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Generic question: which brakes should lock first in an hard/panic stop?

David Womby

David Womby

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Background: My 64 Bristol has discs all round. 2 1/8in calipers on front and 1 1/2in calipers on back. It has a dual master cylinder - one line for front brakes and one for rear. No proportioning valve anywhere. Each set of brakes has a remote booster. The boosters front and rear were both 2x boost and the car stopped fine but needed a LOT of pedal effort. So I replaced the front booster only with one that gives 4x boost. The pedal effort is now much more reasonable and the car's braking is quite impressive for a car this vintage and weight.

The question: I tried a panic stop on an empty, straight, dry asphalt road. Car stopped fast and left some rubber behind it. I am sure all 4 wheels locked up. The car stayed pretty much straight throughout but one pair of wheels left darker rubber lines than the others and appear to have started further back along the road.

I would love all 4 to have locked simultaneously but I don't think I can achieve that. How do you know if front or rears locked first? I am guessing the darker line was the fronts as they carry a lot more weight in a hard stop and that line started further back (about 3 yards before the lighter lines appeared) but does that mean the front locked first? Should I worry about which locked first given it stayed straight and stopped well?

Any thoughts would be appreciated.

David
 
You don't want the rears to lock ever, as that will put you in a spin.
 
padam said:
You don't want the rears to lock ever, as that will put you in a spin.
Click to expand...
Correct - and unfortunately, a lot of these cars did just that in stock form.
Ideally?
There would be zero locking, just a lot of maximum braking on the VERGE of locking up from all wheels...
but if any of them just HAVE to lock for the purpose of answering the OP's question - then my answer
would be "all of them at the same time".

By the way, what's a "Bristol"?
 
moparedtn said:
Correct - and unfortunately, a lot of these cars did just that in stock form.
Ideally?
There would be zero locking, just a lot of maximum braking on the VERGE of locking up from all wheels...
but if any of them just HAVE to lock for the purpose of answering the OP's question - then my answer
would be "all of them at the same time".

By the way, what's a "Bristol"?
Click to expand...

Thanks but I suspect getting all 4 to lock at the same time is probably not attainable. How would you do that? And if it's not attainable and the second best goal is for the fronts to lock before the rears, how do you know if that is what's happening? How do you tell which is locking up first?

BTW, Bristol were an English handbuilt car company that from the early 60s used Chrysler V8s and transmissions. Here's a pic.
s-l1600.jpg


David
 
Watch some episodes of Fantom Works. They always seem to get all the brakes locking up. I’m sure you can google some of the episodes where they are testing the brakes
 
David Womby said:
Thanks but I suspect getting all 4 to lock at the same time is probably not attainable. How would you do that? And if it's not attainable and the second best goal is for the fronts to lock before the rears, how do you know if that is what's happening? How do you tell which is locking up first?

BTW, Bristol were an English handbuilt car company that from the early 60s used Chrysler V8s and transmissions. Here's a pic. View attachment 1111654

David
Click to expand...
Thanks for the info on the Bristol. Very cool! Sort of like the more famous Jensen, eh?
In regards to determining which lock first, that can be done by any number of ways:
- sizes/compounds of friction materials front vs. rear
- weight bias F/R adjustments
But by far the easiest way - and the way it's been done seemingly forever, by both manufacturers and
aftermarket alike, is with the use of proportioning valves.
All cars have one, have for decades now from the factory - but if you want to modify the brake bias,
then that takes installation of an adjustable proportioning valve, with which you can make adjustments
to line pressures fore and/or aft.
 
To determine which wheels are locking a drive on slightly damp dirt makes reading the tire impressions easy, doesn't take much to lock a wheel and you'll have a clear print of what occurred...
 
The reason they developed ABS was you can not account for the weight transfer as that relates to the road surface traction so changes all the time
Best is to have hard braking with no rear lockup as a skidding tire has less stopping power than one that is at breaking point but not yet skidding
 
Not going to write a book here but a couple of things to keep in mind about braking.
Tire and surface friction are load dependent ( to a point ) a tire with more load will generate
greater traction ( again to a point ). When testing brakes you really don't want to apply them
in what I call the "mouse trap " fashion but rather as hard as the suspension will let the load transfer
to the front...lots of factors effect the rate ( skipping those for now ).
I'd suggest repeating your test on whatever passes for an average road where you drive.
Don't go full Hulk stomp on the pedal but bring them on hard to get the weight transferred till
you get some lock up. You ought to be able to easily feel which end is locking first.
Any setup you use is going to be a compromise.
On average I prefer to be able to lock the fronts before the rears for a street car.
Dave
Edit to add...most modern street tires will generate peak longitudinal grip
with somewhere around 10% slip..take that number as a wide average.
 
Last edited:
mpd659 said:
Watch some episodes of Fantom Works. They always seem to get all the brakes locking up. I’m sure you can google some of the episodes where they are testing the brakes
Click to expand...
Wouldn't have to watch many to see Dan doing a brake test, lol. Seems like just about every episode, maybe every other.
 
Thanks, everyone. I will see if I can find some wet dirt or gravel to try again but I am feeling that if I can't get all four to lock simultaneously (which I probably can't), then locking the front first is preferable to locking rears first.

Of course, stopping on the street without ever locking any brakes is best :)

David
 
By testing on wet roads or gravel, both relatively low friction compared to dry pavement you're not really
getting a picture of what happens . You're not able to create enough friction
to create substantial deceleration and the resulting weight transfer to load the fronts.
Example...on a track in the rain myself and pretty much everybody I know increase our rear bias
sometimes a fair amount.
Key element in the whole deal is ..weight transfer...
Dave
 
11” Hemi drums here. All 4. Good modulation and stopping. When necessary I’ve done”impending skid. Guessing the rears would lock first. Haven’t had a full fledged panic stop since my teenage years. Those years ended in 1969.
 
David Womby said:
Thanks, everyone. I will see if I can find some wet dirt or gravel to try again but I am feeling that if I can't get all four to lock simultaneously (which I probably can't), then locking the front first is preferable to locking rears first.

Of course, stopping on the street without ever locking any brakes is best :)

David
Click to expand...
You summed it up nicely, right there.:thumbsup:
 
Dennis H said:
11” Hemi drums here. All 4. Good modulation and stopping. When necessary I’ve done”impending skid. Guessing the rears would lock first. Haven’t had a full fledged panic stop since my teenage years. Those years ended in 1969.
Click to expand...
Reminded me of the old CarToons magazine cartoon - "Panic Stop":
8270865338_c874d31764_o.jpg
 
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