Brake stability

[Vetracr]

I have been tracking my E89 upgraded with 335i calipers, 348mm x 30mm rotors with Cool Carbon ST brake pads. I have been having a problem with braking stability when braking from high speeds, over 100mph. Car gets very unstable and darts left or right on initial application of brakes. Gets quite scary. I never had this problem on any other car. Alignment is right on with 0 toe and monoball suspension parts. Tires are brand new Falken Azeni 651+. Kingpin offset reduced by 6mm with TSW wheels which should make braking more stable. Any thoughts?

[NoQuarter]

Bleeding issue maybe?

[fmorelli]

Can you describe the behavior of braking in more detail? Like is the stability more in the front or the rear? What's your complete suspension setup? Does it happen at lower speeds? You say upgraded calipers ... I suspect you've looked at proportioning and know that's right?

Filippo

[fmorelli]

Vetracr PM me your email address so I can send you something.

Filippo

[Vetracr]

Originally Posted by fmorelli

Can you describe the behavior of braking in more detail? Like is the stability more in the front or the rear? What's your complete suspension setup? Does it happen at lower speeds? You say upgraded calipers ... I suspect you've looked at proportioning and know that's right?

Filippo

Filippo
Car is dialed in right. No air in brake lines. I use a pressure bleeder and Motul 600 fluid. Problem is with front darting left or right under initial hard braking at over 100mph, usually at end of long straight. Doesn't happen at slower speeds. I'm using 335i calipers which have same diameter piston (57mm) as stock Z4 but are mated to 348mm dia., 30mm thick rotors. I'm running Cool Carbon ST pads. Front suspension is all monoball, M3 control arm, Turner monoball tension strut, Koni FSD shocks with stock springs, H&R 27mm sway bar. Tires are new Falken Azeni 651+. Toe is set at zero. Original king pin off set with stock 29mm offset factory wheels is 9.6mm. My TSW wheels are 35mm offset reducing king pin offset to 3.6mm greatly reducing king pin offset effect. Center of tire is only 3.6mm from kingpin ground contact point. It should be more stable under braking, not less stable. My suspicion is a problem with the Cool Carbon pads but I have no way of telling unless I switch to different pad. What pads are you running? Also, any other thoughts.

Larry

[fmorelli]

I'm running the gold brembos (BMW OE kit), so not much help there. I'd be surprised if its cool carbons ... never heard of that and they are well-used pads. I'm not sure how the pads would actually contribute to darting. It sounds more like something in the suspension is moving around under breaking. Very hard to say (especially in forum mode). I'll ask Barry when I see him this week. But I've not heard of any other Z4 folks having this issue.

Filippo

[Asbjorn]

Im running a very similar setup, also with reduced kingpin, M3 arms, koni fsd etc. However I still run OEM EU spec brakes.

I just want to confirm that I do not experience darting unless I turn the wheel while braking with ABS activated. If you have increased the disc diameter up front without changing anything rear, your brake bias has most likely moved forward. But on the other hand, with stickier tires, it sort of moves backwards again (as you pull more braking g's and transfer more weight to the front, the rear may lock up sooner - at least in my understanding). But if the darting only happens during initial application of brakes (ie before ABS is activated), then all of this should be irrelevant.

[NoQuarter]

Will is do it every time? Or does it behave properly after you do it a few times in a row and get the brakes good and warm?

[Lurcher]

Tried this with traction control on/off? Computers may be doing something to the rear....

[Vetracr]

Originally Posted by Vetracr

Filippo
Car is dialed in right. No air in brake lines. I use a pressure bleeder and Motul 600 fluid. Problem is with front darting left or right under initial hard braking at over 100mph, usually at end of long straight. Doesn't happen at slower speeds. I'm using 335i calipers which have same diameter piston (57mm) as stock Z4 but are mated to 348mm dia., 30mm thick rotors. I'm running Cool Carbon ST pads. Front suspension is all monoball, M3 control arm, Turner monoball tension strut, Koni FSD shocks with stock springs, H&R 27mm sway bar. Tires are new Falken Azeni 651+. Toe is set at zero. Original king pin off set with stock 29mm offset factory wheels is 9.6mm. My TSW wheels are 35mm offset reducing king pin offset to 3.6mm greatly reducing king pin offset effect. Center of tire is only 3.6mm from kingpin ground contact point. It should be more stable under braking, not less stable. My suspicion is a problem with the Cool Carbon pads but I have no way of telling unless I switch to different pad. What pads are you running? Also, any other thoughts.

Larry

Did some more research and discovered that factory BMW caliper mounting bolts ride in rubber bushings in the caliper. This could cause the caliper to move or possibly bind on the rotor especially under an initial hard application of the brakes with high friction pads and sticky tires. I found that Bimmerworld sells a brass liner and pin kit to replace the rubber in the caliper. I have them on order. Hopefully that should fix problem. I've never encountered rubber isolated brake calipers so it never entered my mind as a possible problem. BMW never fails to surprise me.

[NoQuarter]

Woah... This is a problem/solution I have never heard of.

Don't forget this thread. Update with some results!

[fmorelli]

Those brass liners have been around for 20 years, I want to say. Curious to see if it makes a difference, and if so, why your car is the only one I know of that suffers this issue. Please keep us posted and good luck!

Thanks,

Filippo

[The HACK]

Originally Posted by Vetracr

Alignment is right on with 0 toe...

There you go.

[Vetracr]

Originally Posted by The HACK

There you go.

I don't understand your comment. Please explain. You're never too old to learn something new. I've been racing SCCA for years from MG Midgets to Corvettes. With the monoball suspension bushings on the car there is no deflection in the front suspension under braking so there is no toe out from suspension bushing deflection. The zero toe stays zero toe. I've reduced the kingpin offset at the contact patch from 9.6mm to 3.6m, another benefit for braking stability.

[The HACK]

Originally Posted by Vetracr

I don't understand your comment. Please explain. You're never too old to learn something new. I've been racing SCCA for years from MG Midgets to Corvettes. With the monoball suspension bushings on the car there is no deflection in the front suspension under braking so there is no toe out from suspension bushing deflection. The zero toe stays zero toe. I've reduced the kingpin offset at the contact patch from 9.6mm to 3.6m, another benefit for braking stability.

On a MacPherson front strut? Bushing deflection isn't the sole cause of increase in toe. As you brake and the front end compresses, the negative camber increase and lead to a toe-out condition from an initial zero toe. Toe-out at high-speed leads to directional instability.

Corvette and MG Midget both use double a-arm front if I'm not mistaken. Double A Arms suspension joint movements are far more predictable than MacPherson strut.

When you initially mentioned high speed instability I thought about a possible seized caliper until you mentioned that the direction it goes in isn't consistent. Then you mentioned zero toe. That was the obvious cause to me because I used to have the same problem with my MZ4 @ Auto Club Speedway, where there are several spots where you have to brake from ~135mph. When I ran a slight toe-out the car would want to follow the camber on the track. Great for quick turn-in, BAD for clean underwear.

Problem is exacerbated by a shortish wheelbase.

[Vetracr]

Originally Posted by The HACK

On a MacPherson front strut? Bushing deflection isn't the sole cause of increase in toe. As you brake and the front end compresses, the negative camber increase and lead to a toe-out condition from an initial zero toe. Toe-out at high-speed leads to directional instability.

Corvette and MG Midget both use double a-arm front if I'm not mistaken. Double A Arms suspension joint movements are far more predictable than MacPherson strut.

When you initially mentioned high speed instability I thought about a possible seized caliper until you mentioned that the direction it goes in isn't consistent. Then you mentioned zero toe. That was the obvious cause to me because I used to have the same problem with my MZ4 @ Auto Club Speedway, where there are several spots where you have to brake from ~135mph. When I ran a slight toe-out the car would want to follow the camber on the track. Great for quick turn-in, BAD for clean underwear.

Problem is exacerbated by a shortish wheelbase.

Great response. Thank you. I never considered the bump steer and wheelbase aspect of running front zero toe. I have ground control camber plates so the top if the strut is also in a monoball bearing. The front suspension is solid. What led me astray is that the factory toe setting for all Bmw's is very small even with the soft factory bushings. Definitely will have to check angle of steering arms at ride height to see if they angle down from rack to spindle. If they do you are right on the money. Thanks again.

[fmorelli]

I just asked my suspension guy, Barry Battle (ex-bimmerworld, ex-RRT), who is building my car. He sent along a few comments. Hope they help:

• The E90 suspension does not toe out under bump.
• It's not kingpin offset. It's scrub radius.
• Toe out bump has nothing to do with the fact that it's a McPherson strut.
• Probably the zero toe. Run either toe out or toe in.
• Could also be a whacky corner weight in conjunction with zero toe. More weight on a cross weight in conjunction with zero toe can cause whacky handling under braking.
• If he is running more than stock camber, he would be hard-pressed to get positive scrub radius which can cause instability. Adding camber makes the scrub radius go more negative.

Best of luck!

Filippo

[Vetracr]

Originally Posted by fmorelli

I just asked my suspension guy, Barry Battle (ex-bimmerworld, ex-RRT), who is building my car. He sent along a few comments. Hope they help:

• The E90 suspension does not toe out under bump.
• It's not kingpin offset. It's scrub radius.
• Toe out bump has nothing to do with the fact that it's a McPherson strut.
• Probably the zero toe. Run either toe out or toe in.
• Could also be a whacky corner weight in conjunction with zero toe. More weight on a cross weight in conjunction with zero toe can cause whacky handling under braking.
• If he is running more than stock camber, he would be hard-pressed to get positive scrub radius which can cause instability. Adding camber makes the scrub radius go more negative.

Best of luck!

Filippo

Filippo
Interesting input. You are right re scrub radius. I just couldn't remember term when I wrote post. I'm running 2 degrees negative camber up front. Car has stock ride height and springs so I'd be hard pressed to think my corner weights are way off. Will add some toe in for next track day to see what happens.

Larry

[Asbjorn]

BMW uses the term kingpin offset for some reason.

I made a quick comparison of the E90 and E89 front suspensions



It looks like BMW M increased toe when they installed the longer wishbone / control arm which adds camber.