Brakes / Spring Rates - Brad? Racers? Options

[Evill Ed]

Hey, I just finished installing BMW 320i calipers on the rear of the 914. I added a Tilton proportioning valve too. In the front, I have vented 911 rotors with M style calipers. They are actually BMW units, not 320i, for a 5 series with vented rotors. The Pistons are a little larger than M pistons.

Anyway here is the deal. My front brakes lock up way before the rears. When braking moderately, with the propotioning valve set in position 5 (of 7) the car pulls down tightly and evenly. If I brake hard, the car starts to slow evenly, but then the fronts lock and skid. If I go to max position on the proportioning valve, when I brake heavy at higher speed, the rear gets light and the nose starts to turn to the right. then the fronts lock.

The right front locks faster than the left. The fronts have always locked like this. I figured the stock rear 914 calipers were too small and kinda beat, even with rebuilds, so I went for the 320i's.

The 911 19mm master is new, the brake lines are new rubber, all calipers are rebuilt and perfect, and the system is bled well.

I'm begining to think this may be related to weight transfer from front to rear.

The front is 21mm torsion bars with a 19mm Weltmeister sway bar with Weltmeister soft bushings.

The rear is 140# Weltmeister springs. Front and rear struts are Bilstein.

Should I go to 165 or 180# rear springs?

Am I on the right path?

I appreciate any input.

Regards,
Ed

[ChrisReale]

I think 21mm bars equal roughly 175 lbs in spring weight. So, you have 175# up front and 140 rear. I would go to 180 rea if you keep the 21mm bars, but then, you might want to upgrade to a 22mm bar??

[Evill Ed]

Thanks Chris. I have a 22mm sway bar too. I had changed to the 19mm becuase the front used to plow really bad. (That right there would indicate that the front is too stiff huh? )With the 19mm, the car would turn.

If I recall correctly, the front brake lock ups started after I went from stock torsion bars to the 21mm.

Anyone else?

Regards,
Ed

[drew365]

The fact that your right front locks and pulls seems to indicate you still have a brake problem. Maybe a pinched line steel line? After I put 22mm torsion bars on my car I had big time understeer until I upgraded the rear springs but my brakes didn't lock up in front.

[Brad Roberts]

Ed..

I'm still taking all this in. Weight transfer happens evenly "normally" so the right front locking up shouldnt have anything to do with the weight. Are the shocks in good shape ?? A bad shock that is screwed up on the rebound side... *might* cause something crazy.

I'll be honest here... this sounds like a rubber brake line problem straight out of the gate.


B

[ChrisReale]

Yea, I was thinking that you have a pretty wickid car, but rubber lines... (IMG:style_emoticons/default/wacko.gif) Maybe an upgrade to steel lines?

[Brad Roberts]

The rubber ones work fine... but they can rip on the inside. Especially if you have done a ton of bleeding and possibly passed something thru the system.

Thinking more on it.

One of the pistons in that right front caliper has an issue. Like the piston is worn or you installed a rebuild seal incorrectly ?? Any of that sound possible ?? Did the caliper have any rust pits in the cylinder bore ?


B

[ChrisReale]

Exactly, they work fine, but they can rip...

[airsix]

If your corner ballance is way off could it cause said condition? ie most weight on an opposing front and rear wheel causing the unweighted front wheel to lock first? All the weight transfer of heavy braking probably overshadows the comparatively small effects of corner ballance, but I'm just brain storming.

-Ben

ps - Ed, how much advance are you running under boost? I'm just looking for some benchmarks. (I'm getting closer)

[Brad Roberts]

He could have it off (balance) but it would be obvious in the rear shock collars or the front torsion bar adjusters. It would take a one inch plus off balance to make a difference. More than visible by the naked eye without measuring it.

B

[john rogers]

If the calipers are rebuilt units I would suspect they haven't seated in the seals yet or one could be bad? Are the pads good and free in the caliper? Are the calipers aligned exactly to the rotor, both axially and parallel? I agree with Brad, chack the shocks to see if one is failing which could cause the side locking. If the fronts are locking then there is not much braking in the rear it seems and I would try to get somemore brake use from the rear calipers. Are you running out of brake pedal travel or close to running out as there might not be enough fluid volume to go to the rears? You might need one of the 23mm m/c and I had to use one when I pur 320 calipers all around on my race car? Good luck

[Qarl]

Chris... interesting info about matching the spring rates to the torsion bars? Do you have any additional information about torsion bar rates and spring rates. What about solid vs. hollow.

I'm putting together my suspension for my street 914-6 GT car right now.

Here's what I have....

Front:

Carerra front end with undermount swaybar and aluminum cross member.

I am probably going to get rid of the undermount bar and put on a Smart Racing swaybar. Brad says 27mm, but I'm wondering if that's too agressive for a street car.

22mm solid torsion bars up front.

Bilsten struts with Bilsten sports shocks.

I considering Mueller's roller bearing upgrade for THE front.


Rear

Rear Bilstein Sports with adjustable spring perch and 175 lbs Eibach springs (per Jim Patrick's recommendations at Patrick Motorsports).

Rear Carerra Brakes

I am also considering Mueller's roller bearing upgrade for the rear.

Car will have the chassis stiffening kit welded in. Steel flares, and 9x16 Fuchs in the rear and 8x16 Fuchs up front.

Other possibilities

I may switch to the Boxster monoblocks all the way around.


Thanks,

Karl

Can you feel the love? (IMG:style_emoticons/default/MDB2.gif)

[campbellcj]

There was a recent thread here about t-bar & coil spring rate equivalents.

The setup I'm about to try is 22mm t-bars, 22mm front Welt swaybar, 225lb rear coil-overs. I have a lot of room for adjustment on the swaybar and can easily swap the rear springs stiffer or softer for approx $100/pr so it shouldn't take long to get this dialed-in. I'll go to a Smart-Racing front bar before too long, probably when we pull the tank to do the cell and some replumbing.

I think 22's with 140's sounds out of kilter. Stock t-bars are a good match for 140 rears IMHO, as long as you have an adjustable front swaybar (19 or 22). My guess is you'd be better off with 180's or 200's.

Jim Patrick seems to favor soft rear springs. He suggested a 21/200 setup to me when I spoke with him last year. A lot of the other 914-6 guys seem to run 225-300 or more in the back.

[tesserra]

Ed you are making me nervous. I am about to install the 320 brake system on all 4 corners. I have an adjustable (knob type) prop valve. It sounds like you installed the fronts and put some miles on them then installed the rears.? Do the rears need some time to "seat" before you start changing any more parts. Weight transfer, front to rear, should not make the car lock one side before the other. How about pad type, are they the same? I have had some funky braking when the wheel bearing were not tight. You should not need to change suspension parts to make your brakes balanced if they were balanced before. Have you figured out a park brake with the new 320i calipers?
Regards,
Tesserra

[Brad Roberts]

I'm from the big bar softer spring school as of late. The large bar allows me to really fine tune the setup compared to changing springs in 25lb increments. Its much easier to adjust the bar than it is to change torsion bars and or rear springs. The large front bar also compensates for lack of torsion bars available for the car. The largest I can get in the control arm is a 26mm bar. Most places dont stock this.. so instead we go to coil-over... well... not everybody wants to go coil-over.. my thought: run a bigger front bar.

I have a car running around right now on the street with 21/27/200 and it ROCKS. 3.2 liter 9146 with Koni adjustables F+R. The car is very well balanced.

B

[Evill Ed]

Thanks for all the quick replies.

The rubber brake lines are brand new factory ATE, I just put them on Saturday to replace my suspect SS lines. One SS line was staining blue,like the ATE brake fluid appeared to be wicking out of the SS cover from the inside line.
The locking fronts existed with the SS lines too, so the rubber lines are not the problem. All the steel lines are new, no kinks, blockages or damage. The master is new too.

The calipers appear to be perfect, pistons, seals and assembly.

The fronts pads have some milage on them, approximately 5k, the rear pads are brand new, and I'm sure that they are not bedded in yet. They seem to grab stronger and the pedal is less spongey as I put some miles on them. Maybe more miles will and full srface contact will make the rears grab quicker and harder.

I will check the struts themselves for wear or damage, although they only have 5k on them also. I will re-check all wheel bearings and components.

I'm not sure, but when I had full pressure from the prop valve last night and the rears were locking, I think that the right rear may have been locking a little early too. This seems like a right to left as well as front to rear problem.

If it is a balance issue, could this possibly be caused by me sitting on the left side, though I only weigh 180 lbs.

Like I said, the braking is strong and I can feel the effects of the prop valve at different pressure settings. The locking fronts at high speed hard braking is unsettling. When they lock, the front gets loose and you don't slow down.


Ed

[Don Wohlfarth]

Ed, sounds like you have 5 series calipers on the front, 320 on the rear?
You want fronts to lock before rear, little easier to control car.
You may (?) have too much caliper on the front, but I'm not familar with BMW calipers. You may need larger front tires for more grip with this setup as you may be overpowering the tires ability to slow the car so it lock the wheel.

[morgan]

This sounds like the same as my SCCA it racecar. I found that when I bleed the brakes , I have lock up at the right front too. after the brakes get hot a number of times, the brake peddle gets a little softer and I can brake much harder with out lockup. AS they said in harlly and the marborel man (SQueeze the trigger)

[john rogers]

tesserra, I am using 320i calipers at all 4 corners and the front and rear lock just about the same time if I make it lock. Normally the braking is really even and smooth so don't worry about it as it is really worth the $$$ and time to convert. I use the 23mm m/c too so I don't have a lot of pedal travel. I am also using Goodyear slicks so I get LOTS of road traction with the tire. I also the small spot caliper that Wilwood sells as a parking brake, one for each side and I had to get custom cables as the stock ones were too short. I still leave the car in gear if I park it just to be safe. Ed, have you tried a residual valve in the rear lines? My brake system smoothed out some when I installed one for the rear brakes. Good luck.

[kdfoust]

I dunno about the F vs R lock up issue.

I was experiencing right front lock up and pulling (it must be a coincidence) when I first bought my car. It was unsettling enough that I sent the car to a p-car mechanic that gave the brake system a clean bill of health (all stock). His determination was that the pads which were relatively new had not fully seated. I drove the car some more and found that the pull/lock up began to improve, but, was variable and appeared that it would never go away completely. The pads were Pagids - don't know the particular compound but very soft. I dumped them figuring that close to 1k miles to "bed" pads is totally unacceptable. Porterfield R4S pads went in all around. The car brakes straight as an arrow with no other changes to the brakes. YMMV.

Good luck,
Kevin