ribs

Well...I have never heard of a suspension set up with an adjustable spring rate. The only things adjustable on most suspensions I have heard of are geometry (camber, caster, tow, ride height if it has adjustable spring perches), adjustable swaybar mounting points, and dampening if it has adjustable shocks (my shocks only have one adjustment). So, like with so many other things, on a long drive with no radio and nothing to do but think about stuff I figured out a way to do it, but somebody has either already done it or it won't work for some obvious reason...but...anyways, I spent 10 minutes with paint and photoshop and drew a little diagram of a sample double wishbone suspension with adjustable spring rate:



Let me try to explain it. First, it isn't a coilover strut design because that would change the dampening rate and wear out the shocks much faster...the spring and shock are seperate, and I arbitrarily chose to mount the shock on the lower arm and the spring on the upper arm.

What makes this special (and you can't see too well in the diagram) is that the spring will be on a laterally adjustable mount on the upper arm, allowing it to be adjusted inward or outward from the rotational point of the suspension, effectively changing the spring rate by changing the length of the lever (the control arm) working on it. The closer to the inner control arm mount (away from the steering knuckle/hub) the spring is mounted, the softer the spring rate, and the further out it is, the firmer the spring rate. The spring's upper and lower perch would both have to be able to rotate to compensate for change in suspension geometry (maybe mount on a super soft short MacPherson strut) and it would have to have an adjustable perch to compensate for load so ride height could be set correctly.

The benefits of this set up would be...only one set of springs needed for a car...it would save racers a lot of time and money if all they had to do was undo a couple of bolts and slide the spring mount inward or outward to adjust their cars for track conditions as opposed to taking the suspension apart to change out springs. If this was put into a production car, a weekend racer could have his springs set further in for his commute to work for a softer ride then put them out further to make it firm for track conditions. The dampening could still be adjusted via shock valving, blah blah blah.

So...did somebody already think of this and employ this idea on something I don't know about? Is there some obvious reason this would never work? Do you think I should see a psychiatrist and get on meds? Do you think my roommates are slipping me crack rocks in my cigarettes? Whats the deal? I also thought of a way to accomplish the same thing using torsion bars (having one of the two mounting points for the torsion bar adjustable along the length of the bar effectively shortening the bar and making it stiffer)...I'm just too lazy to draw another diagram.

mideastmafia

dont bike suspensions have adjustable spring rates?

SHAUN

Ken

I'm not sure if its an adjustable spring rate. The suspension is adjustable for compression and dampening. I'm not entirely sure on how changing the compression will affect spring rate. My friend had this to say on the topic.

"Well, if it's a variable rate spring, then you would change the k by compressing it so that the stiffer zone works more I guess.

In terms of the physics of what I just said, a spring is governed by F=kx where F is the Force applied, k is the spring constant (or not-so-constant), and x is the displacement. If you up the F by compressing the spring more off the bat, you may get into a new k. That's the only way I can imagine the spring compression changing anything."

Changing the force on a motorcycle spring would be much easier as the spring rates are alot less than a cars. I gather it could still be done though.

Here's what i came up with:



You basically tighten down on the upper adjustable nut to compress the spring more and increase the spring rate. Or loosen it to decrease the rate.

What'd ya think?

Doug944s2

Very cool idea, Ribs. And nice artwork, both of you. Hmmm, relatively cushy rates for a long freeway trip, intermediate springing for canyon assaults, and 400 lb./in. for auto-x. Nice!

Seems as though it'd be easy for torsion bars, with a sliding collar arrangement along the length of the bar that would yield infinite adjustment possibilities. Position of collar would alter anchor point of T-bar, and thus spring rate. Would be nice to have cockpit-adjustable shock valving to round out the package, like Mercedes' system on the SL that uses magneto-rheological fluid to vary damping. It's a pretty elegant setup in that the shocks use fixed orifices, and the fluid's viscosity changes by just altering the current.

ribs

[quote]Originally posted by Ken:
<strong>I'm not sure if its an adjustable spring rate. The suspension is adjustable for compression and dampening. I'm not entirely sure on how changing the compression will affect spring rate. My friend had this to say on the topic.

"Well, if it's a variable rate spring, then you would change the k by compressing it so that the stiffer zone works more I guess.

In terms of the physics of what I just said, a spring is governed by F=kx where F is the Force applied, k is the spring constant (or not-so-constant), and x is the displacement. If you up the F by compressing the spring more off the bat, you may get into a new k. That's the only way I can imagine the spring compression changing anything."

Changing the force on a motorcycle spring would be much easier as the spring rates are alot less than a cars. I gather it could still be done though.

Here's what i came up with:



You basically tighten down on the upper adjustable nut to compress the spring more and increase the spring rate. Or loosen it to decrease the rate.

What'd ya think?</strong><hr></blockquote>


Ken,

Thats and interesting proposal...I imagine that you would have to have a second adjustable perch for the spring mount (would work either top or bottom) to set ride height, but adjusting the pre-load on the spring would effectively jack the spring rate as well. The only downside I see to this is that you would limit suspension travel, but if you start with a long enough spring to begin with, it may not be a problem.

I am not sure if my explaination of the whole thing was adequate...I am using the concept of lever length to adjust springing. (Just FYI...its been a while since I took physics and haven't studied lever forces and forgot all of the formulas and stuff, so these numbers are made up, but should at least show the general concept). Just think of it this way: loosening a rusted bolt using a 12" ratchet...you may have to put your entire body weight into the handle of the ratchet to get the bolt free (for example: 170 pounds at 12" from bolt, moves 3 inches vertically). Alternative: loosening a rusted bolt with a breaker bar slid over the ratchet handle...you don't need super amounts of force to move the bolt from a distance, but you do need more travel (example: 50 pounds at 30" from bolt, moves 8 inches vetically).

So, applying this concept to a spring with a laterally adjustable lower perch, a 400# spring set all the way to the outside (next to the hub) of the control arm will yield 400#/inch of force and have this force spread evenly over a, say, 12" suspension travel. Moving the 400# spring to half way between the inside and outside of the control arm, and you end up with say a 200# spring with 24" of travel (travel would be ultimately be stopped by the bump stops in the shock). This is the effect of adding a breaker bar to the ratchet...by moving the spring inwards, you effectively add a "breaker bar" to the spring and this decreases the rate of the spring by increasing the travel. Makes sense?

[quote]Originally posted by Doug944s2:
<strong>Very cool idea, Ribs. And nice artwork, both of you. Hmmm, relatively cushy rates for a long freeway trip, intermediate springing for canyon assaults, and 400 lb./in. for auto-x. Nice!

Seems as though it'd be easy for torsion bars, with a sliding collar arrangement along the length of the bar that would yield infinite adjustment possibilities. Position of collar would alter anchor point of T-bar, and thus spring rate. Would be nice to have cockpit-adjustable shock valving to round out the package, like Mercedes' system on the SL that uses magneto-rheological fluid to vary damping. It's a pretty elegant setup in that the shocks use fixed orifices, and the fluid's viscosity changes by just altering the current.</strong><hr></blockquote>

Doug,

Seems like the torsion bar idea is also a good one. In my example of the double wishbone suspension, you could leave out the coil springs with adjustable perches and instead have one of the control arm mounts to the chassis (on the left) be larger in diameter, containing the fixed mounting point for a torsion bar. In front of the control arm (from rear to front on the car) would be a sliding collar which would bolt to the chassis at different points. You would just have to have splines along the torsion bar for the entire range of adjustability. I could think of an easy way to mount the t-bars where re-indexing them would take 2 or 3 minutes as opposed to 4 hours like our cars. Anyways, I always enjoy hypothetical technical discussions, so keep it coming.

Ken

Ribs,

Yeah, I understand what you're trying to say, and I believe it would work. I was just putting a different spin on things from my motorcycle experience.

My only concern is that the laterally adjustable lower spring mount might move under hard loads. Well, i'm imagining it infinately adjustable along the control arm, but it might be better if there were 3-5 settings. Ala the adjustable rear sway bar, with a pin and holes. The 'idiot factor' says that if it's infinately adjustable, someone could make the change at the track, not tighten it enough, and cause an accident. It would make sense to work out the holes that you could adjust the spring in 50 lb increments.