Rear upright design with new rims offsets

[isaac23]

Hey people in a sort of re-design phase of an old car and so i am trying to maximize things like the rear uprights. there has been discussion within my team as too whether the rear uprights needs scrub radius. The rim offset that we are running is 3:4 inner to outer with 13 inch diameter rims..

Does a rear upright need a scrub radius? From what i can find teams are mostly saying what their preferences are which is usually slightly negative scrub on the rear. Last year we ran plenty of negative scrub on the rear wheels due to large offsets of the rims and very little KPI.

In regards to the front steering system and uprights we have already identified and are fixing the issues present, but im just looking to top things off with this.

cheers

[ChristianChalliner]

Hmm, I'm curious as to why you say the rear uprights have a lot of caster? I can't see a reason for this since the rear upright is not steered is it?

I'd honestly never considered if the scrub radius at the rear made a difference though so you've given me something to think about even though I can't answer your initial question http://fsae.com/groupee_common/emoticons/icon_frown.gif. Personally I'd have just ran a rim offset that gave me the track width I wanted at the rear.

[Racer-X]

My basic understanding of this as an engine guy is that you are only concerned with camber and toe control in the rear. As in you look at roll centers and make sure you don't have toe changes throughout the range of motion of the upright as well. Based on that I would say things like scrub and offset and so on aren't a concern as your goal is to keep the motion as plainer as possible.

EDIT: On second thought offset does play a roll when camber gain comes into play, just not a big one.

[Kevin Hayward]

Terms such as KPI and Caster lose their meaning in the rear assuming you are not rear steering.

You want to avoid scrub in the rear suspension. The larger it is the larger the loads on your toe control link (and a-arms) will be under braking. Do some 3-d statics calcs and it becomes clear.

Think about your rear upright points in terms of reducing link loads and reducing the effect of slop and compliance, while allowing sensible adjustments (i.e. keeping adjustments such as camber and toe separate if possible). With this in mind the three upright points should form as big a triangle as you can fit in the wheel, with the intersection line of the upright and the ground planes passing very close to the centre of the contact patch.

Kev

[Goost]

Our idea has always been to line up the kingpin axis with the center of the contact patch to decreases the torque the toe-link must react.

Not sure how you evaluated that your "toe-link was very un-effective", but if this was based on observations while driving it may that the large negative scrub radius increased the moment that the upright had to react.

We have found in simulation that the contact patch center is on average centered left/right of the tire (with zero camber), ~0.0 to ~1.5 inches behind the wheel center-line - of course at any time this is all dependent on the tire you use and the traction conditions.
Look at the data for your tires to get the specifics, my point is only that some compliance may be removed by clever geometry (though, granted, the total reaction is the same: decreasing compliance-toe this way usually increases compliance-camber).

Otherwise, I think there is something to say for other completely different methods: for instance, if you know you have a very compliant upright, a negative scrub radius will toe-out under acceleration and toe in during braking.

I don't think in general compliance will ever make you quicker though - physics doesn't work like that - unless it saves you enough weight: physics Does work like that.

[Flight909]

We used scrub for getting toe-in on acceleration by having compliance. Not a huge amount but in theory we liked it. Also having toe-out in braking was not a problem because it made the car more quick when entering corners.

I think is a also question that can answered by three letters: FBD