ATV Air Suspension: Is It Suitable?

[monty37]

Is air suspension suitable for All terrain vehicles?

 

[Mech_Engineer]

It depends on what you mean by air suspension.

 

[Ranger Mike]

I suspect the substitution of some type of air bag vs mechanical spring/ shock damper traditional suspension would be
more expensive
more complicated
more difficult to repair in the bush
less rugged and durable
would require air lines ( these are open to snagging on rocks, tree limbs etc)
valving control module ..subject to vibration..and off road that's all that you are going to get
big air tank reservoir..where you going to mount it..limited space on these short wheel base rides
and what is the benefit?
a smoother ride..off road?

on the plus side you got an air compressor available for any flat tire repairs

 

[Mech_Engineer]

Coilless gas charged shocks (a.k.a. air shocks) are a new high-performance option for off-road trucks, but they're somewhat expensive. The shocks eschew traditional coil springs for a unit pressurized with nitrogen (shock forks utilizing similar technology are also available on mountain bikes).

The O.P. doesn't specify what he means by an "air suspension" or "all terrain vehicle" so it's impossible to answer his question more specifically.

 

[monty37]

well i mean using alternatives for springs ,air bag type etc .But a smoother ride can be ensured using this type of suspension ,shocks experienced are less...so i thought if they could be effective in All Terrain Vehicles.But as you listed out all the cons ..though it provides smoother ride it does not suit ATVs.

 

[Mech_Engineer]

Well as you can see in my previous post, air shocks are already being used on customized 4x4 vehicles; there's no reason the same technology couldn't be applied to an ATV.

In addition, most mid to high-end mountain bikes come with air shocks as well- the shock forks use pressized air for front and rear suspensions. Take for example this Santa Cruz Nomad which has front and rear air shocks:

 

[Ranger Mike]

Whoa up fellows...there is no such thing as AIR shocks...even the old style air shocks ( were popular in the muscle car era) were conventional oil filled shocks that had a rubber bladder that could be inflated to raise the ride height of the car. As the air bladder inflated, it limited the shock travel and ride quality. ALL automotive and motorcycle shocks use " shock oil" that is forced through orifices to dampen the rate of compression or rebound.

Lets review. Shocks provide resistance by forcing hydraulic fluid (oil) through valves in the piston as it moves up and down. Because the oil cannot be compressed, only a certain amount of fluid can be forced through these valves, which creates resistance to the vehicle movement. "GAS" shocks are superior to regular hydraulic shocks because air in the shock is replaced by pressurized nitrogen gas. The gas in the shocks is contained in a bladder to prevent the mixture of the gas and the shock oil, which is different from shocks where the oil is stored in a separate chamber or stored in the oil chamber. This advancement in technology prevents bubbles from forming in the hydraulic fluid. These bubbles, called foaming, reduces the ability of shocks to provide resistance and prevent bounce. Gas shocks also quicken the response of a shock's movement thereby increasing comfort and control under all conditions. When you think gas..think Nitrogen because it has less moisture and better properties than normal " air".

We run the Penske "gas" shocks on the race car that achieve their damping with fluid being forced through a series of spring steel washers. Changing the thickness of these washers produces different valvings. Racers themselves can change the valving at the track! It makes these Penske shocks both rebuildable and revalvable. Not only can the racers change the oil themselves, but they can change the valving stack to achieve a different value. To illustrate how easy it is, both the compression and rebound damping can be changed in less than 10 minutes with no special tools (except an inflation valve to pressurize the shock with nitrogen. We carry a bottle of nitrogen in the trailer for this purpose and to inflate the tires.) In the old days we had to carry a whole bunch of conventional shocks and swap them out as they were manufactured with different compression and rebound rates. At $ 100 a shock this adds up! Penske shocks cost $400 and up...each!

Another feature that makes these shocks the hot setup is that since they are rebuildable by the user, they can also be repaired by the user. If you bend the shaft on a Penske shock, you can fix it for about $40. With sealed shocks, you would have to buy another new shock if you bend it and on round track YOU WILL bend quite a few.

So why the switch to gas shocks? Racers over the years found shock consistency to be a problem in two separate areas. First, shocks would not stay consistent ..see above info on "foaming) and secondly, after extended usage (over 30 to 40 hours of race time), the oil inside the shocks tended to break down and thus lose its viscous properties. After finding this out, shock engineers started investigating different oils for use in the shocks. They also investigated oil change intervals to keep the shocks as consistent as possible. The end result points to changing the shock oil after 30 hours of use, which translates into about a third to a half of a short track racer's season. Now a set of shocks, with proper maintenance and rebuilds, will last a racer five years or more.

One final note: unless you come up with some totally new "air suspension" we are stuck with a variant of the tried and true spring/shock design.

 

[monty37]

i was asked to design an off-road quad ,so i thought i could go for air suspension(tapered sleeve type) as double wishbone is conventional .But due to quad dimensions and weight issues i need to reconsider this..

 

[Mech_Engineer]

Whoa up fellows...there is no such thing as AIR shocks...

You're very knowledgeable in the areas of race car suspensions, but let's be honest: you're nit-picking without any useful goal. I posted two perfect examples of what are colloquially known as modern coilless gas charged shocks a.k.a. "air shocks," as in they use compressed nitrogen ("air") in the shock's body to provide the suspension spring rather than a coil or leaf spring. Preload and spring force characteristics can be adjusted by adjusting pressure in the shock.

Air shock tech article: http://www.4wheeloffroad.com/techarticles/suspension/131_0606_air_shock_technical/index.html

 

[Mech_Engineer]

One final note: unless you come up with some totally new "air suspension" we are stuck with a variant of the tried and true spring/shock design.

Please point out the spring in this picture:

Air shocks are a compromise between a shock and a spring. They are very similar to nitrogen-charged emulsion shocks. However, you will quickly notice that air shocks have a much larger chrome shaft that runs out of the body.
...
These new air shocks look very much like the coilover shocks used by desert racers and rockcrawlers, but without the coil springs around the outside of the shock body. What really makes folks scratch their head is that no other type of spring is used with air shocks-no coils, leaves, or airbags. The air shock itself works as both spring and shock in a cheap, lightweight, and simple package...

 

[Ranger Mike]

Here is the spring..

I copied this from the link you kindly provided, thank you...


The problems arise when the air shock tries to do two jobs at once-support the weight of the vehicle like a spring, and damping the movement of the suspension like a shock. The main reason for this is due to the spring rate of the air shock. On a basic coil- or leaf-spring the spring rate is a given amount (for examples sake, we'll say 100 pounds per inch), so to compress the spring 1 inch you need 100 pounds, then to go 2 inches you need another 100 pounds, adding up to 200 pounds and so on. With an air shock the spring rate is very flat for the majority of the compression and then near the end of its compression it ramps up quickly. For example, it may only go from 30 to 200 pounds for the first three-quarters of total compression, but then the final 4 inches will jump up to nearly 7,000 pounds (these numbers will change according to the nitrogen charge, but the general curve will stay the same). This results in a suspension that has little resistance to compression or rebound force for the first 60 to 75 percent of the shock's travel and then the final 30 percent gets exponentially harder to compress and quicker to rebound. This may seem like a great attribute, almost like having a bumpstop built into your air shock, but when this erratic spring rate is combined with the unpredictable damping of an emulsion shock, these magical mystery shocks don't look so wonderful anymore.

Old School Air
This story wouldn't be complete without mentioning load-carrying air shocks such as the Gabriel or Monroe air shock absorbers. These are designed as a supplemental overload spring, and work by adding air to an enclosed chamber much like the airbag overload springs. The benefit to this style air shock is it allows the use of a softer riding spring pack for your truck for off-road use, but then the shock can be aired up for your next trip to Bufford's Rock and Sand to get a thousand pounds of gravel. One concern is how strong the stock shock mounts are for that added load as they are now being given stresses comparable to the spring mounts. We're sure an ingenious wheeler could add these to his old 4x4 and with an onboard compressor or air tank have a trick way to raise the suspension an inch to help get off of a belly skidplate obstacle.

You Need To Be Negative
Fox Racing Shocks has addressed the spring rate issue better than other manufacturers by

adding a negative coil spring inside the shock body around the air-shock shaft.

see photo attached from the link


This in effect adds some spring rate to the compression stroke of the shock. Plus there are rumors of a 4x4-sized version of Fox's famous mountain-bike Float shock that has an internal damping shock built within the shaft of the air shock, plus a more balanced spring rate, but price may be comparable to a current coilover shock.

How And Why
The 2- and 2 1/2-inch air shock's shafts are greater than twice the diameter of the basic 2-inch nitrogen-charged damping shock on the right, but the shock bodies on both the 2-inch air shock and the 2-inch damping shock are equal diameter. Thus as the air-shock shaft moves into the shock body it displaces more than four times the volume as the damping shock does and begins compressing the nitrogen charge much like a tire compresses the air within, and that makes it act like a spring. Imagine a plastic cup three-quarters full of water with a lid on it and a small hole in the lid; now if you push a pencil through that hole into the liquid it will only displace a small amount of fluid, and though the pressure may increase slightly it surely won't pop the top off the cup. This is like the damping shock, only it has a piston with holes drilled through it on the end of the shaft (pencil) to slow it down for damping. Now imagine the hole in the cup's top being just big enough to fit a soda can through; it will displace quite a bit more fluid, and even with one-quarter of the cup's volume filled with air it will definitely pop the top off the cup. This is more like an air shock, the larger volume of the can represents the larger shock shaft, but since the top is firmly attached to the shock body the pressure inside will simply increase until it is enough to support the weight of the vehicle.

What's Right 4U?
Most 4x4s running air shocks only use a small amount of uptravel, where usually only 5 inches of uptravel is available with a 16-inch shock. This doesn't really lend itself to high-speed trail bashing. If you take an airshock-equipped vehicle out for some high-speed jaunts, the shocks quickly heat up and begin to raise the ride height of the vehicle. As the oil heats up the nitrogen begins to expand and forces the shaft out of the shock body. Now before you cancel your order for air shocks let's consider your usage. If you have a tube buggy, an old Jeep, or a lightweight Suzuki project and you mainly rockcrawl, rarely see high-speed action over muddy bogs, never plan to do hours of fast wheeling across desert whoops, and you don't drive this thing on the street much, then air shocks are for you. If you choose air shocks you may also want to invest in a sway bar to help deal with the body roll common to them due to the flat spring rate. But if you are looking for big air, ultimate adjustability, and the proper tool for more jobs, then take the step up to coilovers.

I guess you can use a totally gas shock without the spring but the drawbacks out weight the weight savings,,in my opinion..

 

[Mech_Engineer]

So you copied and pasted the entire article I posted a link to... what point are you trying to make? My point is that suspension systems already exist which have no coil/leaf spring whatsoever. The OP's question is whether an air suspension could be used on an ATV, which the answer is undoubtedly yes- it's already being done!

 

[Ranger Mike]

mech eng
you said i was nit picking without a specific goal...ok, maybe ( engineers and nit picking could take up a whole new thread) i can live with the commnet..
I would have left it alone except YOU asked me to point out the SPRING in the picture..
I did...
nuff said

 

[monty37]

if we do use air shocks type for off-road vehicles won't there be any air leak problems ?

 

[Mech_Engineer]

if we do use air shocks type for off-road vehicles won't there be any air leak problems ?

It depends on the qulity of the shock. As a general rule air shocks don't have problems with leaking when new, but may tend to start leaking as they get older and need the seals replaced. Even then, you can fill them up once a week or so.

 

[monty37]

i am having to design this all terrain vehicle ,but there is a particuler school of thought
who feel that if there are any leakage problems during the final stages when the vehicle
is being tested,the whole suspension would be affected .May i know your view on this

 

[Mech_Engineer]

Yes if air leaks out of the shocks their spring/damping characteristics will change. This happens with any suspension with age though (coils sag, shocks blow out, etc.)

You still haven't told us how big this all terrain vehicle you're designing is, or what kind of terrain it will be navigating.

 

[monty37]

This is according to the rule book :
The vehicle must have four (4) or more wheels not in a straight line. The vehicle must be capable of carrying one (1) person 190.3 cm (6’3”) tall weighing
113.4 kg (250 lb).Width: 162.56 cm (64 in) at its widest point (the widest point of the vehicle and not necessarily the wheel track
width) with the wheels pointing forward at static ride height.

The recommended maximum vehicle length should not exceed 108 inches end to end.

The vehicle must be capable of safe operation over rough land terrain including obstructions such as rocks,sand, jumps, logs, steep inclines, mud and shallow water in any or all combinations and in any type of weatherincluding rain, snow and ice. The vehicle must have adequate ground clearance and traction.

 

[Mech_Engineer]

So, what's the problem? Sounds to me like you basically need to build one of these:

http://www.polarisindustries.com/en...RZR-Side-By-Side-Vehicles/Pages/Overview.aspx