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rustyvw
rustyvw HalfDork
11/15/08 7:01 p.m.

I've had cars that were lowered to the bump stops before. I don't see how cutting a coil or two is going to kill you.

GameboyRMH
GameboyRMH SuperDork
11/15/08 7:18 p.m.

Cutting the springs will make them harder, and isn't really dangerous, but in certain conditions it could make your car handle like crap.

iceracer
iceracer Reader
11/15/08 7:39 p.m.

Once upon a time, SAAB told me to cut the springs to lower the car and stiffen the suspension. Worked like a charm.

doc_speeder
doc_speeder New Reader
11/15/08 7:45 p.m.

I've done it on a Jetta. Worked good to lower it, but it didn't stiffen the spring rate enough to improve handling. The price was right though

I think it really depends on the car. With all the P71 discussions on here lately, I think that may be a good candidate for a spring cut. The springs are already stiffer than a civi version, and longer, so cutting them to bring the car back down to the regular ride height may work very nicely, provided they are evenly shaped on one or both ends.

I think the Hyundai would probably fair similarly to my Jetta experience though.

Volksroddin
Volksroddin Reader
11/15/08 7:45 p.m.

"your gona die" We're all going to die some day.

ditchdigger
ditchdigger Reader
11/15/08 7:52 p.m.

I too have tried to inform people of thier misconceptions about cuting springs using things like math and science but they still insist I am wrong.

People will continue to beleive whatever they want regardless of the facts...kinda like religion

Hasbro
Hasbro HalfDork
11/15/08 8:00 p.m.
ditchdigger wrote: I too have tried to inform people of thier misconceptions about cuting springs using things like math and science but they still insist I am wrong. People will continue to beleive whatever they want regardless of the facts...kinda like religion

Probably because a lot of the newer suspension systems don't have the simpler springs of yesteryear. A 2002 I cut worked really well.

geomiata
geomiata Reader
11/15/08 8:03 p.m.

please lets not make this a religious discussion.

nicksta43
nicksta43 New Reader
11/15/08 8:56 p.m.

I've cut plenty of springs with good results. If you want a slightly lower ride height and a slight increase in rate it's a good option in my opinion. You can go too far, personally I would never go beyond two coils and even that is pushing it depending on the spring. The most important thing is keep them as cool as possible, getting them hot will effect the temper of the steel and could cause them to break. Do not cut them with a torch! It gets the steel way too hot. Even a chop saw or sawzall is a much better option.

See if you can find the book Herb Adams Chassis Engineering. He recommends cutting springs as a way of getting a lower center of gravity and increasing rate.

Think about straitening out a coil spring. What are you left with? A torsion bar. And that's how a coil spring works it actually twists. That little bar of steel can't tell that its coiled up. So if you cut, say, ten inches off of a three foot bar of steel the amount of force it would take to twist that bar the same amount increases. It's the same whether it's a straight bar or coiled.

Keith
Keith SuperDork
11/15/08 10:07 p.m.

You can figure the change in spring rate fairy easily - Pro Shocks will do the hard part for you.

http://proshocks.com/calcs/coilsprate.htm

I think a common concern with cutting springs is that the stock shocks may not be able to properly damp the resulting stiffer spring. That's a problem with installing stiffer springs on any car, of course. But those who are cutting stock springs are potentially more likely to retain the stock shocks as well.

Wally
Wally SuperDork
11/16/08 1:40 a.m.

My father had a collection of cut springs and crank pulleys with the centers cut out for his circle track cars. You couldn't use an aftermarket spring and he couldn't afford to anyway so he would pick out some Cadillac springs, cut them to the rate he wanted and shim the ride hight back up with the crank pulley. Towards the end he also came up with the idea he was most proud of, air shocks with the valves next to the driver and a small airtank so he could adjust the wedge during the race.

chknhwk
chknhwk Reader
11/16/08 6:55 a.m.

Sounds like a great writeup for GRM to do! With as anal as these peoples are with taking data during testing it should make an interesting read...

ncjay
ncjay New Reader
11/16/08 7:05 a.m.

I can speak from experience on this one. Cutting a coil off, maybe even two, is not all that big of a deal, but it can screw things up on some cars. I have a 1978 Monte Carlo that needed to be dropped in the front about 2". I cut approx. 3/4 of a coil off of both sides. The spring rate is much stiffer thankfully. Stock springs are made for cruising around, not handling. It did mess with the suspension angles, but it's probably only the lousy suspension design that brought this on to begin with. Car handles better and sits so much nicer. Cutting coils can be very effective if done properly. Many people recommend using a cutting wheel to keep the heat out of the spring, but I used a torch and tried to do it as quickly as possible. Springs have been in the car for over 10 years now.

11110000
11110000 New Reader
11/16/08 7:14 a.m.

My cut srings work great. People freak out over internet lore, and so the legend continues...

Things that could be bad: - if you cut too much, the spring could end up free at full droop, requiring a limiting strap or something - suspension geometry might change in a bad way (i.e. roll center drops too far) - new spring rate could overwhelm shock

I'd think most GRM folks could spot and avoid these negatives and end up with the two certanties of cut springs: - Spring rate will increase (ask Isaac Newton!!!) - CG will drop

Jensenman
Jensenman SuperDork
11/16/08 8:51 a.m.

Yeah, cut springs will work fine. That's what we did with the LeMons car, it had a set of Monroe SensaTracs and the damn thing handled very well. Of course, there were some additional super secret mods as well...

Cutting them will 1) lower the car 2) stiffen the spring rate.

That second one is hard for people to get their head around, here's the easy explanation: A coil spring works not by bending but by twisting, just like a torsion bar. So if you imagine taking your coil spring and straightening it out and you attach a lever to one end and try to twist it, say, 10 degrees you need X amount of force. Now if you cut the same torsion bar shorter and use the same lever, it takes more force to achieve the same 10 degrees of twist.

The amount of stiffness increase is the same as the amount cut off, if you cut 15% of the length off you add 15% stiffness.

ww
ww Dork
11/16/08 11:02 p.m.
nicksta43 wrote: Do not cut them with a torch! It gets the steel way too hot. Even a chop saw or sawzall is a much better option.

What about a plasma cutter? Is that going to put too much heat into the spring? My thinking is that it's such a concentrated application of heat that it'll cause minimal problems.

Thoughts?

noisycricket
noisycricket Reader
11/16/08 11:26 p.m.

The problem is that the amount of suspension travel lost goes up out of proportion to the amount of rate gained.

For example, let's say I have a car with front springs that are 15" long in the front when free, 9" long at installed ride height, 100lb-in rate, and there's 3 inches of bump travel from static ride height. This is pretty generic for a lot of econoboxes. We therefore have 300lb of reserve load in the spring to handle bumps, braking, cornering. Now, I want to lower it an inch, so we remove 1/9th of the coils to make the installed height 8". The spring rate becomes 9/8ths of what it used to be, or 125 lb-in. Because there's now only 2 inches of bump travel, that means we have only 250lb of reserve load, so the car will bottom out more often. Bottoming out might not make you ZOMG DIE FIERY DEATH but it will cause random severe understeer.

Note that 125 lb-in is still pretty much econobox spring rate.

I have taken springs from other applications and modified them to fit. Rear springs from a Pathfinder turn into 250lb-in springs for the rear of an RX-7, for instance :) I just use a cutoff wheel and if it needs a flat end, I heat one spot on the spring yellow, about 180deg out from the end, and then press it against the ground.

Our Lemons car's suspension consists of springs cut so the car is essentially on the bump stops at ride height. On smooth tracks it doesn't seem to matter, plus the other teams are also doing it so we're not at so much of a disadvantage...

SVreX
SVreX SuperDork
11/17/08 6:46 a.m.
ww wrote:
nicksta43 wrote: Do not cut them with a torch! It gets the steel way too hot. Even a chop saw or sawzall is a much better option.
What about a plasma cutter? Is that going to put too much heat into the spring? My thinking is that it's such a concentrated application of heat that it'll cause minimal problems. Thoughts?

Never cut a spring with a plasma cutter, but I've got quite a bit of time in using one. There's quite a bit of heat generated. Too much to touch 6" away. Don't know if it is more than a sawzall.

Advantage to the sawzall is that you can also cut wet for cooling.

Jensenman
Jensenman SuperDork
11/17/08 7:11 a.m.

I have cut springs with my small plasma cutter, it's time consuming even on the highest setting. I find it quicker to use a whizz wheel (muffler cutter).

According to an engineer buddy of mine, a spring can withstand continuous temps of 500 deg and intermittently as high as 700 deg with no change in the physical properties. Since it's only hot for a few minutes and right at the end of the spring which would be on a 'dead' coil, it's doubtful that using a torch or plasma cutter will introduce enough heat in a 'live' coil to make a noticeable difference.

About the ride height, yeah the roll center at rest will change. Its normal movement after that point due to body roll, etc should not be affected, only the starting point will change. Now, if the suspension travel goes farther than originally intended once the suspension goes past the original stop point the roll center can start doing all kinds of funky things.

The suspension being lower will bring the bump stops closer and the bump stops are like progressive springs. This means the wheel spring rate will change quickly as they are compressed and that can lead to understeer/oversteer. It's possible in many cases to trim the bump stops shorter so this will happen later in the compression cycle. You don't want to trim them so short that the suspension goes metal to metal because when that happens the spring rate go infinite and then you DO have a problem.

Wally
Wally SuperDork
11/17/08 9:27 a.m.
nicksta43 wrote: The most important thing is keep them as cool as possible, getting them hot will effect the temper of the steel and could cause them to break. Do not cut them with a torch! It gets the steel way too hot. Even a chop saw or sawzall is a much better option.

More internet lore. I've only done it with a torch. The ten seconds that it takes to lop off a piece of spring is not going to cause it to shatter into a zillion pieces or collapse under it's own weight. The time spent cutting a spring with a hacksaw would be better spent getting a second job to new springs.

SVreX
SVreX SuperDork
11/17/08 1:34 p.m.

Hmmm...I've had the opposite experience.

When we were prepping the wrecked car for the movie "Fireproof", we removed so much weight that it sat way too high and looked ridiculous.

I intentionally wanted them to sag to look "normal" (it was destined for the scrap heap anyway). It only took 10 or 15 seconds with a torch at each corner to get the car to squat right back down.

Wally
Wally SuperDork
11/17/08 1:52 p.m.

Maybe the torch only works on Cadillacs? Did you heat the springs i the car with weight on them? Maybe that makes a difference. The ones we cut in the garage seemed to work fine.

Chris_V
Chris_V SuperDork
11/17/08 2:06 p.m.

Heating them with a torch will definitely get them to sag down pretty quickly. But, every set I've seen heated to get them to sag ended up breaking in fairly short order (one friend with a BMW 2002 heated his coils to lower the car and drove from his house where he had heated them to my garage, about a half hour away. When he got there, two of the heated coils had actually broken into multiple pieces...). Never saw a problem with ones that were cut.

Jensenman
Jensenman SuperDork
11/17/08 2:15 p.m.

Back during the dropped minitruck craze, the kids around here would go to a local muffler shop, put their (fill in the blank) on the 4 post lift and the guy there would heat the bottom coils until it settled to the height they wanted. I never saw one of those break but I drove a Sidekick done that way over a speed bump and a spring came rolling out from under it.

Wally
Wally SuperDork
11/17/08 2:44 p.m.

Heating and cutting are two different things. I'm not holding the torch to them to cook them, just a quick slice off the end of a foot tall spring.

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