Disc Brakes XK 120

[scotsmaun120]

I have bought a bolt on kit for the front brakes on my 1954 XK 120 roadster the kit bolts on and fits well. Two things are apparently immediately. Pads are rubbing on the disc at all times. Brakes are no improvement on Drums. Has anyone had experience with disc brakes ? If so can they work with the original master cylinder arrangement? I would appreciate any help anyone can give me on this one.

 

[John S Farrington]

Jaguar upgraded to disc brakes on the later XKs because they experienced significant brake fading do the heating during racing events with the 120s and C types. Disc brakes will stand much more useage before significant fading occurs. The cold performance of discs vrs. drums is probably not great as you have observed. It is normal for the pads to maintain light contact with the rotors. If more than that is experienced there is something wrong with the master cylinder. Your original brake master cylinder should work OK with the front discs but if you want to go all the way, upgrade your master cylinder to a tandem.

 

[PhilW]

With discs on the front only, you would get better braking if you added a brake booster to those front discs only (not to the rear drums). I've heard the front brakes do 80% of a car's braking.

Discs need more pressure to get the same results as the drum brakes. Many newer cars and trucks have discs/front and drums/rear with a power booster, but the rear line usually have an in line bias to lower the pressure to the back as compared to the front.

Phil.

 

[Keoke]

/ubbthreads/images/graemlins/iagree.gif Phil W, might as well throw in a vacuum tank too.---Keoke- /ubbthreads/images/graemlins/yesnod.gif

 

[scotsmaun120]

Thank you John. I have a tandam master cylinder already installed. This one has just come back from Engkland where it was completely refurbished. I will get assistance i "bleeding" the system once again. I really do not see why the calipers are not retracting eneough for at least minimium clearance.
Appreciate your input and I will get back to you if I can clear the problem.

Regards, John Christie

 

[70herald]

[ QUOTE ]
Pads are rubbing on the disc at all times.

[/ QUOTE ]
If you are talking about a tiny drag, this in normal. Not sure exactly which brakes you installed, but disk brakes do not have a "return" spring (unlike drums)They work because of the square profile of the O-ring. When you put on the brakes, it slightly distorts to O-ring, and when you release the brakes, the O-ring returns to normal shape and very relieves the pressure on the pad. Basically, you won't really see the motion. When driving, the small amount of friction generated will expand the disk, and push the pads back a tiny bit further. One point, if the disks are not properly flat you will get a pulsing effect on the brake pedal, and more drag.

[ QUOTE ]
Brakes are no improvement on Drums. Has anyone had experience with disc brakes ?

[/ QUOTE ]

Braking is a rather complicated subject, however maximum braking is related to the friction coefficient between the tire and the road. Now the brakes need to be able to control the braking action sufficiently well to allow the tires to reach this point with out skidding, when they loose most of their traction. As has already been pointed out, the main advantage of disk brakes is that they are more resistant to heating and subsequent brake fade.
The front brakes provide a larger portion of braking power for two reasons 1, the weight of the engine up front results in greater traction while breaking 2. weight transfer caused by breaking further increases the percentage of weight (traction generated) by the front wheels. Therefore, they need to dissipate more heat, hence the reason for large drums up front or disks up front.

[ QUOTE ]
If so can they work with the original master cylinder arrangement? I would appreciate any help anyone can give me on this one.

[/ QUOTE ]

In theory yes, and brake line pressure should be about the same. A few concerns, the disk brakes may have a larger quantity of brake fluid, and may therefore need a larger reservoir. It sounds like you already have a later model dual circuit master cyl installed? This probably has a larger reservoir than the original. The dual circuit is a safety feature so that in the event of a leak at least 1/2 the brakes remain. The hydraulic pressure when braking should be the same.
Balance between front/ rear brakes, some cars spec different diameter rear brake cylinders with disk brakes. This certainly could affect the braking ability of the car. Also see if there is simple mechanical wear in the mechanism between the brake pedal and the master cyl. This could take up a large percentage of the pedal travel and subsequent braking applied.

Yisrael