Coil spacers, torsion bars and preload 1

[robinhood4x4]

Hi, new here. I'm an avid off roader and wish to help others in my sport to understand how their suspension works. It has proved to be very difficult so I made a webpage outlining the basics of coilover suspension and how adding lift spacers affect the suspension.

I was hoping you guys could look over my website and let me know if anything is incorrect. The premise of the website is to debunk the myth that adding spacers add preload to the spring.

Also, another debate we're having is why, when spacers are used or torsion bars are "cranked" for lift the ride becomes stiffer. A couple of us engineers are saying it is because of the geometry effect of increasing the angle of the control arms, thus reducing the moment arm on the tbar or coil and also because of increased tire scrub. Why do you guys think the ride gets stiffer?

Here is my website:

http://robinhood4x4.com/suspension1.htm

Please let me know if anything is wrong, but keep in mind this is written for non-engineers and therefore it is a very basic outline.

 

[robinhood4x4]

Can anybody help me out here? I think this is a simple question for you guys, I just need to know whether my website is accurate or not.

Thanks,
Steve

 

[bhart]

Steve,
The information on your website seems fairly straight forward and accurate to me w/ regards to what your are trying to explain.
Here is my attempt on how the myths/confusion got started, b/c I've had to explain this situation before to some friends when dealing w/ our mini-baja project back in college.
I might be going out on a limb here, and I've never seen a technical definition of "preload", but to me, preload only exists when there is a force in the spring while there is zero force on the actual suspension (i.e. suspension is in full droop and shock has bottomed, chain limiting travel, etc). In that case, your are going to have to exert X amount of force on the suspension, before the springs will compress @ all to overcome the "preload" in the spring...the spring rate will appear to be very high (infinite) until you overcome the spring "preload." Once you overcome the preload, of course, the spring rate just goes to whatever it is if you ignore preload all together.
For instance, let's say that you installed your coil-overs onto your truck, and you cranked the your adjuster nuts down until the suspension had bottomed out in droop through the shocks, so that the spring is "preloaded" one inch (assume 100 lb/in so this is 100 lb of preload). Now, let's say that you put a 2" space in the coil-over assembly, and attempt to crank the adjuster nut back down to the same point that you originally had it w/o the spacer. Now there will be 3" of "preload" in the spring, and when you set the truck down, and measure the shock travel, it would read less travel to support the weight of the truck b/c for the first 100[lb/in]*3[in]=300[lb] pounds of force, the suspension did not compress at all, rather the normal force within the shock (internal contact within the shock) simiply reduced until it was zero, then the spring started to compress further. At this point, if you didn't know any better, and didn't think to measure the actual spring deflection, you would say to yourself, "Hey, I just added that spacer and adjusted everything back to where it was, and now my suspension travels less to hold the truck up, it must have made the spring stiffer!"
Well, I'm sure that you wouldn't say that to yourself, b/c you understand the situation, but the average off-roader might not recognize that the length of the spring is the same to support the truck (which you pointed out).
I think that what your are doing w/ your site is good, and could potentially be very educational for those that share your interest in off-roading.
bhart

 

[robinhood4x4]

Thank you very much Bhart. After rereading my preload definition, your explanation of preload is much more accurate than mine. I'll be amending that soon.

Can anybody else add to this? It is very frustrating to explain these concepts to people and yet they still don't believe me (although, I do manage to convert a few of them). The more engineers I can get to agree with me, the easier it will be to convince them.

 

[waskillywabbit]

Lookie what I found searching this morning...

Howdy Steve!

Brian
Pressure Vessels and Autoclave Systems

http://www.mcabeeconstruction.com

The above comments/opinions are solely my own and not those of McAbee Construction.

 

[BillyShope]

Yes, definitions can be tricky. In dragracing, "preload" is an increase in right rear static wheel loading (and, consequently, left front) to counteract the effect of driveshaft torque and thus tend to equalize rear wheel loads during the launch of a RWD beam axle car.

Now, if we can just get those limeys to understand that "bonnets" are found on the heads of women and children and not on automobiles.

 

[c2s]

Hmmmm ... There are two kinds of coil spring spacers. Spacers that go between coils and those that are placed at the ends of the coil pack. You website suggest the later.
As bhart pointed out, the pre-load is determined by the difference between the free height of the spring and it's constrained, unloaded height (call this difference dZ). The pre-load = dZ * rate.
Spacers between coils reduce the effective no. of coils thereby increasing the spring rate. The difference in spring heights may not change significanly (dZ), but the spring rate has increased, therefore the pre-load has increased.
Spacers placed at the ends of the spring pack reduce the constrained spring pack height. So, you have reduced the constrained, unloaded height (increased dZ) and therefore increased the pre-load.
Kevin