Kumba,
You asked me to come over and look at this thread so here I am.
Good Hot Rodding! I’ve worked on Ford projects where they spend hundreds of thousands to do what you did, but in some ways they already did via Jaguar. You took good advantage. :Beer
The DEW98 platform was R&D’d mostly in Europe, and one of the USA complaints you’ve mentioned was brake dust. Europe is not that concerned about dust and tends to use higher friction formulations that are also higher is rotor abrasiveness. That works out well for them and they tend not to have what we mis-characterized as rotor warping. It’s usually DTV (Disc Thickness Variation), which often is solved just by using higher abrasive pads. Well, as long as the rotor runout is within spec.
I didn’t care for the dust either on my wife’s car so changed out to the Hawk HPS pads on both ends, but this is not a wholesale recommendation for the Hawk pads. The OE pads up front were either Jurid or TMD, while the rears were from Ferodo. Aftermarket pads from those companies will not be the OE formula.
As you theorized what you have done is improve the leverage of the brake vs. the tire diameter, which is always an improvement in braking output vs. pedal effort. I’ve thrown together some shorthand calculations of what you have done in the first table. It’s not detailed so ask questions for what I didn’t explain well.
I’m using a SWAG 0.40 Coefficient Of Friction for both the front and rear pads, which would be within a pad’s Edge Code of EE. Edge Coding is required by a few states, not Federal, and each letter spans a value of 0.10 mu. E is between 0.35 and 0.45. It’s been over a decade since we (I) did the testing on the DEW 98, so the pad’s values could just as well have been within the F range – just don’t remember.
So using those values of the front rotor swept radius (the center of the pad’s relative rotor radius location) by the addition of the 20mm to the rotor diameter you increase your output by over 5%. That’s a good change. Now towards the bottom of the table I show that you could have done the same with a higher COF set of front pads, still within the EE rating, but you would not have gained the additional mass of the rotor used as a heat sink. In addition, you now still can use a higher friction front pad and gain more effectiveness, but with a higher shift towards front brake bias.
It was stated in the thread that the DEW98 has an abnormal rear brake bias – not true! We are used to hearing that the front brakes do most of the work, but that was factored more due to rear brake drums and mechanical proportioning valves. The majority of the vehicles on the road today are disc/disc and with dynamic proportioning through the ABS controller, and only at high deceleration values. The brake bias in most cases is more like 60/40%, so the LS is actually more front biased then others. And in fact I can name off a few vehicles that wear out the rear brakes faster then the fronts. This is mostly due to the aforementioned change towards rear bias and the engineers lack of expectation, designing the rear pads to a lower friction material wear volume then it should have been.
It was mentioned about wet braking being more skid prone. That is a valid concern as this vehicle is well weight balanced and shifting the brake bias more to the front means during a higher deceleration rate the front tires could be over-whelmed in their traction capability. ABS will compensate for that the majority of the time. Besides, all you are doing is going to the same setup used on the S-type!
In your tests you indicated the front brakes were not locking as much as you expected, which may be due to the Centric pads you are using. They are not a big player in the market and on a few forums I’ve seen complaints. I would recommend a change to Akebono, Jurid, or maybe Ferodo to see if there is better friction there. Bendix may also be a good source. That extra 10mm in radius should have brought up more of an improvement.
The SS braided lines should also help, something that I have on my wife’s car. Less displacement not only gives you a higher pedal, but it keeps you out of the vacuum boosters knee. Although there are counterpoints to that as well.
I don’t have access to the DEW 98 booster values anymore, but I’ll use another example that I have, seen in the second attachment.
This is pretty typical for most vacuum boosted vehicles. You get to about 50-60 lbs pedal effort and the booster has exceeded it’s travel so the remainder of the hydraulic pressure output is “manual”, no longer boosted. So past the “knee” the driver needs to push harder and farther to get the increase in hydraulic pressure then he did before the knee. With less displacement due to hose expansion or god forbid air in the lines, the more you are in the boosted zone and getting more hydraulic pressure for a given pedal effort.
Here is the counterpoint – if you want good incipient modulation. The shortest braking occurs with about 5-7% tire slip on the road surface. The tires are talking to you but not locked up. It’s been said and it is true that a very good test driver can beat a vehicles ABS system in stopping distance. He stays in the tires incipient skid region while the ABS is varying the hydraulic pressure though the stages of skid, unlock, incipient and then skid again. The distance is much better then a driver who would just lock the tires, you retain steering control, and the vehicle does not go out of control. But if you are better then the average test driver, and much better then the average person on the road, you want the ability to modulate the braking incipient skid.
That is much easier past the booster knee then before it. Before the knee a change in 100 psi hydraulic pressure may only require about a 4lbs differential while after the knee it would require 36lbs in the graphed example. So in years past before ABS we used to adjust friction material COF to achieve incipient skid just within the knee, where today with ABS most people want very easy pedals to achieve tire skid.
Just throwing that out there for reference as with the ABS system on these vehicles that type of engineering thought is really not necessary unless you are shutting off your ABS during Autocross.
On edit I'll throw something else out there. While in racing and for the sales Wow factor having the front caliper pistons different diameters sounds good, in practical application this vehicle tends to wear the front pads with a taper. The rotors normal knockback still occurs and the steelbacks align to the caliper slides perpendicularly. But now with the tapered pads there is more clearance at the higher wear end and this requires longer pedal travel. In my experience with my wife's LS who gets about 40k miles out of a set of pads, soon after 25k I'm noticing longer pedal travel and less braking effectiveness. At this point I change out the pads even though they are only halfway worn.
