CrookedRacer

Hello, everyone! This is my first post here on Rennlist!

Last year I bought a 1988 944 N/A with a cage and a few suspension mods.

https://grassrootsmotorsports.com/fo.../105890/page1/

It has upgraded brake calipers/rotors from a Non-M030 optioned 968 (cross-drilled rotors, four piston calipers all around).

I've done three track weekends with it, and I noticed a repeatable problem with the brakes:

At Summit Point Main, Turn 10 (a right sweeper) is the fastest turn on the course, which dumps onto the main straight before Turn 1.
If I track out onto the rumble strips on the left side of the track at the end of the turn (beginning of the straight), the heavy vibration seems to affect the brakes, such that I need to pump them 2-4 times to get pedal back.
That's a very scary situation coming into Turn 1 and nothing happens!

If I just brush the edges of the rumble strips, I'm ok. But if I really get into them or cross them altogether (more vibration), I lose my brakes, and I need to pump them back up again before the end of the straight.

Once they're pumped up again, pedal feel is fine. So I don't think it's air or contamination of the fluid. It's like the brake pads and/or calipers are being pushed over enough to displace a few pumps-worth of fluid back to the master cylinder.

Has anyone had this issue at all? Thanks in advance for your input on this.

kevin12973

That's called pad knock back. It's a common problem with fixed calipers and sometimes even with floating calipers. I know someone has used springs inside the Pistons to push the pads slightly against the rotors but I don't think that's an option with porsche calipers.
Just saying, the original NA floating calipers are up to anything you can through at them. Ask anyone running SP2 !

CrookedRacer

I just found this when a co-worker mentioned the term "knockback"... I had never heard the term before.

http://www.stoptech.com/technical-su.../pad-knockback

I guess it's pretty severe in my car, which has the fixed calipers now. I'll be going over the bearing situation before the season begins, too.

Thanks!

StoogeMoe

Yup. Just tap the brake pedal with the left foot while going down that long straight so they're ready for turn 1.

Van

I bet you have a bad left rear wheel bearing. It's causing the wheel hub to move around, thus giving the brake rotor enough movement to push the pads back into the caliper.

CrookedRacer

I got into the right rear bearing which was making noise during the last track day. Here is what it looked like...

<img src="https://farm2.staticflickr.com/1581/24528330484_891a650a5d_z.jpg" width="640" height="360" alt="2016-02-20_08-52-43">

Looks like someone had a tough time getting a bearing out before. I bought the Front Bearing Adapter Kit from Harbor Freight which made it easy for me, anyway. I highly recommend the kit. I was going to get the arnnworx tool but the HF kit will do so much more.

Here's where that noise was coming from.

<img src="https://farm2.staticflickr.com/1607/25065757911_072557a444_z.jpg" width="640" height="360" alt="2016-02-20_08-53-05">

It's just this small area of nastiness on the inner race of the outboard half of the bearing. There were no identifying marks on the bearing itself, but maybe when I get this race off, the edge of it will reveal the brand.

In any case, it didn't seem to be a big issue with play, but it was definitely time for a new bearing! More to come.

CrookedRacer

I got it all back together again... once the race was off, it was fairly straightforward to pull the hub into the new bearing. The Harbor Freight kit worked great here too. It was really easy to put it back together again.

<img src="https://farm2.staticflickr.com/1573/25055845252_478b2dc0da_z.jpg" width="360" height="640" alt="2016-02-21_12-52-10">

The brand of the old one was SKF made in Spain, so it's not like it was some sort of cheap part.

The new one that went in was F.A.G. made in Brazil.

Overall it was fairly straightforward. The toughest part was getting the hub out. That was accomplished by pounding on the hub puller from the inside with a draft. Because the tool I borrowed from Advautoreillyboys didn't come with a slide hammer thingy. That might have been the ticket.

CrookedRacer

I lied - I hadn't gotten it ALL back together again.

I still had to torque the axle nut to 350 lb-ft.

So I had a brand new Craftsman breaker bar - what looked like the strongest thing in the store, and as I'm trying to apply 87.5 pounds of normal force to the end of my 4' pipe extension, I get to about 78 pounds and POW!

<img src="https://farm2.staticflickr.com/1655/24544917344_60c1324a91_z.jpg" width="360" height="640" alt="2016-02-21_02-13-18">

The breaker bar breaks.

I got a replacement at Sears, but now I'm scared to use it! What kind of tool do I need to get a full 350 pound-feet of twist on this nut?

Noahs944

That's 3/4" drive not 1/2" drive territory brother I'm thinking.

You did well on the bearing fix & I think your black 944 looks killer.

CrookedRacer

I have a 3/4" drive 32mm impact socket on order from Amazon.

I also have a 3/4" drive breaker bar that I used once in 1986 to remove a crankshaft pulley on my '75 Triumph Spitfire.
That bar has been sitting idle in my toolbox for 30 years, and will finally see some use!

I may purchase a 3/4" drive extension so I can torque the nut with the wheel on the ground against a chock. Safer that way.

Thank you, Noah!

Van

You did both sides, right? Also, make sure you replace the axle nut.

CrookedRacer

Thanks Van,

I only had all weekend, so I only got one side done.

The right rear bearing, which was the one making noise during my last track weekend on left turns, is now a new, quiet one. I replaced the nut with a new one when I put it all back together.

I'll have all the tools and techniques to do the other side, except for a hub puller. I'll have to borrow that again. It would be nice to be able to do the whole job with tools I own, though.

As I recall, the brake pad knock-back only happened when I was WELL over the rumble strips and onto the narrow strip of pavement on the other side of it... far enough to put the RIGHT wheels onto the bumps. So your prediction of a bad wheel bearing might have already been validated, even if the left bearing turns out ok. It's possible the right one was the only loose one.

Thanks for all your videos - I think I've watched every minute of your 944 content. I've learned so much from them. Keep them coming!

CrookedRacer

Also,

I'm nervous about how to keep the rotor from turning when torquing the nut. I saw a video (was it you, Van?) where you put a big screwdriver in the rotor vanes to stop it against the caliper.

On my car I've got 968 drilled rotors and 4-piston calipers, and on the passenger side there's a brake line running right where the screwdriver would have hit it.

So when I put the screwdriver between vanes, I chose a vane that didn't have any holes drilled nearby, and spaced out the screwdriver shaft with another screwdriver inside the brake line so it wouldn't get crushed. That worked for as far as I got torque on it. But I didn't like the whole situation. That's a lot of force.

Van

You can also have someone step on the brake pedal... Or you can put the wheel on, set the car on the ground, and access the axle nut through the center cap hole.

You can also put a large piece of angle iron or other rigid stuff between wheel studs (thread on the nuts to protect the threads).

And, if you can, use a torque wrench. If you don't have one, borrow one from someone with a cup car - they use them for the center-lock wheels.

kevin12973

To get the torque value you can use a torque multiplier. It is a gear reduction type of ratchet so at 4 to 1 ratio, you only need 87.5 lbs to achieve 350 lbs of torque. Its used often on heavy equipment work.