Sorry about sounding like a broken record, but DaWorstPlaya, regarding your mentions of rake. By too much rake, which direction are you talking about, rear raked lower than the front or rear raked higher than the front? Just missing that detail and then I should be able to follow your post

Does anyone have a starting point suggestion for rake? From my measurements, the stock rake is ~3/4" rear fender ride height lower than front fender ride height. I'm assuming thats obviously a good starting point, but have people been more successful with the rear more raked than that (lower) or less raked (higher)?

Seeing as this is my daily driver, I tend to go a while before I make height adjustments as that means I have to realign and now, in a week, recorner weight in addition to realign. Therefore, I like to get close to what works the first time, so I can avoid having to make constant changes.


Devin

 

Devin-

Rake is defined as a jacking measurement on the rear of the car relative to the front or the rear roll center is higher than the front roll center as measured directly over each axle center line. Therefore to increase rake you raise the rear, to decrease you will lower the rear. Mitsubishi did this on the Evo 9.5 (we didn't get this car), they lowered the car 5mm in the rear to attain better dynamic balance.

 

Dave,

Understood, I was just being clear as to DaWorstPlaya's frame of reference. The frame of reference I was using was from the stock measurements on the fenders, implying that less rake would produce more even front and rear ride height measurements and more rake would produce lower rear ride height measurements than the front. Obviously, this is counter to rake's definition, but I thought it would be easier to discuss it in those terms, considering the ride height frame of reference (fender measurements) that we have been going by.

Nonetheless, thank you for the clarification.

Interesting tidbit on the 9.5, though; I never knew that. That would make the difference between fender measurements slightly less than an inch. Very interesting.



Has anyone done any roll center calculations on a stock Evo to see where the front roll center sits in relation to the rear, or are we all assuming that the rear roll center is sitting slightly higher than the front? Just wondering.

Devin

 

Just keep in mind that strut inclination affects roll center height.

 

To what degree Steve? A few of us have additonal caster in the front end of our cars so does that raise the roll center height or lower it?

 

Layback of the strut (caster) does not affect the roll center height (RCH). A change in strut inclination - "inward tilt" - in the front view (as looking at the car from the front), is what affects RCH.

Keep in mind how the RCH is determined on a MacPherson strut: the top construction line to find the instant center (IC) is drawn perpendicular to the strut axis at the point of attachment to the chassis. We can call this the upper ball joint, and it's the center of the spherical bearing in the camber plate. The lower construction line is draw through the inner and outer pivot points of the lower control arm (LCA). Where those two lines meet is the IC for that side of the suspension. From the IC, you draw another construction line to the center of the tire contact patch. Do this for both sides of the suspension and where those lines from each side cross is the RC.

As a consequence, strut inclination can drastically affect the RCH with a typical camber plate. So, you need to consider the interplay of ride height (LCA angle) and strut inclination on RCH, not just ride height.

If you've slammed your camber plates full negative AND your ride height is slammed, then your RC is likely underground. Not good. Understeer city.

I've found the Evo to be particularly picky about front RCH. Rake is always a consideration, also. It's actually easy to tell when you have too much rake on the Evo - the rear end will not settle down in abrupt transitions; it tends to undulate.

One other thing you should take away from this: changing the camber at the upright with the eccentric bolt (or a slot) is not the same as changing it with a camber plate. Changing it at the upright has significantly less affect on RCH, though there is still some effect because the lower ball joint is not coincident with the pivot point of in the upright, which for a strut is the lower-most bolt that attaches the strut to the upright. It is also has the exact opposite effect - more negative raises the RCH, whereas more strut inclination in the negative camber direction lowers the RCH.

 

Steve,

To play devil's advocate here, I've seen arguements for the other way around, as increasing negative camber at the top of the strut will increase roll center.

Just glancing at a roll center diagram again, for a McPherson Strut car, it appears as though the roll center should raise considering the connecting line from the tire patch to the instant center will have to become steeper, thus contacting the vehicle centerline at a higher location.

Jim, from Whiteline, had this to say when talking about his balljoints:


I also found this picture:







Just looking for a clear explanation as I've been pondering this for a bit as well. Pictures help


Devin

 

It depends on the angle of the LCA as seen by the construction line drawn through it. In the first figure, a steeper (more negative) strut inclination "chops off" the LCA line "higher" because the LCA line goes downward to the right.

In the third diagram, the opposite is true - chopping off the LCA line sooner puts the RCH lower, because the LCA line rises to the right.

So it depends on the LCA angle. You certainly don't want the geometry depicted in the first figure. I suppose you could "correct" the RCH of a suboptimal ride height a little with strut inclination, but you'll still have a compromised camber curve.

So, yes Jim is correct given the LCA angle in the first figure, though you really never want to do that to your front suspension.

EDIT: I'll try to play around in WinGEO tonight and post some screen shots to confirm. Dyslexia could own me.

 

Pretty much what Dave Mac said with regards to rake ...

Aaaah ... the roll center diagram for a Mcpherson Strut Design that I have seen so many times. I guess what I'm been trying to do is set up my car with the diagram shown in "figure 2" only problem is the car needs to stay relatively flat when going through corners for it to be effective. Any excessive roll tends to push one to the undesirable portion of the camber curve where we actually start to loose camber and thus contact patch.

I guess the quick and easy sure fire way to make sure the RCH is above ground and the "roll couple" is short would be to follow "figure 3" but that also moves the CG higher than if it were set up in "figure 2" .... as always its a balance and compromise act.

Also I have noted, EVOs tend to have a higher CG in the front given most of our weight distribution is in the front and our engine and turbo are higher than the STis which have a lower placed boxer engine. The more I look into things the more I realize there are a lot of thing working against us:

1. McPherson struts in the front
2. High front weight distribution
3. High CG in the front ...

 

Steve,

Since the "inward tilt" strut position raises your RCH, wouldn't you benefit by moving your strut inward as far as possible and then use the factory camber bolt to add positive camber until you reach your final negative camber position? In otherwords, adding negative camber at the top is better than adding it at the bottom (strut ears), no?......or is the change not big enough to bother with?

Dave

 

Dave - Actually, I'm claiming the exact opposite - when your LCA angles are not "reversed" (inner pivot lower than outer pivot).

Also, lowering your RCH is only optimal to a point. On a strut suspension, based on LCA angles, it's easy to put your RCH below the ground. This also exacerbates lateral migration of the roll center in roll, which is not beneficial.

I'll post up some diagrams tonight and we'll have some better visual aides to work from.

 

Dave, IMHO, I don't think that it makes as much of a difference where you tilt the front strut because the line drawn from the upper ball joint that determines the roll center is taken at a 90 deg angle and thus is more dependant on the tilt of strut. The way I see it, is basically if you lower your car where you the LCA are pointing upward (from the center) then you need more -ve camber to help bring the roll center higher (and hopefully above the ground).

If you want every little bit as possible to move the RCH up then you max the camber bolt at the strut and then tilt the strut. What this basically does is it moves the line taken at 90deg from the upper ball joint that intersects the line from the LCA to intersect sooner. Thus ultimately helping move the RCH up a tensy-winsy bit. But I am sure Steve could clarify better ...

 

Informative thread, keep the info coming. I'd also like to know how changing the camber at the plates or the bottom bolt affects the roll center. As far as rake is concerned it seems that 3/4 to 1 " lower in the rear works best on my bsp evo.

 

Ok, let's start from a different point of view. Let's say the LCA is fixed parallel to the ground. Let's also say the negative camber of the tire is fixed or maintained at -3 degrees. The strut now can be positioned in several ways to raise or lower the RCH based on the 90 degree angle from the strut body, upper ball joint line. This ultimately moves your IC closer or further away from the car. Bringing the line closer raises the RCH, while further lowers the RCH. It would be at an advantage to "tilt" the strut inward at the top ball joint raising the RCH, true? At least that's what my scratch drawings show. Now whether or not your LCA inclines more towards the center of the car while tilting the strut is a seperate issue that can be adjusted via ride height.

Dave

 

Tilting the strut helps move the the IC closer as you said but if you can gain the same "tilt" without having to move the strut @ the upper ball joint on the camber plate more inward but instead at the bottom, what that does is, it moves the strut away in a parallel axis away from the car (when viewed from the front). Which in return gives you the same "tilt" but now the upper ball joint will be farther away from the center of the car than if you were using minimizing the camber bolt and maximizing the camber plate to get the same tilt.

When that happens the new line drawn to the IC will now run parallel to the one you would get if you only used the camber plate to tilt the strut and minimized the bolt. This new parallel line will start slightly further away from the center of the car and will intersect the line from the LCA slightly quicker (closer to the car) thus helping the RCH move slightly higher. All other things staying the same ofcourse ...