Ackerman/Steering Arm vs. KPI

[Sathersc]

Hey,

I hope everyones cars are comming along so far, just a few short weeks until competition. I'm working on the suspension design for our first car in 2007, and I've run into a bit of a problem.

When I use the KPI and caster angles I like, I have to give up anything near a good % of my ackerman angle to get a steering arm I can get any mechanical advantage. So what is your take on increasing KPI vs giving up an ackerman advantage.

As I understand it, I'm increasing my steering effort by increasing KPI, along with gaining adverse camber effects, but if I give in on the ackerman side, my rotation percentage of my outside wheel to my inside wheel will decrese, meaning I have to turn the wheels more at low speed, meaning I increase my camber problem. I feel like I'm missing part of the picture here, so I'm opening it up for discussion.

Sorry for the fact I'm wordy as all get out, but it's 3am on a saturday and, well, you know.

[Sathersc]

Hey,

I hope everyones cars are comming along so far, just a few short weeks until competition. I'm working on the suspension design for our first car in 2007, and I've run into a bit of a problem.

When I use the KPI and caster angles I like, I have to give up anything near a good % of my ackerman angle to get a steering arm I can get any mechanical advantage. So what is your take on increasing KPI vs giving up an ackerman advantage.

As I understand it, I'm increasing my steering effort by increasing KPI, along with gaining adverse camber effects, but if I give in on the ackerman side, my rotation percentage of my outside wheel to my inside wheel will decrese, meaning I have to turn the wheels more at low speed, meaning I increase my camber problem. I feel like I'm missing part of the picture here, so I'm opening it up for discussion.

Sorry for the fact I'm wordy as all get out, but it's 3am on a saturday and, well, you know.

[PatClarke]

Sathersc,
My suggestion would be to go gather up all the SAI you find on your car, design, lying around the shop, on your hard drive as well as any stashed away by team members.
Take it outside and drop it in the dumpster =]

Now, with no (or very little) SAI left on your car, you can find some room for that Ackermann.

While you are gathering up the SAI keep an eye open for any small bit of scrub that might be lying around. A little bit more put into your car probably won't hurt and will make a little more room for that Ackermann stuff ;-)
Regards
Pat Clarke

PS, you might need to look around for a bit of spare caster too!

[Storbeck]

I have no idea how much kpi is acceptable and how much isn't, but one thing that was eye opening for me when I was debating the same compromise was when I actually calculated the camber effects of kpi. I constrained a simple assembly in my favorite cad program and played with changing kingpin, caster, and steering angles to see the results. Helped me make up my mind.

[fsae racer]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Suddenlee:
Sathersc,
My suggestion would be to go gather up all the SAI you find on your car, design, lying around the shop, on your hard drive as well as any stashed away by team members.
Take it outside and drop it in the dumpster =]

Now, with no (or very little) SAI left on your car, you can find some room for that Ackermann.

While you are gathering up the SAI keep an eye open for any small bit of scrub that might be lying around. A little bit more put into your car probably won't hurt and will make a little more room for that Ackermann stuff ;-)
Regards
Pat Clarke

PS, you might need to look around for a bit of spare caster too! </div></BLOCKQUOTE>

Sounds like a winner to me. Also, last time I checked, it was caster that has a larger effect on required steering force (if your spindle is on your steerin axis that is).

[Nick McNaughton]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Suddenlee:
Sathersc,
My suggestion would be to go gather up all the SAI you find on your car, design, lying around the shop, on your hard drive as well as any stashed away by team members.
Take it outside and drop it in the dumpster =]

Now, with no (or very little) SAI left on your car, you can find some room for that Ackermann.

While you are gathering up the SAI keep an eye open for any small bit of scrub that might be lying around. A little bit more put into your car probably won't hurt and will make a little more room for that Ackermann stuff ;-)
Regards
Pat Clarke

PS, you might need to look around for a bit of spare caster too! </div></BLOCKQUOTE>

Pat,
While I can appreciate the intent of your post, is a little KPI really such a bad thing?
With modern software tools the effect of KPI on camber with steering is easily seen, and it's entirely possible to have some KPI to achieve other goals with very small undesireable effects on camber.

Is the anti-KPI sentiment a carryover from times before rapid 3D kinematic analysis was easy? Is it one of those rules of thumb learned by experiment years ago, got a bad wrap and has been passed on as a 'don't do this' since then? Or is it common opinion that KPI is the wrong compromise to make..?

Cheers,
Nick

[Frank]

the only use for KPI...

is to use it in congunction with generous amounts of static negative camber, and castor

and it's much better on a car that dosn't steer much

[PatClarke]

Hi Nick (and Frank)

Yes, you have to take my original advice with a little tongue in cheek.

Sure, a little KPI doesn't hurt, and there are reasons why you would have it. I usually see a little KPI creep back into cars as the team mature and get a handle on what is actually happening. I think your hypothesis on 2D and 3d is valid, but probably more in the mind or imagination of the designer that in software.
In practical terms, when making uprights it might be easier to machine them so the axle C/L is at right angles to the steering axis. That way the KPI would equal the Camber, and as all cars run some negative Camber, they would therefore have a little positive KPI.
Still, you cannot discuss any of the steering angles without taking all the others into account.
What most teams don't look at is the effect of Caster and KPI on corner weights.

While I am on the subject, some Ackermann stuff published out there assumes the car will have no Caster, KPI or Scrub offset, and so only look at toe out gain (or loss) in 2D. I think the weight jacking effect of Caster and KPI are very important!

Something designers of FSAE cars don't take into account is that the corner weights on these cars is (or should be) very low, say 50kg. For the driver to 'feel' the car he needs to be doing a little work, hence the advice to add a little scrub (it would also give him some room to add that Ackermann he was looking for).

In closing, I dont think KPI is a wrong compromise! But I know when I see a car with a huge amount of KPI that the designer hasn't got to grips with either the geometrical or packaging problem. As a Design Judge I would never knock points off a car with KPI as long as the designer can cogently argue in favour of his decision.

Regards to all
Pat

[Storbeck]

Warning! I'm about to ramble:

Pat mentions that a little scrub radius might be a good thing, because it lets the driver feel the car. It seems like the moment arm caused by scrub radius wouldn't do much for driver feel because it does not give much feedback from cornering force, but mostly from braking force. However the scrub radius would contribute significantly to the weight jacking due to caster or kpi, and to the steering effort caused by that weight jacking. So it seems like scrub radius is not a good way to get driver feedback if you are trying to minimize weight jacking from steering geometry. It would raise steering effort and increase weight jacking without significantly effecting feedback.

Now trail (by trail I mean the distance form steering axis to center of contact patch in the direction parallel to the car) seems like it would cause a direct relationship between steering effort and lateral force, which might be what you want for driver feedback. It would also contribute to weight jacking which could be good or bad. So more trail give you more driver feedback?

Or does it? Since actual trail is a combination of mechanical trail from suspension geometry and pnumatic trail from the tires, it seems like more mechanical trail would make it so the pneumatic trail is a smaller percentage overall. It seems there would be more valuable feedback from the variation in steering effort caused by changes in pnumatic trail than the variations of steering effort caused by lateral force. Based on that logic I'd want relatively small trail.

So based on all of that logic, I designed my steering geometry so that it had a half inch of scrub radius, so that it would come out to roughly zero when the tire flexes in a corner (might not be enough?), as little kpi as possible to give me enough clearance for the ackerman I want (how much is debateable but my conclusion was that I want a lot ackerman) and added caster till my camber gain due to steering came out to what I wanted (don't really know what I want, but made some educated guesses based on previous cars), then set my trail to what I wanted, which was not very much (again a guess based on previous years cars)

So that leaves me with the following questions:

Am I way off in my logic? My conclusion was that in the compromise between kpi and scrub radius, I'd rather have more kpi to get the desired scrub radius, as long as other things are adjusted accordingly.

Why do I see a lot of discussion about caster, scrub radius, and kpi, but usually no mention of trail, which seem to be as important or more so than any of that stuff?

Why do people seem to always put the stub axle in line with the steering axis, regaurdless of the caster, kpi, scrub radius and even tire diameter used? If all that stuff is the same, and a smaller tire diameter is used, the trail will get smaller. ("switching from 13 inch rims to 10 inch rims gave us way better feedback, must be the tires...")

What would be the disadvantage to geometry that has a few degrees of kpi (or even a lot of kpi, say 6 or 7 degrees), but the same scrub radius, trail, camber change with steering on the outside wheel(from more caster) compared to one with no kpi, or less kpi?

Is the above explanation the sort of thing the judges are looking for? Probably would be better if some testing on the various things was included, but I simply don't and won't have that by competition. The only one of those parameters adjustable on my car is caster, and scrub radius (with spacers), because those are simple and easy, and we won't be done in time to do that kind of testing anyway (reality sucks).

Why is pondering this stuff so much more interesting than studying for finals, and how the hell am I going to not fail AND get the car done?

[Sathersc]

That sounds like you're kind of going in circles, which is my problem. Only, unlike you, I have no previous cars to base any guesses off of, and no tangible way to predict how any of my calculations will turn out untill we drive the car. The good news is, we should have a rolling chassis by mid-october to early november, so I've got time and a half to test.

At this point I'm fairly happy with my guess on KPI and caster and their respective trails, so what i'm really trying to figure out is since I have a set distance between my upper joint and my rotor, what's the best angle I can get between my a-arm joint and my steering joint, without giving up too much mechanical advantage.

[CMURacing - Prometheus]

When we looked at steering geometries for 06, we set goals based on previous cars:

increase mechanical trail such that "pneumatic" trail is not a driving factor in steering forces (it still plays a role, but the wheel doesn't pull away from you in a full lock scenario, a bad idea)

decrease kpi for camber gain reasons (as small as possible)

scrub radius: "run what you brung" (milliken points out differential braking forces may cause steer effects, but on the new smooth MPG track, we don't have surface imperfections that may cause this), which ended up being small anyway.

remember that the angle between your steering joint and your upper BJ will determine a lot of bump/roll steer and ackerman parameters. its best to use a kinematics program/spreadsheet to do this, becuase the effects are funky.