Rear roll center & sway preload ?

[isrboss]

I have been reading members talking about the KB rear roll center relocation kit, and finding that Steeda also has a similar offering. I read that you can't fit FRPP relocation brackets with the KB, and I don't even care to read up on the Steeda unit.
Here is a picture of the angle of my BMR panhard with approx. -1.3" lowering.

You can see I still have a slight upward angle, and I have read it should be level. Now Is there something special about the KB kit that i'm missing? When I look at the panhard chassis mount, I see at the least about 1" of area below the original mounting hole that could be drilled or slotted. If the idea is to get the panhard as close to level, could we not just simply slot the chassis mounting point?

If this is possible, I don't see why any relocation bracket etc. couldn't be used, while getting the panhard close to level or parallel to the axle. Maybe I'm missing something here, so please educate me.

The other question I have is rear sway preload with driver in seat. I was thinking of shimming the axle mount point of the side that will want to move away from the axle with the driver in the seat. Does anyone have insight on this, is it possible or worth it with the solid rear?

Thanks guys

 

[boro92]

I would ask the question - why not get adjustable endlinks to take out the pre-load?
I'm assuming you're working on swaybar preload because you're corner balancing the car? If one were to do it relatively "properly", you'll need adjustable endlinks on all 4 corners, coilovers and ideally some flexibility in moving some components around in the car.

I'm not sure I'd want to slot the PHB mount, as that area sees some pretty high loads. At some points of suspension loading, that end of the phb is going to want to move up, and at other points in suspension loading, that end of the phb is going to want to move down. If the potential for that to happen is there, I wouldn't want it to be in a location where it's for a critical component which locates the axle!

I want to ask the question as well: lowering the rear roll center reduces the weight jacking effect during suspension loading (the un-loaded side lifts upwards). However, it also increases the distance between the roll center and CG, so you have a longer moment arm. This would then cause more suspension compression during suspension loading (the loaded side compresses). On a lowered car, will you ever see a point where you run out of bump travel in some situations? I have no idea...just a question to throw out there, unless one is running some higher rates to account for the lack of swaybar when running the relocation kit setup, bump travel can be a concern. Looking at my stock height car, it looks like 2 inch bump travel in the back before contacting the bump stop.

 

[isrboss]

I would ask the question - why not get adjustable endlinks to take out the pre-load?
I'm assuming you're working on swaybar preload because you're corner balancing the car? If one were to do it relatively "properly", you'll need adjustable endlinks on all 4 corners, coilovers and ideally some flexibility in moving some components around in the car.

I'm not sure I'd want to slot the PHB mount, as that area sees some pretty high loads. At some points of suspension loading, that end of the phb is going to want to move up, and at other points in suspension loading, that end of the phb is going to want to move down. If the potential for that to happen is there, I wouldn't want it to be in a location where it's for a critical component which locates the axle!

I want to ask the question as well: lowering the rear roll center reduces the weight jacking effect during suspension loading (the un-loaded side lifts upwards). However, it also increases the distance between the roll center and CG, so you have a longer moment arm. This would then cause more suspension compression during suspension loading (the loaded side compresses). On a lowered car, will you ever see a point where you run out of bump travel in some situations? I have no idea...just a question to throw out there, unless one is running some higher rates to account for the lack of swaybar when running the relocation kit setup.

I looked for adjustable rear sway links to no success, please point me to them. Have you seen the KB and Steeda roll center bars? Steeda is slotted KB is drilled, and I mentioned both drilled and slotted mod to the chassis mount. I too thought of the load, but after seeing Steeda slots, and knowing my -1.7* of camber is held tightly by slotted struts, I stopped worrying about that. I'm not on stock springs, almost all of my suspension has been changed. I'm running a 250# rear spring along with a 24mm rear bar so bottoming seems distant.

EDIT: I should have also stated that I intend to stay on street tires for awhile with the spring and bar rate. I'm thinking I'm already running stiff in the rear due to corner entry hunt over steer. Just need to see if a 22mm rear bar will fix that, as I have no dampener adjustments.

 

[Jimmy Pribble]

[youtube]http://www.youtube.com/watch?feature=player_embedded&v=s0-8iQzoY4E[/youtube]

Here. Just because people like to watch TV.

I'm not saying your answer will be found in here, but it might help get people up to speed with what you are talking about.

I'm in the thread, because I would like for somebody to confirm if the FRPP LCA relocation brackets can or can't be used with the KB RCRK.

 

[isrboss]

Great vid I didn't see that yet. I think I'm in the same boat as you to say, if this worked or works with the FRPP relocation brackets I would have purchased it already.

Now in the vid he said this moves the roll center low to the point the sway is not needed. Seeing the brackets drop both mounting points lower than OEM points, maybe this is what I'm missing? Let's just say I want to run my 24mm rear sway though, do I have to lower the mounting point of the axle to acheive the benefit of making the panhard level?

 

[2012Boss302]

Now in the vid he said this moves the roll center low to the point the sway is not needed. Seeing the brackets drop both mounting points lower than OEM points, maybe this is what I'm missing? Let's just say I want to run my 24mm rear sway though, do I have to lower the mounting point of the axle to acheive the benefit of making the panhard level?

I myself am just learning here. I believe the point of the KB kit is to lower the roll center, leveling the panhard is just a byproduct. Roll center is the point where the car will, uh, roll. Our car's rear roll center is where the panhard crosses the centerline of the axle. Am I right ???

Highjack alert !!!!! I asked this very same question before, if rear roll center is changed, ie: lowered, without changing the front, wouldn't the roll axis be too low?? I always thought rear roll should be a few inches above the rear.....

 

[VoodooBoss]

KB likes to lower the RC in the rear and raise it in the front. A friend of mine just installed most of the KB kit including the front LCA's and he was pleased with the results but has only been to the track one time with the new setup. The best thing to do is to call KB and talk to them. In for further updates and explanation.

 

[boro92]

FWIW, some first hand experience from someone who has run a lower rear roll center (via Fays2 watts link) in an autox scenario:

"The other big regret is the Fays2 Watts link pivot bolt location. It is NOT a good idea to slam your roll center all the way down like I have done. I fought understeer all last year because of it. This next year I plan on returning it to the middle position (of a bunch of positions) in addition to the rear sway bar to the middle position and testing from there."

Taken from here: http://www.moddedmustangs.com/forums/2005-2010/293434-suspension-sticky.html

I guess there are some additional nuances with the car being a solid axle...because typically with an IRS front-engined car, having a delining roll axis (rear roll center highter than the front) will yield understeer at the upper limits. This is more confidence inspiring and the car isn't a handful at speed. However, it looks like the guy linked above finds the opposite--a lowered rear roll center on our cars actually will gain a ton of rear grip--so much so that balance is fundamentally altered and there's too much push.
I guess a reduction in weight jacking from the rear end during the roll phase has something to do with it, as I can see some diagonal weight transfer happening when this occurs (transfering more weight to the front tires--more front end bite). A reduction in this phenomenon would mean you're going to need to find more front end grip via other means.

 

[isrboss]

Great info boro92, thanks for helping bring more information to light.

Yesterday I spent some time under the Boss, checking out the effects of my body weight (ballast) in the car on the panhard and rear sway. What I found is the panhard gets very close to level just by my weight and a -1.3-1.4" lower rear ride height. I could probably get it level with a slight lowering, but because I don't want to lower the front anymore due to LCA angle, I'm close enough to on the panhard. From what I have read getting the panhard level minimizes the rear end movement. Now understanding more on the effects of rear roll center, and since I lowered my rear height, I lowered the roll center. I did not factor that in my math when picking my rear sway, now I'm pretty sure I need a 22mm instead of the 24mm I'm running. This could solve my corner entry/ mid oversteer I have be getting when I push over 7/10's.
On the sway, there is not much reaction there, probably due to the solid axle. It did pull away from the drivers side slightly, but I could manipulate it by hand easy. Even with a slight preload, I feel there would not be any effect on handling. The only thing I did differently this time, was to tighten the sway axle mounting nuts with the ballast in the car. There are slots on the brackets for up and down movement, so maybe tightening the sway bar down with driver weight may be better.

Time to shop for a 22mm rear sway bar, my wife loooves when I learn stuff on BMO.

 

[VoodooBoss]

My friend with the KB RCRK is not running any rear sway bar. He needs more track time to determine if he will need one but so far he likes the balance he has.

 

[boro92]

Great info boro92, thanks for helping bring more information to light.

Yesterday I spent some time under the Boss, checking out the effects of my body weight (ballast) in the car on the panhard and rear sway. What I found is the panhard gets very close to level just by my weight and a -1.3-1.4" lower rear ride height. I could probably get it level with a slight lowering, but because I don't want to lower the front anymore due to LCA angle, I'm close enough to on the panhard. From what I have read getting the panhard level minimizes the rear end movement. Now understanding more on the effects of rear roll center, and since I lowered my rear height, I lowered the roll center. I did not factor that in my math when picking my rear sway, now I'm pretty sure I need a 22mm instead of the 24mm I'm running. This could solve my corner entry/ mid oversteer I have be getting when I push over 7/10's.
On the sway, there is not much reaction there, probably due to the solid axle. It did pull away from the drivers side slightly, but I could manipulate it by hand easy. Even with a slight preload, I feel there would not be any effect on handling. The only thing I did differently this time, was to tighten the sway axle mounting nuts with the ballast in the car. There are slots on the brackets for up and down movement, so maybe tightening the sway bar down with driver weight may be better.

Time to shop for a 22mm rear sway bar, my wife loooves when I learn stuff on BMO.

In the case that you're no longer runing the stock Tokicos and can play with rebound only adjustment, run more front rebound to correct your corner exit issue. This'll weight the back tires more as weight transitions rear-ward during on-throttle situations. That can help quite a bit I've found.

The interesting thing with playing with rear roll center on solid axle cars is that when you lower the car (and keep stock roll center), you actually shorten the roll center and cg distance in the rear (think about it -- when the body of the car is lowered, the cg is now closer to the center of the axle. Therefore, the lever arm the roll center has is shortened). The shorter distance causes the car to want to roll less--so the back end is effectively stiffer. And this almost always affects corner exit--specifically, it increases oversteer during the end of the corner and also off-throttle situations. Might also be why you're feeling that it's a bit squirrely. What tire pressures are you running by the way?

Anyway, with what I mentioned above in mind, I suspect the KB kit merely puts rear roll center closer to a stock length (distance between the roll center and cg) to account for cars that have been lowered. As for the other benefits and effects, I have no first hand experience--which is why I found my previous link to be useful.

If you havent already, I'd try playing with shock settings and tire pressures. If you've done as much as you could already, and driving at a mere 7/10ths is getting the back end to get loose...then you have a setup problem that needs to be addressed with additional components, or an alteration of driving style is required

 

[isrboss]

boro, I am having corner entry/mid oversteer. I did go one hole stiffer on my front sway today, and I lost all the loose feeling I was having. I gained a lot more steering feel as well. I will find out soon if the entry oversteer is gone. At Homestead I was twitchy coming off the banking to the infield and that was no fun. If weather looks good I am going back June 29th.