Rubber bushing reading material

[speed]

Hi...
I have been looking for some good info on bushings (rubber) used for suspensions (control arm).
Specifically - bushing properties and their response curves to force application.

Appreciate the replies.
Cheers

[speed]

Hi...
I have been looking for some good info on bushings (rubber) used for suspensions (control arm).
Specifically - bushing properties and their response curves to force application.

Appreciate the replies.
Cheers

[carlosm]

May i ask why you are looking into this? The standard is sphericals or rodends, do you have a reason for not using them?

[speed]

Thanks Carl.
Well... I just needed some reading stuff on all types of bushing (spherical as well).
When I searched around I could not find any except for SAE papers.

[Drew Price]

Firstly, identify yourself, where are you from, what are you looking for this info for? You won't get friendly responses without taking some time to put forth the effort for those little things that will separate your post.... from all the other poorly assembled first posts out there.

********************

That would be because stiffly sprung racing cars do not use compliant bushings for pivots. The forces involved and the level of control of geometry that most designs require preclude the intentional introduction of additional compliance.

Suspension travel is measured on the order of millimeters, and load in each joint on the order of thousands of Newtons.

If you're looking at different designs for cost savings for ease of manufacture, do not use rubber!! Use something like poly-urethane, UHMD, or Delrin, or maybe oil impregnated bronze riding on ground shoulder bolts (but those will probably need grease fittings, and will be heavy... which is why we use sphericals). Same mounting benefits of compliant bushings.... with much less compliance.

Modelling the compliant bushings will depend on the radial thickness of the material compared to the inner and outer diameter, and will probably change with deflection, since good bushing designs have the material bonded to the inner and outer sleeves, and so have to be installed in a particular baseline orientation, or they will introduce some (small) static preload into the system, another generally undesireable thing, since you want it all to be in the springs since those are easily adjustable, and generally linear.


Best,
Drew

[whitenoise]

Road going vehicles use bushings very effectively to control the motion of the tire, specially when subjected to lateral and longitudinal loads. This provides a means of controlling the cornering and accel/braking behavior of the vehicle over and above that provided by the locating geometry of the suspension, which is dictated to a large extent by packaging constraints.

That being said, accurately predicting the effects of adding compliance in an as-yet unbuilt design will require a multi-body analysis tool such as ADAMS and plenty of experience. Go with spherical bearings or rod ends unless you have both of these.

[Courtney Waters]

MIL-HDBK-149B Rubber

...may be a good place to start. I haven't read through it (just came across it the other day) but it may have what you're looking for. It contains loads of technical data. Should be available at assistdocs or everyspec.com

[Adambomb]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by whitenoise:
Road going vehicles use bushings very effectively to control the motion of the tire, specially when subjected to lateral and longitudinal loads. This provides a means of controlling the cornering and accel/braking behavior of the vehicle over and above that provided by the locating geometry of the suspension, which is dictated to a large extent by packaging constraints.

That being said, accurately predicting the effects of adding compliance in an as-yet unbuilt design will require a multi-body analysis tool such as ADAMS and plenty of experience. Go with spherical bearings or rod ends unless you have both of these. </div></BLOCKQUOTE>

If road going vehicles' bushings are "very effective" at controlling motion, then why are companies like Prothane around??? Road going vehicles use rubber bushings for 2 reasons: 1) they are cheap, and 2) whiny consumers don't like to feel bumps. Motion control comes about 9th or 10th on the list of priorities for OEMs when choosing bushing material.

As to predicting the response of rubber bushings, here's my thoughts: there are groups of people who go to great effort of modeling the dynamic behavior of their cars' suspensions, and there are groups of people who use rubber bushings in their suspension. I strongly believe these groups are mutually exclusive.

Rubber is crap for suspension bushings. If you want a compromise, polyurethane has about the same internal damping as rubber, but is much stiffer; I honestly can't feel much seat of the pants ride quality difference between poly and rubber (assuming the job isn't botched...which I have also seen!). If you are serious enough about performance to try to model its behavior, stick to sphericals and rod ends!

[Zac]

In terms of passenger car suspension design, "stiffer" is not always better. OEM's use bushings to be able to tune their handling and ride characteristics in ways that they might not be able to achieve given a rigid suspension and tight packaging constraints.

Stiffening up your bushings in a passenger car will lead to greater force transmissibility and NVH/Ride quality issues. The people installing aftermarket bushings in their street vehicles are making a trade-off that they are probably fine with. Would the typical Camry or Malibu owner fall into the same category? probably not.

That said, I don't think bushings have any place in a FSAE suspension. From what I have seen most of the "rigid" suspensions already have a fairly significant amount of compliance built into the design. There's no need to make further complications.

[whitenoise]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Adambomb:
Road going vehicles use rubber bushings for 2 reasons: 1) they are cheap, and 2) whiny consumers don't like to feel bumps. Motion control comes about 9th or 10th on the list of priorities for OEMs when choosing bushing material.

As to predicting the response of rubber bushings, here's my thoughts: there are groups of people who go to great effort of modeling the dynamic behavior of their cars' suspensions, and there are groups of people who use rubber bushings in their suspension. I strongly believe these groups are mutually exclusive.

Rubber is crap for suspension bushings. If you want a compromise, polyurethane has about the same internal damping as rubber, but is much stiffer; I honestly can't feel much seat of the pants ride quality difference between poly and rubber (assuming the job isn't botched...which I have also seen!). If you are serious enough about performance to try to model its behavior, stick to sphericals and rod ends! </div></BLOCKQUOTE>

You're pretty opinionated for someone who's yet to get out of school. What I posted is hard fact based on industry experience. Take it for what its worth.

Ever heard of a SPMM aka K&C rig? Nearly 75% of the interesting things from a session on an SPMM are compliance based. And guess what controls compliance: rubber bushing properties.

[Adambomb]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by whitenoise:
You're pretty opinionated for someone who's yet to get out of school. What I posted is hard fact based on industry experience. Take it for what its worth.

Ever heard of a SPMM aka K&C rig? Nearly 75% of the interesting things from a session on an SPMM are compliance based. And guess what controls compliance: rubber bushing properties. </div></BLOCKQUOTE>

Just because I happen to be enrolled in a university at the time doesn't mean this is my first rodeo. While I haven't spent any real time in the "auto industry" per se, aside from 7 years doing formula I've worked in a racing fab shop for a couple years and a mechanic shop for another couple years, on top of 15 years worth of hardcore automotive bastardization. And yes, I have "heard of" K&C rigs.

That aside, is this not a racing forum? This topic is under "Open FSAE Discussion," therefore I can only assume this discussion pertains to formula cars, or at least race cars. Rubber bushings are fine for Camrys and Malibus, but they have no place in race cars. That's all I'm saying.

[whitenoise]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Adambomb:
Rubber bushings are fine for Camrys and Malibus, but they have no place in race cars. That's all I'm saying. </div></BLOCKQUOTE>

Which is exactly what I implied in my post that you quoted, so what the hell are we arguing about?

And if you're somehow assuming race cars have no compliance, think again. Like someone said "Everything has a spring rate. Its just a matter of how much".

[Adambomb]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by whitenoise:
Road going vehicles use bushings very effectively to control the motion of the tire, specially when subjected to lateral and longitudinal loads. </div></BLOCKQUOTE>

That's my only beef. Rubber is not "very effective."

So first you assume that I have no experience outside of a classroom, and now you assume that I assume that race cars have "no compliance." What pray tell are these gratuitous assumptions based on? I hope your engineering assumptions are better justified. Looks to me more like you're just making an A$$ out of U and Me.

That's what I'm arguing about.

[Zac]

Yes, rubber is typically not very stiff. But what if you want that deflection? Then I'd say it is highly effective. Or what if you want a bushing that is stiff in one direction and fairly soft in another? Last time I checked, rubber is full of anisotropic material properties that someone could potentially exploit. Depending entirely on the application, rubber can be a fine choice, and this include high performance vehicles.


Also this...

"As to predicting the response of rubber bushings, here's my thoughts: there are groups of people who go to great effort of modeling the dynamic behavior of their cars' suspensions, and there are groups of people who use rubber bushings in their suspension. I strongly believe these groups are mutually exclusive."

...is wrong. Kinematics are easy to model. Compliances are not.

I think it is also worth pointing out that a car with rubber in the suspension has won FSAE in the past.

[whitenoise]

My intention was not to have a pissing match with a guy who has no appreciation for what someone with real industry experience has to say. Not your fault, Adambomb, you'll learn eventually that you don't have all the answers all the time.

Here's the real answer for now, at the risk of repeating what Zac here has already said not once but twice:
If "effective"ness of rubber is measured in terms of its ability to deflect the least under a given load, then yes, it is less "effective" than a polyurethane or what-have-you bushing. This is supposedly Adambombs definition from what I can gather. And it is completely, utterly wrong in this context.

When I, OTOH, said "effectively controlling wheel motion", I meant "causing the wheel center to move in certain directions under certain loads, thus dynamically controlling steer and inclination angles of the tire, that cannot otherwise be controlled kinematically without causing other bad things to happen". Lateral force steering compliance for example, can contribute as much as 50% of the sub-limit understeer in a typical sports car (which is a REAL "performance" vehicle, whatever that means).

In my experience, there are a couple places where intentional compliance due to a elastomeric bushing has benefited lap times (yes, on a REAL race car). But I am not at liberty to divulge where and how.

Thank you for your time.

[Adambomb]

Eegs, experienced some cross-post action. Sent a reply, but missed a response or two while constructing said reply. Anyhoo...

Actually I was in progress to write another response due to the fact that I did recall one instance where additional bushing compliance did give an actual performance advantage, after further consideration of Zac's post. This one happened to be on a car where wedge was desirable...

As for sub-limit understeer, that happens to be something I hate about production "performance" cars, but at least when it is added through easy to replace bushings it's easier to fix! Now, bearing that in mind, is this something you would want in an FSAE car? I'm not touching that one anymore! Not to say that in proper creative contexts it couldn't be made to work in other ways...

As for not respecting the opinion of someone with real industry experience, remember, it is a 2 way street. Please do at least identify yourself next time, as it is nearly impossible to tell who you're talking to otherwise, and to be perfectly honest, well, you have no doubt already discovered the general experience level of the "typical" forum user. I will take it as a further reminder to myself that there is an incredibly large spectrum of knowledge and experience levels on these forums.

[HoggyN]

One of the main reasons rubber bushings are used on mass produced road cars is that they are, in most applications, maintenance free. There are no wearing parts.

If I remember correctly, the rear suspension of the Porsche 928 exploited the compliance of rubber bushes to introduce a controlled amount of rear steer.

[Adambomb]

After further thought, and against my better judgement, I gave this one another look...only because whitenoise was so absolutely intent on proving, for reasons beyond me, to the world that I was "wrong, wrong, wrong."

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by whitenoise:
My intention was not to have a pissing match with a guy who has no appreciation for what someone with real industry experience has to say. Not your fault, Adambomb, you'll learn eventually that you don't have all the answers all the time. </div></BLOCKQUOTE>

Really? Then what's the deal with the personal attacks? Does this really further your position?

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">
Here's the real answer for now, at the risk of repeating what Zac here has already said not once but twice:
If "effective"ness of rubber is measured in terms of its ability to deflect the least under a given load, then yes, it is less "effective" than a polyurethane or what-have-you bushing. This is supposedly Adambombs definition from what I can gather. And it is completely, utterly wrong in this context. </div></BLOCKQUOTE>

I gave this one some more thought too, and as it has been discussed, our definitions of "control" in this instance differed. If you really want to get into semantics, I went ahead and looked up the dictionary definition of control:

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">
1. To exercise authoritative or dominating influence over; direct. See synonyms at conduct.
2. To adjust to a requirement; regulate: controlled trading on the stock market; controls the flow of water.
3. To hold in restraint; check: struggled to control my temper.
4. To reduce or prevent the spread of: control insects; controlled the fire by dousing it with water.
5.
1. To verify or regulate (a scientific experiment) by conducting a parallel experiment or by comparing with another standard.
2. To verify (an account, for example) by using a duplicate register for comparison.
</div></BLOCKQUOTE>

Clearly your definition in this particular scenario meets definition 2 above, while mine meets definition 3. And yet somehow, I was "utterly wrong."

After some further research I see that you do professional vehicle dynamics simulation, and apparently have found your way here looking for some good resources to help you build an A-mod for SCCA competition. Perhaps you do work for an OEM, and any comments bashing compliance bushings hit a little close to home? If so then I apologize, but there are better ways to sort these issues out than starting flame wars. Please show more courtesy next time.

[Wesley]

Hoggy, a few cars exploited deflection in rubber bushings for passive rear steering.

My Volvo 850 Station Wagon, for instance. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif