Advanced Suspension Geometry and Design

so



Edited 1 time(s). Last edit at 02/05/2014 08:23AM by Iowa999.

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Iowa999
I'm sorry; I omitted an assumption.

When you tilt the strut back, the tab on the knuckle for the steering arm will drop (on a rear-steer car, like an Evo) or move up (on a front-steer, like a Scooby). But moving only the top of the strut does not alter the swing-arc of the knuckle. Thus, moving the top of the strut will alter bump-steer.

I could be wrong about this, of course, but that was the bit that I didn't mention that made my napkin suggest the effect.

Ah ok so the tilting of the strut was solely for the purpose of moving the steering ball joint. Although it works seems like a bubba way of doing it. Because you'd have to incline the strut back so much and still only get minimal movement at the joint, and at that point it would have excessive caster, causing weight jacking, unloading of the inner rear wheel and on FWD crazy torque steer.
Shiiiirley using a rod end and stud and a proper spacer would be the best way to tackle the height of the outer tie rod joint.

she



Edited 1 time(s). Last edit at 02/05/2014 08:24AM by Iowa999.

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Iowa999
The point about bump-steer sent me back to my napkins and pencils. Assuming a single, L-shaped lower control arm, if you lower the rear inboard pivot (to alter the anti-dive), then you're pretty much guaranteed to change the bump-steer. Same goes for moving the top of the strut back for more caster. So what is the suggested order of things with regard to tweaking and testing?

Also figure that if the control arm's axis isn't parallel to the ground, Ackerman is going to change at droop vs. bump even after you fix any bumpsteer issues you may have. More Ackerman at droop and less at bump.

Is this good? Is it bad? Is it miniscule enough to not make two shit's worth of difference?

I remember the Fox Mustang guys would go all tech-nerd over changing the various anti's and Ackerman and roll centers, because Mustang suspensions were complete garbage. So the people who play with Mustangs by nature have to become conversant in suspension geometry, whereas the people who have cars that work well out of the box mostly niggle over tiny details like 25lb-in differences in spring rate... (See also: Porsche becoming excellent at suspension kinematics while trying to figure out how to make a car handle despite having a 500lb chunk of metal nestled up to the rear bumper)

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Pete Remner
Cleveland, Ohio

1984 RX-7 (rallycross thing)
1978
Silence is golden, but duct tape is silver.

Good piccie but still too general..General musing arn't gonna lead us to any revelations:

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John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

got



Edited 1 time(s). Last edit at 02/05/2014 08:24AM by Iowa999.

I did my changes in this order.
Lower front and pull forward front pivot (add anti dive and castor, known benefit from previous car)
Set adjustable strut tops in roughly same position as old car.
Test bump steer.
Lower rack mounts 10mm to hopefully correct issue. I knew which way to go but had no idea how to work out how far.
Rechecked and I'd fluked it first go.
I had planed to recheck it once final ride heights, damping, rebound, spring rates and alignments settings etc were worked out but it drove really well and my circumstances changed(eg had to buy my mates rallycar I crashed) so it was sold.
I keep in touch with new owners and they haven't changed any of the settings.

EDIT
if it did need any tweaking I would have changed the tierod ends to something like pictured above.



Edited 1 time(s). Last edit at 02/02/2014 04:17PM by buerckner.

Most uprights are steel and you can put 'em in a press and shove things over this way and that if you have to move them a little bit.

Unless things are WAAAAY out of whack, I am not sure that bumpsteer is going to be a big deal just from changing caster. If you move caster two degrees, the steering arm is going to move not very much at all, less than 1/4" as a rough guesstimate (sine of 2 degrees times six-eight inches, not perfect math but close 'nuff for SWAG purposes). Not ideal but, was it ideal to begin with? CAN you make it ideal, meaning are you going to be able to "fix" bumpsteer or are you just moving things around so that it is at a minimum inside one specific portion of suspension travel?

Then you whang something and it all gets thrown out the window anyway...

---

Pete Remner
Cleveland, Ohio

1984 RX-7 (rallycross thing)
1978
Silence is golden, but duct tape is silver.

Quote
Pete
Most uprights are steel and you can put 'em in a press and shove things over this way and that if you have to move them a little bit.

An awful lot are cast iron. Many recent are cast aluminum.

---

John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

an



Edited 1 time(s). Last edit at 02/05/2014 08:25AM by Iowa999.

I have the pins that paul posted for adjusting bump. $18.00. Those tie rods are nice but i wouldnt want to pay that every time I knocked the toe out.

Ackerman. I havent even checked that on a subie.

I dont believe my issue is in the front its the rear susp. I can change my driving style and make it not spin. Im giving up to much time and the car isnt worth a shit the way it is now.

There has to be a better set up for this style rear susp. I dont have rear struts. These have upper control arms and coil over springs. Ive moved up the spring mount about 6" hoping this would help body roll. It didnt seem to change a thing. I have longer trailing arms that go faward. And I changed the lateral link length to correct bump because of the longer trailing arm. The angles of the upper and the rear arms are the same as stock. Should I put more or less angle on the arms to move roll center up?

Most pics of 08 up cars the ass is on the ground so I think there fix is to lower the rear of the car. This would mean more arm angle but less rear travel.

Ive ran with and with out sways. The car is more stable with sways then with out. I did try un hooking the front sway and it got worse. I started with no sways then added front then added rear then un hooked front got worse. Could un hook rear and test again but I dont think its the fix. It sounds like it is the rear roll center hieght that I need to change.

I am pauls worst enemy when it comes to driver wanting changes. There are many times when the driver is correct. Knowing when this is and dialing the car in is when you have a good team. I do agree thow with Paul leaving a proven set up. The track could have been dirty or who knows what. If the driver came back the second time complaining I would make a change. Hence why im here trying to sort out this car. 6 rallies in different conditions with the same issue. Guess it must be the driver but thats not changing so I need to fix the car for my driving style.

So you guys have found that adding anti dive to the front was better in gravel then less anti dive? Why is this?

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Quote
Iowa999
My first reaction was to point out that the tab on the knuckle is only about 6" from the steering axis, so that 1/4" would be many degrees of toe. Then it soaked in that you'd, of course, realign the car, so the initial effect on toe isn't the issue. The issue is the new angle of the steering arm, which won't be much different from before, so the bump-toe curve won't be altered very much. It really comes down to whether it would make it worse than before or whether it - gasp! - actually helped flatten it out.

You're looking at it wrong. Changing caster affects the vertical height of the end of the tie rod, and 1/4" up and down isn't going to change toe much if at all. Bumpsteer is the toe *curve*.

Now, changing camber WILL affect toe unless you have a car where the steering is down in the same plane as the control arm (and you adjust it at the top). And it sure as hell is also going to be changing bumpsteer because you're making the tie rod shorter or longer as well as slightly changing the arc in which it travels. Very few people seem to be all that concerned with it, since the changes are miniscule relative to the gains achieved with a better alignment.

Heh. I'm taken back to one net.doofus about twenty years ago who was trying to figure out how to make his steering axis inclination (the front-view counterpart to caster) zero, because somehow he thought that the tiny, tiny sliver of camber lost when the steering was turned was ruining his handling...

---

Pete Remner
Cleveland, Ohio

1984 RX-7 (rallycross thing)
1978
Silence is golden, but duct tape is silver.

Quote
Reamer
So you guys have found that adding anti dive to the front was better in gravel then less anti dive? Why is this?

It would vary for different cars, ours were G100 Charades. But I suspect the original geomtery didn't have any antidive as under brakes they pulled the nose down quite a bit.
By reducing the body movement we effectively turn more of the weight transfer from braking into loading the tyres with obvious benefits to front grip and turn in.
Also could run softer spring rates to soak up the bumps better.

If your car doesn't try to do endos when you brake then it may not be a problem you need to look at.

My turbo MX-5 works pretty well with std geometry but it started out as a good handling car, rather than an econobox shopping trolley!

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buerckner
By reducing the body movement we effectively turn more of the weight transfer from braking into loading the tyres with obvious benefits to front grip and turn in.

This is not true. Adding anti-dive puts the forces into the ball joints not the springs, and that makes the tires skid quicker under brakes. I'm sure you gained some benefits but not from braking in rough terrain. UNLESS the car was gaining so much camber that would give you less foot print, therefore less grip. Otherwise more antidive is generally no bueno in rough bumpy surfaces.

Asian



Edited 1 time(s). Last edit at 02/05/2014 08:25AM by Iowa999.