Spring Preload

[Loren]

Started looking into the ups and downs of spring preload a while ago.

I always thought of it more as "adjusting available shock travel with the lower spring perch", and never gave much thought to the actual "preload" aspect of it. Yes, you can raise or lower the car by adjusting the lower spring perch. But, to my thinking, you really wanted to adjust it to get the shock stroke in the right place. If you go too low with it, you have "less than zero preload", and your spring can fall out of the perch at full droop. If you go too high with it, you end up with insufficient compression travel. You can be on or near the bump stop at static ride height, so that ANYTHING you do (bump or steering) puts you hard into the bump stop.

Turns out, there is a little bit more to think about with regards to the actual "preload" tension on the spring!

All of the above is still a factor. You can't adjust the lower perch such that you don't have adequate compression OR droop travel and expect things to work properly. But, what I'd overlooked is how that preload can effect things like turn-in response.

Let's say you're at near zero spring preload. In that condition, as soon as you begin to turn the wheel, body roll happens instantly. Same thing when you hit a bump, compression happens instantly. So, that's good for ride quality, but not so good for crisp turn-in response.

Now, let's say you've got a 300# spring loaded with 600# of static weight. And you raise the lower spring perch by a half inch. You've preloaded the spring by 1/2", so that's 150# of preload. What happens when you turn the wheel now? Well, the suspension isn't going to move AT ALL until you've overcome that 150# of preload! If it's 600# static weight, that means 1/4 of the weight has to transfer (0.25G) before the suspension moves. That's good for steering response... none of your input is going into making the body roll, which means it's going directly into turning the tire! Flip side, if you hit a bump... it might be a tad harsh because the suspension won't move until 150# has been applied to it.

If you go too far with the preload (assuming that you don't go so far as to completely eliminate droop travel... which would present its own issues), you'll make the ride REALLY rough, initial steering response really sharp, but... once you overcome the preload and the suspension starts moving, it might move a lot! (depending on spring rate) So, you could end up giving yourself non-linear steering feel. Crisp initial turn-in, then "blah" until you get into the bump stop, then your feel would return.

Not sure where I'm going with this, I just thought it was interesting. Might have to play with it.

Lots of branches in this rabbit hole of suspension design!

[Loren]

Ooooh, here's some sheep dip:
http://www.apexspeed.com/forums/showthr ... d&p=205922

[CaptainSquirts]

Kinda off top, kinda not. I usually see/read that more roll will net you more grip compared to less roll. Is reducing body roll pretty much giving up grip but in return the car being more responsive?

[Loren]

Yeah, that sounds right. You're in the never-ending pursuit of "balance".

In my case, I'm also thinking of "cheating" a little bit to get something completely different out of playing with the preload.

This might make more sense with a craftily drawn picture, but I'm lazy.

So, I have a main spring and a tender spring. I knew I didn't have "the right" tender spring, but it was cheap, so I went with it. Now that I have things together and have looked at it closer, I realize that it's "more wrong" than I thought it was.

For the tender spring to be of value over just fitting a single constant rate spring, it has to fully coil bind so as to change the rate of the stack to the main spring. And it needs to make that transition in a "useful" place.

My problem is that I have 4.625" of shock stroke and 1.625" of bump stop. And I need about 1.5" of droop travel. That means that at rest, I have 3.125" of compression travel, but only 1.5" before I get into the bump stop. I'd really like the transition point of my spring stack to be somewhere in the first .5 to .75" of travel so that I get some benefit out of that firmer spring rate before getting into the bump stop. Now, the bump stop itself is progressive, and very stiff... I've BEEN cornering on it, that's what this suspension is designed to do. So, even if my transition point is at 1.5", though I won't necessarily feel it as much, it will be working along with the added spring rate of the bump stop and making that last 1.625" of travel even stiffer. Not all bad. But, I just "want" it to engage sooner.

So, as it sits, at rest, the tender is not compressed enough. Which means that with the remaining suspension travel, it DOES NOT bind until I'm well into the bump stop. (even with the heater hose wrap that I put on it that took up an extra half inch of travel)

One solution would be to "pack" the spring with more rubber. They do make parts to do that. But, I ran the calcs on that, and it really doesn't help. Packing the spring in that manner just raises the spring rate, which makes the rest of the tender spring compress much more slowly... and it still doesn't get into coil bind any sooner!

The real solution would be a shorter tender spring. like a 2 or 2.5" instead of the 4" that I have.

Enter the notion of spring preload. If I preload the stack by 1", a good portion of that goes into the main spring. But, maybe 1/3 of it (I have a spreadsheet, I can calculate it) will compress the tender spring... and get it closer to its transition point. My net spring rate is 220, and the weight on the front corner is around 600, so it's only about 1/3 of the weight of that corner in preload. Somewhere in that range will probably work.

Net result is that I get a little better steering response, and get the tender spring compressed a little more so that maybe it will be useful.

If it doesn't work, what I need to do is just get a shorter tender spring and rework the stack. No wonder race car tuners have such big piles of extra springs!

[twistedwankel]

If it doesn't work, what I need to do is just get a shorter tender spring and rework the stack. No wonder race car tuners have such big piles of extra springs!

This what Loren thinks "spring break" is all about? Off the wall but still very interesting stuff.

[dbeng]

I'm looking forward to the next edition; Summer Preload

[Loren]

Everybody needs a hobby, right?

[twistedwankel]

Everybody needs a hobby, right?

Absolutely. Retirement without a hobby is called "deceased". Go Loren. Oh And you cannot retire from anything you've never earned money at. Therefore: autocross is "forever". Long live FAST.

[Loren]

Oh, the weirdness...

So, I drove the car a bit over the weekend, and today. And I sort of got a feel for it. It's got a slightly softer ride than it used to, and it feels a little bit "rolly", but it's VERY predictable. And it came to feel more like that familiar term of "taking a set" in a turn. You toss it, it rolls a little bit, and very predicatbly settles in. It actually felt very good... at least on the street.

Now... let's add some preload. Changing nothing else, just 3/4" of preload added to the front springs. Of course, that involves raising the spring perch, which also raised the front of the car a little bit. Hidden side-effect: it also increased the available compression travel by 3/4".

Quick drive of this setup... it sucks! Steering feels NUMB. Yeah, it's fairly responsive, but it doesn't FEEL responsive. It's sort of like something I'd read about when talking about setting the "roll couple" of the suspension. In that instance, you can set the suspension up such that the roll center and the CG are exactly the same height, zero roll couple, zero theoretical body roll. You could do that, and they say drivers would hate it because it gives them no feedback. That's sort of what this felt like. The car's doing stuff... but, the feel isn't there. Blah.

Before readjusting it, I took some measurements. there's no way I'm going to compress the tender spring far enough for it to bind. Which means as-configured, it's a wasted effort. I'll have to think about that some more.

But, also... I measured exactly 2" of droop travel. Which means 2.5" of compression travel. Let's ponder that.

2.5", but the last 1.5" is bump stop. So, as it sits right now, I've got 1" of free travel (at my 225# spring rate) before touching the bump stop. And then probably an inch of the bump stop being progressive before it gets serious and halts all motion. I've got MORE travel than I had before, but it sure doesn't feel like it.

What I had before... the lower spring perch was 3/4" lower, meaning that I had 3/4" less compression travel. (and 3/4" more droop travel)

With 1.75" compression travel and 1.5" of it being bump stop... that means that I only had .25" of free travel... but, it was with nearly no preload, so it was TRULY free travel, which is why it felt so smooth. Initial turn-in took up that .25" and then the progressive bump stop started kicking in. And it all felt good.

Another interesting thing: With the current "less droop travel" setup, I swear I felt some wheel hop!

So, maybe this particular setup "needs" a lot of droop travel to keep the inside tire planted? And maybe being just off the bump stop is the ideal place to be?

Will have to play with it more. But, as far as this thread goes... man, adding a bunch of preload made the steering feel awful!

[twistedwankel]

This has nothing to do with Loren's intellectual discussion other than the reality of suspension design extremes.

This afternoon on way to get 2 for $6 at Arby's for a light supper.

I was passed on Rt50 by a Mirage loaded with 4 grown men and I think one woman driver? Or another guy with curly long blonde hair. 5 adults for sure. It was an auto (based on brake lights at a stop) and I'm guessing pretty much sitting on the rear bump stops? Maybe not? For sure a low rider with the tires buried well above the fender line.

Go Mirage. I was going 55ish and it drove right by no craziness in handling at all.

[Loren]

Oh yeah, the stock rear springs are crazy soft, and there's about 3/4 mile of suspension travel back there. The automagic is actually a CVT. Saps all the fun out of driving, but apparently does a really good job of keeping the engine in its happy place. You put your foot down, it goes... more or less. Fully loaded, that's what you'll do, too! Foot to the floor. Probably why they were passing you. Driving flat-out from stop light to stop light.