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You could make a housing that is smaller or larger without changing the clamping force of the brake pistons or the size of the brake pad friction surfaces, at least theoretically. But one assumes that LR would have put some effort into a rear caliper that works on all models rather than having to deal with slightly different calipers for the 300 and 400, if they could have met ALL the metrics the brake engineers had to meet and still fit the smaller calipers to the 400. Since they didn’t I have to assume that something forced them to forego the smaller rears of the 300 in order to meet ALL of the requirements, some of which you and I have no doubt never even thought of.
But what I’ve not seen anyone address is this: Why is it the REAR calipers causing the problem with 18” wheels? Front calipers are always the largest, even on mid-engine and even rear-engine cars, where more of the weight is being handled by the rear wheels. But on a nose-heavy pig of a truck like the Defender, the front calipers and rotors ought to be twice as large as the rears, and the ones causing fitment problems.
Anybody know why it’s the rears?
Here are the GT3/GT4 fronts (left) and rears (right). The fronts are 6-piston 380mm, and the rears are 4-piston 350 mm, iirc.
I can only offer speculation and conjecture.
It was a decade or so ago when I noticed that the rear brakes on some of our vehicles (Turbo Beetle, Supercharged Mini Cooper) were regularly wearing out before our fronts.
I chalked that up to the OEMs specing smaller rear brakes on those vehicles.
Thinking about this, if tow capacity is such a priority to JLR, then perhaps, similarly, they've specified larger and heavier rear brakes for a vehicle expected to carry a heavy tongue load/trailer. I would think that would present a similar need like the weight and shifting of momentum would normally excerpt on the front brakes.
As a matter of fact our Defender replaces a Grand Cherokee with an OEM tow package that had massive brakes. And though I replaced the pads about 80k miles in I swore that they were barely halfway worn all around. And the rears were more massive then I expected.
^^ Good thinking about the towing capacity — I’ve never owned a heavy-rated towing capability vehicle before. But still… larger than the fronts? I can see maybe sizing them as big as the fronts, but bigger? And if they’re not actually bigger, I wonder what it is that’s making them be the source of fitment problems. The other possibility is that they are mounted further outboard of the hub for some reason.
Just FYI. These are the winter rated tyres I got. Sipetastic…That specific size shouldn’t rub according to other reports, but then all tyres (and rims) have small differences, so we will have to assess when they go on in a few months.
It's the front and rear rotors (discs) that are the issue, at least in my case. My 90X comes with 376mm (14.8") rear rotors and 380mm (14.96") front. The Alston 18" wheel and caliper package that Lucky8 has is designed for the 350mm (13.77") rotors and will not fit my front or rear rotors. The rotor must fit inside the wheel and have additional clearance for the caliper.
I think the reason for the larger rear calipers is the rise of Dynamic Stability Control (DSC). You likely NEVER know it is going on, but to help the vehicle with over/under steer many of the more performance tuned vehicles are starting to exert extra braking force on the outside/inside wheel, depending on turn direction and which is occurring. I doubt full DSC is happening all the time and the conditions are rarely/uncommonly that extreme - but as the vehicles try to give you a more luxurious ride, I'm sure it is happening more than you realize. Here's a quick vid to help show the forces and where the braking is actually happening. Imagine rather than in a hair raising problem, it is doing this type of effort at the macro level with every turn & brake.
08-05-2021, 10:51 PM
