Suspension geometry/design

JustinEET

Advanced Member
Messages
108
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
109
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
1,433
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

Advanced Member
Messages
108
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
109
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

Advanced Member
Messages
108
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
109
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

Advanced Member
Messages
108
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.
 

JustinEET

Advanced Member
Messages
108
This .pdf has data will further add on to the same findings from Kelvin.
Shouldn't be much difference between RSX and Integra Type R.
The SAI and scrub radius are going to be tad different because the Integra Type R control arm is longer and hub also is longer.
 

Attachments

Jam_DC5

Member
Messages
6
Hi Gents,

New to DC5 ownership, bought this facelift a few months back and the guy before me has done a fair bit of work to it and tracked it occasionally.
IMG_5642.jpg

I'm actually baffled how he's managed to take it on track with the bump steer behaviour on it currently - so i'm just trying to work my way around all of the mods and tweaks to try and cure it. I have recently fitted the following:

Tegiwa adjustable tie rods
Hardrace inverted rod ends
SHG rack slider
Tegiwa RCA lower ball joints

It's sitting on Meister ZetaCRD, lowered around 30mm-40mm, not stupidly low.
Hardrace rear camber arms
No upgraded bushes (yet) but everything seems fairly tight

The car is going in for alignment this weekend so i'm unsure how all of the parts will tie in. After a small drive along the road it feels absolutely awful, near suicidal to drive it but its hard to quantify without having the alignment done first.

Reading through all the posts about amending bump steer and getting the geometry back to a useable level there's so much conflicting information. I've stayed clear of the rack raiser as there wasn't much good press about that. Would you advise to remove the inverted rod ends too?

Is there any definitive answer to the correct angle that the tie rods and LCAs need to be close to? I know it's a bit of fine tuning to get it perfect relative to the IC and the RC but i was just looking at finding a general reference angle to look for while i'm setting it up, then work from that on getting it perfected. I've always thought they should be close to 0 degrees but i've also read that it's not entirely true with this chassis.

Any help would be appreciated!
 

Lemons

Advanced Member
Messages
372
Hi Gents,

New to DC5 ownership, bought this facelift a few months back and the guy before me has done a fair bit of work to it and tracked it occasionally.
View attachment 8713

I'm actually baffled how he's managed to take it on track with the bump steer behaviour on it currently - so i'm just trying to work my way around all of the mods and tweaks to try and cure it. I have recently fitted the following:

Tegiwa adjustable tie rods
Hardrace inverted rod ends
SHG rack slider
Tegiwa RCA lower ball joints

It's sitting on Meister ZetaCRD, lowered around 30mm-40mm, not stupidly low.
Hardrace rear camber arms
No upgraded bushes (yet) but everything seems fairly tight

The car is going in for alignment this weekend so i'm unsure how all of the parts will tie in. After a small drive along the road it feels absolutely awful, near suicidal to drive it but its hard to quantify without having the alignment done first.

Reading through all the posts about amending bump steer and getting the geometry back to a useable level there's so much conflicting information. I've stayed clear of the rack raiser as there wasn't much good press about that. Would you advise to remove the inverted rod ends too?

Is there any definitive answer to the correct angle that the tie rods and LCAs need to be close to? I know it's a bit of fine tuning to get it perfect relative to the IC and the RC but i was just looking at finding a general reference angle to look for while i'm setting it up, then work from that on getting it perfected. I've always thought they should be close to 0 degrees but i've also read that it's not entirely true with this chassis.

Any help would be appreciated!
I don't have any of those mods so can't really comment on them, although I am lowered a bit, I'm sure someone else can advise. Sounds a bit like the previous owner was chasing the issue by throwing parts at it. A DC5 specialist such as TGM would be able to help.

When I first got my DC5 I thought I had a bit of bump steer, fitting new tyres (PS4) and new engine mounts fixed a lot of it. I also realised when I went down the road in a new car, the road was terrible, really bumpy and a lot of camber so not really the DC5 fault as my modern car was doing it's best to cope with the road also.
 

JordanSuth

Advanced Member
Messages
225
I have to agree with the above comment. Throwing parts at a car isn't always going to fix or improve it. People generally think modified = improved, but I'd more often or not I say it doesn't.

I'd take it to a specialist as there so many variables now with the amount of changes from OEM.

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carl hammond

Advanced Member
Messages
3,737
I echo what's been said above, and if you look through any of my build, geo posts or posts as a whole you'll see that I did my car stage by stage with testing of each change completed on track etc before making another changes to ensure the car was perfect and in my whole ownership building mine from an OEM car to a Track Focussed Car I never experienced any bump steer or issues like many seem too.

The first thing I would recommend would be getting in touch with and the car to a specialist to evaluate it, a lot of the time issues like these are due to WORN or POOR quality parts being fitted, cheap suspension parts, cheap bushes or worn oem ones will result in a poor driving car and poor feedback.

Deff avoid the rack raiser and tbh any poorly made cheap copy parts imo, these just add more issues.

Don't go spending more and more by throwing parts at it until a specialist has checked it over and advised on what could be the cause of the issues as it could be as simple as one failing part due to its poor quality or down to age deterioration etc
 
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