Suspension book

[kozak]

What book is the best at conceptually and actually showing me how to design a suspension system for my application. (i'm talking about "the akerman angle shoulod be between bla bla and bla bla" and formulas and such, actual info not just theory of design)

[kozak]

What book is the best at conceptually and actually showing me how to design a suspension system for my application. (i'm talking about "the akerman angle shoulod be between bla bla and bla bla" and formulas and such, actual info not just theory of design)

[TA]

"Competition Car Suspension" by Allan Staniforth
does a decent job at simplifying the complex and provides a lot of useful formulae.

"How To Make Your Car Handle" by Fred Puhn is a nice easy read with a lot of descriptive text and a few formulas. More useful for optimizing an existing suspension.

"Race And Rally Car Sourcebook" by Allan Staniforth is a great book, fewer formulas but has a good section on the "string computer" a very useful and quick way to determine camber curves and roll behaviour.

AFAIK, no book will "tell" you how to design a suspension, Staniforth does a good job of describing the characteristics of various choices though -- up to you to decide what's best for your car.

The Carrol Smith books also provide a good read on Ackerman -- in his first books he's against using any Ackerman but he comes around in later titles.

FWIW, "full ackerman" on a rear steer car (with the rack arms perpendicular to the chassis) has lines through your steering arms intersecting at the rear diff.

According to Smith, adding more Ackerman (sometimes even more than "full") can help make the car turn-in better (reduce entry understeer) he seemed to indicate it was a case by case thing though -- might be best to make steering arms with multiple holes and set your final Ackerman in testing.

Cheers, Ted

[DJHache]

You could also check out Van Valkenburgh's Racecar Engineering and Mechanics. It has a nice rundown of suspension characteristics.
I would definitely recommend the Milliken book, Racecar Vehicle Dynamics. It's by far the most thorough and rigorous book I've seen and it's approchable if you've already read the other books.

[Z]

Quote
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"I would definitely recommend the Milliken book, Racecar Vehicle Dynamics. It's by far the most thorough and rigorous book I've seen..."
--------------------------------

Milliken et al spend 3 pages (p713-p716) on Ackermann steering geometry. After a cursory explanation they recommend that "parallel steer or a bit of reverse Ackermann is a reasonable compromise".

This is a terrible recommendation for FSAE conditions. Try it...

Z

[kozak]

Z wha would you recomend

[Z]

I would recommend, for a start, that anyone in doubt about Ackermann should read through the many "Ackermann/steering-geometry/toe-out/etc." threads on this site. Not all the recommendations in those threads are right, but at least they are based on FSAE conditions, and are not sweeping generalisations taken from 200+mph Indy cars on 500ft radii corners, etc., as in Milliken.

More specifically, FSAE conditions benefit from as much dynamic toe-out as you can get with a conventional steering system. That is, with the outer-wheel at, say, 30 degrees you want the inner-wheel at least at 40 degrees, preferably more. That's a long way from parallel steer.

Z

[DJHache]

If you read the Milliken section carefully they deduce their optimal Ackermann geometry from the tire data. As the load on the inside tire decreases, the required slip angle for maximum lateral force decreases, hence the parallel or negative Ackermann geometry. Unless you're from a very lucky team you don't have that tire data so it is difficult to arrive at any sensible conclusion.

The Millikens also say that at low speeds (ie low lateral accelerations) it is more important to maintain all the tires on (or close to) their turning arc so as to reduce tire drag (=> positive Ackermann).

Obviously the "Recommendations" they give may not apply to FSAE cars specifically, but until someone writes a book specifically about FSAE you would be wise to understand where the recommendations come from before blindly applying them to one of our cars.

[kozak]

Well since we area first year team i'm loking for something that will basically allow me to build a suspension that won't suck, and can be something that we can learn from for next time.

[D J Yates]

Although FSAE speeds are low the corner radii are so small that lateral acclerations should be large. If they're not your not driving fast enough. There should be no need for positive ackermann, unless your tire data says otherwise. We are fortunate enough to have tire data and it does suggest negative ackermann.

Our first car ran negative ackermann (not sure how much). Apparently it was meant to be positive, but when we referenced it for this year's design we found it was negative. Anyhow, the car turns in really well - infact there is so much front grip the rear often steps out mid corner - so proof that negative ackermann works.

Back on topic:

Although Milliken's books (RCVD and Olley) are great, i find them hard to interpret. I prefer Dixon's "Tires Suspension and Handling", it's easier to follow. I also found Matschinsky's "Road Vehicle Suspensions" really useful as it's written purely from an engineering perspective and has some very useful maths in it. Allthough not on suspension, i've found general mechanics and dynamics books essential. A Matlab guide proved usefull too.

[Z]

Quote from D J Yates;
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"There should be no need for positive ackermann, unless your tire data says otherwise."
--------------------------------------------------------------------

Firstly, I've never come across a universal and accurate definition of what zero, +ve, or -ve Ackermann really is. The definition of "100% Ackermann" having the steer-arms pointing towards the diff centre is meaningless, because there are so many other variables involved. Let's say that parallel steer is zero, any dynamic-toe-in (ie. as the wheels are steered) is -ve or anti-Ackermann, and dynamic-toe-out is +ve or pro-Ackermann.

Regarding the different slip-angles for peak-lateral-force at different Fz loads. Fifty odd years ago (or more) when anti-Ackermann was first proposed the peak-force slip angles were around 10 degrees, and the difference at different loads was maybe 2 or 3 degrees. Nowadays, the peak-force is at about 5 degrees (or less) and any difference is barely 1 degree.

However, FAR MORE IMPORTANT are the kinematic steer angles required just to get the front wheels at EQUAL slip-angles on sharp (ie. typical FSAE) corners. On tight corners FSAE cars need around 10 degrees more inside-wheel-steer-angle than outside-wheel-steer-angle. This is very easy to measure on a real FSAE car, or it can be drawn on a scrap of paper. (If you want to sketch this the corner centre should be in front of the rear-axle-line to account for the rear-wheel-slip-angles.)

So, 10 degrees toe-out for kinematic reasons, minus 1 degree (or even 3 if you want) for peak-force-slip-angle reasons, and you've still got at least 7 degrees toe-out. I would call this a fairly large +ve, or pro-Ackermann angle.

Any FSAE car that has its front wheels toed-in, or even parallel, on the sharper corners will have the inner-wheel pushing the nose out of the corner (unless you've managed to lift that inner-front-wheel off the road, or are power-sliding the rear around).


Z

[Patrick W. Crane]

We don't have any tire data (yet) but testing data has proven what Z posted. with parallel steer (or not enough +ve ackermann) you'd have to run some seriously twitchy toe out to get the best turn in.

don't belive it? set up a slalom and try it. you know your turn in is good when it almost feels like the car knows what you are about to do (especialy if you are really used to "bad" turn in).

in the end though, design whatever you think is best, then test it. often things seem like they don't add up, but that's usually cause you forgot something!

now that i have said that, i must add that this was true for our old car, but will NOT nescessariy carry over to anyone elses. there are a 1000 variables and the fun part is optomising them once the car is build. plan on spending more time on testing than design... (how many teams show up in detroit without serious testing - they get ripped apart not because of poor design (like the judges may or may not be telling them) but cause they aren't using the car to the best of it's abilities).

[Z]

Quote from D J Yates;
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"Anyhow, the car turns in really well - infact there is so much front grip the rear often steps out mid corner - so proof that negative ackermann works."
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It is not normally a good idea to assume that some specific aspect of a car is "proved" to be good just because the overall behaviour of the car is ok.

The above quote might be interpreted as a car lacking rear grip, rather than having too much front grip.

Z

[kozak]

ok i now understand why you need the toe out, but how much are you guys talking about, like 1-2 degrees. I just can;t wait to go to detroit and see all these cars for the first time this summer

[D J Yates]

Z, i should probably retract that first statement and learn not to post late at night. Your argument about kinematic steer angles makes a lot of sense. However, FSAE is all about dynamics/transient response and i'm currently only able to visualise a force/turn centre in steady state. When i've got the time i'll have to think about this a bit more, but for the now i'm going to trust our steering designer. And like i said, our first car had negative ackermann and seemed to turn in well. Not conclusive proof as you said, but (as i understand the theory) it does add up.

Your quite right that the oversteer could be due to a lack of rear grip, another dumb statement on my part. But i don't know why i even mentioned it because it only happens on exiting fast corners and is totally un-related to turn in. BTW, is there such thing as too much front grip?

Kozak, we're a realatively new team too. We're aiming for a well balanced car with as many adjustable suspension features as possible. That way we'll learn in practice what works and what doesn't when we test.

[DJHache]

I guess we need a more specific definition of Ackermann. Either we look at wheel alignment or tire slip angles.

As Z points out, the inside tire will have more turn angle than the outside one, suggesting positive Ackermann. This shouldn't be confused with the fact that the inside tire will have a smaller slip angle than the outside tire, which is what the tire data suggest.

On a small radius corner, positive Ackermann seems the way to go, but on a sweeping turn, where steer input is smaller, negative Ackermann is better. Dynamic Ackermann?

[Mark Bacchetti]

The best resource for FSAE specific applications are SAE Technical Papers. Go to sae.org and search for "fsae suspension design" or just "race car suspension" in their Papers section. Though the papers you will find are not as complete as Dixon or Milliken, you will see how some teams have gone about designing their vehicles. I think that if you combine these papers with the books mentioned earlier on this thread, you will be ready to go.

Also, SAE just published a book that includes Caroll Smith's favorite SAE Technical Papers. There are some GREAT papers in there. It is also a great deal for the amount of papers you get (27 papers for $80 for members, this would cost $270 if you bought each paper on its own).

Mark
Cal Poly Pomona

[kwancho]

Brilliant thread so far. Thanks for all the help. A friend and I will start taking a look at these books ASAP.

[Buckingham]

Check to see if your university library has the papers before you buy them on your own. Any student here at UW - Madison can download PDF's of any SAE paper published '98 or later (the earlier ones are on micro at the library). We then print them out and keep a binder of them in our office.

[Mark Bacchetti]

Lucky...

We only have the pre-98 micro. Cal Poly Pomona is too cheap to get the PDF subscription (damn budget crisis!). Too bad a lot of good FSAE papers are post-99.

Mark
-Cal Poly Pomona

[Z]

Kozak,

Another book you might look at is "Chassis Engineering" by Herb Adams. This is only 133 pages (with big print), so it only covers the very basics (nothing on Ackermann). But at about $20 it is a good place to start.


Re: More on Ackermann and static-toe.

Putting static-toe-in on the front wheels (ie. wheels toed-in up to 1 degree at "straight-ahead") makes the car "stable/sluggish", so is good for high speeds and large radius turns. It also suits the anti-ackermann argument regarding peak lateral force slip angles.

Static-toe-in on the rear wheels is also good for stability, and also good for the peak force reasons.

Putting static-toe-out on the front wheels makes the car more "nimble/agile/darty", so gives better turn-in on twisty race tracks.

Static-toe-out on the rear wheels is even better at making a car turn in. In fact it is so good it puts the car somewhere between "white-knuckle" and "deadly". For a really slow, sluggish car you might want to try a tiny bit of rear toe-out. For anything else (including most FSAE) you should keep well away from it. Try driving a car fast in reverse to see what happens.

Positive-ackermann (or "dynamic-toe-out") increases toe-out as you steer away from straight ahead (ie. into a corner). So a car with static-toe-in and positive-ackermann is stable in a straight line, and becomes more nimble/agile/darty as you turn the steering wheel.

Z

[kozak]

wow that was a really good explination. i never would have thought of that thanks for the help. still the only thing i am having trouble with is deciding on actual numbers (how many degrees ackerman, link lengths, etc)

[syoung]

Yeah thanks Z, that's been a big help for my understanding too!

[Z]

There's some more comments and references for Ackermann on the "Ackerman help" thread.

[D-Train]

So "Chassis Engineering" by Herb Adams is about suspension and dynamics rather than the body (what I have grown accustomed to calling the chassis)?
(sorry to dig up another ancient thread)

[Lukin]

I wouldn't recommend the Herb Adams book, there is some slighly dodgy info. If your prepard to spend $20 on it, you might as well spend the extra $10-15 and get Van Valkenburgh or Tune to Win.

RCE Ackerman Articles:
http://www.racecar-engineering.com/content/features.htm

[D-Train]

Yeah, I was actually a little more interested in chassis (i.e the steel space frame for Swinburne) design etc. I've got tune to win already (yet to read through it all...) and plan to go through the milliken & milliken book from our uni library. Someone was telling me the Herb Adams book costs about $70 (australian)...?

[Lukin]

I paid $50 AUD for it, but it's not worth that. The chassis side is mostly related to how triangulating bars makes a chassis stiffer. Big discovery hey! I'll check through it tomorrow at work again and see what else is in there.

If you have Tune to Win there your probably not missing any information. I think most of the information you require will come from the books you have (or plan to get) and the basic mechanics or materials/statics books you have for uni.

[D-Train]

Hmmm. good point. Guess i'll have to dig out those "Mechanics of Structures" books... http://fsae.com/groupee_common/emoticons/icon_frown.gif
Thanks for the replies.