The obvious thing to do is to change to a double wishbone, installing an upper ball joint at the strut to knuckle location.

If you make the upper arm and leading link the same as the lower arm in every respect, and parallel to it, then you will have a zero/zero suspension.

 

Cheers

Greg Locock


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I thought about something like that, but it's not really an option in my situation without chopping stuff up and drag racing rules frown upon that in the classes I would have to compete in if I decided to do so. Right now I'm just looking for better, not necessarily best. I was thinking that maybe I could add a lower ball joint spacer to give my forward bar a bit more antisquat geometry. That will probably have somewhat of a negative effect on the lateral bar though, but maybe I could aleviate some of that by spacing the subframe down some where the lateral arm ties to the body. I'd probably also have to lean the strut back a bit. Or maybe I'm all together wrong haha

very little of that will do what you asked for in the first post.

Perhaps somebody else would like to take on the challenge of extracting what you really want and are allowed to do in a 20 questions fashion, but frankly I'm not.

Cheers

Greg Locock


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Your original question is easy and Greg gave one answer.

In addition to that answer and a system that has no transverse contact patch movement with acceleration, several cars that have already done it years ago are the Mini and it's derivatives, the Citrone D series and the VW Beetle front suspension (pre Superbug at least). They may need minor adjustments, but a simple trailing arm, with adjustments if necessary to get the arm parallel to the car axis and the pivot point axle being perpendicular to the cars axis and parallel to the ground will do it.

As to your second question I am totally with Greg.  

Regards
Pat
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If I were to make the trailing arm level with the car at ride height, would I not lose anti-squat characteritics? A typical drag car with a more conventional straight beam axle can achieve as much as %200 anti-squat forcing the tires into the track and raising the rear of the car. I realize that there are practical limits with IRS setups. I'm just trying to increase the factory amount without making counter productive changes in other dynamics all while keeping within the rules of having to use factory chassis mounting points for all components. I do agree though. A UCA would do wonders for camber issues.

I never said make the trailing arm level with the ground. I said it should pivot on an axis that is level with the ground, like a trailing arm VW, but without the swing axle or twisting in the arm. ie more like an Austin 1800, probably with an A arm more like a Porsche 911, but with the pivot points adjusted as I said so it does not change camber and toe with travel.

Having the arm high at the front will generate anti squat. Anyway, you second post kinda shot down any real modifications that could really fix the problems.

Also I have no idea of the layout of an MX2 suspension other than it accommodates an East/West rear engine, so possibly is built from mainly Corolla front suspension bits. It's got to be over 20 years since I found a Corolla suspension interesting enough to bother looking at.

Regards
Pat
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My turn ...

What's in your car right now for the rear suspension is a MacPherson strut arrangement. It's like what's normally used in the front.

In this vehicle the torque from the drivetrain goes directly into the chassis. There isn't any torque reaction in the drive direction at the hubs - they just carry the wheel bearings. It's not like with a live rear axle, where the torque that's in the direction of turning the drive wheels gets "produced" in the axle and transmitted to the chassis by links. The difference is that this makes it much, much harder to get significant anti-squat. Rear-drive cars with IRS have a similar situation because the diff is sending the torque reactions straight into the chassis.

The only way you can get significant anti-squat is if the geometry is such that the wheel hub moves backwards significantly in conjunction with suspension compression. (The forces at the hub then want to push the hub down in reaction to forward drive force.) If you draw a diagram resolving the components of the applied forces you can see why this is the case. This requires that the trailing link angle down significantly from the chassis locating point to wherever it connects to this hub. The lateral links need to act as a parallelogram in top view if you want to avoid massive rear-wheel-steering effects.

As for the camber change, MacPherson normally doesn't have all that much, but it will have some. You'll have to draw an accurate diagram to establish how much. In front view, if the angle between the top strut mount, the centerline of the strut, and the lateral arm(s) is 90 degrees then there will instantaneously be no camber change (because the strut will be compressing/extending exactly parallel with its axis) but I guarantee that on your vehicle, the strut is inclined, which means an arrangement like this would make the contact patch go in and out with suspension travel if you were to do this - and the roll center would be below ground - overall a bad idea. If the strut is dead vertical and the lateral arm is dead horizontal at nominal ride height then at least in the close neighborhood of that ride height, no camber change and no track-width change. It will not be possible to completely achieve this with practical geometry. Something's gotta give. Normally what gives is that you set the static camber to be such that at your ride height of interest, the camber is where you want it to be. At other ride heights, it will be something else. Can't have it both ways.

The only suspension design that keep the wheels perfectly vertical regardless of up or down suspension movement and regardless of body roll ... is a solid beam axle.

The only independent suspension designs that keep the wheels vertical as long as body roll is not happening are pure trailing arms, double pure trailing arms (old air-cooled VW Beetle front suspension - but not a Super Beetle), and parallel equal-length double wishbone. But if there is body roll, the wheels will tilt exactly as the bodyshell does.

Front-drive twist-beam systems are a combination of pure trailing arm and solid beam axle geometry. Wheels stay vertical as long as there is no body roll, if there is body roll the wheels lean but by less than the amount of body roll.

Whether you want to re-engineer the whole rear of the vehicle in order to achieve this ... is up to you.

Most people just set the camber to where they want it to be at their ride height of interest, and let it be whatever it wants to be at some other ride height where it doesn't matter so much. Tires normally don't completely stop gripping just because the wheel is one degree off perfectly vertical.

All the IRS drag cars I ever saw that launched OK had the driving wheels camber set to be perfectly flat contact patch at the suspension height at full weight transfer.

The down and dirty way is a cable tie around the shock shaft to get maximum compression and a vidio from side on with marks on the tyre sidewall or on a gauge attached to the body , then adjust camber and toe so at the observed ride hight during launch, the wheels are parallel and vertical.

It is also good practice to have no rear end toe out under brakes or when the chute hits if you run a chute. NOTE. Chutes often lift the rear end a bit as it is less problematic than lifting the front and getting light squirrelly steering.

Regards
Pat
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Thanks for the help guys. I'm working with a fabricator (Hux Racing) to get this stuff made.
This was my original plan.
http://i785.photobucket.com/albums/yy138/fobangn/scan0001.jpg
I thought that if I made a ball joint spacer, I could get the more aggressive trailing arm angle but, like a poster commented would put the LCA at a bad angle as far as camber changes were concerned. That was the idea behind a sub frame spacer or drop bracket for the inboard LCA joint. If I start with a bit of negative camber and can get the LCA to go past level when the rear is loaded, the camber should start going back positive to 0 at point of concern.

The Hux racing bars that I was planning on using
http://a7.sphotos.ak.fbcdn.net/hphotos-ak-snc7/387569_166842410078214_100002573682094_278615_732000085_n.jpg
The four arms on the left are the rear suspension pieces. The longer arms are the trailing arms. My only concern was that these bars have no articulating joint where they connect to one another. My brain hurts thinking about what that will cause without having the deflection capabilities of the factory bushing like in my drawing. It will more or less cause the two arms to act like an a-arm on a diagonal axis to the chassis I assume. That doesn't seem good for toe steer, but I guess the toe arms correct that. Unfortunately all design attempts that use a heim at the intersection or a factory style rod through bushing have all resulted in bent trailing arms at the joint because of the shear amount of force the wheel drives forward into those bars when launching. It should be noted though that the fastest MR2 in the world was running these Hux Racing bars and was pulling the tires past the 60-ft and doing 1.43's with his rear tires.
http://www.youtube.com/watch?v=uUtSUApuuNo
http://www.youtube.com/watch?v=hmSF3LsShLg
Sorry if I have a hard time following you guys sometimes, I come more from an engine building/auto restoration background. I'm kinda new to hardcore suspension analysis.
 

Yeah Pat,
Toe out under decell is a major concern of mine. Probably the most major. The biggest hurdle for mr2 drag racing has been keeping the car stable down track, especially when shifting. Rear weight biased, short wheel base cars don't like to have the rear unsettle, especially with any sort of toe out. I compare it to trying to throw a short arrow backwards. That's why my first area of concern is getting as much weight forward as possible and keeping the rear tires a little more inboard than the fronts. I figured a snow mobile is more stable than a 3 wheeler. I think it should make the rear tires have a more difficult time trying to drive around the steering axis. My old buddy Mike says that was part of the reason why they started tubbing drag cars back in the day. He used to be on a team that raced everything from stock style classes up to top fuel. I figured he might have a point, but maybe not.

What are your ET and mph now?    

previous best was 10.85 @ 132mph with 557whp. That was on a motor with a poor power band and a chassis setup for auto crossing. The setup for this season will be all out drag oriented and the new motor has 10% more displacement with a billet wheel, ball bearing, twin scroll, smaller wheel size turbo that should still be able to make 100whp more than previously. When all said and done, I should be sitting at about 2600 lbs. with 600+ whp with gear, rpm and tire size optimized for mid 140mph traps.

Jeeez....I'm gettin' too old for all this. I was thrilled last year to get 120 WHP on the 1275cc Mini...

Rod

But Rod.

You where not using a big rotating carburetor on the Mini, oh and it has an engine life measured in hours, not minutes.

Regards
Pat
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The last motor (557 whp 2.0 Liter 8500 rpms 26 psi, 25 mpg) went for 6 years seeing drag and auto cross events and was occasionally driven to work. There are Honda K series 4 cylinders that are street driven making as much as 300 whp naturally aspirated. You guys need to get with the times. LOL Turbos are nice since you don't have to drive around at "kill mode" boost when you're just going to the grocery store.

And how many minutes did it spend at over 500hp during those 6 years

Regards
Pat
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Which component are you calling the "trailing arm", that bends?
48780 or 48710/20
http://opc.mr2oc.com/online_parts_catalog/Handling/rear%20suspension.GIF




 

Got any time slips with 60 foot times? Small engine with turbo powerband strikes me as a car that might be hard drive off the line effectively, consistently.  Have others had good drag race results with a light flywheel (I think you mentioned that.)

 

I'm also pondering about Sacrificing all of the lovely Rear weight bias' traction advantage just to compensate for twitchy "high speed" handling, then trying to buy back some traction by keeping the tires at zero camber.  If the instability is the result of rear suspension compliance/steering responding to power interuption when shifting I'd expect it to be worse for the 1>2 and 2>3 shift than "down the track."  

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.

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.

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

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%20suspension.GIF
 

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.

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.

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

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.