ideas for improving IRS for drag racing

[Wolfenstein]

Hi guys,
I'm new here. I'm having some problems figuring our what is ideal for drag racing from a not ideal setup.

I'm in the process of building a '93 Toyota MR2 in an attempt to set a new drag ET world record. I'm trying to figure out how to reduce the amount of toe/camber changes without essentially locking the rear suspension with very stiff spring/damping. Also, any additional anti squat changes would be very welcome. The car is mid engine rear drive. Most Mr2s will have a rear biased 60/40 weight distribution. I am trying to get mine at least 50/50 with the hopes of having a front bias for high speed stability and to keep the front steerable at 140+ mph. The type of IRS I am dealing with is a strut rear with one lateral lower control arm tied to one forward arm and a toe arm on the rear of the spindle. Any Ideas for planting a flat contact patch with minumal camber/toe changes during launch/shifting dive and accell with added anti- squat would be very very appreciated. Here is a link to an exploded view of the rear suspension.

http://opc.mr2oc.com/online_parts_catalog/Handling/rear suspension.GIF

and another of a real world view from the rear looking forward

http://www.mr2turbo.info/pics/800x600/dcp_2105.jpg

and another from the front looking back

http://www.mr2turbo.info/pics/800x600/dcp_2099.jpg

 

[Tmoose]

Perhaps you are inadvertently moving the wheel with the non-shifting hand. What is your safety harness like, and how tight do you run it. I'm guessing on rce day a slowed steering ratio (longer steering arms), a stout steering damper and a batch of caster would make it harder to twitch the wheel when shifting, and reduce the amount of steering change for a given twitch.

 

[BrianPetersen]

This is essentially the front suspension of a front-drive car that has bee put in the back and with the tie-rod attached to a fixed point instead of to a steering rack.

On the diagram posted above, 48780A is a trailing link that locates the main lower control arm 48710A fore-and-aft through bushings, and 48703 is the steering rod.

It looks like the lower arm is shorter than the steering rod. With this type of arrangement, there will (or should) be a certain ride height where the lower arm and the steering rod are both aimed straight at the instant-center. Slightly above and below this ride height it's designed to have (almost) zero rear-steering effect. With significant changes from nominal ride height, this drives the geometry towards toe-in, and that's normally what you want in this situation.

But, if you are running significantly different from nominal ride height, this relationship could be out of whack.

The other thing that happens with these, is that fore/aft compliance in the trailing link will let the whole hub move fore/aft a little bit. Depending on the relationship of the lower link and the steering link, this could affect the toe. If they're parallel, there shouldn't be rear-steer with fore/aft compliance. But if they're non-parallel, you could obtain some toe-in or toe-out in response to fore/aft compliance - and you can play with it by moving the steering arm attachment location fore/aft.

 

[patprimmer]

Also, everything is mounted on rubber bushes so there will be considerable compliance when sticky tyres are hit with 600hp, so theoretical rigid arm geometry calculations won't represent reality

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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for site rules

 

[Wolfenstein]

my entire suspension will be made up entirely of solid heims and chromoly tubes. Bushings need not apply

 

[Tmoose]

Hi Wolfenstiein,

Which component are you calling the "trailing arm", that bends?
48780 or 48710/20

http://opc.mr2oc.com/online_parts_catalog/Handling/rear suspension.GIF

 

[BrianPetersen]

See post above. 48780A is the trailing link, 48710/20 is the lateral link. The trailing link triangulates the lateral link through a rubber bushing to control its fore/aft motion. Nothing (approximately ...) "bends" except for the rubber bushings. The trailing link takes pure tension or compression only (in theory). There will be a bit of bending on the lateral link because the trailing link attachment point isn't all the way out at the ball joint but it should be pretty minimal. Everything in stock form is pivoting on rubber bushings, though.

 

[Wolfenstein]

the trailing link itself will bend whether it's in s bushing or mounted solidly in a bracket at the lateral arm. When the car launches the trailing link will go to a pro squat angle (front down/rear up) and the wheel will drive forward so hard on that angle that there has been cases of breaking the heims in half on a completelty aftermarket chromoloy solid setup. On my last setup, I was using an aftermarket trailing arm that had a heim at the chassis in front and a threaded rod that went through a urethane bushing in the lateral arm. I was able to bend it on the threaded portion going through the lateral arm. That's part of the reason that I was thinking that a ball joint spacer to drop the rear of the trailing arm down a bit would be good. It should reduce the amount of pro-squat and make the angle straighter when the wheel is pushing forward against the arm. The lateral lower control arms still need to be relatively level though if I want to have minumal camber changes.

 

[Wolfenstein]

Tmoose,
60 ft times were previously 1.61. I should have no problem seeing 1.4 or better with the next setup. This is the kind of power bands we are seeing with turbo 4 cylinders now days. I expect 400whp by 4500-5000 rpms with my motor. I can launch with full spool at well over that rpm if need be on a 2 step. I have a tilton twin disk for slipping needs. The last motor was launched off the 8000rpm limiter and could barely break the tires free with 1.61 60 fts. on bogus suspension, too big of tires, and no spool enhancers at the line.

http://i1130.photobucket.com/albums/m533/Benjamin_Denkers/Robi_Denkers_Red_RS.jpg

 

[BrianPetersen]

In plan view (top), where (in the side-to-side direction) is the chassis attachment point of the trailing link relative to the chassis attachment point of the lateral link?

From the way the forces are transmitted (not the way they deflect!) the trailing link and the lateral link are triangulated and act as an A-arm. If that combined A-arm has an effective pivot axis that is (close to) parallel with the centerline of the car, which is VERY common when installed in the front, as opposed to acting kinda like a semitrailing arm (attachment points further outboard at the trailing link than at the lateral arm) then the net forces on the chassis might not be what you think they are.

Moving the chassis-end attachmennt point of the trailing link outboard (and up), so that the trailing link IS a trailing link instead of at an angle, may have some effect that you are looking for.