Suspension Design 4

[Tsiolkovsky]

Hello fellow engineers.

I am tasked to design a complete new suspension system for a concept race car. With no background in suspension design, I am currently in the self teaching/literature review phase (for 4 months and counting). I have done cad models but only as concept designs with little calculations backing them up.

This thread serves for me to ask, along the way, any questions I have regarding design of suspension. I am hoping there may be altruistic souls out there willing to shed light on some concepts that I do not yet grasp.

For today's question:

Why are there bushings that connect the control arm to the chassis mounting point? What would be the consequence if I directly bolt together the control arm to chassis?

I am assuming, firstly, the connection point would be un-damped so the ride comfort will feel more jolted.
However I don't see why this would be the case. After-all, all displacement, jolts and stresses are eventually transmitted to the spring damper-system that dissipates all shocks. Having said that,why not hammer in a close tolerance pin connecting the two. There would be no play between components whatsoever. Also, some of these sophisticated bushings do not deform nearly as much as old rubber bushings so the dampening effect is not there anymore. Ultimately, how justified is the dampening effect of bushings?

I also assume secondly, the other reason bushings are used is due to their classical definition i.e. A plain simple bearing that consists of a shaft and a journal (smooth hole). i.e. The control arm is constantly rotating with respect to the chassis mounting bracket and the bolt connecting the control arm to the chassis. However this rotation never does full circle and is at low speeds. Because of this, I cant see why a single pin that's greased can be used at the connection point instead of separate bushings.

Cheers

 

[BrianPetersen]

The control arm needs to pivot. It has to pivot on something! So you can't just "bolt" it.

The normal original-equipment style bushing actually doesn't slide. The rubber deforms (the inside steel bushing twists relative to the outer shell). This eliminates the need for lubrication. Thus, there is no grease that needs to be sealed inside. Pins and bearings require lubrication and seals. More initial cost, more maintenance. Urethane bushings are usually designed to pivot, in addition to deforming when the joint has to deflect in other directions than the pivot. They squeak if you don't lubricate them periodically.

You CAN build suspension systems with all bearings and "hard" pivots. There are kits for some cars that involve replacing bushings with spherical joints, or replacing rubber/urethane bushings with aluminum or other metallic bushings, to eliminate deflection. Motorcycle rear suspension always uses "hard" pivots because the physics of motorcycle rear suspension would have severe bad side effects if too much flexibility and "give" were permitted in other directions than the suspension is allowed to move.

The motion at a control arm pivot is not always a pure rotation around the pivot point. Often the geometry of the suspension dictates that the joint twists or moves in other planes as the suspension moves. Rubber bushings often act as substitutes for spherical joints in these applications.

Damping by the joints is normally not significant relative to damping by the actual dampers.

Rubber bushings can be designed to take up a fair amount of road shock ("noise, vibration, harshness").

Bushings allow a fair amount of manufacturing tolerance to be taken up. Bearing housings require precision machining. Costs more.

 

[Tsiolkovsky]

Good points.

I was indeed aware that the rubber bushings twisted along their axis thus eliminating the need for lubrication. I know that a certain custom kit car built used Teflon bushings that eliminated the need for lubrication. They were poor in absorbing bumps however.

I guess it comes down to cost and maintenance as you say.

I attached an image of what I was alternatively thinking of. Basically, the control arm is aligned with the chassis bracket, then a parallel pin is greased and hammered in the aligned holes and secured on either side by retaining rings/circlips.

Am I right in saying this would work, however the cost and maintenance (constant lubrication?) would render it easier to use bushings instead. (The pin idea would also have a rougher ride and no lateral deflection ability).

Ofcoarse, automobiles use bushings for a reason and if there was any better solution, it would be utilised. However I got fed up of paging through catalogues of bushings that I couldnt make out heads from tails, and decided to ponder if theres any other feasible way.

I guess ill have to continue paging through the DMR bushing catalogues and see if I can find one to use.

Cheers.

 

[LionelHutz]

With a race car you should be looking at things like rod ends, spherical bearings or Currie Johnny Joints.

You could do a pin and hole type of joint but I would line it with something, like an aluminum bushing or a teflon sleeve, or else you'll find that the sleeve wears a bit and reders the whole arm garbage. With some kind or bushing or bearing you can rebuild it.

Don't put rod ends into an application with much side load.

 

[MintJulep]

If you don't already have them I recommend that you get Carroll Smith's three excellent books Engineer to Win, Prepare to Win, and Tune to Win.

Although they may be a bit outdated from the current state of the art, they remain excellent resources for the fundamentals of race cars.

They're a good read too.

 

[GregLocock]

Way back when, say 1900-1960 ish, it was not unusual to use proper greased plain bearings (looked like main crankshaft bearings) in suspension arms. The maintenance schedule included regreasing these bastards every 1500 miles. If you didn't grease them then the shells would wear, if you were lucky, or the arm would wear and then break.

For a race oriented car use rod ends. You will find that getting the actual rates of available rubber bushings is virtually impossible.

Agree with MJ, Carroll Smith tells you enough to do a good job.


Cheers

Greg Locock


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[MintJulep]

Agree with the spherical rod ends. In addition to providing a perfectly fine rotational mount, the usual way of installing them is to have a female thread in the end of your control arm and use a male threaded rod end. This gives your some adjust-ability of your suspension geometry.

 

[Metalguy]

>"I am tasked to design a complete new suspension system for a concept race car. With no background in suspension design,----"<

Just wondering how something like this happens----.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[MacGyverS2000]

Metal,

Usually school projects... no company serious about their product would put an engineer in charge of an area they have zero knowledge in.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[murpia]

MacGyver,

Unfortunately I've had to deal with a few companies who are NOT serious about their products (according to your definition)...!

Regards, Ian

 

[MacGyverS2000]

And those companies typically fail in the marketplace. As an employee, if you don't feel qualified to do what you've been tasked with, and you believe the company's health lies on your shoulders, I would start polishing your resume.

Sure, I've seen companies do what I've suggested, but they don't last long after it hits the prototype stage.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[pgpada]

Ok then if it is a sport's vehicle and it's the first time then you should probably use spherical bearings on the wheel side and rod ends on the frame side. You could watch Formula Student photos. They could be helpful.

 

[Tsiolkovsky]

"With a race car you should be looking at things like rod ends, spherical bearings or Currie Johnny Joints.

You could do a pin and hole type of joint but I would line it with something, like an aluminum bushing or a teflon sleeve, or else you'll find that the sleeve wears a bit and reders the whole arm garbage. With some kind or bushing or bearing you can rebuild it.

Don't put rod ends into an application with much side load."

What I was thinking. I'm considering to use the existing picture and add a Teflon bushing at the interface and secure it with retaining rings. I prefer ciclips over bolts, should be mechanically sound.

"If you don't already have them I recommend that you get Carroll Smith's three excellent books Engineer to Win, Prepare to Win, and Tune to Win.

Although they may be a bit outdated from the current state of the art, they remain excellent resources for the fundamentals of race cars.

They're a good read too. "

Thanks alot, I welcome all literature review with open arms.
Currently im going through Allan Staniforths "competition car Suspension, design construction, tuning".


"Way back when, say 1900-1960 ish, it was not unusual to use proper greased plain bearings (looked like main crankshaft bearings) in suspension arms. The maintenance schedule included regreasing these bastards every 1500 miles. If you didn't grease them then the shells would wear, if you were lucky, or the arm would wear and then break.

For a race oriented car use rod ends. You will find that getting the actual rates of available rubber bushings is virtually impossible.

Agree with MJ, Carroll Smith tells you enough to do a good job."

Ha! Thats exactly what I needed to know! So it comes down to frequently re-greasing them that's the issue. Thought as much. But do rod ends not require a little maintenance of their own when used as a link for control arms?

"Agree with the spherical rod ends. In addition to providing a perfectly fine rotational mount, the usual way of installing them is to have a female thread in the end of your control arm and use a male threaded rod end. This gives your some adjust-ability of your suspension geometry. "

Apart from adjustablity, are there any advantages rod ends offer over Teflon and other non rubber bushings?


"Just wondering how something like this happens----. "

"Metal,

Usually school projects... no company serious about their product would put an engineer in charge of an area they have zero knowledge in. "

"And those companies typically fail in the marketplace. As an employee, if you don't feel qualified to do what you've been tasked with, and you believe the company's health lies on your shoulders, I would start polishing your resume.

Sure, I've seen companies do what I've suggested, but they don't last long after it hits the prototype stage."



A colleague of mine who is designing a concept race car. He needed someone to do the suspension system. I have 4 years to design it, that should (I hope) give me ample time to learn just enough to design this specific suspension. It will not interfere with my usual mech eng daily work because its a non professional design task as we are doing it for a hobby. So there can be no failure for any company or any loss of money. Please be pragmatic and less ideological on this topic, I need facts, design corrections and suspension engineering tips, not demotivational drivel.


Thanks all for the help.

 

[murpia]

"I need facts, design corrections and suspension engineering tips, not demotivational drivel."

OK then:

Start with your load cases - accel, braking, cornering, bump, aero and all combinations of those.

If you or your colleague cannot define those, you will not find it easy to select a bushing type or any other design detail.

Regards, Ian

 

[patprimmer]

Learn how to use the the Quote feature in the process TGML as it will make your posts a LOT easier to read.

Some of the people who offered you the de-motivational drivel have worked for many years for companies like Jaguar and Lotus where they designed suspensions for a living and most would say did a very good job of it.

Despite some very wordy posts, actual information required to offer more than very general advice is missing.

I am still not sure what you are asking other than what material might be used foe bushes in suspension joints.

While you mention Teflon (Actually just one brand name for PTFE) there are quite a few plastics that can be used depending on the nature of the load, the environment of use and the details of the design.

Most obvious choices are:-
UHMWPE
HDPE
XlinkablePE
Nylon 6.6
Cast nylon
Acetal
Moulded Polyurethane
Cast Polyurethane
PET

Many of these may also be filled to modify impact strength, abrasion resistance, PV value, compressive strength, elongation, hardness, creep resistance or reduce coefficients of dynamic and static friction.





Regards
Pat
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[NormPeterson]

Why are there bushings that connect the control arm to the chassis mounting point? What would be the consequence if I directly bolt together the control arm to chassis?

There's a rather practical reason why you don't see much other than bushed pivots or some sort of spherical joint.

As soon as you admit that whatever is built will be subject to both build tolerances and that there will be chassis deformations in service, you're stuck with some amount of multiaxis rotation and non-concentricity of pivot axes. Bushings accommodate this via compliance, and sphericals do so by the geometric uniqueness of a sphere. Everything else will put up a fight and generate unintended loads and greater wear rates when it is asked to move.


Norm

 

[Alec186]

Mr. Tsiolkovsky - I think the inner attachment point of the upper suspension arm of the Morris/BMC/Leyland etc. Mini is probably of the type you are referring to - it is rigidly mounted through a hefty (about 1 inch diameter from memory) pivot shaft with needle roller bearings. The lower arm attachment points are conventional rubber-bushed types. Overall it is a very good system if the greasing of the bearings is not neglected.

 

[Tsiolkovsky]

Thanks alot folks. Im going with non deformable bushings. The exact calculations to justify what material comes at a later stage. So far ive immersed myself with literature however one thing is nagging me badly and that is the kinematics of SLA suspension:

Consider a suspension with negative camber gain (accomplished most easily by using short-long-arm suspension) and has already negative camber at rest (static). This means, the more the wheel jounces the higher the negative camber becomes as quoted in literature:

"The upper arm is usually shorter to induce negative camber as the suspension jounces (rises)"

This means using this suspension setup, when you hit a turn, your center of gravity drifts to the outside and compresses the outer suspension and "slackens" the inner suspension. This means the inner suspension becomes less loaded and gains more positive camber thus becoming perpendicular to the ground. Correct? The problem is now onto the outer wheel. This side is more loaded because the outer side sustains the rolling moment due to the center of gravity. This being the case, it jounces and gains EVEN MORE negative camber. So now the camber is more negative and far from making the tire perpendicular to the ground. I must be terribly confused, this is clearly wrong and not making sense, I'm missing something so fundamental I feel embarrassed to ask, but alas, the literature and the google never answered my concerns.

 

[patprimmer]

If the camber gain matches the body roll, the wheel remains at the same angle.

As you turn the wheel, the camber also changes due to castor angle and king pin inclination.

You do not want the wheel perpendicular, you want the tread flat on the road. The tyre distorts from side loads. The amount of distortion depends on many factors, so you are juggling several variables to offset another variable.

Every time you change one it impacts on the others in several ways.

Working out the balance with regard to various compromises is the black art aspect of suspension design.

Regards
Pat
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[BrianPetersen]

I think you have the sense of "negative" and "positive" camber "gain" or "loss" reversed - or maybe you have "double-negatived" yourself into confusion.

A double-wishbone suspension with the top arm shorter and / or the chassis-side A-arm pivots closer together (in the top-to-bottom direction) than the ball joints, will pull the top of the wheel inward as the suspension compresses. If that suspension compression is because of body roll, the body roll is carrying the top of the wheel outward. Superimpose the two, and they (more or less, depending on the geometry) offset each other. Independent suspensions of all designs ordinarily aren't designed to fully offset the camber-change effects of body roll, because doing so is associated with a whole bunch of other bad side effects. But, they often partially compensate.

"Non-deformable" bushings will get you in trouble unless everything is milled-with-a-CNC precision (not just an ordinary stamped and welded structure) and all the links and arms involved are in pure rotation around the axis.