or so. A compression of three inches to installed height would give you a 150 pound increase in load-carrying capacity per spring.

I suspect the originals were 200 or 220 lbs. I'll check to see if there are any markings.

I believe the Aldans have an adjustable spring seat (at least the ones I almost bought at Stafford last week did) so you should be able to set your ride to correct height - even with slightly stiffer springs.

Correct.

Me again.. I talked with pete and jakes about my viper coilovers and i was told if i didnt have to dont crank the springs up to correct the ride height. Also jerry said if i needed to do it dont go over 1 inch of thread showing at the bottom of the shock under the adjusters. I lowered mine back to the bottom and if i dint hit the frame with the rr thats where ill leav it although im assuming after a few miles ill have to crank it up a bit to adjuts the frame to rear end clearence. I have chrome springs on my vipers. Jerry told me that the were probably aldan springs cause pete and jakes were just starting to offer chrome springs. If i had it to do over id have gone with the black springs. Number 1 they arent changed by the chroming process and number 2 they are marked so you know what rate they are. The chrome ones arent marked..
Im driving it to franks tomorrow for the bbq build and i need to get some miles on it. Ill let ya know how it rides but i know around here its kinda choppy. I do have the rebound set at the max adjustment and that helped.
How many adjustments do the aldans have ? mine has 6. I looked at the bilstiens also and im confused at how any shock can automaticaly adjust to road conditions. I dont care what fat jack says i just dont believe it LOL...
Dave
After that im going to take a spelling class........................ Ha Ha

[DRD57]

Aldans have 6 settings. The installation sheet says most guys run them at #1 or #6, rarely anything in between.  

I got the ones with the black springs. You can't see them when I'm hauling down the road anyway.

[Bob Paulin]

......

>>SNIP<<

I looked at the bilstiens also and im confused at how any shock can automaticaly adjust to road conditions. I dont care what fat jack says i just dont believe it LOL...

Ah, yes.....

The magic of advertising hyperbole.....

Years ago - when the earth was still cooling, and I was working as a line technician - Monroe Shocks came out with the advertising hook that they were "self-adjusting" to road conditions.

Well, the truth of the matter is that, due to valving design, a shock absorber DOES react differently to a small smooth bump than it does to dropping a wheel into a pothole.

You can compress a shock by hand, but you cannot compress it quickly due to the hydraulics involved internally, so it "self-adjusts" to resist the amount of pressure you are applying in trying to collapse it.

And, even though you can compress it the full-length by hand with moderate effort, if you hit the top of the shock with a hammer, it will not completely collapse.

IOW - it has "adjusted" itself to handle the two different inputs - light, smooth input from your human muscles, and heavy hammer impact.

In this sense, *ALL* shocks are "self-adjusting." Some companies' ad agencies latch onto this and exploit it with the careful application of the English language, smoke and mirrors.

External adjustments are actually used to fine-tune the level of hydraulic resistance, but externally adjustable shocks are also "self-adjusting" in the above sense.

Bob Paulin

[Phat]

You should rate any spring you use as they are allways mis marked or not a good match.  Bob can tell you this as a racer....i bet a have 5 or 6. 200lb coil over springs and some rate at 185 up to 230.  Thats repeatable.  Most good chrome springs we use are engraved with a rate.  But i guess i get most of my spring from race supply houses that tend to be a little more carefull with spring rates as racers tend to check and send stuff back.  Good idea to scale your car ...you wont believe the diff in the way it drives.  Just think ever turn extra that you put in the left rear coil over takes weight off the  the left front and puts it on the right front.  Wish more guys would scale cars as they would not believe the diff in the handling and balance. Would have to guess but on most coupes(steel) i would run 175s to 225s across the rear depending on the wheel weights.
Did you weld that old housing up LOL

[flt-blk]

Once you weight the corners how do you achieve a starting spring rate.
I have access to scales, how do I translate corner weights to a coiliver
spring rate.  

The car is a Model A pickup, gas tank in back.  I currrently have 250#
springs and the shocks are leaned in at 25 deg. so they are much less
than 250#.

Not trying to steal your post Don, I just want to understand the reasoning
and learn a little from the race car guys.

Thanks,
TZ

[Bob Paulin]

Once you weight the corners how do you achieve a starting spring rate.
I have access to scales, how do I translate corner weights to a coiliver
spring rate.  

The car is a Model A pickup, gas tank in back.  I currrently have 250#
springs and the shocks are leaned in at 25 deg. so they are much less
than 250#.

Not trying to steal your post Don, I just want to understand the reasoning
and learn a little from the race car guys.

Thanks,
TZ

TZ:

You might want to pour yourself a fresh cup of coffee before you start reading this one....

You really don't translate corner weights directly into spring rates - but you need them to help determine weight transfer.

You must first figure out your motion ratios......

On the front end, that would be the distance between the inner pivot and the ball joint divided by the distance between the inner pivot and the spring mounting point.

The rear motion ratio on a standard rear axle is determined by dividing the distance between spring mounts by the distance between wheel centers. You would also use this formula for a straight front axle.

IRS is more like the independent front suspension formula described above

On the IFS, if the first measurement was 12 and the second measurement was 8, you would have 8/12 for a motion ratio of .666.

That means that a 250 pound spring would have an effective rate of 166.5 pounds (.666 X 200) if it were mounted straight up.

Now, tipping the spring in 25° would, according to the Pro-Shock catalog, cause our spring to work at 82 percent of its rate - or .82X166.5 for an effective spring rate of 136.53 pounds from a 250 pound spring.

Now, we have determined that the 250 pound spring will compress approximately one-inch for each 136 pounds of weight on that corner.

Stand the spring up straight, and it will only compress one-inch for every 166.5 pounds of weight on that corner.

So far, though, all that has been determined is the changes that known weight will cause for that spring mounted in that position.

But, you can achieve the same sort of ride height by using double or half the spring rate, and adjusting the coilover sleeve for more or less compression due to spring rate, so we really haven't figured out what the car actually needs.

A spring with half the effective rate will compress twice as much as the above spring in the same application. Vice-versa for a spring with twice the effective rate.


• To come up with a more accurate calculation on what the car will actually need for springs, you will need to determine the car's Center of Gravity height and its front and rear roll centers.

The relationship between the CoG height and RC determines how much weight is being transferred to the outside in a turn, and predicts exactly what sort of spring rate is needed to handle that amount of weight transfer.

The CoG height acts as a lever arm against the RC height, so the further apart these numbers are, the more roll the car will experience.  

If 300 pounds is being transferred from the inside suspension to a 300 pound outside spring that spring will compress one-inch, but if the same 300 pounds is being transferred to a 100 pound spring, that spring will compress three-inches - which creates a huge difference in how far the acr rolls in a turn.

There are formulae that I used to use with a pocket calculator to figure CoG height, RC height and weight transfer, but I now use a chassis analysis program into which I simply plug raw numbers, and it does the figuring for me.

If you could borrow a copy of "Race Car Engineering" by Paul van Valkenburgh, then you would find all the formulae you need. Maybe a library in your area has a copy.

Now, depending upon how much you enjoy crunching numbers, you can come up with something pretty close using various formulae.


• Having said all that, I can suggest a simpler, somewhat less technical method of determining if you have enough spring on any particular corner of the car.

Place a tie-wrap tightly on the shiny chrome piston rod of each shock. Trust me! It won't hurt anything. We do it all the time on racing shocks.

Slide the tie-wrap along the piston rod until it contacts the shock body. (Note that I didn't say up or down since monotube coilovers are run with the shock mounted upside down.)

Now, simply drive the car through some moderately severe turns. As the shock compresses in the turns, the tie-wrap will be forced up the shaft.

You can then measure how much the tie-wrap has moved, and determine if you need more or less spring at that particular corner.

Keep in mind, and apply your motion ratio at the shock to more accurately determine how much the wheel itself is moving up into the fender/wheelwell.

Also, keeping the effective spring rate in mind, you can pretty closely determine just how much weight is being transferred by factoring effective spring rate and shock movement.

For example, given the above example, we know that a spring with an effective rate of 100 pounds moving three-inches would tell us that we were transferring 300 pounds of weight to that corner in that particular turning situation.

If you wanted the shock to only move 1.5 inches - thus reducing the car's roll in the turn -  then you could replace that spring with one having a 150 pound effective rate.

A series of tests with your current springs could help you in determining whether or not you want/need to change spring rates - and help you to determine exactly how much of a spring rate change you want.

I am slowly building an inventory of springs which I can loan/rent to a customer to try before he buys the new set.

On an oval-track Pro-Stock with a front motion ratio of between .8 and .9, and a pretty straight-up shock mount (approximately 5° tip), and, if I haven't actually measured it, assuming a 1g cornering force rate, I like to see about three-inches of shock compression - but I "adjust" my thinking in a number of different cases such as extremely flat or extremely high-banked tracks.

Even on a big coil car, with a motion ratio closer to .5 or .6, if the shock is mounted outward with a .8 or .9 motion ratio on the shock (which we always try to do when the rules allow it), I still look for approximately three-inches of shock movement.

Just so you'll know, it is not unusual to leave the shop with the "correct" setup in the race car for a particular track's turn radii and banking as determined by the setup package, only to change one-or-more springs and/or shocks at the race track to compensate for a number of
things including driver preference, track conditions, and a stackup of tolerances that occurred when measuring various inputs for the setup program, thus skewing the numbers slightly.

I guess what I'm saying is that no matter how exact you can be with engineering calculations, the real world can often be different from what "...it says right here in the book...."

Hope this helps.....


Bob Paulin

[1FATGMC]

Great information Bob and written up very well.

Thanks,  Sum

[flt-blk]

I really needed two cups of coffee, but I read it 3 times.

I followed most of that.  I have flipped through the Race Car Engineering
book a the book store, I guess I need to buy it before I build my track car.

As for the current ride I think I will crunch some numbers then try the tie-
wrap idea to see how close I was.

Thanks for the input.  
TZ