suspension bushings

[TMSengineer]

I am working on a street/performance shock mount for a sports car. The original problem that needs redesign is that the rubber bushing wears out very quickly. I do not have much experience with flexible materials ie plastic/rubber. So, how do I go about figuring out what material I need to make the bushing out of? Fatique is my biggest concern.

Thanks

 

[patprimmer]

What is the nature of the failure, is it failing in compression or from tension through twisting.

If it is from compression, can you increase the bearing area of the pivot point thru bolt by useing a thick wall sleeve, thus increasing the ID of the bush.

If it is from tension while twisting, can you increase the OD of the bush, and therefore have a thicker wall, so the %age elongation in the material is lower per deg of rotation

Regards
pat

 

[GregLocock]

I'm intrigued. How are you making the bush? Can you give the dimensions? Why aren't you using spherical joints?

Cheers

Greg Locock

 

[wade66]

TM engineer

Bonding the bushing to the inner and outer metal will help. Rubber is a fluid and will flow under high loadings unless it it constrained.

fyi

 

[TMSengineer]

Pat, there shouldnt' be much twist or torsion on the bushing. It is basically failing because of compression. At "failure" the metal sleeve around the shock can move around a lot because the rubber hole is so loose. Thus, not a good situation.

Greg, the bushing at this point will be two piece. Upera and lower spererated by a metal piece to servea as somewhat of a pivot point. The shock is then clamped by washers upper and lower. The OD is 1.25" and ID is .5". We don't like to use sphericals unless it is a pure racing application due to quick wear, and harsh ride.

Wade, the stock bushing was bonded, and didn't last long. The one we buy now is only bonded to the inner sleeve, and is more free to move on the OD. I guess this helps not stetch the hole out?

Thanks for the input....I would greatly appreciate some more

 

[GregLocock]

That sounds like the old compression type of shock mount, which are nice because you can change the stiffness of the rubber by changing the preload on the nut.

Making the rubber bigger is always a good move, and make sure the washers are cupped, not flat.

So far as materials go most performance parts seem to use a polyurethane or a silicon rubber. We tend to use normal black stuff.

Any chance of posting a drawing? there's instructions if you hit the preview button instead of submit.





Cheers

Greg Locock

 

[patprimmer]

OK
I'v got the picture now

It's simply a compression pad between the shock and the chassis.

The top mount will be a shoulder on the shock shaft with a washer seated on the shoulder, rubber washer, shock shaft through hole on bracket attached to chassis rail, rubberwasher, steel washer, nut on shock shaft holding it all together. Preload on rubber controlled by how far you screw the nut down and the thickness of the washers.

My advice

The washers should be cupped to retain the rubber dureing compression.

The washer should be substantial so that it does not flatten out and allow the elastomer to stretch radially.

If it is rubber, you need to ask a rubber expert, which I am not.

If it is polyurethane it should be a hard, fully crosslinked material (thermoset, not thermoplastic urethane). Fork lift truck tyre grade comes to mind.

Increase the OD of the pads and the washers if there is room.

Remember the surface area of a circle increases as the square of the dia, so a little in dia gives a lot in area



Regards
pat

 

[wade66]

TMSengineer

Start with a high tensile natural rubber - ASTM M4AA624A13B13C12f17K11 This is a specification for a 60 duro material. You can go harder and reduce the strain level at the loads you are seeing to get it to live longer. However, it will be a tradeoff in ride to do so. Increasing the area (shape factor) is always a good option.

Dense Thermoset Urethane will also work, but if you are looking for any ride characteristics at all you might as well use wood.

 

[GregLocock]

60 duro is very soft for that part. You actually want damping typically, as you use the damping in the rubber to improve micro shake, where the damper is locked due to stiction.



Cheers

Greg Locock

 

[wade66]

Dynamic stiffness is the determining factor (typically 500-1000 N/mm), not durometer. Dynamic stiffness is a function of the material and the shape factor of the design. Typically a dynamic stiffness 2 to 3 times the vertical dynamic rate of the tire is sufficient to react the shock.

 

[TMSengineer]

Ok, you guys have given me some great input. I am going to try and post a drawing. We'll see what happens. As far as the actual shock mount goes. Right now, the bushing extends beyond the upper and lower cylinder portion. Or should I surround the rubber in the cylinder almost completely? I also gave the cylinder portion a 10 degree misalignment to allow the bushing to give some radially....good or not? The washers used are already cupped and pretty substantial in thickness...and obviously right now the bushing is not bonded to the shock mount itself just to the inner sleeve. Also, what is good to use to bond the inner sleeve to the bushing? Thoughts?

img

http://www.turnermotorsport.com/ben/mount_1.jpg

img

http://www.turnermotorsport.com/ben/mount_2.jpg

 

[GregLocock]

Depends on the application, but I think you'll find that for many suspensions that that design is overconstrained.

The rubber does not need to be surrounded by metal, in fact it can't work if it is is. A simple flat plate with a hole in it works fine!


Cheers

Greg Locock

 

[TMSengineer]

Ok, so last question. I have had a hard time finding a company with any sort of bushing material with the properties that have been described in stock....as far as it just has to be round "bar stock". Any suggestions? Oh and any good bonding ideas for metal to rubber?

Thanks for your help everyone

 

[wade66]

Bar stock natural rubber would be hard to come by and it is difficult to cut/machine. Depending on the volume, there are quite a few rubber molding companies who could get you something to try and then mold up what you need. rubberworld.com is a good resource.

These same companies routinely bond rubber to metal using a thermoset polyurethane adhesive system. This can be bonded during the molding process or after molding with the application of heat (300 F) and pressure.

We are typically a high volume supplier, but I would be willing to help if you cannot find a source. We have a good prototype department and we may be able to work something out.

Let me know.

 

[patprimmer]

Mind you I think rubber is the better way to go, but if you want one off items, you can buy castable PU by the pint (or 500 ml if your in the rest of the world)

you simply make a mould out of anything from a pice of wood to an old mug, mix the two parts, then pour it in and wait for it to set. You need to choose the grade with the strength/hardness properties you want though. be carefull not to get a foaming grade.

You will need to treat the surface of the mould with a release agent as PU makes an excellent adhesive.

If you want to go to a bit of trouble to make a proper mould, you could incorperate the washers into the bottom of the mould, and use no mould release on the washer to get a bond

Regards
pat

 

[GregLocock]

Pat, designing the fillet at the rubber/metal/air interface is very difficult for rubber parts in a bonded system. Is PU more tolerant in this regard?

Also we found that when using two part PUs it was vital to degas the mixture using a vac pump/ultrasonic shaker.

The saving grace with this design is that the elastomer is always in compression, I'm not sure you'll gain much life by going to a bonded part.



Cheers

Greg Locock

 

[patprimmer]

Greg,

Thermosets are not really my area, I picked up a bit of knowlege by cross fertilisation while at Bayer. The guy at the next desk sold thermoset PU.

My real area is thermoplastics.

I just accepted from the replies that bonding was benificial, but I did wonder why.

If the washer had the right amount of cupping to give enough restraint to radial expansion so as to prevent failure, but not so much so as to overly restrict compliance, the system should work without bonding.

I would think that it is a ballancing act to get it right, and if it was right, no bonding is necessary, nor desirable

Every shock I ever installed with this mounting system, including Beilstiens and Koni ajustables, had washers that were NOT BONDED.

I think the secret is in getting the right ballance between radii of the washers, projected areas of washers and bushes, duro and thickness of the bushes, but hey, I'm only a plastics technician, not a suspensions systems engineer.

Regards
pat

 

[TMSengineer]

You know really...natural rubber looks like it would have the qualities needed in this application. I have looked into PU, but feel is it to rigid for this application, we are just looking for a certain comfort level in the vehicle ride and life of the bushing.

I agree, that the washers should not be bonded to the bushing, but wouldn't bonding the inner sleeve help from wear?

 

[GregLocock]

Pat - I agree, that design has always been unbonded in my experience too.

TMS - are you getting wear at the centre? That seems a bit odd. I agree, in this application I have always seen rubber, not PU. Developing a PU shape that is soft enough in 'coning' might be more trouble than it is worth. I'm in two minds about the dynamic rate - there is certainly an argument that it might be worth making one end very stiff(ie a spherical joint) and then making the other end of the shock somewhat softer. This is used on some BMW and/or Merc front ends.





Cheers

Greg Locock

 

[TMSengineer]

Greg, yes the main problem is that the center "cones" out. Then there is just way too much play. I think part of this problem is from what you said earlier is that the stock mount is overly constrained....the bushing is completely encased on the outside.