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Author: Subject: Suspension Design - how to?
Puk

posted on 7/1/08 at 02:17 PM Reply With Quote
Suspension Design - how to?

I've been iterating through different ways of attacking the design of the suspension. I've read most of the reference books excepting Kimini one and the Miliken one (Santa proved to be a disappointment on that front - but thanks for the jumper mate). Rather than continue to lie awake in bed pondering this, can I ask for some feedback:

My thoughts are that you'd want the tyre to be as close to upright as possible under conditions of maximum load and it would be ok to compromise on this when the tyre was under little load.

If that is an acceptable idea (pipe up anyone with some experience) then the following ought to hold true:

1. When cornering the outside wheels would be upright, or leaning in slightly at the top like a motorbike type (but definitely not leaning out) AND it would be an acceptable compromise for the inside wheel to be less than upright because it is lightly loaded.
2. Under brakes the front tyres are most heavily loaded, so aim to keep them upright or leaning inwards at the top slightly (but avoid leaning out) AND it would be an acceptable compromise for the rear wheels to be less than upright - again because they're lightly loaded.
3. Under acceleration keep the rear wheels upright or leaning inwards at the top slightly (but avoid leaning out) AND it would be an acceptable compromise for the front wheels to be less than upright - again because they're lightly loaded.

So if all that hold true then it is just the small matter (gulp) of establishing the dip of the chassis at the loaded end under these 3 cases of maximum acceleration. If I know the target acceleration in any direction (1.5G in all directions is about the best I would except from a set of race tyres on car without downforce) then if I also know the spring rates, the location of the CofG, the tracks and wheel base, then it ought to be possible to calculate the roll of the chassis under acceleration (acceleration being positive when the car gets quicker, negative under brakes and lateral in a corner).

Knowing the dip of the chassis tells me the amount that the loaded wheel has been compressed by. For the sake of argument if the car dips 50mm braking at 1.5g then I want the front wheels to be upright when the suspension is compressed by 50mm.

Ideally I'd like the outside wheel in a bend to be compressed be the same amount when achieving 1.5g of lateral acceleration. Though quite how that is achieved is a bridge that is yet to arrive - maybe that is where rollbars come in?

Ok fire away!





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clockwork

posted on 7/1/08 at 07:10 PM Reply With Quote
Puk, if you are in the UK you can pop to the library and request a book. If they don't happen to own a copy in any of their libraries they often will buy it for their shelves. I've done this a couple of times (when not in full time employment). I've got every book I requested for the princely some of 50p + late fines :-)
While you won't own it, it is very cheap, and a lot of susp books are broadly similar.





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Puk

posted on 7/1/08 at 07:37 PM Reply With Quote
Cheers - I'll try it but I'm based in Denmark so not sure of the local rules nor how they would react to a request for a none-Danish book!





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clockwork

posted on 7/1/08 at 10:06 PM Reply With Quote
I did wonder about your location...
Unfortunately I have the knowledge from 20 books but zero practical experience. I do intend to have a front ant-roll bar though, whereas most designs don't.

Nitram 38 has an analogue computer model shown here if you want to have a visual play.
http://www.locostbuilders.co.uk/viewthread.php?tid=80560
Print it out and use drawing pins to follow see the camber through various arcs.

Good luck.





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Puk

posted on 8/1/08 at 10:26 AM Reply With Quote
Hi Clockwork, thanks for taking the time to suggest some books. Over the years I've read pretty much all the classics - Staniforth, Smith, Van Valkenburgh, Costin, Tony Foal and the Brian Beckman online articles. I must admit that I haven't seen some of them for a few years but I can't recall one which tackled designing the suspension from scratch - but I could be mistaken. I know that there is plenty of stuff describing how to calculate roll centres, how to find weight transfer but not anything that outlined a sequence of steps that would allow you to start with what you know and then allow you to findthe missing pieces.

So for example, knowing the weights and dimensions of a 7 what amount of front end dip under braking is to be expected or what amount was optimal? It is pretty straighforward to calculate the weight transfer, and then combining that with the desired spring rate should allow the suspension compression to drop out. Just not sure if this is a good way to go about the design!

Have you seen anything in your books that hints at that?

I've ordered Kurt Bilinski's "How to design and build a mid-engine sports car - from scratch" book so hoping that there might be some sequence of steps outlined in that.

Cheers,
Puk





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clockwork

posted on 8/1/08 at 04:34 PM Reply With Quote
I think you are asking for the holy grail
Every book I've read states that suspension design is all about compromises. Don't forget cars are in a transient state a lot of the time.

If you can get hold of the Carroll Smith book "Tune to win" he does an excellent job of explaining why everything is a compromise.

I know this is probably not what you wanted to hear (and I'm surprised you haven't had more responses to be honest), but your best bet is to copy a well regarded 7 for the rough measurements, and then fine tune until you get what you want. On the plus side, this information is freely given.

I am in the process of designing my own front suspension for my locost, but don't expect to get it right on my first go.

If you want me to email you some scans from Tune to Win send me a u2u.





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Puk

posted on 21/1/08 at 01:15 PM Reply With Quote
Clues from the tyre suppliers

I spoke to Alan Meaker at Toyo who is their technical engineer in the UK and one time Locost racer. I asked what tyres and camber values would be suited to a Locost type car. He suggested two options the 888 for a track car that did a little road work or a T1R for a road car that did a little track work. He wasnt able to provide a graph that showed a link between camber, tyre load and grip but did suggest that the value would likely lie between 0* and -4*. A road biased car would be set with a camber values closer to 0 which would likely sacrifice ultimate cornering grip for tyre longevity.

Yokohama's rep who suggested Yokohama, AO48 - R for a track day car. Like Allan Meaker he urged caution when trying to design for the optimum camber, but suggested that suspension geometry that could present the tyre at a negative camber of 2.5* when cornering would be a good starting point. Track testing will reveal whether more or less camber is required to get the best out of the tyre.

I had been naively hoping to design the suspension geometry to produce 0* ish camber at the target braking and the same camber at target cornering acceleration. But this little idea is shattered when it became clear that optimum camber for cornering is not 0 deg. So my current objective is to design for the cornering requirement, as in that mode the outer tyre is doing the lions share of the work, where as under braking the load is spread across both wheels on the axle.

Maybe caster and king pin inclination can be contrived to introduce negative camber in the outer wheel in a bend - at least a the front...





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Fred W B

posted on 21/1/08 at 05:13 PM Reply With Quote
quote:

one which tackled designing the suspension from scratch



You need to read Staniforth again

Cheers

Fred W B





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Puk

posted on 21/1/08 at 05:33 PM Reply With Quote
quote:
Originally posted by Fred W B


You need to read Staniforth again

Cheers

Fred W B


Hi Fred, funnily enough I've been reading Competition Car Suspension over the weekend, and have it in my hand as I write this but can't find anything in it which indicates that this is a good or bad idea. He does indicate that KPI is often in around 7* and that caster can be 0* (Chapter 5 - "The Winning Package"). But that chapter often references F1 practice.

Is that the section you had in mind, or is it something in Staniforth's Race & Rally Source book?

Thanks,
Puk





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Fred W B

posted on 21/1/08 at 06:11 PM Reply With Quote
Try Chapter Seven

Once you have selected a tyre, an upright and taken an educated guess as to what SAL to use to it is pretty much done.

He does go into a bit more detail in Race & Rally Source book

Cheers

Fred W B

[Edited on 21/1/08 by Fred W B]





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Puk

posted on 21/1/08 at 06:21 PM Reply With Quote
quote:
Originally posted by Fred W B
Try Chapter Seven[Edited on 21/1/08 by Fred W B]


"The amateur at work" - I'll flog through it tonight - thanks for the steer.





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Puk

posted on 21/1/08 at 09:57 PM Reply With Quote
Good paper on the subject

For anyone else hitting this thread in the future, here is a useful paper on the subject:



Introduction to Formula SAE Suspension and Frame Design





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NS Dev

posted on 22/1/08 at 10:43 PM Reply With Quote
heh heh!

You are working on exactly the right lines, nice to see "from scratch" thinking.

The main constraint you face from that "pure" point of view will be wishbone lengths.

If you can make extremely long wishbones compared to the required wheel travel then you can make the wheels do whatever you like.

In the same way with semi-trailing arm rear suspension, make the arm long enough and you can have whatever you need.

Unfortunately bits of car get in the way of this ideal and you suddenly end up with what looks like a horrible compromise. Then you take it to the track and realise that everybody else has as bad if not worse a compromise, and they are still faster than you, then you learn to drive better, get in an even more horrid car and go really fast..............

sorry, off track but I'm sure a few on here reading this will have some sympathy with my thoughts! lol





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Puk

posted on 23/1/08 at 08:06 AM Reply With Quote
Thanks for the vote of confidence NS Dev!

I did suspect that some bugger will be driving his monstrosity quicker than my thing of engineering beauty. But hopefully I'll be in a better position to tune it.

Failing that I'll throw my toys and pram at him. The git.





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Tralfaz

posted on 23/1/08 at 12:40 PM Reply With Quote
If you are as serious as you seem with your design, I would recommend Susprog 3d as a design tool.

While expensive it saved me countless hours.


T

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Puk

posted on 23/1/08 at 12:58 PM Reply With Quote
Thanks for the tip Trafaz, I think I'm going to try and do some geometry experimentation in CAD first (I was formerly a CAD jockey) to allow my self to get the feel for how adjustments in SAL, RC and upright dimensions affect things. Once I'm reasonably sure that I know what is going on I'll be more comfortable about using a more automated approach.

This is a bit of a learning exercise for me - as much as a project to build something. I had quite a fright yesterday evening when trying to work through stress loads when I realized just how much calculus I'd forgotten.

I intend to post design developments up to allow them to be toughened up under a spot of peer review - so your input will be welcome.

Cheers,
Puk





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NS Dev

posted on 23/1/08 at 01:27 PM Reply With Quote
if you draw it all up on cad its great, you can play pretty easily and soon see from the relevant arcs what is going on.





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t.j.

posted on 23/1/08 at 08:59 PM Reply With Quote
And after all thinking and reading and seeing you will come to the conclusion:

It is one big compromise!

I'm happy, we are just above the surface, and have not much movement in the suspension.





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Puk

posted on 23/1/08 at 10:01 PM Reply With Quote
Compromise is fine - just want to have the set of "least bad" decisions.

And be quickest!





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Wolf HR

posted on 24/1/08 at 10:37 PM Reply With Quote
Some time ago, I did make a small flash analyzing geometry of double wishbone suspension (only in 2D), I may try to dig it out if it's of any ineterst...

Currently, I'm trying to make one with full 3D analysis (anti- angles and all).

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chrisg

posted on 25/1/08 at 02:16 PM Reply With Quote
I'd be interested to see that.

cheers

Chris





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Wolf HR

posted on 25/1/08 at 03:40 PM Reply With Quote
ChrisG- here it is. Despite the text on the page, data page is functioning (right click on any of the fields*, select all, copy and paste wherever you want- I think it will paste directly into Excell spreadsheet as a column). I've put a link to flash file on bottom so it could be saved to your computer more easily.

http://tpetricevic.110mb.com/index1.html

There is a small glitch or two on page where one defines geometry, but it's fine otherwise (but still please treat it as beta program). Most of dimensions can be changed with either drag & drop or editing the text of the dimension.

* vd/vJ is damper speed vs. jounce speed ratio at that point

Currently, I'm working on a version where wishbones would pivot around arbitrary axis (not just perpendicular to the drawing plane as in this version) and would be moveable fore/aft relative to each other. It would also incorporate steering geometry (for bump steer) and more details on springs/coilovers.

HTH

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Johan

posted on 28/1/08 at 11:00 AM Reply With Quote
When it comes to recommended reading I'm very fond of Mark Ortiz "Chassis newsletter". Mark is a chassis consultant that has a column (drawn from his newsletters) in Race Car Engineering
I guy at corner-carver.com has all of his articles hosted:
http://www.2kgt.com/MarkOrtiz/
(There's a zip-file at the end with all articles)

I've categorized the articles I found most useful with links to the directory mentioned above:

Oval track concepts
June 2002
Some basic oval track concepts for road racers.

May 2001
Rear Stagger versus static cross.

Chassis troubleshooting guide
July 2001
Chassis troubleshooting guide.

July 2003b
Chassis troubleshooting guide.

August 2001
Things that make spring changes work backwards.

October 2001
Balancing the car with camber and tire pressure.
June 2001
Weight transfer in winged over sprint cars

November 2001
Selecting springs.

February 2002
Things that make spring changes work backwards, revisited.

April, May, June 2003
Shock and spring forces.

August 2003
Load transfer basics.

September, October, November 2004
Tire load sensitivity and weight transfer in trail braking

September, October 2005
Load transfer question.

April 2006
Springs, bars and load transfer.

June 2006
Effects of spring rates and damper settings during cornering.

August 2007
Is it bad to lift a wheel?

Front/Rear weight distribution
March 2002
Racing front-drive cars.

October 2002
Move the rear wheels back to improve traction?

February 2003
More on rear wheel placement and traction.

November 2003
Weight distribution and tire size.

December 2003
More on racing front-drive cars and load transfer.

April 2004
More on weight distribution.

November, December 2005
Optimizing engine-over-drive-wheels layout.

Corner weight
January, February 2004
Managing the behavior of the unbalanced.


Roll center and roll axis
June 2004
More on F1 control arm angles.

August 2004
Roll center migration, some more.

December 2004
Roll center below ground.

February 2006
Roll axis inclination.

September 2006
Roll center in trailing arm front suspension.
More on roll axis and related analytical concepts.

May 2007
Roll axis for live-rear-axle road/hillclimb car
Roll axis of the axle

December 2004
Roll moments from longitudinal anti.

RollCenter Myth and Reality (Mitchell)


Polar moment of inertia
July 2000
Polar moment of inertia.

September, October, November 2004
Rear percentage versus yaw inertia.

May 2006
Rear percentage or polar moment?

Steering geometry
August 2002
Steering geometry variables.
Importance of steering rack placement.

November 2002
Trail and steering feel.

January, February 2005
Ideal spindle or steering geometry.


Ackermann
December 2002
Ackermann Recommendation.

January, February 2004
Reverse ackermann or toe-in on ovals.

Shocks
August 2000
The shock dyno lies!

December 2001
Analyzing shock traces.

January 2002
Suspension Natural Frequencies.

February 2002
Some Basic shock questions.

May 2002
Natural Frequencies revisited.

February 2003
Shock research update.

September, October 2003
Shock research update.

November 2006
Stiff rebound damping.

Tires
September, October, November 2004
Why are wide tires better?

January 2007
Tall vs. short sidewalls

June 2007
Large vs. small tire and wheel diameter

Beam axle
March 2003
Independent rear suspension for dirt?

January, February 2004
Independent or beam front axle suspension for paved oval?

July 2006
Beam axle pros and cons.

October 2006
Twist beams and jointed de dion tubes.

June 2007
Another triangulated four-link axle

May 2004
Arm angles in triangulated four-bar beam rear axle.


Panhard bars
December 2000
Shorty panhard bars versus long ones.

January 2003
Roll center with a j-bar/panhard bar

August 2006
Panhard bar longitudinal location.
Panhard bar jacking forces.

July 2007
High or low panhard bar

Frame stiffness and steel quality
February 2001
Required frame stiffness.

May 2001
Chromemoly in stock cars.


Differentials
December 2002
Torque, RPM and power distribution in differentials.

December 2003
Single or dual brakes for FSAE

March 2004
Differentiating differentials.

March 2006
Diff differences


Zero droop
February 2007
Zero-droop setups

July 2007
More thoughts on zero-droop setups, and related oval track spring split issues

Center of gravity calculation
September 2002
Front and rear center of gravity.

Winter reading
November 2000
Winter reading material.

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Puk

posted on 28/1/08 at 11:54 AM Reply With Quote
Thanks for sharing those - I feel like Jonny Five

JonnyFive
JonnyFive


[Edited on 28/1/08 by Puk]





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