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tension on elastic rope / rubber instead of springs?

tension on elastic rope / rubber instead of springs?

(OP)
As a general question, why don't suspension systems use tensioned materials as opposed to flexing bits of metal in compression?

The specific application I have in mind is motorcycling. I'm going working up a custom motorcycle design and build. One feature I've got is the design would make it pretty easy to use separate dampers and springs instead of the common coil over spring & dampers seen on motorcycles. Because of this, I've considered a tensioned material for the suspension springinstead of the common compression spring. (Note that this would apply both rear and front - I will be building a Hossack fork, which supports the steering head on double wishbones and turns it via a linkage.)

I think it would look pretty slick (and it may even be easier to build that way) to do away with normal spring, but is it possible to match / exceed typical spring performance with such a design? I can configure mountings and leverage ratios any way I'd need, within reason (something that is perfect but only has .5% elongation at extreme force wont work, the leverage needed to achieve useful travel would require the pivots and mountings be much to beefy). What I'd be looking for is a material that (in tension) matches or beats a steel compression spring in terms of the load it can handle for a given weight of material. Kevlar rope seems like a likely candidate - can take high loads and can safely handle 5% or even more elongation. Something with more elongation (like common rubber bungee) would be easier to use (and may offer aesthetic advantages for my purposes) but I'm wondering if it would end up weighing hugely more, or failing to easily.

If its not possible to match steel spring performance in terms of weight, is it at least possible to match it for ride quality and safety, or would the loading / fatigue just be to much? A bit of extra weight isn't the end of the world for a custom bike. For that matter, many are downright awful in terms of handling - I don't want to go that route, I want a practical daily rider, ideally one with sporting potential.

RE: tension on elastic rope / rubber instead of springs?

(OP)
Nope, not a new idea at all. In my research on suspension, I've read about motorcycles that used rubber (unsure if in tension or compression) as the spring element. I've seen proposed (and actual) suspension designs that pull against leaf springs (of various materials) rather than pushing. For example - http://www.bitterendchoppers.com/sitebuilder/image...

My question wasn't why it's never been done, but why its never been POPULAR. (My guess is because the layout is space consuming, leading to higher weight, both static and dynamic.) Also, of course, what would be a good material selection to use. A metal tension spring makes sense, but if I'm gonna go to the trouble, seems looking at other materials.

RE: tension on elastic rope / rubber instead of springs?

One of the reasons is packaging. To get a spring rate equiv to a wound steel compression spring takes a lot more rubber cross section. If you go higher in durometer to reduce the cross section the travel and ride go all to heck. Look at the hockey pucks in the old style Mini.
I would not want to put them in tension; they would be extremely susecptable to tearing and cuts under load; leading to total collapse of the suspension...not a good thing.

RE: tension on elastic rope / rubber instead of springs?

(OP)
Yeah, the failure mode was a concern; I'd figured to have safety cables to prevent suspension collapse and a wrap around the rubber, and in any case the damping unit would likely be run under compression and limit the max suspension compression.

I'd suspected the total mass of rubber needed would be fairly bulky, but don't really know the math / figures needed. Probably worth picking up a small sample and figuring out just HOW much I'd need. But yeah, even if it fits well in the design (which is seeming less and less likely) excessive bulk itself would increase the moving mass in the suspension.

Looks like I'll just be shopping for some nice coil overs or air shocks. To bad, because in my case, pulling on something when the suspension compresses is actually easier than pushing.

RE: tension on elastic rope / rubber instead of springs?

My real point was it was known and used over a century ago but has never been popular. That indicates some significant draw backs are probable.

I think it was popular on some horse drawn carriages. It may even be where the name suspension comes from where a carriage was suspended on ropes or leather straps or whatever.

Anyway, once dynamic forces became higher and space smaller and quality spring steel available, the swing seems to be extremely strongly toward compression, but hey, research is about rethinking old and new ideas and to see if previous trends are still valid for your application.

Regards
Pat
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RE: tension on elastic rope / rubber instead of springs?

Look at all the places rubber (or elastomers) are used in a car for suspension and powertrain mounts. Now look at all the places where they are used in tension. Even where it looks like they are in tension they are usually precompressed so that they aren't.

Screwman1 has identified the reason, rubber in tension fails catastrophically when a crack forms.

However there is no real reason why you couldn't use shear or compression mounts to get your springing . What spring rate do you want at the wheel? what is the motion ratio of your spring mechanism?

Cheers

Greg Locock


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RE: tension on elastic rope / rubber instead of springs?

Incidentally in a production car at least you tend to try not to use rubber as springs as they have a rather high damping around zero velocity, so it makes tuning the ride more difficult.

Cheers

Greg Locock


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RE: tension on elastic rope / rubber instead of springs?

(OP)
> What spring rate do you want at the wheel?

I'm looking for 5-7" travel, 300-400 lbs per wheel. Target weight distribution is 50/50 front to rear, probably with a bit more travel in front than rear.

> what is the motion ratio of your spring mechanism?

Entirely TBD, as its a custom design, but a good bet would be anything between 2:1 and 3:1 if using compression springs - it would fit the layout and is a pretty common range for motorcycle mono-shocks, afaik. I could probably go as low as 1:1 if using tension springs.

So, for a compression spring, I'm looking at a minimum of 600 lbs per inch, probably more like 900. For tension spring, could be as low as 300, but would need more elongation.

> Screwman1 has identified the reason, rubber in tension fails catastrophically when a crack forms.

Makes sense, especially for a commercial application. I think its safe to say I'll be doing a LOT of inspections regardless of the spring material, and any place I had rubber in tension it would be designed for easy replacement. Indeed, ease of replacement is one of the things that makes it appealing; I can buy 150 feet of 11mm rubber rope of $60 and play with all sorts of configurations. If it looks worn while out on the road, any truck stop has more.

> Incidentally in a production car at least you tend to try not to use rubber as springs as they have a rather high damping around zero velocity, so it makes tuning the ride more difficult.

Yeah, that and other non-linear elastic behaviors in rubber do seem an issue.

RE: tension on elastic rope / rubber instead of springs?

(OP)
By the by, are there better elastic tension materials than rubber (or steel spring)? I know there climber (Dan Osman) who did some massive free jumps using climbing rope, which typically has an elongation of up to 30% and (obviously) huge strength to weight, so it seems to have potential. Not sure how it would hold up to repeated stretching, though, and wear, weather, and heat would all be concerns (more so that with rubber even).

RE: tension on elastic rope / rubber instead of springs?

OK

I missed the bit about Kevlar and rubber bungee.

Kevlar rope has some real limitations here. It has the tensile strength of steel but at 1/7th the weight BUT if it prone to fibrilate down to individual crystal level and fail when compressed or bent. Bending typically compresses one side and stretches the other. Any twist in a rope compresses individual fibres from the side as the rope is stretched. That is the point of twist in a yarn or rope. To create side compression to create friction to hold the fibres together. Under a powerful microscope the individual filament failures will look like a wooden yacht mast that faile under compression with splinters everywhere.

To be effective and durable, a Kevlar rope needs continuous filaments from one end of the rope to the other and needs to fasten the filaments at each end so they are straight and parallel to the axis of the rope and with equal tension on each filament and no squeezing together under load. This is necessary for even load sharing between filaments and not easy to achieve. Because of the low elongation, it is very easy for one filament to break before another is even at half load unless preload is very precise.

My data suggests an elongation at break (not yeild) of 2.5% for high modulus fibres and 3.7% for std modulus.

It is also really poor to UV light and Kevlar ropes are normally enclosed in a sheath of a tougher and UV resistant material like nylon or polyester.

Rubbers and other elastomers like polyurethane can have very high elongation, like over 300% but they tend to gradually take on permanent extension and fail catastrophically.

Mixing materials with about 3% elongation and about 300% elongation has very significant design limitations.

Rubber springs tend to be much larger than steel unless designed as a solid compression type like in a Mini or on car bump stops.

Regards
Pat
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RE: tension on elastic rope / rubber instead of springs?

(OP)
Er- whoops, my spring rates assume only 1 inch of sag. With that much travel, I'd probably run more like 1.5 - 2 inches sag. So, rates would be maybe 75% given; 450-650 for compression, and maybe 250 for tension.

RE: tension on elastic rope / rubber instead of springs?

(OP)
Well then, kevlar is right out; I can't see how I'd fasten it without bending or compression. I wasn't referring to mixing it WITH rubber, though!

300% elongation seems hard to use effectively in a system with ~6" travel. I'd need "springs" that were a few inches long even if doing 1:1 actuation. That makes my intended source problematic, and removes much of the aesthetic motive. Such a short length or material also makes inspection more difficult, etc.

I looked at compression type rubbers, but they don't seem to offer me any design advantage vs a coil over shock. An air bag style would probably be preferable.

RE: tension on elastic rope / rubber instead of springs?

In a rope it is the construction as much or more than the material itself that controls elongation. The construction is about how many fibres twisted together and how many turns of twist per unit length and how many yarns then twisted together and whether twist against twist or twist on twist for instance. Generally the greater the number of levels in the structure ie thin yarns twisted together to make medium yarn etc etc to make thick yarn to make real thick yarn and the closer the individual fibres or filaments get to perpendicular to the axis of the rope the greater the elongation but also the greater the chance of snarling or looping into a tangle.

Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
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RE: tension on elastic rope / rubber instead of springs?

FYI: I love my Mini Cooper...but, no way am I in love with those damning little rubber cones. Rules are rules or they would be neat little 'paper weights' and the springs made of STEEL!

Rod

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