Panic Stop - Great Scott what just happened!

Sacto Mitch posted:

 

 

My car (and almost every other Speedster I've heard about) locks the fronts way before the rears. And the brakes on most modern cars are purposely proportioned to do the same because, again, understeer is easier for the average driver to control.

 

 

Agreed.  Cliff was there, so he knows best what happened, but the rears locking first on any of our cars would be HIGHLY unusual and really begs the question of whether or not Cliff may have been mistaken about which wheels were locked.  Again, he was there, so he knows best.

DannyP posted:

Mitch and Justin, nicely explained.

Troy, a locked wheel has less traction than a rolling wheel under braking. I think that explains it right there.

Agreed Danny, they did do a good job of explaining.  This makes the most sense to me.  Like Justin said "rear drag wants to pull the car straight."  In other words, if this was a pure physics experiment the rear would remain at the rear, but in a moving car the other factors of steering, road conditions, braking, etc., are always present and they are what could cause the car to swap ends with locked rear brakes.

I think I'm done beating this dead horse.

You guys lost me at "...Do you know when the rear wheel cylinders were last replaced?". 

Cliff, as always, lots of great info here for trouble-shooting! I hope you get it all sorted out soon so you can get that bad boy back on the road.  

@Cliff Presley - Charlotte, NC

Shoot, Jim.....I've been in and out of airports and reading this on a friggin "smaht phone" so this thing lost me at Cliff's first post!

Now that I've gone back through some of it (certainly not all of it) Justin makes the most sense by far (great post Justin!), but I'll add just this:

Yes - under severe braking, the car, en toto, will try to follow the front wheel trajectory if they haven't locked up.

However!

You must remember that these cars only have rudimentary rear brake adjusters that most-of-the-time get rusted or gummed up and don't "adjust" even remotely as they should.  

The chances are, with a drum brake rear that neither of those drums have been adjusted perfectly to the characteristics of the front wheels, nor are they perfectly adjusted side-to-side, so THAT means that when applying the brakes, one rear wheel will engage first and harder, given the same pedal pressure, and THAT will cause a pull to that side and will try to come around first in that direction.  This is totally unrelated to what happens when applying the emergency brake.  

For example, back when I was still lazy about it, applying the brake pedal on my speedster would make my car pull slightly to the right> because the right rear drum was adjusted very slightly tighter than the left.  Harder application of the pedal made it pull even harder to the right.

If I were to pull on the handbrake instead, the car would try to pull to the <LEFT because that side's cable was adjusted slightly tighter than the right.  It consistently pulled to the left until I adjusted the e-brake cables to even things out (tested on a long, straight stretch of back road and confirmed in a relatively level parking lot).

So there is a balancing effort required between the front and rear brakes, and then between both sides in the rear as actuated by the pedal to balance left-and-right, and again between both sides at the rear to balance the hand brake.  They are totally different braking systems.

Get all of those things balanced and the car will stop sort-of straight, forgetting all of the effects of different tire adhesions, pavement irregularities, humidity, where the front wheels are pointed and all of that other stuff.  

The bottom line is; Cliff should take it to his mechanic, not give them ANY of the stuff presented in this thread and ask them to check out the brakes, adjust the rears and balance them left-to-right via a set of panic-stop tests (there is no in-shop test to do this...It must be done as a road test), then do the same for the hand brake with another set of panic stop tests.  Any decent "older car" mechanic knows that this is the ONLY way to test drum brakes for balance and that it is a trial-and-error process.  Get all that done and it might still try to swap ends in a panic stop because of pavement irregularities or operator moves or whatever - you just don't know.

Geez....You guys typed a whole LOT while I was away!

This afternoon I stop by the shop that is working on the car to check on their progress. 

They showed me a number of issues with the rear drum brakes such as the shoes had not been meshing with the drums properly. 

I was also not happy to learn that when the car was built the builder had installed used drums and plates, and that after they had been refinished the drums had not been refinished correctly.  Rump sputter.

No need to cry over spilt milk at this point, and since I have every reason to trust the company doing the work I've asked them to use their best judgement and to refinish or better yet replace anything with new parts if they feel it would be appropriate.  

Once that work is finish, and depending on the weather, we will start the road tests sometime next week. 

Before they go crazy changing all the parts I wonder if you shouldn't just upgrade to disc brakes or at least consider it... assuming the master cylinder is the right size. 

That might cure the whole issue for you once and for all and give you better braking... just saying. 

IAM, I don't disagree but the senior mechanic thought that the only things they might have to replace would be the two drums. . . which to me looked worn out. 

There is no question that they had been turned (machined) several times in the past and who ever did it the last time did a poor job; therefore, those drums probably never should have been reused again. . . but the builder did anyway.   

"There is no question that they had been turned (machined) several times in the past and who ever did it the last time did a poor job; therefore, those drums probably never should have been reused again. . . but the builder did anyway."

Vintage Speedster, did you say?  

I agree, converting to disc will be a decision you will never regret.

4X discs really are the only way to fly.  Could conceivably save your life.  If you really gotta have the look, you can get drum skins and slap those on. Just sayin' . . .

Totally agree on the 4-wheel disks.  I have to wonder if I might have been able to stop in time had my car been so equipped.  The best part could be never having to adjust them because I think my poor job of doing that contributed to the accident..

I love having four wheel disc brakes.  Of course, they came with all the Porsche suspension parts, but I would recommend this set up to anyone.  Good, solid, smooth stops every time.

So what happens to a car with four wheel disc brakes, without ABS, when you lock all 4 wheels?

You still skid, but if the brakes are properly balanced you can modulate them to the point just before lockup and get a good, straight stop.

It does depend on the car.  I guess mine is a little heavier than others, and I get no skidding on a panic stop.  I did get it with my previous Speedster - front wheels locked up.

As for ABS, I seem to recall we drove without that system for years, and we seemed to manage okay.  People have lost driving/braking ability with all the computer assist on cars now.

And Lane, when a little kid runs out in front of your car, you aren't thinking 'I must modulate my braking...'    It's a great theory, and maybe some can do it.

I ordered my new VS with four-wheel discs (in 2013), mainly to avoid the hassle of adjusting rear drums. I ended up, not surprisingly, with the full Empi treatment - Varga calipers up front and no-name mystery components on the back.

They do stop OK, but I don't know if any better than a properly adjusted front disc/rear drum setup.

I've never braked hard enough to lock up all four, as the fronts lock very easily and I usually ease up on the pedal when that happens. They do pull pretty evenly, are easy to modulate, have a firm pedal, nice feedback through the pedal, and generally perform well.

But because of general suspension and traction limitations with this car (compared to a modern 'performance' car), I drive it pretty gingerly and try to avoid aggressive braking.  I think it's a car that rewards a smoother driving style - advance planning into corners, gentle inputs, gradual transitions, and looking as far ahead as possible.

The Empi rear discs weren't without problems - mine made awful noises until I found better pads, VS installed them so they hit the frame, and one of the cheesy stamped brackets caused an axle leak - but we eventually got them to work. As usual, there's better stuff out there for more dough.

PS: I've got four-bolt wheels and swing axles.

Troy wrote: "So what happens to a car with four wheel disc brakes, without ABS, when you lock all 4 wheels?"    Well, you keep skidding in the direction of the force vector of the mass of the car (see Justin's "Automotive Physics during Braking 101", above) just like a disk/drum car with locked up wheels would.

Wait - aren't you somewhere sorta close to my age (later 60's)??   When you were going through "safe driving school" in the 60's, did they teach you to "pump your brakes" if you detect a skid during a panic stop?   Oh, wait - maybe they only do that in snowy states for when we're skidding uncontrollably on ice and snow, like this year, even into June for all I know.

So if you're starting to skid and the wheels are locked up (regardless of disk or drums), you let off the brake a bit to let them roll again and then re-apply the brake on and off and on, "pumping it", simulating what ABS units do (you'll probably be slower than an ABS at several times a second) but that should help a LOT to start to stabilize the vehicle and hopefully let you regain control.  You should try this in a school bus on an icy road - Gives you a Death Grip on the steering wheel, for sure.

You will then have to get your heart out of your mouth and shovel out your jeans, but that's to be expected, too.

Sacto Mitch posted:

 

They do stop OK, but I don't know if any better than a properly adjusted front disc/rear drum setup.

 

 

Careful Mitch.  You might start the "locked brakes is locked brakes" regardless of disks or drums debate again.

Gordon Nichols posted:

Wait - aren't you somewhere sorta close to my age (later 60's)??   

 

Sorta, but 60 is the new 40.

All this hype on disc brakes - just had another caliper seize on my Yota 4Runner.   95k miles and I've replaced all 4 and now starting again on front.  Lifetime rebuilt from AdvanceAuto but still several hours to swap one out.  At 2 tons - I wouldn't want to try to stop it with drum brakes though. (No off roading so not a case of mud or water damaged).

Speedies are light up front, and will lock up there first, in my experience., and under even moderate braking conditions. If road bumpy at all, they will chirp as you bounce.  This is an alert to let up just a tad, and pump if you have time.  A situation can happen where there is no time to think, crash is eminent and reflex just mashes the pedal and hopes for the best.  Only did that once a long time ago, and never in the Speedster, so far.  With all four locked up, unless your tires are really worn differently, you will go in the direction old Mo dictates, and all you get to do is hope all comes to rest before the crumple or the cliff's edge.