Questions about Shock Absorber Application

[vospertw]

My company sells motorcycle shock absorber springs as an alternative to the OEM spring. Our manufacturer recently sent us a batch of springs that we've had some problems with, and I'm trying to ascertain the real culprit. Some of the springs have a bow to them that is apparent to the naked eye. Additionally, they are closed and ground springs but at least one end on each spring is so out of square that when the spring is placed over the shock body and on the base ring, the spring will lean into the shock body. As a comparison, the OEM spring will stand straight when placed similarly. Some of the springs will contact the shock body under compression. Obviously, we are trying to work on a solution with the manufacturer. I want to be fair and hate to return springs that would function correctly; is the lack of squared ends a primary suspect, or do we need to focus on the springs that are not visibly straight? These springs are 235mm long and at the maximum would have 25mm-30mm preload and be subject to no more than 70mm of compression. Solid height is approx. 100mm so they are not being fully compressed.

I've also contacted a second manufacturer. When we received their design, it contained only 3.7 active coils, compared to 8-9 active coils on our current springs (which seems to be much more common across several other companies as well). I have to assume there is some reason that most spring manufacturers producing springs for shock abosorbers are using 8-9 coils instead of 4. The data used for this spring desing was .312 music wire, 9.270" free height, 2.25" ID, 2.85" OD, and a 450lb/in rate. Any advice, including any recommended publications to learn more on the subject, would be appreciated.

 

[israelkk]

Are you sure the rate is 450 lb/in?

"....The data used for this spring design was .312 music wire, 9.270" free height, 2.25" ID, 2.85" OD, and a 450lb/in rate....."

Is this refers to your company spring design or to the other spring with the 3.7 active coils?

As far as I recall music wire maximum wire diameter available is 0.25" beyond that it has no better properties than other wire materials.

Can yo give full details for both springs?
Material type?
Wire diameter?
Outside diameter?
Load at first compressed length?
Load at second compressed length? (or spring rate)
Solid length?
Required life cycle (under what loads and deflection?)

 

[vospertw]

Yes, the rate was 450lbs/in. I do have the design parameters for both spring desings, however, they are currently sitting on my desk in San Diego and I'm in Phoenix. I will post them as soon as I can.

The .312 music wire design was the one with 3.7 active coils.

Obviously, if there were no drawbacks to using 3.7 active coils with the .312 wire (our current springs use wire approx .432 or so), the advantages would be increased clearance between spring and chassis components as well as a significant decrease in weight. I have to wonder then, why no other springs for the same application use so few active coils.

 

[israelkk]

In my calculations I am getting wire diameter similar to yours and number of coils similar to yours. However, to my calculations such a spring doesn't have enough life at all. More than that only spring intended for static application can be achieved and this is with preset!!.

My calculations are based on the minimum guarantied properties of the wires but you can never assume better properties unless you make a special order to sort the wires for you.

When you will have the info I can check again.

 

[desertfox]

Hi vospertw

Firstly let me say the dimensions you have quoted with the 3.7 active turns gives a rate value of 218lbf/in and not 450lbf/in and I have checked this with a hand calculation and an online calculator which I post the link here:-

http://www.efunda.com/DesignStandards/springs/calc_comp_designer.cfm#calc

Now the reason you might not see any other designs with 3.7 active turns is that with a free length of 9.27" the pitch angle of the spring is almost 17.5 degree's which is quite a large angle and the standard formula used for the spring rate is now subject to error (as are my calculation and the online calculator mentioned above) and ultimately results in errors of the calculated stress values.A good reference to this is the book by A.M.Wahl Mechanical Springs (McGraw Hill book company)

 

[sbozy25]

I think I can take a stab at your problem. We just did a market study of similar products.

One of our divisions make high performance springs for cars and bikes. We obtained similar springs from other companies and did an extensive market study. Many manufacturers have real issues keeping parts square, as most requirements are 1°, also spring bow is a big problem. We have found that if we use high tensile chrome sil, and grind each end independant of the other we can usually get an 80% sucsess rate.... As far as the bow issue, they should not have sent you parts that were that bad.... a good spring manufacturer would have caught that problem....

I have an idea of what company they came from... but dont' think it would be appropriate to call them out on here.

 

[vospertw]

First of all, I appreciate the information I've received on this forum. Thank you very much, each of you. I apologize in advance since I now see that I misstated some of the information on active/total coils. I didn't have the data sheets with me and transposed them in my head.

Here's some detailed info on our springs. I will first list the information from our current manufacturer, including the problem-spring in question, as well as a proposed heavier rate spring we were looking to produce. After that I will list the design proposal for the new manufacturer for a spring similar to our current problem spring. I had intended to include their proposal for the heavier spring, but as I look at the data sheet, they failed to use the correct spring rate for their calculations.

Current Manufacturer, problem spring:
Design rate: 426
Wire: .437 music wire
ID: 2.235
Max OD: 3.119
First load: 366@15%
Second load: 2100@85%
Solid Ht: 3.687
Free length: 9.37
Active coils: 6.4
Total coils: 8.4

Current Manufacturer, proposed spring:
Design rate: 475
Wire: .469 chrome sil
ID: 2.235
Max OD: 3.173
First load: 349@15%
Second load: 2000@85%
Solid Ht: 4.409
Free length: 9.27
Active coils: 7.4
Total coils: 9.4

New Manufacturer, proposed spring:
Design rate: 420
Wire: .312 music wire
ID: 2.25
Max OD: 2.874
First load: 340@8.46
Second load: 1702@5.22
Solid Ht: 1.155
Free length: 9.27
Active coils: 1.93
Total coils: 3.7

The new manufacturer seems very aware of the requirement to have the springs squared and spec'd a requirement of 1 +/- .5 degrees, but I still question the very low number of active coils.

As a related question, our problem springs needed to be pressed twice - does this increase the chance of spring bow?

Todd

 

[sbozy25]

You could cause coil blow out when pressing them. That happens on our floor all the time. The thing is, designs for this sort of application are pushed to the limit, on purpose... but as a result, they are rather difficult to product.

 

[sbozy25]

Sorry forgot to type this part before clickign submit...

I looked at the designs, all three are overstressed and as a result you will take set and have problems with coil blow out during manufacturing.

The 3rd design is a p.o.s. You are almost 250% overstressed, that design will never work... dont' even try it...


If you use the second design and go with high tensile chrome sil your design stresses come within an acceptable range. You still will have an issue with "bow" from time to time, it will all depend on the pressing operation itself. Could depend on press speed, and or how it's pressed... I would say that is your best bet. Also, they will want to scrag grind each end first to get it within 4°, and then put them on a tandem grinder to finish it off. This will help you get within the 1° you need, and keep the end coil from kicking out and opening up on you.

 

[vospertw]

Sbozy, thanks again. Seems that from a design perspective, our current manufacturer is on track, but maybe quality control isn't what it should be?

 

[israelkk]

vospertw

In the spring information you didn't include any required life cycle which is odd.

The "Current Manufacturer, proposed spring" can be manufactured but only for static use it has no useful life cycle if the spring deflects between 15% to 85% of the deflection. The total deflection is shorter than the "Current Manufacturer, problem spring".

As to the "Current Manufacturer, problem spring" someting doesn't adds up. According to my calculations such spring can not be achieved based on minimum material strength as allowed by the spec even with Chrome Silicone or Chrome Vanadium. The fact that you have an actual spring is because the material properties are probably better than the minimum allowed by the spec. I would test this spring and press it to solid height for 24 hours to see if it returns to its previous free length.

You mentioned that your spring needed to press twice, what did you mean by that? Did you mean the preset (scragging) process or it will be comressed twice in its entire life?

If you mean preset than to stabilize the spring it should be pressed to solid height (or to the minimum expected compressed length) 12 times it is preffered to compress it to solid height. As to bowing, the preset operation if it was done without an accurate guide bar to avoid bowing can cause bowing or side buckling of the spring especially if the grounded edges are not precision squared.

The common tolerance or edge squareness is +/-3 degrees. The tolerance on forces at loading points is +/- 10%.

It is better to leave the free length as a reference number and specify the first load at specified spring compressed length, the same for the second load point.

For example:

Free length 9.37 REF
First load point 366 +/- 37 lb when the spring is compressed to a length of 8.517 in
Second load point 2100 +/- 210 lb when the spring is compressed to a length of 4.87 in

As to the "New Manufacturer, proposed spring" I suggest you call the manufacturer and ask what are the stresses he gets in his spring calculations. If he can find a wire spring that can suffer over 400 ksi at this diameter I will be glad to here it. This is a bad design approach to design a spring that uses only small part of the deflection and especially if when pressed to soild it will have a permanent set.

The only spring which seams to be correctly designed from the three you listed is the "Current Manufacturer, proposed spring". However as I mentioned it is an illegal spring for cyclic use, it may good only for static use if relaxation with time is allowed.

http://israelkk.googlepages.com/home

 

[vospertw]

I don't have adequate information to define the required life cycle. As a shock absorber spring, it will compress many, many times. However, the compression is limited by the shock absorber shaft travel, in this case roughly 90mm. However, that's further limited by a bump rubber, so true compression would be closer to 70mm, with maybe 25mm preload.

As far as testing the current spring, we could certainly do that. We had the spring rate verified by a third party tester to ensure it was correct. The "pressed twice" that I mentioned was indeed to preset the spring. Maybe they've always done that, but it was only mentioned during our discussions with the manufacturer concerning our current problems.

I received the print outs from the SMI software used for the springs. Our 426lb spring did include a caution about not being suitable for cyclic operation. When I ask the manufacturer what the software considers "cyclic" they responded that it was really intended for valves and other very rapid cyclic operations.

The manufacturer was recommended by other people within the suspension industry, and they provide springs for some major suspension outfits. Of course, that doesn't mean they are always correct, but I'm concerned that even their designs seem to be pushing the envelope for the material/application.

 

[sbozy25]

Don't put much stock in the SMI software. It is very, very conservative... Which is not necessary...


There are many companies out there that claim to have good high performance springs. But really, there are only 2 worth anything... Eibach, and Hyperco.... If you haven't spoken with either of these companies, you are cheating your self. They have degreed engineers with many years of experience. They only use SMI and the IST software as a starting point, they then work form there for a good design.

You would be supprised what you can push the material to....

 

[vospertw]

Sbozy,

I'm familiar with both companies. Unfortunately, neither makes a spring for this particular application. As you know, you can sometimes find something that will "come close" but often require adapter rings, or the length doesn't really match up with normal preload ranges, etc. I'm assuming that both companies have their springs manufactured for them, do they not? I didn't think they made springs "in house."

 

[israelkk]

vospertw

With all do respect to the manufacturers they are wrong about the cyclic term. To make it simpler, the spring as designed will not last even 1000 compressions if compressed from the 15% to 85% at least from the calculations based on standard minimum material properties. To "receive" the spring the designer ignored the Wahl factor completely. This can be done only for "static" springs that have gone the preset process. Not all spring design professionals will agree with that even for static springs. As far as I recall this subject is covered in depth in Berry's book SPRING DESIGN 1961.

I assume some dedicated shock absorber spring manufacturers purchase special wires to their specs probably sorted for the maximum tensile strength. They can do it because they manufacture large quantities normally. This is why they can claim having their springs better than others.

When you design a spring and manufacture it in a common spring manufacturing house you can not expect to have those special selected wires unless you order it yourself and have the time, money and volume justification to do it.

 

[sbozy25]

Well I am not so sure about Eibach. I only deal with them on a limited basis. Technically you are correct, Hyperco does get their springs manufactured by another company. They are manufacured by their sister company Matthew Warren Spring Division, which I work for. We are less than 1 block apart, and the engineer for Hyperco sits 50 feet from me.

You are probably correct about them not having what you want, but they do make parts specially as needed. I'm sure Eibach is the same. I bet if you contacted either you could get better answers and designs...

 

[israelkk]

vospertw

Can you place the SMI print outs so we can look at them. I do not believe the SMI program approve this spring even for static use. I have a suspicion that the SMI software will report an error about the high stresses beyond the allowable loads even after preset. I have encountered in the past designers that use the SMI and the IST software incorrectly and ignore warnings they do not like or understand.

 

[vospertw]

israelkk,

Here's a scan of the print out for the problem spring. The proposed 475 spring had the same comment about cyclic service.

 

[israelkk]

vospertw

As I said you can see the blackened area which is the area of excessive stress. If you look carefully a load over 1350 lb is excessive (even after preset). See also the warning about the cyclic operation and about the tensile strength calculations.

The wire diameter is more than the 0.25" which is the maximum listed in ASTM A 228. Therefore, the ultimate stress calculated by the program for the 0.437 diameter is not guarantied at all.

What is the point to build a spring that will yield when pressed over the 1350 lb. It is better to make it shorter such that when it reaches its solid height it will not yield.

If the spring rate is 426 lb/in and the spring is compressed 1" when installed than the allowed extra deflection is no more than (1350-426)/426 = 2.17 inch during the shock absorbing process.

There is still the issue of cyclic (fatigue life cycle).

 

[vospertw]

israelkk,

We don't have much room to manuever on spring length since it needs to fit the OEM shock absorber and still provide a useful preload range so the rider can set the proper ride height for the load carried.