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Pros/Cons of progressive rate springs?

 
Old 11-02-2002, 10:37 PM
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wannabefaster
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Post Pros/Cons of progressive rate springs?

what are the pros and cons of progressive rate or main/tender spring combo in a track car suspension?
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Old 11-02-2002, 11:47 PM
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Steve Weiner-Rennsport Systems
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Wannabe:

Great question, and there are no firm answers as there are many conditions and situations that can require either suspension setup.

For the average race car, one really wants a linear-acting suspension that has even dynamics from full droop to full bump to keep the tires on the ground. In most cases, this requires a linear constant-rate spring. While not absolute, progressive springs are better suited for street cars to retain some ride quality while using a firm, performance spring. Rising rate spring setups have had mixed success on race cars and in most cases do not work as well as a linear spring package.

Race cars need springs that will support the static & dynamic weight of the car and keep the tires on the ground, even with aerodynamic (downforce) loads. Big, wide sticky tires add to chassis loads and sometimes those stiff springs needed to handle the full range of loads load, can make the suspension system too stiff on bumpy tracks and uneven surfaces. In these cases, you will suffer understeer or oversteer, depending upon the spring rates at each end.

Now, there are two kinds of tender springs; passive and active. Passive ones are pretty soft and mainly serve to keep the main springs tensioned so that they do not fall out of the perches at full droop. Active ones are stiffer and serve as an integral part of the overall spring rate package at each corner. This allows one to use the proper (and sometimes stiff) spring that might be too firm at low speed and during low chassis loads on small bumps. In these cases, the tender spring permits enough suspension movement and compliance to keep the tire patches on the ground until the main spring compresses with load. In short, an active tender allow you to use stiff springs without paying the price of poor grip on small bumps and in slow corners.

What to do,.......right?

This depends on your car, its weight, your wheel and tire size, the aero package (downforce), and engine power. Other factors are the characteristics of the tracks you run. Its not a one-size-fits-all, and this is why serious racers carry and use different spring setups for different tracks. LOL,...another good argument for double (and triple) adjustable shocks.

I've offered a pretty simplistic answer to give you an idea of what things do but I'd need the information I listed above to tell you anything specific, and it would only be a good starting point.

I do hope this helped a little.
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Old 11-03-2002, 12:19 AM
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Danno
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The other thing about progressive springs is you can't vary your damper's valving to match the springs through the entire length of the stroke. If you want progressive action, you want to devise a rising-rate linkage that acts on a coil-over shock with linear-rate spring, like on motorcycles (on cars you can do it with push/pull rods). This way, as the ratio of shock-compression to suspension-movement increases, the damping will increase along with the spring-rate to match.
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Old 11-03-2002, 11:04 AM
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Steve, Danno thanks

You have both confirmed what i was thinking, regarding the progressive springs.

But it seems as though there might be something to explore with the main/active tender spring combo.

My car is a 951 with 350lb coilovers in the front and the 968 MO30 setup in the rear with welmeister adjustable bars front and rear. I run Koni yellow sports all around usually set about a turn loose from full firm. Wheels and tires are the same all around 18" x 8.5" (heavy) and and 245x35 Hoosiers. A pro who drove the car at Watkins Glen commented that it was "wonderfully" neutral. The next lap, when i was driving, i promptly spun in the "toe of the boot"... demonstrating that smooth is important. Besides the driver, the probem is the car tends to bottom out at places like the downhill at Lime Rock.

Steve you say:

[quote]In short, an active tender allow you to use stiff springs without paying the price of poor grip on small bumps and in slow corners.
<hr></blockquote>

and Danno commented that we need to keep in mind damping rates and spring rates.


So....
Assuming you stay off the turtles and don't hit any furry speed bumps. It seems like an reasonable setup might be an active tender spring with shocks valving ideal for their rate and main springs that are say 20% stiffer and consequently slightly under damped. This would allow good performance on the most uneven surfaces and small bumps and still allow for handling situation like the downhill at Lime Rock where the higher rate is required to keep from bottoming out.

The Main spring will be underdamped but i was thinking that if the large suspension deflections happen relatively slowly compared to bumps this might not be so bad.

If this thinking seems plausible then the remining question is: At what point in the spring travel should the Main rate be dominate?

BTW, do you guys have any favorite books on suspension tuning? I'm an engineer (EE) so i don't mind some tech stuff.

Peter
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Old 11-04-2002, 12:14 AM
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"A pro who drove the car at Watkins Glen commented that it was "wonderfully" neutral. The next lap, when i was driving, i promptly spun in the "toe of the boot"... demonstrating that smooth is important."

That should be all you need then; the car's set up close to optimally.

"Besides the driver, the probem is the car tends to bottom out at places like the downhill at Lime Rock."

Alright, now we're getting somewhere. But first, let's back up and make sure we're talking about the same things here. When you talk about 'progressive springs' are you referring to spring that look like this?



These are really 'dual-rate' springs, not really progressive. That's because the tight coils at the top are all evenly spaced apart. Under normal operation, you have a 12-coil spring that's soft. But as the suspension compresses, the tighter half will coil-bind all at one time, resulting in a 6-coil spring that's twice as stiff. This is a sudden transition with a dramatic increase in spring-rate. These springs are typically available in a range of 150/350-lb/in to 200/550-lb/in or so.

Or are you talking about a dual-spring configuration that Steve is talking about?



This would appear to work similarly to the dual-rate spring set-up. However, it is far, far from that in operation. Due to the physics of a spring-on-top-of-a-spring configuration, the resulting spring-rate will be even softer than the softest coil as given by the spring-rate equation:

1/EffectiveRate = (1/coil#1rate + 1/coil#2rate)

Meaning if you put a 400-lb/in spring on top of a 200-lb/in spring, the actual spring-rate will only be 133-lb/in. Until the softer spring goes into coil-bind, at which time, you'll have a 400lb/in spring. So the effective spring-rate will be like a dual-rate 133/400-lb/in spring. This isn't even the typical way these springs are configured. They are usually set up so that the soft spring is used only when you jack up the car, to take up the extra space created by going to short, stiff coilover springs. Then when the car is lowered, the softer spring is completely compressed, contributing nothing to suspension control.

The only real progressive-rate springs were made by Progressive Suspensions a couple of decades ago. It was wound in such a way that the distance between coils were widened progressively as you move down the spring. That is, the spacing between each and every individual coil was different (got progressively wider). Thus the spring would go into coil-bind only one coil at a time as the suspension compressed. This is a real progressive-rate spring, but I don't know of anyone making such a creature (if anyone knows, please email me their contact info, thanks).

Again, this brings up the issue of matching shock-valving mentioned earlier. Even if you set up the shock-rebound for a rate right in the middle of these extreme ranges, it will be too stiff over small, high-velocity bumps. Yet on large-displacement, low-velocity bumps, it'll be too soft. The 'velocity' I'm talking about here is the wheel's vertical speed in hitting bumps, not the car's horizontal speed. And, the higher spring-rate you encounter on those big bumps (like the one you bottom-out on) will have the car bounce up even faster creating a wallowing effect if you hit a bump in mid-corner.

However, we're actually looking at completely the wrong areas in tuning your suspension. The issue is your Koni shocks. What you really want is shocks with more low-velocity damping for smooth large-displacement bumps like coming down a hill. Koni makes a triple-adjustable racing shock that can be fine-tuned for this exact kind of issue. They are adjustable for rebound to match your spring-rates. As well as having low & high-velocity compression-damping to deal with the smallest & largest bumps you'll encounter on any given track.



You'll want to have the red curve as shown on this shock-dyno as opposed to the blue one you have now. High-velocity compression-damping is close to the same, so you'll good response on small quick bmps. Low-velocity compression damping is increased about 20-50% more so those big bumps won't bottom out the car.

I think <a href="http://www.paragon-products.com" target="_blank">Paragon Products</a>has the triple-adjustable coilover struts & shocks you would need to make this happen.

Besides the shock-valving match issue, the other is front/rear balance. If your car is handling neutrally now, progressive or dual-rate springs will mess up that balance depending upon the amount of cornering force you're under. If you only have these springs in front, as you build up more and more cornering, the front spring-rate gets higher and higher and you'll have more and more of the lateral weight-transfer going to the front wheels. Thus as cornering forces build up, your car will understeer more and more. The only way around this would be to install the same springs in the rear that has the exact progressive action.

And I haven't even gone into motion-ratios, effective spring-rates, wheel-rates, resonance-frequencies and their front-rear balances yet! You can read up on that here: <a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic&f=15&t=012370" target="_blank">Topic: spring rates vs wheel rates? </a>

Here are some books get you started:

"How to make your car handle" - Fred Puhn
"Performance Handling" - Don Alexander
"... To Win" series by Carroll Smith
"Race Car Vehicle Dynamcis" by Milliken
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Old 11-04-2002, 10:03 PM
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Danno,

At this point i'm definitely thinking that a relatively small and easily controlable change is in order....kinda the corollary to the old mechanics rule - "if it ain't brokem don't fix it".

Sounds like shocks should probably be the first "tweak"... along with a bunch more reading.

Thanks again for your help

Peter
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