Suspension geometry/design

JustinEET

Active Member
Messages
83
Hi guys,
i will race my dc5 on rally-events next season so i try to dig deep in the suspension geo thingy.
Im aware of the Scrubradius. I think thats my first issue. Im running 8,5J with ET-35. I swapped Topmounts for Caster and i grinded the topmount holes more the the inner back. Im at -2,2° Camber at the moment, so with the increased camber i should be changing the SAI as well. Im unsure of how much Scrubradius i have left. I tried to measure it but its close to impossible with a cord and an McPherson Strut. I might just change to a rim with less offset and see how it drives.

Now to the Bumpsteer.
I also was in the believe that manufactors know what they are doing, and fitted Hardrace shperical inverted track rod ends. So I heard alot of negativety about this stuff and i am sure that ive done nothing good with that mod. BUT, i just dont understand why. If the tie rods are at an angle and move on the arc (=circle), the more they are on an angle, the more they move inwards (view from infront)! Hence changing toe, hence producing bumpsteer. If they are flat - or better, slightly leaned down, and the coil compresses, the movement ist mostly traveling up and down, and not inwards, without changing the toe that much.

I asked that in the facebook group but nobody could explain me exactly why it is. As i said. I am certain that its not good from what you heard from very clever people, but i cant get my head around it whys that.

So any help would be appreciated!
Cool youtube channel you have.

Hello,
As Sam said with you need to concern with Instant Center. Instant Center is where the Upper control arm intersects the lower control arm. On the DC5 the upper control arm would be the top nut of the strut tower. When you lower our car you will notice that the lower control arm starts to change its angle. But the upper angle doesn't change because its not like a wishbone that has a upper control arm. So now you have an ARC of travel based on where these arms intersect(if you were to extend them with imaginary lines). Typically you have the steering rack mounted low down where the lower control arms. So with a low rack mount tie rods can be flat like the lower control arm(when you lower your car). But for the DC5 the steering rack is mounted up high on the strut. So if you were to lay that Tie Rod flat it would NEVER intercept the Instant center. Thus its traveling on a different axis/arc the control arms are not. So you must give it an angle so it can intersect the lower control arms imaginary line with its imaginary line coming from the tie rod. And you want this intersection to be as close as possible to instant center(having a little bump steer is not bad). If you take a look at the stock RSX you'll notice that the tie rod still has a slight angle to it.
 

Tuneboy

Advanced Member
Messages
102
Hey you two, thanks for the reply!
With Roll Center <-> Center of Gravity, it was easy to understand, the distance between these two acts as a lever. But with the pivot point of the Steering arms its a different story :)

So did i understood it correct? You want the steering arm pivot as close to the rollcenter as possible? Or do you want the Steering arm pivot on the same angle?
I do understand that they travel on an arc, but i still dont understand why the steering arm / tie rod needs to intercept the IC. What would be the benefit or disadvantage of this if its not intercepting the IC? They would roll on the ARC around different pivot points - gotcha - but whys that causing bumpsteer o_O
 

SamDC5

Advanced Member
Messages
1,414
Hey you two, thanks for the reply!
With Roll Center Center of Gravity, it was easy to understand, the distance between these two acts as a lever. But with the pivot point of the Steering arms its a different story :)

So did i understood it correct? You want the steering arm pivot as close to the rollcenter as possible? Or do you want the Steering arm pivot on the same angle?
I do understand that they travel on an arc, but i still dont understand why the steering arm / tie rod needs to intercept the IC. What would be the benefit or disadvantage of this if its not intercepting the IC? They would roll on the ARC around different pivot points - gotcha - but whys that causing bumpsteer o_O
"The instant center is an imaginary point that is created by drawing a line from the upper a-arm ball joint through the a-arm pivot where it is intersected by an imaginary line that extends from the lower ball joint through the inner control arm pivot. Where the two imaginary lines intersect is the instant center. "

"The tie rod must travel on the same arc as the suspension when the car goes through travel. Simply matching lengths and arcs to prevent any unwanted steering of the front tires."

"To exaggerate, if the tie rod were only 10" long and the suspension were 20" long then when the suspension traveled the tie rod angle would shorten much quicker than the suspension arc."


Off a page I found which puts it nicely into simple terms.

Sent from my SM-G975F using Tapatalk
 

JustinEET

Active Member
Messages
83
Hey you two, thanks for the reply!
With Roll Center <-> Center of Gravity, it was easy to understand, the distance between these two acts as a lever. But with the pivot point of the Steering arms its a different story :)

So did i understood it correct? You want the steering arm pivot as close to the rollcenter as possible? Or do you want the Steering arm pivot on the same angle?
I do understand that they travel on an arc, but i still dont understand why the steering arm / tie rod needs to intercept the IC. What would be the benefit or disadvantage of this if its not intercepting the IC? They would roll on the ARC around different pivot points - gotcha - but whys that causing bumpsteer o_O
You want steering arm pivot to Instant center. Roll center is where the Moment arm between center of gravity of the car and the Intersection of the imaginary arm that goes from the tires contact patch center to Instant Center.

If they have different swing arcs this causes a Toe out/In as they travel through this arc. Since its not on the same arc with upper control arm and lower control arm. It will want to pull the wheel in or push it out. Something that throws me off is the pivot point for the tie rod being in the center of the engine bay. Its pivot points aren't intersected by the lines between upper control arm and lower control arm pivots. You can only match the pivot point at Instant Center.

For Macpherson just change the upper control arm in diagram below to the top nut of strut tower.


 

Tuneboy

Advanced Member
Messages
102
cheers! I think i got it now. So basically you want to have it on the same angle as the LCA. Thats doable :)

Second: Whats it the Scrub Radius by the factory? Since im running ET-35 rn, i think i need to dial the ET in by some mm. Im just unsure by how much. Im running around -1,4° of camber. Also i thought that i try to go slightly negative, since i want to use the car on tarmac rallys and running a negative scrub radius might be useful there
 

JustinEET

Active Member
Messages
83
cheers! I think i got it now. So basically you want to have it on the same angle as the LCA. Thats doable :)

Second: Whats it the Scrub Radius by the factory? Since im running ET-35 rn, i think i need to dial the ET in by some mm. Im just unsure by how much. Im running around -1,4° of camber. Also i thought that i try to go slightly negative, since i want to use the car on tarmac rallys and running a negative scrub radius might be useful there
Well the only info I have is stock the SAI is 18degrees.
But depending on your wheel offset, tire sidewall, LCA(which most of us are using Type-R control arm) and how much you moved your strut... Will determine your scrub radius. So no telling without getting down there and measuring it yourself. ;)
 

krsx

New Member
Messages
1
Hey there, I am the author of the analysis posted earlier in the thread. I noticed the discussion here and thought to drop in. Glad to know there was value in sharing it. I finished writing up the analysis on the rear suspension if anyone is interested. Most of the action is related to the front suspension so I guess the rear analysis is for completion sake. You can check it out here: Camber/toe curves, Motion ratios, Kinematic roll centre

You can approximate where the steering axis lands on the ground (ie. approximating the scrub radius) by steering the wheel while the car is stationary, lifting the car and checking the marks on the tire tread.
 

Tuneboy

Advanced Member
Messages
102
Hey there, I am the author of the analysis posted earlier in the thread. I noticed the discussion here and thought to drop in. Glad to know there was value in sharing it. I finished writing up the analysis on the rear suspension if anyone is interested. Most of the action is related to the front suspension so I guess the rear analysis is for completion sake. You can check it out here: Camber/toe curves, Motion ratios, Kinematic roll centre

You can approximate where the steering axis lands on the ground (ie. approximating the scrub radius) by steering the wheel while the car is stationary, lifting the car and checking the marks on the tire tread.
Thanks!
I will check it out. A rought indication is sufficient.

What do you guys think with my plan to drive negative scrub on tarmac rallys?
 

JustinEET

Active Member
Messages
83
Hey there, I am the author of the analysis posted earlier in the thread. I noticed the discussion here and thought to drop in. Glad to know there was value in sharing it. I finished writing up the analysis on the rear suspension if anyone is interested. Most of the action is related to the front suspension so I guess the rear analysis is for completion sake. You can check it out here: Camber/toe curves, Motion ratios, Kinematic roll centre

You can approximate where the steering axis lands on the ground (ie. approximating the scrub radius) by steering the wheel while the car is stationary, lifting the car and checking the marks on the tire tread.
Wow!
Excellent information.
Thank you sir.
 
Top