Tech Tuesday - What a Negative Spring is and Why it Makes the Coil-Spring Nearly Obsolete
Enter the Negative Spring
Suspension engineers discovered that placing an opposing spring that engaged at the end of the rebound stroke would fool an air-spring into acting like a coil type. In short, the negative spring counters the static pressure of the main air-spring as it reaches the end of its stroke and causes it to hover in place a small distance before full extension. Essentially, the air spring performs exactly like a coil spring at the beginning of the shock's (or fork's ) travel, giving it equal sensitivity over small bumps, and smoothness moving into the mid-stroke where the coil-spring once dominated. Fox and RockShox use an air negative spring, but a hybrid design using a coil spring to oppose the static pressure of the main air spring is used effectively by X-Fusion.
This illustration depicts a simple air spring - a piston in a cylinder - to explain how a negative spring functions. (From left) Without a negative spring, 200 psi in the air canister forces the piston to the end of its stroke, hard against the travel limiter. The addition of a second air valve (center) below the piston, like the RockShox Dual-Air forks, equalizes pressure of the positive spring and suspends the piston slightly above the end-stroke limiter. As the positive air spring is compressed (right), the minimal volume of air in the negative spring quickly loses pressure and has no effect on the positive spring rate.
The boxing match is not over, however, because the coil spring is essentially linear. For instance: compress a 300-pound spring one inch and it stores 300 pounds. Compress it three inches and it stores 900 pounds. Simple math. Gasses, by nature, compress in a non-linear fashion. Left to its own devices, an air spring ramps up considerably near the end of its stroke. As it turns out, however, some degree of a rising rate spring is beneficial in both forks and shocks, and suspension engineers have learned to add or subtract air volume from the air spring to tune exactly the right spring rates for mountain bike suspensions. With the tables turned, suspension designers now use an end-stroke damping booster or add an air-spring in series with the coil (like Marzocchi's 55 and 888 forks) to emulate the air-spring's beneficial rising rate.
Fox Racing Shox provided this cut-away illustration of its Float RP23 shock. The air canister above the shock body replaces the coil spring of a standard damper. The negative spring sits just below the air-spring piston. Air pressure from the positive spring is fed to the negative spring through the air-volume sleeve via a tiny transfer hole near the bottom of the can. Every time the shock is compressed, the air piston passes the transfer hole and the negative air spring is recharged with the static pressure of the positive spring.
Coil-Spring Fails
Unless you are rich, the big fails for coil spring are cost and weight. The linear nature of a coil spring, and the fact that it derives its rate from a combination of coil-spacing and wire diameter, means that coil springs are only useful for a narrow range of rider weights and/or riding intensities, so you'll have to purchase a new spring to experiment with different preload and spring rates. Most riders use the spring that came on the shock - some are willing to buy one with a different rate, and darn few own more than two springs. An air-spring can be set to many rider weights and preferences - with a air pump. Last time I checked, air was a bit less expensive than coil springs sold by Fox and RockShox. Weight? Well you can figure that one out. Coil springs weigh between 400 and 600 grams (more than a dropper seatpost).
In this simplified graphic, RC illustrates the function of the Fox Float RP23 negative spring.(From left) The negative spring is formed by the space between the main air piston and a seal at the lower end of the air can. An air sleeve surrounds the positive air spring. When the shock is compressed beyond a certain point, a pair of holes in the air-can (center) allow air to bypass the main air piston and fill the negative chamber. As the main air piston returns and passes the transfer hole (Right), it traps air in the negative chamber, and because the effective piston area of the negative chamber is smaller than the positive side, the air is compressed slightly. The activation zone (where the negative spring kicks in) is about one centimeter of the stroke, from the transfer hole, to the point where the negative spring equalizes with the positive spring.
Air-Spring Fails
Air springs have a couple of fails as well. Heat can expand the gas inside the spring canister and increase the spring rate midway through a ride. Most riders, even downhill racers rarely heat their shocks up enough to make this an issue. Perhaps in a 45-minute Super D situation, where a rider is using an under-sized XC air shock like the Fox RP23 for an extended super-techy descent would create a heat problem, but that said, it's a rare descent when an RP23 damper gets beyond warm to the touch. By nature, an air-sprung shock has more seals than a coil-over type, so starting friction is greater. Low-leverage suspension designs are susceptible to starting friction issues, as well as some fork designs. That said, super-slick coatings like the (dare I say the word?) Kashima treatment used by Fox have narrowed the disparity between coil and air-sprung suspension.
So, Coil-Over or Air-Sprung - Which is Best?
The bottom line, at least for mountain bikes, for the air-vs-coil-spring debate is that the performance gap between the two has been whittled down to nearly nothing by the invention of the negative spring and air canisters with variable volume. The versatility of an air spring simply kills a coil-over type. If you love coil-over shocks, by all means, ride one. If you want to save 400 grams and have the ability to change your suspension setup anytime you want for little or no cost, the modern air-sprung shock and fork is your first pick.
Past Tech Tuesdays:
How could you debate this setup for a long ride Shark555?
At the end of the day you just want to make sure like cydaps says you don't have too much sag and that your properly sprung
why do you think santa cruz went to a 3" stroke shock on the v10 after using a 2.75 for years? cos they were constantly blowing them up in the syndicate. with heat being a part of the problem.
air will work for xc but never dh racing, far too much depends on suspension remaining consistent.
as for people not changing coils? its the first thing every one i know does when they get new suspension, normally they just to a straight swap on coils for free. hardly a problem now is it.
I never read the whole article, just a brief scan over.... Coil's aren't overly expensive to buy anyway and I do have 2, one on the bike and one as a spare which is a different weight...
Its a brave statement to say that a negative spring makes the coil spring nearly obsolete. This is only true where the overall weight of a bike is of more importance than the performance of the suspension.
When I see top flight Rally cars and MX bikes running coil free air suspension, I'll be ready to believe there is no compromise in performance.
Does anyone know if companies give different compression/rebound tunes depending on spring type?
But other than that, if you want less dive and have a fork at less than its max travel, make the negative air chamber smaller by putting the largest diameter custom spacer you can get. I have a Reba lowered to 50mm for street- the air spring feels awful. In fact, it never even gets progressive, it is stuck in the digressive part of the stroke because of the ratio of the size of the negative chamber to the positive chamber. Customizing the size of the negative chamber is one more way to tune your ride.
If I'm wrong please explain.
As far as I am aware, a frame's leverage ratio imparts a force on the shock (spring) for a given rear axle movement. The shock (spring) will provide a resistance to the applied force. The combination of the frame leverage ratio and shock resistance characteristics give the wheel rate - which is essentially how the bike will 'feel' under compression through the stroke. Some bikes have a progressively stiffer resistance (bottomless travel), whilst others may be more compliant mid-stroke.
Frame leverage ratios can be linear or non-linear, therefore applying a different force onto the shock through the stroke. Most bikes tend to have a linear progressive rate these days. A traditional coil spring has a linear resistance (Hookes Law), as RC explained in the video. Air shocks (springs) have traditionally had a non-linear, exponential resistance, i.e. the rate of resistance increases the more load that is applied.
So, if you combine a coil shock (linear) with a linear leverage ratio frame you'll get a linear progressive wheel rate.
If you combine a coil shock (linear) with a non-linear leverage ratio frame you'll get a non-linear wheel rate.
Etc...
That's my understanding - hope it makes sense!? ;+)
I can save double that by taking a dump before I ride. I'm very happy with my 'obsolete' coil fork mainly because I doubt I could get anything near as reliable or smooth in an air fork in my price range (think £200/$300ish). I'm sure that a super kashima coated uber air fork/shock is fantastic if you can afford one. But for me I just want something simple that works reliably....and air suspension is anything but simple.
In my opinion, 15 grams is only important when you are a pro racer... and for people who brag with their bikes, not their riding.
Keep up the good work!!
That also applies when you lower a fork, you clip to the rod, next to the negative spring some internal travel spacers to prevent the forks from getting its original full travel back.
I hope that answered
@The-Banders the newer Rockshox stuff has set-up guides available on the SRAM website. I just use my weight and dial in the settings according to the charts to get a good starting point. Every fork and shock should come with something like this as far as I am concerned!
Coil and air both have advantages and disadvantages. One of the biggest advantages of coil/oil suspension is durability. Durability as in a coil and oil shock or fork can last years with zero maintenance. Bike fanatics wouldn't do this but the vast majority of riders aren't bike fanatics.
A. Compared to the lower pressures required for a fork the high pressures of a shock require much tighter seals and higher friction on the shaft and the protruding stanction. One can argue about the Kashima or special oils with friction additives like I ve used in air Totems with success. I ve tested an Intense tracer with Kashima shock and one without and could not notice a difference either when pedaling or on the trail. On a 36 fork I think I could, but also note that the non kashima was well prepared for a ride.
Negative and Positive piston area ratio create a slope of the force as the shock compresses and it would be nice for the manufacturers to post that as well the spring rate so we can choose accordingly.
So what existed before the negative chamber? was there vacuum or was there a rubber bump or just a hole?
Unless you try a bike and continuously bunny hop and feel the extension of the suspension , on the trail you are always somewhat compressed and the feel there is what is more important, not how it feels when you drop the bike in a LBS.
Agreed that the weight difference for a trail bike is considerable with a steel spring.
Regarding tunability, the negative pressure is dependent on the positive pressure so back to the fixed ratio. Scott with their equaliser shock had separate adjustment for positive and negative pressures as does the new Cannondale. Read some reviews about them and the riders comments about small bump compliance. Forks generally have pos and neg adjustments so are more tunable.
I have a Tracer 2 on which I changed the rp23 to RC4. The bike was a joy to ride but with the rc4 it can do everything. You could go to maximum sag with the pr23 and for example go relatively comfortably on a rocky trail but then forget any drop more than 4-5 ft. One could argue using a DHX air with bottom out compression but this shock has even less mid stroke support.( the ratio we talked before)
Going to the lesser sag ( within allowance) , meaning higher pressure on the rp 23 led to stiff bike feeling it has less travel. For example I ride a Tazer vp with minimal sag and on drops the two bikes did not feel far apart.
The tracer with the rc4 is simply amazing. You can blast trough any trail or dh track which indeed is not what it is supposed for, with comfort and surefooting.
Indeed the rc4 is heavier, but one can also bump up the pressure in the rear tire which reduces its friction and makes for a faster bike balancing some of the weight handicap.
But I gotta know...Do those guys who go out and just rip, and you all have the friends I am talking about, really spend time thinking about stuff that is this tech? Perhaps... but my guess is they are too busy riding to worry about it.
most mountain bikers may not be engineers and therefore their knowledge of such things limited to the sport of mountain biking.
but the air spring will never make the coil over obsolete....what a stupid thing to even concider implying!
the matter of fact is unless your pro and every little gram counts in a race then to the rest of us it doesnt matter.
ok there are your weight weenies and bike tarts that MUST have what is marketed as the BEST shock ever, wether it makes a difference in their riding or not.
how many of you guys have been out on your bike (not top of the range) and seen other guys with TOP OF THE RANGE bikes and shocks and body armour and jerseys then been astounded at how badly they ride?
it really only comes down to the user.
plus the reliability and cost of a coil over seems to far out weigh the cost and reliability of an air shock.
Really enjoyed this article, more like it please pinkbike. I'll still most likely always be coil, purely from a reliability standpoint, but educating people is always good!
BTW, you might be surprised by all the unaggressive sports for which Fox makes air suspension, their website is only clicks away.
on forks its much the same but is less significant because lower pressure
1>the leverage ratio is 1:1 meaning significantly lower pressure is used
2> as stanction size increases the volume increases allowing lower pressures to be used
3> much larger surface area allows heat to be dissipated more efficiently
lower pressure reduces heat build up and so is better for air really its as simple as that really
this means that where a rear shock might need to be a coil at 160mm of travel you may be able to use an air fork for a 180mm bike without any problems
Think this why Fox added the "Boost Valve" to some of their air shocks,
anybody know about this?
Waiting on a CCDB air and i'm hoping that "air is the new coil"
Also, does elevation effect air sprung shocks? (8000' vs sea level for example).
Elevation does not make a difference since the shock is an isolated system.
so he aint far off
I use both for Dh and limit the use for air for shortstroke parkbikes where you want to filter forcespikes so as not to damage the frame. Coilspringdamper effectively split the forces between damper and spring - protecting each other and the frame and the rider. That is why they rarely fail.
Air is fine for light-use-xc-bikes.
And I just got rid of my last airfork yesterday - a Marzocchi 66 - that was a crappy piece of equipment. Set when cold, nosettings when hot, reset when hot - oh you need to bring a pump with you on the track... Now its coilsprung Domain and Boxxer allover. There is a reason why dh is the pacesetter - junk has no acceptance with the community.
Domain is a perfectly capable and durable dc-fork with long service intervalls. Try to see beyond the hype. Also run Boxxer - give and take.
Again - airshocks are prefered by the industry because they are cheap to make and sell for a better price and there is no stocking cost with various coilsprings. OEM like airshocks because they only need one shock and no springs. Resellers like airshocks because they do not have to stock springs. Who fixes and services airshocks? Specialized aftermarket outlets. Are airshocks better for riding? Hell, no.