Suspension Basics

[Lemon Lime]

Hello Everyone , I'm really new to FS and I was just wondering if you can help me sorting this one out ; Alright I was wondering if a great caster angle ( Say like 11-14 degrees ) for the front wheels would affect the understeering value or whether gonna change steering at all ?! and I was wondering if a great kingpin inclination would help in steering ( WHY USE KINGPIN OTHER THAN THE FACT THAT IT HELPS TO REDUCE STEERING FORCE ) One last thing ; I was looking for good books about suspension and kinematics any help on that ?! ... Lots of questions I know ... Sorry !! ... Thanks alot in advance and Thanks alot for your attention and your time . http://fsae.com/groupee_common/emoticons/icon_smile.gif

[paufdencamp]

For a cursory treatment of the topic, Caroll Smith isn't bad, though personally I found him kind of hand wavey at times. Milliken's book is great, RCVD is what I live by when it comes to this competition. The problem you have with excessive kingpin inclination is in extreme cases it can lead to excessive camber which can reduce grip, but the more immediate problem in my opinion/experience is the reduction of valuable steering feedback. Read Chapter 2 of RCVD where he talks about aligning torque as a function of slip angles and lateral G's, that should clarify why excessive kingpin angles aren't that great for this competition.

[BillCobb]

Caster has a large effect on vehicle understeer but it MAY be the only understeer component in your car.

[Lemon Lime]

@ paufdencamp ... Thanks alot for the valuable information ... infact I've read Carroll Smith's Tune to win and I'm currently reading RCVD ( Race Car Vehicle Dynamics) but I was lookin' for more advanced tips and more about kinematics .. Anyway , Thanks for your valuable time Apperciated http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Lemon Lime]

@ BillCobb .. Well Thanks alot for your reply ... But I may find that your reply is rather a bit twitchy or not clear enough ... If the caster angle is increased does that mean that the car will tend to understeer more ?! Thanks in Advance and Thanks for your reply and for your valuable time http://fsae.com/groupee_common/emoticons/icon_smile.gif

[BillCobb]

The car will understeer more because there will be a 'great' (your words) increase in the steering system loads. This mainly causes flexure in any of the steering system components: steer arms, tie rods, steering gear teeth and gear separation mechanisms. gear mounts and structure, steering shaft, U-joints and on and on upwards. (That's why I always recommend you pull on a tierod, pushing is nicht so gut).

The effect on tierod load gradient with lateral acceleration is the 'driving force' (that's a good one) to add caster so that the tendency to mistake max steering effort with max lateral g is avoided. The use of the camber by steer mechanism (which is the true way to measure and indicate caster) is somewhat dulled in FSAE because of the low steer angles and the pronounced weakness of camber gain of the 'radial' tires.

Since tierod load relaxes the front slip angle gradient, its 'understeering'.

FSAE cars are rear weight biased (oversteering with same wheels, tires and pressures front & rear), they don't roll a lot (so no effective roll understeering geometry), and a short wheelbase (so low Ackerman gradient) there's not a lot left to understeer the car other than the heavily biased forward TLLTD from springs, bars and force based roll centers. Yes, the longitudinal components for sideforce can help you at low speed. (In God we trust, all others bring Data). But the faster you go the less this plays out.

There is an optimum caster setting, but this requires YOUR tire's data, the roll system effects (caster rocks) (did I say that?), and a decent simulation with a proper test procedure to analyze it.

10 --> 12 deg. is a LOT of caster. You're gonna need power steering if your steering ratio is under 10:1 AND a steering wheel diameter that will hit the ground running (OUCH !!). Even Danica couldn't 'handle' that one...

[Z]

Lemon Lime,

There is a thread permanently at the top of the first page of threads that give many links to useful resources. There is also the "Find" button (at top-left) that will help you search through the many posts on this forum. Use keywords like <trail caster castor offset scrub steer-axis king-pin kpi> etc., and note the different possible spellings.

You will find a lot of very useful information on this forum, but unfortunately it is buried under a whole lot more that is less than useful. It is your job to sift the wheat from the chaff. So start unlocking those locks. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

PS. I have to disagree with the gist of Bill's post above. Once again Bill is confusing the effect of increased Castor with that of increased Trail. Large changes to Castor, in itself (ie. with Trail, Offset, and Steer-Axis-Inclination (= KPI) all set to zero), only gives relatively small changes to steering linkage loads. However, increasing Castor in such a way that you also increase Trail, does result in large changes to steering loads.

PPS. Just as a starting point you might try;
Offset (= scrub radius) = 0,
SAI (= KPI) = 0.
Trail = 0 - 3 cm (0-1"),
Castor = 0 - 3 degrees.

[Tim.Wright]

...confusing the effect of increased Castor with that of increased Trail.

I see this confusion quite a lot. I think its due to the fact that while theoretically caster angle and trail are different things, in the space of a small wheel its often not possible to change one and not the other.

I'd say that added caster angle would make the car less understeering because you have the increased the camber thrust coming from the front tyres.

[Z]

Also adding greatly to the confusion are those little wheels found under office chairs, etc. They are called "casters" (or "castors"???), their behaviour is called "the castor effect", but in the terminology of vehicle steering systems they usually have zero Castor-angle, and a whole lot of Trail!

Ain't words wonderful! http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

Z

[Pete Marsh]

Also common, confusion between "understeering" and "Understeer gradient"

Bill is referring to the change in understeer gradient in deg/G as measured at the steering wheel. A big deal in road car feel. He makes no comment on which tyre pair may reach their traction limits first.

Tim, on the other hand, doesn't care how much he has to move the wheel, but but would like to know if his imminent crash (assuming we are going to find the traction limits) will damage the front or back of his car.

Pete

[Tim.Wright]

I'd say the camber effect on the front axle would decrease the understeer gradient at the steering wheel.

In steady state cornering, the total front axle and total rear axle lateral forces depend only on the longitudinal CG location. They are largely independent of camber angle and other small details.

So for arguments sake consider you are cornering at 1G and have 1500N of lateral cornering force on the front axle. Say this uses 5deg of slip angle.

If you increase the caster induced camber of the front axle, you are increasing the cornering force contribution coming from camber thrust, but still at 1G you will have 1500N of lateral force on the front axle. Only in this case, more of that is coming from camber thrust and less is coming from the slip angle. So therefore, the slip angle will be less.

Less slip angle on the front means less understeer because you need less steer angle to track the same radius as the case with less camber.

Tim

[Lemon Lime]

WOOOOOW ... That's a subject's drift here ... Well Thank you all a lot for the valuable information and help ... Sorry if my question was really bad and a bit pointless ... FIRST YEAR IF YOU KNOW WHAT I MEAN !!! http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[Tim.Wright]

As usual in vehicle dynamics, you have as many different answers to your question as people who answered it.

Enjoy!