Steering, Suspension, & Brakes

I'm being informed by several sources, that from static to accelleration, the rear rear wheels will move to toe out, (semi trailing arm). No one says why. I'm trying to wrap my head around this statement. How ? Centerline of wheel is approx 8+ inches outboard of STA. and outer front pivot STA and outer front pivot(bushing), will tend to compress, under accelleration. As I'm seeing this, toe-in gain should be the result. Help ! Thanks, roverman.



Edited 1 time(s). Last edit at 04/17/2012 11:27AM by roverman.

With a semi trailing arm the geometry goes towards toe out as the suspension compresses, that may be the cause.

Art, with the STA suspension you need a link between the diff and the hub carrier and as the suspension compresses that distance gets shorter due to the arc of that link. Because it is pulling the rear of the trailing arm inwards and the front is held in place on the chassis it tends to toe out the rear tire.

Clan, Porsche 911 STA uses no transverse link. It will be using (2) pivot bushings at front, approximately 10" apart. Pivot length is 17". This resembles an assemetrical shaped A-arm. Cheers, roverman.

Art, in effect the rear leg of the A arm is a transverse link. That causes the rotation of the arm to be at an angle to the center line of the chassis, and if you compress or extend that A arm from it's normal location it will tend to toe out the tire. The only IRS arrangement that does't display toe steer on compression is similar to that used by Jaguar where the lower control arm controls the vertical location of the upright causing it to move parallel to the chsassis centerline. A trailing arm or semi trailing arm will induce bump steer into the rear suspension because the effective pivot axis is no longer parallel to the chassis centerline. A lot of modern IRS systems use a ball joint to attach the upright and a seperate steering link to control toe steer. Miata and Corvettes are two that had variations on this design.





Edited 1 time(s). Last edit at 04/18/2012 11:03AM by Bill Young.

Bill, Your showing RX7 rear,(dynamic steering) and Vette front. Left hand turn, RX7 goes into "roll oversteer" ? When outside rear tire receives toe out gain,(left hand turn),= roll oversteer ? Thanks, roverman.



Edited 1 time(s). Last edit at 04/18/2012 01:26PM by roverman.

The caption on the vette photo said rear suspension, C6 if I remember correclty. The whole point is that you can play with the linkage and design in what ever characteristics you want with that design, add more toe out for roll oversteer to compensate for an understeering front or dial it back for a more neutral steer if the front is more neutral. I've been toying with some IRS designs for my Midget over the years and while certainly not an engineer or any type of expert I have read a few things about some of the handling characteristics with various designs. My plan was to use FWD spindles in the rear that wiould clear my 13" wheels and build from there using a steering link design with a trailing arm mounted from the original front spring brackets. About the simplest design I can build at home that will offer enough clearance in the Midget chassis. Now I have another project in the wings that will probably get the IRS first.

To demonstrate the toe out of a semi trailing arm suspension, fold a piece of ppaer on a diagonal; then move the paper up and down at the fold; the toe out effect will be obvious.

What a great way to visualize that Larry!
Mind if I use that for instructional purposes?
Cheers
Fred

Ok, If I understand this correctly, outside rear, in a turn, will have toe-out gain while inside rear will achieve toe-out reduction ? This being as a result of body roll, while inducing "roll oversteer" ? Therefore, having 30-40% more tire contact patch in rear vs. front, this roll oversteer could be beneficial to help balance the car front/rear ? There will be adjustable static toe. Obviously, I want as much front bite as possible, then reduce rear side bite, with adjustable rear anti roll bar, to balance car front/rear. It's much simpler to just drag race ! Cheers, roverman.



Edited 3 time(s). Last edit at 04/21/2012 05:00PM by roverman.

510 trailing arm IRS needed about twice the spring rate as the Jag IRS also.

Jim N., what is the relavence ? I'm trying to understand the suspension dynamics of STA, for now. I would rather not find out the hard way, that this woun't do the job. As a longer route, I "could" use a variant of Boxter rear suspension,(SLA) ? I have the uprights, but unable to find suitable pictures of suspension layout. Onward, roverman.

I think it is the long lever effect that makes the heavier spring rate necessary for the trailing arm IRS. Toe out is like Bill said due to the angle of the trailing arm acting like an A arm. The stiffer springs help reduce that by limiting suspension travel.



Edited 1 time(s). Last edit at 04/22/2012 11:41PM by mgb260.