measuring spring weights - again

G

ghwallice

Hero member
Location
Ipswich
I've got a selection of front springs that I want to measure the weight of as I am looking for a pair about (based on good advice) 90kg over 10cm. I've read a few past posts.

I've been thinking about a way to measure without having to hold the spring straight, and using compression, not extension.


If I cut a piece of electrical conduit to put the spring in to keep it straight, cut another piece to slip inside to compress the spring, put the bathroom scales under the workshop press and adjust to zero, stand the conduit on the scales, compress the spring 100mm with the press, and then read the scales, would that give me a relatively accurate reading of the type I require?

Regards, Greg
 
If you have a fork leg available, use that as your guide tube. But if you're going to use PVC conduit, I'd suggest you make it heavy duty (thicker wall) as close as you can get to the spring OD. And select a nice straight piece (they often have a slight bend in them where they've sagged while stored in warehouse racks. You don't want the whole contraption buckling and flinging itself across the workshop.
 
Greg, I wouldn't go into the hazzle to measure the rate directly on them. Just give me the wire gauge, outer diameter and total number of windings and I will calcute the rates for you in a few minutes. Would bet it being even more accurate than measuring the rate by using your bathroom scales. :D

As you are looking for an initial spring rate of 90kg/cm, it doesn't matter for the calculation if it's a progressive or non-progressive spring.

Regards,
Stefan
 
The springs in the 3cl with Ceriani's when I bought it. 31.1 spring diameter, 5.3 wire diameter, 56 coils, 500mm overall.

These were quite stiff and I had compensated by using 5w oil. I recently changed to 2.5weight oil and have suffered seal leakage and piston binding. I don't think the seals are designed to handle such a thin oil, and I am of the opinion that 2.5w does not have enough lubricating "quality" for a shock that is meant to run 10 to 15w.

So...I'm now looking at chasing springs, getting it functional near oem, then save and fit racetech emulators.

Another set of springs I have, 23, 4.0, 65, 457. What is the formula???:)

Does anyone know what the shock piston clearance is for a Ceriani. mine are 6.2thou and 4.2 thou.

Regards, Greg
 
Just google "spring rate calculator"

Here's an example http://www.thespringstore.com/spring-calculator/spring-rate-calculator.html

Plugging your figures in gives 8.127 N/mm
or 82.9 Kg per 100mm

Another on-line calculator http://www.reliablespring.co.uk/calculator.htm gives 8.133 N/mm, so they agree within an acceptable error margin.

Such calculators assume a modulus of elasticity for the steel, which is probably where minor differences creep in.
Your springs may or may have exactly the same elasticity as the calculator assumes, but would be pretty close.
 
I thought I remembered the green book had cer fork springs at 1.2
and they calculate out at 8 and I remember my std springs as being pretty strong.
 
drikko said:
Wouldn't other things come into play like exactly what quality steel is used, how it is heat treated etc?
Click to expand...

No. For the calculation only the shear modulus of steel is required, which is a constant value for ferritic and martensitic steels (austenitic steels, e.g. stainless are different), independent of steel grade and heat treatment.
 
ghwallice said:
Another set of springs I have, 23, 4.0, 65, 457.
Click to expand...

23, 4.0, 65, 457.  = .58kg or something.

I'm wondering if the bloke that gave me the above set of springs out of Ceriani forks was taking the piss out me.....):

If so, well done :) thanks ..d!
 
Gravelroad said:
No. For the calculation only the shear modulus of steel is required, which is a constant value for ferritic and martensitic steels (austenitic steels, e.g. stainless are different), independent of steel grade and heat treatment.
Click to expand...

Well fuck me dead ya lern summit evry doi!
 
Gravelroad said:
No. For the calculation only the shear modulus of steel is required, which is a constant value for ferritic and martensitic steels (austenitic steels, e.g. stainless are different), independent of steel grade and heat treatment.
Click to expand...

The clapped out springs in the Betor forks of my 1973 Puch 125 beg to say otherwise .........
 
Jim, the still have the same shear modulus as when brand new. They are just sagged, read have become slightly shorter over time. Mostly caused by small, localized plastification, usually in places where residual stresses from manufacturing have been left in the spring. Consequences are quite dear.
Given Gregs example of the Ceriani springs, a sag of 1% aka 5mm means you'll loose 90kg of preload on the fork.
One might consider to shim this out by a 5mm washer, but then he'll miss travel, i.e. fork might bottom out on next pothole. Springs might run on block due to reduced travel by shimming, in which case you'll get kicked over the bars/off the bike.

Another reason for fork springs becoming "weak" over time could be the coils being locally ground off at places where they have been in contact with the fork tube walls. This reduces wire cross section, and causes local degradation of spring rate. Found this to an astonishing extent on the old Ceriani springs in my 3C. On some of the flat spots found on the outside of the spring coils, nearly 1mm of wire thickness had disappeared. >:(
 
Gravelroad said:
Jim, the still have the same shear modulus as when brand new. They are just sagged, read have become slightly shorter over time.
Click to expand...

Just messing with you ......... I believe that to be only part of the problem though. The increase in mass (me) causes a greater force upon the springs unless I can somehow reduce the effects of gravity (I can't seem to get the mass to change).
I have added a couple of valve springs to increase the lenght and that has improved the situation but new springs are definitely in order.
Thank you for your excellent information!
 
Jim, finally I messed it up myself, it's not 90kg but 9kg preload loss in above example, now corrected. :-[

With respect to gravity, you might consider to ride the bike only during full moon nights days. :D
 
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