dapinky wrote:isuzuowner wrote:If you have data to include and a the correct coefficient to multiply the rear force number by to add that, please post it.
There is no simple coefficient of multiplication - far to many variables, including (but not exhaustively) temperature of brake components, material specifications, weight distribution (which is non-constant, depending upon rate of decelleration), specification of brake fluid (degeneration), etc etc...
...so I'm not sure what advantage publishing lots of meaningless data may prove.
Anyway, my brakes work well enough for me - I can lock wheels if I want to (which i don't!), and it passes the annual MOT test. I have used the car on road and track, and never had any great concerns about the efficiency of my (mainly) OE system.
At the same time, I fully accept that the system can be improved, and an 'off=the-shelf' kit from the few specialist suppliers will appeal to many owners who want them to feel a bit better, and be less of a regular maintenance regime.
Basically, if I can lock the wheels on an OE system, then the car won't stop in a shorter distance with ANY other callipers fitted, as the limiting factor becomes the friction between tyre and tarmac.... the difference is in the force required to engage the wheel lock, and the time at which that rotation is stopped after hitting the 'oh s**t' pedal hard.
dapinky wrote:I merely made comments in the chronological order you posted, with no importance to be ellicited from the order.
It wasn't me who mentioned "that's why we have an MOT test", so why ask me about it?
I am fully aware of the published coefficients of friction of various brake materials from different manufacturers, but not quite sure how they are relevant to your points - it is easy to quote things from the interweb, but lists of numbers are pretty useless.
You mention PNM, Hi-Spec and BB kits - and say that you can only find information on one of them......... perhaps because they are all the same (callipers manufactured by Hi-Spec, and sold by PNM under their own name.... the rest of the hardware developed by Bob and utilised in the 'kits'.)
As for your suggestion that proportioning valves are a 'band aid' to cover poor design/selection of components..... well, that means that every manufacturer in the world makes poor choices. You show me ANY car (production) which doesn't use some sort of pressure proportioning system (usually part of the ABS manifold or master cylinder on modern vehicles) and I'd be surprised!
They are fitted to allow a 'standard' shelf part to be used for multiple applications, otherwise every vehicle would have individually made components, even down to different variants of the same car to allow for the extra weight of the original equipment... that would be absurd. It is only to get the bias/split to fall into an acceptable range for expected/intended use.
Anyway, i'm bored now, and shall give up with this pointless diatribe - it is impossible to have a meaningful discussion with someone who either chooses to not listen, or who feels that by regurgitating 'facts' from the interweb, they must be right....after all, we all know that 86.7% of statistics found on the internet are purely inserted to make a point appear valid.
Your post about proportioning valves was after my last comment, I will start there. I mentioned "If you have a coefficient to correct for the proportioning valve, please post it", which was a bit of a trap, because I had already researched that topic rather thoroughly and found none.
In the course of the discussion of other discussions, there is reference to the Wilwood proportioning valve, and some comment that it has a larger range of adjustment than what Lotus used on the Elan. I have direct experience using the Wilwood proportioning valve, in attempting to resolve a brake biasing problem in a car put together by someone who did not do the math. The front brakes were a little better than the originals, and the rears were quite a bit better than the originals. And the car would spin pretty prolifically every time the brake pedal was used.
Wilwood states that their valve can provide a "57% reduction in line pressure". That is not a 57% reduction in braking force. Only the hydraulic pressure. And that is not a reduction from zero, it is a curve. Brake force increases normally at lower pressure, and when the line pressure meets the resistance of the spring inside the valve, it continues to increase at a slightly reduced rate. The adjustment knob moves the transition point up or down. So full pressure with partial application of the pedal, slightly reduced pressure under full application of the pedal. But the rotor diameter and brake pad friction compound remain the same, and this is why that proportioning valve have just a very small range of impact on the overall braking.
I mentioned that car with the excessive rear braking force. I started with higher friction front pads, changing from .38 to .54 cf. That didn't solve the problem. Next I installed that same Wilwood proportioning valve, and after small adjustment steps, had the valve turned to the fully off position, with the maximum limit to the rear brake circuit. That almost reduced the rear braking force enough. Better, but not enough reduction in the rear braking force.
At that point, I sat down and did the math. And the solution was to change to larger piston calipers, and drop the pad cf down to .44. That was a 12% increase in front braking force. The brake proportioning valve was turned to the fully open position, and is no longer needed.
The range of adjustment of the brake proportioning valve was less than 12% of the rear braking force. And a 12% change in braking force was enough to change a car from spinning to dead stable.
And that's how inputting all those specs and figuring out what the actual braking force is, and being able to compare the braking forces produced by different combinations of parts, is useful.
The calculation is for braking force at the rotor.
Weight distribution and weight transition are not part of the braking force. These are considerations when deciding what braking force is needed or desirable, but they do not affect the braking force that the parts are able to generate between the rotor and the pad.
The temperature of the pad does affect the braking force. For the sake of comparison, I have used the coefficient at ambient temperature, because this is going to be how the brakes act upon panic stop in traffic. For determining braking force under constant heavy braking, you have to find the operating temperature of the rotor under those conditions, and a graph showing cf across the temperature range. That is a little beyond attempting to put together a simple comparison of three or four combinations of parts (brake kits). But it does point out that a pad with a flat curve that is consistent across its temperature range is desirable. Most of the pads mentioned with these kits fall into that category. And any of the pads that would be streetable would fall into that category.
"Quoting numbers is useless". Well, if those numbers are not understood, perhaps. If you want to correct those numbers, please point to what you believe is in error and state your case.
No one has provided any reason the specs I have found, the method of calculating the braking force, or the results is in any way inaccurate or erroneous.
The response is a flat refusal to even look at or consider the data. Similar to a refusal to believe that the world is round.
I mentioned that PNM appears to be using Hi-Spec calipers.
I also pointed out that Hi-Spec do not state what size pistons are in the calipers they provide with their kit for the Elan. And neither does PNM.
This site has no FAQ. I can find no statement posted by someone who purchased any of these that they measured what was delivered to them.
Additionally, I pointed out that both Hi-Spec and PNM may very well be providing calipers with different piston sizes to difference customers, if they tailor their kit to what they believe the customer wants/needs.
That may be good for the individual customer, but makes comparison of the product performance from one customer to the next a problem. A customer wants the performance recommended by another customer, and receives something different.
Proportioning valves in pre-ABS production cars are rare. Adjustable proportioning valves in a production car is not just rare, it is a liability problem. Anything the customer can adjust that might result in making the vehicle dangerous to operate, is something that the customer can screw up, cause damage or injury, and then the customer can sue the vehicle manufacturer for delivering a product that is unsafe or can be made unsafe with the adjustment provided. This is why almost no one manufactures and sells a car with adjustable spring perches, camber plates, and the full compliment of suspension adjustment like threaded diagonal arms and multiple suspension mounting points.
Please do not confuse residual pressure valves with proportioning valves. A residual pressure valve keeps a small amount of pressure in the rear hydraulic circuit, to compensate for the additional length of the pipes attached to the rear brake calipers. This prevents the rear brakes from disengaging too much so that they are ready to push the pads against the rotors at the same time as the front brakes.
Just about every car has a residual pressure valve. It is located in the master cylinder. It is not adjustable, and may be easily confused with a proportioning valve. But it does not do the same thing.
Most auto makers sit down and calculate out the braking force similarly to the method we are discussing, using the rotor diameter, caliper piston size, and brake pad coefficients. And they select those parts to work together as they are and without something like an adjustable proportioning valve.
You mention changes within variants of a vehicle, but you fail to recognize that the standard practice is to specify different parts for the different vehicles, such as the common practice among auto makers of specifying a different brake pad compound, in the same pattern, between a base model and a upper level model. This is easy to see with Brembo's popular brakes, and cars using the same brake pad pattern, such a the four piston front found on Evo, Sti, Mustang, Camaro, Corvette, several Cadillacs, and even a few Vipers. They do make this caliper in different piston sizes. But each of those vehicles has a different pad compound provided to the vehicle manufacturer to tailor that caliper to their use. And some of those models have changes in the pad compound over the range of model years, when the vehicle manufacturer decides something needs to be changed. They all have the same shape and thickness, and are available from just about every performance pad maker. But what performance compound is desirable for one specific vehicle to match the original or make an improvement, can be very different from another vehicle using that same pattern. And most good pad makers will know which compound not to use with which vehicle, because it does not even meet the minimum friction level of the original pad for that vehicle.
As far as proportioning valves on production vehicles, I know that none of the Isuzu vehicles made between 1972 and 1995 have proportioning valves. I haven't seen any on Honda, Nissan, Toyota, Mitsubishi, Chevrolet, Ford, or Dodge.
On this side of the world anyway, that particular item on the Elan is an anomaly.
A pig in the poke.
Relation to other idioms and expressions. The English colloquialisms such as turn out to be a pig in a poke or buy a pig in a poke mean that something is sold or bought without the buyer knowing its true nature or value, especially when buying without inspecting the item beforehand.Without quantifying the braking force of a brake system (stock or big brake kit), there is no way to compare and make an educated decision.
It is like buying a pig in the poke.