How do you measure your fork spring rate on the bench?

['03KDX200]

I see all kinds of videos on fork springs and what rates you should use, and how to calculate fork spring rates and whatnot, but what I don't see is how you're supposed to measure the spring rate is of your existing springs! A good reason to do this would be if you bought a used bike that's had several owners and you want to know exactly what your current springs are so that you can decide if you need to replace them with different ones or not, or add a spacer, etc. I'd imagine even springs that started out at the perfect rate years ago may have drifted out of range.

So is there a fairly simple tool or jig out there that you can make, for putting your fork spring on the bench and measuring the exact Kg/mm?

I was thinking about something that uses a regular bathroom scale or food scale, a couple C clamps, and a ruler. I mean all you really need is something that holds the spring horizontal on the bench, where one end is anchored, the other end is pushed on with a lever that you can lock and let go, and then simply measure the force required and the distance the spring is compressed. Like say the EXACT number of grams it takes to compress the spring about an inch. That's all it would take but it would have to be a very precise distance measurement down to the mm. I tried to do this by hand vertically by holding the digital food scale in my palm and pressing down on the end of the spring. What I found was that it was impossible to hold the scale at anything resembling a constant pressure, and impossible to measure accurately with my hand moving all around trying to compress the spring. There needs to be some kind of jig for this so you can do it on the bench.

[KDXGarage]

workout weights!

Get something made (vise / plate / rod combo) to be similar to the following:

Place the fork spring over a vertical rod. Measure the length, preferable with a meter stick (yard stick). Write that down.

With your workout weights that have been verified for exact weight, add enough to compress the spring about 25 mm (or an inch). Write that down. Now add enough weight to compress it 25 mm more (or to go from one inch to two inches). Write that down. Do it again to get to 50 mm or 3 inches. Write that down.

For inches, one wants to know how much weight it took to make it go from one inch to two inches, then again from two to three. They should be nearly the same on a linear rate spring. This method will allow one to determine how many POUNDS PER INCH rate the spring is. Use conversion factors to convert to metric if you want.

Measure the weights in Kg, then the spring compression in millimeters to find out Kg / mm rate without converting.

I did this back in 2005?? and got 0.357 Kg / mm rate for a set of 1995+ springs.

DO NOT use the zero - one inch measurement as the rate. Just use it to make a starting point.

BE CAREFUL!! Stacking up 100 pounds is only as safe as your jig / apparatus is sturdy.

Do use some sort of bathroom scale or such to verify the workout weights are accurate.

This method will get you pretty darn close. The accuracy of measuring the weight of the weights and the amount of compression will effect results.

['03KDX200]

Ok cool thanks that gives me some good ideas. But why wouldn't you count the resting point of the spring as zero?

[kdxdazz]

Don't have the formula on me right now but a Google search will find it. You have to measure spring diameter, the amount of coils, thickness of spring wire, length, then put that information into the calculator

[KDXGarage]

03kdx200, it is not reliable. It has to be preloaded some to get it into the "consistant range" of sorts.

['03KDX200]

03kdx200, it is not reliable. It has to be preloaded some to get it into the "consistant range" of sorts.

I see. That makes sense.

['03KDX200]

Don't have the formula on me right now but a Google search will find it. You have to measure spring diameter, the amount of coils, thickness of spring wire, length, then put that information into the calculator

I was thinking about doing something like that but there are two huge problems with that method: number one is you don't know the exact steel alloy that is used in this spring or it's mechanical properties in terms of hardness, stiffness, etc. Even if you had all the physical measurements of the spring, the composition of the steel used will give it wildly different parameters in terms of it's specific behaviors as a spring.

The other issue with this method is drift over time. It may have started out life with .35kg/mm 20 years ago when it was new, but now after 20 years of hard use it may be sagging, may only be giving .29kg/mm of pressure, that kind of thing. You have to be able to measure what you have in front of you and not have to rely on the specs for the steel being what they're supposed to be on paper.

[KDXGarage]

Post up some measurements of your springs. If they are stock length, then most likely they are stock springs.

I don't think age will lower the rate as much as you posted.

Measuring all the dimensions, number of active coils, etc. was quite tricky to me.

Beyond workout weights, one can buy spring rate testers that are "store bought", not home made.