Cut Down OD of MC Shock Spring on Lathe?

[mcman56]

I have a motorcycle mono shock fitted with a spring that is a bit too stiff and have not been able to find a softer one. It is around 3" OD with 1/2" wire diameter. It seems that I could soften the spring by fixing it on a mandrel and taking a cut off of the OD. The wire would no longer be round but the section modulus would be reduced so the spring would be softer. Has anyone tried this? Do large coil springs have a really hard surface?

[Carm]

Idle time presently, I'm shooting from the hip.
The surface will be as hard as spring temper (assuming carbon steel) so yeah, fairly hard.
If your cut chatters, is that a bunch of stress risers?

All the foregoing implies you have a lathe. Thought about making your own?

[Magicniner]

if all you need to do is shorten the spring and a cut end, at whatever angle, will work then an angle grinder with a cutting blade would be better than a lathe.
Mask the section of the spring not being cut off.

- Nick

[Russ Hanscom]

Even on a mandrel, parts of the spring are not likely to be tightly held, and that is going to cause vibration and a poor finish.

I have made wedges out of flat automotive springs with a carbide tool so spring steel can be machined.

When my brother wanted softer springs on his autocross vehicle, he started cutting off the end of the coil spring, maybe a half coil at a time, then testing the results.

[Bob D.]

Reducing the length of the spring will increase the spring rate, not make it softer as mentioned in an earlier post. Reducing the cross section would reduce the spring rate. How much for a given reduction is a big guess. Once not circular is beyond me to figure.
Maybe turn a tight fit wood dowel to support the spring for turning. Carbide will cut it ok.
I'd figure out what you have now and keep hunting for another spring to replace it.

Bob

[SteveM]

I'd be worried that any nicks in the finish could be stress raisers.

Steve

[liveaboard]

I've had motorcycle and other springs custom wound; it wasn't all that expensive. Powder coat or just paint it afterwards .
I converted an old twin springer to monoshock and it took a few tries to get the right spring.
As stated above, shortening a spring makes it harder, not softer. OP probably knows that. Anyway, this sort of spring has flat ground ends.

I doubt you'll succeed in re-profiling; cutting hard material when you don't have a really solid mounting is not good. Maybe if you made a steel mandrel that fit really tightly. You'd have to force the spring on and off with a press or large vice.

[shootnride]

I would respectfully disagree with the idea that a coil spring gets stiffer by shortening. Springs are rated by pounds per inch of compression. A spring of a given design (diameter of wire, spring OD, coils per inch, etc.) compresses at the same rate regardless of the number of coils. A 300 lb/in spring takes 300 lbs to compress it one inch whether it has two coils or twenty coils. Of course, the number of coils will determine the available compression distance before reaching coil-bind.

I agree with others here in that you should keep looking or have one custom made.

Ted

[CaptonZap]

One other thought is to heat treat, (temper) the spring, test it for rate, increase tempering temperature, test, etc.
A press with a bathroom or other suitable scale, and a ruler to measure the compression would be all you would need, other than the heat treating oven.
I would start off at 600 deg F for 20 minutes, allow to air cool. Since you don't know the composition of the metal, this is all a guess, and by golly. Increase by 50 degrees and try again until you get what you want.

CZ

[hanermo]

I don´t know if its a good idea to do this or not.

However, a good/large flat belt sander or stationary sander or flap wheel on a 41/2" grinder will easily, and in a very controlled manner, reduce the OD.

I am experienced, and use a festool sander, best there is, 1400W.

Nevertheless, using permanent markers and dtis etc, its fairly easy to take off stuff in 0.02 - 0.01 mm thicknesses, in a controlled manner.
Edges get rounded, but wont matter for you.

More aggressive discs/pressure/power makes it faster and less controlled.

I would be worried, I think, about weakening it too much.
What if it fails in use ?
Sounds like a catastrophic failure, to me. I ride, fwiw.

Also, if you turn or reduce the OD otherwise, it will rust.
Hardening/temper/surface hardening effects will be unpredictable, imho.

If I had to do it, I might maybe use ferrobend on a mandrel or superglue/epoxy to first make it semi rigid in a removable manner.
Then process the part.
Melt with steam or solvent.

Just some ideas.
Does not sound $$ worth it, imho.

[J.Ramsey]

I would respectfully disagree with the idea that a coil spring gets stiffer by shortening.
Ted

It is true that the rate goes up as the spring gets shorter whether coil, leaf or torsion bar.

I spent 20 years working for one of the top Modified builders in the country and we had spring rate checkers for coil and leaf both and a easy way to fine tune the front coils springs was to cut
a quarter turn of a coil at a time to get the rate that was desired.


https://www.eatondetroitspring.com/cutt ... culations/

[SteveM]

I would respectfully disagree with the idea that a coil spring gets stiffer by shortening.
....
A 300 lb/in spring takes 300 lbs to compress it one inch whether it has two coils or twenty coils.

Yes, two 300 lb per inch springs will compress at the same rate even if one has two coils and the other has twenty.

But, when you take the 20-coil spring and cut 10 coils out of it, the spring rate changes.

I used the calculator at
http://www.engineersedge.com/calculator ... _k_pop.htm
to calculate a hypothetical spring with 20 coils and then recalculated when I took out half the coils.

Steve