Drifting rex

iTrader: (17)

I just recently came upon studying MacPherson strut suspension geometry and read that FC rx7's when lowered develop positive camber as the suspension is compressed from the way the MacPherson suspension is designed. I have been thought of some ways to "correct" this but haven't tested any of them yet so I don't know if they will work. I'm wanting to start a topic on this to see if anyone has or would be willing to share what they have done to correct this.

My first idea was to add a "spacer" between the spindle and lower control arm. I have already heard of companies doing this and making products that possibly work. I have found this thread and website/picture that kind of touches on the subject.
https://www.rx7club.com/race-car-tec...ension-699811/
Suspension Diagram Photo 9
Suspension Tech - Sport Compact Car Magazine

My second idea stemming from this website (Suspension Geometry) was to change the SAI (Steering Axis Inclination). It seems that the camber begins to go positive under suspension compression when this angle goes to 90 degrees. So why not change the angle of the strut to the spindle to a smaller angle?

My third idea was to relocate the mounts for the lower control arm to higher up and the frame but this might not be the ideal fix as it may require some heavy fabrication to make it work.

I also got to reading about adjusting the caster more positive and found that this will cause the outside wheel in a corner to develop negative camber and the inside wheel to develop positive camber as the wheel is turned, which is exactly what you want.

I also wanted to discuss roll center and if there is an optimum place a the roll center. i have read that the roll center will produce less weight transfer of unsprung weight (chassis) into the sprung weigth (suspension) as the roll center is lowered. Is this correct? and does it still do this as the roll center is lowered below ground level? If not what effects does it have?

Just wanting to start a discuss on if my ideas are possible corrections for the lowering of a MacPherson strut design and to discuss creating better suspension geometry for the FC.

REAmemiya_fan

Mazdaspeed sells struts with the spindle mountes 1.5" higher which will allow for that same height in a body drop without affecting the bumpsteer at the very least

Drifting rex

iTrader: (17)

I actually already have coilovers for my car and I'm going to try and come up with a way to still use them if possible. Wouldn't just raising the spindle mounts keep the steering axis inclination angle the same and the camber would still go positive once the lower control arm is compressed passed 90 degrees? It would essentially be the same as shortening the strut/spring, but not changing the angle between the strut and lower control arm.

j9fd3s

iTrader: (3)

i actually measured the camber curve of the FC, quickly, and found that it basically doesn't have one. at the extreme ends of travel it does, but for the portion of suspension travel you'd use, its basically flat. which is ok, not as good as camber gain, but it doesn't do anything weird.

for roll center, there is kind of an optimum range, however its hard to say where that is, i suspect having it near stock is probably best, although most of us trade off roll center for lower ride height. AWR does make roll center spacers, which would probably be good to have!

i looked at moving the arm up in the subframe, and in the front there is a little room, but the way they did the rear there isn't anything "easy"

eage8

iTrader: (9)

The sport compact car article you refferenced is continued here by the same author (SCC is no longer around)
The Ultimate Guide to Suspension & Handling

REAmemiya_fan: moving the ears up the strut does nothing for bump steer or geometry in general, it just allows you to get more compression travel before you bottom out the strut and allows the car to be lower.

I have AWR RCA Balljoints:


They don't raise the roll center that much because they're designed to fit under 15" wheels, but it's better than nothing.

you'll need adjustable tie rods if you do something like this and fix your bump steer afterwards because it'll be all messed up...

an easy way to change SAI is simply to get camber plates, they add SAI and static negative camber at the same time.

Drifting rex

iTrader: (17)

j9fd3s: Thats good news! So will lowering the car extreme amounts (3-4" or more) and suspension travel cause it to go positive? Or i guess I should ask when did it start to go positive? More than 90 degrees at the Lower control arm. Also if you don't mind sharing your results with us, how did you measure the camber? Camber gauge on the rotor and springless strut?

eage8: Those AWR balljoints look awesome, would they be height adjustable where they bolt into the spindle?

I'm thinking of possibly increasing the SAI by relocating the top bolt of my strut to spindle mount and dialing in more negative camber in my camber plate to account for it. I also would like to try dialing in some more positive caster, to see the effects of positive camber on the outside wheel?

Drifting rex

iTrader: (17)

Also j9fd3s i've read that roll center can also be corrected through spring rates and sway bar stiffness? Is this correct and is this the correct way to fix it or more of a band aid?

eage8

iTrader: (9)

they're not really adjustable, you might be able to make an adjustable shank though...



I think you're better off just adding a lot of static camber along with the SAI opposed to taking it back out again at the hub... How much camber are you running?

the camber curve will go positive when the angle between the control arm (actually the line between the control arm mount, and the ball joint pivot) and the SAI go past 90*. but it won't be all that bad for a while after that....

I think your tire will hit the frame rail before anything really bad happens to the camber curve, I'd be more worried about bump steer.

eage8

iTrader: (9)

No, stiffer springs and sway bars just bandaid the problem... a very low roll center will cause the car to roll more in corners, stiffer springs and bars will bandaid that but not solve the problem.

Drifting rex

iTrader: (17)

Does AWR still make those ball joints? I couldn't find them on their website?
I do plan to address bump steer with heim joints/spherical bearings and spacers. Do you know of an easy way to measure the differneces in arc between the lca and steering arm?
Also you say the SAI angle is determined by the line from the lca mount and ball joint pivot so adding a spacer between the ball joint and spindle will raise the roll center and decrease SAI but will adding a spacer between the two bolts where the ball joint mounts to the lca have the same effect? I'm gunna assume so, but right now I can't seem to picture it in my head

Drifting rex

iTrader: (17)

I would and will add some static camber but I don't want my tires to wear unevenly as my car will be somewhat daily driven and I don't wanna buy expensive tires more often than I don't have to. I figure if theres a way to add negative camber as the suspension is compressed and the wheels are turned then that would be the best of both worlds. Flat tire wear in a non corner condition and increased grip as the car corners. Do you have any experience with adding positive caster?

eage8

iTrader: (9)

you'll have to call AWR to get a set, they're not on the website.

I measured my bump steer with a longacre kit:
Longacre Racing - Online Catalog: Electronic Wheel Scales, Gauges, Pyrometers, Chassis Setup and More!

you can make your own for cheapish though.

SAI is determined by the line between the ball joint pivot and the top strut mount bearing. Roll center is determined by the line of the control arm mount and balljoint.

and no, a spacer between the control arm and balljoint it won't have the same effect. the line between the balljoint pivot and control arm mount is still in the same place. the movement of the arm will still be the same as the stock control arm.

eage8

iTrader: (9)

camber doesn't wear tires nearly as much as most people think. Most of the tire wear that's perceived as "camber wear" is actually toe wear... most likely from adding camber without getting an alignment. a not extreme amount of negative camber (something like -2*) I think will actually increase your tire life because you wont' were the shoulders away while driving hard.

as far as caster goes, I like most people, just maxed out the ground control camber plates:
Ground Control - Camber/Caster Plate - '86-'91 RX7 (Pair)

I run ~7* and it's fine.

Drifting rex

iTrader: (17)

Ok, thanks! Was doing some searching and ran across GM's new strut designed suspension, pretty interesting so thought I would share
GM Introduces HiPer Strut On 2010 Buick LaCrosse CXS | GM Authority

Edit: Just realilzed this design was more for FWD and scrub radius lol

eage8

iTrader: (9)

Looks a lot like Toyota's Super Strut design from the 90s.

Drifting rex

iTrader: (17)

Yeah it does, I think the whole design is oriented towards getting the steering axis closer to spindle so there is a smaller scrub radius for front wheel drive cars. I'm not sure if it helps with camber variation or roll center?

j9fd3s

iTrader: (3)

yep thats exactly what i did. you should measure it yourself, it actually never goes positive.

nope roll center is just a geometrical place. like eage says, the car reacts to the roll center thru the sway bars and springs. "correct" is relative. suspension tuning is a compromise. the Rx8 manuals actually make a big point about drawing a line between the front and rear roll centers, and how the line moves thru the cars roll in cornering. since the Rx8 feels great to drive, maybe its important?

actually as a part 2, we played with ride height on the honda, and its faster when its lower. even when the roll center gets funky, its faster when its lower.

part 3. i set the ride height on my 1st gen by eyeballing the factory GTU car, and its actually pretty high up, its actually still in stock spec! and the handling is totally benign, it actually understeers a little, which is weird in a 1st gen

true! the stock alignment of 0 camber will burn the outside of the tire off at the track.... in fact we ran an E46 BMW and with the maximum negative camber we could get -3? it was still wearing the outside of the tires, it wanted more

Drifting rex

iTrader: (17)

Do you guys know a good store/website to buy good spherical bearings from?

eage8

iTrader: (9)

Just plan old generic bearings? Qa1.net is a good place to start.

RockLobster

iTrader: (7)

How in the hell do you get 7° out of an FC?!

eage8

iTrader: (9)

Of caster? pretty easily stock is like 6* isnt' it?

REAmemiya_fan

AWR sells some good kits, but they are very pricey.

RockLobster

iTrader: (7)

aH sorry i read camber, that makes mucho more sense.

Travis R

Another source for all things mechanical is McMaster-Carr

But as far as your original post...
Struts are not ideal, so you work with what you have.
Don't lower the car too much. The further your RC gets from your CG the more weight transfer you have.
Camber recovery is bad with struts, so you try to match the static camber to the expected chassis roll, so your outside tires are flat-ish in the turn. That means that camber/caster are also tied to springs/roll bars and even tires and surface (grip).

Drifting rex

iTrader: (17)

Ok, so you want your roll center be as close as possible to your center of gravity to decrease the amount the body rolls/ suspension moves in a corner with out the use of stiffer springs/swaybars... Is this correct reasoning?
Also what do you mean by camber reconvery?