suspension dimensions

[mm davari]

as a fresh designer we calculated dimensions below :
caster angle:4.5deg
camber change:.03deg
scrub radius:4mm
trail:3cm
king pin icl.:3deg
i eager to know about the other teams dimensions and i need some statistics about it?

[mm davari]

as a fresh designer we calculated dimensions below :
caster angle:4.5deg
camber change:.03deg
scrub radius:4mm
trail:3cm
king pin icl.:3deg
i eager to know about the other teams dimensions and i need some statistics about it?

[magicweed]

I'm sure you're just looking for help, by asking for other peoples numbers, but don't expect many of them. The reason this is a design competition is the one must design the car, not copy, or adapt other peoples ideas. As long as you're confident in what you calculated, go with it. Who are we to judge. I realize that the level of automotive knowledge on this forum is vast, as this is a passion for most of us. However, we aren't who you're trying to sway with your design, except maybe Pat. You'd be better off coming up with your own justifications for your design than seeing if its close to someone elses. This is just my 2 cents though.

[kwancho]

Also, when I was starting out suspension design, I read a ton of different papers and asked a bunch of different people, and I got all different answers. Everybody has a different design, and there are different ways to attack the same problem.

[SpdRcr]

stop wasting our time.

[kwancho]

Dude, at least he's taken a first pass at designing it himself. He's not asking "how do i design suspension". It's progress. I'd have asked a similar question last year.

[CMURacing - Prometheus]

The perennial answer in our team to "are these suspension numbers good?" is "you'll find out when you build it." Suspension design is a continuous process, i.e. you design it, build it, improve it, redesign it, rebuild it, until you've found what you think is the right answer. Maybe this is grip at all costs, making the car a mean one to drive. Maybe the answer is make it all as simple as possible, so its easier to analyze, drive, and iterate. But there's no one golden rule.

[KU_Racing]

I would say that exact suspension characteristics are probably the most proprietary design element on a racecar. No way I would divulge any of that. On top of that, as several people have said, different characteristics are better for different cars, different tires, different drivers, different weather, different tracks, different design goals... different judges.. whatever.

[murpia]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by mm davari:
as a fresh designer we calculated dimensions below :
caster angle:4.5deg
camber change:.03deg
scrub radius:4mm
trail:3cm
king pin icl.:3deg
</div></BLOCKQUOTE>

If you are prepared to tell us how you calculated these dimensions and why you chose them over alternatives, then we can offer a critique of the process and the validity of the results. That is how the design judging process works.
Regards, Ian

[John_Burford]

I think some of the suspension parameters MM Davari asked about are fair territory for discusion. I mean how many of you are going to biuld multiple uprights to explore variations in kingpin inclination. Some of the other parameter can be best understood by learning it at the track.

So on the topic of KPI. Can anyone give a reason to use anything other than zero. Excluding using KPI to fix a packaging problem.

John Burford

[Bill Kunst]

John and all,
This is my first post, and maybe I could give some insight to the KPI. Excluding packaging: great little document I found by searching the net, and it came up as either the 2nd or 3rd item.
http://www.eng.fsu.edu/~hollis/eml4930-f04/Chapter_19-a.doc (http://www.eng.fsu.edu/%7Ehollis/eml4930-f04/Chapter_19-a.doc)

I don't know what book this is from, but besides your KPI issues, Davari, it has other useful information on why you have your number(i.e. we wanted the car to have neutral steering).

Hope this helps,
Bill

[KU_Racing]

John: the kingpin angle effects the angle of the axis about which the wheel rotates when it is steered. That means it affects caster, caster trail, scrub radius, camber gain, etc. I would say 0 kingpin inclination would not be a terrible good idea- think about the wheels of a shopping cart. When they are turned, they stay in the vertical axis. That camber gain from steering is necessary to help fight roll, as well as giving the tires grip in steering.

[KU_Racing]

just now read bill's post. he beat me to it.

[^_^]

KU_Racing, you have no idea what you are talking about. KPI addresses the front view angle of the kingpin axis. KPI causes adverse camber gain that contributes to compliance camber gain. And explain to me how camber helps fight roll.

[KU_Racing]

think about why you run camber in the first place. But you are right, I was thinking about KPI from a side view plane, along the caster axis. Sorry.

[Patrick W. Crane]

The most obvious answer to the question about kpi is probably that an idealized scrub radius (whatever that may be for you) is more important that having exactly zero kpi. Also, you can add kpi if you are running lots of caster with offset to reduce your camber gain. So, say for example that you want zero scrub, but need a pile of kpi to make it work, you could throw a bunch of extra caster at the car and off set the caster axis to keep your mechanical trail smaller. In the end it's not nealy that simple, but it ilustrates the point.

[jonnycowboy]

thanks that helps!! http://fsae.com/groupee_common/emoticons/icon_smile.gif

[^_^]

Patrick is leaving out a very important issue that a lot of people seem to overlook. A lot of the justification for zero scrub radius is to reduce the kickback through the steering wheel on rough terrain. Sure, adjusting the KPI you can get to zero scrub radius, but SCRUB RADIUS IS NOT THE INPUT MOMENT ARM FOR LONGITUDINAL ROAD EXCITATIONS. When you hit bumbs in the road, they are reacted at the wheel center rather than than at the contact patch (when not braking). This means that the spindle length is actually the input arm, not the scrub radius. High KPI will get you to 0 scrub radius, but you probably won't change your spindle length much in the process- so don't make extreme design concessions unless you can fully justify them.

[Patrick W. Crane]

are you coming from Baja? cause most fsae cars aren't tuned to deal with large bumps. they are more geared towards lateral acceleration.

just a thought

also, changing kpi will affect your effective spindle length at the wheel centre. it doesn't matter where the force acts, it still has to turn the upright about the ball joints..

[^_^]

I never said whether or not it was good or bad, I merely made a distinction between the different moment arms over which a longitudinal disturbance acts.

[Patrick W. Crane]

fair enough

[Andrew Nabb]

I can't speak for everyone, but I did have a rather detailed conversation with a design judge about this in 2004. We run some KPI to get the scrub radius down. We do have to run some significant castor to get back to the camber change required when turning the wheel. Just to sum it up for the new team, i would reccomend keeping your low caster trail and scrub radius, however i think you can go slightler larger on these values if needed. Remember if you have zero trail and zero scrub, it will be pretty hard for the driver to feel any feedback through the wheel. One of the old cars before my time had somewhere between 1.75 and 2 inches of scrub, and it was nearly undriveable. Pick something imbetween.

I am not sure if teams are really keeping these values low because they are worried about kickback to the wheel.....I think most teams (like us) are worried about having hideous steering effort. We often have to turn the wheel with the car stopped, and if you have a hand clutch (most teams do) you will want to make sure the driver can turn the wheel with the clutch pulled in (ie with one hand). Of course if you have chuck norris driving......

Scrub also comes into effect when braking on split mu (ie ice on one side, pavement on the other) surfaces, but unless your scrub radius is really large (inches) then you probally will not have much of an issue. If you are driving the front wheels, there are also some important issues with scrub radius ....but not many teams have awd.

If anyone is interested i have a Mark Ortiz newsletter from august 2002 which talks alot about these steering parameters.

[kwancho]

I'd take it.
kwancho@stanford.edu

[Marshall Grice]

you can have it and more from his forum.

click here (http://www.auto-ware.com/ubbthreads/postlist.php?Cat=0&Board=knowledgebase&page=)

[kwancho]

Aww shiet. Well there goes all my spring break free time. http://fsae.com/groupee_common/emoticons/icon_wink.gif