Suspension Caster/Kingpin Offset?

[Jackson]

Anybody have any good places to start? I was thinking of starting with 2.5 degrees of caster and around 7 degrees of kingpin offset with an inch or two of scrub radius.

Anyone care to clue me in on whether or not this is a good first iteration? I am not looking for solid numbers (no team secrets or anything, just ballpark figures). Of course the best numbers will be different for every config, but it would be nice to know where my current design stands before I start making the metal chips fly...

Brian
Wazzu Formula

[Sleeque]

Hello there Brian,
Swap those two angles over and you will be MUCH closer to what you need =]
There is a good paper available on the FSAE Australian website that will give you good tips.
Slan leat
Slique

[John Gregor]

At the comp in May i saw teams with no caster and some with as much as 12 deg.
As for kpi, have a good read of the vehicle dynamics books that are out there.

A really good start is Pat Clarke's FSAE guide http://www.sae-a.com.au/fsae/tech_specs.htm .

[Frank]

no kingpin

0 to -1 deg camber

5 to 10 deg castor

20-30mm scrub
20-30mm trail

100% ackerman

no static toe

$0.02

[dancin stu]

just a question about scrub radius, i was thinking of running close to zero on next years car, last couple of years have tried to get as low as possible to zero but have ended up with 20mm cos of 10 inch wheels. any reasons why people are running 30mm??

[Sam]

dude, if you had close to zero scrub radius you'd have close to zero feedback. may as well be driving with a playstation controller!

Sam Graham
Engine Group Leader 2003
UQ Racing

[Denny Trimble]

Here is my understanding of caster, KPI, scrub radius, and mechanical trail.

Camber Affects:
<UL TYPE=SQUARE>
<LI>More Caster = good camber affects (negative on outside, positive on inside) as more steering is added. So, it's a very-low-speed-corner affect only (Mr. Smith, rest his soul, hadn't heard of this in '99, or at least he didn't believe me, but he was praising it by 2002. Real race cars see ~5 degrees steering; we see 20+).

<LI>More KPI = positive camber change with any steering input (bad for outside tire)
[/list]

Weight Affects:
<UL TYPE=SQUARE>
<LI>Caster * Scrub Radius = diagonal weight jacking in the oversteer direction, as steering is added. Good for getting the car to rotate in very tight corners, when yaw accelerations are larger therefore understeer normally prevails.
Note that this is also dependent on the torsional stiffness of your frame+suspension.

<LI>KPI * Scrub Radius = weight lifting (jacking up the front end of the car), increases as steering is added. Not much reason for this.
[/list]

Lateral Force Affects:
<UL TYPE=SQUARE>
<LI>(Mechanical Trail + Pneumatic Trail) * Lateral Force <contributes to> steering wheel torque.
[/list]

Braking Affects:
<UL TYPE=SQUARE>
<LI>Scrub Radius * Braking Force Difference between Left and Right Front Wheels = Steering Wheel Kickback. This is an indicator of one wheel locking up or grabbing harder then the other due to surface irregularities. Too much can overwhelm, just right will give good feedback to help the driver brake at the limit.
[/list]

Southern Hemisphere Affects:
<UL TYPE=SQUARE>
<LI>KPI*Track Width/Sqrt(atan(RC Height/pi)) = probability that a drop bear will win the event.
[/list]

University of Washington Formula SAE ('98, '99, '03)

[Charlie]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Denny Trimble:
Southern Hemisphere Affects:_

+ KPI*Track Width/Sqrt(atan(RC Height/pi)) = probability that a drop bear will win the event.
<HR></BLOCKQUOTE>

Denny-

Good thing you are not going to the FSAE-A event, because this is dead wrong. We have been doing testing simulating the 'down under' environment by filling our tires with salt water, and driving in reverse. From this simulation we've found a KPI of -5 to -28 degrees is optimal.

As for scrub radius, our tires are already round, they come that way from the factory. http://fsae.com/groupee_common/emoticons/icon_wink.gif

-Charlie Ping
Auburn University FSAE (http://eng.auburn.edu/organizations/SAE/AUFSAE)
5th Overall Detroit 2003
? Overall Aussie 2003. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Jackson]

You guys are hilarious... Or maybe I just enjoy engineer humour... Go figure.

What is a drop bear anyway, I see everyone making fun of the aussies with that, but have NO idea.

Thanks for the Suggestions. I'll Bump up the caster and try to get rid of the KPI. The finals will all be in the packaging of course!

I find it funny that someone from our major rival school (in sports anyway) gave one of the more engaging answers-- Thanks Denny.

Brian
Wazzu Formula

[Charlie]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Jackson:
I find it funny that someone from our major rival school (in sports anyway) gave one of the more engaging answers-- Thanks Denny.

Brian
Wazzu Formula<HR></BLOCKQUOTE>

It is a trap-- Go with my advice. http://fsae.com/groupee_common/emoticons/icon_wink.gif

Sorry, I have been drinking. Gotta prepare for Australia you know. http://fsae.com/groupee_common/emoticons/icon_wink.gif

-Charlie Ping
Auburn University FSAE (http://eng.auburn.edu/organizations/SAE/AUFSAE)
5th Overall Detroit 2003
? Overall Aussie 2003. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[woollymoof]

Drop bears are little creatures, a bit bigger than a Koala that hang about in the trees. If you walk under a tree that they are in they'll drop out, land on your head and suck your brains out. One of Australia's most feared native animals, especially by the tourists. They're somehow related to the hoop snake.

Cheers,

Kirk Veitch
Swinburne University of Technology

[Denny Trimble]

Brian,
I like to share... and the execution (packaging, manufacturing well, testing, tuning, driving) is still the hard part, IMHO. Plus, I learned a heck of a lot last year just talking to other teams in Pontiac. The judges are a pretty good source of info too, especially after the design judging. Bounce your ideas off them!

All About Dropbears (http://fsae.com/eve/forums?a=tpc&s=763607348&f=125607348&m=5996039723&r=6946014853#6946014853)

University of Washington Formula SAE ('98, '99, '03)

[Engelbert]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Sam:
dude, if you had close to zero scrub radius you'd have close to zero feedback. may as well be driving with a playstation controller!

Sam Graham
Engine Group Leader 2003
UQ Racing <HR></BLOCKQUOTE>

SamG,

zero scrub would cause (close to) zero feedback in braking loads only, not cornering.

Cornering feedback is (mostly) due to trail, not scrub.

I say 'mostly' and 'close to', due to large steering angles seen in FSAE.

But you are right, zero scrub would be a biartch to drive with....

SamM.

[Frank]

one of the most important things i picked up from the Claude Roulle seminar... had to do with the notion of tyre deformation.....

and that "scrub radius" and "castor trail" is usually expressed as a static measurement....

while in reality the tyre deformation (especially with as little pressure as we run) is HUGE...

therefore the "dynamic scrub radius / castor trail" might not be at all what you think it is...

say a car with 30mm scrub / 30mm trail... at 0.7g cornering and 0.7g braking simultaneously (ahhh "trail braking" http://fsae.com/groupee_common/emoticons/icon_smile.gif whatever)...

the dynamic castor trail might be more like 55mm and the dynamic scrub radius near 0mm

my gut feeling is that these static values (and the "feedback" they give) are....

A: difficult to mess with (in most cases a certain amount of re-design)... in many designs changing JUST ONE variable is difficult to achieve

B: effected by more than just g's present... for example i think the "ergonomic factors" with the controls play a big part

C: im also wary that deflection / free play in the whole car might well be causing geometries that you don't think are there (ie shouldn't be there)

$0.02

if you added up all the $0.02 cents I've dropped here this year, you could by a few beers... but not drop bear insurance

Regards all

Frank

[sTaRk]

can any one provide any formula or relation between castor, kingpin, steering angle and camber change.

I tried to search a lot but didn't find ny thng.

[Tim.Wright]

yes

[sTaRk]

what is it?

[Francis Gagné]

You should be able to work formulas or write a program for this using the following principles :

http://en.wikipedia.org/wiki/Trigonometry
http://en.wikipedia.org/wiki/Rack_and_pinion
http://inside.mines.edu/~gmurr...bitraryAxisRotation/ (http://inside.mines.edu/%7Egmurray/ArbitraryAxisRotation/)

That should get you started. Start with simple simplification assumptions and make more "precise" if needed.

[Warpspeed]

Originally posted by sTaRk:
can any one provide any formula or relation between castor, kingpin, steering angle and camber change.


The key to this is finding an Astrologer that you trust.
He/She can work out the planetary alignments for your car and give sound advice.

[Tim.Wright]

Originally posted by Tim.Wright:
yes
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by sTaRk:
what is it? </div></BLOCKQUOTE>

Ah sorry I meant no. Stupid keyboard...

[Tim.Wright]

But seriosly, did you really look? There was exactly this discussion on this forum about a year or so ago. Somone even wrote out the formula.

[Moreboost]

If there are 5 drop bears 7 trees and a galah then KPI is 3, for all other conditions its 7.