24 replies to this topic

[Andy69]

I've been living with this issue for a while now but now that I'm starting to get down to brass tacks regarding car setup, I need to address it.  It's making the car slower as I have to watch how hard I hit the brakes even with the R comps.  The result is I can't brake as late as I know the car should be capable of doing.

 

1987 Si with 27.5 mm up front, 550 lb with GC coilovers in the back, Illumina dampers, and 13x7 Keizers with 225 Hankook Z214 (soon to be Hoosiers as the Hankooks are no longer available in 13", and soon to be 13x9 Keizers with 255 A7s).  It's also about as low as I can get it without a huge amount of tire rub.

 

As far as I know, only the calipers and rotors were swapped out.  The MC, lines, PV, and rears are still stock CRX.  when I got the car, it had Porterfield pads.  I've tried Hawk pads, but didn't like them because they took forever to heat up and started to fade almost immediately.  I never had a chance to see if the fronts locked up because I had to brake way early either because the brakes were cold or they were hot.  This was at a track day, not autocross.  At the moment I just have so called premium semi metallic replacement pads from Autozone.

 

I've perused the threads regarding the swap, and it seems that the MC should be changed out to the Integra version (or the Prelude, which i guess fits better), but not the PV unless also swapping in the Integra rear brakes.  But will that cure the brake bias problem?  It would seem to me that with the bigger caliper piston on the Integra brakes there is a mismatch between the volume of fluid needed by the calipers and the volume of fluid supplied by the MC, resulting in greater pedal travel to supply the same volume.  I see then how changing the MC will affect the brake feel but I don't see it affecting the bias at all.
 

I have no idea what the rear shoes are.  I'm assuming they are bone stock.  If they are, one would think upgrading to a better friction material would help with the bias. 

 

Any other ideas?  Perhaps just swapping back to the original CRX brakes?

[cbstdscott]

I am also using Integra front brakes and rear drums. And,I am using a "performance" street pads rather than racing pads for autocross.

Normally, all of these cars will unload the weight over their rear wheels under braking. This cacaus the rear brakes to lock up first, not the fronts.

Because you are not 100% certain of the mods made to this car, I suspect that a previous owner tweeked brake bias forward to stop rear brake lock up.

I would also check your rear shoe adjustment, you want them close to the drums.

On my car I am using the stock M/C and booster, Integra discs, I was using Integra cailpers but moved to twin piston calipers from a JDM Accord and aluminum drums. I have not had a lock up problem front or rear.

[Andy69]

Do you also have the Integra MC and PV?

 

I think the first step should be at least to make sure everything is working as it should, rear brakes included.  Pretty sure they are, but it won't hurt to double check.

[cbstdscott]

I am using the stock CRX M/C and portioning valve. My bakes work great.

Maybe the previous owner put in an Integra portioning valve, but never got around to putting in the rear discs.

[anjin]

I am a bit puzzled. There isn't a lot of weight on the back when braking.  So you should just be able to go for it when you want and brake at the limit of the front brakes. Something isn't adding up.

 

Your comment that you are having to brake early as the pads are too cold or too hot leads me to the problem lying in the pad material, and not the bias. You need something that is good when warm and hot, and doesn't fade. Should be something out there.

[Andy69]

I am a bit puzzled. There isn't a lot of weight on the back when braking.  So you should just be able to go for it when you want and brake at the limit of the front brakes. Something isn't adding up.

 

Your comment that you are having to brake early as the pads are too cold or too hot leads me to the problem lying in the pad material, and not the bias. You need something that is good when warm and hot, and doesn't fade. Should be something out there.

 

That's just it.  I'm hitting the limit of the front brakes (actually the limit of the front tires) way earlier than I think I should be.  I'm having to brake earlier and with less pedal pressure because of it.  My first instinct is too much front bias, given that it has Integra brakes and stock CRX everything else. 

 

I ordered up a mess of brake parts after talking with the PO.  She never did any of the maintenance on the car and so she didn't know exactly but she was reasonably sure the rear drums were totally stock, right down to the shoes, and they had never done much to them except adjust once in a while.  So I figure they are in need of refreshing even though they are not squealing and don't appear worn out.  They could be working OK just not up to 100%.  It's cheap and I learned a long time ago to make sure everything you have is operating properly before trying any upgrades or other changes. 

 

My first change might be to swap back to the CRX brakes to restore the factory balance and go from there.

 

My reference to the pads being too hot or too cold was in reference to the Hawk street pads, which I no longer have.

[anjin]

It is a few years since I did any of this, but I remember being surprised that the piston diameters didn't change as much as I expected  between the various calipers, There was more variation in pad size, the mass of the caliper itself, and the disc diameter.

One of reasons we needed to change the bias proprtioning valve when going to the integra brake set-up was the change from drum to disc in the rear, where the drum piston to disc caliper piston diameters really did change a lot.  It wasn't actually the front brakes change driving the need to change the proportioning valve.

 

What I am leading to is to stop thinging about the bias itself, and start thinking about  effect of differences in the pad area and disc diameter of the integra and crx systems.as affecting your braking.  And yes, check the rear efficency as it all helps.

[Andy69]

It is a few years since I did any of this, but I remember being surprised that the piston diameters didn't change as much as I expected  between the various calipers, There was more variation in pad size, the mass of the caliper itself, and the disc diameter.

One of reasons we needed to change the bias proprtioning valve when going to the integra brake set-up was the change from drum to disc in the rear, where the drum piston to disc caliper piston diameters really did change a lot.  It wasn't actually the front brakes change driving the need to change the proportioning valve.

 

What I am leading to is to stop thinging about the bias itself, and start thinking about  effect of differences in the pad area and disc diameter of the integra and crx systems.as affecting your braking.  And yes, check the rear efficency as it all helps.

 

The PO never had issues like this with this setup.  Then again, I think I drive the car a lot harder than she did.  I'm going to make sure everything is working correctly WRT the brakes, then go from there.  That means new fluid, rebuild the rear brakes, and check over the fronts.  I'm pretty sure they are good but I'll make sure.  Then I'll see if they still lock up in the front and go from there.

 

I get what you're saying - the fronts have more stopping power due to the larger surface area of the pads and rotors.  That's thrown the brakes out of balance from stock.  Plus the weight reduction has undoubtedly had an effect too.

 

I've spent a lot of time reading and learning about the rest of the car, now it's time to do that for the brakes I think.

Edited by Andy69, 07 April 2016 - 07:03 AM.

[Screech]

Lighter foot on brake, leave clutch engaged, I found that the engine rotate helps alot with preventing front wheel lockup. 

[cbstdscott]

Let's get back to basic physics. As you step on the brakes, the car's weight shifts forward. As weight comes off the rear tires, their contact with the road decreases. At this point the rear brakes should have stopped the wheel, but the rear tires can not contribute much stopping power as they are skimming over the surface of the road, creating a non traction "lock up" condition.

If this does not happen with your car, the chances are very good that your rear brakes are not stopping the wheel.

Easy test: Pull up on your parking brake. If it "clicks" more than 2-3 times, your rear shoes need to be adjusted.

The great advantage of the larger Integra disc is that they have a slightly better leverage on stoppping the wheel and can absorb more heat than the smaller stock disc. As pointed out, the caliper pistion size is not that much bigger, but the pads have a bigger face.

On my car, I have a set of JDM-only twin piston calipers. They use the same pads and discs as the Integra. The greatest difference is the,area size of the pistons are obviously bigger and provides a more even clamping on the pads.

I used to get a lot more rear brake lock up before I made my suspension modifications, the stiffer set up prevents as much weight shifting and more weight is staying over the rear.

Your front brake lock up is a factor of not enough rear brakes due to misadjustment or portioning valve issues.

[Andy69]

A proportioning valve problem would result in the rear brakes locking up quicker, rather than the problem I'm having.

 

Well, sort of.  It depends on the problem.

 

Proportioning valve is a bad term.  It doesn't proportion anything.  It's purpose is to limit line pressure to the rear brakes to a given PSI.  Drum brakes lock up at much lower pressures than disc brakes, plus under heavy braking the weight shifts off the rear to the front.  Unregulated rear drum brakes would lock up under heavy braking conditions as the brake line pressure exceeds the grip of the rear tires.  The purpose of the proportioning valve is to limit how high the pressure goes and thus prevent rear brake lockup.  If the proportioning valve has gone bad, the rear brakes would lock up easily - the opposite of what I am experiencing.  So if the PV problem we are thinking about is  a bad PV, then that is not the problem I am having.

 

If you consider the problem might be the WRONG PV, and that I need one from an Integra, one must think about it differently. The Integra has rear discs.  Discs are much happier with higher line pressures.  In fact, they need much higher line pressures in order to work properly.  So the Integra PV has a higher terminal pressure than the CRX PV.  If you used an Integra PV with a stock CRX, the rears would lock under heavy braking.  In a CRX equipped with Integra front brakes, it gets a little muddier.  The precise balance between the front and rear brakes engineered by the factory has been upset. 

 

Now the first instinct is to assume that the imbalance is caused by the addition of the larger front brakes - that the fronts have more stopping power in relation to the rears and the rears can no longer do their job.  But, they are still the same rear brakes, with the same PV, and should still lock up at the same pressure.  If aythig, under that scenario, if the front brakes are now more effective, there should be more weight transfer off the rear, and potentially cause the rears to lock up where before with CRX fronts they did not.

 

That's what led me to believe that the benefit of the larger calipers, rotors, and pads is not an increase in stopping power but a decrease in component wear and and increase in heat dissipation.  I started poking around and found this article from StopTech

 

http://stoptech.com/...grade-selection

 

 

 

Only increasing the effective radius of the disc, the caliper piston area, the line pressure, or the coefficient of friction can increase brake torque. Increasing the pad area will decrease pad wear and improve the fade characteristics of the pads but it will not increase the brake torque.

 

So, my hunch was only half true.  The pads are larger, but the diameter of the rotor is also larger.  So I should have better fade resistance and also slightly better braking.

 

So, it would seem that the Integra brakes do increase torque.  By how much would require a calculation of the increase effective surface area of the rotor.  My guess is maybe 15-20%.  Then add to that the increased coefficient of friction of the better pads I generally use for racing  I haven no idea what that may be

 

So given that the fronts now have slightly more brake torque than they had with the CRX brakes, the rears, if anything, should be MORE prone to locking up, not less, provided the front braking power does not exceed the ability of the tires to stop the car.  That seems to be what is happening here, although the CRX brakes are designed to stop the car effectively in full panic mode on dry pavement with as little drama as possible.  That owuld mean the Integra brakes have to have enough stopping force to create that drama not only on dry pavement but with sticky racing tires.  Do they?  I kind of doubt it.

 

So, something else must be going on.

 

WRT the proportioning valve, and bad one and an Integra one should cause the rears to do the opposite of what I'm experiencing.  It could be that the PV was replaced with one that has a lower cutoff pressure, but I can't see why that would happen.

 

In addition, since what really stops the car are the tires not the brakes, the upgrades I have made to the tires - moving from street tires to 100 tw tires to 40 tw r-comps - should have resulted in a greater tendency, all else being equal, for the rears to lock up as the fronts would more stopping power.  It seems like they have, although I still get front lockup even with the autocross compound 225 Z214s.

 

Suspension changes also affect the brake balance.  That's where my looking into this has come up short. But it seems to me that a stiffer suspension does not result in the transfer of less weight, but rather increases the speed with which the existing weight transfers.  It only SEEMS like less weight because the side or end of the car the weight is being trasferred to does not compress as much.  The only things that affect the actual amount of weight transfered are physical speed and the initial weight itself.  The more initial weight, or the faster the turn (or more rapid the stop) the more weight is transferred. 

 

Now, as my confidence and driving ability increases, so does the speed at which I drive, stop, and corner.  So theoretically there should be MORE weight transferred to the front wheels during braking, which would result in less weight over the rear wheels, which again should result in the opposite of what I'm experiencing.

 

But it seems to me that what's happening is the rears just don't have enough stopping power.  If there is still a problem after I make sure the brakes are a peak operating condition, then I need to look at other options including increasing the rotor surface area or upgrading to shoes with an increased coefficient of friction, which should both increase the brake torque.

 

The other option might be to swap in a rear brake valve that will allow me to fine tune the pressure to the rears.

Edited by Andy69, 07 April 2016 - 12:11 PM.

[Andy69]

It could be a bad metering valve.

 

The metering valve delays the pressure to the front disc brakes to allow the slower rear drums to start to apply first.

 

Symptoms of a bad metering valve include diving front end and increased pad wear.  The car does that, but because I;m racing :-)

 

Still, that implies more weight to the front wheels, which may be causing them to lock up.  Worth looking into if redoing the brakes does not fix the problem.

[cbstdscott]

Have you done the hand brake test? How many clicks?

[cbstdscott]

Have you done the hand brake test? How many clicks?

[cbstdscott]

Have you done the hand brake test? How many clicks?