Separated steering and suspension geometries

[murpia]

I was thinking a bit about the design of some radio controlled model cars, where the steering geometry is separated from the suspension geometry by the use of an intermediate carrier between the wishbones and the upright. Seems to offer some geometric advantage possibilities, at the obvious expense of some weight and complexity.

Does anyone know of a use of this in full-size racing or FSAE or similar? The only one I am aware of (anecdotally) is the 'super strut' (I think it's called) designed to get around the KPI problems of MacPherson strut suspension on a touring car (Peugeot and Toyota, I think).

Regards, Ian

[murpia]

I was thinking a bit about the design of some radio controlled model cars, where the steering geometry is separated from the suspension geometry by the use of an intermediate carrier between the wishbones and the upright. Seems to offer some geometric advantage possibilities, at the obvious expense of some weight and complexity.

Does anyone know of a use of this in full-size racing or FSAE or similar? The only one I am aware of (anecdotally) is the 'super strut' (I think it's called) designed to get around the KPI problems of MacPherson strut suspension on a touring car (Peugeot and Toyota, I think).

Regards, Ian

[flavorPacket]

unless your family is in the spherical bearing business, I don't think it's really worth it. I have worked with the Peugeot system a bit, and it is extremely costly, but it does get the motion ratio close to 1, which is nice. Anything with the caveat of "weight and complexity" probably shouldn't be on a FSAE car (unless you're competing in design at Formula Student, of course).

[drivetrainUW-Platt]

Pics of this????

[flavorPacket]

it's called FRIP (don't ask me what it stands for, I'm not French)

http://faimg1.forum-auto.com/mesimages/149175/trainavant407.jpg

[Ben Beacock]

interesting!

FRIP=Fonction Rotulant Inferieure Pivot

from an SKF news item:

SKF CARB Bearing In Peugeot 407 Suspension Design

07/03/2006

SKF supplies the patented CARB bearing for Peugeot 407 in their new suspension design, building on the lower rotation and pivot function bearing (FRIP).

). The design allows a lower torque/weight on the steering through the separation between steering and suspension functions.

The SKF lower rotation and pivot function bearing mount (Fonction Rotulant Inferieure Pivot, FRIP), is a sealed CARB bearing encased in a cast housing that can accommodate to both rotational and vertical movement while also allowing for small amounts of swiveling.

The Peugeot 407 uses an independent hub carrier with upper and lower wishbones, where the CARB bearing allows for misalignment and axial movements in the ball joint assembly. In the suspension design, the hub carrier does not pivot directly on the wishbone ends when steering, but on a tall aluminum support which bridges the large space between the wishbones.

In addition, SKF also supplies the Peugeot 407 coupe with belt tensioner units including ABTU for the timing system, valve stem seals and clutch bearings for manual transmissions.

[murpia]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by flavorPacket:
unless your family is in the spherical bearing business, I don't think it's really worth it. I have worked with the Peugeot system a bit, and it is extremely costly, but it does get the motion ratio close to 1, which is nice. Anything with the caveat of "weight and complexity" probably shouldn't be on a FSAE car (unless you're competing in design at Formula Student, of course). </div></BLOCKQUOTE>
Seems to me you'd only need 2 extra sphericals? At least given the setup on the car I was looking at recently here (http://www.racing-cars.com/images/Technical_info/Schumacher/Current%20Cars/Mi3/Exploded%20Diagram/Mi3_exploded.pdf). The major issue would seem to be the need for the lower wishbone to react torque. Or something could be done with 2 tie rods and 'standard' A-arms which would then need 4 extra sphericals.

I'm wondering if anyone has experienced geometry packaging issues (maybe with 10" rims) that could be helped by this arrangement? Getting rid of KPI would seem to be the main one to me, or maybe getting the right camber gain. Those appear to be the reasons for the Peugeot system in that image, although as it's a road-car system that fits in a Mac strut packaging space it has those insane 'gooseneck' wishbone things.

Care to elaborate on your comment regarding Formula Student judging? I'm in the process of collating my feedback on the event to pass to the IMechE.

Regards, Ian

[flavorPacket]

That's right, you need 2 more per corner (i.e. double a normal SLA). And I can tell you that running 10" wheels doesn't mean you can't get good steering geometry with a traditional setup, it just takes some work with the brakes.

As for Formula Student, I'll PM you my thoughts. This isn't the right place for comments.

[Storbeck]

Not really relevent to this thread, but it occured to me while looking at that FRIP setup that it isn't actually a mcpherson strut suspension, but an A-arm suspension rearranged to fit where a mcphersion strut goes. Upper A-arm is above the wheel rather than inside it.

Now about the original post, my imagination can't come up with a reason that doing something like this would improve your options for steering geometry. You can fit a spherical attatched to an A-arm anywhere you can fit one attatched to something else. It might have to be a strangly shaped A-arm though, but that's still better than an extra joint (probably would have something strangely shaped with the extra joint anyway)

But some situation might come up where you would need it, like the FRIP pictured above, upper A-arm is in completely the wrong place.