Rear Caster

[Carlos Torres]

Hi... Does caster have any influence in the rear suspension design?? http://fsae.com/groupee_common/emoticons/icon_confused.gif

I´ve seen teams that use positive, no caster and negative... so I thing there´s no influence, can you you confirm it to me?

Thanks

Carlos Torres
FSAE-LUZ
Venezuela

[Carlos Torres]

Hi... Does caster have any influence in the rear suspension design?? http://fsae.com/groupee_common/emoticons/icon_confused.gif

I´ve seen teams that use positive, no caster and negative... so I thing there´s no influence, can you you confirm it to me?

Thanks

Carlos Torres
FSAE-LUZ
Venezuela

[Buckingham]

Castor is roughly defined by the angle between the steering axis and vertical, in the sideview.

If there is no steering axis in the rear, it might become rather difficult to define a castor angle in the rear.

If you cannot define rear castor, it probably can't effect much.

[Wilso]

Ah, but is there a steering axis? Think about rear toe and bumpsteer. Because castor determines the aligning force of the wheel, you must consider this when designing the toe base and link. Although I guess dhaidinger is correct that it has little effect.

Stupid cars, something always changes something else...

[jack]

how flexable are your rear a-arms and toe-link? http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Buckingham]

Bumpsteer/toe:

Keep in mind that the upright probably doesn't care whether you call the control arm the toe link, or the toe link the control arm. For example, if you are measuring bump steer, which link is actually doing the steering?? You COULD adjust rear toe by adjusting the control arm instead of the toe link. It might be just as likely that it is actually the control arm which is out of alignment, causing the bump steer.

Flex:

The upright lies in a plane defined by three points. Unless one of those points is a few orders of magnitude more flexible than the other two, it probably wouldn't be correct to say that any one point will exactly rotate about the axis created by the other two points, under loading.

With those two arguments in mind, it still appears difficult to define a steer axis.

[UTA racer rikki]

dhaidinger,

You have to remember that not all ball joints are loaded equally.

Rear Castor exists but only has influence if, as someone already mentioned, you are running an all wheel steering system or a rear steer system.

If you have major amounts of castor and major amounts of bump steer in the rear, then the castor could mess with some cross weight transfer under corner exit.

The rear castor would be defined as the front is. You invariably have two fixed ball joints on your rear upright, the third will be your toe link (aptly named to adjust your toe setting). The castor angle would be defined by the two fixed ball joints. The third point would act as your "steering input." It just so happens that it is fixed if you aren't running 4 wheel steering.

Anyone thought about what kind of castor you should run for a 4 wheel steering sytem in the rear???

[Denny Trimble]

Way back in the Fall of '98 when I was designing my first FSAE suspension, I set the rear "steer axis" to intersect with the force centroid of the tire during max cornering and driving cases. Since the theoretical moments about the steer axis were zero (or close to it), I used a very small toe link and toe base.

Well, the car worked until I whacked a cone in testing, at which point the toe link rodend snapped (3/16" shank). And toe stiffness wasn't even in our minds, either.

I share this story so that hopefully nobody else will carry out this same thought process to fruition... http://fsae.com/groupee_common/emoticons/icon_smile.gif

[UTA racer rikki]

Denny,

I doubt the cone actually impacted the point where the "steering axis" interesected the ground. While the design was probably great for cornering loads, a cone impact at high g's was probably a little less pleasant.

[fade]

it depends on where your toe ball joint is situated. in plane with one of the a-arms makes it easier to control bump steer but places it closer to the wheel shrinking your moment arm. I would think ideally the toe link would be in the center of the wheel with the a arms with no caster (unless bump steer is desired for tuning). Camber adjustments would be affected and your chassis would need a suitable hard point.

[Big D]

Fade..... we do it like that.... the toe link comes off the upright directly behind the axle, and goes off to it's own point at the frame, partway between the two A-arm mounts. I don't see a lot of teams running this, but I don't know why.

another related thing I have seen is a lot of rear scrub radius on some cars. Wouldn't this produce gigantic forces in the toe links? (i.e. the wheel tries to drive a circle around the ball joint axis, and tugs on the toe link)

[Denny Trimble]

I think the "Y" upright layout (upper balljoint and toe link at the same elevation, spaced fore and aft equally from the wheel centerline, and as close to the rim as possible) is better than what you've just described. There's more "toe base", and instead of the UBJ taking most of the load and the toe link taking very little, the two upper balljoints share the load equally. Camber and Toe adjustments need to be made on each UBJ in this case, but that's not a big deal.

Yes, scrub radius is an issue in the rear relating to toe stiffness, especially with outboard brakes. With a inboard brakes, there's no reason to have any scrub radius in the rear. Unless you're planning on running 4WS and want the weight jacking to go a certain way...

[jack]

listen to all you guys and your silly 13" wheels...

...just run 10" wheels, and you get what the packaging lets you have. http://fsae.com/groupee_common/emoticons/icon_wink.gif http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[Big D]

Ok, I have a few more questions about the common "Y" layout. More force should be taken by the lower joint(s), so why is the double side of the upright put on top? are you just trying to keep the lower A-arm (and chassis) lower by setting it up as a "Y" rather than an inverted Y? Does anyone run it as an inverted Y?

Also, how do you define all the suspension variables - caster, scrub radius, etc. with such a system? Probably a plane containing the 3 points, rather than an axis?

[Denny Trimble]

Well, the Y configuration usually has a single balljoint on the bottom so it can be closer to the contact patch, to better resist lateral forces. The upper balljoints usually are closer to the hub vertically so they can be spaced more widely fore and aft, for a larger toe base.

If you invert that, your lower balljoints need to be close together if they're very low, or high up if they're spaced very far apart. The single upper ball joint can be as high as you want, but there's not much need to make it very high because of the lower forces acting on it.

This is all qualitative, and would be a good study for somebody to do some FEA on.

[Big D]

yeah, that's what I figured was the reason..... and also that it would bring your lower arms, and chassis up the same amount, bad for CG.

speaking of gravity, do you see any weight reduction from this setup?

[Denny Trimble]

Mike T could probably answer that. I know our stiffnesses (in camber and toe) have at least doubled over the last few years, with very little weight change, by going to better geometry of the upright and the control arm layout.

[Big D]

that's good to hear..... maybe we will have to make the switch to that system. Our current car has a certain rear wheel power shimmy that might go away if we went that way. That, and lose the Hummer-like track width

[Carlos Torres]

guys what do you think about this..

Use 0 caster at the upright, but using an adjustable upper rear leg of the wishbone, and when moving it (smaller linK) the caster will increase from 0 - 6 deg

The only problenm is that the camber will increase too...

What do you think? Can it be done?

[Mike T.]

Camber change via a bracket on the upper part of the 'Y' affcets the scrub radius much less that does a brack on the lowers side, since you are pivoting about the lower ball joint, and the angle change of the tire over the shorter distance to the ground results in less lateral movement of the contact patch. Also, the bracket does not need to carry as much load, and the interface to the upright body itself can be much slimmer. In terms of weight reduction, our 2003 rear uprights had an inverted Y layout, and no integrated camber adjustment, and our 2004 rear upright had a Y layout, with integrated camber adjustment, lost a few tenths of a pound, and was over 50% stiffer. This was likely due to the designs themselves,round tube vs. boxed sections, and not as much with the layout. I'm sure either layout could be designed to match the other.

Mike T.