21 replies to this topic

[cheapracer]

I have heard that rubber (neoprene) itself contains some damping qualities.

Never having tried it I ask the experts in the area, maybe Greg, if you feel a small vehicle such as a golf cart when using rubber in torsion (like those trailer axles) could suffer having no shockers?

[Greg Locock]

High Shure hardness rubbers have a lot of damping... compared with a steel spring. My gut feel is that the ride would be too bouncy, Moulton tried an all rubber suspension for one of his bikes and it was bouncy.

[mmmcurry]

Mountain bike forks used to use a mis of springs and elastimers in suspention forks years ago. The elastimers were supposed to offer damping, however for most forks oil is used these days.

Steve.

[kikiturbo2]

Mountain bike forks used to use a mis of springs and elastimers in suspention forks years ago. The elastimers were supposed to offer damping, however for most forks oil is used these days.

Steve.

manitou forks and rear suspensions were 100% elastomer only..

[zac510]

I was going ot mention the MTB springs! That really was the early days though. The cheap forks were all-elastomer with different compounds stacked on top of each other but the higher-end forks had elastomer spring but with an oil damping unit, despite the damping property of the elastomer. But it wasn't long before everyone pulled them out and replaced them with aftermarket springs and retaining the oil damping unit..
I haven't been in the scene for a long time but pretty sure anything in the high level is not elastomer damped.

[CWard]

Neoprene rubber does have damping qualities which can be quite high and useful in low amplitude applications. However as we know damping generates heat and with high damping neoprene rubber this results in internal melting that can severely affect the life of the material. From experience trying to use neoprene applied to cab and engine mounts resulted in failure as mount life was extremely short. The mounts ended up as a horrible "gooey" mess.

[DaveW]

All polymer materials absorb energy when they are worked mechanically, I think, ranging from almost none (remember "superballs"?) to lots (e.g. sorbothane, as used in expensive insoles). A few questions to ponder are:

1. What happens to the heat generated (most polymers are good insulators)?
2. Is the heat generated, or mechanical action, destructive (it appears to be so for the long bump rubbers used in road vehicles)?
3. What is the nature of the damping (tyre damping, for example, appears to be mainly hysteretic with a small viscous component)?
4. Is the damping temperature and/or load sensitive?

Both cellular & solid polymer bump rubbers are used widely in aero race vehicles. They are rarely used without "real" dampers, however.

[JtP1]

Richard Longman tried these units on the back of a mini in a weight saving exercise. Apparently worked quite well for the first lap or 2 and then the handling got a bit erratic. So they stopped and checked the car, nothing found wrong, so back out with the same result. He eventually worked out that the rubber when soft with heat of operation and this allowed the toe in on the unit to get somewhat inconsistent.

Did Harvey Postlewaite not try rubber springs on the last Hesketh with disasteous results fo Frank Williams who bought the cars. Great idea, but only seems to have worked effectively in a Mini and De Havallind Mosquito.

[gruntguru]

The Mini rubber springs were large, the car was light and most of the damping was performed by conventional hydraulic dampers.

I think some of those trailer springs (AL-KO) incorporate friction damping as well by using interfernce fit rather than bonding the elastomer to the metal.

[cheapracer]

Great answers all, thanks for the effort :-)

[Villes Gilleneuve]

Hesketh F1 had rubber suspension in the early 70s.

[macoran]

Hesketh F1 had rubber suspension in the early 70s.

Photo of the rear setup here
http://www.flickr.co...os/cx500/page5/

[Greg Locock]

Probably worth differentitaing between elsatomer springs, which have been used all over the place, and elastomers as replacements for both springs and dampers. I think the Hesketh is basically a rubber coilover ?

[CWard]

The springs used on the Hesketh were Aeon springs and were rubber only. Aeon springs are now produced by a Company called Timbren (Timbren.com) based in Canada. Aeon springs were originally from Firestone and in the 60's were manufactured in the Birmingham area. I was for a short time employed by Aeon at that time but was not involved with the Hesketh installation.

[Catalina Park]

The trucks that I drive use rubber springs on the rear end. They are like a big laminated engine mount and they mount them so that they work in compression and sheer. They are quite effective on very rough ground.
If you feel the need take a look at http://www.hendricks...df/HNSeries.pdf for more info.

[OfficeLinebacker]

The trucks that I drive use rubber springs on the rear end. They are like a big laminated engine mount and they mount them so that they work in compression and sheer. They are quite effective on very rough ground.
If you feel the need take a look at http://www.hendricks...df/HNSeries.pdf for more info.

That is really neat!

[Tenmantaylor]

My first tamiya off road buggy, Mad-Cap http://media.photobu...67/IMG_0126.jpg had rubber damping sheaths around the suspension pushrods instead of oil pistons (inside the blue plastic pistons). One of the reasons when I started racing it I was nowhere near the competition of mostly team associated and team losi oil damped buggies.

[REN_AF1]

The springs used on the Hesketh were Aeon springs and were rubber only. Aeon springs are now produced by a Company called Timbren (Timbren.com) based in Canada. Aeon springs were originally from Firestone and in the 60's were manufactured in the Birmingham area. I was for a short time employed by Aeon at that time but was not involved with the Hesketh installation.

But it still uses conventional shock absorbers for dampening.

It would seem that a working "pure" elastomere design, without some kind of friction dampening build in, is really to be seen yet. Certainly and as mentioned, the traditional AL-KO rubber spring seems to suffer from a pronounced lack of dampening, especially unloaded.

R

Edited by REN_AF1, 26 October 2009 - 19:46.

[CWard]

About 10 years ago a 30 year old concept of providing a form of damping to a rubber mount was successfully developed by the Lord Corporation as a result of developments in materials and lubricants (SAE952666). An application of the hysteretic damping using this principle was to a truck cab mount (Hystec mount) – see pictures.




The mount consists of a low rate rubber element that is integrated with a rubber-lined canister. Within the canister are plastic washers having a small interference fit with the rubber that lines the wall of the canister. A special lubricant is applied to the rubber to dissipate the heat generated by the friction between the rubber – plastic interface. The connection between the rubber mount and the plastic washers incorporates a de-coupler, similar to a hydraulic mount. This is achieved by using a spacer between the steel retaining washers for the plastic element, which is longer than the thickness of the plastic washers. As a result of this simple construction, the amount of damping and control of the decoupling can easily be tuned by varying the number or thickness of the plastic washers, in combination with the length of the spacer for the amount of decoupling. Compared to the hydraulic mount, the de-coupler of the Hystec mount can be controlled very precisely. The following shows the cross-section of the Hystec mount and the characteristics of the Hystec and hydraulic mounts are shown in figure 2.








Elastomeric mounts have a limitation on the amount of static deflection they can handle, thus limiting the lowest frequency that can be used, typically 5 Hertz. This mount had a natural frequency of 5 hertz. The damping element of the mount was tuned to provide sufficient control of the cab motion relative to the frame vibrations. The de-coupler tuning is critical to the performance of the mounts, and the gap was set to 0.5mm. It was found that the human body can discriminate between very small de-coupler gaps. The small change from 0.5 to 1.0mm noticeably changed the perception of ride in the cab. In addition to providing isolation from some of the frame beaming vibrations, the Hystec mount, as a result of the de-coupler element, provided excellent isolation of engine vibrations. The level of isolation was typically in the region of 20dB.

The plot below shows the effect of using the Hystec mount on a dump truck. The graph is of the fore-aft acceleration measured at the seat belt mounting point on the B pillar of the cab. The acceleration level is plotted relative to the wheel rotation speed, i.e. vehicle speed. With the base mounts, red trace, the vehicle has strong cab shake vibration at 60mph, which can be seen at a wheel speed of 470 rpm. The response with the vehicle modified by installing Hystec front mounts and a rear mount constructed from ribbed anti-vibration matting, blue trace, shows the elimination of the 60mph vibration.

[Greg Locock]

Nice one. But the cruel question is, how many horsepower can your little plastic discs absorb, for how long?

[CWard]

Greg,

In response to your query re horsepower and life the following are extracts from the 1995 SAE paper by W. Flower (Lord Corporation) who developed this type of mount.

]Rubber Hysteresis Friction (RHF) DAMPING

An enlarged view of the interface between the elastomeric substrate and the slider indenter is shown in figure 19. The slider indents and travels along the elastomeric layer. Ahead of the slider, a small "bow wave" is built up; and behind the slider, a "trough" exists which gradually recovers from the slider indentation. The magnitude of the bow wave and trough depend on the indentation, velocity of sliding and various elastomer properties. A non-symmetrical, non-uniform pressure field is build up about the slider/elastomer contact patch area opposing slider motion. This results in a motion retarding force composed of frictional and hysteretic
components. This is the force which provides the damping in the RHF damped mount system.
The situation is more complex for the case of multiple, closely spaced sliders, since the recovery properties of the elastomer become critical. If elastomer recovery is too slow, following sliders do not indent the elastomer to the degree that the initial slider does, and the damping from friction is lessened. However, if elastomer recovery is too rapid, the bulk hysteresis loss in the elastomer is reduced and the damping effect from hysteresis is reduced. Both friction and hysteresis damping effects are important.




TEMPERATURE SENSITIVITY OF RHF DAMPING
The temperature sensitivity of elastomer damping is well known]. Tests were conducted on a RHF damper to determine the stiffening influence of low temperatures. Figure 20 shows the ratio of loop area at -40°C to loop area at 20°C obtained at a common input amplitude and frequency. For the first cycle of excitation, at the beginning of the test sequence, the loop area at -40°C is almost three times the loop area measured at room temperature. After accumulating 100 cycles, the loop area ratio drops to about 2.7, the cycling is then discontinued until the next test time. Following one hour from time 0, the loop area ratio for the first cycle is about 2.8, and 2.5 for the 100th cycle. These ratios are approximately constant with accumulated time. Figure 21 shows the same loop area ratio information, but obtained at -20°C relative to 20°C. This data does not include the heating effect of continued cycling and the resultant reduction in loop area ratio.



FATIGUE OF RFH DAMPER MOUNT
Single axis fatigue tests were conducted using the dynamic load and test frequency versus cycles spectrum shown in figure 22 which is typical of the vertical loads seen on a medium duty truck engine mount. The dynamic loads were superimposed on a static load of 408 kgm to simulate engine weight per mount. Figure 23 indicates how the room temperature loop area is retained as fatigue cycles are accumulated. The retention of loop area with cycle accumulation suggests consistent dynamic performance is feasible.


For the RHF damped mount the hysteresis loop curve in figure 13 shows a nonlinear curve suggestive of a mount with supplemental snubbing. Actually the non-linear effect is due to the onset of RHF damping. Up to about ±1.0 mm, the damping function is disconnected or decoupled from the simple rubber mount. The loop area is only 0.09 kN-mm/cycle, but at inputs above ± 1.0 mm, damping from RHF components is added to damping from the simple rubber mount. This is shown more clearly in figure 14 which was taken at ±5.0 mm and 1 Hz. The shaded area indicates the contribution of the RHF damper. The loop area becomes 10.12 kN-mm/cycle.









SUMMARY
RHF damped mounts are shown to provide damping performance somewhat different than similarly sized fluid mounts. They are primarily amplitude sensitive and not strongly frequency sensitive. They will not provide a minimum stiffness at certain frequencies and amplitudes; but neither do they develop significant stiffness increase at high frequency and low amplitudes. Both types of mounts offer amplitude decoupling for enhanced low amplitude isolation.


I hope this is of help.

[gruntguru]

Greg,
In response to your query re horsepower and life the following are extracts from the 1995 SAE paper by W. Flower (Lord Corporation) who developed this type of mount.

I don't think the answer to Greg's question is in there - although the work per cycle (integral of the hysteresis loop) is given. The power dissipation capability will depend on a lot of other factors including:
- temperature limit of the elastomer
- ambient temperature
- thermal dissipation characteristics of the mount