A bit more technical this one, so better put here on the technical forum!
The big idea
This brake upgrade is about a number of things. Partly, and I can't deny it, it is aesthetic. Who doesn't like the look of AP 4-pot calipers on large diameter brake rotors? No one, that's who. But there are other "man maths" concepts at play here. One of the principle drivers is that the AP calipers weigh less than the standard Lucas Girling sliding calipers that are doing a perfectly satisfactory job on Project Shed at the moment. Combined with 280mm rotors, I have yet to experience any issues with brake fade. Saving a kilo or two of unsprung weight is no bad thing IMO. However, the standard AP set up mandates either specific wheels or spacers. With spacers one starts to have concerns regarding thread engagement and therefore safety, although the wheel studs can be replaced with longer items. But it could be a bit of a pain, but what about changing the standard MG AP racing caliper from lug mount to radial? That would open a huge range of alternate fitting options - and this is what I am enaged with at the moment, with the plan to try and enable the AP race set up with any wheel without the need for spacers. The catalyst for this was buying a pair of calipers that someone had crudely cut up already!
Background
As it turns out, this idea is not quite as crazy as it seems - see below is a very similar AP CP6607 - see the radial mounts - and how this can be adapted to suit the "modified" caliper I purchased:
A little more digging provides the equivalent AP part number for the MG 4-pots - it is a CP7601-17SORD, and the rebuild parts for it are available from AP distributors if you use the CP7601 caliper part number. In general, the CP7600 series calipers are matched to 295mm rotors, so I am hoping I can retain the 280mm rotors on Project Shed. But if I need to swap over to 295mmx24mm rotors, that would not be a big deal.
Progress so far
The slow-burn AP brake project is making progress. I sent the manky calipers shown in the photos above to BG Developments after a thorough clean and along with a spare front hub carrier, so that they had all the physical components to work out how to make a suitable adapter. With the ready availability of 3D printing these days, prototypes can be made very quickly and cheaply.
I was sent some prototype 3D printed parts just before Christmas and I played with these on a TF front subframe that I will be refurbishing. The parts permit the caliper to the spaced to suit either the 280mm rotor that I currently have on Project Shed, or the 295mm rotor that is the more natural compliment to this caliper (and actually is the same diameter rotor used on the MGF Cup cars).
Here's what the parts look like fitted up, along with a new radial mount AP caliper that I shall return to BGD when the prototype work is complete:
Note also the 3D printed spacer on the hub - this is to mimic the material thickness of the rotor bell/ top hat that will be used when it the project work is finally finished and ensures that the wheel fits as it should so as to correctly assess clearances with the different styles of wheel.
Fitting up a freshly refurbished and straightened ULW - a 16" wheel that was designed from the outset to be used with the AP caliper, there is no surprises there that the rotor fits perfectly (and looks resplendent!):
Equally encouraging was the masses of clearance this wheel design offers - at least 10mm to the spokes:
Things then get complicated. Project Shed currently has a set of 16" Abingdon (aka MGF Cup) wheels - so I offered up a spare rim to the hub:
Looks okay from a distance, but it fouls the inner edge of the wheel spokes, particularly where the spokes curve inward towards the centre of the wheel where they meet the mounting boss:
This wheel would still need at least 5-7mm of spacing to get the the 295mm rotor to fit with sufficient clearance for the caliper - although ironically, the wheel *almost* clears the caliper if the caliper is spaced out to match a 304mm rotor!
Based on this, I didn't have high hopes of clearing the 15" wheel:
Good news, bad news. Good news is that the caliper actually fits within the 15" 5-spoke VVC rim, even with the caliper spaced out to suit a 295mm diameter rotor! The bad news is that the caliper severely fouls the inner face of the spokes. As in "this wheel will need more than a 10mm spacer" fouling of the caliper... May be 15mm.
I couldn't even get the second wheel nut on!Conclusion
The caliper in the 295 and 280mm rotor positions clear the ULW wheel comfortably - perhaps no great surprise as the wheel was designed to clear the AP racing brakes from the outset - albeit with a different rotor offset and with a 304mm rotor diameter. As I'll be likely swapping Project Shed over to these wheels (because they are usefully lighter per corner than the current wheels and therefore further reduce un-sprung weight), I could just go with this prototype solution right now and run with it! However, this would be a fail on the "make the brakes clear many different wheel designs" ethos of the project.
16" Abingdon/ MGF Cup wheels foul the caliper in the 295mm position - and that interference would only increase if the rotor diameter were reduced to 280mm. However, modifications to the caliper mounting bracket would be fairly minimal, with a change of caliper offset inwards of about 5-7mm. Even 280mm could be accommodated if needed, albeit with a deeper rotor bell housing.
15" VVC style alloys could accommodate the AP racing 295mm set up, but this would require an even deeper rotor bell and caliper offset. However, this would be potentially do-able I think...
Plan
Not quite sure which way to go. Here are my thoughts so far:
- I don't want to go with the full-sized 304mm rotors because of (1) weight and (2) the limitations it places on wheel options.
- Ideally, I would go for 280mm rotors because of weight, but this would necessitate a much deeper bell (or top hat) for the rotor for all wheel styles bar the ULW tested.
- 295mm rotors are the ideal size for the MG AP rotors, so for purity and increased front leverage, this would be a good option - but even if using a 295mm rotor, I would still need a different offset rotor bell/ top hat. However, any option other than the ULW-mandating current set up would necessitate this anyway!
- If I ever wanted to drop to a 15" wheel diameter (cheaper track day tyres for this rim diameter with arguably better availability in suitable sizes), then 295mm with the AP caliper appears to offer the best chance of working with the 15" VVC wheel style.
- If going to 295mm front rotors, I think I would need to consider the knock on impact of altering the balance of the car's braking and the need to upgrade the rear brake arrangement in parallel (perhaps also to 295mm? perhaps also change the rear caliper? - all these are adding £££s to the original project budget, not to mention the associated "project creep" that this would provoke).
- How likely am I to go down the 15" rim route given that the ULW 16" is actually (paradoxically) so much lighter than the 15" wheel design?
What are your thoughts? Anyone else considering fitting AP racing brakes but under a 15" wheel or one of the many 16" wheel styles that are non-compatible with the standard AP brakes? Remember that what I am working on is a long way away from being a simple bolt on option with the standard AP racing calipers and would necessitate conversion to radial mount calipers - an option I doubt that many would be prepared to make?
