OT: Changing capacity of a motorcycle spring

SteveF · Feb 5, 2019

I know we have some very smart auto types in this group.

A mid size Honda motorcycle has a single shock enclosed within the spring on the rear. Let's say you wanted to make the spring softer. Would it be possible to machine off maybe 1/8" (or less) of the outside diameter of one or two of the lower coils to accomplish this?

Penske will make a nice custom, fully adjustable one for $950 but I'm not that interested.

For the sake of making this simpler - let's say the shock looks like this one:

2014-2016 Honda Ctx700 Rear Back Shock Absorber Suspension OEM | eBay

Steve

Tony Quiring · Feb 5, 2019

Lowriders use a torch to heat up a spot and allow spring to settle.

Possible...?

Need to check motorcycle experts but shaving material not likely correct.

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Cole2534 · Feb 5, 2019

With the shock off the bike- I'd measure the installed length, then pull the spring and measure it's free length. Then use a press and a scale to calc it's rate and preload value.

If I had to do anything, I'd remove some of the end of the coil to lessen preload.

Or order a custom spring. If you have the data above it shouldn't be hard for an experienced spring maker to get you a custom relatively cheap.

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J.Ramsey · Feb 5, 2019

Shortening the spring it will increase its rate not reduce it.
Buy a softer rate spring usually less than $75.

john.k · Feb 5, 2019

The pics on fleabay show a pair of adjustment nuts with more than a 1/8 of travel.......anyhoo,the spring looks to be compound coned ,not easy to replicate..........but if you can get a used one for $40,I d have a go at grinding something from the outside of the spring,and reducing the volume of wire /as it were........its highly likely the damper inside is also involved in setting the spring rate...........is there a linkage you could alter in the suspension.....maybe even alternative mounting to change the leverage.

kustomizingkid · Feb 5, 2019

Race Tech, go find a proper spring.

CalG · Feb 5, 2019

Removing material from the spring wire is an excellent way to reduce a spring's RATE.

Smaller wire is less stiff. You could even use any of the on line spring calculators to get an approximation of how much material might need be removed.

Preload adjustment only effects RIDE HEIGHT. NOT RATE.

As mentioned, cutting springs only INCREASES the spring rate.

I would use a grinder to modify the spring coils.

For a linkage style motorbike rear end, softening even 3/4 " of compression would be very noticeable.

ratbldr427 · Feb 5, 2019

Reducing the diameter by maybe grinding will soften the spring rate.Maybe putting on a mandrel would give be the easiest way to hold it.Rig up a scale and press to measure your progress and buy another spring because you will most assuredly cut to much!
Springs have been made with tapered wire to give a variable rate,wound with a different pitch at the end also.
Remember a spring is just a torsion bar in a circle.

SteveF · Feb 5, 2019

Spring rate is what I'm trying to change. I checked Race Tech's web site, unfortunately they sell different springs for the front but not the rear. At least I now know the rear spring rate is 19 kg/mm. The fronts they "recommend" are .79 kg/mm versus stock of .57 kg/mm so I'm not thinking their idea of softer is the same as mine.

Hesistant to take a torch to the springs because I'd like to be able to control the process.

Thanks for the ideas. Keep 'em coming. According to Alibaba there are 9,856 shops that will make me custom springs!!

Steve

kustomizingkid · Feb 5, 2019

Another option that some cheaty people have done is acid dip stock springs and repaint them... that practice is more common in the "spec" classes that don't allow changing any components.

johansen · Feb 5, 2019

SteveF said:
Spring rate is what I'm trying to change. I checked Race Tech's web site, unfortunately they sell different springs for the front but not the rear. At least I now know the rear spring rate is 19 kg/mm. The fronts they "recommend" are .79 kg/mm versus stock of .57 kg/mm so I'm not thinking their idea of softer is the same as mine.

Hesistant to take a torch to the springs because I'd like to be able to control the process.

I thought about suggesting acid dip as kustomizingkid did but I would still be hesitant to do such a thing given the lethality of failure.

as for a torch, it won't affect the spring rate it will only lower the ride height or whatever you call it. (unless you are acid diping the springs and then heating them with a torch while stretching them to compensate for the softer spring!)

9100 · Feb 5, 2019

Acid dipping can lead to failure from hydrogen embrittlement. If you do it, it must be baked to drive the hydrogen out. Why take the chance?

Heating with a torch is equally bad. You can either harden the spot if it cools rapidly and make it brittle or anneal it if it cools slowly and cause a fatigue failure. In any case, you will destroy the original heat treat.

It is a common practice with die springs made of square wire to grind the OD to get the desired spring rate, described in the spring manufacturer's literature.

You can measure the spring rate by putting it on a bathroom scale and pressing it to the working length. Then put it on a mandrel and grind the OD and test until you get the rate you want.

You can also wind your own with hardened spring wire on a lathe but you will need to stress relieve it by baking around 500F for an hour. Google spring stress relief and get the temperature for your alloy, it varies quite a bit. Stress relieving is an absolute necessity. I have made lots of coil springs and tested them with and without stress relief. You can also wind it with soft wire and have it heat treated.

Stretching the spring to take a set at a new length is also bad, same reason. It will leave residual stress. The point is, any time you deflect a spring past the yield point, it needs stress relief to restore it to full performance.

Bill

Jim Williams · Feb 6, 2019

Reducing the cross sectional area of the spring wire is the only practical way of decreasing the rate. You might fabricate a mandrill fitting the inside diameter of the relaxed spring closely. Then you could use a cylindrical grinder, a tool post grinder, or some other means to remove some material from the outside diameter of the spring. There is no practical way to calculate the rate of the resulting odd shaped spring, so you would have to complete the task empirically.

Bobw · Feb 6, 2019

Just a thought here..

I don't know your motorcycle, but instead of playing with the spring, how
about moving the mounting point on the swing arm? Maybe even an offset bushing??

DDoug · Feb 6, 2019

What are the springs dimensions ?

You-would-think...that a die spring could be found that would fit/work.

henrya · Feb 6, 2019

Buy a spring.

Motorcycle Springs | Dirt Bike Springs | Street Bike Springs

These folks will make what you want.

IWUP · Feb 6, 2019

Try this guy, He reps a lot of the big brands, his work is mostly geared toward race and track bikes but he might have something for you.
thermosman

crossthread · Feb 6, 2019

I don't know if this would work on a motorcycle spring but at Wright Patterson we recalibrated some springs by sandblasting them.

SteveF · Feb 6, 2019

Re-engineering the lower linkage isn't happening because 1-there really isn't any room to move things around and 2-Waaaaay more work than making the spring wire thinner.

The wire is 16mm. A good deal of metal to work with.

I just ordered the used shock. For $38 I can play with grinding and see how that works out and still have the original if I screw it up.

Having one made is a good backup option.

Thanks for the ideas.

Steve

9100 · Feb 6, 2019

digger doug said:
What are the springs dimensions ?

You-would-think...that a die spring could be found that would fit/work.

Its a dual rate spring. Extended it has all the turns in play to give a softer ride and then on a bump it closes the bottom ones and becomes stiffer.

When I opened my email this morning, there was an add for the shock assembly used as an example here. Big Brother is watching too damned closely.

Bill

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