Suspension Design
Suspension Design
(OP)
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
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





RE: Suspension Design
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.
RE: Suspension Design
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.
RE: Suspension Design
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.
RE: Suspension Design
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.
RE: Suspension Design
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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RE: Suspension Design
RE: 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
RE: Suspension Design
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
RE: Suspension Design
Unfortunately I've had to deal with a few companies who are NOT serious about their products (according to your definition)...!
Regards, Ian
RE: Suspension Design
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
RE: Suspension Design
RE: Suspension Design
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.
RE: Suspension Design
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
RE: Suspension Design
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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RE: Suspension Design
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
RE: Suspension Design
RE: Suspension Design
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.
RE: Suspension Design
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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RE: Suspension Design
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.
RE: Suspension Design
Got it. However we must pay special attention to nomenclature. "Rolling moment" refers to the rotation of the sprung mass and the "non-rolling moment" refers to the rotation of the automobile as a whole: sprung and unsprung masses. Just to clarify: The effect of camber gain is offset by the the non-rolling moment of the vehicle.
I am assuming that's the reason SLA suspension is preferred over parallel-equal length wishbone configuration: The former has the property of camber gain/loss thus offsetting the non rolling moment thus keeping its original static camber angle through corners. The former, however, does not have the property of camber gain, thus the non-rolling moment cannot be offsetted by camber gain/loss. Correct?
RE: Suspension Design
Front suspensions on mass-market production vehicles designed to be driven by ordinary people are deliberately designed to lose grip at the front first. A good many front MacPherson suspensions have very little camber change in compression. Often the rear suspensions on the same vehicles have a double-wishbone or multilink arrangement with shorter non-parallel upper links to get camber change in compression, so as to have lots of rear grip. Can't be having oversteer in a production vehicle these days.
Also, as mentioned by someone else, the caster and steering axis inclination angles have an influence on the camber when the steering is turned to the side.
RE: Suspension Design
Then read the rule book though for the class rules very very carefully.
Then look very closely at the cars currently winning in that type of racing.
And beating the very best in the business at their own game, is not going to be all that easy.
But the winning cars define the current state of the art, and you could do far worse than use them as a case study, and a basis for YOUR design.
RE: Suspension Design
In other locations (front control arms on Formula SAE, for example), we have (many times) used rod end bushings (spherical bushings with a threaded rod extending from one end. They largely work well, but have problems when loads are applied in the wrong direction.
All this being said, if your current primary concern is what type of bushing to use, or what type of material to use in your bushing, then you should either be well into your design, or you are approaching the design problems in by far the wrong order. The first tasks should be to determine general types of suspension to use on the front and rear of the vehicle, and then determining approximately what behavior (camber gain, trail, total travel, natural frequency, spring/damper mounting) you want, and then looking at general geometry, and finally starting to look at actual structural designs.
Finally, as a previous poster already mentioned, it would behoove you to look at existing race cars in the class in which you wish to compete.
RE: Suspension Design
working on vw's all of them use a trailing arm style setup. id imagine its similar for other manufacturers?
RE: Suspension Design
Here is my reasoning, correct me if I'm wrong:
1) It depends on the tire properties. An easier flexing carcass means the tread will conform more easily to the ground even with the smallest of lateral tire loads. This in turn means that if the tire carcass is "soft" (don't know the correct term for this), the camber gain must be more pronounced. If it wasn't more pronounced, the tire tread will "over" distort with a given lateral load through cornering.
2) It depends on the non-rolling moment of the automobile (which in turn is dependent on the position of CG WRT RC). If the non-rolling moment is high (the automobile-as-a-whole rolls easily), then you would want a higher camber gain to "offset" this disturbance in camber angle.
If the above is correct (there may indeed be even more factors), then how would you quantify and calculate it?
Am I correct in saying the answer is: You dont? The reason being is that its an experimentally deduced number. You set it to an arbitrary value, run the car on corners and then bring it in to measure the tire temperature along the tire tread. If its hotter in the inside, you have too much camber gain. If its hotter on the outside, you have the camber gain set to too low a value. This makes sense to me, I think I even read it in some literature (the experimental nature of determining camber gain). Adjusting the camber gain itself is of coarse only a function of SLA geometry so you could incrementally lengthen/shorten the bottom wishbone and then do another test run)
Ultimately, how true is the above?
RE: Suspension Design
We don't even know what sort of "race car" we are talking about here. World Rally cars have very compliant suspensions and with that and the nature of the rally stages, comes a lot of suspension travel during operation, and with *that*, comes not wanting too much camber change with suspension travel. Subaru cars use MacPherson front and rear, and the rears have long lower links - a recipe for not much camber change.
Open wheel cars designed for smooth pavement race tracks don't need much suspension travel and generally have to be designed with ground-effect aerodynamics in mind. With that, comes a need for not letting the ride height vary too much no matter what happens with the car. With that, comes very stiff spring and antiroll rates. With *that*, comes very little camber change regardless of how the suspension is designed, because it hardly moves. Formula 1 cars have very visible upper and lower A-arms, and they're visibly almost horizontal, suggesting minimal camber change with what little suspension movement that there is.
Ordinary road cars that are expected to have an acceptably smooth ride and still have decent cornering properties and have varying height due to cargo loading ... are a tricky application, but even then, there is a wide variety of suspension designs, particularly for the rear, some with no camber change (trailing arms), some with wheels perpendicular to the ground at all times (beam axle), some with a lot of camber change on bump compression (late model Honda Civic upper and lower arms), some with not much (MacPherson strut).
RE: Suspension Design
The whole thing is a rather complex process of evolution.
Choosing the correct compromise for the actual racing conditions is what wins races.
RE: Suspension Design
Don't overlook the static camber setting as a means of adjusting where the camber under operation ends up. Hopefully we aren't looking at extremely large camber changes.
Norm
RE: Suspension Design
Front hub height
Rear hub height
Wheelbase
Track
Front RC
Rear RC
Type of Front Suspension
Type of Rear Suspension
Relative position of outboard pivots (upright/knuckle control arm pickup)
wheel travel
suspension unit travel
ROUGH Geometry of control arms (length, angle, position, pickup outboard & inboard)
Undertray Line
Pause: Feed this information, and the following information to chassis designer
return any input from chassis designer
Locations of all internal components
Bulkheads Main/subsidiary structural members
General traverse members
more detailed susp Design Begins:
Plot Wheel Limits of Travel
Plot roll centers during bump/droop
Plot camber change during bump/droop
Move wishbone pivots to constantly control this
Fix maximum desirable roll angle
Hence wheel inclination at maximum roll
Virtual swing arm length (rule of thumb/baseline: 1.5-3 X track)
Static camber angle agjustments
Through RC & tire contact & outboard pickups, control arm front view angles are found
Inboard pickups
again, Roll centre height change on bump-droop
Keep roll centre movement to minimum without keeping camber gain too low
Detailed factors calculations: Determination of Roll Angles and Wheel Loading (by calculating the roll moment)
Inboard pickups (to be fed directly into chassis)
Mountings for susp units
before wishbone pickups, determine Type of mountings/bearings
Steering linkages (anti ackerman and toe in/out under bump/droop)
Castor/KPI angles
Anti features and other non standard features
Repeat and constantly increment all steps in order and feed all these factors into each other until good compromise values are found