Kingpin axle axis

[Goyds]

G'day everyone

One day I hope to be able to contribute to the forum rather than always asking questions but i have one that I’ve been unable to find a decent answer for.

I was looking through a couple of textbooks and over some of our older designs when i noticed that on some of our vehicles the front axle line is forward of the kingpin axis. It would seem to me that this would make the steering unstable, kind of like a shopping cart wheel facing the wrong way. I would have thought that having the axle and kingpin axis intercept or slightly trailing would be better for stability.

Those of you who saw our vehicle during the acceleration event in the 2011 Australian comp will have seen how unstable the vehicle was under brakes and so I wonder if this may have been the cause.

Thanks again for the help
Lloyd Morrissey
Chief Engineer
Team Swinburne 2012

[PatClarke]

Draw it up in side elevation and all will become clear!

Cheers

Pat

[Goyds]

G'day Pat

That’s exactly what I had done and I wanted to check my reasoning. Thanks for the reply.

Oh and I just noticed the little bug animation under your name, I’ve been watching it on the forums for a while assuming it was on my screen and I only just tried to squash it...

Lloyd Morrissey
Chief Engineer
Team Swinburne 2012

[Goyds]

G'day everyone

I thought I’d post this in case anyone is interested.
Thanks to Pats encouragement I’ve been back over my drawings and I can see that having the axle axis forward of the kingpin axis will make the steering behave similar to having negative caster and will make the vehicle unstable. This also explains why the vehicle began to settle down when we started to run (what we thought was) huge positive caster when in reality it probably only just hid the bulk of the problems. This also explains our front tire wear patterns. Thanks again for the help Pat.

Lloyd Morrissey
Chief Engineer
Team Swinburne 2012

[rmk36]

How must caster angle does your current set-up have?

[Z]

Lloyd,

A kingpin axis behind the wheel axis is quite common. Nothing bad comes of it, IN ITSELF. In fact, it can work quite well.

Any problems you had were not JUST because the KP was behind the axle. Best you look elsewhere...

Z

[Goyds]

G'day Z

Good to hear from you as well.

I suppose I was hoping for there to be one discrete mistake that we made last year however it is beginning to look like it was several mistakes that added together to a very major one.

The exact problem I am trying to diagnose is the steering tended to oscillate under braking as the driver turned in. Depending on the strength and skill of the driver it was either a minor problem or broadly dangerous.

The observations we have made about the 2011 car’s front end is basically: the front end is too soft with the suspension hitting the bump stops under brakes (very sever dive), the steering toes out under bump (and dive/brakes), The steering arms are very short which magnifies the steering compliance and bump steer into major wheel movements and the above mentioned concern about the kingpin and axle axis.

During post comp testing we found that the vehicle behaved much better with 10+ degrees of caster as well as 4mm static toe in to address the bump steer. This reduced the oscillations however this lead to very uneven tire wear. Also while the car started to behave well I always felt it was lacking pace and I couldn’t help but feel that we sacrificed a huge amount of grip to make the car drivable.

Essentially these are all problems which we have considered and attempted to fix in our 2012 design however I have been trying to figure out which problem(s) is(are) most likely to be the root cause as there are a few design ramifications of “fixing” the above which severely limit us in other areas. I appreciate there are few absolute truths in racing car design however I would like to know if in anyone’s experienced mind these are likely to result in the behaviour I’ve described or are we barking up the wrong tree.


Thanks for the help
Lloyd Morrissey
Chief Engineer
Team Swinburne2012

[Z]

Lloyd,

Diagnosis is easier when more details are available, so could you post some more numbers/drawings/pics, etc.

Meanwhile, IMO, the two most likely causes of your problems are "bump-toe-out" and "short steer-arms". If you have (enough) bump-toe-out at the rear, then the car is completely undriveable. At the front you can get away with it because the driver can compensate, as you noted. Short steer-arms (or "short toe-base" at the rear) is madness. There is negligible cost to having longer steer-arms (or wider rear toe-base), and lots of benefit, mainly precision of control.
~~~o0o~~~

Regarding kingpin (steer-axis) geometry, there are some things that happen at the wheel-axle, but most of the action is at ground level. Here are the four main parameters, taken wrt the wheelprint centre:

1. Scrub radius, or lateral offset (y-axis) of KP passing through the ground plane.

2. Trail, or longitudinal offset (x-axis) of KP...

3. KP Inclination, or front-view slope of KP from vertical.

4. Castor, or side-view slope of KP...
~~~o0o~~~

I would suggest that the best starting point for the above is all zeros! That is, a vertical steer-axis passing through the wheelprint centre. This works remarkably well (yet, for whatever reasons, people always want to complicate it!!! http://fsae.com/groupee_common/emoticons/icon_smile.gif).

For more steering feel (ie. self-centering), especially at the limit of grip, add more Trail. For both wheels leaning into a corner, add more Castor. Note that with most suspension setups, when you adjust one of these, the other also changes. Try to keep Scrub and KPI at zero, or as small as you can. What you now have is like that on a bike, with steer-axis typically passing behind wheel-axle.

I suspect that whoever did your 2011 car was trying to get lots of Castor-camber effect, while keeping smallish Trail, hence the "KP behind wheel-axle". But with 10+ degrees Castor, you get 5+ degrees "negative" camber at 30 degrees steer-angle, which is probably too much for good grip or wear.

Z

[Goyds]

G'day Z

Thanks for coming back and checking this post, i appreciate the help.

(Hopefully) Attached are some images of what i can measure from our 2011 design. They don’t seem to match our models very well which is worrying...

As a note for other readers: I’m happy to post these as they didn’t work for us however I can't recommend copying this design or looking to much at it and saying "well it didn’t work for them so we shouldn’t do it". There are a lot of parameters in these drawings and in different designs they might work more successfully (or not).

The arrow points forward. Sizes are approximate but should be accurate to within 1-2mm/degrees or so.

I think the scrub radius may actualy be larger than shown but we are missing a few parts from the old car so i'm having difficulty measuring it. The scrub radius was a victim of the rims hitting the a-arms. Oh and the 245mm dimension in the first drawing is actualy 260mm.

http://i50.tinypic.com/651tno.png

http://i49.tinypic.com/n4wpad.png

[Z]

Lloyd,

Thanks, this is always easier with some hard numbers.

1. The 16mm of KP behind wheel-axle is NOT a problem (IMO).

2. Your bump-toe-out is, in general, a BAD thing, but it depends on how big it is. However, it should be easy to fix. Either lower the R&P, or raise the tie-rod outer-BJs (where they attach to the steer-arms). It is a good idea to design-in this sort of adjustability, if only to compensate for build errors. Using a variable number of washers to "raise or lower" works well enough.

3. The 52mm steer-arms make me queasy! You say that the Scrub may be bigger than the 35mm shown, which means that the load on the steering tie-rods, their BJs, the rack/pinion teeth when one wheel lifted, etc., is almost as much as the TOTAL REARWARD BRAKING FORCE AT THE WHEELPRINT! (Apologies for the shouting http://fsae.com/groupee_common/emoticons/icon_smile.gif, but that is a frighteningly big load!). As you noted, this causes compliance, which exacerbates the toe-out, which...

I strongly suggest at least 100+mm steer-arms for your next car. This means a longer travel rack, but it is more lightly stressed. Or try a 90 degree bevel gear box, with rearward facing Pitman arm at the bottom, tie-roded to your forward steer-arms, as I have described elsewhere (it gives really good Ackermann). BTW, with R&P you want about 60 degree angle between the forward facing steer-arm and the tie-rod for acceptable Ackermann.

4. The combination of quite large Scrub + quite large Castor gives go-kart like "wedging" of the chassis (increased load on front-inner and rear-outer wheels), which affects handling and is hard work for the driver. To fix this, and other such interelated problems, I again suggest minimising both Scrub and KPI, to zero if possible.

Your 260mm outer wishbone spacing means you have plenty of room for discs inboard of the uprights. These are quite common in FSAE nowadays, and you may already have them? These make "centre-plane" KPs easy. If you have outboard discs, then you should still push the KP further out, even if it means bending your upright around the disc. If this also means "y" shaped wishbones, then so be it. You might suffer 1 kg total weight gain from heavier wishbones, but the car will be driveable. Scrub = 0, + KPI = 0, = Good!

5. Errr..., that's all I can think of for now. Any experienced "Steering Guys" want to comment? http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

[Jay Lawrence]

I am not a steering guy at all, let alone an experienced one, so feel free to laugh heartily at my folly (and perhaps point me in the right direction), but wouldn't a 'backwards' caster angle like the one shown create inherent instability? I look at it and imagine that everytime the contact patch moves 'forwards' the tyre would try to turn around on itself (as a typical caster wheel does).

Definitely agree with the scariness of the toe base...

[Kevin Hayward]

Some comments that may help:

The caster offset (KP behind wheel-axle) is not a problem, and I would go as far as argue it is probably a good thing. You could probably even put a bit more in to make it easier to get a longer steering arm, which is your biggest issue.

The steering arm at 90 degrees to the caster line will most likely give you bump steer if you have the rack aligned with the lower arm. A better approximation would be this line being horizontal. This is hard to tell without seeing rack mounts etc.

The length of the tie rod is critical for bump steer. If it is aligned with the lower wishbone it should be the same length (or incredibly close to).

Scrub radius is high. Probably only want 10-15mm max.

The steering arm is very short, With this layout you should be able to get 70mm at least. Longer is better, but will mean switching to a layout similar to what Z suggests. The pitman arms with a steering drop-box can be difficult to get a template through.

Having done both inboard and outboard brakes I would say it is harder to get Centre-Plane KPI with the inboard brakes mainly due to arm-wheel clearance issues for the large steer angles.

Kev

[acedeuce802]

You have to understand that the forces don't act through the front axle line, they act through the tire contact patch. Since the steering axis is forward of the tire contact patch (and will be even more forward when you consider the pneumatic trail), the "caster effect" is acting in the correct direction.