Bigger Brakes - Huh ? why ?
To recap, my car is running standard front discs/calipers with a rear disc conversion using 240mm MGF discs & Renault calipers. Mintex 1144 pads up front, EBC Ultimax in the rear, no servo assistance. Although my brakes are probably better than they’ve ever been, they aren’t “modern” brakes where you get oodles of stopping power simply by brushing the brake pedal with your big toe.
I could re-fit a servo but that just changes the perception of braking effort, not actual stopping power.
But the trouble with this interweb thing is that you learn new stuff; I noticed some comment about Fred Puhn’s "Brake Handbook" and eventually tracked down a copy. Although some parts are irrelevant for our cars it does give you a good insight into designing and upgrading brake systems.
And that’s how it all started. So I’m blaming Fred.
I was interested in how you decided the diameter of the disc and it seems to be “as big as you can fit under the wheel”. I read about the design of the Elise S1 which uses 288mm vented discs and likewise they “used the biggest one that would fit in case the MMC discs don’t work as efficiently as iron.” So perhaps the selection isn’t magical, it’s just whatever you can fit in there ?
So.... for the first stage I'd retain the front OEM and rear calipers as they're almost new. Why not 4-pot or larger front calipers as I did with the Elan ? Well, it surprised me to find the Elise S1 with 288mm discs has calipers with 44mm pistons and yet we have 48mm pistons on 232mm discs, so perhaps I don't need more ? More piston area means more pedal travel which implies even more changes, so the logic for going to larger calipers isn’t there yet.
Secondly I’ll have to change the diameter of all 4 discs if only to preserve balance. I don’t race or do track days and I’ve yet to experience brake fade on the roads so I’m not convinced a vented disc is essential, I’ll stick with solid discs and save some weight. I can’t fit vented discs anyway without changing or spacing out the front calipers and as Puhn says in his book “one step at a time”.
So, what discs to use ? Theoretically 288mm will fit under the rims as I’ve got 15” wheels but there’s where you hit the first snag; the caliper is designed around a 232mm disc and once you increase the disc radius you start to hit problems.
OEM Type 14 Caliper superimposed on 232mm and 280mm discs
The image above is a schematic courtesy of Photoshop, but it serves to show the problem. The two discs are aligned at the top, where you want the top of the brake pad to fit. On the front 232mm disc you can see the cut in the caliper body just has clearance where the disc exits the caliper, which is as you’d expect.
But there’s no room to fit a very large disc without cutting back the caliper body. Hmmm, that doesn't sound a good idea.
Simply put, to stop either end of the caliper body catching on the disc you must raise the caliper and in turn raise the piston. Which is ok until you get to the point where the tops of the pads are above the disc face and that isn’t good news.
Drawing it out my best guess was around 260mm should be ok, a healthy 28mm increase over the 232mm OEM. The rear discs aren’t so much a problem as the Renault calipers I’m using were designed for a 270mm disc, so 260mm should be ok there as well. Time to look through catalogues and see what I can get.
Short Cut to the Final Selections
So this is where we ended up, with a 265mm front disc from a Citroen, and a 259mm rear disc from the modern BMW Mini. Both choices were made on the basis of having similar offsets (dimension "D" in the chart) to the OEM Lotus and 240mm MGF discs that I'm currently using.
The snag is that neither disc appears to be a shoe-in fit for the hub or stud patterns. Hmmm, ok, time to get these new discs to fit.
Click on the button for the Disc Modifications
