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TR6 TR6 Brake Balance Problem

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rooster

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Hi,

I recently did the mod to the rear brakes where the stock rear brake cylindera are replaced with the ones from a Morgan. At the same time I also put new rear brake pads, and new flexible brake lines all around. I did not put the Toyota brake calipers on the front, since I have heard that they don't make a difference in stopping distance (according to the tests that some folks have done).

The problem I am having is that the rear wheels will always lock up before the front.

What I have done so far:

1. Bled the front and rear brakes (I've done this several times on the TR and other vehicles so I think I have a reasonable technique) From what I can tell the brake fluid in the system is all fresh.

2. Played with the adjusters on the rear, and road tested. This doesn't seem to have made a difference, the rear still locks up first, even with the adjusters backed off almost all the way.

3. Checked for leaks, none found.

4. Adjusted tire pressure, all tires are equal.

Is there something that I am missing? Do I need to also have the Toyota front calipers to make this mod work?

Thanks in advance for any help! /bcforum/images/%%GRAEMLIN_URL%%/thankyousign.gif
 
I never heard of anybody adding the 7/8" rear cylinders without add the Toyota 4-pot calipers. So that maybe what you're missing. You can always an inline proportioning valve to the rear circuit and turn it down until you get the balance you want.

You're probably setup pretty good for downhill road courses right now though. /bcforum/images/%%GRAEMLIN_URL%%/grin.gif It's always good to have more rear brakes on downhill corners. (joking)

As far as test results comparing all stock versus Toy fronts and stock rear versus Toy fronts w/morgan 7/8" rear cylinders, according to John Lye's test, total stopping distance went from 262 feet to 215 feet. This sounds fairly significant to me and the 4-pots are supposed to be more fade resistant.

John Lye said:
EARLY (prior to metric calipers) Stock configuration with semi metallic pads, composite shoes and 0.70" rear wheel cylinders: 262 feet with a standard deviation of 19; 6 data points, tossed out both the minimum and maximum measurements.

Toyota front calipers with semi metallic pads, composite shoes: 243 feet with a standard deviation of 9; 8 data points, tossed out minimum and maximum.

Toyota front calipers with semi metallic pads, composite shoes with a 7/8 inch brake cylinder:
215 feet with a standard deviation of 9; 9 data points, tossed out minimum and maximum.
Click to expand...

YMMV of course
 
Somewhat the same to what I did, but I used Nissan rear wheel cylinders. Query, how did you replace rear pads? I replaced the rear shoes and got lockup also, after brake flush, rebuild and steel lines. So went with a different shoe compound. Think it was Mintex, and my locking up went away.

So, either of two courses of resolution, try different rear shoes or install a proportioning valve.
 
Good point about the rear shoes.

If you used Kevlar, did you know that you have to bed them in gradually with several successive almost stops?

Standard shoes won't need bedding in usually.
 
Interesting point about the brake shoes. I didn't think about that. I used the standard shoes from V.B on the rear. I don't think that they are anything other than standard (definitley not high performance) pad material. I replaced the front pads a couple of years ago, but I don't remember what type they were. I did do many hard stops while playing around with the balance so I don't think that bedding in is the problem.

At this point I'm tempted to remove the Morgan cylinders and go back to stock, at least until I get a pair of the Toyota fronts. My car is a daily driver and I do a lot of highway driving so having the rear lock up isn't an option. It's not a big deal at 40 mph (I know from practice) but at 75, I don't think I want to find out.

Thanks for the help so far!
 
Actually, if they were high-performance shoes like the Kevlar shoes Ted S. (TSI) sells, hard stops would have killed them.

Better to err on the side of safety as you say. Did you rough up the inside of the drums to remove glaze?
 
No I didn't rough up the drums. I don't think there was any glaze in there.

Thinking about it though, it makes sense from the data in the test by John Lye (I hadn't seen that one before) that the front would need to be changed over to the Toyota's at the same time as the rear is being changed to the Morgans. The Toyota's are probably a little bit larger in volume compared to the TR front calipers, so you would have to change the front in order to maintain the correct brake balance if you increase the diameter of the rear cylinders.

I'm going to go back to the stock for now and get the Toyota's and upgrade everything at once (when I have more time to fiddle around).

Thanks again for your help!
 
I have the Morgan rear cylinders and stock front calipers with no problems (brakes seem pretty balanced). I do however have the Hawk front pads, which provide greater friction than the semi-metalics. Perhaps this is all you need?

As I understand it, the total cylinder area of the Toyota dual pistons is about the same as the stock calipers, although the pad area is greater. So with the Toyota conversion, you get pretty much the same stopping power, but with less fade.

Rob.
 
rooster said:
Hi Rob,

Where did you get the Hawk front pads? Are they the same as the TRF green ones?

Thanks
Click to expand...

Moss sells them - p/n 585-600, but I only see pre-71 listed.

Rob.
 
Just a question but could the brake warning valve PDWA( I guess thats its proper name) be causing the problem? if it were shifted toward the side of the front causing more pressure on the back?
 
hondo402000 said:
Just a question but could the brake warning valve PDWA( I guess thats its proper name) be causing the problem? if it were shifted toward the side of the front causing more pressure on the back?
Click to expand...

From what I know of the PDWA, its function is only to sound the alert when the front or rear hydraulics has failed by turning on a dash light and has nothing to do with brake proportioning. I thought the same a you and was corrected by someone who knew more about the thing than I do.

You are correct:
PDWA = Pressure Differential Warning Actuator (it's fun to say this like Marvin the Martian, especially Act-U-aaa-t-oor!)
 
I don't think the PDWA is the problem. The brake warning light is off, so it's pretty close to being centered if not centered.

I am wondering if my front pads are shot. Last year I replaced the brake servo and while I was adjusting the new one (a nissan unit) I had the part that pushes on the master cylinder sticking out too much so that the front brakes were dragging causing them to overheat quite a bit.

Could that cause the front pads to loose their effectiveness?

Is there any way to tell by looking at them if they are still good? (edit... there is still a lot of "meat" on them)

I can pull them off later this evening, and take a look.
 
Your Brake servo is a Nissan unit? That may be part of the problem right there if it doesn't match the stock units output. There could be another culprit as well.

Under heavy braking more weight is transferred to the front and takes weight off of the rear tires. More weight on the front tires will mean that it will take a lot more for the front tires to lock. Less weight on the rear tires will allow the rear tires to lock up easier. Especially if you are travelling in a straight line (Dynamics change while cornering). If the rears are locking up under heavy straight line braking (like emergency heavy braking) and as long as they don't lock up immediately, then it could actually be a suspension problem. Stock TR6 suspension is actually pretty soft for a sports car. Too soft of a suspension will allow too much weight transfer under heaving braking and allow early rear lockup.

This is also why early ABS systems actually increased your braking distances. Early ABS wasn't actually designed to reduce stopping distance but to allow control during an emergency stop so you could steer around trouble. They just forgot to train drivers to steer around trouble as most people just stand on the brakes and pray. Early ABS would reduce braking if any one of the tires in the system started to lose adhesion and lock. The easiest tires to lock are actually doing the least amount of braking when they reach the point of locking. 2-Channel systems at least separated front and rear axles. Modern 4 channel ABS controls each tire separately so that all 4 tires can be held at the threshold of maximum braking efficiency dependent upon each tires available traction. This dramatically improved braking distances and maintained the ability to maintain control and steer around trouble.

If the rears are locking because of weight transfer, then you need to look at your springs and shocks. Your suspension can have more effect on braking performance than most realize. Primarily through allowing too much weight transfer and/or not maintaining tire to pavement contact.

For the record, I have the 7/8" wheel cylinders on the rear with standard calipers and pads up front and it is more balanced the stock. More rear braking but not too much. My rear brakes do not lock early (if they do, it is a fraction of a second).

Just another iron in the fire that could be helpful. I hope it is.
 
Hi Shawn

I don't think the brake servo is the problem. The reason is that I didn't change the master cylinder so that would not affect the proportioning between front and rear. The brake servo is only applying force to the master cylinder but not changing the ratio of fluid to the front and rear brakes because I am still using the stock master cylinder.

The Nissan brake servo does apply more force than the stock unit ( at least my old barely functioning one) but to lock the brakes takes the same amount of force applied to the master cylinder as before. The reason I changed to the Nissan is because the stock one just wasn't working any more. (the Nissan one was only 50 bucks)

I do hear you about the suspension though. I have the TRF uprated springs all round, koni shocks in the front and koni tube shock conversion in the rear. The rear shocks are set 1 turn stiffer than the front (I got this setting from some one else here on the forum). This could also be my problem, too soft in the front, too hard in the rear. This might be fine for stock brakes but might not work for uprated rear brakes with more force.

I used to have the koni's set up with the front and rear at the same setting, and not had a problem with the rear brakes locking first but that was with the stock rear brake cylinders. I can try that again by putting the rear shocks to be the same as the front (1 turn softer,cause adjusting the rear is much easier than the front) and then road test, and see what happens.

If anyone who races reads this, does this make sense to do?

Shawn what suspension setup do you have?

Thanks

Todd
 
If you like to stay with the Morgan cylinders at the rear you might be able to solve the problem with installing steel flex lines to the front brakes and the normal rubber lines to the back brakes.
The rear system get the brake impulse with a slight delay that might be able to avoid locking.

The Morgan cylinders normally used in combination with an upgrade of the front brakes pad material to keep the balance.

Without the upgrade of the front the Morganeerd rear will always lock.

One other way to go is with an adjustable brake system.....

waagebalken.jpg
 
MadMarx said:
If you like to stay with the Morgan cylinders at the rear you might be able to solve the problem with installing steel flex lines to the front brakes and the normal rubber lines to the back brakes.
The rear system get the brake impulse with a slight delay that might be able to avoid locking.

The Morgan cylinders normally used in combination with an upgrade of the front brakes pad material to keep the balance.

Without the upgrade of the front the Morganeerd rear will always lock.

One other way to go is with an adjustable brake system.....
Click to expand...

Thanks for the info!

I'll try the suspension change first since it's easy enough to do. If that dosn't work then I'll see if I can get a set of high performance pads for the front in the next few days. Hopefully that will do the trick.

Unfortunately (or not) my TR becomes my only daily transportation on the weekend, as the insurance on my winter car ends then. The insurance here is ludirously expensive so I can only afford to insure one car at a time. This means that I have a limited time to experiment with the brakes before I have to go back to the stock setup.

I'll post an update once I've tried the change to the rear shocks. It might take a couple of days due to the rainy weather in the forcast for the next while.

Thanks
 
The mention of a Nissan booster is interesting. Is this an easy replacement, and does it have advantages other than availability?
-Dennis
 
rooster said:
Hi Shawn

I don't think the brake servo is the problem.

If anyone who races reads this, does this make sense to do?

Shawn what suspension setup do you have?

Thanks

Todd
Click to expand...

My bad, I read servo and thought M/C. I've seen other TR6's with Japanese M/C's and made an assumption.

Christian does mention a different way to 'fine tune' the balance through the different flex hoses. Though it would probably be easier to upgrade the front pads. At least you wouldn't have to re-bleed. /bcforum/images/%%GRAEMLIN_URL%%/wink.gif

It could be down to your shock valving. You might also lower your rear tire pressure a tad bit, maybe two pounds, and see what happens.

I have TriumphTune Fast Road Springs and KYB's at all four corners. With 195/65R15's my car sits 1.5 inches lower than stock (which greatly changes the dynamics of the car by itself, higher/stock ride heights will see greater weight transfer). My tire pressures are 30lbs in the front and 28lbs in the rear. I have braided stainless hoses front and rear. Standard rear shoes and front pads. Standard Calipers and Morgan wheel cylinders. (Better pads are on the 'to do' list.)

I guess when I think about it, my rears do lock before the fronts. However, they basically just give up a quick chirp of lock up right before I stop. Not enough to shorten my stopping distance or lose control. Not even enough to hurt the tire by overheating or flat-spotting. So it really hasn't bothered me.

One more thing, if your tires do not have a high traction rating (if their a high mileage tire or the like) then that can also be part of the problem. Redlines are like stones for braking and handling. Tires are what actually stops the car. Through the co-efficiency of grip from the rubber and how well the suspension keeps the rubber in contact with the road surface. The Brakes turn kinetic energy into heat.

I understand your comment about the Toyota calipers and the comments of others. On Lee Jansen's web page he states in big bold letters, "Addition of Toyota 4 piston calipers will NOT improve braking distances. The Toyota calipers may be more fade resistant, however, if all other variables are fixed, the braking distance will remain the same." However, on the VTR page (Which was authored by the same people) they show the following:

1. Early (prior to metric calipers, Girling 16PB) stock configuration with semi metallic pads, composite shoes and 0.70" rear wheel cylinders: 262 feet with a standard deviation of 19; 6 data points, tossed out both the minimum and maximum measurements.

2. Toyota front calipers with semi metallic pads, composite shoes: 243 feet with a standard deviation of 9; 8 data points, tossed out minimum and maximum.

3. Toyota front calipers with semi metallic pads, composite shoes with a 7/8 inch rear wheel cylinder: 215 feet with a standard deviation of 9; 9 data points, tossed out minimum and maximum.

So you have two contradictory conclusions from the same group of people. Which can be confusing. What changed?

They also mention that some may need a proportioning valve to completely balance the system. Even with the Toyota calipers.

Cheers,
 
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