Author Topic: Independent Suspension System  (Read 601 times)

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Offline SylvestreTopic starter

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Independent Suspension System
« on: February 19, 2014, 07:29:00 PM »
Hello,

I'm building a robot with tank tracks and I'm using a independent suspension system where each idler is connected to a shock absorber. There are issues with the shock absorbers though.

1) It is a 1/2" too long so the track is very tight, which is drastically decreasing the efficiency of the track.
2) They require too much force to compress.  I probably only need 3/4 the required force needed. 
3) They are very hard to find.  I don't know if I'm searching for the right terms or not, but I only could find one http://www.pocketbikeparts.com/Shock_Front_Adjustable_11_1_2_inch_6mm_spring_p/atsh303238.htm
And there are hardly any specs regarding the force needed to compress

I've determined that my tracks aren't running as smoothly and efficiently because the track is too tight so I think I could kill two birds with one stone if I purchased new shock absorbers or gas struts or whatever they're called.

So my question is do you guys have any suggestions on what terms to search for if I want to replace the shocks? What should I be looking for? Gas struts? Shock absorbers? Hydraulic shock? I dont know. It just seems like the shock absorbers I have would rebound too quickly also. 

When I look at high speed tanks like this, it looks like they use hydraulics, but I'm not sure.  Something like that except if it were 11-11.5 inches (hole to hole) would be perfect.
http://waronterrornews.typepad.com/.a/6a00e551d9d3fd883301156fca796f970c-pi

And do you have any suggestions where I could buy these?  I'm a student too so I would prefer not to spend a fortune on these.

Below is a video describing my problem in further detail 
Track Analysis


Thanks for taking the time to read this

Offline jwatte

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Re: Independent Suspension System
« Reply #1 on: February 19, 2014, 09:01:55 PM »
Shocks are typically only movement dampers; the extending force comes from springs. if your shocks come with built-in springs, you may be able to find different springs that have the same dimensions.

Another source for suspension parts is Go-kart places (for heavier parts) and RC car/truck places (for lighter parts.)

Offline bdeuell

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Re: Independent Suspension System
« Reply #2 on: February 20, 2014, 12:04:54 AM »
While a shock adsorber technically only refers to the damping element, in practice the entire assembly including spring is commonly referred to as a shock adsorber. I think think you are on the right track with searching terms like "strut", "shock adsorber" and "shock". The key will be finding a market where something with the specs you want are used and produced in large quantities. Based on your link it looks like you have been searching in the ATV market. You might find some good products in the mountain biking market as well but I know things can get really pricey there. Try searching by specific applications that use similar products such as "mini ATV strut" or "rear mountain bike shock".

If you can disassemble them or even  get them apart in a manner that you would be able to reconstruct them replacing the springs might be a good way to go and keep costs down.

As far as specs go there are a lot of variables but the main ones I can think of are: spring rate (which may vary along the stroke), preload, damping coefficient (may be different for extension and compression). Any of these may also be adjustable depending on what shocks you have. I'm not surprised you are having difficulty finding specs as I suspect most of the shocks for ATVs have had little testing outside of does the thing work in application (if they are made in china the manufacturer may not even have an idea what the target specs are ... just what the design they copied looked like  :)) It looks like the ones you are currently using have a progressive spring rate as well as an adjustable preload. Have you reduced the preload to the minimum setting?

Another idea is to change the mechanical advantage of the suspension mechanism. You could move the mounting point of the shock closer to the pivot of the suspension arm. You could also mount the shock so its axis is more perpendicular to the path made by the point where the shock attaches to the suspension arm.

It sounds like the track is putting a lot of perload on your  shocks, can you make the tracks longer?

Offline SylvestreTopic starter

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Re: Independent Suspension System
« Reply #3 on: February 20, 2014, 12:17:14 PM »
Thanks for the response JWatte and Bdeull.

Quote
Shocks are typically only movement dampers; the extending force comes from springs. if your shocks come with built-in springs, you may be able to find different springs that have the same dimensions.

Another source for suspension parts is Go-kart places (for heavier parts) and RC car/truck places (for lighter parts.)

I'm wondering if the rebound is almost too fast with the shock absorbers that include the coils.  Howe and Howe Tech which is a company known for the high speed track systems looks like they don't use coils in their suspension system. Take a look at this video and you can see what I mean
Mini Ripsaw UNREAL CRAZY VIDEO of the Most Kickass ATV EVER Built! Newest Vid never released


Maybe its good to not have much rebound?

Quote
The key will be finding a market where something with the specs you want are used and produced in large quantities. Based on your link it looks like you have been searching in the ATV market. You might find some good products in the mountain biking market as well but I know things can get really pricey there. Try searching by specific applications that use similar products such as "mini ATV strut" or "rear mountain bike shock".

Yeah its very tough to find shock absorbers that are just the correct size.  Its right inbetween standard sizes for rc vehicles and atvs and go karts

Quote
If you can disassemble them or even  get them apart in a manner that you would be able to reconstruct them replacing the springs might be a good way to go and keep costs down.

I agree. I'm trying to figure out how to disassemble these to see if that's an option. It is very cumbersome and requires some advanced tools that I don't have.

Quote
As far as specs go there are a lot of variables but the main ones I can think of are: spring rate (which may vary along the stroke), preload, damping coefficient (may be different for extension and compression). Any of these may also be adjustable depending on what shocks you have. I'm not surprised you are having difficulty finding specs as I suspect most of the shocks for ATVs have had little testing outside of does the thing work in application (if they are made in china the manufacturer may not even have an idea what the target specs are ... just what the design they copied looked like  :)) It looks like the ones you are currently using have a progressive spring rate as well as an adjustable preload. Have you reduced the preload to the minimum setting?


I have put it to the minimum setting and it is still extremely stiff for my application.

Quote
Another idea is to change the mechanical advantage of the suspension mechanism. You could move the mounting point of the shock closer to the pivot of the suspension arm. You could also mount the shock so its axis is more perpendicular to the path made by the point where the shock attaches to the suspension arm.

Right now, the shocks are mounted perpendicular to the doglegs and there is hardly any wiggle room in terms of creating a new mounting point on the suspension arms so I don't think this is really an option.

Quote
It sounds like the track is putting a lot of perload on your  shocks, can you make the tracks longer?

Its one of those things where one more link would make it too loose and one less link would make it too tight.

Offline bdeuell

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Re: Independent Suspension System
« Reply #4 on: February 20, 2014, 05:34:07 PM »
Quote
I'm wondering if the rebound is almost too fast with the shock absorbers that include the coils.  Howe and Howe Tech which is a company known for the high speed track systems looks like they don't use coils in their suspension system. Take a look at this video and you can see what I mean

I am not able to see exactly how the suspension in the video operates but my guesses are:

- they may be using a torsion bar  for the spring, which is not uncommon in larger tanks

- the strut may have an internal spring (like a screen door closer)

- the strut may actually be a gas spring (this would mean they do not have a damping element) ... at first i thought this would be unlikely but then i  realized they also have a couple of "boogies" in their suspension which most likely pivot freely and have no damping. Thinking about this a little more the lack of any damping component (at least in the extending direction) may work well on tracked systems as it would allow for the fastest return speed when the load is removed making sure the track remains taught.


Quote
Quote
If you can disassemble them or even  get them apart in a manner that you would be able to reconstruct them replacing the springs might be a good way to go and keep costs down.

I agree. I'm trying to figure out how to disassemble these to see if that's an option. It is very cumbersome and requires some advanced tools that I don't have.

It looks like there is a hex on the bottom of the rod eye (inside the spring) is the rod eye threaded onto the rod? If they are permanently attached (or require tools you do not have access to) perhaps you could cut the end of the rod off and either modify the rod eye, manufacture a new one, or purchase an off the shelf component and then reattach the parts.


Offline SylvestreTopic starter

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Re: Independent Suspension System
« Reply #5 on: February 21, 2014, 12:47:32 PM »
Quote
It looks like there is a hex on the bottom of the rod eye (inside the spring) is the rod eye threaded onto the rod? If they are permanently attached (or require tools you do not have access to) perhaps you could cut the end of the rod off and either modify the rod eye, manufacture a new one, or purchase an off the shelf component and then reattach the parts.

Thank for the advice Bdeull. I found a thin wrench that fit the nut and was able to get the coil off as you can see in the images below.  Looks like the only thing that does damping is the actual spring itself. That's not right, is it? I thought the coils were only responsible for the rebound. 

IMAGE- https://drive.google.com/file/d/0B83RdnxAI6xveXlmWWgzV05zRDQ/edit?usp=sharing

Offline bdeuell

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Re: Independent Suspension System
« Reply #6 on: February 22, 2014, 01:17:48 AM »
Quote
Looks like the only thing that does damping is the actual spring itself. That's not right, is it? I thought the coils were only responsible for the rebound.

You are correct springs provide very little damping, in terms of system design the damping of a coil spring can usually be neglected. The damper is the chromed cylinder object in your picture. The cylinder is likely filled with oil, typically a piston attached to the rod has a small orifice in it that restricts flow of oil as the rod move in and out. This restriction is what provides the dissipation of energy and the damping. Because the volume inside the cylinder changes a the rod moves in and out shocks are usually "charged" with a compressed gas (nitrogen is often used because it is inert) to take up the remainder of the space not filled with oil. It looks like your cylinders have been assembled by welding the bottom end-cap on so it is unlikely you will be able to see any of the inner working without cutting the cylinder in half. You should be able to feel this damping tho if you move the rod in and out with the spring removed.

Offline SylvestreTopic starter

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Re: Independent Suspension System
« Reply #7 on: February 22, 2014, 11:28:53 AM »
Quote
You are correct springs provide very little damping, in terms of system design the damping of a coil spring can usually be neglected. The damper is the chromed cylinder object in your picture. The cylinder is likely filled with oil, typically a piston attached to the rod has a small orifice in it that restricts flow of oil as the rod move in and out. This restriction is what provides the dissipation of energy and the damping. Because the volume inside the cylinder changes a the rod moves in and out shocks are usually "charged" with a compressed gas (nitrogen is often used because it is inert) to take up the remainder of the space not filled with oil. It looks like your cylinders have been assembled by welding the bottom end-cap on so it is unlikely you will be able to see any of the inner working without cutting the cylinder in half. You should be able to feel this damping tho if you move the rod in and out with the spring removed.
Yeah I think these are poorly constructed shocks. There is no resistance when I move the piston up and down. As you can see in that picture above, the shaft is pushed all the way down.   Right now it seems the only purpose of that shaft and piston is linear motion .

Thanks for the explanation.

 


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