Effect of trimming coils from springs
 

Now I know that trimming a coil or 2 from a spring will make it shorter - but will it also make the rate stiffer? (this is something i read elsewhere in relation to full-size cars, but is it true?).

don't know about stiffer..but you may end up with 2 springs with different rate on each.

It shouldn't change the rate by any noticable amount, since the rate comes from the thickness and material properties of the metal used for the spring. You'd just need more spacers to get your ride height right.
There could be a small change from the fact that the ends of a spring have less of an angle to the spiral as it is made into a flat end, the flatter angle is bound to compress differently from the steeper angle in the niddle section, so cutting it off will change the overall effect, but probably far too minutly to notice.
Cheaper to buy new springs of a different rate.

I thought it might be the angle of the last coil that was causing the confusion - thanks for clearing it up.

PS I need to trim a coil to get the ride height on a buggy down lower.

I think the problem would be the end of the spring if you cut it, there won't be a ring for the ends of the shock to rest on, it might try and bend the shock.

yes. given all other parameters constant, the rate changes linearly, but inversely, with the percentage of length removed. if you trim a spring of given spring rate X in 1/2, your rate will be doubled or 2/1 times X. and so on. so to find your new rate you take

original rate x original length / new length = new rate.

voila.

I know I'm new to the forum and all, and I haven't driven an R/C car in about 10 years, but I'm technically sound in disagreeing with seaball.

Trimming a spring won't change the rate a bit (provided it's linear and not a progressive rate spring). It will change the ride height, which is what the original poster appears to desire.

I'd want to bend the top coil to make it reasonably flat so as to provide a good contact with the shock, or provide enough extra so that you compress at least one coil top and bottom at your desired ride height.

You can tell whether a spring is progressive by looking at the coils, do they get tighter towards the top and bottom? If so it's progressive. Are they consistent across the springs length (besides the very top and bottom where they are wound very tight to provide a good flat base)? Then they are linear.

In full scale car apps there are also progressive springs which are varying thicnkess throughout the length, but maintain the same wind. I'd doubt such springs are common in R/C applications. You can probably just look at the winding.

The basic premise of a linear rate spring is just that. The rate of the spring is linear. As you compress it, the rate of force increase is linear with the amount of compression. The rate does not change.

There would be no more compression if you were to cut the spring than if you did not, as the force will remain unchanged for springs of different length, you'd just be changing the ride-height. It would react just the same as longer spring, just with a different range of available ride hieght adjustment

My first car (real car!) was bought with a couple of coils cut off the rear springs. In its free range of travel (not hitting bump stops), they acted similarly to the much stiffer springs i installed later on, although the higher-rate and properly lowered springs did offer greater control.

Try applying that if u can. I know its a different example, but I began in R/C because I like big cars, so I try to find connections.

Hm, I think this change in handling in 1/1 cars is due to stock springs always being loaded, and varying in their natural range of movement, whereas cut springs are usually fully extended when the car is at rest, only to compress when in travel. This may explain the pogo effect of cut springs.

Or, may it equate to less coils=stiffer, more coils=softer? Maybe the less lenge of 'wire' there is to cause deflection, the greater the force upon any given section of it, thus increasing rate? Put a bar in a vice and try bending it at the end, it should deflect a bit, then try bending it halfway along its length and it should require a proportional amount of effort.

food for thought, try not to shoot me down if im flying too high

In full scale cars, comfort is usually a major concern and stock springs are almost always progressive, meaning if you cut them you do change the rate. How did you know they were not hitting the bump stops, were they cut also as is customary when lowring a full scale vehicle? Also do you know that the struts or shocks were providing linear damping? That is not always a safe assumption either.

Try it with a spring in a 'clicker' ball point pen. Cut a few coils off and tell me if you feel a difference. Or look in a physics book and read up on how (linear) springs act.

In a linear rate spring, the number of coils compressed will be the same in both cases. The length of a spring determines ride height, the wieght of the car is still on the spring, and this determines teh force from the weight of the car determines the amount of compression. unless the spring has a progressive rate, which would create more compressed coils at full length when at rest than cut springs would.

For a given stiffness wire this is absolutely true, and how most progressive rate springs are made.

Thanks Concillian good points, I'll add the mechanics behind this so people can work it out for themselves, I encourage enquiry based teaching and learning.

This here statics book says that the force the spring exerts is equal to the spring constant (k, which equals force/length), multiplied by the change in length (of the spring as a unit).

The spring constant equals force/length.

My cars springs were linear, and my little toy car springs are linear but they have a coil at the top and bottom that is 'levelled out' so that the springs remain in their correct upright position in relation to the shock/strut, as well as helping them remain captive. I'm thinking that it is the cutting of this coil that may have the most significance. May these 2 coils with decreasing distance between eachother have differing properties to the rest of the spring? There may be some method to my madness.

I dont wanna break my ball pen to do this! And to check for any interference or travel limits I put paint on the ends of the bump stops, hehe. They didn't hit the axle at all during normal driving.
I'm glad the new car has coilovers, much nicer to work with.

A coil spring will get stiffer if you remove a coil. The way that a progressive coil spring (that has some coils more closely spaced)works is that as it compresses, the closely spaced coils bind together first. This effectively removes the coils from the spring thus making the spring stiffer. If you get no coil bind the spring won't be progressive.

If you take two coil springs and place them end to end the spring rate of the combination will be half the rate of each spring alone. You have doubled the number of coils (and spring wire length). There is a formula in Herb Adams Book "Chassis Engineering" that can be used to calculate the spring rate of a coil spring given the length, number of coils, and spring wire diameter. Decreasing the number of coils decreases the wire length and increases the spring rate.

After some further research, indeed you are correct and I must eat crow. The spring will get stiffer if you trim coils. I apologize for any confusion I caused.

:)

Hmm, so Aerospace Engineering actually teaches you stuff, cool!