The fact (not opinion, not theory, not speculation, but fact) is that the maximum deceleration the brakes can give you is limited simply by the force between the road and tire at which it stops rolling and transitions to sliding friction. That's because sliding friction provides a lower force than rolling friction for a given downforce on the wheel. Another fact is that this limit is determined simply by the tire and the road surface it's on and the weight on the wheels at that time, not the force on disk or the ratio between the force on the pedal and the force on the disk.
But my issue was about the things that change whether the front wheels lock first, as they should. And that's something that can change in a more complex way than may always be obvious. That it can change as a result of increasing the power of the back brakes (or reducing the front ones) may be obvious. But then there must be an issue that the problem itself is not obvious. Otherwise why would there be folks who think switching a road car to rear disks without considering the need to re-balance the brakes, i.e. turn back down the effort from the back brakes they just spent so much money increasing?
It's also a possible problem that some see dive on the front suspension as causing the weight transfer, not a response to it. The way the downforces on the front and rear brakes change as you decelerate is determined only by the height of the center of gravity of the car and the total amount of decelerating force between the road and the tires. It's not affected by how much of that decelerating force comes from the front or the back, only the total. So fitting bigger brakes at the back will not reduce the dive, only increase the risk of the back wheels locking first and the possible laundry problem that Steve mentioned.
But one of the less obvious things that change the risk of the rear wheels locking first is that if you increase the grip from the tires. In that case, you can decelerate more quickly, so more weight is transferred onto the front wheels and you reduce the likelihood that the front wheels will lock. But, perhaps counterintuitively, even though you've increase the grip at the back, you also reduce the weight on the back wheels at the limit, and
increase the likelihood that the rear wheels will lock first. However, if you've increased the grip by much, you probably have to increase the brakes to get to the new limit at a reasonable pedal pressure, and increasing just the front brakes is enough to get you into that risk situation - but only with more grip.
So, ignoring changes in grip and to back brakes, upgrading the fronts is only about how hard you have to press to get to that point where the front wheels lock. And that is purely a matter of personal preference.
My opinion is that the brakes on the Sprint take me as near to locking the front wheels as I want them to for the amount of force I'm used to applying. Also, I don't go even to that point except in more extreme situations, so brake fade is not an issue. If I went racing, I'd have a different opinion, but I don't.
I don't think I can do better here than to quote Jon Tilson:
Quote:
As I have explained and will repeat here one more time, standard Dolomite brakes are quite capable of slowing the car in a continuous emergency stop from 90 mph to zero maybe twice in relatively quick succession, ergo they are demonstrably adequate for normal road use. Brakes used to be engineered so that lock up on a dry road could be achieved with pedal pressures ranging from 60 to 100 lbf at the peddle. This is the sort of effort that a normal woman driver can just about achieve in emergency at the top end of the range.
The problems come or are compounded because of bad maintenance allowing partially seized callipers and discs to go thin, or even fitting el cheapo brake pads from Moprod etc. I have personally transformed the brakes of numerous dolomites (some of them owned by forum members) by servicing calipers and fitting decent pads. I sometimes shudder to think what people are actually driving about with and wonder if a lot of the bad press and hot air generated about dolomite brakes on various forums is down to ignorance of what the barkes really should be like.
Now when on track or rallying or regularly needing to whiz up and down mountain passes with a full load of passengers I quite accept that the standard setup will cause some fade. In these cases by all means upgrade if you want to.
But please dont subject normal users to wrong advice and make them spend money they don't need to and will see little benefit from when a 20 quid set of caliper seals and some proper pads will see them stop as they should.
Jonners
I'd only add that giving the servo a coat of looking at never hurts.
Graham