why do some jag xkr brakes feel harder than another xkr

 

There is no way to give you a specific answer because there are so many variables. Since there are no brand new XKRs, every XKR is a used car and they will drive somewhat differently from each other due to different levels of wear on the brake system components, including the pads, rotors, and brake fluid. Note that the 4.2L and 5.0L XKRs have different brake components and that the Alcon Performance Brake Package was available only on 4.2L XKRs. Also, some XKRs have aftermarket pads and rotors, and I'm sure that some XKRs have contaminated brake fluid. All of these affect pedal feel.

 

Different brake feel among the same model is not unique to an XKR. Most XKR's are not doing annual brake fluid flushes like one would do on an Aston, Lotus, Ferrari, etc... Makes a difference; that alone.

 

As I just looked at my pads and rotors as I cleaned my wheels I don't need to replace them for at least another 5-10K miles. But what this brought me to was wondering if there was a difference in pads for the XK series. While looking at the Akebono site it said they didn't have pads for my car. They did have pads for a 2012 XK but not for a 2012 XKR. So the question is are the brake calipers and pads the same for the 2010-2015 XK and XKR.

 

I am by no means a brake expert, but my 2010 -R has the optional "Brake Upgrade Package" and has 380mm(???) front disks. I do believe that other models have a bit smaller disks (other than Alcon 400mm).

 

I believe the "Brake Upgrade Package" is actually standard for an XKR (i.e. it's an upgrade from the XK brakes). Here are the only brake systems that I'm aware of:

4.2 XK (ATE*): 326x30mm front, single piston, floating calliper; 326x20mm rear, single 45mm piston, floating calliper
4.2 XKR (ATE): 355x32mm front, single piston, floating calliper; 326x20mm rear, single 45mm piston, floating calliper
4.2 XKR (Alcon): 400x36mm front, six pistons (38.1, 34.9 & 30.2mm), fixed calliper; 350x28mm rear, quad 32mm pistons, fixed calliper
5.0 XK (ATE): 355x32mm front, single 60mm piston, floating calliper; 326x20mm rear, single 45mm piston, floating calliper
5.0 XKR (ATE): 380x36mm front, dual 42mm pistons, floating calliper; 376x26mm rear, single 45mm piston, floating calliper
R-S GT (Brembo): 398x38mm carbon-ceramic front, six pistons (38, 34 & 30mm), fixed calliper; 380x34mm carbon-ceramic rear, quad 30mm pistons, fixed calliper

* ATE = Alfred Teves, part of the Continental group

 

OK good info as I have the 380 's . So when the time comes I'll be looking for some ceramic replacements .

 

Brake pads are a compromise, just like tires. There is no "one size fits all" for every purpose. Standard OEM pads are designed with a soft compound for street use and to provide excellent initial "bite", longevity and quietness at the cost of excessive black brake dust. Aftermarket ceramic pads usually lack the same initial "bite" but produce less noticeable brake dust because it's not black. And they might not be as quiet as the OEM pads.

 

Brake pad can definitely be a part of it.

It also might be possible that one car has a problem with the power assist/vacuum.

Couple this with different braking systems and you can have a lot of reasons.

Same system to same system how hard a brake pedal is can be from one of two factors.

A: Less compression in the system resulting in higher braking pressure for the same depth of brake pedal movement.

B: Less brake assist resulting in more force required from you for the same depth of brake pedal movement.

Now I'm not sure what exactly you meant by hard, but different brake pads would effect the braking pressure required, in the same system this would result in the need to press the brake pedal to different depths for the same braking force. This would not feel the same as a harder pedal but with the same travel.

If you aren't comparing the same system, there can be a ratio of motion that changes similar to changing the steering ratio on a car. Lighter pedal, more travel, less travel, stiffer pedal.