gracer7-rx7

iTrader: (16)

A few manufacturers make these anti knock back springs and they are available in 4, 6 and 7 lb configurations. They would mount behind the caliper piston.

I actually called and spoke with the guys at HRP and they also suggested that a WILWOOD RESIDUAL PRESSURE VALVE might help the situation.
http://www.hrpworld.com/index.cfm?fo...action=product

I personally dislike the 'tap up' method. I'd prefer to get rid of the knockback issues if possible. I often get this happening on the street also which is annoying.

gracer7-rx7

iTrader: (16)

It seems that all of the issues are really around the pads.

At first there were fitment issues caused by some incorrect assumptions and incorrect information in general. The rotors and calipers and the hardware in general appear to be of high quality.

Since I haven't personally driven these, I'll let hyperion and adax comment further on that.

I would like to point out something though - It seems like most BBK vendors do not really do much instrumented testing on the kits they sell. Stoptech is an obvious exception to this as they do perform some rigorous real world testing on the car to ensure that the piston sizes provide the kind of balance they want and that everything works properly in the real world. Most BBK vendors rely on the engineering and math to dictate what the correct sizes of the various parts should be. I don't see any other vendors providing specifics on their test programs (if they have any).

adax

iTrader: (2)

This potential solution looks attractive! Did they recommend one per wheel or just one off the MC? Since it is not designed for the application we are discussing, I assume they have some experience with it being used for this?

GooRoo

iTrader: (4)

I agree with all of this. I think that the 'mistake' that people made was changing to 2 new setups here. 1) the caliper/rotor setup 2) an unknown/tested pad. I use mistake there not to point blame at the buyer, but to point at the issue people had. I'm a *very* strong believer in only changing one unknown thing at a time. Yes, it is slower, but you have a verifiable trail that you can go back over to try to troubleshoot a problem. Switching both the brake setup and pad compound to something new and untested is where you guys got into trouble. I realize that sometimes this is impractical too, and I've ignored that rule in certain situations myself, but here I think it would have served everyone well.

Now, that said, I understand that you guys were buying into what RB and Howard said and you believed that it would be track ready. However, if you go into the history of the thread I think you will find that Howard never actually took the setup to the track. No track = no useful information for me.

For myself, right there, I got red flags and said that I'm going to stick with pads that I know are good (HT-10's in my case) until I verify that the new calipers are solid, and then once I get back to a stable state I can experiment with pads. Lucky for me you guys have done testing and found they're junk.

This part I think is more complicated. I have met and talked with Howard on several occasions and I don't think he did anything here that was out of line. Since I didn't get the front calipers I haven't followed that part of the discussion as much, but it seems like there was some problems with the measurements. It seems like RB should have had several people doing measurements with different cars to check that they were all the same. This also should have been caught by RB and Howard once they sent out the 'test' Production calipers for fitment. If I was RB I would have found 5 people who are major track addicts and given them the first 5 sets at a discount and get feedback before going full production. As Hyperion said, this is how engineering is done.

I think we can all agree that this was going to be a V1 product and everyone should know that there is a good chance in all V1 products that something will be imperfect. That is true for brakes, turbo kits, motor builders, etc etc etc. If you want to be safer, buy only products that have been on the market a long time and that you can read numerous reviews/experiences with. Even then it doesn't always work out with custom/aftermarket/racing gear.

Again, not trying to beat anyone up, and it's really all moot at this point for this brake situation.

What isn't moot:

1) Make sure you get the revised kit if you get the fronts, the rears are fine.
2) Don't use the RB pads for track use, stay tuned for rx7club user recommendations (mine is HT-10s, for low to mid powered cars)
3) Use the caliper and brake pad paints (like hyperions stuff from above) to determine temps if you're going to track to the car to make sure you're in the right range.

hyperion

iTrader: (2)

I don't think this would work. The force of the knockback is huge, but the displacement is small. Which is to say that anti-knockback springs work because they allow the pads to knock back but then return them to their proper location once the car is no longer under cornering loads.

The hydraulic valve won't hold at only 2psi. Like the product blurb says, it's really designed to combat hydrostatic flow when the MC is lower than the calipers. It won't return the pad to a location because once the 2psi is overcome, the fluid flows back into the lines.

But who knows? The only way to be sure is test.

-ch

hyperion

iTrader: (2)

I think the responses to this question above are spot-on. I'll re-iterate where I stand:

1) I think the caliper design appears sound, and the SS pistons are a big plus
2) I would stay away from the pads. Go with Hawk instead.
3) Knockback is an issue the is inherent in the FD, and this kit doesn't address it--but most won't even notice it
4) If--and it's a big if--there is a heat soak issue, the RB rotor design will benefit less from ducting due to the way their rotors and rotor hats interface. Ducts take advantage of the fact that the outside of the rotor is almost sealed from the backside so the air is forced out through the cooling vanes. The RB design has big gaps, so if testing shows that you absolutely must run ducts, it may prove challenging to get them to work properly. (TBD.)
5) Let's face it: the RB kit is about 50% the cost of any other kit out there. A set of StopTech fronts are over $2100. A set of RZ rears can cost upwards of $1000. If this kit can be made to work, it's a great value. For some that matters, others not.

More to come,

-ch

GooRoo

iTrader: (4)

Well, 10 pounds of residual pressure will generate something on the order of 60-80 pounds of clamp load on the front caliper (depending on whose front kit you're using)... which divided by the 4 pistons means something like 15 lbs per piston. (ignoring compliance) So I think that would actually be quite a bit more force than the 6 lb knock back springs. I would think you'd want to use the least effective force to get the pistons back since the excess basically just wears your pad for no reason.

So it would seem that the 10 pound residual valve would be an option over the 6 pound springs inside the caliper if you were still having knockback.

I can see where it would be nice to be able to connect/disconnect a valve without having to rebuild the calipers (like you have to do to install/uninstall springs) for a street driven car.

The 2 pound residual valve would be about 3 pounds per piston... so that might actually be a first option to try. I have no experience to know whether or not that will be enough to stop the knock back.

hyperion

iTrader: (2)

I'm not sure, but what I'm arguing is that the systems work in two different ways. The Wilwood is a check valve. Once the valve is overcome (10psi) the fluid can flow. Does the bending of the upright and bearings generate more than 50-100lbs of force on the pads? I'm pretty sure it does. So the valve will be overcome, fluid will flow back into the lines, and the pads will knock back. After this, there is no force pushing the pads back into place.

The springs on the other hand deflect into the bores during knockback, but then push the pistons back out. The springs need only overcome the force of the drag of the piston seals and any hydraulic losses to pump fluid back into the calipers. You try to set the spring force so that you're just barely overcoming these forces to return the pad into place after knockback has already occurred.

In other words, you can't stop knockback, you can only reverse it after the fact.

Or so I think.

-ch

adax

iTrader: (2)

^ Well put hyperion, that makes a lot of sense. On with the springs.

Regarding only changing one variable at a time, I couldn't agree more being a scientist by training. I would have used my usual pad (Performance Friction 01's) for this very reason but based on the earlier statements about RB not warranting their rotors when used with pads other than theirs or Hawk's, I decided against it. Good point none-the-less and lessons learned.

Further thoughts on knock-back:

I believe some of the observed effect may due to the pads and rotor expanding when hot and contracting when cold. I've observed this when I've adjusted by brake pedal return to keep the free-play as small as possible. I can adjust free play such that there is no drag when cold but after a mile or so of driving, the drag will become significant. I actually got stranded 100 yards from my house with locked brakes. Admittedly the effect is probably small compared to knock-back, and I don't understand why the fluid doesn't flow back into the reservoir (maybe because I did it with new pads and the pistons were bottomed out). The reverse could be happening on the track. As components cool, they contract and suddenly you have free pedal travel.

gracer7-rx7

iTrader: (16)

They are not familiar with the FD braking system but they had suggested that normally these are mounted "after the MC and before the Y". That assumed one exit from the MC where we have two. I would think that it would be installed at the front brake line exiting the MC.

Unfortunately, this is well beyond my scope of knowledge so I can't contribute anything useful beyond that.

Googling around reveals that oftentimes sanctioning bodies and race car prep shops and even motorcycle racing use anti knockback springs to deal with this issue.

GooRoo

iTrader: (4)

Yeah, I don't think that's the way it works. I think it lets 100% of the pressure past from the MC to the caliper, then when you release it lets all that pressure back, except the last x psi. So yes, the caliper then deflects and pushes a small amount of fluid back (since we now have over x psi again due to the pistons being pushed) however there will then be some residual pressure still in the calipers to push the pistons back to the face of the rotor. I think some of this could also be mitigated since the opposite pistons will be pushed out due to the residual pressure.

What would be ideal is a adjustable valve, so that you could turn in just enough residual pressure to get the pistons back to the rotors, while not so much that you cause alot of pad drag.

ptrhahn

iTrader: (9)

Funny, HRP probably suggested these because I called them and Wilwood last winter about them, and both indicated they'd never thought of them in relation to knockback, and weren't sure if they'd work.

Long story short, I couldn't source adapter fittings that would allow me to splice them in at the junction of the hard line and braided line in the fender well. Wilwood sent me to a guy in Cali who supposedly makes half of the BBK brake lines for folks, including StopTech, and he couldn't do anything for me.

Honestly guys, unless you've got a pure track car, I'm not sure how much you want antiknockback springs. You're essentially dragging your brakes at all times.

I'll tell you what WOULD be cool, is an anti knockback button. A thumb-operated device on your steering wheel that, when pushed, creates a single, measured brake application... just enough to set the brakes back up. I find that, depending on the track, corner, speed, stress level, etc.,my right foot "tap up" isn't very consistent... sometimes just right, other times heavy enough to lurch the car a little. I also worry about the fact that anyone following me sees my brake lights flash after i've tracked out... which I would think could be disconcerting if you weren't expecting it.

billyboy

If indeed spindle rigidity is causing knockback problems on the FD, might be worthwhile sourcing some stub axle stiffeners. A pretty common fix over here - at least for previous generation rotors when neg is cranked in.

ptrhahn

iTrader: (9)

You have a link to someplace with pictures of a stub axle stiffener? I wouldn't even know how it would work.

cabaynes

iTrader: (4)

It's funny you guys are mentioning this, I was just recently watching one of those Best Motoring DVD's from Japan with Tsuchiya and other pro racers. They were doing their usual tests of new/tuned cars and racing them around Tsukuba. I noticed that almost EVERY driver would "tap up" the pads before entering every corner, on just about every car. This usually happened as a quick tap about 1 or 2 seconds before the braking zone began, at the end of the straight. It didn't seem to phase anyone, as I would have thought it might due to the close nature of the approaching braking zone, but I guess they're just all so used to it from the other drivers by now that they can recognize it.

For what it's worth they were doing it on just about every car in the videos I've seen, from tuned FD, Silvias, GTR's, even the handful of Porsches and Ferrari's they've done. Not sure if the problem was evident or if it's just a habit they've developed, but it seemed to be common practice.

adax

iTrader: (2)

The easiest way to tap up for me is to do it with my left foot while still on the gas well before the braking zone. It doesn't disrupt my rhythm this way as it does if you try to do it right footed immediately before braking.

hyperion

iTrader: (2)

This is how I do it too. This way, you also don't lose 0.5 sec in the braking zone while your throttle foot is priming the brakes.

-ch

hyperion

iTrader: (2)

Hmm, sounds like a technique that you might want to perfect for just about any road car. As you pointed out, you can do your tap-up any time after the previous corner and/or rumble strip. A second or two before the braking zone makes sense.

Great info!

-ch

ptrhahn

iTrader: (9)

^^^^^

You don't have to wait that long, in my experience. I tap 'em up with my clutch foot right after I track out of the prior corner, or right after I upshift, and it "lasts" until the next corner. I don't like to do it later near the brake zone because by that time you're going much faster, and concentrating on the marker, etc. It really requires almost no pressure, so little in fact that it's hard to do it that lightly.

billyboy

Disregard, got my thoughts on tapered roller bearing hubs, where you basically preload the bearings with a close fitting spacer, induce a tension load which in turn reduces the deflection of the spindle.

hyperion

iTrader: (2)

Well, it looks like it's going to be damp at times tomorrow at Laguna, but let's hope we can get a few dry sessions in. This test follows my previous at Infineon two weeks ago.

This time around, I'll have a few more diagnostic tools at my disposal:







The red dots are temperature paint that reads up to 1471 degrees F. The Hawk DTC-70 pads are rated up to 1600F, but if the rotor edges and backing plates are getting to over 1400F that means things are very close to the failure point and some additional cooling (ducts) or thermal capacity (bigger brakes) will be required.

The temperature strips are by Alcon and read a pretty wide range. According to them, if the outside face of the caliper reaches 370 degF you need more cooling and must using racing brake fluid. If the calipers are operated at 410 degF for more than 30 minutes Alcon suggests checking for damage at the caliper mount points and replacing the hydraulic seals. Alcon along with AP and Brembo are the leaders in racing brake development, so they know their stuff.

It's pretty impressive to think that the pad/rotor interface could be 1400+ degF while the caliper body is only in the mid-300's. Shows how much things like piston material, backing plates, venting, and forced cooling can affect heat transfer.

Anyway. My tires are starting to get a bit heat cycled, but they should still generate enough grip to give the brakes a good workout. I'm running the Hawk DTC-70 up front and HT-10 in the rear.

Will report back after the test,

-ch

sorosiej

Thanks for all the info. For the simple job that brakes perform, there sure is a lot of design and development involved!

GooRoo

iTrader: (4)

Very cool. That looks very similar to what Damian did at NASA nationals when he/we finally got his brakes working right. I'm very interested to see what you think in terms of braking feel, and what data you end up with from all that paint.

cabaynes

iTrader: (4)

Speaking of Damian, what brake setup is he running? Or has that already been answered?

m5q8

any body try to use 350z brembo kit for fd ?????